1988 Volume 37 Issue 414 Pages 289-294
The formation mechanism of surface cracks leading to creep fracture was investigated through microscopic observations and measurements of grain boundary sliding during creep, in a 321 stainless steel.
Creep tests were carried out in argon containing 4% hydrogen at 923K under the stresses of 196, 176 and 157MPa. The grain boundary sliding during the creep tests were 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. The surface cracks were formed when the grain boundary displacement reached a critical value of about 0.3μm. This suggests that the surface cracking depends mainly on the grain boundary sliding. The microscopic observation indicates that the surface cracking process is as follows.
(1) Small cavities form at the grain boundaries on 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 grain size and wedge-like shape.
This process is caused mainly by the grain boundary slinding.
