A study of precipitation in a 12 %Cr-Co-Mo steel

A STUDY OF PRECIPITATION D. J. DYSON?

IN A 1233--Co-M0

STEEL*

and S. R. KEOWNT

The structural changes occurring during the tempering of a 12 %Cr-6%Mo-10 %Co-0.1 %C steel have been investigated. It has been shown that an intermetallic compound R-phase is responsible for the marked secondary hardening but that the carbides M,X and M& are also precipitated. The R-phase contained iron, chromium and molybdenum but little or no cobalt. By determining the orientation relationship between R-phase and ferrite it has been shown that the precipitation of R-phase in ferrite is theoretically favourable. ETUDE

DE

LA

PRECIPITATION

DANS

UN

ACIER

1233-43~-MO

Les variations de structure se prod&ant au tours du revenu d’un acier 12 %Cr-6 %Mo-10 %Co-0,I %C ont et& Qtudiees. Les auteurs montrent qu’une phase oonstituee par un compose interm&allique R est responsable du durcissement seoondaire prononce, mais qua les carbures M,X et M& prcicipitent bgalement. La phase R contient du fer, du chrome et du molybdene mais peu ou pas de cobalt. En determinant la relation d’orientation entre la phase R et la ferrite, les auteurs montrent que la precipitation de la phase R dans la ferrite est favori& par la theorie. EINE

UNTERSUCHUNG

DER

AUSSCHEIDUNG

IN

EINEM

12 %Cr-Co-Mo-Stahl

Die w&rend des Temperns von 12 %Cr-6 %Mo-10 %Co-0,l %C-Stahl auftretenden Strukturanderungen wurden untersucht. Es wurde gezeight, da13eine R-Phase einer zwisohenmetallischen Verbindung fur die ausgepragte sekundare Verfestigung verantwortlioh ist; jedoch scheiden sich auoh M,X- und M,C-Karbide aus. Die R-Phase enth< Eisen, Chrom und Molybdlin, jedoch wenig oder gar kein Kobalt. Durch Bestimmung der Orientierungsbeziehun~ zwisehen R-Phase und Ferrit wird gezeigt, dal3 die Ausscheidung der R-Phase in Ferrit theoretisoh giinstig ist.

1. INTRODUCTION

Only recently have steels been developed whose properties depend upon the precipitation of intermetallic compounds. Early work on the aging reactions in lZ%Cr steels with large MO and Co additions attributed the high strengths achieved on tempering to the precipitation of Laves and chi phases(ig). In addition an ordered phase, thought to be Fe&o, has been reported. (3) In all this work the precipitates were identified from polycrystalline electron diffraction patterns or, when possible, from X-ray diffraction patterns. With recent advances in the identification of phases by selected area electron diffraction in the electron microscope(4) it has become possible to identify individual precipitate particles from single crystal electron diffraction patterns. The purpose of this work, therefore, was to investigate more fully the nature of the precipitating phases responsible for the strengthening of a stainless 12 %Cr--CO-MO steel. The secondary hardening characteristics of five steels with varying cobalt and molybdenum additions to a basic 12 %Cr-O.l %C composition have previously been reported,(5*6) and the hardness curves are reproduced in Fig. 1. It is clear that the S%Mo100/,0 and 9 %iVZo-15 %Co steels age-harden appreciably more than the steels containing smaller

amounts of molybdenum and cobalt. Increasing the molybdenum content of these steels above 4% appeared to give the maximum hardening and the cobalt was added in the amounts required to balance the ferrite-forming tendencies of the molybdenum. While the two steels with the highest cobalt and molybdenum additions gave similar age-hardening effects, the 9%M?3-15 %CO steel contained an appreciable amount of undissolved chi phase. The steel with the smallest alloy additions i.e. the 6% No-10 %Co steel, was therefore chosen for the present investigation. 2. EXPERIMENTAL

PROCEDURE

The steel was manufactured as a 25 lb. air-melted commercial purity cast of the following composition (wt. %) : -

C

Mn

Si

Cr

MO

co

0.11

0.51

0.26

11.50

6.05

9.60

The ingot was forged to +j in. diameter bar which was then hot rolled to 0.075 in. strip. Specimens were solution treated at 1050°C for 1 hr and water quenched before tempering for 1 hr at temperatures between 450’ and 700°C. Extraction replicas were prepared by etching * Received November 19, 1968. polished specimens in 5 % HCl in Picral and stripping t Swinden Laboratories, Midland Group, British Steel in 5 *A HCl and 10% HNO, in alcohol. Some replicas Corporation,Moorgate, Rotherham, Yorkshire. ACTA METALLURGICA,

VOL.

17, AUGUST

1969

1995

ACTA

1096

METALLURGICA,

VOL.

Ii,

1969

(....

“-y..

/

/.”

FIG. 1. Tempering

‘. \

curves of 12%Cr-CO-MO

steels.

were examined by electron probe microanalysis as electron microscopy. fluorescence

analyses

potentiostatic foil

microscopy

of

suitable

diffraction

residues

techniques”)

specimens

material

X-ray

extracted

were for

also

a thickness

transmission

of

using

made.

were prepared by machining to

as well

and X-ray Thin

electron

the 0.075 in.

0.030 in.

before

heat

treatment, and finally chemical thinningf8) and electropolishing in a chromic-phosphoric 3. MICROSTRUCTURAL

electrolyte.

the

structure

solution

OBSERVATIONS

consisted

stringers

treated of

increase in hardness. 600°C

to

positive Fig.

produce

5(a),

separate identified

Specimens slight

microstructural and

precipitate

martensite

the

with

of delta ferrite and undissolved

micro-

the

first

particles

of

two

at very

quite high

The acicular particles were

carbide

cipitates as the intermetallic condition

gave

for precipitation,

were detected

Fig. 5(b).

as the M,X

aged for 20 min at

overaging evidence

morphologies

magnification,

3.1. Solution treated specimen In

FIG. 2. Optical microstructure of solution treated specimen, showing delta ferrite (f) and chi-phase (c) in heavily segregated regions x 1000.

and the angular pre-

R-phase at later stages of

aging when the particles were larger and more widely

occasional

spaced.

chi phase,

more clearly shown in Fig. 5(c).

The two types of precipitate

morphology

are

It can be seen from

Fig. 2. The shape and size of the ferrite areas enabled

Figs. 6(a) and (b) that there was a very dense distri-

them

bution of precipitate

to be easily

phase.

The

occasional surface

distinguished

ferrite

and

merely

showed

ferrite, chi and martensite dislocation

chi were

those

only

readily

of chi

present

more heavily segregated regions.

replicas

microscopy

from

in

Although

the outline

of the

phases, Fig. 3(a), thin foil

showed

the

vast

difference

in

density between the ferrite, Fig. 3(b), and

part#icles in this slightly overaged

condition. With aging temperatures

up to 700°C the particles

of R-phase slowly increased in size, Figs. 7(a)-(c)

and

then at 750°C the particle size increased quite rapidly. The relationship between the precipitate size of Rphase and hardness is shown in Fig. 8 from which it

the martensite,

Fig. 3(c).

can be seen that there was a linear decrease in hardness

Tempered

specimens

but a further increase in size to about 3000 A did not

3.2.

On tempering,

very

with increasing precipitate little microstructural

was observed until just after maximum hardness. Tempering for short periods

change secondary at 600°C

indicated that the time required for maximum agehardening was very critical. The isothermal aging curve for the steel tempered at 600°C is shown in Fig. 4 and it can be seen that increasing the aging time from 5 to 10 min resulted in about 100 HV

size from 100 A to 1000 A,

cause much further change in hardness. The acicular M,X phase persisted to 65O”C, but at 700°C the needles were replaced by angular particles identified as the M,C carbide. The morphologies of the M,C and the R-phase were clearly very similar at 7OO”C, and M,C could only be positively detected by electron diffraction analysis. As the structure

of the martensite

became

more

DYSOS

AND

KEOWN:

PRECIPITATION

IN

A

lZ”/;Cr-CO-MO

1097

STEEL

FIG. 3(a) FIG. 3(b)

rT “”

; ?

560

,? I

540 520 500

I 10

Time

FIG. 4. Isothermal

4. THE

tempering curve for 12%Cr-6%MolO%Co steel.

be explained

FIG. 3. Microstructure of solution treated specimen. (a) Surface replica showing delta ferrite and chi-phase in martensite. x 4000 x 6. rho Dislocations i” delta ferrite. Thin foil. , . (c) Dislocations in martensite. Thin foil. x 60,000.

unit cell.

in terms of a hexagonal

The parameters (a) hexagonal :

to

the

coarsening

of

apparent that there was a marked decrease in dislocation density compared with the original martensitic The precipitation of R-phase in the delta structure. ferrite was much denser than in the martensite,

see

diffrac-

system can

or rhombehedral

are :

a, = 10.903 A : a, = 9.005 .& x

the

precipitates at 650°C it was possible to detect the remnant dislocations in the structure, Fig. 9. It was

Fig. 6(a).

R-PHASE

ca = 19.342 A (b) rhombohedral

due

OF

et aZ.(s) have shown that the X-ray

tion pattern of R-phase in the Mo-Cr-Co

FIG. 3(c)

resolvable,

thnutes

CRYSTALLOGRAPHY

Komura

clearly

-

50

4u?

30

20

= 74°31.2’

The space group is thought to be R 3-C$. The X-ray powder diffraction patterns of the residue extracted from the specimen heat-treated at 700°C showed broad diffuse lines indicative of a particle size of ~500 8. This was confirmed by the electron microscope observations. The lines were identified as the strong lines of R-phase.

EO

ACTA

1098

METALLURGICA,

VOL.

17,

1969

Fra. 5(b)

Fra. 6(a)

structural

similarities

compounds

between

systems,(g-12)

see

Table

tion for certain R-phase

intermetallic

of the electron

phase

M,Moso3)

explain

offer an explanation Figures diffraction

10 and

diffraction

in terms

patterns

of a second

However,

some patterns,

11 were typical obtained.

whilst

this

R-phase

can

of the electron

Both

show

The patterns as indexed,

illustrate the space group proposed The reflections

careful

for them all.

patterns

Laiie zone orders.

a

Indeed a possible explana-

can be found

can reasonably

intermetallic

in ternary alloy

1, necessitates

analysis of such patterns. from

several

which occur particularly

forming

several serve to

by Komura

Fig. 10 define a [22.1] crystal zone as indexed. curvature

FIG. S(c) FIG. 5. Microstructure of specimens showing M,X and R-phase precipitation. (a) Precipitated particles in tempered martensite. Thin foil. Aged at 600°C for (b) Acicular and spheroidal pre20 min. x 80,000. cipitates in tempered martensite. Thin foil. Aged at (c) Acicular and spheroidal 600°C for 20 min. x 200,000. precipitates in tempered marten&e. Thin foil. Aged at 600°C for 1 hr. x 100,000.

With such large lattice parameters it is expected that electron diffraction patterns from simple index [UV.W] zones will contain several Laiie zone orders since the planes of the reciprocal lattice normal to these directions will be very close together. The close

et al.

the zero order Laiie zone of The

of the median line of this zero order Laiie

zone suggests a tilt of -12”

from [2%1].

reflections

in higher

orientation

is nearer [X.2].

Laiie zones suggest the This is lO”43’ from [22.1].

order

The stronger

The ferrite pattern in Fig. 11 comes from both the [113] and [115] crystal zones which are 9’27 The

orientation

relationship

using the intermediate

has been

[114] zone.

Laiie zone of the precipitate

pattern

apart.

determined

The zero order defines a [12.i]

crystal zone, although its curvature suggests an orientation -12’ from this. The stronger reflections from higher order zones taken together define a [27.6] crystal zone which is 12’43’ from [12.i]. Figure 12 shows the stereographic projection corresponding to this pattern. The orientation relationship common to the solution

DYSOR

AND

KEOWh’:

PRECIPITATION

IN

A

12%Cr-CO-MO

STEEL

FIG. 6. Microstructure of specimen aged at 600°C for 1 hr. (a) Extraction replica showing dense precipitation in martensite and even heavier precipitation in the delta ferrite. x8000. (b) Extraction replica of M,X and R-phase in the marten&e. x 100,000.

of all the electron diffraction p&terns (10.0) R-phase -//

taken was:

(32i) ferrite

(01.0) R-phase -_I/ (213) ferrite (li.0)

R-phase -_I/ (132) ferrite

(00.1) R-phase

FIG. 6(b)

// (111) ferrite

In general, exact parallelism of precipitete and matrix planes is seldom seen in the diffraction patterns. The ferrite planes of simple indexing which almost form a set of hexagonal axes and which are closest to the unit cell faces of R-phase were chosen to define the above relationship. The sign ambiguity associated with the indexing of single crystal patterns(4) gave rise to several different possible relationships. Only the above one was common to all the patterns.

ACTA

1100

METALLURGICA,

VOL.

15,

1969

Fla. 7(b)

FIG. 7(a)

600

I

4ool 0

SW

VXO Meen

FIG,

from

8. The

2,000 Diameter

3,000

2.500

1500

8,

relationship between hardness and precipitate particle size.

a sample

following

1,500 Preclpltaie

treated

composition

at 700°C for 1 hr gave the (at. %) :

Fe

Cr

MO

co

44.8

25.6

27.8

1.8

FIG. 7(C) FIG. 7. Effect of increasing aging temperature on pre-

cipitate particle size. (a) Aged at 600°C for 1 hr. Extraction replica x 16,000. (b) Aged at 650°C for 1 hr. Extraction replica. x 16,000. (c) Aged at 700°C for 1 hr. Extraction replica. x 16,000. 5. ELEMENTAL

Electron together

ANALYSIS

probe microanalysis

with X-ray

fluorescence

OF R-PHASE

of extraction

replicas

analysis of some of

the extracted residues separated for X-ray diffraction analysis has enabled the approximate composition of R-phase to be established. X-ray fluorescence analysis of a residue extracted

Electron probe microanalysis of an extraction replica from a specimen tempered at 700°C for 1 hr confirmed

that the phase was rich in iron, chromium

and molybdenum and contained little, if any, cobalt. These analyses showed that although the steel had a very large cobalt addition, the cobalt content of the precipitating phase was very small. Thus the R-phase in this steel was an Fe-Cr-Mo compound. However since the residue used for fluorescence analysis and the replica used for microanalysis both contained carbides it was not possible to define the exact chemical composition

of the R-phase.

DYSON

KEOWN:

AND

PRECIPITATION

FIG. 9. R-phase precipitates and dislocations in martensite aged at 650°C for 1 hr. Thin foil. x 120,000. 6. DISCUSSION

6.1.

Relationship

OF

RESULTS

between microstructure

and hardness The extensive age hardening and retarded softening in this 12 %Cr-6 %Mo-10 %Co stainless maraging steel can be attributed to the very heavy dispersion of precipitates produced during tempering. Initially there is a solid solution hardening effect produced by the high alloy content, augmented by a strengthening contribution from the high dislocation density and the fine grain size of the martensite. On tempering a two stage hardening reaction occurred, M,X precipitating at 550” to 600°C followed by the intermetallic R-phase at 600°C. Maximum hardness at 600% coincided with the heaviest dispersion of carbide and intermetallic particles. The softening of the alloy with further tempering was associated with the growth of the precipitate particles, Fig. 8. The M,X carbide TABLE 1. Comparison

of lattice parameters

IN

A

12%Cr-CO-MO

STEEL

1101

eventually transformed to M,C but it is not known whether on high temperature tempering the R-phase particles merely grew in size or whether they transformed to a different intermetallic phase such as chi or sigma phase. Certainly at very high temperatures chi is the stable phase since it was detected after solution treatment at 1050°C. The microstructure of the specimen tempered to give maximum hardness was extremely complicated. The dislocations were unresolvable and diffraction patterns failed to show any evidence of precipitates or pre-precipitation zones. There was no streaking of the matrix reflections which eliminated the possibility of disc or rod-shaped zones, but not spherical zones which could have been present. Although diffraction effects were obtained from the precipitates present after tempering at 600°C for 1 hr, the particles were too closely spaced to enable single crystal patterns to be recorded. It was not possible to identify the spherical particles as R-phase until the alloy wa,s tempered at 650°C when single crystal patterns were obtained. A noticeable feature of the thin foil microstructure was the absence of dislocations in the overaged condition. It is possible that the misfit between the precipitate and the matrix is sufficiently large for the dislocations to be accommodated at the growing precipitate-matrix interface. 6.2. The significance of the orientation relationship

It can be seen from Fig. 14 that the precipitation of R-phase in ferrite according to the derived relationship requires relatively small atom movements, and therefore represents a low energy transformation. In this diagram the outline of the base of the hexagonal unit cell of R-phase has been superimposed on the projection onto (111) of the atoms in ferrite. The cell faces have been set parallel to (321) ferrite planes. The repetitive volume in ferrite defined by the projected of some transition

metal intermetallic

phases

Crystal structure Phase system Iron-Chromium (T Iron-Molybdenum o Fe,Mo, (p) Fe,Mo, (1~) Fe,Mo Laves Phase R Phase (Mo-Co-Cr) R Phase (Mo-C+Cr) 6 Phase (Mo-Ni) P Phase (Mo-Ni-Cr) x Phase (Fe-Cr-Ni) 12

Tetragonal Tetragonal Hexagonal Rhombohedral Hexagonal Hexagonal Rhombohedral Tetragonal Orthorhombic Cubic

%A 8.800 9.188 4.746 8.928 4.727 10.903 9.005 9.108 9.070 8.860

%?A 8.800 9.188 4.746 8.928 4.727 10.903 9.005 9.108 16.983 8.860

%A 4.544 4.812 25.78 8.928 7.704 19.342 9.005 8.852 4.752 8.860

c(

30”47’

74”31’

llQ2

ACTA

METALLURGICA,

VOL.

.

ZONE

AXE -

FIG. 10, Electron diffraction pattern from

17,

1969

l

N 12’p

[744 FROM

(25.2

R-phasssnd its interpretation.

DYSON

AXTI

KEOWN:

PRECIPITATION

IN

A

12’+oCr-Co-Mo

STEE

ZERO LAUE

ORDER ZONE

F<

PRECIPITATE

SOL”’

Fra.

TO PATTERN PRECIPITATE

11. Electron

FROM

diffraction pattern from R-phesc

and fcrrite and its interpretation

ACTA

METALLURGICA,

VOL.

17,

1969

..OO.tR

111F

---I

--

I

FIG. 12. Stereographic projection corresponding to Fig. 11.

--...

ATOMS

I@;i Wa. 13. Projection of atoms in a-iron

ABOVE

onto (I 11).

PLANE

AT

OF

A

DYSON

base of R-phase R-phase

KEOWN:

and a height ?

contains

favourably

AND

the c,, parameter

168 metal atoms.

This

of

compares

with the 159 metal atoms in the R-phase

unit cell.

The a,, parameter

indicated

10.725 11 and thus an expansion necessary to fit in the intermetallic perpendicular

to

corner distance contraction R-phase

PRECIPITATION

this

plane,

of only 1.66 % is unit cell. Similarly,

the

cell

corner-cell

is 19.861 b thus requiring

to fit the R-phase cell

in Fig. 13 is

cell.

necessary

shows

that

many

of the movements

Other

atoms

which

approach

distances.

require

larger

the

However

magnitude

evidence of the orientation

the

movements

atom

necessary

for

explains

required to convert the transition

strains

cell of ferrite to the

the preferential

Further evidence

theoretically

are found to be large.(14) This precipitation

of the validity

relation is found in terms of the Patterson

precipitate

For a phase whose atomic

if the metallurgical

con-

ditions are favourable. The

atom

Komura

positions

position in

R-phase

et aZ.(g) have been plotted

according

to

as full circles in

Fig. 14. This diagram is one of a series of four which divide equal

the height sections.

tionship,

of the unit cell of R-phase

Using

the derived

the ferrite atoms

have

orientation

into rela-

been superimposed

on Fig. 14 in their relative positions

and are denoted

by open circles. These atoms define planes. Since these planes have been four groups, each open circle represents different heights. The height of the

the 24 (111) divided into two atoms at “first” {ill}

plane above the basal plane of the R-phase cell was chosen such that the sum of the magnitudes of the vertical movements of all atoms to change from their positions in ferrite to those in R-phase, was a minimum.

of two atoms

of R

of the orientation

R-phase cell are thus very small and the latter should in the former

per

between sigma phase and ferrite is known

rather than sigma phase in ferrite.

lattice

moved

relationship

probably

The

of

inter-atomic

the average distance

consist of 24 equally spaced layers of atoms, i.e. 8 of 13.

of

atom remains small.

the

Fig.

distances.

movements-some

relationships

in

1105

are much less than interatomic

Whilst no experimental

a 2.61%

The height of the

ferrite

This diagram

STEEL

proposed

shown

be 4 [ill]

lB%Cr-Co-MO

A

and will thus

units

will

IN

structure

projection.

is based on the

the Patterson

projection

certain planes shows peaks in the same positions

on as

the atoms projected onto that plane. Such is the case for the (111) projection of a-iron, Fig. 13. The close similarity between such a projection projection further

for

R-phase

confirmatory

an

and the Patterson

(00.1)

evidence

of

Fig.

15cg) offers

the

orientation

relationship. 6.3. The role of cobalt It is interesting to outline the role of cobalt in this 12 %Cr-6 %Mo-10 %Co steel. The cobalt is added as a control element to offset the ferrite-forming tendency of molybdenum. Cobalt is an efficient austenite forming element with the added advantage of not depressing

the M, temperature.

It has been shown

ACTA

1106

METALLURGICA,

VOL.

17,

1969

FIG. 15. Patterson projection of R-phase on (00.1) (after Komura et cd.(“)).

that with cobalt

additions

of up to 15 % there is a

hardness

increment

of approximately

wt. %Co

in

steels,(Q

maintained extensive 75 %Co) y-Fe

12 %Cr

at all tempering solid

solubility

and complete

so that

this

9 HV

increment

temperatures.

of cobalt

all the cobalt

1

being

There is

in a-Fe

solid solubility

per

(up to

of cobalt

in

in the 12 %Cr-6%Mo-

10 %Co steel is easily accommodated

in solid solution.

and perhaps this contributes solid solution hardening, Bannerjee

et ai. (15*16)have shown that cobalt increases

the precipitate steels

in some way, other than

to the strength of the steel.

and

nucleation

have

suggested

stacking fault energy number of dislocations tion by discouraging

that

cobalt

maraging lowers

the

of the matrix, increasing the available as sites for precipitacross slip.

suggested

possible

hardening

molybdenum(17~18) or that cobalt orders the matrix.(19)

can

The present work has merely shown that cobalt remains in the matrix and does not enter into the

application if

cobalt

as

a precipitation

intermetallic

compounds

be

produced in steels. From the results of the present work it appears that little or no cobalt is entering into the precipitating cobalt

which

hardening

R-phase

initially

and therefore

imparts

in the solution

useful

treated

the 10% solid

condition,

of

solution main-

tains the solid solution hardening effect throughout the tempering reactions. It is interesting to note that on tempering at 600°C or at higher temperatures, when there is a large volume fraction of precipitate particles and a corresponding decrease in the matrix volume, the matrix will contain considerably

more than 10 % of

cobalt so that cobalt will contribute extra solid solution hardness. However the iron, chromium and molybdenum concentrations in the matrix are being reduced because

as the amount of R-phase increases and molybdenum is a much more potent solid

cobalt

Other workers have

Cobalt does not form carbides in steels but it has a agent

that

rate in 18%Ni

decreases

the

solubility

of

It is suggested however that precipitating phase. cobalt has two distinct effects on the matrix. Firstly the cobalt

affects

the precipitation

of intermetallic

phases by stabilising the nucleating dislocations, not necessarily by lowering the stacking fault energy, at temperatures where normally they would begin to anneal out. Secondly there may be a strengthening effect such as matrix ordering due to the matrix containing cobalt concentrations cobalt content of the steel. 7. SUMMARY

AND

in excess

of the

CONCLUSIONS

An investigation changes occurring

has been made into the structural in a 12 ‘ACr-6 %Mo-10 %Co stainless maraging steel. From the results of this work the

solution hardener than cobalt the total solid solution hardening is not likely to increase. It is not known

following conclusions have been reached : (a) The extensive age hardening and retarded softening was mainly due to the precipitation of the

however what properties a high cobalt matrix possesses

intermetallic

compound

R-phase.

DYSON

AND

KEOWN:

PRECIPITATION

(b) The normal carbide reaction produced in 12 %Cr steels, namely I$X + M,C is superimposed on the intermetallic compound precipitation reaction. (c) It was shown that the R-phase contained iron, chromium and molybdenum but little or no cobalt. (d) The orientation relationship between R-phase and ferrite was determined to be : (10.0) R-phase N /I (3x) (01.0) R-phase -

ferrite

// (213) ferrite

(1i.O) R-phase N // (152) ferrite (00.1) R-phase

// (111) ferrite

(e) From the above relationship it has been shown that only minimal atom movements and small lattice strains are required for R-phase to precipitate from ferrite. Thus R-phase precipitation appears more favourable than the precipitation of sigma phase from ferrite containing suitable alloying elements. ACKNOWLEDGMENTS

The authors wouId like to thank Mr. F. B. Pickering for numerous discussions and continued encouragement and also Dr. F. I-I. Saniter, O.B.E., Director of Research for permission to publish this work.

IN

A

12~oCr-Co-Mo

STEEL

1107

REFERENCES 1. FL J. IRVINE, J. Iron Steel In&. 200, 820 (1962). 2. A. KASAK, V. K. CHANDHOKand E. J. DULIS, Trans. Am. Sot. Metals56, 455 (1963). 3. E. DIDERRICH,D. COUTSOURADIS and L. HABRAKEN,MBm. scient. Revue. M&all. 61. 655 (1964). 4. K. W. ANDREWS, D. J. *Dvsok and S. R. KEOWN, Interpretation of E&xtron Diffraction Patterns. Hilger & Watts (1967). 5. K. J. IRVINE and F. B. PICKERING,Ivwn and Steel Iv&. Special Report No. 86 (1964). 6. K. J. IRVINE, Proc. Brzcssels Int, Conf. on Applications of Cobalt i 1964 1. H. HU&Es,‘J. Iron Steel Inst. 204, 804 (1966). i: S. R. KEOWN and F. B. PICKERING,J. IronSteel In&. 200, 757 (1962). W. G. SLY and D. P. SHOEMAKER, Acta 9. Y. KOMURA, crystallogr. 13, 575 (1960). 10. D. P. SHOEMAKER,C. B. SHOEMAKER,and F. C. WILSON, Acta cry8taZZogr. 10, 1 (1957). and D. P. SHOEMAKER,Acta crysta2Zogr. 11. C. B. SHOEMAKER 10, 997 (1963). 12. C. B. SHOEMAKER,D. P. SHOEMAKERand J. MELLOR, Acta c~~~l~og~. 18,37 (1965). Third Eur. Conf. os Electron 13. S. R. KEOWN, Proe. Microscopy. Prague ( 1964). 14. D. J. DYSON, Unpublished work. 15. R. R. BANNERJEEand J. J. HAUSER, Tv-ansfoymation and Hardenability in steel, p. 133, Climax Molybdenum Co. (1967). 16. B. R. BANERJEE,J. J. IEAUSERand J. M. CAPENOS,Metal Sci. JI. 2, 76 (1968). 17. G. P. MILLERand W. I. MITCHELL,J. Iron Steel Inst. 208, 899 (1965). 18. S. FLOREEN and G. R. SPEICR, Trans. Am. Soe. &letaZs57, 714 (1964). 19. R. F. DECKER, J. T. EASH and A. J. GOLDMAN, Trtaw, Am. Soe. Meta.% 55,58 (1962).