Thermomechanical treatment of steel

FORE IGN TE CHNOLOGY

T H E R M O M E C H A N I C A L T R E A T M E N T OF STEEL

G. I . C h e r e p a n o v a and V. D. K a l ' n e r

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 58-62, April, 1963

A well-known American professor, R. F. Mehl, gave a lecture on thermomechanical treatment of steel at the Moscow Institute of Steels and Alloys, June 5, 1962. Professor Mehl indicated that up to the beginning of 1962 the work on thermomechanical treatment in the United States was being done essentially by two companies: United

States Steel Corporation and Ford. High-temperature thermome- TABLE 1

d

c co

1 0,98 2 0,28 3 0,35 4 O,40 ]

5 O, 49 6 0,31 7 0.41 '8 0.47

Concentration of elements,%

Cr

M

1,40 1,45 1,45 1,44 2.21 2,23 2,27

NI

3,96 4,55 4,60 4.75 4.50 1,03 1,07 1,05

Sl

1,78 1155 1,43 1.59 1,64 1,46

Mo

0.31 0,34 0,34

m

0,31 0,32- 0,32

k g / m m ~

250

10~ ~-----o--- .~----o 50 ._ .o .~ .~....~ O0

3O

6: ~ 1o 1oo - - - - . - o

5G Z'/f 3 ZS- /

O ZO tO0 200 300 ttOO *C Tempering temperature

Fig. ! . Effect of tempering temperature (for 1 h) on the mechanical properties of steel after low-temperature thermomechanical treatment at different degrees of swaging, i) 75% deformation; 2) 25% deformation; 3) 0% deformation.

chanical treatment has been investigated primarily at United Steel by Grange and his co-workers, while work on low-temperature thermomechanical treatment has been done essentially at Ford by Zeckley.

The first work on low-temperature thermomechanical treatment was published by Zeckley in 1958. He worked with various steels of the compositiom given in Table 1. The composition of the steel was chosen so that the austenite was highly stable in the intermediate region. The alloys were rolled at 316-538~ quenched in off, and tempered for 1 h at different temperatures. The mechanical properties of these steels are shown in Figs. 1 and 2.

The ductility and strength increase after low-temperature thermomechanical treatment and quenching. Swaging of 25% increases the strength considerably.

Later, the following steels were subjected to low-temperature thermomechanical treatment: Vasco MA high-speed tool steel (0.55% C and 12% alloyed elements), N-11 steel (5% Cr, 1.3%oMo, 0.5% V, and 0.4% C), and 300M steel (4340 type) with a high concentration of silicon. Figure 3 shows that Vasco MA has optimal properties when it is swaged 90%. After tempering at 538~ and rolling at 593~ the characteristics are as follows: o s = 298 kg /mm 2, o b = 323 kg/mm 2, 6 = 8%, r = 35 %. In this ease only swaging of 50% or more produced a significant improvement in the mechanical properties.

N-11 steel, used for hot stamping, is also used as a structural steel, and although it contains a relatively small amount of carbon the ultimate strength after low-temperature thermomechanical treatment reaches 263-280 kg /mm 2, the ductility being either equal to or higher than after ordinary treatment (Fig. 4). The effect of the deformation temperature on the ductility was investigated at different degrees of swaging (Fig. 5). The deformation temperature must be determined for each particular steel to obtain the optimum combination of strength and ductility for a given degree of defor-

mation.

Fatigue tests were run on N-11 steel after ordinary treatment and after low-temperature thermomechanical treatment. Large numbers of samples were tested on the standard R. R. Moore machine (10,000 cycles/rain). The statistically treated results are given in Fig. 6.

236

0.3! % C

2 2 0 l ~ E s �9 b

~ ' ,

180

~ o

~ 0 :', '~ 30 Z30 430

a

0.6! ~ C

t ,as r~.~

n

30 230 430 b

i tl�9

30 230 ~30 ~ C

Fig. 2. Variation of the mechanical properties after low-temperature thermomechanical treatment as a function of the concentration�9 of carbon, a) 0.31% C; b) 0,41% C; c) 0.47% C; upper curves: 93% deformation; lower curves: 0% deformation.

HRC

-D- 2 k g / m m 2 ~ . . . - - - o - 4 _ -

210

140

70

~0

20 - ' " - , o ,=

0 zoo 3o0 ~,oo 500 6oo 7o0 'r,

Tempering temperature

Fig. 3. Effect of the tempering temperature on the properties of Vasco MA steel after low-temperature thermomechanical treatment. 1)Undeformed; 2) 91% deformation at 593~

"\6j

I # "~

Ob, kg /mm 2

28o I I t z 6 o - - ~ - ~ . - ~ . . . . . ~ - " ' ~ - - - : ' : ~ ~ , ,

Z20 Os.i / m

z o o I - 400 500 600 700 800 *C

Rolling temperature

Fig. 4. Effect of the degree of swaging and the deformation temperature on the properties of N-11 steel tempered at 510~ after deformation. 1) 94% deformation; 2) 75% deformation; 3) 50% deformation; 4) 30% deformation.

Low-temperature thermomechanical treatment increases the fatigue resistance of N-11 steel 20-30% as com- pared to ordinary treatment and tempering.

The fatigue l imit of this steel - 11,7.4 kg /mm 2 - is the highest ever recorded in the scientific literature.

The impact strength of 5M21 steel (13% Mo, 3% Ni, 0.2% C) after low-temperature and high-temperature thermomechanical treatments was determined with Charpy samples at the Ford laboratories. However, Professor Mehl has doubts about the very high values obtained in these tests.

The Ford laboratories and Manufacturing Engineering and Development have studied low-temperature thermomechanical treatment by extrusion. In this case all the stresses during deformation are compression stresses, and

237

238

~0

30

2G

tO 6~ tO

0 l t l ~00 500 ~00 700 800 *c

Rolling temperature

Fig. 5. Effect of the deformation temperature on the ductility of N-11 steel subjected to dif= ferent degrees of deformation . . . . . ) 75 and 94% deformation; . . . . . ) 50% deformation; - ' - . . . . . . ) 34% deformation.

kg/mm2

I00

8O

III!IIJ -I I-Jill los /O r 10 a N

Number of cycles

Fig. 6, Fatigue resistance of steels after different treatments. ) N-11 steel after low=temperature thermomechani= eal treatment; . . . . . ) N-11 steel after quenching and tempering; . . . . . . ) SAE 5160 steel after quenching and t em- pering.

Ob,Os kg /mm z 286

Z56

350 aO0 050 500 550 350 400 ~50 500 550 Extrusion temperature

a b

Fig. 7. Strength of N-11 steel after low- temperature thermomechanical treatment by extrusion at different temperatures, a) 70~ deformation, tempered at 510~ b) 70% deformation, tempered at 538~

kg//mm 2

~ S

k g / m m 5 /60

f40

120

6~ I0

/ / / I / / /

/////// ////I//

//////1 / / / / / / / / / / / / / / / / / / / / i

1 / / / ' i i i W////A

760 ~ 538 ~ "c

Deformation temperature

Fig. 8. Effect of the deformation temperature on the mechanical properties of E steel subjected to 5 0 %

deformation (see Table 2). (The horizontal dashed line represents the mechanical properties of undeformed E steel.)

Ill Iit VIIIIIIII ,01 l llllIll I I I I Itf

10 I00 1000 1000~ sec T i m e

Fig. 9. Time required for complete recrystallization at different degrees of deformation of 51860 steel heated to 1205~ rolled at 927"C, and recrystallized at 760= 982~ 1) 50% deformation; 2) 65% deformation; 3) 75% deformation.

TABLE 2

Concentrat ion of e lements , % Steel

C Mn P S Si NI Cr No V

A B C D E

0,26 0,57 0,87 0,41 0,25

0.36 O, 82 2.07 O, 72

0.007 0.016 0,009 0.002

O, 032 0.019 0.010 0,010

0,28 0.30 0.32 O, 26

0.52 1,16 4.95 1,90 0.59

1,32 1,07

072 0.73

1,00 0,26

0~s

0.35

0.27

therefore the probabi l i ty of cracks is reduced to a min imum. Also, in the case of extrusion i t is very easy to hea t the instruments and change the deformation rate . The investigations were made on N-11 and Vasco MA steels. The

mechan ica l properties are shown in Fig. 7.

High- tempera ture thermomechanica l t rea tment has been studied at the United States Steet Corporation.

The composi t ion of the steels invest igated is given in Table 2.

i l!! llt 0 10 T ime 100 1000sec

Fig. 10. Effect of the ini t ia l grain size ( t e m -

perature) on the recrys ta l l iza t ion of 51B60 steel at 816~ 1) After heat ing at 927~ the grains are grade 7; 2) after heating at 1200~ the grains are grades 4 and 5.

Fig. 11. Undeformed tempered mar - tensite, x15,000,

Fig. 12. Martensite deformed 93%. Fig. 13. Block structure of the slip system in grains, x 15,000.

The samples were rolled with smooth rollers 127 mm in diarneter . Samples 50 mm long, 6 mm wide, and 2.5 m m thick were subjected to tension tests. The degree of swaging was 50-88%. The samples were rolled in one or several passes. They were heated to 985~ cooled to the desired temperature in a lead bath, rol led, and quenched in oil . The properties of E steel obtained after this t rea tment are shown in Fig. 8.

23 9

High-temperature thermomechanical treatment increases the strength and yield strength, while the ductility remains the same as after other treatments. Apparently a high rolling speed prevents recrystallization during high- temperature thermomechanical treatment; partial decomposition may occur during low-temperature thermomechani- cat treatment.

R. A. Grange and R. S. Mulhauser showed that for most alloyed steels the t ime required for complete recrystallization is rather long. Steel 51B60 (Fig. 9) is an example. In the same investigation it was also shown that even when there is partial recrystallization (during the early stages) the properties of the steel change very little. The authors conclude that the rolling temperature either has no effect or very little effect on the recrystallization process, while the initial grain size and the degree of defon-nation are the decisive factors (Fig. 10).

Fig. 14. Block structure of the steel containing 0.3% C. The slip planes are outlined by the products of isothermal transformation, a) 0% deformation; b) 89% deformation after low-temperature thermomechanical treatment.

Fig. 15. The shapes and distributions of carbides precipitated on the replica: a) After ordinary heat treatment; b) after low- temperature thermomechanical treatment of steel containing 12% Cr, 4% Ni, and 0.35% C tempered at 427~

Professor Mehl indicated the following possible reasons for strengthening as the result of thermomechanical treatment: strengthening of austenite and the transfer of its dislocational structure to martensite; a considerable decrease in the size of martensite platelets and their preferential orientation; and a change in the type of carbides precipitated during tempering.

Professor Mehl described some results of electron microscope investigations (Figs. 11-15): the martensite structure changes; the martensite platelets are bent and broken by the slip planes; the large number of slip systems result- ing from thermomechanical treatment leads to considerable decrease of the size of martensite grains; small spherical carbides accumulate between large needlelike carbides in the martensite formed from cold hardened austenite; the carbides precipitate preferentially along the slip planes; the martensite is heterogeneous, and its strengthening by oriented dispersed carbides apparently leads to improved mechanical characteristics which are retained at high tempering temperatures.

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Thermomechanical treatment of steel