Environment-Assisted Cracking in Custom 465 Stainless Steel
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RODUCTION
BECAUSE of its good corrosion resistance and mechanical properties, Custom 465 stainless steel (SS) has been widely used for aircraft and other structural components. Its excellent cold workability and age hardenability have expanded its usefulness further. However, the effects of cold work and aging on the environment-assisted cracking (EAC*) resistance have *EAC is a cracking process in which the environment promotes crack growth or higher growth rates than would occur without the presence of the environment.
not been fully identified. In addition, the effect of corrosive medium concentration on EAC has not been well understood. The effects of cold work, aging, strength, pH, and concentration of the corrosive medium on EAC
E.U. LEE and M. JONES, Materials Engineers, are with the Naval Air Warfare Center Aircraft Division, Patuxent River, MD 20670 e-mail: [email protected] R. GOSWAMI, Senior Scientist, is with the Naval Research Laboratory, Washington, DC 20375, and the Science Application International Corporation, McLean, VA 22102. A.K. VASUDEVAN, Scientific Officer, is with the Office of Naval Research, Arlington, VA 22203. Manuscript submitted December 9, 2009. Article published online September 14, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
resistance of steels have been studied by a number of investigators, resulting in some conflicting trends. Cold work has been found to improve the EAC resistance for some steels,[1,2] but reduce it for others.[3] Tempering and aging have been known to influence the microstructure, strength, and EAC resistance of steels.[4–9] It has been reported that the susceptibility to EAC is greater at higher yield strengths.[1,10–18] Generally, highly acidic conditions promote cracking and highly basic conditions reduce or even prevent cracking. However, over a certain range of pH, there is no great change in susceptibility to EAC.[4,9] It has been reported that KIEAC values of some steels are affected by the NaCl concentration of the aqueous test solution, while for the other steels, there is minimal or no effect.[19–21] This study is aimed at providing insight into effects of prior cold work, subsequent aging, and NaCl concentration of the aqueous test environment on EAC resistance. Furthermore, an attempt is made to identify the controlling mechanism for the EAC.
II.
EXPERIMENTAL PROCEDURE
The chemical composition of the custom 465 SS is shown in Table I. The material was initially solution annealed at 1255 K (982 °C) for 1 hour, rapidly cooled, and refrigerated at 200 K (–73 °C) for 8 hours (referred
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VOLUME 42A, FEBRUARY 2011—415
Table I. Ni 10.75/11.25
Chemical Composition of Custom 465 SS in Weight Percent
Cr
Mo
Ti
Mn
Si
C
S
P
Fe
11.00/12.50
0.75/1.25
1.50/1.80
0.25
0.25
0.020
0.010
0.015
bal
(1/4 in.) were prepared and subjected to fracture toughness testing in a laboratory atmosphere, following ASTM Standard E399 – 08.[24] The crack paths and fractographs of the fatigue-precracked and EAC-tested
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