Astm stp 672 1979

The first objective of the alloy design program was, therefore, to establish whether this inverse relationship of high strength and low toughness in secondary hardening steels could be o

AMERICAN SOCIETY FOR

TESTING AND MATERIALS

Houston, Tex., 3-5 April 1978

ASTM SPECIAL TECHNICAL PUBLICATION 672

Halle Abrams, Bethlehem Steel Corp

G N Maniar, Carpenter Technology Corp

D A Nail, Cameron Iron Works

H D Solomon, General Electric Co

editors

List price $53.50

04-672000-28

I / 1 9 1 6 Race Street, Philadelphia, Pa 19103

Library of Congress Catalog Card Number: 78-74560

NOTE The Society is not responsible, as a body, for the statements and opinions advanced in this publication

Printed in Baltimore, Md

July 1979

The symposium on MiCon 78: Optimization of Processing, Properties,

and Service Performance Through Microstructural Control was held in

Houston, Texas, 3-5 April 1978 Sponsored by Committee E-4 on

Metal-lography of the American Society for Testing and Materials, the

sympo-sium was also cosponsored by The Metallurgical Society of the American

Institute of Mining, Metallurgical, and Petroleum Engineers, the

Interna-tional Metallographic Society, and the Houston Chapter of the American

Society for Metals Dr Halle Abrams, Bethlehem Steel Corporation, G

N Maniar, Carpenter Technology Corporation, D A Nail, Cameron Iron

Works, and Dr H D Solomon, General Electric Company are editors

of this publication

The success of the First MiCon Symposium, on which this ASTM

special technical publication is based, was the outgrowth of two years of

effort on the part of several individuals and technical societies The

MiCon Organizing Committee was the driving force behind this

undertak-ing, and thanks are due to members of this committee, in particular to Dr

Charles Hays, General Chairman of MiCon 78, J A Richardson, IMS

Liaison, and J D Blanchard, ASM Houston Liaison Thanks are also due

to P S Gupton, ASM Houston Chapter, Dr A G Gray, ASM, J J

Palmer, ASTM, R J Gray, IMS, and Dr Kinrad Kundig, TMS/AIME

Finally, an expression of appreciation goes to Dr Dan Albrecht, IMS, for

his invaluable help in the formative stage of MiCon

Committee The members of the committee for MiCon 78 and their

responsibilities were:

Dr Halle Abrams, Bethlehem Steel Corporation, Chairman, Steels

Session

James D Blanchard, Rolled Alloys Inc., ASM Houston Liaison

Dr William D Forgeng, Jr., U.S Steel Corporation, ASTM

Commit-tee E-4 Liaison

Dr Charles Hays, Dept of Mechanical Technology, University of

Houston, General Chairman

Gunvant N Maniar, Carpenter Technology Corporation, Chairman,

High Temperature Alloys Session

Don A Nail, Cameron Iron Works, Technical Chairman and

Organiz-ing Committee Chairman

James H Richardson, The Aerospace Corporation, IMS Liaison and

Organizing Committee Secretary

Dr Harvey D Solomon, General Electric Company, Chairman,

Stain-less Steels Session and TMS/AIME Liaison

Dr Martin G H, Wells, Colt Industries

Buehler, Ltd

Cameron Iron Works, Inc

Carpenter Technology Corp

Cooper Industries, Cooper Energy Services Division

Deere and Company

General Electric Company,

Corporate Research and Development Center

Houston Lighting and Power

Ladish Company

Shell Development Company

Sun Petroleum Products Company

Universal-Cyclops

Cyclops Corporation

Wyman-Gordon Company

Related ASTM Publications

Unified Numbering System for Metals and Alloys, DS 56A (1977), $49.00,

This publication is made possible by the authors and, also, the

unheralded efforts of the reviewers This body of technical experts whose

dedication, sacrifice of time and effort, and collective wisdom in

review-ing the papers must be acknowledged The quality level of ASTM

publications is a direct function of their respected opinions On behalf of

ASTM we acknowledge with appreciation their contribution

ASTM Committee on Publications

Editorial Staff

Jane B Wheeler, Managing Editor Helen M Hoersch, Associate Editor Ellen J McGlinchey, Senior Assistant Editor

Helen Mahy, Assistant Editor

High-Strength Microalloyed Pipe Steels Resistant to Hydrogen-Induced

Failures—c PARRINI A N D A DE VITO 53

Evaluation of Steels for Arctic Line Pipe—HALLE ABRAMS AND G J ROE 73

Control of Microstructure by the Processing Parameters and Chemistry in

the Arctic Line Pipe Steels—CHIAKI OUCHI, JUNICHI TANAKA,

iSAO KOZASU, A N D KOSHIRO TSUKADA 105

Structure-Property Relationships for Pearlite-Reduced Mo-Nb Steels

Finish-Rolled Moderately Below Ara^A p COLDREN, G T E L D I S ,

AND G TITHER 126

Controlled Processing of Molybdenum Bearing Line Pipe Steels—o w

DELVECCHIO, J E HOOD, AND D B MC CUTCHEON 145

Influence of Microstructure on the Temper Embrittlement of Some Low-Alloy

Steels—R VISWANATHAN 169

Effects of Composition and Gage on the Microstructure of A533-B Steels—

R P SMITH AND R A SWIFT 186

High-Hardenability Carburizing Steels for Rock Bits—D E DIESBURG 207

Discussion—Steels Session 230

STAINLESS STEELS

Summary—Stainless Steels Session 261

Relationship Between Microstructure and Properties in Stainless Steels—

F B PICKERING 263

Possibilities for Microstructural Control During Hot Working of Austenit

Stainless Steels—BERTIL AHLBLOM A N D WILLIAM ROBERTS 2%

Stainless Steel Pipe Joint—Y G NAKAGAWA, T KAWAMOTO,

Microstructural and Microchemical Studies in Weld Sensitized Austenitic

Stainless Steels—PRAKASH RAO 321

Microstructures Versus Properties of 29-4 Ferritic Stainless Steel—o AGGEN,

H E DEVERELL, AND T J NICHOL 334

Effect of Heat Treatment and Microstructure on the Mechanical and

Corrosion Properties of a Precipitation Hardenable Stainless Steel—

T KOSA AND T A DE BOLD 367

Influence of Hydrogen on Age-Hardening Processes in 15-5 Precipitation

Hardened Stainless Steel—^J MURALI, M R LOUTHAN, JR., AND

R P MC NITT 382

Structure and Properties of a 19Cr-25Ni-Mo-Ti Steel—T ANDERSSON,

H TORNBLOM, A N D A SAMUELSSON 393

Microstructure and Related Material Characteristics of Some Duplex

Austenitic-Ferritic Alloys with Less Than 40 Percent Ferrite—

G C BODINE, JR., A N D C H SUMP 406

Influence of Microstructure on the Mechanical Properties and Localized

Corrosion of a Duplex Stainless Steel—H D SOLOMON AND

T M DEVINE 430

Discussion—Stainless Steels Session 462

HIGH-TEMPERATURE ALLOYS

Summary—^High-Temperature Alloys Session 475

Microstructural Objectives for High-Temperature Alloys in Advanced

Energy Systems—c T SIMS 480

Melting of Superalloys—L w LHERBIER 514

Physical Metallurgy and Effects of Process Variables on the Microstructure of

Wrought Superalloys—D R MUZYKA 526

Forging and Processing of High-Temperature Alloys—A J DE RIDDER

Review of Superalloy Powder Metallurgy Processing for Aircraft Gas

Turbine Applications—^J L BARTOS 564

Application of Superalloys in Internal Combustion Engine Exhaust Valves—

J M LARSON AND L F JENKINS 578

Reformer—w L MANKINS AND D E WENSCHHOF 616

Discussion—High Temperature Alloys Session 633

Index 643

Introduction

This symposium was organized under the egis of the E-4,

Metallog-raphy Committee of the American Society for Testing and Materials and

was cosponsored by the Houston Chapter of the American Society for

Metals, the International Metallographic Society, and the Metallurgical

Society of the American Institute of Mining, Metallurgy, and Petroleum

Engineers

The E-4 Metallography Committee is concerned with the

microstruc-tural aspects of materials Its members recognized the importance of

microstructural control to the processing, properties, and service

per-formance of materials; hence, their desire to hold a symposium devoted to

this topic

No problem is more central to metallurgy than the relationship of

microstructure to properties The symposium was organized to provide a

forum for discussing new results in this important area of study Since the

topic of microstructure and properties is so broad, it was decided to

narrow it down a bit by focusing on materials used for a specific type of

application—^in this case, materials for energy generation Future

sym-posia may deal with materials for land or sea transportation, or with

aerospace or electronic materials

The symposium brought together about 150 participants from

univer-sities, producers of materials for energy generation, and users of such

materials There was a lively discussion of many aspects of the

relation-ship of microstructures and properties The discussion together with the

papers presented at this symposium are included in this special technical

publication It is hoped that these proceedings will be of value to those

who use the alloys discussed here, to those who produce them, and to

those of the research community who study them Some care was taken to

balance the presenters among these three areas The resulting set of

papers therefore covers a spectrum of viewpoints, from those who must

Steels

Summary—Steels Session

The papers for the Steels Session of MiCon 78 were chosen to be

consistent with the objectives of MiCon and to emphasize the theme of

the symposium, namely, energy generation and related applications The

Steels Session had ten invited papers, of which six addressed the subject

of high-strength low-alloy (HSLA) steels for Arctic linepipe application

The papers were complementary to one another in terms of the overall

theme, and each one dealt with specific topics such as: alloy composition

and processing, microstructural control, stress corrosion cracking and

hydrogen embrittlement, accelerated cooling, and regression analysis and

prediction of plate and pipe mechanical properties The keynote paper

discussed the need for improved wear- and abrasion-resistant steels for

components in advanced fossil energy conversion systems such as coal

gasification plants In the area of turbine materials, which are susceptible

to temper embrittlement at the operating temperatures involved,

Vis-wanathan's paper reviewed the most common steels used for this

applica-tion (Cr-Mo, Cr-Mo-V, Ni-Cr-Mo-V) and provided an insight into the role

of microstructure in interpreting the embrittlement susceptibility as

effected by transformation product and strength level In the area of

pressure vessel steels, the Swift and Smith paper considered A533-B steel

and describes a procedure for predicting the microstructure of heavy-gage

plates as a function of carbon equivalent and cooling rate The final paper

by Diesburg considered the problems of drilling large-diameter deep oil

wells and, because of the high stresses involved, the need for

rolling-cutter rock bits having high levels of hardenability

The intent of this summary is not to abstract each of the papers

presented, but rather to comment on how their salient highlights fit into

the overall theme of the symposium In Prof Zackay's keynote paper, he

described the utilization of composition control to provide the desired

combination of mechanical properties required in advanced fossil energy

conversion systems In this context, high hot strength up to 593°C

(1100°F) with adequate room temperature toughness is required for screw

feeders Secondary hardening alloys with combinations of up to 3Si and

3A1 retard the tempering reactions, and optimum improvement in room

temperature toughness is obtained by combining 1.5A1 and 1.5Si

How-ever, additions of more than 2Si cause intragranular fracture after

tempering at 550°C

Cohen and Hansen's paper, "Microstructural Control in Microailoyed

Steels," was based on the premise that a fine ferritic grain size is essential

to develop the best strength toughness combination in microailoyed

steels Structural refinement of the ferrite was shown to depend on

control of the austenite structure during roUing, coupled with control of

the austenite-to-ferrite transformation kinetics The finest ferritic grain

sizes evolve on transformation from unrecrystallized austenite, and the

mechanism and control of the austenite recrystalhzation reaction were

discussed The subsequent transformation of unrecrystallized austenite

was followed, and it was stressed that any structural refinement gained

during rolling may be further maximized by appropriate control of the

transformation kinetics via alloying or process controls

The paper, "Evaluation of Steels for Arctic Line Pipe," by Abrams and

Roe was a practical demonstration of microstructural control to improve

the strength and toughness properties described by Cohen and Hansen

Extensive property data were presented from seven full-scale mill trials,

which were used to predict the plate and pipe properties based on

chemistry and processing The improved strength and toughness

properties are associated with a fine ferrite grain size and a high percent of

fine-grain ferrite patches (fgfp) Specifically, for a vanadium-columbium

(VCb) grade, to assure an 85 percent shear fracture appearance below

-23°C (-10°F) in the pipe, the control-rolled plate must have a grain

size number greater than ASTM 11 and a percent fgfp value greater than

75 percent Lower slab reheating temperatures reduce the amount of

duplex ferrite microstructure common to severely control-rolled steels

and provide further improvement in toughness as characterized by the

Battelle Drop Weight Tear Test

In the paper, "The Control of Microstructure by Processing Parameters

and Chemistry in Arctic Line Pipe Steels," by Ouchi et al, the authors

evaluated a series of columbium (0.02 to 0.05 percent) and vanadium (0.03

to 0.09 percent) steels and studied the effects of controlled rolling and

quench and temper heat treatment (after rolling) on microstructure and

mechanical properties In the control-rolled condition, these steels exhibit

ferrite/pearlite or acicular ferrite microstructures and develop yield

strengths of 70 to 75 ksi Accelerated cooling after control-rolling

in-creased the yield strength to 80 to 85 ksi due to a more refined structure

consisting of ferrite and bainite The quench and temper heat treatment by

induction heating of pipe produces a ferrite-bainite-martensite

microstruc-ture, retaining a fine grain size and further increasing the yield strength to

100 ksi This heat treatment eliminates variations in toughness across the

heat-affected zone (HAZ) and results in improved resistance to

hydrogen-induced cracking and HjS stress corrosion cracking

presented the paper, "High-Strength Microalloyed Pipe Steels Resistant

to Hydrogen-Induced Failures," by Parrini and DeVito of Italsider The

susceptibility to H2S stress corrosion cracking was found to increase with

increasing tensile strength Above 120 ksi tensile strength, the failure was

always intragranular, whereas below 80 ksi tensile strength, the failure

was always ductile This susceptibility was also related to steel

cleanli-ness and the degree of inclusion elongation Lower levels of MnS via

desulfurization and higher finish-rolling temperatures reduced the degree

of cracking, but the major improvement was associated with rare earth

additions Another approach to the hydrogen cracking problem in sour

gas environments was to make the weld metal cathodic with respect to the

HAZ, thereby preferentially attracting the hydrogen to the weld metal

This cathodic protection was accomplished by additions of 0.5Mo and

0.4Cr to the welding rod

Processing, mechanical properties, and microstructure

interrelation-ships for an acicular ferrite steel containing Mn-Mo-Cb were described in

the Stelco paper, "Controlled Processing of Molybdenum Bearing Line

Pipe Steels," by Delvecchio, Hood, and McCutcheon The acicular

ferrite grades are attractive for Arctic line pipe because in the plate form

the yield strength is relatively low, about 60 ksi, which upon forming to

pipe and hydrauHc expansion achieves X70 to X80 levels due to the

continuous yielding behavior and appreciable strain hardening However,

these grades typically are very low-carbon and high-manganese and offer

the disadvantage of BOF melting problems and added alloy costs

Delvecchio and co-workers found that both the low- and high-manganese

grades are suitable for X70 Arctic applications However, at the more

economical 0.2Mo level, the higher manganese grades consistently

pro-vided higher yield strengths and required less low-temperature rolling

Coldren et al reported on their laboratory study of Mn-Mo-Cb steels

finish-rolled moderately below Ars In this study, they rolled 19 mm (3/4

in.) plate into the two-phase region to determine the relative effects of

ferrite grain refinement, dislocation substructure and Cb (C, N)

precipita-tion strengthening on the strength and toughness They found that

stress-assisted precipitation and dislocation substructure can effectively

increase the strength without adversely affecting the toughness, and these

mechanisms were most efficient in plates with 30 to 40 percent deformed

ferrite that was given a 20 percent reduction on the last pass Increasing

the molybdenum content from 0.2 to 0.4 percent reduced the yield point

elongation, and in plates with as little as 11 percent deformed ferrite, there

was continuous yielding This behavior, as mentioned previously, offers

the capability of making an X75/X80 expanded line pipe from a

molybdenum-containing alloy grade However, the effect of this work

hardening on notch toughness at these high-strength levels would have to

be more fully studied before an X80 grade could be exploited

One of the many factors that influence toughness is reversible temper

embrittlement, which manifests itself as an increase in the

ductile-to-brittle transition temperature of the steel The problem has assumed even

greater importance in recent years in view of the findings that the

susceptibility of steels to cracking in hydrogen and stress corrosion attack

is also increased due to prior temper embrittlement The critical

tempera-ture range over which embrittlement occurs often coincides with the

operational or heat treatment temperature for many of the steels used by

the petrochemical and other energy-related industries Results have

recently been reported relating microstructural variations produced by

varying the transformation product and/or the tensile strength level to

embrittlement susceptibility and in turn to the susceptibility of cracking in

adverse environments In his paper, "Influence of Microstructure on the

Temper Embrittlement of Some Low-Alloy Steels," Viswanathan

pro-vides a critical review and interpretation of the results for Cr-Mo-V,

Ni-Cr-Mo-V, and 2.25Cr-lMo steels

The mechanical properties of heavy-gage plate for pressure vessels is

determined by the microstructure and tempering parameters The

micro-structure is in turn controlled by the hardenability and post-austenitizing

cooling rate Using the carbon equivalent to account for the hardenability

effects of the steel chemistry, Swift and Smith studied the effects of

composition and cooling rate on the microstructure of A533-B steel

Equations were developed from the experimental data to predict the

microstructures, and comparison with the microstructures to commercial

heats showed that the predictions were in good agreement

In view of the energy shortage and its increasing cost, it is now

economical to recover oil and gas from known deep reserves, which

require large-diameter shafts The replacement of worn or broken rock

bits during the drilling of these deep shafts is obviously undesirable from

the standpoint of cost and productivity Accordingly, the rolling-cutter

rock bit is an integral part of making the drilling operation efficient In

most instances, the best combination of properties for these rock bits is

obtained by carburizing and quenching and tempering In his paper,

"High Hardenability Carburizing Steels for Rock Bits," Diesburg

de-scribes the excellent performance of EX55 (0.87Mn, 0.58Cr, 1.85Ni,

0.75Mo) in impact fatigue, high-cycle fatigue, impact fracture stress, and

plane strain fracture toughness tests Comparison of EX55 grades with

high-nickel SAE 4800 grades indicates that the improved hardenability of

the EX55 grades would be suitable for rolling-cutter rock bits for deep

shaft drilling

In conclusion, I would like to stress the excellent participation of the

authors and others attending the symposium The knowledge and

en-papers themselves, there are the fine points and interplay of ideas in the

discussions, which have been grouped together and appended to the

volume of Steels Session papers

I would also like to take this opportunity to thank each of the authors,

reviewers, and participants for making the Steel Session of MiCon 78 such

a rewarding experience for all of us

Halle Abrams

Homer Research Laboratories, Bethlehem Steel Corporation, Bethlehem, Pa., ses- sion chairman

Design of High Hardness, Tough

Steels for Energy-Related Applications

REFERENCE: Zackay, V F., "Design of High Hardness, Tougli Steels for

Energy-Related Applications," MiCon 78: Optimization of Processing, Properties,

and Service Performance Through Microstructural Control, ASTM STP 672, Halle

Abrams, G N Maniar, D A Nail, and H D Solomon, Eds., American Society

for Testing and Materials, 1979, pp 10-33

ABSTRACT: The need for improved wear and abrasion resistant steels for

components in advanced fossil energy conversion systems is described Desirable

combinations of mechanical properties for these components are enumerated A

critical component, coal feeders, in coal gasification plants requires adequate room

temperature toughness and high strength at both room and moderately elevated

temperatures Through modification of both composition and heat treatment, it has

been shown that commercial secondary hardening matrix steels are promising

candidates for this application It is further shown that improvements can be

achieved by the synthesis of new secondary hardening steels A key feature of the

design of these steels is the suppression, by composition control, of solid-state

tempering reactions that (in commercial secondary hardening steels) lead to

inadequate toughness In other components for advanced coal technology, hot

strength is not required but hardness and impact strength are Modified

medium-alloy, ultra-high-strength steels are described with combinations of strength and

toughness achievable only in the high-alloy (and expensive) maraging steels

KEY WORDS: steels, microstructure, high-strength steels, secondary hardening

steels, coal gasification, abrasion resistance

The widespread recognition of the diminishing supply of certain fossil

fuels, notably oil and natural gas, has resulted in the initiation of research

and development programs in advanced energy conversion systems

throughout the world While many of these systems are in an early stage

of planning, others have advanced sufficiently to enable designers to

specify performance criteria and to suggest materials of construction

While it is economically desirable to utilize commercially-available

mate-rials for these advanced systems, it is not always possible to do so In

'Professor of Metallurgy, University of California, Berkeley, Calif 94720

10

properties The Fossil Energy Research and Nuclear Research Groups,

Energy Technology, Department of Energy, have recently initiated

projects at the University of California, Berkeley, to address some of

these alloy design problems For example, a major interdisciplinary

research effort is being made to build devices utilizing metals as energy

absorbers These devices are intended to enhance the safety of nuclear

reactor piping systems under cyclical (seismic) and impact (water

ham-mer) loading

A major feature of the national energy plan is a doubling of the use of

coal as a source of energy [7,2].^ The execution of this plan will pose

many challenges to the engineer and scientist and, in particular, to those

in the field of materials Major forthcoming materials problems have been

identified in the mining, transporting, and processing of coal Some of

these problems are a direct consequence of the massive materials

handling aspects of new technologies such as large-scale coal gasification

and liqiiefaction Although the present program at Berkeley is primarily

concerned with coal handUng equipment, some of the results are relevant

to mining and earthmoving equipment

The mining, sizing, and transporting of large tonnages of coal call for

materials with improved wear and abrasion resistance In some of these

applications, unusual combinations of mechanical properties are often

required For example, high hardness and toughness at room temperature

as well as strength at elevated temperatures may be specified Where

existing commercial steels are not adequate, new steels have to be

designed, tested, and introduced into the technology This paper

dis-cusses the preliminary results obtained in a study of the design of alloys

for improved wear and abrasion resistance

Materials Requirements

The wear and abrasion resistant alloys being investigated are intended

for use in the following: (1) coal feeders; (2) coal moving equipment, such

as chutes and loader shovels; and (3) coal crushing and milling equipment

The operating conditions of such components are different and, in the

case of the coal feeder, not completely established However, sufficient

information is available in all instances for the initial formulation of

desirable alloy compositions, mechanical property requirements, and

wear test procedures

An evaluation of the various designs of dry coal feeders and expected

performances was made recently [?] Certain types of feeders were

selected for further study and development One type, the screw feeder,

^ The italic numbers in brackets refer to the list of references appended to this paper

was judged to have the highest probabiHty of eventual commerciaUzation

PreUminary tests with laboratory-size feeders revealed that severe wear

could be expected at several places, namely, at the outer edges of the

screw tips, where the velocity of the feed material is high, and at the exit

end, because of the high pressure and velocity in this region [4,5] It was

concluded [5] that the abrasive wear observed was due to high stress, dry,

three-body abrasion (that is, coal particles, 6.35 mm (1/4-in.) in diameter

or less, being wedged between two metal surfaces, namely, barrel and

screw) A study of the performance of screw feeders in injection molding

machines [6,7] revealed that adhesive wear also occurs due to such

factors as misalignment, uneven feeding, and nonuniform heating,

al-though under normal operating conditions the screw is expected \.ofloat in

the center of the barrel A critical feature of the screw feeder in the coal

gasifier is the temperature of operation Depending on the particular

design, the device may operate at temperatures of 538°C, (1000°F) or it

may be subject to a temperature gradient, that is, only that part of the

device nearest to the gasifier vessel may be heated

An analysis of the performance criteria for screw feeders suggests that

an unusual combination of mechanical properties is desirable These

include hardness and adequate toughness from room to the maximum

operating temperature In addition, enough hot strength at the maximum

operating temperature is required to resist plastic deformation caused

either by the abrading particles or by the unit loads on the sliding metal

parts

The materials requirements for coal handling equipment, such as chutes

and loader shovels, are better known than for coal feeders and are less

demanding These components are subject to relatively low-stress dry

abrasion of a two-body type (contact between two materials having

relative motion) with the abrading material varying widely in size, shape,

and hardness These components are also subject to the multiple impacts

of falling rock and coal Alloys for parts exposed to these conditions must

have adequate hardness, toughness, and, in some instances, impact

fatigue resistance Because they are used in large volumes and must be

replaced periodically, economy of manufacture is also essential

The components of coal crushing and milUng equipment are subjected

to wear and environmental factors that include dry abrasion and impact

Therefore, the properties desired are adequate hardness, impact fatigue

resistance, and toughness

It is emphasized that the optimum combination of mechanical

properties of an alloy for any wear application, regardless of its nature, is

unpredictable Although there is no consensus among experts as to the

precise combination of mechanical properties that control the different

types of wear, there is general agreement that microstructure is an

important variable [8-15] Most, if not all, of the design for wear and

field evaluation aspects of the current program are still in an early stage

and will be described in more detail in later publications

Feeder Components

Alloy Design Criteria

The distinguishing metallurgical feature of materials used in screw

feeders is the requirement for both hot strength and adequate

room-temperature toughness Either static (compact tension) or dynamic

(notched Charpy impact) tests can be used to measure the toughness of

metals The applications of concern, that is, coal handling, screw feeders,

etc., involve dynamic loading and, for this reason, the Charpy impact test

has been used as the criterion of toughness in the initial phase of these

studies The compact tension test (characterized by static loading in the

presence of a sharp crack) will also be used in the latter phases of the

research program to obtain the fracture toughness of selected alloys

Secondary hardening steels have been used in certain manufacturing

industries for many years because of their ability to retain their strength

to moderately elevated temperatures, that is, 538 to 593°C (1000 to

1100°F) However, at the optimum tempering temperature for hot strength

(and peak hardness), the room-temperature toughness is discouragingly

low In fact, the toughness of these steels is characteristically so poor that

it is often not reported The usual relationship between strength (hardness)

and toughness is revealed in a study of 5Cr-Mo-V steel by Contractor

et al [19] As shown in Fig 1, the maximum in room-temperature hardness

and the minimum in room-temperature toughness occur at the same

temper-ing temperature, 538°C (1000°F) At a tempertemper-ing temperature less than

538°C, (1000°F), the steel would not have its maximum hot strength The

composition-microstructure-mechanical property relationships of these

steels will be described later

In contrast to the secondary hardening steels, many low- and

medium-alloy quenched and tempered steels are known to possess excellent

combinations of strength and toughness through microstructural control

These ultra-high-strength steels are widely used in the aerospace,

trans-portation, and manufacturing industries However, it is unlikely that these

steels can be used at temperatures exceeding about 482°C (900°F),

because a pronounced degradation of properties, especially hardness,

almost invariably occurs above this temperature

The first objective of the alloy design program was, therefore, to

establish whether this inverse relationship of high strength and low

toughness in secondary hardening steels could be overcome Before

FIG 1—Hardness and impact strength versus tempering temperature relationships for a

5Cr-Mo-V steel from Ref 19

describing the experimental program, it is useful to examine both the

tempering sequence of secondary hardening steels and the basic factors

contributing to the enhancement of toughness in ultra-high-strength

steels

Speich [20], among others, has established the sequence and chemistry

of solid-state reactions occurring in steels containing only iron (Fe) and

carbon (C) Briefly, for steels having greater than 0.2 C, a transition

carbide, called epsilon-carbide, first forms as a decomposition product of

martensite Epsilon-carbide forms at temperatures up to 200°C (392°F)

Above 250°C (482°F), FcgC precipitates are formed at lath boundaries and

at former austenite grain boundaries The cementite also precipitates

within the laths initially as plates that grow and spheriodize rapidly at

higher temperatures Also, any retained austenite that is present will

decompose into ferrite and cementite in the temperature range of 230 to

280°C (446 to 536°F) From 400°C (752°F) to 600°C (1112°F), recovery of

the martensite defect structure occurs The hardness drops continuously

following tempering at temperatures above about 200°C (392°F) to 300°C

(572°F), and this decrease is often associated with the formation of iron

carbides in the martensite lath boundaries [21-24]

The hardness versus tempering relationship and the associated

solid-state reactions in a highly alloyed, secondary hardening steel are similar

to those of the plain carbon or low-£illoy steels for temperatures below

about 450°C (842°F) Above this temperature, any highly alloyed retained

austenite that is present decomposes to form low-alloy retained austenite

chromium to form complex iron carbides At about 540°C (972°F), finely

dispersed complex refractory element carbides such as (MoWjgC form,

completely replacing the complex iron carbides The formation of these

carbides and the resultant increase in hardness is referred to as secondary

hardening At a sufficiently high temperature, these refractory carbides

grow, the hardness drops, and the steel is said to be overtempered

Goolsby [21] and Tom [25] attempted to relate the various features of

the tempering temperature versus toughness relationship to

microstruc-tural features The reladonships between tempering temperature and

hardness Fig 2a, and toughness Fig 2i, in a 0.3C-5Mo secondary

hardening steel, are taken from Goolsby [27 J^He ascribed the first drop in

toughness, between 225°C (437°F) and 300°C (572°F), to the precipitation

of iron carbides at martensite lath boundaries He attributed the second

drop, at about 600°C (1112°F) to the precipitation of refractory carbides at

the same sites Clearly, the minimization of deleterious solid-state

chemi-cal reactions during the tempering operation is desirable The

precipita-tion of brittle compounds at heterogeneous nucleaprecipita-tion sites, such as lath

or grain boundaries, is obviously a major reason for the lack of toughness

in secondary hardening steels

Goolsby [21] also Usted some of the factors that might account for the

high toughness of the martensite tempered at low temperatures These

were: (1) the microstructure was relatively free of minor amounts of weak

and ductile phases such as ferrite or retained austenite; (2) there were no

extensive lath or grain boundary precipitates; (3) there were no

undis-solved carbides, that is, carbides not disundis-solved during austenitization and

embedded in the martensite after quenching Tom [25] has also shown

that there is a threshold size for these undissolved carbides above which

the fracture toughness abruptly decreases for a given strength leveL

To these resuhs might be added the well-known fact that an excessively

large prior austenite grain size can degrade impact toughness The

temperature of austenitization is therefore a very important factor in the

heat treatment of secondary hardening steels The temperature must be

sufficiently high to remove undissolved carbides, or at least to minimize

the number of such particles, and also it should be below that which

causes excessive grain coarsening

The relationship between toughness and austenitizing temperature may

also vary according to the type of toughness test, as shown by Ritchie et

al [26] In recent work, Ritchie and Horn [27] have shown that an

austenitizing temperature intermediate between a conventional and a high

(grain-coarsening) one results in an increase in the fracture toughness

without a concomitant decrease in the Charpy impact strength

These considerations and others, which will be discussed later, provide

2 0 0 3 0 0 4 0 0 5 0 0 6 0 0 7 0 0 TEMPERING TEMPERATURE, °C,(I HR)

8 0 0

0 ^ 100

AS-QUENCHED

200 3 0 0 4 0 0 TEMPERING TEMPERATURE,' 500

C (I HRJ

FIG 2—(a) Microhardness versus tempering temperature curve for the tempering of a

OJC-SMo steel [21], (b) The variation of Charpy impact energy with tempering

tempera-ture for a OJC-SMo steel [21]

explored and are described in the following sections

Modified Commercial Matrix Alloys

Early in the research program, a commercially available steel

(Vasco-MA), having a composition and microstructure that appeared to meet

some of the basic requirements, was selected This steel is one of a family

of matrix steels whose matrix is that of the popular M-2 tool steel The

composition of M-2 and the recommended heat treatment have been

adjusted to minimize the volume fraction of primary carbides The

relative absence of large brittle primary carbides is purported to confer

enhanced toughness and high hardness on these secondary hardening

steels

The hardness and impact strength of Vasco-MA were measured, and

the relatively poor toughness (about 8.13 J (6 ft 1 lb) at 56 HRC)

suggested that, contrary to expectation, the matrix was not free of large

primary carbides This was confirmed by metallographic and fracture

surface examination of specimens austenitized over a wide range of

temperatures, 900 to 1200°C (1652 to 2192°F) Large, undissolved primary

carbides are clearly shown in Fig 3, in a specimen austenitized at 10(K)°C

(1832°F) Scanning electron microscopic (SEM) examination, in

conjunc-tion with energy-dispersive X-ray analysis, established that tungsten was

the major, and vanadium and molybdenum were the minor, alloy

con-stituents of these carbides

The observations formed the basis of a program encompassing one of

the two approaches mentioned earlier, that is, modification of an existing

commercial steel A series of steels whose compositions are shown in

Table 1 was made based on the base composition on the Vasco-MA steel,

designated Bl in the table The analysis consisted of chemical

determina-tions (using energy-dispersive X-ray analysis) of the complex alloy

carbides after each of several austenitization treatments The analyses

provided the guidance for the formulation of new matrix compositions

that might possess better combinations of hardness and toughness than

those of the base Vasco-MA steel

The results of studies to date are presented in Figs 4 through 7 Useful

information can be obtained by qualitative comparisons of the hardness

versus austenitizing and hardness versus tempering temperature curves of

the several steels The hardness versus austenitizing temperature curves

of the three steels, Bl (2 W), B4 (1 W), and B6 (0.5 W) are shown in Fig 4

The significantly higher hardness of B4 and B6, at temperatures between

1000°C (1832°F) and 1100°C (2012°F) suggests that (prior to reaching a

temperature of 1200°C (2192°F)) the tungsten carbides in these steels

i«L

, " • "

L:Mr:^:

FIG 3—Optical micrograph showing large undissolved primary carbides in a Bl steel

specimen austenitized at lOOO'C (1832°F)

TABLE 1—Nominal composition of matrix-type steels

0 5 0.5

Si 0.2 0.2 0.2 0.2 0.2 0.2

Cr 4.5 4.5 4.5 4.5 3.5 2.5

Mo 2.8 2.8 2.8 2.8 2.8 2.8

V

1

1 0.5

2

2

Fe balance balance balance balance balance balance

have largely gone into solution while those in Steel Bl have not The

hardness and Charpy impact energy versus tempering temperature data

for these three steels, austenitized at 1000°C (1832°F), is shown in Fig 5a

and b, respectively, in the tempering range of 450°C (842°F) to 650°C

(1202°F) From Fig 5a, it is observed that Steel B4 hardened to a greater

extent than the base steel, Bl However, Steel B6 with only 0.5 W had

lower hardnesses than the Steel Bl

The secondary hardening peak of all three steels is at a tempering

temperature of about 550°C (1022°F) In Steels B4 and B6, the Charpy

impact energy, associated with the peak hardness, is either a maximum

(B6) or at a plateau (B4) with tempering temperature, as shown in Fig 5b

Steels B4 and B6 possessed much higher Charpy impact toughness than

the base steel, Bl The combinations of hardness and Charpy impact

toughness for these steels at the peak hardness, that is, 52 HRC and 27,1 J

(20 ft • lb) for Steel B6, and 54 HRC and 20.3 J (15.50 ft • lb) for Steel B4,

are far superior to those of the commercial steel, Bl

A similar comparison of the influence of vanadium on mechanical

properties can be made by comparing Steels Bl (1 W) and B5 (0.5 W), as

shown in Fig 6a and b The flatness (and position) of the hardness versus

austenitizing temperature curve for Steel B5, Fig 6a, indicates that most

of the carbides are in solution above about 1000°C (1832°F) The higher

hardness on tempering this steel in the 450 to 650°C (842 to 1202°F) range,

as shown in Fig 6b, substantiates this conclusion In Fig 7, the Charpy

impact energy of Steels Bl, B4, and B5 are plotted as a function of

FIG 5—fa) The variation of hardness with tempering temperature for Steels Bl, B4, and

B6, austenitized at lOOO'C (1832°F) (b) The variation ofCharpy impact energy with ing temperature for Steels Bl, B4, and B6, austenitized at J0OO°C (1832°F)

temper-tempering temperature It is seen that Steel B5, with the lower vanadium

content, had a higher Charpy impact toughness than Steel Bl Both the

lower tungsten (B4) and lower vanadium (B5) steels showed an increase in

both the hardness and toughness as compared with Steel Bl, supporting

the conclusions mentioned previously From a study of Steels B9 and BIO, it appeared that at least 4.5 Cr is desirable in these steels for

obtaining a sufficient secondary hardening response [4]

FIG 6—(a) The variation of hardness with austenitizing temperature for Steels Bl andBS

(b) The variation of hardness with tempering temperature for Steels Bl and B5

The best hardness and toughness combinations obtained for this first

group of steels are shown plotted in Fig 8 The results of this initial study

suggest that one of the research objectives, that is, a secondary hardening

steel with a hardness of 55 HRC and a Charpy impact energy of at least

20.3 J (15 ft • lb) is attainable

New Secondary Hardening Alloys

Encouraging as the results on the modified commercial matrix alloys

were, scanning and transmission electron micrographs of fracture

sur-faces of those steels with the best combinations of hardness and

FIG 8—Representative combinations of hardness and Charpy impact energy for several

secondary hardening steels

B4, as shown in Fig 9 The fracture of this steel was complex, involving intergranular, quasi-cleavage, and dimpled rupture modes Some undis-

solved primary carbides were also observed in Steels B4 and B5 As shown in Fig 10, these fine carbides were often associated with the dimpled rupture facets of the fracture These and other observations strongly suggested that new balanced compositions should be formulated

One intent in the design of the new steels is to explore the possibility of

minimizing those solid-state reactions that lead either to embrittlement or

to marked decreases in hardness As mentioned earlier, these undesirable reactions usually involved the formation of FcsC at intermediate temper-

ing temperatures The subsequent precipitation of FejC in grain and lath

boundaries inevitably leads to a degradation of mechanical properties

Independent studies in this laboratory and others have convincingly demonstrated that silicon, and especially silicon plus aluminum additions

can alter the kinetics of formation of FeaC from

epsilon-carbide [9,16,28,29] Bhat [28] observed that the softening that normally

occurs on tempering AISI 4340 steels beyond 200°C (392°F) was retarded

by the additions of aluminum or combinations of aluminum and silicon, with the combined additions being more effective The tempering behav-

ior of some of these steels is shown in Fig 11 from which it is observed that the tenipering was retarded to temperatures as high as 400°C (752°F)

FIG 10—High magnification scanning electron fractograph of Steel B4, showing

carbides associated with dimples

Even beyond this temperature, the modified steels maintained higher hardness, presumably due to lower growth rates of carbides in the

presence of silicon and aluminum [28] From an investigation of the

tempering response and the microstructure, at the electron optical level,

of these steels, it was concluded that additions of aluminum and

combina-tions of aluminum and silicon to AISI 4340 steel resulted in: {a) the

extension of the first stage of tempering to higher tempering temperatures,

Q}) an increase in the temperature for the second stage of tempering, (c)

the retardation of the third stage of tempering to temperatures greater than about 350 to 400°C (662 to 752°F) depending upon the alloy content,

and {d) an inhibition of the growth rate of carbides

Figure 12 shows a comparison of one of the experimental steels with base AISI 4340 steel showing the following effects: (a) an increase in

strength, {b) an increase in toughness, and (c) a shift in the tempered

martensite embrittlement range to higher temperatures Optimum yield strength and fracture toughness combinations of 245 ksi and 80 ksivInT^

600

FIG 11—The influence of aluminum and silicon additions on the tempering behavior of

AISI4340 steel

significantly higher than those achievable in commercial low-alloy,

ultra-high-strength steels, were attained

The microstructural study of the modified steels led to the conclusion

that optimum combinations of strength and toughness are obtained in the

presence of: (a) fine dispersions of carbides in dislocated martensite, {b)

retained austenite films at lath boundaries that are stable to stress/strain,

and (c) smaller prior austenite grain sizes

The study of the possible influence of silicon and aluminum on the

tempering response of secondary hardening steels was an objective in the

design of the new steels Another intent in the design of the new steels

was to achieve balanced compositions with respect to the austenitic and

ferritic alloying elements The carbide-forming elements, such as

chromium, molybdenum, and vanadium, and the noncarbide formers,

silicon and aluminum, are all strong ferrite stabilizers The desirable

composition of a secondary hardening steel is one that provides a large

enough austenite phase field at solutionizing temperatures to dissolve all

the carbides present, and that also has enough carbon and

carbide-forming elements to provide the desired hardness

The steels listed in Table 2 represent an initial attempt to achieve the

600

FIG 12—Plots of yield strength, ultimate tensile strength, and fracture toughness versus

tempering temperature for AISI4340 and modified AISI4340 steels [28]

TABLE 2—Nominal compositions of new steels

Mn 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

0.25 0.25

on

Cr 1.0

1

1

1

Si 1.5

' Weight % of carbon was obtained by chemical analysis

The influence of silicon content on the tempering behavior of several

molybdenum-nickel-chromium steels is shown in Fig 13 The hardness of

the non-silicon steel (A14) decreases continuously to about 400°C (752°F)

followed by a flattening of hardness curve occurring around 500 to 600°C

(932 to 1112°F) indicating some secondary carbide precipitation The

hardness of Steel A16 (2 Si) is nearly constant with tempering temperature

to 500°C (932°F) with a nearly imperceptible minimum at 400°C (752°F)

The analogous curve for Steel A17 (3 Si) is displaced to a higher hardness

and exhibits a dip in hardness at 200°C (392°F) for reasons probably

related to the higher carbon content (relative to A14 and A16) The

tempering behavior of Steel A15 (1 Si) is consistent with the trends shown

for A14, A16, and A17, but is not plotted in Fig 13

Bhat's [28] studies with low- and medium-alloy, ultra-high-strength

steels revealed that a combination of aluminum and silicon was more

effective in enhancing the strength (hardness) and toughness than either

aluminum or silicon alone The tempering behavior of a steel containing

combined additions of aluminum and silicon is compared with that of a

steel containing no aluminum or silicon in Fig 14 The effect of the

combined aluminum plus silicon addition on the level and shape of the

hardness curve, especially above 200°C (392°F) is striking The effect of a

combined aluminum plus silicon addition on the tempering behavior of a

steel (A21) containing small amounts of another carbide former

TEMPERING TEMPERATURE , °C

700

FIG 13—The variations of hardness with tempering temperature for Steels A14, A16,

and A17, austenitized at 1100°C (2012°F)

O AI4 0.2eC 0.5Mn ZMo I Cr 3Ni

V A19 0.34C O.SMn 2Mo I Cr 3Ni I Al ISi

AS QUENCHED TEMPERING TEMPERATURE.*C 2 0 0 4 0 0 6 0 0

FIG 14—The variation of hardness with tempering temperature for Steels A14 and A19,

austenitized at UOO°C (2012°F)

dium) is similar, as shown in Fig 15 It is well known that the element

vanadium increases the temperature at which the hardness drops The dip

in the hardness versus tempering temperature curve at intermediate

temperatures is not always eliminated by the combined aluminum plus

silicon addition However, the minimum hardness is invariably raised

The hardness and toughness combinations of the new steels have not

yet been completely determined, but a few selected measurements have

been made and the properties of Steel AlO are worthy of comment This

chromium-nickel-molybdenum-vanadium-aluminum-silicon steel has a

relatively high hardness versus tempering temperature curve, as shown in

Fig 16 At the secondary hardening temperature, 550°C (1022°F), it has a

hardness of about 58 HRC and a measured room-temperature

Charpy-impact energy of 20.3 J (15 ft • lb) Although these data are from small

ingots and from single rather than multi-specimen tests, the attainment of

these properties suggests that the combination of hardness and toughness

specified for the feeder components will eventually be met

55 i!^ A 21 0.36 C 2 Mo 0.2S V I Al I Si

AS 200 400 600 QUENCHED

TEMPERING TEMPERATURE, C

FIG 15—The variation of hardness with tempering temperature for vanadium-containing

Steels A20 andA21, austenitized at llOO'C (2012°F)

Coal Handling and Moving Equipment

Modified Commercial Alloys

In the section on materials requirements, it was stated that the combination of mechanical properties thought to be required for compo-

nents in coal handling and moving equipment consisted of adequate hardness, toughness, and in all probability, impact fatigue resistance The

elimination of hot strength as a requisite property in these components

considerably lessens the difficulties in the design of these steels