The calculation of transition temperature changes in steels due to temper embrittlement
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grain size. Such data would be useful in a number of ways, such as: 1) Comparing the effects of various impurities; 2) Estimating the value of transition temperature at any level of impurity segregation as a function of hardness and grain size; and 3) Estimating the beneficial effects that would accrue from reductions in one or more of the important factors. EXPERIMENTAL PROCEDURE Since the objective was to obtain information with the minimum amount of ambiguity, vacuum-melted and argon-cast laboratory heats of steel with carefully controlled composition were used. The nominal base composition of the steel was 3.5 Ni, 1.7 Cr, 0.3 C. Three heats were doped with 0.07 P, 0.06 Sn, or 0.6 Si and compared with an undoped heat. The compositions are given in Table I. The steel was received in the form of forged and hot-rolled plates, which were subsequently surface ground, cut into Charpy specimen blanks, and vacuum heat-treated as shown in Table II to produce specimens with three prior austenitic grain sizes in the range ASTM no. - 2 to 8 (designated coarse C, medium M, and fine F, respectively). Two tempering treatments for each grain size yielded nominal Rockwell C hardnesses of 30 and 20. Sets of 5 to 8 specimens in each condition
Table I. Alloy Compositions (wt pct) Designation
Ni
Cr
C
Sn
P
Si
P-doped Sn-doped Si-doped Pure
3.5 3.5 3.5 3.5
1.7 1.7 1.7 1.7
0.30 0.29 0.30 0.30
-0.0610 ---
0.070 0.004 0.005 0.006
--0.61 --
ISSN 0360-2133/80/0911-1513500.75/0 M E T A L L U R G I C A L T R A N S A C T I O N S A 9 1980 A M E R I C A N SOCIETY FOR M E T A L S A N D V O L U M E 11A, S E P T E M B E R 1980--1513 T H E M E T A L L U R G I C A L SOCIETY OF A I M E
Table II. Heat Treatments Condition
Austenization
Mean Grain Diameter (ram)
Tempering
ASTM No.
P-doped H20 H30 M20 M30 C20 C30
900 900 1050 1050 1300 1300
~ ~ ~ ~ ~ ~
1h 1h 2h 2h 1h 1h
670 620 670 620 670 620
~ ~ ~ ~ ~ ~
7h 1h 7h 1h 7h 1h
0.04 0.04 0.17 0.17 0.65 0.65
6.8 6.8 2.6 2.6 - 1.5
Sn-doped F20 F30 M20 M30 C20 C30
850 850 1050 1050 1300 1300
~ ~ ~ ~ ~ ~
1h 1h 1h 1h 1h 1h
670 600 670 610 670 600
~ ~ ~ ~ ~ ~
4h 1h 4h 1h 4h 1h
0.03 0.03 0.06 0.06 0.11 0.11
7.8 7.8 5.3 5.3 1.8 1.8
Si-doped F20 F30 M20 M30 C20 C30
880 880 1100 1100 1300 1300
~ ~ ~ ~ ~ ~
2h 2h 2h 2h 2h 2h
670 620 675 630 680 640
~ ~ ~ ~ ~ ~
7h 1h 7h 1h 7h 1h
0.04 0.04 0.21 0.21 0.42 0.42
7 7 1.8 1.8 -0.2 -0.2
Pure M20 M30
1025 ~ 1025 ~
1h 1h
670 ~ 600 ~
3h 1h
0.12 0.12
3-4 3-4
were then aged in vacuum for various times at 480 ~ to allow the segregation of the impurities to grain boundaries. The blanks were then finish-ground and notched to produce standard Charpy V-notch specimens. They were then tested in a calibrated impact machine to measure the fracture energy at various temperatures: the latter were obtained by prior immersion of the specimens in baths of liquid N:, 2-methyl butane, ethyl alcohol, silicone oil, or molten salt. Notched cylindrical specimens (3.5 mm diam) were machined from fractured Charpy specimens and were used for Auger e
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