GRAIN-BOUNDARY SLIDING AND SURFACE CRACKING DURING CREEP OF 321 STAINLESS-STEEL

The formation mechanism of surface cracks leading to creep fracture was investigated by microscope observations and measurements of grain boundary sliding during creep, in 321 stainless steel. Creep tests were carried out in argon containing 4% of hydrogen at 923 K under stresses of 196, 176 and 157 MPa. The grain boundary sliding during the creep tests was measured by the displacements of scratched lines and interference fringes at the grain boundaries on the crept specimen's surface. The grain boundary displacements could be detected clearly after the primary creep stage, and increased in proportion to the creep strain during the secondary stage, and the initial part of the tertiary stage. Surface cracks were formed when the grain boundary displacement reached a critical value of about 0.3-mu-m. This suggests that surface cracking depends mainly on grain boundary sliding. The microscopic observations indicate that the surface cracking process is as follows. (1) Small cavities form at the grain boundaries on the surface. (2) The cavities grow along the grain boundaries and become crack-like due to their coalescence. (3) The crack-like cavities grow into cracks of about grain size with a wedge-like shape. This process is caused mainly by grain boundary sliding.