I Resistance fo
R. E. LOCHEN and E.
R.
MILLER
Allis-Chalmers Research Laboratories, Milwaukee, Wis.
...
Stress Corrosion of 12% Chromium Stainless Steel is a complex problem related to
b b
b
Composition
HARDENED
Hardness
Surface finish
Applied stress
and tempered 12% chromium stainless steel (Type 403) has been used for many years, but few cases of stress corrosion have been reported. In 1953, failure of air craft compressor blades tempered to a relatively high hardness (300-362 Brinell) was reported (7). Although stress corrosion failures of 1270 chromium stainless steel were first reported as early as 1932 (4), most reports of service failures of stainless steels have involved the austenitic type (3, 6-9, 72). For martensitic stainless steels, failures were probably attributed to causes other than stress corrosion because of the inability to crack hardenable steels in corrosion media which cracked austenitic stainless steels (7 7, 73). More recent studies have reported stress corrosion cracking of martensitic stainless steels (2, 5, 70). I n the few failures reported for 12% chromium steel, the material had been heat treated to a relatively high hardness-i.e., above 300 Brinell. Because these 12Y0 chromium steels are used widely, the effect of applied stress, influence of hardness and heat treatment, and the effect of material composition on stress corrosion were investigated.
Tempering temperature
where Y = deflection, inches; S = stress, p s i . ; L = gage length, inches; E = modulus of elasticity, p.s.i.; c = one-half specimen thickness, inches. Stressed test specimens were exposed to a corrosion medium of 0.50j, acetic acid saturated with hydrogen sulfide. The bath was maintained at 80' =k 5' F. Hydrogen sulfide concentration was maintained by a continuous, metered flow bubbled through the acetic acid solution. This medium was selected because it produced stress corrosion cracking with a minimum of general corrosion. Four test specimens were exposed under identical conditions in accordance with the 'Yeast of four" statistical method ( 74). An electrical timing circuit was used to measure the exact time to failure. The test was terminated a t the time of failure of one of the four specimens or after 500 hours. Preliminary tests indicated no galvanic effect between test specimens and holders, and it was not necessary to insulate the former.
the vapor blasted specimen (curve 200A, Figure 1) was probably caused by introduction of desirable compression stresses on the surface by the vapor blasting operation. As applied stress decreased, time to failure increased. Time to failure also increased with the addition of secondary alloying elements. Type 403 stainless steel containing 0.50% molybdenum ( B , Figure 1) was more resistant to stress corrosion than the 0.0570 molybdenum Type 403 material ( A , Figure 1). Type 422 stainless steel (C, Figure I ) , which contained approximately 1% molybdenum, 1% tungsten, and 0.2570 vanadium, was more resistant to stress corrosion than both heats of Type 403 stainless steel a t equivalent hardness and stress levels. Time to failure and impact strength increased slightly with a n increase in tempering temperature u p to 500' F. Above 500' F., time to failure and impact strength decreased to a minimum at about 800' F., but at still higher temperatures, both time to failure and impact strength increased (Figure 2). A correlation also existed between hardness and time to failure a t tempering temperatures above about 1000" F. At temperatures below 1000° F., there was little change in hardness, and hard-
Results Stress corrosion data for Type 4030.05 molybdenum, Type 403-0.50 molybdenum, and Type 422 stainless steel are summarized in Figure 1. Improvement in resistance to stress corrosion of
Experimental Materials. Materials were commercial heats of hot-rolled and heat-treated bars (table). Test specimens were airhardened and tempered to a range of hardness levels. Procedures. Flat, surface-ground test X 0.1875 3= specimens (0.5 f 0.0005 X G inches) were stressed by bending in a constant load fixture. Amount of deflection required to develop the desired stress for a given test was calculated from: y = -SL2 SEc
Mill and Laboratory Check Analyses of Test Specimens Were Compared
Element C
Mn
P S
Si
Ni Cr Mo
W V
Type 403-0.05 Mo Lab. Mill 0.11 0.48 0.022 0.011 0.28 0.29 12.10 0.07
... ...
0.10 0.47 0.026 0.010 0.34 0.31 12.45 0.03
...
6 . .
Amount Present, To Type 403-0.50 Mo Lab. Mill 0.12 0.44
... ...
0.13 0.27 11.80 0.45
... ...
0.10 0.44 0.014 0.019 0.26 0.26 12.62 0.50
... ...
VOL. 51, NO. 6
Type 422
Lab. 0.17 0.98 0.020 0,020
...
0.79 12.72 1.09 1.15 0.23
Mill 0.18 0.86 0.027 0.015 0.25 0.78 12.66 0.95 1.12 0.27
JUNE 1959
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Figure 1. Type 4 2 2 stainless steel was more resistant to stress corrosion than both heats of Type 403. Arrow shows test discontinued without failure; surface of 2 0 0 A ground and vapor blasted, all others as ground; air quench after 0.5 hour at 1725” F. for series A and 8, 1900” F. for series C Draw, 4 Hr.,
Draw, 4 Hr.,
Series Type 403-0.05% Mo 1 OOA 200A 300A 400A 500A 600A 700A
F.
Brinell
Brinell
Type 403-0.50% M o
.. ..
372 372 332 270 250 230 205
995 1035 1065 1100 1250
ness could not predict susceptibility to stress corrosion. Metallographic Examinations. Stress corrosion cracks occurred on the tension surface of test specimens and propagated in the plane perpendicular to the applied stress. Generally, there were small cracks parallel to the main fracture which appeared to pass through or start at corrosion pits. The large number of cracks across the tension surface indicated that the stresses had
F.
Series 1006 2008 3008 4008 5008
a .
1010 1050 1125 1250
Type 422
..
1 ooc 200c 300C 400C
1075 1175 1300
372 332 270 250 230
509 382 322 276
been uniformly distributed along the gage length of the specimens. The stress corrosion cracking was predominantly intergranular.
design of test equipment and planning of this investigation.
Acknowledgment
(1) Badger, W. L., S.A.E. Trans. 62, 307-10 (1954). (2) Bloom, F. K., Corrosion 11, 39-49 (1955). (3) Ellis, 0. B., “Symposium on Stress Corrosion Cracking of Metals,” p. 421, .4m. SOC. Testing Materials and Am. Inst. Mining Met. Engrs., New York,
Nathan Vahldieck contributed to the
literature Cited
1 Odd
Figure 2. Tempering temperature had a significant effect on time to failure and impact strength
(4j’Ffikld, P. D., “The Book of Stainless Steels,” pp. 679-86, Am. SOC. Metals, Cleveland, Ohio, 1935. (5) Fontana, M. G., WADC Tech. Rept. 56-242 (August 1956). ( 6 ) Franks, R., Binder, W. O., Brown, C. M., “Symposium on Stress Corrosion Cracking of Metals,” pp. 411-21, Am. SOC. Testing Materials and Am. Inst. Mining Met, Engrs., New York, 1944. (7) Heger, J. J., M e t a l Progr. 66, 109-16 (1955). (8) Hodge, J. C., Miller, J. L., T r a n s . A m . SOC. M e t a l s 28, 25-82 (1940). (9) Hoyt, S. A., Scheil, M. A,, Ibid., 27, 191-226 (1939). (IO) Lillvs, P., Nehrenberg, A. E., Zbzd., 48, 327-46 (1956). (11) Lochen, R. E., unpublished reports, 1945. (12) Rees, W. P., Inst. Metals (London), Monograph and Rept. Ser. No. 5, 333-6 (1947). (1 3) Scheil, M. A, Zmeskal, O., Waber, J., Stockhausen, F. Weidzng J . (IV. Y.) 22, 493s-503s (1943). (14) Schutte, E. H., Am. Sod. Testtng Materials Proc. 54, 853-64 (1954). RECEIVED for review November 10, 1958 ACCEPTED February 24, 1959
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INDUSTRIAL AND ENGINEERING CHEMISTRY
