The use of laser surface-annealed treatment to retard fatigue crack growth of austenitic stainless steel

Fatigue crack growth behavior of an AISI 316 austenitic stainless steel (SS) annealed using a CO2 laser was evaluated under various environments--lab air, gaseous hydrogen and saturated hydrogen sulfide solution. The laser-annealed specimen revealed no change in microstructures in various regions of the specimen. The results of fatigue crack growth tests indicated the laser-annealed specimen had a higher resistance to fatigue crack growth in the region preceding the laser-annealed zone (LAZ) independent of the test environments. Meanwhile, crack growth results also suggested that AISI 316 SS showed a low level of sensitivity to hydrogen-accelerated crack growth. X-ray diffraction pattern of the fatigue-cracked surface revealed that partial austenite to martensite transformation occurred within a narrow depth. The presence of residual austenite in the highly strained region trapped a large amount of hydrogen, which helped reduce hydrogen embrittlement susceptibility and hydrogen-accelerated crack growth in the alloy. Fatigue fractography of the specimens tested in air showed predominantly transgranular fatigue fracture with some flat facets (FFs). In case of specimens tested in the H2S solution or gaseous hydrogen at low loading frequency, quasi-cleavage (QC) fracture was correlated with hydrogen-enhanced crack growth. Moreover, the presence of obvious striations on the fracture surface of embrittled specimens could be attributed to the hydrogen-activated slip processes ahead of the crack front