Experimental and numerical investigation on shot peening of low alloy steel

INTRODUCTION

B ACKGROUND

Shot peening is a cold working technique that improves material properties, particularly extending fatigue life by creating significant compressive residual stresses within the subsurface This process is increasingly utilized across various industries, including aerospace, automotive, and construction During shot peening, numerous balls strike the material's surface, leading to plastic deformation and the introduction of compressive stresses throughout the material As a result, the upper layers of the surface become hardened, compressed, and exhibit a rougher appearance.

Shot peening is a sophisticated process influenced by various parameters, including media type, velocity, nozzle pressure, media shape, and target material properties The effectiveness of shot peening relies on two key factors: intensity and coverage Intensity, measured by the Almen strip method, quantifies the energy of the shot stream impacting the surface, while coverage indicates the extent of the original surface affected by shot peening indentations Visual inspection is commonly used to assess coverage, alongside methods like the blue-ink technique, replicas, fluorescent tracers, and video analysis Both intensity and coverage are measurable factors that also depend on additional variables such as ball velocity, shape, nozzle pressure, shot peening duration, shot type, and material properties.

Ultra-fine grain materials have garnered considerable attention in both industry and academia due to their potential to provide enhanced mechanical properties compared to standard grain size specimens Research indicates that surface properties significantly influence the failure of engineering components, particularly in cases of fatigue fracture, fretting fatigue, wear, and corrosion It is often unnecessary for entire components to possess an ultra-fine grain structure; instead, achieving this microstructure at the surface or sub-surface level can suffice This can be accomplished through a process known as severe shot peening, which requires higher intensity and coverage than traditional shot peening methods.

M OTIVATION

Shot peening is extensively utilized across various industries, including aerospace, automotive, and construction, because it induces compressive residual stresses that effectively prevent surface crack formation and significantly enhance the fatigue life of mechanical components.

Recent studies, encompassing both experimental and numerical simulation methods, have demonstrated that shot peening can enhance material properties Nevertheless, there remain research gaps that must be addressed to gain a comprehensive understanding of the shot peening process and to achieve improved control over its effects.

Recent simulation studies have utilized two types of models: single shot impact and multiple-shot impacts The multiple-shot impact model primarily focuses on the effects of the shot peening process, analyzing a set number of shot impacts and their specific locations on the surface.

3 of the target material area is random in the reported experiments Therefore, it is necessary to employ random functions for proper shot peening simulations

Research on shot peening dates back to the 1920s, but the advancement of severe shot peening to create an ultra-fine grain (NC) layer on material surfaces emerged in the early 21st century This ultra-fine grain layer is anticipated to offer enhanced mechanical properties compared to the conventional coarse-grained interior However, despite its potential benefits, severe shot peening has not yet been adopted in industrial applications due to increased processing time and surface roughness concerns.

Shot peening is an effective technique used to restore materials with micro cracks and extend their lifespan During operation, fatigue fractures initially develop on the surface and can propagate into the material The compressive stresses generated by shot peening help inhibit both crack formation and propagation, significantly improving the material's fatigue life This process serves as a reliable method for preventing cracks and enhancing overall durability.

AISI 4340 low alloy steel is a high-strength material widely utilized in the automotive, construction, and mechanical industries due to its excellent plastic deformation capabilities Its applications include power transmission gears, shafts, aircraft landing gear, and various structural components, making it a key choice for machine tool applications This study focuses on the properties and uses of AISI 4340 steel.

O BJECTIVE

This research aims to experimentally and numerically examine how the shot peening process impacts the fatigue life properties of low alloy steel.

The scope is as follows:

The shot peening process was conducted under different conditions to examine how various operating parameters influence the mechanical and tribological properties of low alloy steel.

This study aims to explore the formation of ultrafine grain structures in the subsurface layer of materials subjected to severe shot peening It also compares the mechanical and tribological properties of conventional low alloy steel with those treated through severe shot peening, highlighting the effects of this advanced surface treatment on material performance.

A three-dimensional finite element method (FEM) model for shot peening has been developed to elucidate the shot peening process and predict how various parameters influence material properties This new approach includes a proposed process to validate the FEM results through experimental work Once validated, the FEM model can be utilized to optimize shot peening operating parameters for experiments in other case studies.

R EPORT OUTLINE

The structure of the document is organized as follows: The literature review relating to this research is presented in Chapter 2 Chapter 3 outlines the investigation of the effects of

This article explores the impact of shot peening parameters on the mechanical and tribological properties of materials Chapter 4 presents a comparison between the properties of conventional and severely shot peened steel A novel method for estimating the number of shots and their coordinates to ensure full coverage is introduced in Chapter 5, alongside a Finite Element Method (FEM) model developed to simulate the shot peening process Chapter 6 outlines a new process for comparing the FEM model results with experimental data The primary objective of shot peening is to enhance the fatigue life of components, and Chapter 7 investigates how various shot peening parameters affect the fatigue life of treated samples Finally, Chapter 8 concludes the research and outlines potential future work.

LITERATURE REVIEW

I NTRODUCTION

2.1.1 History of shot peening process

Shot peening has its origins in ancient craftsmanship, where artisans and blacksmiths utilized hammers to shape and strengthen armor The modern shot peening technique emerged in the 1920s, employing metal, ceramic, and glass shots in a process similar to sandblasting to create a thicker, hardened layer on various workpieces.

In 1943, Almen developed a non-destructive method for measuring the intensity of shot peening by assessing the arc height of an Almen strip post-process This technique became the most widely used for evaluating shot peening intensity By 1945, Milburn introduced X-ray diffraction as a means to measure the residual stresses generated by shot peening, enabling a connection between the intensity of the process and the resulting residual stresses.

Numerous studies have investigated the impact of shot peening on residual stress distribution and surface properties of treated materials Research indicates that shot peening significantly alters various characteristics, including residual stress, hardness, surface roughness, microstructure, and resistance to fatigue, wear, and corrosion.

In recent decades, ultrafine-grained materials have garnered significant interest from scientists due to their unique properties Among various production methods, severe plastic deformation has emerged as the most prominent technique for creating these materials.

The seven peening methods effectively create ultrafine-grained structures on the target surface layer Research indicates that the mechanical properties of materials subjected to severe shot peening undergo significant changes; however, comprehensive studies on severe shot peening remain limited.

An air-blast shot peening machine, as shown in Figure 2-1, utilizes a nozzle to propel media at high velocities, which are achieved through an air compressor system After the peening process, the used media is collected and recycled back into the system for continuous operation.

Figure 2-1 Schematic of shot peening equipment [21].

S HOT PEENING PARAMETERS

The overview of the whole shot peening process is described in figure 2-2 There are two outputs that are used to control the operating process, e.g.intensity and coverage Media

The intensity of shot peening is influenced by several key parameters, including media size, shape, hardness, shot flow rate, shot velocity, nozzle-workpiece distance, and impact angle In contrast, coverage is affected by factors such as the type of system (nozzle or wheel), nozzle geometry, shot velocity, angle of impact, mass flow rate, shot peening duration, the media used, and the properties of the workpiece Additionally, Figure 2-2 illustrates the various measurement methods employed to assess both intensity and coverage, as well as the changes in material properties resulting from the shot peening process, along with the relevant techniques for examining these properties.

Intensity Process parameters * Visual inspection

* Types systems: air nozzle/wheel

* Scanning electron microscope / Optical microscope

* Microhardness test (Knoop, Vickers, Brinell)

Figure 2-2 Theoverview of the whole shot peening process

2.2.1 Intensity: Intensity, arc height, saturation

Intensity serves as an indirect metric for assessing the energy transferred to a workpiece's surface, with the Almen strip and Almen gauge being essential tools for measuring this intensity.

Almen strips, specifically types N, C, and A, are crafted from SAE 1070 cold rolled spring steel and possess a standard hardness of 44-50 HRC While these strips share identical length and width dimensions, they differ in thickness, making them essential for measuring intensity in various applications.

Figure 2-3 demonstrates the process of measuring the arc height of a single Almen strip The strip is secured in a holding fixture and subjected to peening under specific conditions Once removed, the non-peened side of the strip bends, and an Almen gauge is used to measure the resulting curve height This data is then plotted against exposure time to create a saturation curve.

Figure 2-3 The Almen strip measurement system [36]

Saturation refers to the initial point on the saturation curve, where the arc height increases by 10% or less when the peening time is doubled In this context, intensity is characterized by the arc height of the strip at the saturation point.

Coverage is a critical parameter defined as the percentage of the peened area relative to the total surface area of a specimen Quality variations in coverage can lead to significant damage and component failure In many industrial applications, optimal coverage ranges from 100% to 200%, ensuring uniform residual stress without surface damage However, recent research indicates that achieving an ultrafine-grained structure on the specimen's surface may necessitate a coverage exceeding 1000%.

Several techniques can be used to assess the degree of coverage in shot peening, including visual inspection, blue-ink, replicas, fluorescent tracers, and videos, with visual inspection being the most common method Coverage tends to increase with peening time, although not in a linear fashion For instance, a coverage curve illustrating the relationship between shot peening time and degree of coverage is depicted in figure 2-5b Visual inspection is effective only when the coverage is below 100% To achieve a higher degree of coverage, such as 300%, the peening time should be estimated at three times the duration required to reach a coverage of 98%.

(b) Figure 2-5 Coverage (a) definition and (b) as a function of the shot peening time [1]

In the theoretical model proposed by Kirk [37], the shot size indentation, peening rate and exposure time to predict the degree of coverage was considered based on the Avrami equation (2-1)[38]

The coverage at any given time, denoted as C(t), is influenced by several factors: the average radius of indentations from a single shot (R), the mass flow rate of the shot (𝑚̇), the exposure time (t), the area over which the shot spreads (A), the average radius of the shot (r), and the density of the shot (𝜌).

R ESIDUAL STRESS

Residual stresses in mechanical parts can greatly extend their lifespan, as cracks typically do not form or spread in compressive zones Most fatigue and stress corrosion failures occur at or near the surface, making the compressive stresses generated by shot peening crucial for enhancing the durability of mechanical components.

Figure 2-6 Illustration of material deformation as a result of a peening process showing: a) initial material before peening, b) surface deformation during peening media impact, and c) resultant deformation after spring back [1]

Figure 2-7 Resultant residual stress gradient after shot peening [1]

Peening a flat sheet causes it to deform into a convex arc shape on the peened side, indicating that the top surface layers undergo plastic stretching This plastic deformation results in a permanent bend curve in the sheet, which can be explained by the underlying mechanics of the process.

During the shot peening process, a flat sheet is compressed and bent into a convex arc around the impact area, resulting in either elastic or plastic deformation based on the energy of the shot impact If the impact energy is low, the sheet returns to its original shape without generating residual stress Conversely, higher energy levels cause plastic deformation, primarily affecting the surface and near-surface regions, while the subsurface experiences elastic deformation Upon release of the impact load, the near-surface area partially springs back but remains elongated due to plastic deformation, whereas the subsurface attempts to revert to its original state but is hindered by the surface's plastic changes.

After shot peening, residual stresses are introduced in the subsurface of the treated material, with the maximum compressive residual stress typically occurring just below the surface This peak value is influenced by the impact energy and the mechanical properties of the peened material As the depth of the specimen increases, the magnitude of the compressive stress diminishes To maintain equilibrium, the compressive residual stress is counterbalanced by tensile stresses in the subsurface, which completely offset the compressive residual stresses created by the shot peening process.

2.3.2 Practical Residual Stress Measurement Methods

Residual stresses, often referred to as "lock-in" stresses, remain in materials after external loads are removed These stresses are balanced throughout the component, with the total of tensile and compressive stresses equaling zero As illustrated in Figure 2-8, residual stresses typically form in the sub-surface of materials following the release of these external loads.

Residual stresses are generated by nearly all manufacturing processes and can either develop or relax over the lifespan of components The mechanisms responsible for the formation of these residual stresses can be categorized, as illustrated in figure 2.9.

1 Non-uniform plastic deformation: including processes such as rolling, drawing and extrusion, forging, bending, cutting, turning, etc These methods are employed to change the shape of the material [39]

2 Surface modification: including some techniques such as machining, grinding, plating, peening, carburizing, etc [39] These methods are used to improve the surface properties of the material

3 The changes of material phase and/or density due to the presence of large thermal gradients also can create the residual stress in the material These methods are heat treatment, casting, quenching, phase transformation in metals and ceramics, welding, etc [39]

Figure 2-8 Schematic diagram of the cross-section of a material showing the formation and distribution of the residual stresses in a material [39]

After the manufacturing process, both tensile and compressive stresses develop in the material's sub-surface For instance, in the shot peening process, compressive residual stress zones are generated near the surface, while tensile stresses occur beneath these zones.

Figure 2-9 Examples of some common processes in which residual stresses are created in the material [39]

Residual stresses in materials can be categorized into three types: macro stress (Type I), micro stress (Type II), and a combination of both (Type III), which can occur simultaneously.

+ Type I: Macro residual stress This type stress develops in the component with the scale larger than grain size of the material [41];

+ Type II: Micro residual stress This stress alters on on the individual grain scale [39, 41];

+ Type III: Micro residual stresses exist within a grain of material due to the presence of dislocations and crystalline defects [41]

Over the past few decades, numerous methods have been developed to measure residual stresses in various materials Most of these techniques focus on Type I residual stresses and are categorized as either destructive or non-destructive methods.

Semi-destructive and non-destructive techniques, depicted in figure 2-10, are classified as mechanical or stress-relaxing methods used to evaluate stress-relaxation in components during material removal These techniques assess material deformations resulting from the release of residual stresses In contrast, non-destructive methods focus on measuring parameters related to stress, such as the interplanar spacing (d) compared to its stress-free value (d₀), allowing for strain analysis through Bragg’s law in X-ray diffraction Additionally, figure 2-11 and table 2-1 provide insights into the penetration depth, spatial resolution, and a comprehensive overview of the advantages and disadvantages of various residual stress measurement techniques.

Figure 2-10 Residual stresses measuring techniques [41]

Table 2-1 Comparison of the residual stresses measurement techniques [41]

Ductile Generally available Wide range of materials Hand-held systems Macro and Micro RS

Lab-based systems Small components Only basic measurements

Fast, Easy use Generally available Hand-held

Interpretation of data Semi destructive Limited strain sensitivity and resolution

Macro and Micro RS Optimal penetration and resolution 3D maps

Only specialist facility Lab-based system

Very quick Wide sensitive to Microstructure effects especially in welds

Only ferromagnetic materials Need to divide the microstructure signal from that due to stress

Limited resolution Bulk measurements over whole volume

Wide range of material Economy and speed Hand-held

Destructive Interpretation of data Limited strain resolution Contour

High-resolution maps of the stress normal to the cut surface

Hand-held Wide range of material Larger components

Destructive Interpretation of data Impossible to make successive slices close together

Deep interior stresses measurement Thick section components

Interpretation of data Semi destructive Limited strain sensitivity and resolution

Improved penetration and resolution of X-rays Depth profiling Fast

Only specialist facility Lab-based systems

Figure 2-11 Residual stresses measuring techniques [41].

S URFACE TEXTURE PARAMETERS

The surface texture parameters of shot peened materials are defined by the ISO 25178 standard, which outlines a vast array of field parameters calculated from the coordinates (x, y, z) of each data point within a defined area Unlike feature parameters that focus on specific points, field parameters assess surface characteristics such as heights, slopes, complexity, and wavelength content across the entire sampling area These parameters are categorized into four main types: height (amplitude), spatial, hybrid, and functional Additionally, typical applications for various 3D parameters are illustrated, highlighting the specific 3D measurements necessary for effective control and assessment.

Height parameters are crucial in the manufacturing industry as they offer straightforward standards for assessing the quality of surface finishes on specimens.

Figure 2-12 The surface texture parameters according to ISO 25178

Figure 2-13 Typical applications for various 3D parameters[44]

N UMERICAL SIMULATION OF SHOT PEENING PROCESS

Modeling and simulation of the shot peening process offer a cost-effective and efficient method for understanding and predicting outcomes without the need for costly experimental trials Additionally, the insights gained from simulations can enhance the design, control, and improvement of experimental procedures.

The dynamics of the single shot peening model have been extensively studied by researchers including Meguid, Hong, Bhuvaraghan, and Bae Their investigations focus on how various shot peening parameters—such as shot velocity, size, shape, and impact angle—affect the distribution of residual stress.

Figure 2-14 Single shot model: (a) Meguid [11], (b) Hong [45], (c) Bhuvaraghan [46], and (d) Bae [21]

Recent advancements in the modeling of multiple-shot impacts have been documented in various studies Research by Meguid and Eltobgy explored the influence of distance between shots, while Majzoobi investigated the impact of varying the number of balls on target materials, specifically comparing four and nine balls Additionally, Kim examined the effects of different cycles and impact sequences in multiple-shot scenarios Miao employed a random function to determine shot locations, yielding more reliable predictions compared to traditional fixed and symmetrical arrangements.

17) However, from the literature review, the number of balls required to achieve 100% coverage and the coverage of the shot peening process have still not been well documented

(a) (b) Figure 2-15 FE model of multiple impingements of multiple-shot impacts: (a) full model and (b) discretized symmetry cell in Meguid research [12]

Figure 2-16 Multiple-shot impact model of Majzoobi [48]: (a) four-shot model, and (b) nine-shot model

Figure 2-17 Multiple-shot impact model of Miao [49]

2.5.3 Numerical model of severe shot peening model

The research by Umemoto [33] concluded that the most important condition to produce nanocrystalline (NC) structure is a strain, with the minimum amount of strain

A necessary strain level of approximately 7 mm/mm is essential for effective severe shot peening, which also benefits from repetitive or cyclic deformation Bagherifard developed a model to predict the formation of a nanostructured surface layer, maintaining the same boundary conditions as traditional shot peening that utilizes random multiple-shot impacts However, to achieve the required strain, the severe shot peening process demands a significantly higher number of shots and increased velocity.

Figure 2-18 (a) Severe shot peening and (b) equivalent plastic strain (PEEQ) profile within the target measured from impacted surface of Bagherifard’s model [50]

The distribution of equivalent plastic strain (PEEQ) as a function of depth, illustrated in Figure 2-18b from Bagherifard's research, reveals that with 134 shots, the PEEQ exceeds 7 mm/mm This significant finding confirms the formation of an ultrafine layer on the material's surface.

2.5.4 An analytical model for the shot peening process: the elastic plastic with kinematic hardening model

Numerous material models have been developed for numerical simulation, each with unique attributes and applications Selecting the appropriate model is crucial and depends on the type of physical materials and the treatment processes involved Shot peening, a cold working technique, induces plastic deformation in the surface and sub-surface layers of materials Among the various models, the elastic-plastic model with kinematic hardening is particularly cost-effective for calculations This model is suitable for analyzing isotropic and kinematic hardening plasticity materials, including those that exhibit rate effects.

Figure 2-19 Elastic-plastic behavior with isotropic and kinematic hardening [51]

The parameter β, which ranges from 0 to 1, is essential for determining isotropic, kinematic, or a blend of both isotropic and kinematic hardening As illustrated in Figure 2.19, isotropic hardening occurs at β = 1, while kinematic hardening is observed at β = 0, with l0 representing the initial state and l indicating the current state.

In isotropic hardening, the yield surface's center remains constant while its radius varies with plastic strain Conversely, in the other scenario, the radius of the yield surface is constant, but the center shifts in the direction of the plastic strain.

[51] The yield condition is defined as [51]:

The co-rotational rate of α ij is p ij p ij  E 

  ij n  ij   ik n  kj   n jk  ki   n n ij t n n n  

Then, the Cowper Symonds model is employed to calculate the strain rate The current radius of the yield surface,  y , is calculated by [51]:

The Cowper-Symonds relation defines the static yield stress (σo) in terms of constant material parameters (C and p) Additionally, it incorporates strain rate (ε̇), plastic hardening modulus (Ep), and effective plastic strain (εeff p) to characterize the material's behavior under dynamic loading conditions.

The plastic strain rate is determined as [51]: e ij ij p ij  

In the material model implementation, the total strain rates (ε̇_ij) and elastic strain rates (ε̇_ij^e) are defined, with the deviatoric stresses calculated elastically as represented by the equation σ_ij = C_ijkl ε̇_kl This formulation is crucial for understanding the behavior of materials under stress.

    (2-12) where  ij , ij n ,C ijkl ,and kl are the trial stress tensor, stress tensor, elastic tangent modulus matrix and incremental strain tensor, respectively

The calculation concludes when the yield function is satisfied; however, if it is violated, an increase in plastic strain is computed During this process, the stresses are adjusted to align with the yield surface, and the center of the yield surface is updated accordingly We will examine the trial elastic deviatoric stress at the n+1 stage.

 >0 for plasticharding loading neutral or elastic for

For plastic hardening then p eff p eff p p y eff p eff n n

Scale back the stress deviatiors

E XPERIMENTAL STUDIES

Severe shot peening (SSP) effectively creates ultrafine layers on various materials, including pure substances, alloys, and intermetallic compounds Research primarily focuses on how these ultrafine layers enhance the mechanical properties of materials To analyze the microstructure of peened materials, techniques such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are commonly employed Additionally, X-ray diffraction (XRD) is utilized to assess residual stress and the full width at half maximum (FWHM) of diffraction peaks in components post-peening Recent studies have highlighted the benefits of properties in severe shot peened materials.

Most research has concentrated on the benefits of ultrafine layers created by severe shot peening in low alloy steel, particularly regarding fatigue life, residual stress distribution, surface roughness, and subsurface hardness variation However, there is a lack of studies exploring the impact of severe shot peening on other materials such as aluminum, titanium, and nickel alloys, as well as its effects on additional mechanical properties like friction, wear, scratch resistance, corrosion, and nitriding.

2.6.1 Effect of shot peening on fatigue

Fatigue failures typically begin at or near the surface of components and can spread inward, making surface treatment crucial for longevity Conventional shot peening has been widely utilized to mitigate these failures Additionally, the creation of a nanostructured surface layer through severe shot peening is anticipated to enhance the fatigue life of components significantly This improvement is attributed to two primary factors: first, the initiation and propagation of fatigue cracks are highly influenced by the surface structure and material properties; second, the substantial compressive residual stresses generated can effectively halt or slow down the initiation and spread of these cracks.

In a study by Bagherifard, the low alloy steel 39NiCrMo3 was examined under both conventional and severe shot peening conditions The results indicated that severe shot peening could significantly enhance fatigue life due to the development of an ultra-fine-grained surface layer However, the process also led to a notable increase in surface roughness, which diminished the expected improvements in fatigue life during rotating bending tests when comparing severe shot peened specimens to both conventionally peened and non-peened counterparts.

To mitigate the adverse effects of surface roughness from severe shot peening, Bagherifard [58] explored various methods, including extending treatment time, removing a thin surface layer through grinding and electropolishing, and repeening with smaller media The effectiveness of these techniques on surface roughness and fatigue life is illustrated in Table 2-2 and Figure 2-20 The findings reveal that polishing is the most effective method for reducing surface roughness, and while these approaches can enhance the fatigue life of severely shot-peened materials, the improvement is modest, showing only a 10% increase in fatigue strength compared to non-peened specimens.

Table 2-2 Surface roughness parameters of shot peened specimens [58].

Treatment l t (mm) l n (mm) R a (àm) R q (àm) R z (àm) R t (àm)

Figure 2-20 The true stress-strain curve in log-log axis (NP: not peened, SSP: severe shot peened, RSSP: re-peened, and GSSP: ground by abrasive wheel [58])

2.6.2 Effect of shot peening on residual stress distribution

X-ray diffraction (XRD) analysis is a widely used method for studying residual stress Research conducted by Bagherifard and Miková indicates that while the maximum stress values in severe shot peening and conventional shot peening are similar, the depth of the compressive stress layer in severely shot-peened materials is twice that of conventional shot-peened materials.

(d) Figure 2-21 Residual stress distribution: (a) Bagherifard’s report [63], (b) and (c) Bagherifard’s report [58], and (d) Miková’s research [64]

2.6.3 Effect of shot peening on surface hardness

The micro-hardness of specimens subjected to shot peening and severe shot peening was assessed using a diamond Vickers indenter with forces of 25 gf, 200 gf, and 50 gf The analysis revealed that the hardness peaked at the surface and decreased inward Notably, materials subjected to severe shot peening exhibited greater hardness compared to those that underwent conventional shot peening.

(c) Figure 2-22 Microhardness of peened specimens: (a) Bagherifard’s report [58], (b) Bagherifard’s report [54], and (c) Miková’s research [64].

S UMMARY

This chapter provides a concise history of the shot peening process, tracing its evolution from ancient artisans and blacksmiths to contemporary techniques It outlines key shot peening parameters and systems, offering a comprehensive overview of the process Additionally, the chapter explains the mechanisms behind the deformation zone generation and the resulting residual stress gradient following shot peening Finally, it reviews the current status and limitations of existing research in both simulation and experimental work related to shot peening.

EFFECTS OF SHOT PEENING PROCESS PARAMETERS ON THE MICROSTRUCTURE, MECHANICAL AND TRIBOLOGICAL PROPERTIES OF

I NTRODUCTION

Engineering materials are prone to major failure mechanisms like fatigue fracture, fretting fatigue, wear, and corrosion, which are highly influenced by their surface conditions and properties To enhance the fatigue life of metallic components, compressive residual stresses are introduced to both the surface and subsurface, effectively preventing the initiation and growth of fatigue cracks Common techniques for inducing these compressive residual stresses include shot peening and severe shot peening.

Shot peening is a widely utilized method across various industries, including aerospace and construction, with approximately 75% of components in airplane engines undergoing this process Other techniques such as laser shock peening, deep cold rolling, and roller burnishing are also employed in manufacturing, but shot peening remains the most prevalent.

Despite being utilized for over fifty years, comprehensive data on the mechanical and tribological properties of shot peened components under various conditions, including double shot peening, remains insufficient AISI 4340, a low alloy steel, is recognized for its exceptional strength and plastic deformation capabilities This steel is extensively employed in power transmission gears, shafts, aircraft landing gear, automotive applications, machine tools, and various structural components.

In this study, AISI 4340 low alloy steel was selected as a material for investigation Shot peening process with media S230, S110 and combining both these media (double shot

The mechanical and tribological properties of AISI 4340 steel samples, both as-received and subjected to shot peening at various pressures, were systematically investigated This study focused on key factors such as microstructure, surface roughness, micro-hardness, and ball-on-disc microtribological testing.

E XPERIMENTAL DETAILS

For the shot peening experiments, commercial AISI 4340 low alloy steel samples with a thickness of 8 mm were selected The nominal chemical composition of AISI 4340 steel is detailed in Table 3-1 Before conducting the shot peening, the steel samples underwent polishing with sandpaper of grit sizes 240, 600, and 1000 to remove surface oxide layers and contaminants.

Table 3-1 The nominal chemical composition of AISI 4340 steel (weight %)

C Si Mn Cr Ni Mo Fe

Before the shot peening process, the intensity was assessed at varying pressures ranging from 69 to 552 kPa using a standard Almen test A strip (SAE 1070, dimensions 76.1 mm × 18.95 mm × 1.29 mm), an Almen gauge, and a holding fixture The Almen test strip was subjected to different cycle times at each pressure level, with corresponding arc heights measured This data was used to create a plot of arc height versus time, which facilitated the generation of a saturation curve to accurately define the shot peening intensity.

In this study, all steel samples underwent a uniform shot peening process lasting 20 seconds, examining three test series at varying shot peening pressures Two widely used steel media, S230 and S110, were employed, with specific shot peening conditions detailed in Table 3-2.

Table 3-2 Shot peening process parameters used in chapter 3

Samples Media type Pressure (kPa)

The S230 and S110 media have the mean diameter of about 600 àm and 300 àm respectively and hardness of about 500 ± 30 Hv To investigate the effects of combining both

In a study on AISI 4340 steel, experiments were conducted using two sizes of media: larger media (S230) followed by smaller media (S110) for shot peening Coverage, defined as the percentage of the peened area relative to the total surface area of the specimen, was assessed The shot peened samples were examined under a 50x magnification optical microscope to evaluate the coverage level.

The experimental velocities of shots were measured using a ShotMeter G3 - Particle Velocity Sensor, positioned 150 mm from the nozzle outlet, while maintaining a constant mass flow rate of 3 Kg/min.

Scanning electron microscopy (SEM, JEOL-JSM-5600LV) was utilized to investigate the surface and wear morphologies of the samples, while optical microscopy (OM, Zeiss Axioskop 2, JVC color video camera) was used for analyzing the cross-sectional microstructures Before SEM and OM characterization, the samples underwent a chemical-mechanical polishing process, with detailed procedures outlined in Table 3-3.

Table 3-3 Sample preparation parameters for microstructural observation

MD-Largo MD-Dac MD-Nap MD-Chem

Suspension Largo 9 μm Diapro 3μm Diapro 1μm OPS

The Vickers hardness of the samples was assessed using a Future-tech FM-300e tester, applying a total force of 100 g An average hardness value was calculated based on twenty indentation measurements taken from each sample.

A nanoindenter (Agilent G2000) was utilized to assess hardness variation up to a depth of 2000 nm To analyze hardness as a function of depth from the surface, a line indentation with a spacing of 25 µm between two points was conducted The cross-sectional hardness was determined by averaging five measurements at each location.

The surface roughness of the samples was assessed using a Taylor Hobson Talyscan 150 surface profilometer equipped with a 4 µm diamond contact stylus To determine the average three-dimensional surface roughness parameters, five areal measurements were taken per sample, calculating metrics such as average areal roughness (Sa), root mean squared roughness (Sq), maximum peak height (Sp), maximum valley depth (Sv), and maximum surface height (Sz) These height parameters are essential for evaluating the quality of the material's surface finish and are defined according to ISO 25178-2:2012.

The tribological properties of the samples were analyzed through a ball-on-disc micro-tribological test using a 6 mm diameter 100Cr6 steel ball The tests involved a circular path with a 1.5 mm radius, conducted over 40,000 laps at a sliding speed of 50 mm/s and a normal load of 5 N, all at room temperature (approximately 22-24°C) After the completion of the tests, wear volumes were measured, with five measurements taken for each sample to ensure accurate average tribological results.

R ESULT AND DISCUSSION

Figures 3-1a and 3-1b illustrate the spherical shapes of media S230 and S110, which have mean diameters of approximately 600 μm and 300 μm, respectively Additionally, Figure 3-2 demonstrates the relationship between shot peening pressure and the velocities of S230 and S110 steel shots The data indicates that higher pressure leads to increased velocities for both types of steel shots However, as the pressure rises, the stability of the shot stream decreases, resulting in greater variability.

The dynamic energy of each shot was calculated using the formula E = ½ mv², where m represents the mean mass and v the mean velocity The mean mass for shots S230 and S110 were found to be 8.8 ± 0.5 x 10⁻⁴ g and 1.1 ± 0.1 x 10⁻⁴ g, respectively Despite shot S230 having a lower velocity than shot S110, its dynamic energy is significantly higher under the same shot peening pressure conditions, with this difference becoming more pronounced at increased pressures Consequently, the greater dynamic energies of the shots result in deeper and wider dimples on the peened materials.

Figure 3-3 illustrates the shot peening intensities measured at various shot peening pressures, showing a linear increase in intensities for both S230 and S110 steel shots The intensity of the shot peening process is assessed using three standard Almen test strips: N, A, and others.

C The N strip is employed at low intensity levels, the A strip is the most common and used in the medium range, while the C strip is used for very high intensity level only In this experiment, the standard A strip was used to measure the intensity of the shot peening process Therefore, the unit of intensity is A Besides, it is also indicated that the intensities measured in the case of S230 steel shots are higher than that for S110 steel shots, due to S230 steel shots having higher dynamic energies under the same pressure conditions

Figure 3-1 SEM micrograph showing overview of media: (a) S230 and (b) S110

Figure 3-2 The velocity of shot and dynamic energy of each S230 and S110 steel shot measured under different shot peening pressures

Figure 3-3 Intensities of S230 and S110 shots measured under different pressures

Figure 3-4 Surface topographies (above) and morphologies (below) of as-received AISI 4340 steel sample

Figures 3-4 to 3-7 illustrate the surface topographies and morphologies of shot peened AISI 4340 steel under various conditions The as-received sample (figure 3-4) exhibits a smooth surface, while samples treated with S230 steel shots show increased coverage from 60% at 68.9 kPa (figure 3-5a) to nearly full coverage of 98% at 206.8 kPa (figure 3-5c), achieving total coverage at 275.8 kPa (figure 3-5d) In contrast, samples treated with S110 media achieve full coverage even at the lower pressure of 68.9 kPa (figure 3-6a) Larger shot sizes, like S230, create deeper and wider dimples, increasing surface roughness compared to S110 (figures 3-5 and 3-6) To mitigate this roughness, a double shot peening process using smaller shots (S110) was implemented, resulting in improved surface finish only after the initial treatment at 206.8 kPa (figure 3-7).

Figure 3-5 Surface topographies (above) and morphologies (below) of the shot peened AISI

4340 steel samples treated by the media S230: (a) S230-10, (b) S230-20, (c) S230-30, (d) S230-40, (e) S230-50, (f) S230-60, (g) S230-70 and (h) S230-80

Figure 3-6 Surface topographies (above) and morphologies (below) of the shot peened AISI

4340 steel samples treated by the media S110: (a) S110-10, (b) S110-20, (c) S110-30, (d) S110-40, (e) S110-50, (f) S110-60, (g) S110-70 and (h) S110-80

Figure 3-7 Surface topographies (above) and morphologies (below) of the shot peened AISI

4340 steel samples treated by the double shot peening process: (a) DP-10-20, (b) DP-20-20, (c) DP-30-20, (d) DP-40-20, (e)DP-50-20, (f) DP-60-20, (g) DP-70-20 and (h) DP-80-20

(e) S z Figure 3-8 (a) S a , (b) S q , (c) S p , (d) S v and (e) S z parameters of the shot peened AISI 4340 steel samples as a function of shot peening pressure

Figure 3-8 shows the areal roughness parameters for the shot peened AISI 4340 steel samples under different shot peening conditions The average areal roughness as denoted by

The roughness parameters S_a and S_q demonstrate an increase with higher shot peening pressure and larger media size, with a notable peak S_a value of approximately 10μm observed for larger shot sizes This trend is mirrored in the peak and valley components of the surface, with S_p and S_v also increasing due to the impact of the shot on the surface Higher pressure results in greater surface deformation depth (S_v), while simultaneously pushing the deformed material upwards (S_p) The parameter S_z represents the cumulative effect of these changes.

S p and S v , a similar trend is observed as is the case for S v , which can be considered an average

The S z value for the surface, as shown in figure 3-8e, indicates that employing double shot peening significantly decreases overall roughness This improvement is particularly evident when a second peening stage utilizes a smaller shot size.

The cross-sectional microstructures of shot peened AISI 4340 steel samples reveal significant changes under varying conditions, as illustrated in figures 3-9 After etching with a mixture of 4% nital and 2% picral, the ferrite (α) grains appear white, while pearlite is shown in black The as-received samples, depicted in figures 9a and b, exhibit relatively uniform ferrite grain sizes with a mean diameter of 3 μm Notably, the analysis indicates that ferrite grain sizes decrease as shot peening pressure or media size increases, as shown in figures 3-9c-f.

Figure 3-9 Cross-sectional microstructures of the shot peened AISI 4340 steel samples under different shot peening conditions: (a) As-received, (b) Coarse-grained interior, (c) S110-40, (d) S110-80, (e) S230-40 and (f) S230-80

Figure 3-10 illustrates the surface hardness of AISI 4340 steel samples subjected to various shot peening conditions The shot peening process induces cold work hardening, resulting in increased surface hardness of the steel after treatment.

52 received sample is about 318 ± 8 Hv, while the hardness of the steels after shot peening with S110, S230 steel shots and double shot peening under the pressure of 551.6 kPa are 400 ± 25,

Shot peening treatment induces plastic deformation in samples through the impact of shot media, with varying deformation depths observed for S110, S230, and double shot peening at different pressures This process results in cold work hardening, leading to maximum hardness at the surface, which gradually decreases with depth Specifically, the hardness values for samples S230-80, S230-40, S110-80, and S110-40 are 440 ± 11, 386 ± 15, 412 ± 16, and 361 ± 18 Hv at the surface, respectively, and decrease to approximately 315 ± 20 Hv after a depth of 300 µm.

Figure 3-10 The surface hardness of the shot peened AISI 4340 steel samples under different shot peening conditions as a function of shot peening pressure

Figure 3-11 Cross-sectional hardness variations of the shot peened samples as a function of depth from the surface

The friction coefficients of AISI 4340 steel samples, both as-received and shot peened, were tested against a 100Cr6 steel ball under dry conditions, revealing a clear trend: increased shot peening pressure correlates with higher friction coefficients, ranging from 0.47 for the as-received sample to 0.51, 0.55, and 0.52 for the S110-80, S230-80, and DP-80-20 samples, respectively Notably, larger shot sizes (S230) consistently produced higher friction coefficients across all pressures Additionally, a second shot peening process using S110 at 137.9 kPa was effective in reducing the friction coefficients initially increased by the larger S230 shots The findings further indicate that the rougher surfaces of the shot peened samples enhance friction through mechanical interlocking between the contacting surfaces.

54 increased shot peening pressure can be correlated to the significant increase in their surface roughness (figure 3-8)

The friction coefficients of shot peened AISI 4340 steel samples, tested against a 100Cr6 steel ball under 5 N in dry conditions, varied with treatment conditions and the number of laps, as shown in Figure 3-13 The as-received sample exhibited the lowest friction coefficient, while the S230-80 sample had the highest throughout the sliding test Initially, all samples experienced an increase in friction coefficients during the first 5000 laps, known as the running-in period, due to the removal of surface asperities; this was followed by a stabilization in friction during prolonged sliding.

Figure 3-12 The friction coefficient of the shot peened AISI 4340 steel samples under different shot peening conditions as a function of shot peening pressure

Figure 3-13 Friction coefficients of the shot peened AISI 4340 steel samples tested against a 100Cr6 steel ball under different shot peening conditions as a function of a number of laps

Figure 3-14 Wear volume of the shot peened AISI 4340 steel samples tested against a 100Cr6 steel ball under different shot peening conditions as a function of shot peening pressure

Figure 3-14 illustrates the wear volume of as-received and shot peened steel samples under varying shot peening pressures The as-received sample exhibits a wear volume of approximately 25.3 ± 1.9 x 10^-3 mm³ Under dry conditions, the wear volume decreases with increasing pressure across all tested media (S110, S230, and double shot peening), attributed to enhanced wear resistance from higher hardness At lower pressures (68.9 kPa and 137.9 kPa), smaller shot sizes (S110) demonstrate better wear resistance, while larger shot sizes yield lower wear volumes at pressures exceeding 206.8 kPa This discrepancy is due to incomplete coverage in samples treated with media S230 at lower pressures Additionally, the data shows that second shot peening significantly reduces wear volume for samples initially treated with S230 across all tests.

The wear topographies and morphologies of the as-received and shot peened AISI

Figure 3-15 displays 4340 steel samples subjected to various shot peening conditions The as-received sample, when tested against a 100Cr6 steel ball, shows a larger wear track than the shot peened samples, indicating superior wear resistance in the latter Notably, some dimples created by the shot peening process remain visible on the wear track, as seen in figures 3-15c, d, e, and g This observation suggests that the dimples formed under high pressure are deeper than the wear depth caused by the sliding ball.

Figure 3-15 Wear topographies and morphologies of the shot peened samples: (a) As- received, (b) S110-40, (c) S110-80, (d) S230-40, (e) S230-80, (f) DP-40-20 and (g) DP-80-20

Figures 3-16a and b show the wear morphologies of the 100Cr6 steel balls rubbed on the surfaces of the as-received sample and S230-80 sample, respectively The larger wear

A scar measuring 59 was observed on the 100Cr6 steel ball that slid over the treated sample, highlighting that the increased hardness and roughness of the shot-peened sample significantly contribute to the elevated wear experienced by the 100Cr6 steel ball.

Figure 3-16 Wear morphologies of 100Cr6 steel balls slid on AISI 4340 steel samples: (a) As-received and S230-80 sample.

C ONCLUSION

The microstructure, mechanical and tribological properties of the shot peened AISI

4340 low alloy steel under different shot peening conditions were systematically investigated The following conclusions can be drawn from the obtained results:

 Under the same nozzle pressure, the smaller S110 steel shots have higher velocities but smaller dynamic energies which result in lower measured intensities compared with S230 steel shots

At low shot peening pressures of 68.9 kPa and 137.9 kPa, samples treated with S230 steel shots fail to achieve full coverage, whereas those treated with S110 steel shots do Full coverage with S230 steel shots is only attained when the shot peening pressure exceeds 206.8 kPa.

 The hardness of the shot peened sample increases with the increasing shot pressure and media size due to higher cold working

Increasing shot pressure and media size during shot peening leads to greater surface roughness in the treated samples This phenomenon occurs because larger steel shots, propelled at higher velocities, create deeper and wider dimples on the target material, enhancing its surface characteristics.

 The shot peening process influences the microstructure of shot peened steels by reducing the grain size on the sub-surface depth of influence

Increased shot peening pressure and larger media sizes enhance the friction coefficient while simultaneously reducing the wear volume of shot peened samples This effect is primarily attributed to the substantial improvements in surface roughness and surface hardness achieved through the shot peening process.

 The larger wear scar found on the 100Cr6 steel ball rubbed on the rougher and harder shot peened sample

 The double shot peening process not only reduces the surface roughness but also improves the tribological properties of the shot peened material

INVESTIGATION OF THE PROPERTIES OF CONVENTIONAL

I NTRODUCTION

Ultrafine grain materials, characterized by their nanometer-scale size, have garnered considerable attention from both industry and academia due to their exceptional mechanical properties The surface characteristics of these materials significantly affect the failure modes of engineering components, such as fatigue fracture and corrosion As a result, it is often advisable not to fabricate entire components from ultrafine grain materials; instead, enhancing the subsurface near the surface can markedly improve surface properties without altering the material's chemical composition or shape Achieving ultrafine grain structures requires an intense form of shot peening known as severe shot peening (SSP), which has proven effective in creating ultrafine grain layers across various materials, including alloys and intermetallic compounds Notably, this high-intensity process not only generates these fine grain layers but also induces substantial compressive residual stresses However, a downside of severe shot peening is the increased surface roughness it creates To mitigate this issue and enhance the surface finish, a secondary shot peening process with smaller media and lower intensity, referred to as re-severe shot peening or double severe shot peening, is recommended.

Singh et al [78] reported that the wear volume loss of quenched and tempered SAE

The shot peening process has been shown to enhance the properties of various steel alloys, including 6150 steel Research by Mitrovic et al demonstrated that shot peened 36CrNiMo4 and 36NiCrMo16 alloy steels exhibit improved wear resistance in both dry and lubricated conditions Additionally, Matsui et al found that steel samples treated with pre-shot peening showed better tribological performance under dry rolling/sliding contact conditions Unal et al further investigated AISI 1017 steel and concluded that severe shot peening significantly increased surface hardness, stiffness, and wear durability.

This chapter examines the mechanical and tribological properties of heat-treated AISI 4340 low alloy steel samples subjected to both conventional and severe shot peening conditions The hardness distribution was assessed using a nanoindentor, while friction and wear characteristics were analyzed through ball-on-disc micro-tribological experiments.

M ATERIALS AND METHODS

This study utilized AISI 4340 steel for all experiments, which has a nominal chemical composition of approximately 0.34% carbon, 0.24% silicon, 0.51% manganese, 0.91% chromium, 1.54% nickel, and 0.25% molybdenum The steel underwent heat treatment at 815 °C, followed by oil quenching at around 20 °C and subsequent tempering.

Table 4-1 Shot peening parameter conditions used in chapter 4

Shot size Coverage Peening times

Re-Severe shot peening (Re-SSP)

As-received NA NA NA NA NA

The samples were mounted on a fixture and subjected to shot peening under various conditions, as detailed in table 4-1 To achieve a coverage of 120%, the specimens underwent shot peening for 150 seconds, necessitating a total peening time of 1500 seconds to reach a coverage of 1200% The media utilized in this study were S230 and S110, which are high-quality round steel shots commonly used in industrial applications.

R ESULTS AND DISCUSSION

The cross-sectional optical microstructure of the as-received steel sample, depicted in Figures 4-1a and b at various magnifications, reveals a 20 µm thick iron oxide layer formed during the heat treatment process This iron oxide layer negatively impacts the mechanical properties of the steel, rendering it unsuitable for mechanical components To mitigate the formation of this detrimental layer in critical mechanical parts, it is advisable to utilize a vacuum furnace during heat treatment or to perform a polishing process afterward Additionally, the mean grain size of the as-received sample, as measured by optical microscopy, is approximately 10 µm Figures 4-1c and d illustrate the cross-sectional microstructure of conventionally shot peened samples.

The study of sample 2 (CSP2) at various magnifications reveals that the iron oxide layer on conventional shot peened samples is partially removed, maintaining a depth of approximately 3-5 μm The shot peening process effectively reduces the mean grain size of the second layer, located 100-150 μm deep, to about 5 μm This indicates that shot peening is an effective method for eliminating the iron oxide layer from surfaces In severe shot peened (SSP) samples, the high-energy impact from the shots completely removes the iron oxide layer, as evidenced by specific magnifications in the microstructure The cross-sectional analysis shows three distinct grain size layers, with an ultrafine grain size layer on the top surface, approximately 20 μm deep, resulting from significant strain during severe plastic deformation.

The 2μm layer, measured at a depth of 150μm using an optical microscope, experienced a high strain process that resulted in a reduction of grain size compared to the original In contrast, the final layer retains the original grain size, averaging around 10μm, where the impact of deformation is significantly diminished.

Hardness tests conducted with an Agilent G200 Nanoindenter revealed the microhardness distribution along the sample's depth The heat treatment process in the normal furnace resulted in the depletion of elemental carbon from the surface to a depth of 300-500 μm, with the amount of escaped carbon decreasing from the surface into the interior material This phenomenon led to a reduction in hardness.

65 in the hardness of heat-treated sample at the surface and near surface layer (up to 500 μm)

Figure 4-1 Cross-sectional microstructure of shot peened AISI 4340 steel at different magnifications: (a) and (b) as-received, (c) and (d) CSP2 sample, (e) and (f) SSP sample

(b) Figure 4-2 (a) Micro-hardness distribution along the depth from the surface of the sample and (b) Surface roughness of AISI 4340 steel after undergoing different shot peening conditions

The shot peening process effectively induces plastic deformation on the surface of the target, leading to work hardening in the treated samples Notably, the near-surface hardness of the SSP, Re-SSP, CSP1, and CSP2 samples significantly increased compared to the as-received specimen, with hardness values measuring 6.5, 6.0, 4.8, 3.7, and 3.1 GPa, respectively A distinct difference in hardness profiles between severe and conventional shot peening is observed within approximately 50 μm from the surface, where ultrafine grains form in the severely shot-peened samples due to the intense deformation process.

The surface roughness of AISI 4340 samples was evaluated using a Taylor Hobson Talyscan 150 profilometer The results, illustrated in Figure 4-2b, indicate that the treated samples exhibit significantly higher areal arithmetic average (S a), root mean squared (S q), maximum height of surface (S t), and average distance between the five highest peaks and five lowest valleys (S z) compared to the as-received samples Notably, the differences in S t and S z values highlight the sensitivity of these measurements to peak heights and valley depths resulting from the shot peening process Furthermore, employing a smaller shot size after intensive shot peening effectively reduces the surface roughness across all measured parameters: S a, S q, S t, and S z.

(e) Figure 4-3 Surface morphologies and topographies: (a) SSP, (b) Re-SSP, (c) CSP1, (d) CSP2 and (e) as-received AISI 4340

Figures 4-3a-d illustrate the surface morphology and topography of shot peened samples, while figure 4-3e depicts the as-received sample, which exhibits a relatively rough surface due to an iron oxide layer formed during heat treatment The shot peened samples display distinct dimples on their surfaces, with deeper dimples and rougher textures observed at higher shot peening pressures Additionally, the re-severe shot peening process effectively smooths the surface topography by reducing peak heights, a finding that aligns with the surface roughness measurements shown in figure 4-2b.

The tribological properties of shot peened samples were investigated by the ball-on- disc micro-tribometer (CSM) machine The sliding friction coefficient was measured by the

In this study, the friction force was analyzed with a ratio of 70 to the normal load on the pin The pin stylus remained stationary while samples were tested on a rotating disk The tribological properties of the samples were examined by sliding them against 100Cr6 steel balls, each with a diameter of 6 mm, along a circular path with a radius of 1.5 mm for a total of 40,000 laps at a consistent sliding speed.

In a sliding test conducted at a speed of 50 mm/s and a normal load of 5 N at room temperature (22–24°C), wear tracks were observed on the surfaces of samples mounted on a rotating disc and a stationary pin, indicating material loss due to friction and wear This material removal is attributed to abrasive wear mechanisms, including both two-body and three-body wear The average friction coefficients for each sample were determined from three wear tests, with surface profilometry used to measure the width and depth of wear tracks, allowing for the calculation of average wear volume The friction coefficients for the shot peened samples against 100Cr6 steel balls were found to be 0.53 for SSP, 0.51 for Re-SSP, 0.47 for CSP1, 0.48 for CSP2, and 0.46 for the as-received sample Results indicate that conventional shot peening (CSP1, CSP2) slightly increases the friction coefficient compared to the as-received sample, while SSP and Re-SSP samples exhibit significantly higher values, likely due to increased surface roughness that enhances mechanical interlocking between the rubbing surfaces Notably, despite the Re-SSP sample having a surface roughness similar to CSP1 and lower than CSP2, it still demonstrated a higher friction coefficient, suggesting that the ultrafine grain layers formed on SSP and Re-SSP samples possess distinct properties compared to the original material.

The highest friction coefficients observed in Re-SSP are attributed to the ultrafine grain layer of the material As shown in Figure 4-4b, the friction coefficients of shot peened steels against a steel ball under a 5N load increase gradually during the initial 10,000 laps, known as the running-in period, due to the wear of surface asperities Beyond 10,000 laps, the friction coefficient stabilizes throughout the sliding test.

Figure 4-5 displays the wear volume of shot peened AISI 4340 steel samples subjected to varying peening conditions, tested against a steel ball under a normal load of 5N The wear volumes were determined by analyzing the width and depth of the wear track profiles measured after 40,000 laps using a Taylor Hobson Talyscan 150 surface profilometer The results indicate that the observed reduction in wear volume is likely attributed to enhanced wear resistance linked to increased surface hardness This correlation is further supported by the hardness distribution of the samples, where the sample with the highest surface hardness (Re-SSP) exhibited the lowest wear volume of 8x10^-3 mm³, while the sample with the lowest hardness (as-received) showed the highest wear volume of 27x10^-3 mm³.

The friction coefficient of shot peened samples was evaluated in dry sliding wear tests against 100Cr6 steel balls Results indicated that the friction coefficients of the shot peened steels varied with the number of laps under a load of 5N.

Figure 4-5 Wear volume of ASIS 4340 at different shot peening conditions

Figure 4-6 shows the wear track morphology and topography of the shot peened AISI

4340 steel exhibits significant abrasive wear characterized by grooves on wear tracks created by the repetitive sliding of a steel ball under a load of 5N The as-received sample demonstrates the largest wear track and the highest material loss In contrast, samples subjected to severe and re-severe shot peening show reduced wear tracks and volumes, attributed to their enhanced wear resistance Additionally, some dimples formed during the shot peening process retain greater depth than the material removed by the sliding ball, indicating their persistence despite wear.

(e) Figure 4-6 Surface morphologies and topographies of wear tracks: (a) SSP, (b) Re-SSP, (c) CSP1, (d) CSP2 and as-received AISI 4340 steel.

C ONCLUSION

This chapter examines the mechanical and tribological properties of low alloy steel AISI 4340 after quenching and tempering, subjected to both conventional and severe shot peening Following heat treatment, a 20 µm iron oxide layer developed on the surface of the samples The conventional shot peening process partially eliminated this layer and enhanced the material's properties, whereas the severe shot peening process completely removed the oxide layer from the surface.

Severe shot peening produces an ultrafine grain layer with a depth of 20 µm, significantly enhancing the surface and subsurface hardness of the material through a cold work hardening effect This increase in surface hardness, achieved via both conventional and severe shot peening processes, improves the material's wear resistance by reducing the wear volume of peened samples.

The severe shot peening process significantly increases the surface roughness of samples due to deep indentations, which in turn raises the friction coefficient against steel balls This high surface roughness is viewed as a detrimental side effect of the process To mitigate this issue, an additional shot peening treatment using smaller media was implemented, successfully reducing surface roughness and enhancing the wear resistance of low alloy steel (Re-SSP).

The severe shot peening process effectively removes the iron oxide layer and creates an ultrafine grain layer on low alloy steel surfaces Experimental findings indicate that this newly formed layer significantly impacts the mechanical and tribological properties of AISI 4340 low alloy steel specimens.

Chapter 5: Three - dimensional modeling of shot peening process

I NTRODUCTION

Computational modeling is essential in manufacturing processes like shot peening, as it helps avoid costly and time-consuming experiments for process optimization Various approaches have been proposed for the numerical simulation of shot peening, focusing primarily on the development of stress states during impact rather than the coverage parameter Previous studies, including a review by Bagherifard et al., have examined multiple-shot impact models that utilize uniformly distributed positions and prearranged sequences While symmetry patterns can reduce computational costs, they often fail to capture the random nature of actual shot peening, leading to incomplete simulations of full coverage conditions.

A random impact model was developed by Miao et al.[49], where the center of shot coordinates was generated by equation (5-1)

(5-1) where rand(1,1) is a uniform pseudo-random number generator in the interval [0,1] and N is the number of impacts

Bagherifard et al [50] developed a multiple-shot random impact simulation model to predict the generation of the nanostructured surface layer In this research, to obtain the

78 coverage parameter, the Kirk and Abyaneh model [95] given by the Avrami equation (equation 5-2) was applied

 (5-2) where N is the number of shots, d is the diameter of a dimple caused by a single shot;

In shot peening experiments, achieving a diameter (D) that ensures 98% coverage of the target material is crucial, as this level of coverage is visually assessable and deemed full To reach this 98% coverage, an Ar value of 4 is required.

High shot intensity from large, high-velocity shots effectively induces significant compressive stress in target materials; however, it often results in increased surface roughness To enhance the surface finish of shot peened components subjected to high intensity, a technique known as double shot peening employs lower shot intensity with smaller shots at reduced velocities to minimize roughness While experimental findings on double shot peening have been documented by Vielma and Scuracchio, there remains a notable gap in modeling and simulation knowledge in this area.

This chapter presents a novel approach for estimating the number of shots and arrangements necessary for complete coverage of target materials Utilizing a multiple-shot impact model within the LSDYNA commercial explicit finite element analysis program, the study examines how various shot peening parameters influence the distribution of compressive residual stresses in the material Additionally, the simulation explores the impact of double shot peening on the residual stress distribution.

M ATERIALS AND METHODS

This research presents a novel simulation model to estimate the coverage of target material during the shot peening process, based on the Kirk and Abyaneh model The model simplifies the process by assuming a fixed diameter of circular impressions created by uniformly sized spheres impacting a flat surface The new approach calculates the necessary number and arrangement of shots to achieve 98% coverage To facilitate this estimation, a Matlab code has been developed, following the flow chart outlined in figure 5-1.

The initial step in the flowchart involves calculating the diameter of a dimple created on the surface of the target material from a single impact As illustrated in Figure 5-2a, the target area is segmented into several small cells, each assigned a value of 1, indicating that the node is not peened When the first ball, with radius "R," randomly strikes the target (with its center positioned within the target area), it produces an indentation with radius "r." Consequently, the values of all nodes within the circle transition from "1" to "0."

The value "0" indicates that the node is peened and remains unchanged (figure 5-2b) After each shot, the degree of coverage is calculated using equation (5-3) If the coverage ratio falls below 98%, an additional ball will impact the area (figure 5-2c) until the ratio exceeds 98% Once this threshold is met, the program will cease operations and provide the coordinates of all impacted balls along with the final coverage results.

Figure 5-1 A sequence of multiple-shot impact shot peening process with the aid of Matlab TM

Figure 5-2 A sequence of a new method: (a) an original target surface, and shot peened surfaces after (b) first impact, and (c) first two impacts

To validate the newly introduced method for estimating the coverage of the shot peening process, a multiple-shot impact finite element model (FEM) was developed The steel target material parameters were based on Majzoobi's study, featuring a density of 7800 kg/m³, a Young's modulus of 210 GPa, and an initial yield stress of 1500 MPa The shots were modeled as rigid with radii of 0.15 mm and 0.3 mm, while the target material dimensions were set at 2 mm x 2 mm x 1.5 mm, with a shot peened area of 1 mm x 1 mm Both the target material and shots were meshed using an eight-node brick element with a size of 0.04 mm, and the dynamic friction coefficient was established at 0.1 The plastic strain-rate dependent behavior adhered to the Cowper-Symonds law.

 (5-4) in which σ o is static yield stress, C and p are material constants The values C and p of steel are 2.1e+11 and 3.3, respectively [48]

The rigidity of shots is determined by the need for their yield strength and hardness to exceed that of the target material In the simulation, interactions between shots were not considered, and all shots were modeled as perfectly spherical, impacting the target at uniform velocities ranging from 30 m/s to 50 m/s Additionally, the bottom surface of the target was fully constrained during the simulation.

R ESULTS AND DISCUSSION

Figure 5-3 presents an example result of the introduced method, i.e an indentation with r = 0.064 mm caused by the impact between a 0.3 mm diameter shot at 40 m/s on 1 mm

The results of the shot peening on an 82 x 1 mm target show a coverage of 98.11% after 311 shots, as illustrated in Figure 5-3a Figure 5-3b demonstrates the relationship between the number of shots and the target surface coverage, indicating that the curve closely resembles the Arvami curve when the area of a single dimple (πd²/4) is significantly smaller than the total area of the shot-peened target (πD²/4) This small ratio of areas (d²/D²) suggests a high number of shots (N) is necessary for effective coverage.

Figure 5-3 (a) Coordinates of impact shots employed to obtain coverage of 98%, and (b) dependence of the degree of coverage on a number of shots

This study utilized a random function to determine shot positions, focusing on varying the number of shots and their locations to assess coverage For instance, with a ratio of R = 0.15 and a velocity of v = 40 m/s, five consecutive runs yielded coverage percentages of 98.11%, 98.05%, 98.03%, 98.12%, and 98.01%, requiring 311, 309, 315, 314, and 310 shots, respectively Notably, the variance in the number of shots needed to achieve 98% coverage became more pronounced with an increased (d/D)² ratio.

The shot peening process is inherently random, resulting in a variable number of shots required to achieve complete coverage of a target surface area.

The Kirk and Abyaneh model lacks the standard normal distribution, as evidenced by equation (5-2), which yields a single result that only becomes accurate when the sample size N is sufficiently large In this scenario, N approximates the mean value μ of the standard normal distribution Thus, when the number of shots N is large enough, the model aligns closely with the mean value μ, demonstrating consistency with the proposed method This relationship is illustrated in Figure 5-3.

The Kirk and Abyaneh model proves inaccurate when the number of shots (N) is low For instance, a shot with a diameter of 1.2 mm, impacting the material at 115 m/s, resulted in a single indentation of 0.58 mm in diameter According to equation (5-2), achieving 98% coverage in a 1 mm² area would require 15 impacts, indicating a lack of accuracy in the model Analysis of six consecutive runs reveals varying shot counts—23, 24, 20, 14, 17, and 26 impacts—yielding coverages of 100%, 98.30%, 99.98%, 99.24%, 98.82%, and 99.89%, respectively, showcasing significant variability due to the randomness of the shot peening process Figure 5-4g illustrates the relationship between coverage and the number of shots, demonstrating that while increasing shots enhances coverage, the proposed method's results align closely with the Arvami curve.

Figure 5-4 (a-f) Coordinates of impact shots employed to obtain coverage of 98% in six consecutive repeated runs, and (g) dependence of the degree of coverage on a number of shots

5.3.2 FEM multiple-shot impact model

This study explores the impact of full coverage in the shot peening process using a novel method applied to a multiple-shot impact model As illustrated in Figure 5-5, multiple shots were directed at the surface of a kinematic plastic model The research examines how factors such as coverage, shot velocity, and shot size influence the distribution of residual stress and surface roughness in the target material under varying process conditions.

Figure 5-5 Finite element model of multiple-shot impacts

In finite element modeling, a balance must be struck between achieving greater accuracy through smaller mesh sizes and the computational resources required Evaluating the impact of mesh size on simulated results is crucial to determine the optimal number of elements that ensure sufficient simulation time without sacrificing accuracy This study examines the distribution of residual stresses in the center plane (y = 0) and along the z direction at the contact point, utilizing five different mesh sizes (0.06 mm, 0.05 mm, 0.04 mm, and 0.03 mm) The results indicate convergence when the mesh size is reduced to less than 0.05 mm, leading to the selection of a 0.03 mm mesh size for the simulation.

The residual stress distribution is analyzed on the plane y = 0 using various mesh sizes: 0.05 mm, 0.04 mm, and 0.03 mm, as illustrated in Figure 5-6 Additionally, the distribution is examined along the z direction beneath the contact point, highlighting the impact of different mesh sizes on the results.

5.3.2.2 Validation the studied FEM model with the reference model

To validate the studied FEM model, a comparison was made with a reference model at a velocity of 50 m/s Figures 5-7a and b illustrate the multiple shot impact model and target plate with indentations from Majzoobi's research The results from the numerical study of Majzoobi and the experimental work of Torres, when compared to this FEM model, show a close correlation, indicating a similar compressive residual stress profile across all studies.

The compressive stress measured is approximately 1.2 GPa, with a compressive residual stress zone depth of around 200 µm Notably, a key difference between this study's findings and previous research is observed in the residual stresses located beneath the 200 µm depth Chapter 2's literature review highlights that during the deformation process, the presence of compressive residual stress in the subsurface leads to the development of tensile stresses throughout the material to maintain equilibrium.

(a) The multiple shot impacts model of

(b) The target plate with indentations created by shot impacts of Majzoobi’ model [48]

(c) The residual stresses profile distribution at different points of Majzoobi’s report [48] and

Torres’s work [98] at the velocity of 50 m/s

(d) The residual stresses profile distribution in this study Figure 5-7 The residual stresses profile distribution in this studied FEM model at the velocity of 50 m/s

5.3.2.3 Effect of shot peening process parameters on the residual stress distribution

The influence of coverage on the residual stress of the target material was analyzed and illustrated in Figures 5-8 This study involved multiple impacts from a shot with a 0.3 mm radius, striking the target material at a velocity of 50 m/s.

The residual stress distribution is analyzed across various coverage degrees, showcasing the following: (a) at 100% coverage on the center plane (y = 0), (b) at 200% coverage on the same plane, (c) at 300% coverage also on the center plane, and (d) the average residual stress along the z direction (x = 0, y = 0) within the target area at different coverage levels.

Figures 5-8a-c display that, on the centre plane (y = 0), the compressive zone becomes slightly deeper, when the coverage increases from 100% to 300% The number of

In this study, 91 shots were used to achieve coverages of 100%, 200%, and 300%, resulting in 69, 137, and 208 shots, respectively The distribution of average residual stress along the z direction on the peened area is illustrated in Figure 5-8d The maximum compressive residual stress recorded for 100% coverage was -1.15 GPa, which is lower than the -1.40 GPa achieved with 300% coverage (as shown in Figure 5-6d) Additionally, the depth of influence of these compressive stresses increases with higher coverage levels The simulation results indicate that increasing the coverage is an effective method for enhancing compressive residual stress in the target material.

The study analyzed the impact of shot velocity, specifically with a shot diameter of 0.3 mm, using a multiple-shot impact model at velocities of 30, 40, and 50 m/s Findings revealed that as shot velocity increases, the size of the indentation from each shot also increases Consequently, the number of shots required to attain 98% coverage decreased with higher velocities, resulting in 82 shots at 30 m/s, 73 shots at 40 m/s, and 69 shots at 50 m/s.

Increasing shot velocity enhances the compressive residual stress region, as illustrated in Figures 5-9a-c Specifically, Figure 5-9d indicates that shots at 50 m/s generate greater compressive residual stress compared to those at 30 m/s Moreover, the compressive stress zone expands from 0.125 mm to 0.200 mm when shot velocity rises from 30 m/s to 50 m/s This demonstrates that higher shot velocities effectively create a deeper compressive stress zone in the target material Similar findings were reported by Meguid et al and Majzoobi et al., confirming the impact of velocity on residual stress distribution in shot-peened materials.

C ONCLUSION

A new approach was introduced to estimate the number of shots and their arrangements to obtain the desired coverage This method could provide the number of shots,

A finite element simulation was conducted to study multiple-shot impact models of the shot peening process, revealing that increasing coverage, shot velocity, and shot size effectively enhances compressive residual stress and deepens the compressive stress zone in the target material The results indicated that smaller shots could be applied after peening with larger shots, slightly reducing the compressive residual stress without affecting the depth of the stress zone This second shot peening process, when applied at low intensity, is a simple and cost-effective method for improving the surface finish of shot peened components.

EXPERIMENTAL AND NUMERICAL INVESTIGATION OF THE

FATIGUE LIFE OF SHOT PEENED LOW ALLOW STEEL

CONCLUSION AND FUTURE WORK