Application of Hardenability Concepts in Heat Treatment of Steel DOUGLAS V. DOANE New information on hardenability is available to the heat treater, in the form of a critical review of existing hardenability data, more accurate predictions of harden- ability from chemical composition, and new data on austenite phase transformation kinetics. This paper summarizes this new information and provides examples as well as references to more comprehensive coverage of several subjects. These subjects include 1) correlation of end-quench hardenability bar cooling rates with cooling rates in fully immersed rounds, 2) documentation of the strong influence of carbon on the effectiveness of alloying elements on hardenability, 3) alloy interaction effects, and 4) various aspects of austenite conditioning. Examples of the use of hardenability in metallurgical design emphasize the use of actual heat treating practice to establish hardenability requirements, the need to consider both core and case properties when heat treating carburized steels, and the role of hardenability in induction and flame-hardening. Extensive work is in progress in Europe and North America to devise and perfect systems for predicting hardenability, microstructure and mechanical properties from chemical composition, section size and heat treatment parameters. These systems hold great promise, but important considerations are the limitations of such systems because of variability in composition due to inherent segregation in steel ingots and in the products of those ingots, variability in heat treating conditions, and, of course, the limitations imposed by the empirical nature of the predictive systems. Nevertheless, such systems provide steel producers, heat treaters and machine designers with useful new tools. THERE has been considerable activity throughout the world, evident in publications during the last five years, to take advantage of the wealth of available information relating the composition of steel to microstructure and properties. The advent of computers has made the task considerably easier. Metallurgists and heat treaters should be aware of the data available and the methods used to analyze and correlate the data, so that maximum DOUGLAS V. DOANE, Climax Molybdenum Co. of Michigan, a Subsidiary of AMAX Inc., Ann Arbor, MI J. HEAT TREATING benefit can be taken of the results. It is also important to understand the limitations of these correlations, so they can be applied realistically. A review of published data on hardenability and much of the work on application of hardenability concepts is contained in two recent books ~,2. It is the purpose of this paper to direct the readers attention to information that should be of particular interest to heat treaters and those concerned with the profound effect of heat treatment on the properties of steel. It will not be possible, of course, to treat any subject in detail, so this paper is, in a sense, an overview. ISSN 0190-9177/79/0806-0005500.75/0 1979 AMERICAN SOCIETY FOR METALS VOLUME 1, NUMBER 1--5
Transcript
Application of Hardenability Concepts in Heat Treatment of Steel
D O U G L A S V. D O A N E New information on hardenability is available to the heat treater, in the form o f a critical review of existing hardenability data, more accurate predictions o f harden- ability f rom chemical composition, and new data on austenite phase transformation kinetics. This paper summarizes this new information and provides examples as well as references to more comprehensive coverage o f several subjects. These subjects include 1) correlation o f end-quench hardenability bar cooling rates with cooling rates in fully immersed rounds, 2) documentation o f the strong influence o f carbon on the effectiveness o f alloying elements on hardenability, 3) alloy interaction effects, and 4) various aspects o f austenite conditioning. Examples o f the use o f hardenability in metallurgical design emphasize the use o f actual heat treating practice to establish hardenability requirements, the need to consider both core and case properties when heat treating carburized steels, and the role o f hardenability in induction and flame-hardening. Extensive work is in progress in Europe and North America to devise and perfect systems for predicting hardenability, microstructure and mechanical properties from chemical composition, section size and heat treatment parameters. These systems hold great promise, but important considerations are the limitations o f such systems because of variability in composition due to inherent segregation in steel ingots and in the products o f those ingots, variability in heat treating conditions, and, o f course, the limitations imposed by the empirical nature o f the predictive systems. Nevertheless, such systems provide steel producers, heat treaters and machine designers with useful new tools.
T H E R E has been considerable activity throughout the world, evident in publicat ions during the last five years, to take advantage o f the wealth of available informat ion relating the compos i t ion of steel to microstructure and properties. The advent o f computers has made the task considerably easier. Metallurgists and heat treaters should be aware o f the data available and the methods used to analyze and correlate the data, so that max imum
DOUGLAS V. DOANE, Climax Molybdenum Co. of Michigan, a Subsidiary of AMAX Inc., Ann Arbor, MI
J. H E A T T R E A T I N G
benefit can be taken o f the results. It is also impor t an t to unders tand the l imitations of these correlat ions, so they can be applied realistically.
A review of publ ished data on hardenabi l i ty and much of the work on appl icat ion o f hardenabi l i ty concepts is conta ined in two recent books ~,2. It is the
purpose o f this paper to direct the readers a t tent ion to in format ion that should be of part icular interest to heat treaters and those concerned with the p ro found effect o f heat t rea tment on the propert ies of steel. It will not be possible, o f course, to treat any subject in detail, so this paper is, in a sense, an overview.
ISSN 0190-9177/79/0806-0005500.75/0 �9 1979 AMERICAN SOCIETY FOR METALS VOLUME 1, NUMBER 1--5
Table I. A Summary of the Observed Correlation of the End-Quench Bar with Center Positions of Rounds Quenched in Oil and Water (Carney + SAE Handbook)
End-quench Ideal distance, diam,
1/16 in. in.
Round size, equal half- H-band values,
tempera ture SAE Handbook , time, in. in.
Round size, equal microstructures , in.
95 pet martensite 80 pct mar tens i te 50 pct martensi te
Wate r Oil Water Oil Water Oil Water Oil Water Oil
CORRELATION OF HARDENABILITY TEST RESULTS WITH ACTUAL QUENCHED PARTS
The end-quench hardenability bar does not provide the same heat transfer characteristics and internal stresses as an immersion quenched part. Yet correla- tions are imperative to be able to specify steels accord- ing to their hardenability characteristics. Many investi- gators have attempted to establish this correlation, and to explain the variations that are evident, but the work of Carney 3 was extensive. A summary of his correlations between equal microstructure, as well as equal half- temperature time,* in the end-quench bar and the corre-
*Half - temperature time is the t ime to reach a temperature halfway between the quench and room temperature .
sponding positions in rounds quenched in water and oil is given in Table I. An additional column has been in- cluded in the table showing Jominy distance v s diameter taken from the H-band charts published in the 1975 SAE Handbook. Carney's data for equal half-tempera- ture time are fairly comparable to the H-band values, indicating that the severity of quench was the same for both evaluations. However, the sizes of rounds based on
equivalent microstructures is not the same, reflecting other influences such as heat transfer and internal stresses. The SAE Iron and Steel Technical Committee, Division 8 on Hardenability, recently reviewed the cor- relations between end-quench distances and diameters of quenched rounds of equivalent microstructure, and has issued a more realistic correlation, as shown in Figs. 1 and 2. These correlations reflect the differences ob- served in practice, even under carefully controlled quenching conditions, and show that correlations are less accurate with larger bar sizes.
THE HARDENABILITY EFFECT OF ALLOYS INCLUDING CARBON-ALLOY AND ALLOY-ALLOY INTERACTIONS
The ASM book 1 extensively reviews the metallurgical factors influencing hardenability of steels. After dis- cussing the effects of grain size, austenitizing tempera- ture and time, the many important studies of the effects of austenite composition are reviewed. For the presenta- tion of alloy effects, steels are grouped as follows:
Carbon-manganese steels
6 - - V O L U M E 1, NUMBER 1 J. H E A T T R E A T I N G
Fig. 1--Correlation between location in a water-quenched round bar and position (dis- tance from the quenched end) on the Jominy bar (SAE).
Medium-carbon low-alloy steels Carburizing steels High-hardenabi l i ty steels
These classifications reflect the fact that alloys affect hardenabil i ty to differing degrees depending on the car- bon content o f the steel and the other alloys present. In the following discussion it will be assumed that austeni- tizing temperatures are high enough so that all alloy ad- ditions are in solution, to maximize their hardenability effect, but not high enough to coarsen the austenitic grain size to the point that it detracts f rom toughness of the hardened steel.
Grange 4 recently devised a new hardenabil ty test
(called the " h o t - b r i n e " test) for shallow hardening steels and presented new data on the effects of carbon and alloys on hardenabil i ty, in terms of "hardenable d iamete r"* expressed by the equation:
*Hardenable diameter is that diameter of water quenched round that will exhibit 90 pct martensite at the center.
D n = D c + ADMn + A D p + ADsi + ADc~ +
zXDNi + ADcr + ADMo + AD v
Da ta for use with this equat ion are shown in Figs. 3 and 4. The Grange test can be applied to steels which are normal ly water quenched in sizes up to those equivalent to 23 m m rounds. These include section sizes for which Jominy end-quench data are not applicable.
The ideal critical d iam, Dr, concept has been so uni- versally accepted as a measure o f hardenabi l i ty that a lmos t all studies o f alloy effects are expressed in terms of Dr. The definition, for reference, is the diameter o f a given steel bar which will exhibit 50 pct martensi te at the center when subjected to a hypothet ica l quench which reduces the surface t empera tu re o f the steel to the ba th t empera tu re in zero time. It is impor t an t to r emember that the definit ion is a micros t ruc tura l one, and makes no reference to hardness. The work o f Gros smann 5 established the fact that the effects o f alloying elements on hardenabi l i ty in medium ca rbon steels are multiplica- tive and the effects can be expressed as "mul t ip ly ing f ac to r s " related to a base/)1. Subsequent work by many investigators conf i rmed the soundness of this concept, but provided evidence that the appl ica t ion of the prin- ciple was complex. Some o f the following problems
J. HEAT TREATING VOLUME 1, NUMBER 1--7
Fig. 2 - -Cor re l a t i on between location in an oil- quenched round bar and posi t ion (distance f rom the quenched end) on the Jominy bar (SAE).
0.(:
0 .5
d
ua 0 . 4 I - - ILl ~E
r '~
~o.3
/ 0.1 / 0
0
/ t t
16
14
ne"
lO N W
8 " W - J
6 z
r , .
0.2 0.4 0 .6 0 .8 1 . 0
CARBON, 7. Fig. 3 - -Ha rdenab i l i t y o f Fe-C alloys (90 pct martensite, water quenched , no. 4 A S T M Grain Size) (Grange).
0 6 i! 0.5
0.4
d0.3
0.2
0.1
0 0 0.2 0 .4 0 .6 0 .8 1.0 1.2 1.4 1.6 1.8 2 .0
ELEMENT, wt % Fig. 4 - - C h a n g e in hardenable diameter (AD) with C, Mn, Si, Cu, Ni, Cr, and Mo (Grange).
were encountered:
1) A multiplying factor for a given element was not always directly proportional to the percentage of that element.
2) Interaction effects occurred at times; that is, the multiplying factor for a given percentage of an element
8 - - V O L U M E 1, N U M B E R 1 J. H E A T T R E A T I N G
0.~
0.8
0 . 7
0 . 6 t~_
>- 0 . 5
0 .4
0 . 3
0 .2 0 .2 0 . 3
t J 4
0.4 0.5
ASTM Grain Size
CARBON, Y~ Fig. 5--Multiplying factor for carbon plus grain size (empirical exten- sion of Kramer factors by deRetana and Doane).
1.00 t I I i i I / I
~STM Grain Size ~ I ~ t ~ 1700 F 0.90 ~ ~ 5 c , 0.80
u. 0.70 , .
~- / / / / ~ " 152s F (83o c ~ > ~ . ~ ' 0.60 / / , / I " / i 147,5 F (800 C)" ~ TM
/////," i I--
-~ 0.50
0.40
0.20 0.40 0.60 0.80 1.00
CARBON, %- Fig. 6--Multiplying factors for carbon at each austenitizing condi- tion. Data plotted on background of original Kramer data for medium-carbon steels with grain size variation from 4 to 8 ASTM (Jatczak).
may not be the same when added in conjunct ion with another element as when used alone in the steel.
3) The accuracy of using hardness measurements to determine the 50 pct martensi te position in the hardened pattern of the test piece is open to question. This is com- plicated by the fact that the steel may exhibit either pearlite or bainite as the nonmartensi t ic structure. The use of micros t ructure in place of the hardness test ap- pears to have some merit in determining the 50 pct mar- tensite posit ion.
With these precaut ionary statements, let us proceed to review some o f the alloy effects.
The studies of alloy effects by Kramer , Siegel and Brooks 6 appear to be more adaptable to modificat ion
,,<
5 . 0 -
4.0
OFrom Data of Kramer, Siegel and Brooks
�9 From Data of Crafts and Lamont /
From deRetana and Doane (for Low C Steels
- - - - -From Jatczak (for High C Steels)
.! 3.0 i II D f
J /
r r
/ !
2.0
1.0 ~ ~ #
J
0 1.0 2.0 3.0 MANGANESE, g
Fig. 7--Multiplying factors for manganese.
and compar i son with later studies by others than the alloy studies by Grossmann . In fact Grossmann , in his classic text, 7 states that the data of Kramer are p robab ly more generally useful. As a way of summariz ing the alloy effects, and the influence of ca rbon content on these effects, the following diagrams are presented. In these diagrams, the Kramer multiplying factors for use in medium carbon steels are compared with those o f deRetana and Doane s for low carbon steels and with those of Ja tczak 9 for high carbon steels. (It should be pointed out that Ja tczak used 90 pct martensi te as his hardenabil i ty criterion rather than 50 pct martensi te , for the good reasons which he points out in his paper.) First, the strong effect o f ca rbon on hardenabil i ty is shown in Figs. 5 and 6. The effect o f austenitizing tempera ture is impor tan t in considering steels o f high carbon content . For the sake o f simplicity in further compar isons , the data of Ja tczak for high carbon steels will be limited to those for steels quenched f rom 925 ~ (1700 ~
As shown in Fig. 7, the effect of manganese up to 1 pct is s tronger in low and high carbon steels than in medium ca rbon steels.
The effect o f silicon on hardenabil i ty is much less than that o f manganese and varies widely with ca rbon
content and with other alloys present, as shown in Fig.
J. HEAT TREATING VOLUME 1, NUMBER 1--9
2.8
2.6
2.4
~ 2.2
2.0
~ 1.8
~ 1.6
I
F L
1.4 / /
1.2 / /
1.o ~ / 0
0.2
Jatczak (multi-alloy -high C steels) /
d a t c z a k (single-alloy high C steels)
/ / / / /
/ /
i f ~ / J
f l
Kramer, Siegel apd Brooks
deRetana and Doane {low C steels)
2.0 0.6 1.0 1.4 1.8
SILICON, % Fig. 8--Multiplying factors for effect of silicon on hardenability.
8. Sil icon is relatively ineffect ive in low carbon steels, but quite effective in high carbon steels.
The effect o f nickel on hardenability is also af- fected by carbon content , and, as s h o w n in Fig. 9, is greatest in medium carbon steels. Jatczak 9 reports an in- teraction between manganese and nickel which needs to be taken into account when steels are austenitized at lower temperatures than 925 ~ (1700 ~
In general the effect o f c h r o m i u m on hardenability is greatest in medium carbon steels, as shown in Fig. 10. In low carbon steels, and in carburized steels, the effect is less, but quite marked. Jatczak 9 points out, as did Grossmann, 7 that at lower austenitizing temperatures, chromium is less effective because o f the stability of car- bides.
The work o f Jatczak 9 showed a much greater effect o f m o l y b d e n u m in high carbon steels than in the medium carbon steels studied by Kramer, 6 as shown in Fig. 11. The work o f deRetana and D o a n e s indicates a lesser but still marked effect on hardenabil ity in low carbon steels. Their work also revealed a definite synergistic effect
4.0
3.0 I---
F-- N 2.o
1.0
datczak (high C Steels)"
~Kramer, Siegel ] and Brooks " t ~ @ ~
-T
-deRet (low
/
1.0 2.0 3.0 NICKEL, %
Fig. 9--Multiplying factors for effect of nickel on hardenability.
and Doane steels)
5 . 0
4.0
/ r
/
o Z u
Kramer, Siegel / i w & Brooks ( a v g 4 /
3.0 I j r
I J = / 'll
J a t c z a k / / ~ ' l h i g h C ste
2.0 / . " / I I I I
i / . , I I I I
~i " p" I I I I I I
" �9 i ' d e R e t a n a and Doan ( low C s tee ls )
z.o " I I I I I I 0.2 0.6 1.0 1.4 1.8
CHROMIUM~ %
s)
0
Fig. 10--Multiplying factors for effect of chromium on hardenability. The Jatczak data for carburizing steels are given.
with nickel, if nickel was present in amounts greater
than 0.75 pct. Boron is being used to an increasing extent, at least in
the US, to enhance the hardenabi l i ty of both carbon- manganese steels and of low alloy steels. The work of Lewellyn and Cook , 1~ one curve f rom which is shown in Fig. 12, demonstrates the marked influence of carbon
on the boron hardenabil i ty effect in a plain 0.8 pct Mn steel. The effect is somewhat different in alloyed steels, resulting in a smaller bo ron multiplying factor which reaches 1.0 at the lower ca rbon content associated with eutectoid composi t ion in the alloyed steel. Kapadia , as
1700 F - (925 C)
5.o / I Jatczak �9 . . . . . / deRetana & Doan
(hign ~ s ~ e e l s ) / l ( l o w c s t e e l s 4.0 ; - - / w i t h >.75% Ni)
I
, , , J , / ' ~ ' ~ deRetana & Doane 2.0 " ~ (low C s tee ls
1 .0 ~ ! ! 0 0.25 0.50 0.75 1.00 1.25
MOLYBDENUM, % Fig. l 1--Multiplying factors ~ r molybdenum.
(-D
!
1.50
10--VOLUME 1, NUMBER 1 J. HEAT TREATING
4.0
3.5
3.0
2 . 5 BORON
M U L T I P L Y I N G FACTOR
2.0
1.5
1.0
0.5
| I I ( l I I I I
r3 90% Mar tens i te
o �9 80% Mar tens i te
O 50% Mar tens i te
~XxO
0 I I I I I I I I I
0 0.2 0.4 0.6 0.8 1.0
CARBON, % Fig. 12--Effect o f carbon on bo ron mult iplying factor in 0.8 pct Mn steels (Lewellyn and Cook) .
well as Maitrepierre et al, have summarized the effects of boron in considerable detail in the recent TMS/ A I M E book, 2 and provide some structural explanations
for the influence of ca rbon and alloy content on the
boron hardenabil i ty effect . The interaction of alloys in enhancing and suppress-
ing hardenabil i ty has been shown by several investiga- tors. As cited above, the Mo-Ni interaction in low car- bon steels was observed by deRetana and Doane a and the Ni-Mn interaction in high carbon steels was observed
E 120 I
E 100 /Z.
I - - s " S p ~ ,,, 80
60 " 1 t J . . " -" '
u
40 ,,,6 ~. t ~ Au~tenitizing Temperature j , ~ 50 C (90 F) above Ac 3
CHROMIUM, % Fig. 13- -The effect o f c h r o m i u m and austenit izing condit ion on the ideal critical diameter in s imple c h r o m i u m steels and in Cr-Mo steels (Moser and Legat).
by Ja tczak . 9 In earlier work , Moser and Legat" observed interactions between ch romium and molybdenum, as shown in Fig. 13. Also, Glen ~2 observed an interaction between nickel and manganese in med ium carbon steels with and without bo ron as shown in Fig. 14.
All o f the above discussion o f the hardenabil i ty mul- tiplying factors of var ious elements, and the impor tan t influence of carbon, and alloy env i ronment on the mul- tiplying factors should not be considered confusing, or adding to the "b l ack a r t " o f hardenabi l i ty and heat
t reatment . Rather , this new in fo rmat ion can be used to advantage if one recognizes the necessity of classifying steels and developing alloy effects for each class of steel. For tunate ly , with computers available, this approach is
not burdensome, as will be discussed later.
AUSTENITE CONDITIONING
The ASM book ~ reviews the influence of austenite condit ioning on hardenabi l i ty and t r ans fo rmat ion in steel. Included in the review is in fo rmat ion on melt con- ditioning, t r ans fo rmat ion f rom worked austenite (as in the controlled rolled H S L A steels) and the influence of rapid heating. The high p roduc t ion techniques of f lame hardening and induction heating are being widely used, and the parameters for effective austenitizing in these processes are usually worked out on an individual par t basis. The heat treater is aware in a qualitative way o f the influence of t ime and t empera tu re on austenite homogenei ty and of the effectiveness of self-quenching f rom the unheated por t ion o f the part . The work o f Or l i ch) 3 summar ized in Fig. 15, demonst ra tes the im- por tance of heating rate and holding t ime on austenite
condit ioning in a 1 pet C-1.5 pet Cr steel. Specimens were p rogram heated at varying rates then immediate ly quenched to provide the data in the left por t ion of the figure. Another series of specimens was heated at 130 ~ to several different isothermal tempera tures
and held for various times to provide the data in the right por t ion of the d iagram. The schematic curves A and C in Fig. 16 display the extremes in shape of the hardness-depth curve which might be encountered in in- duct ion hardening to a given depth. Curve B is more likely the desired pat tern , in which heating rate, holding t ime and hardenabil i ty of the steel are properly con-
trolled to give substantial surface hardening and a grad- ual transit ion to the unhardened core.
TAILORING THE STEEL TO THE APPLICATION AND HEAT TREATMENT
The C H A T system developed by Breen and coworkers
at Internat ional Harves ter represents an advanced use
J. H E A T T R E A T I N G V O L U M E 1, N U M B E R 1--11
W
l.IJ Z
1_9 Z
2.0
1.5
1.0
0.5
IDEAL DIAMETER, mm
25 50 75 100 125 150 175
_._.
i / & " ~ ~
i I l f / , , , ' / , / i ..~ i I
i 1 ~ . . " ~'. J ~..--. 4 - ' - - - I / 1 6 ' , / / I i / - ' I ~ "7 2.o~' I l//fAY , �9 / ~ No Additions l lllY / _1 I I ,, 1 .0~ N i
+ o - - - - - - / ; ' p / ; T S T r ------ Add!,,O~ O " .o~,~ . ,
/ 1 / ~ , ~ " �9 1 . 0 1 ~ , Ni �9 i
/ / / , F ~ / ' - I 2.0% INi I
I 2 3 4 5 6 7
IDEAL DIAMETER, in,
Fig. 14--Relat ionship between ideal d/am and manganese content with various nickel con- tents (Glen).
1300
1200
i i00
& r ~
i000 o.-
t.iJ
t,l
~- 900
800
/ / / / / / / / / ~ A c L ~///~ "////////~
- - I ~ ."Homogeneous Austenite"
"Inhomogeneous " ~ Austenite" \
HEATING RATE, C/sec 2400 300 30 3 0.22 0.05
?'///r~Austenite + Liquid~,//'/'~j Ac c
: ' t ! l If ,' J ~ i Nk''HOm~ Austenit~"
I ~ 1, I I - I \ I I i
"Inhomogeneous ~ r ~ l I - I Austenite' I \ I ~ J i i I " a " I . . . . I , _
"%Ill I I I I I - I I I---' ~lFerrite + " ' I - . ~ I
~Ib_ I ~ A u s t e n i t e + ~ -- i -
~ J - - I I - I I gerrite + Carbicle i ~]
I I I I I I I -
i i0 102 103 104 CONTINUOUS
Austenite + tiquid~/
Austenite + Carbide
ACle
Ferrite + Austenite +
iAClb ~ Carbide
Ac2 Ferlrite + Carbide I I 70O
0.i 0.01 0.I 1 i0 I0 ~ ISOTHERMAL
TIME, see
N
2300
2200
2100
2000
1900
1800
1700 w
1600
- 1500
- 1400
1300
10 3
Fig. 15--The effect o f rapid heating (left) and varying isothermal holding (right) on the achievement o f homogeneous austenite in a 1.0 pct C-1.5 pct Cr steel (Orlich).
12 - -VOLUME 1, NU M BER 1 J, HEAT TREATING
~E] A
Heated Depth
Surface ~,. ~-
DEPTH BELOW SURFACE Fig. 16--Range o f gradient hardnesses produced by induction harden- ing.
Part Cross Section Jominy Hardenability
~ k T A ~ _ 7 ~ 7 ~ a r t Thermal History)
I mended~7 er'a't u r e ) Depth from Surface Jominy Distance
" \ - ~ m I
-- ~-! I \ ~ = I , I.
\ . . ~ - - - - 4 - - u " \ w , / \ ~ ._ | I /
\ \ ~ _ _ _ / C r o s s Section Jec(c>
Jomi ny Distance Fig. 17 - -De te rmin ing Jominy equivalent condi t ion (Jec).
50
o t~" -I-
. 4 0
L~J Z C~ n~
3O - r
20
DISTANCE FROM QUENCHED END, mm
5 !
\=
\!
I0 15 20 25
I i I
\ I !
\I
4 8 12 16
DISTANCE FROM QUENCHED END, sixteenths in.
Fig. 18--Variations in core hardness resulting from variations in quenching conditions and steel hardenability.
of hardenabi l i ty in format ion in which "app l i ca t ion ta i lor ing" (AT) is combined with " c o m p u t e r harmoniz- ing" (CH) to provide the designer and manufac tu re r with least-cost steels or a choice of a l ternate s tandard steels which can meet the proper ty requirements in a given part . The system is discussed in some detail in both Refs. 1 and 2. This paper will indicate briefly how hardenabil i ty requirements for a given par t are estab- lished in the A T por t ion of the system. Rather than use the established correlat ions between end-quench bar positions and diameter of quenched bar using s tandard- ized austenitizing and quenching condit ions (see Figs. 1
and 2), the " J o m i n y equivalent cond i t ion" (Jec) is determined by experimental means but under produc- tion condit ions. Figure 17 graphical ly illustrates the
process. The Jec is determined experimental ly by compar ing
hardness values, obta ined on the cross section of a part or parts subjected to the product ion heat and quench cy- cle, with hardness values obtained on Jominy bars of the exact same steel. Four critical items mus t be consid- ered when determining the experimental curves:
1) The Jominy bar should be given the same thermal cycle as the par t because hardenabil i ty can be influenced by thermal history.
2) For carburizing applications, the par t or parts should be copper plated if processed with a product ion load, to retain the same carbon as the end-quench bar. Also, if extremely long heat cycles are employed, the depth of flat g round on the end-quenched bar may have to be deeper than usual to assure that there is no effect
o f decarbur izat ion. 3) Enough parts should be examined to evaluate the
effects o f load size, posit ion in the load, and changes in quench severity of the coolant with usage.
4) The steel used for Jec evaluation must have an end-quench curve which drops steeply through the criti- cal cooling rate range so that hardness changes give a
sensitive indication of cooling rate.
The range of cooling rates to be encountered in pro- duction is impor tan t . This var ia t ion coupled with the var ia t ion in hardenabi l i ty of the steel being used ac-
counts for the range in hardnesses experienced. Figure 18 demonst ra tes these principles graphically. In this sit- uation, since this is a carburizing steel, a core hardness var ia t ion of 29 to 48.5 H R C would be expected if the Jec varied as indicated. I f this was not acceptable, either the quench consistency would have to be improved or the steel H-band would have to be restricted to a nar- rower width. A lower carbon steel with higher harden- ability could be used but at a cost penalty.
The system becomes somewhat more complicated
J. H E A T TREATING VOLUME 1, NUMBER 1--13
c~
-r"
--'r- I---
F -
Sl
I..--
._.1
__J _..I
Gradient / Allowable Contact Stress
(Pitch Line)= StrenQthi~HardnesShLne)
Gradient = Strength
DEPTH BELOW SURFACE
Fig. 19- -St ress v s s t rength re la t ionship (carbu- rized gear roo t and pitch line).
when establishing the hardenability requirements for
carburized parts. Here the base hardenability and hard~ enability throughout all portions of the case must be considered so that the strength gradient can be matched to the stress gradient. The diagram in Fig. 19 shows schematically the stress and strength gradients at the pitch line of a gear and at the root of the gear tooth. With patience the necessary base and case hardenability requirements can be established. Details are given in Refs. l and 2.
PREDICTING HARDENABILITY
The foregoing discussion of methods and data avail- able to correlate hardenability test results with actual quenched parts, of the wealth of data on hardenability
M,-~rtensite 8
60 [ ~ -
. . . ~ . , . o ~ - L / " s "~'- .9" ~ 507~ Martensite
= 3 0 - - ~ ' ~ T - ~ ~ ' " - - - - ~ - - - - -
20
10 0.10 0.20 0.30 0.40 0.50 0.60 0.70
CARBON, %
Fig. 2 0 - - H a r d n e s s o f mar tens i te p roduc t s as a func t ion o f carbon conten t (Hodge and Orehoski ) .
effect of carbon and various alloys, of the importance of austenite conditioning, and of the methods available to define hardenability requirements, leads to a review of the methods to predict hardenability from composi- tion. As part of the Symposium on Hardenability Con- cepts with Applications to Steel 2 this author reviewed several hardenability predictors available in the open lit- erature as a prelude to detailed presentations and discus- sions of computerized methods of predicting harden- ability. As part of that review, three methods were used to predict D~ values and hardenability curves of nominal compositions of several steels for which hardenability "bands" have been established by the American Iron and Steel Institute (AISI) and the Society of Automotive Engineers (SAE). The relationships established by Hodge and Orehoski 14 for 50 and 99.9 pct martensite at various carbon contents, Fig. 20, and those established by Grossmann 7 for D r and distance from the quenched end of the hardenability bar, Fig. 21, were used with modifications of AISI published tables relating D 1 to hardness at various positions on the end-quench bar.
Examples of the comparison of predicted/91 values and predicted hardenability curves with actual harden- ability bands are given in Figs. 22 to 24, which are taken from the Appendix of this author's TMS/AIME paper} Note here that O~/I represents calculation by the method developed by the Climax Molybdenum Company, /9~ 2 represents calculations using the US Steel slide rule which is one of several using Grossmann's hardenability multiplying factors, and D~ 3 represents conversion of hardenability curve data calculated using the following equation developed by Just: 15
1 4 - - V O L U M E 1, N U M B E R 1 J. H E A T T R E A T I N G
DISTANCE FROM QUENCHED END, mm
I0 20 30 40 50
7 ~ 1 7 5
i~4 [ i!i!~iiiiin m ~ l O 0
Fl-tC/rtC2
2 4 6 8 lO 12 14 16 18 20 22 24 26 28 30 32 DISTANCE FROM QUENCHED END, sixteenths of an inch
Fig. 21--Relationship between end-quenched distance and hardenability (D/).
HRCat J4-40 = 88~,/C- - 0-0135EV~ + 19Cr +
6.3Ni + 16Mn + 35Mo + 5St - 0-82KAsTM --
2 0 " ~ + 2 .11E - 2 HRC
where: HRC = hardness in Rockwell C
E = Jominy distance in sixteenths o f an inch K = A S T M grain size numbers
The two values for D~ represent calculations of D~ from the lower and upper bands of the A I S I / S A E hardenabil- ity band. Hardness data points designated (El) were ob- tained with the Climax method, points designated (&)
Fig. 22--Comparison of calculated hardenability with AISI/SAE H- band for 4820 H.
HARDENABILITY BAND Ni
' L /3< I /3 oI 0.20 0.60 0.23 3.50
4820 H
~[ M~ ~---G'C; i I .2o / / .30
0 0.25 7 DIAMETERS OF ROUNDS WlIR SAME AS QUENCHED HARDNESC LOCATION IN ROUND ~UENCH
3 , 8 SURFACE MiLD I . I 2.0 2.9 3.R 4.B 5.B 6.7 31~ RADIUS FROM CENTER WATER 0.7 1.2 1.6 2.0 2.~ 2.8 3,2 3.6 3.g CENTER QUENCH
1 I l I I I l I I - 0 .8 1.8 2 .5 3.0 3.~ 3 .8 0.5 1.0 1.6 2.0 0.2 0 .6 1.0 1.4
e'-, I
6C ;
. J :
U i m SC !
,';, -~ �9
J ac
o
~ z o
SURFACE MILD 2.~ 2.8 3,2 3.6 ~.0 3/4 RADIUS FROM CENTER OIL 1 .7 -2 .0 " 2.~ 2.8 3.1 CENTER IOUENCH
>__
: ,._.~ i : _ _
,
2 4 6" 8 IO 12 14 16 18 2 0 2 2 2 4 2 6 2 8 D I S T A N C E F R O M Q U E N C H E D E N D - - S I X T E E N T H S OF AN INCH
30 32
D cl= 3.2 I
DI2= 2.8
DI3= 3.2
=2.7 min
=4.0 max
J. HEAT TREATING VOLUME 1, NUMBER 1--15
Fig. 2 3 - - C o m p a r i s o n o f calculated hardenabil i ty with A I S I / S A E H- band for 1330 H.
H A R D E N A B I L I T Y B A N D 1 3 3 0 H Mn i S, [
t I �9 I / . 3 o 1 O. 30 1 . 7 5 O. 23 0 0 0 7
DIAMETERS OF ROUNDS WITH SAME AS QUENCHED HARDNES ~ LOCATION IN ROUND ~UENCH 3.8 SURFACE MILD
I. I 2.0 2.9 3.8 ~.8 5.8 6.7 3/)4 RADIUS FROM CENTER WATER
0.7 1,2 1.6 2.0 2.~ 2.8 3.2 3.6 3.9 CENTER QUENCH 1 1 I I I I I 1 I
0.8 I .8 2.5 3.0 3.t l 3.8 SUR F AO, E MILD 0.5 I.O 1.6 2 . 0 2.LI 2;8 3.2 3.8 14.0 ~/U, RADIUS FROM CENTER OIL 0.2 0.6 1.0 I . t l 1.7 2.0 2.1,1 2.8 3. I CENTER QUENCH
DIS'TANCE FROM QUENCHED END - S I X T E E N T H S OF AN INCH
cl 2 5 D I = .
D =1.7
D; 3 2 3
= 2 . 0 rain
- - 3 . 8 m a x
H A R D E N A B I L I T Y
.2 .3o / . , 5 / / . . 3o I 7 / / - 3 3 ~ .70
B A N D 4 1 3 0 H
7 / / I , 20 25
65 --~
*a 80 i
, ~ 5 5 --'~-
o
" \ Z 4 ( _~_ a \ n, :, < 3: \ T : \
" J 3 0 " .J ',
~ 2 5 ;
0 2 0 I
0.30 0.50 0.23 0 0.98 0.20 7
DIAMETERS OF ROUNDS WiTH SAME AS QUENCHED HARDNES': LOCATION IN ROUND QUENCHJ 3.8 SURFACE MILD I , I 2 .0 2 .9 3 .8 ~.8 5 .8 6.7 3/1,1 RADIUS FROM CENTER WATER 0 .7 1.2 I .G 2.0 2 .4 2.8 3,2 3.6 3 .9 CENTER QUENCH
I I I [ [ I I 1 [ 0.8 1.8 2 .5 3.0 3.1.1 3.8 SURFACE MILD 0 .5 1.0 1.6 2.0 2.~ 2.8 3.2 3.6 11.0 3/15 RAOIUS FROM CE'NTER OIL 0 .2 0 .6 1.0 I .11 I .7 2.0 2.1.1 2.8 3. I CENTER QUENCH
\
iN
2 4 6 8-- I 0 12 14 16 18 2 0 2 2 7'4 7'6 7'8 3 0 37' D ISTANCE F R O M Q U E N C H E D END - S I X T E E N T H S OF AN INCH
DII= 2.:.3
c2 2 8 D I = .
:3= 2.7
= 1 . 9 rain
=3.3 max
Fig. 2 4 - - C o m p a r i s o n o f calculated hardenabil i ty with A I S I / S A E H- band for 4130 H.
16- -VOLUME 1, N U M B E R 1 J. H E A T TR EA TI N G
5.0 E
>_- F- -
. . J
.~4.o
2 EXA~IPLES
Climax 2
5 EXAIIPLES
Climax 2
US Steel 4
7 EXAMPLES
Climax 6
9 EXAMPLES
Climax 9
5 EXAMPLES
US Steel 3
8 EXAt'IPLES
Climax 6
Just 4
US Steel 3
7 EXAMPLES
--175
C1 imax 3
Just 4 Just 3
US Steel 1 US Steel 1
I I I I 0.2 0.3 0.4 0.5 0.6
Low Medium High
CARBON, PERCENT
5 EXANPLES
US Steel 5
2 EXAMPLES
US Steel 2
- - t 5 0
-125 "~
. . . /
100 ~--
75
50
25
o I i
(. ] .
Fig. 25- -Evalua t ion o f three hard- enability predict ion systems.
Z
I--"
7.0
6.0
High
qedi um
3.0
2.0
Low
1.0 0.1
Fig. 26- -Cons t ruc t ion for locating the inflection point o f a Jominy curve (Feldman).
VELOCITY {LINEAR)
J. HEAT TREATING
TIME ( L O G A R I T H M I C )
V O L U M E 1, NUMBER 1--17
A P P R O X I M A T E C O O L I N G R A T E . "F P E R S E C O N D A T 1 3 O O F
=i M d TO
e 8 ~
,,-~4=_-..4~.-.~.1 _ _ I ; . ~ _ _3~ , . , . . . . I
z -
Ir 4 0 < ~ - - - Z 36 ~ * - - " - - -
I ; 4 " - " ~ ' ~ 6 8 t O 12 1,4 t e 111 2 0 2 2 2 4 2 6 211 3 0 3 2 3 4 3 6 3 8 4 0
D I S T A N C E F R O M Q U E N C H E D E N D O F S P E C I M E N I N S I X T E T P ' T H S O F I N C H Fig. 27--Empirical family of Jominy curves for 0.43 carbon H-steels (Feldman).
were obtained using the U.S. Steel calculator, and points designated (e) were calculated using the Just equation given above.
The examples given above were part of a larger exer- cise in which actual hardenability was compared with
r 4(2 4330 h- I
3C ~ . . . ~ . ~ ~ ~ . ~ -4130
l
0 ' 8 ' 1'6 ' ~;4 32
DEPTH ( SIXTEENTHS ) Fig. 28--Comparisons of observed and predicted Jominy curves for 4427, 5130, 4130, and 4330 steels (Feldman).
hardenability calculated from actual composition and grain size for 50 standard and nonstandard steels vary- ing in carbon content and hardenability. A summary of the work is given in Fig. 25, in which regions of low, medium and high hardenability (expressed in terms of D1), and regions of low, medium and high carbon con- tent are delineated. In each region, the systems that were capable of predicting hardenability are indicated in terms of the number of examples in which this author considered the prediction of both Dt and hardness in the 50 to 90 pct martensite range to be adequate�9 These pre- dictors are underlined. While the results certainly pro- vide evidence that no one predictor can be used with all steels, the comparison does provide encouragement that there are useful methods of predicting hardenability-- methods which can be used selectively to define the probable hardenability of a given composition.
The newer computerized methods take advantage of selective application of available data. The Minitech system described in Feldman's paper in the TMS/AIME book z was developed originally by Kirkaldy to use ther- modynamic principles in establishing critical cooling rates for formation of ferrite and pearlite, and, from
18--VOLUME 1, NUMBER 1 J. HEAT TREATING
700 , , , , , / m 500~
,oo
2 0 0 A ~ = I i 0'1 012 i 0=3 0 4 05
DISTANCE FROM QUENCHED END (,nches)
Fig. 29--The effect on the precisely determined Jominy curve of successive additions of trace elements (Brown and James).
these, to es tabl ish in f l ec t ion poin ts in the ha rdenab i l i ty
curves. The m e t h o d is i l lus t ra ted graphical ly in Fig. 26.
Note that the p o i n t o f m a x i m u m t r a n s f o r m a t i o n veloc-
ity establishes the coo l ing rate co r respond ing to the in-
f lection po in t on the ha rdenab i l i t y curve. This me thod
of predic t ing in f l ec t ion po in t s was tested agains t a large
b a n k of ava i lab le h a r d e n a b i l i t y da ta and modi f i ed era-
pirically to increase the accuracy o f predic t ion . Subse-
quent ly , ent i re J o m i n y ha r de na b i l i t y curves were pre-
dicted us ing the fact tha t for a given c a r b o n c o n t e n t the
m a j o r i t y of J o m i n y curves genera te a ra t iona l , n o n i n t e r -
secting set as i l lus t ra ted in Fig. 27. O f course, such a set is no t un iversa l ly app l icab le ; a l loying in f luences the
shape of the curve. Here again , empir ica l da ta are used
0 6
0 5
0 4
~ 0 3 Fig. 30--Calculation of Jominy ..d distance to 450 VPN using Gross- mann factors. High purity steels ~- (Brown and James). < 0 2
OI
O0
!
o
Oo o / o
~ o
o/
J !
O l I I I 1
03 05 P R E D I C T E D ( i n c h e s )
! !
o o
I !
0 7 0 9
J. HEAT TREATING VOLUME 1, NUMBER 1--19
Table II. Alternates for AISI /SAE 8620H
8620H EX 10 EX 15 EX24
Midrange Compos i t ion
Pct C 0.20 0.22 0.20 0.20 Pct Mn 0.78 1.10 1.05 0.88 Pct Si 0.23 0.28 0.27 0.28 Pct Ni 0.55 0.30 0.03 - - Pct Cr 0.50 0.33 0.50 0.55 Pct Mo 0.20 0.08 0.16 0.25
Midrange Hardenabi l i ty
DtB 1.7 1.6 2.1 1.8 D1c 5.4 3.8 4.7 5.5
in the Minitech propr ie tary system to classify steels by their characteristic Jominy curve shape and predictions are accurately made within that classification. An exam- ple of the success of the method is given in Fig. 28.
Brown and James ~6 at GKN developed a systematic prediction system, first using a series of high purity steels, the composit ions and hardenabili ty curves of which are given in Fig. 29. Brown and James used a hardness level of 450 HV as the hardenabili ty criterion and at tempted to calculate Jominy distances to that hardness using Grossmann techniques, then with a " s imple" computer regression analysis developed on the basis of a series of 24 labora tory steels. The results of comparisons with actual hardenabili ty data are given
in Figs. 30 and 31. Encouraged by the good correlation obtained in Fig. 31, Brown and James applied their equations to composit ions of a random selection of commercial steels, but met with much less success. Ad- ditional work led them to develop a system of empirical equations, one for each of the several standard British steels. This approach worked quite well, and GKN has adopted it for their propr ie tary prediction system.
The "compute r h a r m o n i z e d " (CH) port ion of the International Harvester C H A T system can be used to define a least-cost steel to meet established hardenability requirements, or to define a replacement steel which will duplicate the carbon content , hardenabili ty characteris- tics and t ransformation characteristics very closely. Computer methods make it possible to take into account several objectives simultaneously, such as equivalent base hardenability (Dis) and equivalent case hardenabil- ity (D1c) at least cost. To do this by trial and error would require considerably more effor t . The C H A T system
uses the base hardenabili ty factors of deRetana and Doane, 8 case hardenabili ty factors of Jatczak, 9 and Ms temperature equations developed by Andrews ~7 for the base composition and by Payson and Savage ~8 for the case composition. Examples of the use of the C H A T system are given in Refs. 1 and 2. In one example the following requirements and restrictions were placed on the steel to be used for a heavy rear axle hypoid pinion as a result of AT considerations:
Carbon content 0.26 pct min. (0.31 pct max.)
06
04
(.l t-
_I
F-
~02
OC
I I I l I
I 1
o - o /
o,o " 0
0 2 I I I
0 4 0.6 PREDICTED (inches)
Fig. 31- -Cor re la t ion o f Jominy distance to 450 VPN us- ing " s i m p l e " compute r regression equat ion (Brown and James).
2 0 - - V O L U M E 1, N U M B E R 1 J. H E A T T R E A T I N G
Dia = 3.8 in. (97 mm) Dic = 5.0 in. (127 m m ) Chromium content 0.90 pct max. Manganese content 1.00 pct max.
With this informat ion , the compu te r selected the follow- ing least cost analysis: 0.26 pct C, 1.0 pct Mn, 0.15 pct
Si, 0.90 pct Cr, 0.36 pct Mo and no nickel. Other examples of " r ep l acemen t steels" are the SAE
" E X " grades developed in recent years using any of several empirical methods. Four a l ternate steels have been proposed for the popu la r Ni -Cr -Mo carburizing steel A I S I / S A E 8620H. The al ternates are compared in
midrange composi t ion and midrange base and case hardenabi l i ty in Table II. It is evident that the proposed steels are nearly equivalent in DtB but differ considerably
in D1c. To check the adequacy o f these steels to meet case hardenabi l i ty requirements , Hal lock ~9 prepared exper imental heats o f " l o w s ide" and "h igh s ide" com-
70
65
60 LAJ
55 r
to 50
ua 45 Z
"~ 40
35
~ 3o
~ 25
~ 20 15
10
DISTANCE FROM QUENCHED END OF SPECIMEN IN MILLIMETERS
DISTANCE FROM QUENCHED END OF SPECIMEN IN SIXTEENTHS OF INCH
J. HEAT TREATING VOLUME 1, NUMBER 1--21
.8 ~ C u ' ' ; ' '
Z ~
I
0 5
IZ~)O
,•1200
~- 1 0 0 0
/ l I I I I
J f
I f
; I
ALLOYING
_--- ~ , - - - . - _ ~ N i M n
.. " J " : - ' - - . , " - - ,"7" 5
ELEMENT WT-%
- - . . . . .
Fig. 3 4 - - T h e influence of the alloying elements on the eutectoid temperature and carbon con- tent. Solid lines are representing the equilib- r ium state according to the reviewed calcula- t ions whereas the dashed lines are taken f rom Bain (Uhrenius) .
positions o f the four steels,* and determined harden-
* " L o w s ide" compos i t ion is one-quar ter o f the no rma l range o f each element below the midrange composi t ion and " h i g h s ide" com- posit ion is one-quar te r o f the normal range o f each element above the midrange compos i t ion .
ability o f carburized end-quench bars. The results, when the bars were ground to depths corresponding to 0.9 pct C in the case, are given in Figs. 32 and 33. The results conf i rmed the calculations that the EX10 is not an ade- quate " r e p l a c e m e n t " for 8620, on the basis o f case hardenabili ty.
PREDICTING CRIT ICAL T E M P E R A T U R E S AND C A R B O N G R A D I E N T S
Quite useful guidelines for heat t reatment can be found in the studies of phase equilibria in iron-base al- loys by Hillert, by Uhrenius, and by Kirkaldy et al in the T M S / A I M E book. 2 The thermodynamic studies of
Uhrenius resulted in new data on the influence of the alloying elements on the eutectoid temperature and car- bon content. A comparison of the Uhrenius results with those published by Bain and Paxton 2~ are given in Fig. 34.
Among the several useful predictions available in the Minitech system, Feldman 2 described the capability of using diffusion data to predict carbon distributions resulting from carburizing steels of various alloy con- tents. Two examples are given here. In the first example, Fig. 35, a series o f carbon gradients were predicted for a Cr-Ni steel, and then checked against observed data resulting from a standard carburizing operat ion. It is possible to predict carbon gradients for a two-step car- burization process as well. The results of predictions of both carbon distribution and hardness resulting f rom a two-step carburizing process are shown in Fig. 36. The predicted carbon gradient curves follow the observed data quite well through the first port ion of the case, and fall only slightly below the actual data at intermediate carbon levels.
2 2 - - V O L U M E 1, N U M B E R 1 J. H E A T T R E A T I N G
Z 0 r ~r
u 0 4
I -
, . - , 02
�9 , , , - 1 7 0
o~ ~ _ ~ ~~4,o J tlJ
3O �9 ~J
2o ~ CHROMIUM -NICKEL STEEL o e % CARBON PGTENTIAL IN GAS
0 8 ~ PREDICTED I0 �9 o OBSERVED
O
0 6 o
o 4 IO MRS
CASE D E P T H BY H A R R I S F O R M U L &
I IB , ~ 5 . b O A N D 7 9 T H O U , R E S P E C I I V E L Y
0 [ l 1 1 I 0 20 4 0 6 0 BO IO0
Pf NET RAT ION ( T housondlh$ ) Fig. 35--Comparison of observations and predictions for the surface carbon dependence of diffusion penetration (Feldman).
13)
(1) CL E O F-
30 NC 11 AfterlRSID
I CO, o ~Mo%i s,ojo i s% l po;o i N,o,o i c,o,o ,Moo,,;ooo, o~ As~ I 2.95 [ 0.69 ?,056 I !o3o o ,o To 7 i 0,3, O32
Seconds Time lmn 2mn 15ran lh h 4hBh 2 h Fig. 37--I.R.S.I.D. CCT diagram of 30 NC 11 steel.
12
- i t !
CHROMIUM-NICKEL STEEL
PREDICTED
B• �9 A o OBSERVED
~ o a 2 HRS AT 1 3 % C
J I H R AT 0.7 %C
2 H . S . T , 3 . C
~ 0 %C
HRS. AT I3 ~C ~ ~ ' ~ - . . _ ~
CASE DEPTHS BY HARRIS FORMULA 3 5 , 4 3 AND 50 THOU, RESPECTIVELY
I I I I 2 0 4 0 6 0 80
PENETRATION (Thousondth$) 0
, 70
60
50
40
30
20
I0
0
IO0
u~
-= ( J
_J
( J o
Fig. 36--Comparison of observations and predictions for a two-step carburization process (Feldman).
PREDICTING AND USING CONTINUOUS COOLING TRANSFORMATION DATA
The study of transformation kinetics under condi- tions of continuous cooling has rewarded the investiga- tors and users with data which can be applied to real heat treatment conditions. The ASM book ~ explains how continuous cooling transformation (CCT) dia- grams are constructed and how they may be used. The paper by Eldis in the TMS/AIME book 2 summarizes the sources of CCT diagrams and discusses the application of CCT diagrams to commercial practice. Throughout the world there are now a very large number of CCT
diagrams available. Many of these were generated in response to a need for transformation data on a specific composition or commercial grade of steel, but many diagrams were generated to study the effect of varying alloying elements. In several publications, the latest be- ing in the TMS/AIME b o o k , 2 Maynier and coworkers have developed a systematic analysis of CCT diagrams and have generated regression equations from European diagrams which make it possible to predict transforma- tion behavior, for a given composition, grain size and austenitizing condition. A typical diagram, and charac- teristic critical cooling programs, from the work of Maynier et a! are shown in Fig. 37. The critical cooling programs, designated by V, the rate of cooling at 700 ~ are:
J. HEAT TREATING VOLUME 1, NUMBER 1--23
VI, the martensitic critical cooling velocity; the mini- mum quench rate to give an entirely martensitic struc- ture (or a mixture of martensite and retained austenite),
I/2, the bainitic critical cooling velocity; the minimum quench rate to give a structure entirely free f rom ferrite or pearlite (a structure of bainite with or without mar- tensite and retained austenite),
V3, the critical cooling velocity to attain a fully an- nealed structure free f rom bainite (a structure compris- ing entirely ferrite and pearlite)
Other intermediate critical velocities can be useful. For example V~ (50) is the critical cooling velocity which results in 50 pct martensite and 50 pct bainite (or other higher tempera ture t ransformat ion products). Maynier and coworkers have developed regression equations which can be used to predict these critical cooling veloci- ties f rom composi t ion. The form of the equat ions can best be shown by example. The martensitic critical cool- ing velocity in ~ with composit ions in wt pct, and P, (the austenitizing parameter , a separate equat ion in- volving tempera ture and time) is given by the equation:
log V l = 9.81 - (4.62C pct + 1.05Mn pct + 0.54Ni pct +
0.50Cr pct + 0.66Mo pct + 0.00183 P, )
Coefficients for several such equations are summarized in Table Il l . The influence of elements on various trans- format ion reactions is indicated by the magni tude of the coefficients. These are revealing. Carbon, of course, is most effective in inhibiting t ransformat ion and least in-
fluential in controll ing complet ion of the bainitic reac- tion to permit fully annealed structures of ferrite and pearlite (I/3). Molybdenum is shown to be more effec- tive in suppressing ferrite or pearlite (V 2) than in sup- pressing bainite (V 0.
A somewhat similar analysis of the US data has been undertaken by Eldis 2 but is at present in the preliminary stages. Nevertheless, these preliminary equations, shown in Table IV, provide additional and comparat ive information regarding the influence of alloys on inhibit- ing nonmartensit ic reactions. Shown in the table are equations for Ac,, Ac3, Ms, as well as expressions simi- lar to those designated V~ and I"2 by Maynier.
It is, of course, impossible to provide in this paper the detail required to use the results of the extensive work that has been done in the last several years in defining CCT characteristics of steels. This author can only urge review of the diagrams and the papers describing their
use.
PREDICTING MECHANICAL PROPERTIES
In the T M S / A I M E book, 2 papers by Pickering and by Krauss describe the relationship between microstruc- ture and mechanical properties. Predicting mechanical properties of steels as a function of their chemical com- position and heat t reatment is seen by several workers as the next step in utilizing the fund of knowledge that has been developed in metallurgy over the years. The results of work by Maynier and coworkers in France, Kirkaldy
Table III. Constants and Coefficients in Critical Velocity Equations (Maynier et al)
Pa No. Cons t an t C M n Ni Cr Mo ~ h Equat ions 2a
Log 1/700 or = (Kp.P pct) with P = C, Mn, Ni, Cr, Mo, V, Pa . Valid for all low alloyed steels (except XN, the nickel steels)
2 4 - - V O L U M E 1, N U M B E R 1 J. H E A T T R E A T I N G
Table IV. Results of Multiple Linear Regression of Data for Microalloy Free and Boron Free Steels (Eldis)
C = 0 . 0 9 / 0 . 8 1
D e p e n d e n t V a r i a b l e
R e g r e s s i o n Mode l : Y = 1 . C + rn.Si + n . M n + p . M o + q . C r + r . N i + c o n s t a n t
C o m p o s i t i o n R a n g e o f O b s e r v a t i o n s (wt pc t ) :
Si = 0 . 0 2 / 1 . 4 9 M n = 0 . 3 5 / 1 . 7 9 M o = 0 / 0 . 9 2 C r = 0 / 1 . 5 5 N i = 0 / 4 . 5 6
E q u a t i o n a n d S ta t i s t i c s ( A l l o y C o n t e n t s in wt pc t )
ACl, ~
Ac3, ~
Ms~ ~
log Ba in i t e T i m e b in s
log P e a r l i t e T i m e c in s
A c I = 20.1 Si - 17.8 M n - 9 .8 M o + 11.9 C r - 19.1 Ni + 712
No. o f O b s e r v a t i o n s = 174
M i n i m u m S i g n i f i c a n c e Leve l a = 0.01
Mul t i p l e C o r r e l a t i o n C o e f f i c i e n t = 0 . 9 2 5
S t a n d a r d E r r o r o f E s t i m a t e = 11.8 ~
A c 3 = - 2 5 4 . 4 C + 51 .7 Si - 14.2 Ni + 871
No . o f O b s e r v a t i o n s = 174
M i n i m u m S i g n i f i c a n c e Leve l = 0 .001
Mul t i p l e C o r r e l a t i o n C o e f f i c i e n t = 0 . 8 3 9
S t a n d a r d E r r o r o f E s t i m a t e = 23 .6 ~
Ms = - 3 9 1 . 2 C - 43 .3 M n - 16.2 C r - 21 .8 Ni + 531 N o . o f O b s e r v a t i o n s = 142
M i n i m u m S i g n i f i c a n c e Leve l = 0 .001
Mul t i p l e C o r r e l a t i o n C o e f f i c i e n t = 0 . 9 4 8
S t a n d a r d E r r o r o f E s t i m a t e = 15.6 ~
log Ba in i t e T ime = 1 .094 C + 0 .321 Si + 1 .407 M n + 1 .772 M o + 1 .050 C r + 0 . 6 3 2 Ni - 1 .849
No . o f O b s e r v a t i o n s = 126
M i n i m u m S i g n i f i c a n c e Leve l = 0 .025
Mul t ip l e C o r r e l a t i o n C o e f f i c i e n t = 0 . 8 9 9
S t a n d a r d E r r o r o f E s t i m a t e = 0 . 4 6 8
log P e a r l i t e T ime = 1 .259 C + 1.231 M n + 2 . 3 3 9 M o + 0 . 4 4 5 C r + 0 . 4 8 4 Ni - 0 .711
No . o f O b s e r v a t i o n s = 150
M i n i m u m S i g n i f i c a n c e Level = 0 .001
Mul t i p l e C o r r e l a t i o n C o e f f i c i e n t = 0 . 8 5 4
S t a n d a r d E r r o r o f E s t i m a t e = 0 . 5 0 6
a M i n i m u m s i g n i f i c a n c e level = s i g n i f i c a n c e level o f leas t s i g n i f i c a n t c o e f f i c i e n t .
bBa in i t e t ime = p o s i t i o n o f b a i n i t e n o s e o n t ime axis . cpea r l i t e t ime = p o s i t i o n o f t he n o s e o f h igh t e m p e r a t u r e t r a n s f o r m a t i o n ( f e r r i t e / p e a r l i t e ) o n t i m e ax i s .
and Feldman in Canada , Hernacki and Ward as well as Breen and coworkers in the US have also been included
I Thickness - Diameter 1 Quenching Severity
Chemical Composition
Cooling Rate [Critical Quench Velocities I
I I
Quenched Structure
~ [ Addition Rule
Product Characteristics HV-H RC-H B-YS-YS.2%-UTS
E%, Red. Area %
Hardness Martensite Bainite Ferrite Pearlite
Hardness Martensite Bainite Ferrire Pearlite
Before Temper
After Tem per
Fig . 3 8 - - F l o w d i a g r a m o f t h e p r e d i c t i o n sys t em ( M a y n i e r et al).
in the T M S / A I M E book . 2 The Creusot-Loire system
described by Maynier et al can be used to define p roduc t characteristics, as indicated by the flow diagram shown in Fig. 38. The cooling rates for various quenched sec- tions have been reasonably established, as shown in Fig. 39 for the center o f quenched rounds and in Fig. 40 for the midthickness of plates.
As a first step in predict ing mechanical proper t ies Maynier has developed equat ions for predicting hard- ness before and af ter tempering, with the knowledge o f cooling rates and composi t ion . The hardness of as- quenched martensi te is expressed by the equation:
HVmartensi te = 127 + 949C + 27Si + 11Mn +
8Ni + 16Cr + 21 log V r
where V, is the cooling rate in ~
(2a is 26 H V for this equation)
J . H E A T T R E A T I N G V O L U M E 1, N U M B E R 1 - - 2 5
100000
0 O v
�9 10000 O
o
6 1000 O
c
o 100 O O
R E F E R E N C E S
~ R S K D ~NCO GARCON STEVEN & MAYER CREUSO~- LOIRE
#. Water
1 0 t - r"-- 0 20 40 100 200 400 1000 2000
Diameter in m.m. Fig. 39--Cooling rate at the center of bars cooled in air or quenched in water and oil as a function of diam in mm.
10 5.
O
O r-.- 104" c~
%
CI2 ~ 103. -6 o 0
102
A i r
6 10 20 40 60 100 200
T h i c k n e s s in m m Fig. 40--Cooling rates of plates in air, and water, as a function of gage in ram.
A second f o r m u l a for the h a r d n e s s o f ba in i t e (free f r o m
ferr i te or mar t ens i t e ) was m o r e d i f f i cu l t to deve lop , bu t
with ca re fu l ly chosen d a t a t a k e s the fo rm:
HVba~.i, e = 323 + 185C + 330Si + 153Mn +
65Ni + 144Cr + 191Mo + log V r
(89 + 53C - 55Si - 2 2 M n - 10Ni - 20Cr - 33Mo)
(2o is 20 H V for this equa t i on )
A th i rd f o r m u l a for the h a r d n e s s o f fer r i te p lus pea r l i t e
takes the fo rm:
H V ( f e r r i t e + pearlile) = 42 + 223C + 53Si + 30Mn +
12.6Ni + 7Cr + 19Mo + log II,
(10 - 19Si + 4Ni + 8Cr + 130V)
(2a is 13 H V for this equa t i on )
F o r t e m p e r e d steels M a y n i e r d e v e l o p e d o the r equa-
t ions which t ake in to a c c o u n t the " s o f t e n i n g p o i n t "
which he descr ibes as c h a r a c t e r i s t i c and re la t ive ly inde-
penden t o f the q u e n c h e d s t ruc tu re . A sa t i s f ac to ry con-
densa t ion o f this w o r k is no t poss ib l e in this review
paper .
M a yn ie r and c o w o r k e r s go on to deve lop regress ion
equa t ions for tensi le s t r eng th a n d yie ld s t rength . A n in-
d ica t ion o f the success wi th the i r m e t h o d s is given in
Fig. 41 in which c a l c u l a t e d y ie ld s t reng th is c o m p a r e d
E
r
O'3 >-
C)
80.
70
60 �84
50 �84
40
40
�9 Bamdlc Steels
O Ba~mtlc C N D Steels
�9 Balmtcc C M D Steels
A Bamthc MOV Steels
N - - 9 5 X = 1 , 3 2o-= 7,4 k g / m m ~
50 60 70
M e a s u r e d Y S i n k g / m m 2 Fig. 41--Application to plates (quenched and tempered). Structure is bainitic.
26--VOLUME 1, NUMBER 1 J. HEAT TREATING
|, , MILL DATA
! LOW
CAR8OR
OUENCH AND
1EMPER
I I THERMALLY
DEVELOPED
( I RAW
MATERIAL
... J
1 M~ I CARBON .
i
I 1 ~MECH ANICALLY [ o,vEto,~o [
I [ [ pR0ctss,,o ]
Fig. 42- -Chrys ler computer ized steel selector.
with measured yield strength in quenched and tempered bainitic plates.
Hernacki and Ward of Chrysler Corporation in the US have adopted the Minitech system and have ex- panded it to provide mechanical property data. Their system is described in the TMS/AIME book. 2 A sche-
matic of the Chrysler system is shown in Fig. 42, and an example of mechanical properties print-out from the computer is shown in Fig. 43. This is a good example of providing ready access to data in response to questions raised by designers and materials engineers.
Hildenwall and Ericsson of Link6ping University,
rHI5 SECTION ENABLES THE USER TO PICK THE TEST DESIRED TO SELECT THE TEST, TYPE IN THE NUMBER ~IUEH BELOW
MECHANICAL PROPERTIES = TYPE ) QUENCH AND TEMPER PROPERTIES = 3 HADENABILITY �9 H-BANDS �9 4 COST FOR 1 STEEL = 5 COST FOR NORE THAN 1 STEEL =6 . . . . 1
C MN P S SI HI CR MO .36e .65e .825 .928 .159 .e86 .915 .984
UTS BH FSN FSS K
HOT ROLLED PROPERTIES
69 .458 162.269 1.131 157.761 124.577
SPHEROIDIZED PROPERTIES
5 9 . 8 5 3 118 .448 .917
N
.243
11e .955 1 1 9 . 9 3 2 .247
R ~
4 6 . 7 3 7
53 .~38
YS
43.331
2 9 . 9 3 5
WOULD YOU LIKE TO RUN RNOTHER TEST ON THIS CHENISTRY? IF YES ENTER 1 , IF NO ENTER 2 . . . .
Fig. 43--Mechanica l properties ou tpu t (Hernacki and Ward).
J. HEAT T R E A T I N G V O L U M E 1, N U M B E R 1--27
Sweden, in consultation with Volvo, have developed a program which incorporates mechanical and thermal properties of steels and which generates stress distribu- tions in carburized and heat treated parts. This is a pow- erful method for new designs and may eventually lead to a model for predicting distortion resulting from heat treatment. Two figures help to illustrate the capability of the system. Diagrams of CCT behavior as a function of carbon content have been developed for some steels by Rose and Hougardy. n Knowledge of the effect of alloys on transformation of the base steel and the effect of carbon on the Ms temperature at various locations in the case, as shown in Fig. 44, is the first step in estimat- ing stresses.
For quantitative prediction of residual stresses one must consider temperature gradients developed by quenching (influenced by quenching severity, thermal conductivity, heat capacity, heat of transformation and density of the steel), the phase transformations, and the mechanical properties at various temperatures. All such considerations, carefully integrated, result in calcula- tions of residual stress that take the form of Fig. 45, which shows the calculated residual stresses in plates of AISI/SAE 1321 steel carburized to various surface car- bon contents. The technique also permits estimating re- tained austenite in a carburized case. The calculated stresses have been shown to be in good agreement with experimental experience, and confirm earlier experi- mental work and theoretical developments.
S U M M A R Y
It is the author's opinion that there are several fairly accurate procedures available for estimating heat treat- ment response in steel from hardenability data and from continuous cooling transformation data, and for esti- mating critical heat treatment temperatures from chemi- cal composition. The methods available for predicting hardenability and hardness from composition and grain size are also very useful if developed for a limited class of steels, and are quite accurate if used to predict the ef- fect of minor changes in composition for a given steel. The newer methods for predicting mechanical proper- ties from composition and microstructure hold great promise, but require more development and testing be- fore accurate predictions are possible.
More studies of the interaction of alloys and the influ- ence of minor elements on hardenability are required before a comprehensive set of predictive equations for hardenability can be established. These studies will take two forms. One is the detailed statistical analysis of ex- isting hardenability data, and the other involves gener- ating new data using statistically designed grids of alloys. This author is aware of plans for and progress in both types of studies. It is hoped that others will see the need for such data and that the work can be shared on a worldwide basis.
It is evident, from work to date, that the metallurgist involved in alloy design cannot consider hardenability
900
8O0
700
o 6 O O
d = 500
a. 400 E I , , - ,
3O0
200
100
0 0.1 1 10
Time, seconds
100
1500
1300
1100 t,.
IV
70O ~. p -
500
300
200 1000
Fig. 4 4 - - T r a n s f o r m a t i o n in carburized 50 m m diam bars o f Ck 15, 20 MoCr 4 and NiMoCr 6 steels (Rose and Hougardy) .
2 8 - - V O L U M E 1, N U M B E R 1 J. H E A T T R E A T I N G
4 0 0
"I =:
,d
e e - l , . ,-
,=C
== LL'
-tOO
-101
.5 ( ".." . I . f 2.
/ / , , | ' ! /
I : I : e ; ] i
i41/ ' , / .2%C
, .8%C I P I
I ! / ,. t i s I/
2.5
Fig. 45- -The calculated residual stresses in a carburized plate of SAE 1321 with different surface carbon contents (Hildenwall and Ericsson).
DEPI"H.mm
as an isolated p roper ty of steel. The effect of alloy mod- ifications to enhance hardenabil i ty or to provide the same hardenabi l i ty at lower cost must be considered in the full knowledge of the effects of alloys on the proper- ties of the different microstructures generated in the steel. Such propert ies include machinabili ty, fatigue, toughness, s t rength at elevated temperatures, as well as interactive propert ies such as distortion and thermal fatigue.
R E F E R E N C E S
I. C. A. Siebert, D. V. Doane, and D. H. Breen: The Hardenability of Steels--Concepts, Metallurgical Influences, and Industrial Ap- plications, American Society for Metals, Metals Park, Ohio, 1977.
2. Douglas V. Doane and John S. Kirkaldy, editors: Hardenability Concepts with Applications to Steel, The Metallurgical Society/AIME, New York, 1978.
3. D. J. Carney: Trans. ASM, 1954, vol. 46, pp, 882-927. 4. R. A. Grange: Met. Trans., 1973, vol. 4, p. 2231. 5. M. A. Grossmann: Trans. A1ME, 1942, vol. 150, pp. 227-55. 6. I. R. Kramer, S. Siegel, and J. G. Brooks: Trans. AIME, 1946,
vol. 167, p. 670. 7. M. A. Grossmann: Elements ofHardenability, ASM, Cleveland,
1952. 8. A. F. deRetana and D.V. Doane: Met. Progr., 1971, vol. 100, p.
65. 9, C. F. Jatczak: Met. Trans., 1973, vol. 4, p. 2272.
10, D. T. Lewellyn and W. T. Cook: Met. Tech., 1974, p. 517. 11, A. Moser and A. Legat: Berg Huettenmaenn. Monatsh., 1967,
vol. l l2 , pp. 321-31. 12. J. Glen: British Iron and Steel Institute Special Report 36, p.
14. J. M. Hodge and M. A. Orehoski: Trans. AIME, 1946, vol. 167, pp. 627-42.
15. E. Just: Met. Progr., 1969, p. 87. 16. G. T. Brown and B. A. James: Met. Trans., 1973, vol. 4, p. 2245. 17. K. W. Andrews: Am. Iron Steel Inst., 1975. 18. P. Payson and C. H. Savage: Trans. A S M , 1944, vol. 33.
19. J. W. Hallock: Climax Molybdenum Co. Progress Report L-193-77, April 5, 1971.
20. E. Bain and H. Paxton: Alloying Elements in Steel, 2nd Ed. ASM, Metals Park, OH, 1961.
21. A. Rose and H. P. Hougardy: Atlas zur Wiirmbehandlung der Sti~hle, vol. 2, Verlag Stahleisen M.B.H., Dtisseldorf, 1972.
