Volume 16 - Machining Part 2 docx

Table 7 Summary of possible surface alterations resulting from various material removal processesR, roughness of surface; PD, plastic deformation and plastically deformed debris; L & T,

Table 7 Summary of possible surface alterations resulting from various material removal processes

R, roughness of surface; PD, plastic deformation and plastically deformed debris; L & T, laps and tears and crevicelike defects; MCK, microcracks; SE, selective etch; IGA, intergranular attack; UTM, untempered martensite; OTM, overtempered martensite; OA, overaging; RS, resolution or austenite reversion; RC, recast, respattered metal, or vapor-deposited metal; HAZ, heat-affected zone

Process

Conventional Nontraditional

Material

Milling, drilling,

or turning

Grinding Electrical

discharge machining

Electrochemical machining

Chemical machining

Hardenable 4340 and D6ac steels

Type 410 stainless steel (martensitic)

Type 302 stainless steel (austenitic)

Table 8 Comparison of depth of surface integrity effects observed in material removal processes

Maximum observed depth of effect(a) Turning or

milling

Drilling Grinding Chemical

machining

Electrochemical machining

Electrochemical grinding

Electrical discharge machining

Laser beam machining

Property and type

of effect

Condition

mm in mm in mm in mm in mm in mm in mm in mm in

Mechanically altered material zones

Finishing(b) 0.043 0.0017 0.020 0.0008 0.008 0.0003 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c)

Plastic deformation

Roughing(d) 0.076 0.0030 0.119 0.0047 0.089 0.0035 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) Finishing (c) (c) (c) (c) 0.013 0.0005 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c)

Plastically deformed debris

Roughing (c) (c) (c) (c) 0.033 0.0013 (c) (c) (c) (c) (c) (c) (c) (c) (c) (c) Finishing 0.013 0.0005 0.025 0.0010 0.038 0.0015 0.025 0.0010 0.036 0.0014 0.018 0.0007 0.025 0.0010 (c) (c)

Hardness alteration (e)

Roughing 0.127 0.0050 0.508 0.0200 0.254 0.0100 0.079 0.0031 0.051 0.0020 0.038 0.0015 0.203 0.0080 (c) (c) Finishing 0.013 0.0005 0.013 0.0005 0.013 0.0005 (c) (c) 0.008 0.0003 0.000 0.0000 0.013 0.0005 0.015 0.0006

Microcracks or macrocracks

Roughing 0.038 0.0015 0.038 0.0015 0.229 0.0090 (c) (c) 0.038 0.0015 0.025 0.0010 0.178 0.0070 0.102 0.0040 Finishing 0.152 0.0060 (c) (c) 0.013 0.0005 0.025 0.0010 0.000 0.0000 0.000 0.0000 0.051 0.0020 0.005 0.0002

Intergranular attack

Roughing (c) (c) (c) (c) (c) (c) 0.152 0.0060 0.038 0.0015 (c) (c) (c) (c) (c) (c) Finishing 0.010 0.0004 (c) (c) 0.005 0.0002 0.015 0.0006 0.010 0.0004 0.003 0.0001 0.013 0.0005 (c) (c)

Selective etch, pits,

(b) Finishing, gentle, or low-stress conditions

(d) Roughing, off-standard, or abusive conditions

(e) Depth to point at which hardness becomes less than ±2 points HRC (or equivalent) of bulk material hardness (hardness converted from Knoop microhardness

measurements)

(f) Depth to point at which residual stress becomes and remains less than 140 MPa (20 ksi) or 10%, of tensile strength, whichever is greater

Surface Alterations Produced in Traditional Machining Operations Traditional machining methods, which

are the principal means of metal removal, include chip cutting (such as milling, drilling, turning, broaching, reaming, and tapping) and abrasive machining methods (such as grinding, sanding, and polishing) These machining operations can produce surface layer alterations when abusive machining conditions are used (Fig 10 and 11) In general, abusive machining promotes higher temperatures and excessive plastic deformation Gentle machining operations occur when a sharp tool is employed and when machining conditions result in reduced machining forces

Fig 10 Surface alterations produced from drilling with dull tools (a) Section perpendicular to the drill-hole axis

in a 4340 steel (48 HRC) Abusive drilling produced a cracked untempered martensite surface alteration Also note the softer overtempered zone below the untempered martensite layer (b) Cross section through a hole drilled in type 410 stainless with a dull drill (1.5 mm, or 0.060 in., wearland) The drill broke down at the corner during the test and friction welded a portion of the high-speed steel drill bit to the workpiece The base metal exhibits a rehardened and subsequently overtempered zone as a result of the high localized heating 20× Source: Ref 9

Fig 11 Surfaces produced by the face milling of Ti-6Al-4V (aged, 35 HRC) (a) With gentle machining

conditions, a slight white layer is visible, but changes in microhardness are undetected 1000× (b) With abusive conditions, an overheated white layer about 0.01 mm (0.0004 in.) deep and a plastically deformed layer totalling 0.04 mm (0.0015 in.) deep are visible 1000× (c) Microhardness measurements show a total affected zone 0.08 mm (0.003 in.) deep from abusive conditions Source: Ref 9

Surface Effects in Grinding In grinding, there are five important parameters that determine gentle or abusive

conditions: wheel grade, wheel speed, downfeed or infeed, wheel dressing, and grinding fluid As grinding parameters become more aggressive (that is, harder wheels, higher wheel speeds, increased infeed, and so on), the grinding process becomes more abusive and therefore more likely to produce surface damage Gentle, or low-stress, grinding conditions for a variety of alloys are summarized in Table 9

Table 9 Low-stress grinding procedures

Grinding parameters Steels and nickel-base

high-temperature alloys(a)

Titanium Surface grinding

Wheel speed, m/s (sfm) (b) 13-15 (2500-3000) 10-15 (2000-3000)

Downfeed per pass, mm (in.) 0.005-0.013 (0.0002-0.0005) 0.005-0.013 (0.0002-0.0005)

Table speed, m/min (sfm) (c) 12-30 (40-100) 12-30 (40-100)

Crossfeed per pass, mm (in.) 1-1.25 (0.040-0.050) 1-1.25 (0.040-0.050)

Grinding fluid Highly sulfurized oil (d)

Traverse cylindrical grinding

Wheel speed, m/s (sfm) (b) 13-15 (2500-3000) 10-15 (2000-3000)

Infeed per pass, mm (in.) 0.005-0.013 (0.0002-0.0005) 0.005-0.013 (0.0002-0.0005)

Work speed, m/min (sfm) (c) 20-30 (70-100) 20-30 (70-100)

Grinding fluid Highly sulfurized oil (d)

Source: Ref 10

(a) For a wide variety of metals (including strength steels,

high-temperature alloys, titanium, and refractory alloys), low-stress grinding practices develop very low residual tensile stresses In some materials, the residual stress produced near the surface is actually in compression instead

of tension

(b) Low-stress grinding requires wheel speeds lower than the conventional 30

m/s (6000 sfm) To apply low-stress grinding, it would be preferable to have a variable-speed grinder Because most grinding machines do not have wheel-speed control, it is necessary to add a variable-speed drive or

to make pulley modifications

(c) Increased work speeds even above those indicated are considered to be

advantageous for improving surface integrity

(d) Cutting fluids should be nitrate free for health reasons Some

manufacturers also prohibit cutting fluids with chlorine when machining

titanium

Figure 12(a) illustrates the surface characteristics produced by the low-stress grinding of AISI 4340 steel quenched and tempered to 50 HRC The low-stress condition produced no visible surface alterations, while the abusive grinding condition (Fig 12b) produced an untempered martensite layer 0.03 to 0.13 mm (0.001 to 0.005 in.) deep with a hardness

of 65 HRC Below this white layer there was an overtempered martensitic zone with a hardness of 46 HRC The hardness returned to its normal value at a depth of 0.30 mm (0.012 in.) below the surface (Fig 12c)

Fig 12 Surface characteristics produced by the low-stress and abusive grinding of AISI 4340 steel (a)

Low-stress grinding produced no visible surface alterations (b) The white layer shown from abusive conditions has a hardness of 65 HRC and is approximately 0.025 to 0.050 mm (0.001 to 0.002 in.) deep (c) Plot of microhardness alterations showing a total heat-affected zone of 0.33 mm (0.013 in.) from abusive conditions (d) Plot of residual stress (e) Effect on fatigue strength Source: Ref 11

Abusive grinding also tends to produce a residual stress within the altered surface layer A residual stress profile can be readily obtained by using x-ray diffraction techniques both at the surface and then by stepping into the surface with repeated x-ray diffraction readings after successive surface etching The abusive grinding produced high tensile stresses in the altered zone, while low-stress grinding produced a surface with small compressive stresses (Fig 12d)

Fatigue tests conducted on flat specimens indicate that abusive grinding may seriously reduce the fatigue strength, as shown in Fig 12(e) In this example, the abusive grinding depressed the endurance limit from 760 MPa (110 ksi) for low-stress grinding to 520 MPa (75 ksi)

Surface Alterations Produced in Nontraditional Machining Operations Nontraditional machining includes a

variety of methods for removing and finishing materials Examples of nontraditional operations are electrical discharge machining, laser beam machining, electrochemical machining, electropolishing, and chemical machining

Electrical discharge machining (EDM) tends to produce a surface with a layer of recast spattered metal that is

usually hard and cracked and porous to some degree (Fig 13) Below the spattered and recast metal, it is possible to have some of the same types of surface alterations that occur in abusive or traditional machining practices

Fig 13 Surface characteristics of cast Inconel 718 (aged, 40 HRC) produced by EDM (a) Finishing conditions

produced a variable recast layer 0.005 mm (0.0002 in.) thick 860× (b) Roughing conditions produced an extensively cracked variable recast layer up to 0.05 mm (0.002 in.) The random acicular structure is not related to the surface phenomena 660× (c) The recast structure has a hardness of about 53 HRC, and slight overaging due to localized surface overheating was also noted Source: Ref 9

The effect is more pronounced when EDM is used under abusive or roughing conditions Figure 14 shows a surface produced by EDM under finishing and roughing conditions Under roughing conditions, globs of recast metal are spattered onto a white layer of rehardened martensite An overtempered zone up to 46 HRC is also found beneath the surface The surface produced under finishing conditions contains discontinuous patches of recast metal plus a thin layer

of rehardened martensite 0.003 mm (0.0001 in.) deep

Fig 14 Surface characteristics of AISI (quenched and tempered, 50 HRC) produced by EDM under finishing and

roughing conditions (a) Finishing conditions produce discontinuous patches of recast metal plus a thin layer (0.0025 mm, or 0.0001 in.) of rehardened martensite 620× (b) Roughing conditions produce globs of recast metal and a white layer of rehardened martensite 0.075 mm (0.003 in.) deep 620× (c) Microhardness measurements show a total heat-affected zone approaching 0.25 mm (0.010 in.) Globs of recast and the white layer are at 62 HRC An overtempered zone as soft as 46 HRC is found beneath the surface Source: Ref 9

Laser beam machining (LBM) tends to produce the same types of surface alterations as EDM Figure 15 illustrates

the heat-affected zone produced by LBM on Inconel 718 The intense heat generated by the laser beam resulted in a recast surface layer at the entrance and exit of the hole produced by the laser beam

Fig 15 Surfaces from the laser beam drilling of Inconel 718 shown at magnifications of 185× (a) and 1160×

(b) Note the grain structure in the heat-affected zones of the entrance (A) and the exit (B) Source: Ref 9

Electrochemical machining (ECM) is capable of producing a surface that is essentially free of metallurgical surface

layer alterations However, when ECM is not properly controlled, selective etching or intergranular attack may occur (Fig 16) Abusive ECM conditions also tend to degrade surface roughness (Fig 17)

Fig 16 Surface characteristics of Waspaloy (aged, 40 HRC) produced by ECM (a) Gentle conditions produce a

slight roughening of the surface and some intergranular attack (b) Abusive conditions produce severe intergranular attack (c) Microhardness is unaffected by the abusive conditions Source: Ref 9

Fig 17 Surface characteristics of 4340 steel (quenched and tempered, 30 HRC) produced by ECM (a) Gentle

conditions produce slight surface pitting but no other visible changes (b) Abusive conditions produce surface roughening but no other visible effect on microstructure (c) Gentle and abusive conditions both produce a slight hardness loss at the surface Source: Ref 12

Electropolishing (ELP) and Chemical Machining (CM) Surface softening is produced on most materials by

electrochemical machining as well as by electropolishing and chemical machining, also referred to as chemical milling Figure 17 illustrates the surface softening produced by both gentle and abusive ECM on 4340 steel The surface is about 5 HRC points lower in hardness than the interior to approximately 0.05 mm (0.002 in.) in depth With CM, the same steel had its surface softened by about 5 HRC points to a depth of about 0.05 mm (0.002 in.) (Fig 18) This softening may be severe enough and deep enough to affect the fatigue strength and other mechanical properties of metals and may necessitate postprocessing

Fig 18 Surface characteristics of 4340 steel (annealed, 31 to 36 HRC) produced by CM (a) Gentle conditions

produce no visible surface effects and a surface finish of 0.9 m (35 in.) (b) Abusive conditions produce a slight roughening and a surface finish of 3 m (120 in.) (c) Gentle and abusive conditions both produce a softening at the surface Source: Ref 9

Residual Stress Machining processes impart a residual stress in the surface layer In grinding, the residual stress tends

to be tensile when more abusive conditions are used (Fig 19) By using gentle grinding conditions, the stress can be reduced in magnitude and can even become compressive In milling, the residual stress tends to be compressive (Fig 20)

In facing milling 4340 steel (Fig 20), the stresses are tensile at the surface but go into compression below the surface

Wheel A46K8V

Wheel speed, m/min (ft/min) 1800 (6000)

Cross feed, mm/pass (in./pass) 1.25 (0.050)

Table speed, m/min (ft/min) 12 (40)

Depth of grind, mm (in.) 0.25 (0.010)

Grinding fluid Soluble oil (1:20)

Specimen size, mm (in.)

1.5 × 19 × 108 (0.060 × × 4 )

Fig 19 Residual stress from surface grinding of D6AC steel (56 HRC)

Tool 100 mm (4 in.) diam single-tooth face mill with Carboloy 370 (C-6) carbide

End cutting edge angle 5°

Peripheral clearance 8°

Cutting speed, m/min (ft/min) 55 (180)

Feed, mm/tooth (in./tooth) 0.125 (0.005)

Depth of cut, mm (in.) 1.0 (0.040)

Width of cut, mm (in.) 1.9 (0.750)

Cutting fluid Dry

Specimen size, mm (in.)

1.5 × 19 × 108 (0.060 × × 4 )

Fig 20 Residual stress from surface milling 4340 steel (quenched and tempered to 52 HRC)

Distortion and residual stress are two related results from abusive machining conditions In grinding, for example,

a more abusive machining condition (such as an increase in downfeed) increases the distortion of the workpiece (Fig 21) and creates more residual stress at the surface (Fig 19) Figures 22 and 20 illustrate a similar relation in milling As the machining condition becomes more abusive with a duller tool, the distortion (Fig 22) and residual stress (Fig 20) become greater Residual stress and distortion thus exhibit the following relation: The greater the area under the residual stress curve, the greater the distortion of the workpiece

Cross feed, mm/pass (in./pass) 1.25 (0.050)

Table speed, m/min (ft/min) 12 (40)

Depth of cut, mm (in.) 0.25 (0.010)

Specimen size, mm (in.)

1.8 × 19 × 108 (0.070 × × 4 )

Grinding fluid Soluble oil (1:20)

Fig 21 Change in deflection versus wheel speed and down feed in the surface grinding of D6AC steel (56 HRC)

Tool 100 mm (4 in.) diam single-tooth face mill with Carboloy 370 (C-6) carbide

End cutting edge angle 5°

Peripheral clearance 8°

Cutting speed, m/min (ft/min) 55 (180)

Feed, mm/tooth (in./tooth) 0.125 (0.005)

Cutting fluid Dry

Fig 22 Change in deflection versus tool wearland for the face milling of 4340 steel (quenched and tempered to

52 HRC)

Mechanical Property Effects

The surface alterations produced by abusive metal removal conditions generally have a minor effect on the static mechanical properties of materials A major exception to this, however, has been a marked decrease in ductility and tensile strength on materials that have been processed using EDM, followed by stress-relief heat treatment

Ductility and Tensile Strength Figure 23 illustrates the change in ductility and tensile strength from an EDM

application It was found that a carbon deposit produced on the surface during the EDM operation was diffused into the grain boundaries during a subsequent stress-relief treatment and caused an excessive grain-boundary precipitation of carbides These precipitates were responsible for reductions in ductility and strength, the extent of which was found to be

a function of surface roughness (Fig 23) Reductions in ductility as high as 80% were noted after the heat treatment of

Inconel specimens that had been EDM-machined to a surface finish of 16.5 m (650 in.) R

Fig 23 Loss of tensile strength, ductility, and yield strength versus surface finish of Inconel 718 after EDM

Source: Ref 13

Fatigue Strength Surface alterations produced in machining are known to affect the fatigue and stress-corrosion

properties of many materials Extensive investigations have been performed on high-strength steels, and data illustrating the effect of some machining methods on fatigue strength are given in Table 10 The electropolishing of 4340 steel resulted in a 12% decrease in fatigue strength compared to gentle grinding Chemical machining of Ti-6Al-4V resulted in

an 18% drop in fatigue strength compared to gentle grinding, while the electrochemical machining and electrical discharge machining of Inconel 718 produced a 35% and a 63% drop, respectively, in endurance limit compared to gentle grinding Generally, ECM, ELP, and CM produce a stress-free surface These surfaces exhibit reduced fatigue strength when compared to a gently ground surface because the fatigue enhancement by the compressive stress associated with low-stress grinding is not present

Table 10 Effect of method of machining on fatigue strength

Endurance limit in bending, 107 cycles Alloy Machining operation

MPa ksi

Gentle grinding, %

Gentle grinding 703 102 100 Electropolishing 620 90 88

4340 steel, 50 HRC

Abusive grinding 430 62 61 Gentle milling 480 70 113 Gentle grinding 430 62 100 Chemical milling 350 51 82 Abusive milling 220 32 52

Ti-6Al-4V, 32 HRC

Abusive grinding 90 13 21 Gentle grinding 410 60 100 Electrochemical machining 270 39 65 Conventional grinding 165 24 40

Inconel 718, aged, 44 HRC

Electrical discharge machining 150 22 37 Source: Ref 9

Effect of Grinding Endurance limits vary with selected changes in grinding parameters (Fig 24) When abusively

grinding, there is a tendency to form patches or streaks of untempered martensite (UTM) or overtempered martensite (OTM) on the surface It has been found that the presence of either one of these two microconstituents is usually associated with a significant drop in fatigue strength For example, the presence of a depth of UTM as little as 0.01 mm (0.0005 in.) or as large as 0.09 mm (0.0035 in.) produces a decrease in endurance limit from 760 MPa (110 ksi) down to

480 to 520 MPa (70 or 75 ksi) (Fig 24) Typically, residual tensile stresses are involved in this condition

Fig 24 Loss of fatigue strength from the abusive grinding of 4340 steel (quenched and tempered to 50 HRC)

Fatigue tests involved cantilever bending at room temperature and zero mean stress Source: Ref 9

Retempering of the workpiece does not correct the problem Although it tempers the UTM and reduces its brittleness, it does not restore the softening of the OTM to its prior condition In addition, the tempering cycle does not reduce the residual tensile stresses formed in the abusive grinding operation

The effect on fatigue strength from EDM and grinding of aged Inconel 718 is illustrated in Fig 25 After gentle grinding, the Inconel 718 had an endurance limit of 410 MPa (60 ksi) With either gentle or abusive EDM, the endurance limit dropped to 150 MPa (22 ksi)

Fig 25 Effect of EDM and grinding on the fatigue strength of Inconel 718 Fatigue tests involved cantilever

bending at room temperature and zero mean stress Source: Ref 9

The effect of ECM on the fatigue properties of Ti-6Al-4V is shown in Fig 26 The endurance limit after low-stress grinding was 460 MPa (67 ksi) The tests were conducted on flat specimens that were longitudinally surface ground to a surface finish of 0.35 m (14 in.) Specimens that were electrochemically machined by a frontal or pocketing operation had an endurance limit of 410 MPa (60 ksi) for the same surface finish When the titanium specimens were electrochemically machined by a trepanning operation, the endurance limit was reduced to 280 MPa (40 ksi), a 40% drop with respect to the low-stress grinding conditions

Fig 26 Effect of ECM on the fatigue strength of Ti-6Al-4V Fatigue tests involved cantilever bending at room

temperature and zero mean stress Source: Ref 9

Improving or Preserving Mechanical Properties With Shot Peening Shot peening provides the most

practical technique for enhancing the mechanical properties affected by surface alterations It is frequently used in industry to improve the fatigue strength and stress-corrosion properties of most structural alloys subjected to harsh environments and high stress Shot peening not only enhances the properties of gently machined materials but also improves the properties of metals that have been processed by machining techniques that tend to abuse or degrade fatigue strength and other mechanical properties Shot peening parameters should be selected to meet surface finish specifications

Fatigue properties can also be improved by other cold-work finishing processes, such as the burnishing of holes and the roll burnishing of fillets The advantage of shot peening is that it can be applied to large surfaces and to contoured or irregular surfaces With shot peening, the magnitude and depth of the compressive stress layer can be regulated by controlling the type and size of the shot, the intensity, and the coverage However, the compressive stress produced by shot peening can be reduced in service by prolonged exposure to elevated temperature

Improving Fatigue Strength Shot peening can improve the fatigue strength of machined metals (Table 11) On

4340 steel at 50 HRC, for example, shot peening improved the fatigue strength of a gently ground surface by 10% (Table 11) Shot peening after roughing EDM increased the fatigue strength to 130% of the gently ground level Similar improvements in fatigue strength by shot peening occur on surfaces produced by ECM and ELP (Table 11)

Table 11 Effect of shot peening on fatigue strength

Inconel 718, solution treated and aged, 44 HRC

ELP and shot peened 540 79 132 Note: Shot size: S110; shot hardness: 50-55 HRC; coverage: 300% Source: Ref 14

Retarding Stress-Corrosion Cracking It has been shown that the compressive stresses introduced by shot peening

retard stress-corrosion cracking (Ref 14) For example, shot peening increased the stress-corrosion life of 4340 steel specimens that had been prestressed to 75% of the yield strength and exposed to 3.5% sodium chloride solution at room temperature Shot peening increased the life of gently ground specimens from 400 to 850 h (Fig 27) On abusively ground specimens, shot peening increased the life from 200 to over 1000 h, while on an electropolished specimen, the life was increased from 300 to 400 h by shot peening

Fig 27 Effect of shot peening on the stress-corrosion resistance of AISI 4340 steel (50 HRC) Source: Ref 14

Abrasive tumbling is also an effective technique for improving fatigue and surface properties Abrasive tumbling can

be used to reverse unfavorable tensile stress by inducing a compressional stress in the surface layer

Measuring Surface Integrity Effects

In a systematic study of surface integrity, there are two recommended sets of data: a minimum data set and a standard data set

The minimum data set is used for initial screening and consists of measurements of surface texture, macrostructure,

microstructure, and microhardness alterations The minimum recommended data set consists of the following (Ref 12):

preferred)

with a series of photographs at increasing magnifications (preferably 20, 200, 1000, and 2000×)

The standard data set in measuring surface integrity consists of the above minimum data set plus the following

additional tests (Ref 12):

Residual stresses in surfaces can be determined with x-ray diffraction techniques A residual stress profile requires a determination of stress not only at the surface but also into the surface This can be done by taking multiple x-ray readings after successive layers are removed by etching Full-reverse bending at room temperature using tapered flat specimens taken to 107 cycles is recommended for fatigue testing

Macroscopic examination involves some type of visual inspection for detecting visible cracks and other surface

defects Die penetrants are generally applied to nonmagnetic materials, while magnetic particle inspection can detect small cracks in magnetic materials Parts manufactured from martensitic high-strength steels can be visually inspected after a macroetch for evidence of untempered or overtempered martensite resulting from grinding or machining A dilute solution of nitric acid is usually used to etch the damaged areas so that they can be visually detected Typically, untempered martensite appears white, and the overtempered areas appear darker than the background material A specific etching technique for detecting grinding damage in hardened steel is given in Table 12

Table 12 Etching techniques for the detection of grinding injury in hardened steel

Double-etch method

1 Etch No 1 4-5% nitric acid in water Until black, 5-10 s; do not over etch

3 Rinse Methanol or acetone(a) To remove water

4 Etch No 2 5-10% hydrochloric acid in methanol or acetone(a) Until black smut is removed, 5-10 s

6 Neutralize 2% sodium carbonate + phenolpthalien indicator in water To neutralize any remaining acid

9 Oil dip Low-viscosity mineral oil with rust inhibitor To enhance contrast, prevent corrosion

Nital etch method

1 Etch 5-10% nitric acid in ethanol or methanol Until contrast is evident

2 Repeat steps 5-9 above Dark areas are overtempered, light areas are rehardened, uniform gray indicates no injury

Source: Ref 15

(a) 4% nitric acid in water for etch No 1 used with 2% hydrochloric acid in acetone for etch No 2

sometimes gives greater sensitivity on high-carbon hardened steel It is important that appropriate precautions be taken to avoid fire hazards, and good ventilation must be provided

Microscopic examination of a cross section of an altered surface is used to detect the following microstructural

When polishing the cross section of an altered surface, special techniques for mounting and polishing are used to reduce edge rounding Edge retention is important because surface alterations are only about 0.025 to 0.075 mm (0.001 to 0.003 in.) deep and because depth of field decreases as magnification factors increase One technique is to vacuum cast the metallurgical mount and to use special procedures for polishing Polishing units with a horizontal specimen holder, which

is rotated with a preselected force against various grinding and polishing surfaces, have also been successfully used for edge retention An additional mounting and preparation technique for edge retention is as follows (Ref 7):

burring Band sawing or hacksawing is preferred A minimum of 0.75 mm (0.030 in.) is then removed from the cut surface using a 120-grit silicon carbide paper on a low-speed polisher

pallet approximately 125 mm (5 in.) in diameter The inner surface of the molds and the surface of the pallet are previously sprayed with a silicone releasing agent

aluminum oxide sufficient to produce a layer 6.4 to 9.5 mm ( to in.) is poured over the specimen The ratio of resin to hardener is 4 to 1 The amount of pellets added is in the range of 10 to 15 g (0.35 to 0.5 oz) The hardness or abrasive level of the pelletized material used (low, medium, or high fired) is strictly a function of the alloy to be prepared and its hardness characteristics

to degas the mixture, thus improving the adherence of the epoxy and pellets to the surface of the specimen When vigorous bubbling of the mixture decreases after vacuum impregnation, sufficient resin and hardener (4 to 1 ratio) is added to produce a mount approximately 25 mm (1 in.) high

the mounts is accomplished during the latter portion of the laboratory workday so that curing occurs overnight

they are removed from the molds

positive-positioning automatic polishing unit, using the side of a 25 × 330 mm (1 × 13 in.) aluminum oxide 320-grit grinding wheel as the grinding medium Water is used as a coolant

600 grit

polisher using a polishing cloth with a soft nap texture and 6 m diamond paste The final polish is achieved using deep nap or pile cloth similar to billiard cloth with a suspension of 0.1 m or finer aluminum oxide in water Titanium and refractory alloys require an etch-polish cycle (using a slurry of hydrogen peroxide, water, and 0.1 m or finer aluminum oxide), which is accomplished between a diamond polish and a final polish procedure The final polish for titanium and refractory alloys is accomplished on a vibratory polisher using a deep pile cloth with a suspension of 0.1 m or finer aluminum oxide in water

Material Etchant

Steels 2% HNO 3 and 98% denatured anhydrous alcohol

Nickel-base alloys 100 mL HCl, 5 g CuCl2 ·2H 2 O, and 100 mL denatured anhydrous alcohol

Titanium alloys 2% HF and 98% H 2 O or 2% HF, 3% HNO 3 , and 95% H 2 O

Guidelines for Material Removal, Postprocessing, and Inspection

The following guidelines are meant to serve only as general or starting recommendations for the machining of structural components More detailed information on guidelines for surface integrity is given in Ref 16

Data and experience indicate that these practices will lead to increased surface integrity in some important applications However, the current state of knowledge of surface alterations is such that general recommendations are not always applicable for all specific surface integrity situations For highly critical parts, it is mandatory to make individual, specific evaluations It is also important to note that these guidelines were formulated based on experience with structural surfaces

as opposed to mating or bearing surfaces However, many of these guidelines are also applicable to the mating surfaces of bearings, cams, gears, and other similar parts

Surface integrity controls often result in increased manufacturing costs and decreased production rates Therefore, surface integrity practices should not be implemented unless the need exists Process parameters that provide surface integrity should be applied selectively to critical parts or to critical areas of given parts to help minimize cost increases

These surface integrity guidelines are primarily intended for application to metal removal processes used for final surface generation rather than roughing cuts It is important, however, to know the type and depth of surface alterations produced

during roughing so that adequate provisions can be made for establishing surface integrity during finishing operations by removing damaged surface layers

Abrasive Processes Grinding distortion and surface damage can be reduced by using low-stress grinding conditions

Low-stress grinding conditions (Table 9) differ from conventional practices by employing softer-grade grinding wheels, reduced grinding wheel speed, reduced infeed rates, chemically active cutting fluids, and coarse wheel dressing procedures Low-stress grinding and other guidelines should be used as follows:

0.25 mm (0.010 in.) of finish size if the materials being ground are not sensitive to cracking

should be finished with low-stress grinding instead of conventional grinding

thus helping to reduce temperatures at the wheel/workpiece interface

gal./hp) per minute is needed

make proper stock allowances for subsequent cleanup by suitable machining

structural parts unless a standard data set is developed

when employed for the manufacture of aerospace components

Chip Removal Operations For turning and milling, there are at least two very important steps that will improve

surface integrity First, machining conditions should be selected that will provide long tool life and good surface finish Second, all machining should be done with sharp tools Sharp tools minimize distortion and generally lead to better control during machining The maximum flank wear when turning or milling should be 0.13 to 0.20 mm (0.005 to 0.008 in.) A good rule of thumb is to remove the tool when the wearland becomes visible to the naked eye Guidelines for other chip removal operations are as follows:

deburred and chamfered or radiused

the reaming of straight holes On tapered holes (using power-driven machines), hand feeding is permissible, but power feeding is preferred

operation At the first sign of chipping, localized wear, or average flank wear beyond specification, the reamer should be replaced, and the hole should be inspected In addition, regardless of the hole and reamer condition, a maximum number of holes should be specified for reamer replacement

preferred; heads with steel shoes and/or steel wipers are not recommended

engineering limits The tool wearland in finish boring should be limited to 0.13 mm (0.005 in.), but it should often be far less than this in order to achieve the desired accuracy and surface finish

Electrical, Chemical, and Thermal Material Removal Processes Guidelines for these processes are as follows:

produced should be removed Generally, during EDM roughing, the layer showing microstructural changes, including a melted and resolidified layer, is less than 0.13 mm (0.005 in.) deep During EDM finishing, it is less than 0.025 mm (0.001 in.) deep

critical parts

operations such as steel shot or glass bead peening or mechanical polishing Some companies require the peening of all electrochemically machined surfaces of highly stressed structural parts

electrochemical machining of aerospace materials Hardness reduction for CM and ECM range from 3

to 6 HRC points to a depth of 0.025 mm (0.001 in.) for CM and 0.05 mm (0.002 in.) for ECM Shot peening or other suitable postprocessing should be used on such surfaces to restore mechanical properties

suggested that critical parts made by LBM be tested to determine if surface alterations lower the critical mechanical properties

Postprocessing guidelines are given below:

should be removed from critically stressed parts In addition to conventional machining processes, chemical machining and abrasive flow machining are successfully used for this purpose

electrical, chemical, and thermal removal processes should be evaluated Shot peening has been shown

to be extremely effective in improving the fatigue life of specimens processed by ECM, EDM, and ELP (Table 11) Component tests are recommended to confirm the favorable trends shown in laboratory tests

soften the hardened layers produced during the grinding of steels, do not restore the hardness of overtempered layers present immediately below the damaged surface layer In addition, heat treatment does not repair any cracks produced during material removal

Abrasive tumbling can be used to reverse unfavorable tensile stresses by inducing a stressed surface layer

fluids, which may cause stress corrosion Typical suspect compounds are sulfur compounds on aluminum- and nickel-base alloys and chlorine compounds on titanium alloys Some companies use this precaution only for parts subjected to temperatures over 260 °C (500 °F) For applications at less than

260 °C (500 °F), carefully controlled washing procedures are often used to remove the chlorinated and sulfurized cutting oils

Inspection practices should be reviewed and amplified to meet surface integrity requirements Some inspection

practices include the following:

type of surface layer being produced and its depth This method can be used to check for such defects as microcracks, pits, folds, tears, laps, built-up edge, intergranular attack, and sparking

immersion etching using a 3 to 5% aqueous nitric acid solution

• Magnetic particle, penetrant inspection, ultrasonic testing, and eddy current techniques are recommended for detecting macrocracks

References

1 "Surface Texture (Surface Roughness, Waviness and Lay)," ANSI/ASME B46.1-1985, American Society

of Mechanical Engineers, 1985

2 "Surface Texture Symbols," ANSI Y14.36-1978, American Society of Mechanical Engineers, 1978

3 "Surface Integrity," ANSI B211.1-1986, Society of Manufacturing Engineers, 1986

4 J Peters, P Vanherck, and M Sastrodinoto, Assessment of Surface Typology Analysis Techniques, Annals

of the CIRP, Vol 28 (No 2), 1979

5 Machining Data Handbook, Vol 2, 3rd ed., Metcut Research Associates, 1980, p 18-15 to 18-37

6 G Bellows and D.N Tishler, "Introduction to Surface Integrity," Technical Report TM 70-974, General Electric Company, 1970, p 3

7 L.R Gatto and T.D DiLullo, "Metallographic Techniques for Determining Surface Alterations in Machining," Technical Paper IQ 71-225, Society for Manufacturing Engineers, 1971

8 Machining Data Handbook, Vol 2, 3rd ed., Metcut Research Associates, 1980, p 18-58

9 M Field and J.F Kahles, Review of Surface Integrity of Machined Components, Annals of the CIRP, Vol

20 (No.2), 1971

10 Machining Data Handbook, Vol 2, 3rd ed., Metcut Research Associates, 1980, p 18-87

11 M Field, J.F Kahles, and J.T Cammett, A Review of Measuring Methods for Surface Integrity, Annals of

the CIRP, Vol 2 (No 1), 1972

12 W.P Koster et al., "Surface Integrity of Machined Structural Components," AFML-TR-70-11, Metcut

Research Associates, March 1970, p 2

13 A.R Werner and P.C Olson, Paper MR 68-710, Society of Manufacturing Engineers, 1968

14 W.P Koster, L.R Gatto, and J.T.Cammett, Influence of Shot Peening on Surface Integrity of Some

Machined Aerospace Materials, in Proceedings of the First International Conference on Shot Peening

(Paris, France), Sept 1981, Pergamon Press, p 287-293

15 W.E Littman, "The Influence of Grinding on Workpiece Technology," Paper MR 67-593, Society of Manufacturing Engineers, 1967

16 J.F Kahles, G Bellows, and M Field, "Surface Integrity Guidelines for Machining," Paper 69-730, Society

of Manufacturing Engineers, 1969

Tool Wear and Tool Life

L Alden Kendall, Washington State University

Introduction

CUTTING TOOLS WEAR because normal loads on the wear surfaces are high and the cutting chips and workpiece that apply these loads are moving rapidly over the wear surfaces The cutting action and friction at these contact surfaces increase the temperature of the tool material, which further accelerates the physical and chemical processes associated with tool wear In order to remove the unwanted material as chips, these forces and motions are necessary; therefore, cutting tool wear is a production management problem for manufacturing industries

To successfully manage machining processes, production engineers and managers need to establish a system that:

machined from a particular material

the surfaces machined by the tool

economic objective for the machining system

Such a system can provide the necessary information to determine when a tool should be changed Unfortunately, there are many variables to consider; thus, it is not surprising that tool wear assessment and tool change decisions are difficult problems

The Wear Environment

Cutting tool wear occurs along the cutting edge and on adjacent surfaces Figure 1 shows a view of the cutting process in which the rake and clearance surfaces intersect to define the cutting edge Figure 2 in the article "Mechanics of Chip Formation" in this Volume shows a similar view but adds the stress and strain states that are present in the material and chip; Fig 4 from the same article also shows the machining forces and velocities that produce this state of stress and strain These are average forces and velocities Cutting tool wear is localized on specific surfaces where stress, strain, velocity, and temperature are above critical levels It is important to understand where these critical conditions exist and how they interact to cause tool wear

Fig 1 Chip, workpiece, and tool relationship

Wear Surfaces. Figure 2 shows how the sharp tool of Fig 1 may wear Along the rake surface, the chip motion and

high normal stress have produced a wear scar called crater wear Along the clearance surface, the tool motion and high normal stress have increased the area of contact between the tool and work, producing flank wear Lastly, the cutting edge radius has increased Figure 3 shows the characteristic wear surfaces on a turning tool insert, end mill, form tool, and drill The cutting edge view shown in Fig 1 and 2 is identified as section A A in Fig 3

Fig 2 Typical wear surfaces

Fig 3 Wear surfaces on common tools due to the tool motion, V

These figures show how the wear process changes the geometry of these different types of cutting tools Flank wear decreases the diameter of the end mill as well as the depth of cut for a lathe tool Both of these changes in the geometry of the cutting tool could produce out-of-tolerance dimensions on machined parts The edge wear and crater wear on the rake surface alter the state of stress and strain in the cutting region, thereby changing cutting forces and the mechanics associated with the chip-making process Severe geometric changes that decrease the angle between the rake and clearance surfaces can weaken the tool so that the edge may suddenly fracture

It should be apparent that the location and size of these wear surfaces play an important role in determining the useful life

of the cutting tool Localized stresses on cutting tool surfaces are a major contributing factor in regard to location and size

of wear surfaces

Stresses on Tool Wear Surfaces Figure 4 shows the approximate distribution of normal and shear stresses on the

tool wear surfaces The normal stresses, n, are caused by normal forces acting along the rake surface, the cutting edge surface, and the clearance surface In addition to the normal stresses, Fig 4 shows the shear stresses, , that act along the surface of the tool and are associated with sticky and sliding shear processes For the sticky zone, the normal force has a magnitude that results in a shear stress component that equals the shear yield strength, y, of the strain-hardened workpiece material Rather than sliding along the surface, the chip tends to adhere and periodically separate along shear fracture planes The existence and size of this sticky zone is dependent upon the magnitude of the normal force and the friction coefficient along these surfaces The sliding zones have friction forces and associated surface shear stresses that vary according to the normal force and coefficient of friction The state of strain created by the shear stresses associated with the sticky and sliding zones on the rake surface is generally called the secondary shear zone and is identified in Fig

1 Tool surface roughness and lubrication conditions affect the magnitude of these surface shear stresses

Fig 4 Wear surface stresses

The primary shear zone shown in Fig 1 extends from the cutting edge to the surface and is the zone where the chip material is plastically deformed and sheared from the work material The complex state of stress along the cutting edge is caused by the strain associated with the chip separating from the work and moving along the rake surface of the tool and the strained material that remains a part of the work material This conflict between chip motion strain and stationary workpiece strain produces a plowing action by the cutting edge Normal stresses can become very high and exceed the strength of the tool material, causing plastic deformation or fracture of the cutting edge A sticky zone may not exist for certain cutting conditions; however, the plowing action along the cutting edge always exists to some degree because it is impossible to create a cutting edge with no radius and with a primary shear zone that is a perfect plane The magnitude of the state of stress in the cutting region also varies with time and creates a potential fatigue failure environment

Motion Along the Wear Surfaces One way to increase machining productivity is to increase the volumetric chip

removal rate Volumetric chip removal rate is the product of the engagement area of the tool with the work material times

the cutting velocity, V; thus, one option the process planner has is to increase the cutting velocity This productivity gain

must be balanced against increased tool wear caused by higher cutting velocities

In the article "Mechanics of Chip Formation" in this Volume, the chip velocity, Vc, and the shear velocity, Vs, were shown

to be functionally related to the primary shear angle, , the rake angle, , and the cutting velocity, V The cutting

velocity is the relative velocity between the clearance surface of the tool and the work, while the chip velocity is the relative velocity between the chip and the rake surface of the tool The magnitude of these two velocities and the related shear stresses at these interfaces determine the amount of thermal energy released per unit of contact area The magnitude

of the shear velocity causes a high strain rate in the primary shear zone and in the sticky zone The volume of material strained by this strain rate releases additional thermal energy

This thermal energy is the heat source that causes the temperature of the workpiece, cutting tool, and chip to increase The productivity gain from increasing the cutting velocity proportionately increases wear surface velocities, strain rates, and release of thermal energy, thus increasing the wear environment temperature

Temperatures in the Wear Zones The difference between the thermal-energy release rate and the thermal-energy

dissipation rate determines the temperature of the material in these wear zones Thermal-energy dissipation is a function

of the thermal-conductivity properties of the tool material and workpiece material Additionally, the workpiece size and specific heat determine the workpiece heat capacity; to a lesser extent, the surface area plays a role in convective-heat transfer to the surrounding air If a cutting fluid is used, its conductivity and convective-heat transfer boundary coefficient with the hot tool and workpiece play an important role

The development and ultimate selection of tool materials are based on the ability of the tool to maintain hardness, toughness, and chemical stability at high temperatures Even with the best of tools, these properties eventually adversely change with increasing temperatures Figure 5 shows the change in yield strength as the temperature changes for three common cutting tool materials

Fig 5 Cutting tool materials yield strength as a function of temperature Lower curve is high-speed steel

Upper two curves are tungsten carbide Source: Ref 1

There have been many studies of temperatures in the cutting tool Most commonly, average temperature conditions are approximated using the following type of relationship (Ref 2):

where T is the mean temperature (°F) of tool-chip interface, u is the specific cutting energy, which is the energy used per unit volume of material removed, V is the cutting speed, h is the undeformed chip thickness (see Fig 1), and k, , c are

the conductivity, density, and specific heat, respectively, of the workpiece material