Effect of Deep Drawing and Subsequent Aging Condition on Delayed Cracking of Stainless Steel

Abstract

The effects of the conditions of deep drawing and subsequent aging treatment upon a delayed cracking of meta-stable austenitic stainless steel sheets are studied. The incubation time for the initiation of cracking is measured at various aging temperatures and in the various atmospheres. The results are summarized as follows:
(1) A relation between aging temperature and incubation aging time is represented by a "C-curve" with an asymmetrical form. Consequently, the "nose" where the delayed cracking takes place most quickly is found on each C-curve.
(2) With either lowering of deep drawing temperature or increasing of deep drawing ratio, the C-curve moves toward the region of higher aging temperature and shorter incubation time, showing the expansion of the area where the delayed cracking is induced.
(3) The C-curve is markedly affected by the kind of lubricant used for drawing and the method of removing it after drawing as well as aging atmosphere. This suggests the significance of a certain chemical reaction on the surface of the drawn cup to the delayed cracking phenomena.
(4) The temperature dependence of the incubation time has been studied on assumption that the delayed cracking is initiated when the concentration of diffusing atoms of a certain element at some special microstructure site reaches a critical value. As a result, the apparent activation energy for delayed cracking is estimated to be about 13 kcal/mol, which is approximate to that of the activation energy for the volume diffusion of hydrogen in γ phase of type 304 stainless steel. Then it is suggested that the diffusion of hydrogen also makes a significant contribution toward the delayed cracking phenomena of meta-stable austenitic stainless steel.