Assessment of surface relief and short cracks under cyclic creep in a type 316LN austenitic stainless steel

Abstract

Formation of surface relief and short cracks under cyclic creep (stress-controlled fatigue) in type 316LN stainless steel was studied at temperatures ranging from ambient to 923 K using scanning electron microscopy technique. The surface topography and crack distribution behaviour under cyclic creep were found to be strong functions of testing temperature due to the difference in strain accumulation. At 823 K, surface relief mainly consisted of fine slip markings due to negligible accumulation of strain as a consequence of dynamic strain ageing (DSA) which led to an increase in the cyclic life. Persistent slip markings (PSM) with distinct extrusions containing minute cracks were seen to prevail in the temperature range 873–923 K, indicating a higher slip activity causing higher strain accumulation in the absence of DSA. Besides, a large number of secondary cracks (both transgranular and intergranular) which were partially accentuated by severe oxidation, were observed. Extensive cavitation-induced grain boundary cracking took place at 923 K, which coalesced with PSM-induced transgranular cracks resulting in failure dominated by creep that in turn led to a drastic reduction in cyclic life. Investigations on the influence of stress rate were also carried out which underlined the presence of DSA at 823 K. At 923 K, lowering the stress rate caused further strengthening of the contribution from creep damage marked by a shift in the damage mechanism from cyclic slip to diffusion.

Keywords:

• cyclic creep
• type 316LN stainless steel
• persistent slips markings
• extrusion
• intergranular cracking

Acknowledgements

The authors acknowledge the help rendered by Sri. M Arvinth Davinci in carrying out the surface profilometry. They also thank Dr AK Bhaduri, AD, MDTG and Dr T Jayakumar, Director, MMG, IGCAR, Kalpakkam for their encouragement.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. The accumulated strain is calculated as difference in the creep strain between last and first cycle. Strain in the first cycle is omitted since it is only instantaneous strain depending only on σ_a − σ_m combination and is not indicative of any specific mechanism like DSA.