Thermal Fatigue of Austenitic and Duplex Stainless Steels

Thermal fatigue behavior of AISI 304L, AISI 316, AISI 321, and AISI 347 austenitic stainless steels as well as 3RE60 and ACX-100 duplex stainless steels was studied. Test samples were subjected to cyclic thermal transients in the temperature range 20 - 600°C. The resulting thermal strains were analyzed with measurements and numerical calculations. The evolution of thermal fatigue damage was monitored with periodic residual stress measurements and replica-assisted microscopy. The elastic strains of the ferrite phase in duplex stainless steels were studied using Barkhausen noise. Finally, destructive analyses including fratographic scanning electron microscopy (SEM) studies and transmission electron microscopy (TEM) analyses were performed. The surface residual stresses changed markedly during the first load cycles. In the austenitic stainless steels yielding during the rapid cooling resulted in compressive residual stresses from -200 MPa (20 - 300°C temperature cycle) to -600 MPa (20 - 600°C temperature cycle). After 10 cycles the residual stresses stabilized and then started to relax due to crack formation. Cracks were seen to initiate from persistent slip bands (PSBs) and in 3RE60 from MnS inclusions. In duplex stainless steels the phase boundaries retarded crack growth markedly. In the austenitic stainless steels, the fracture surfaces of thermal fatigue cracks showed extensive striation formation, i.e. they were similar to mechanical fatigue. The dislocation density was lower than expected based on mechanical fatique data. Dislocation tangles and occasional cell tendency was observed. In duplex stainless steels the plastic deformation concentrated to the austenite phase. The obtained thermal fatigue data were compared with mechanical fatigue data from literature and with the ASME design curve. The ASME design curve was found to give safe design life, although the remaining safety factor on strain is decreased to 1.5. The total strain (elastic+plastic) caused by thermal loading was solved with linear-elastic Fe-analysis. Thermal fatigue crack growth was predicted successively using total strain solution of an uncracked component and a strain-based growth model: da/dN = C₇Δε_tot^m7 a, where C₇=1.6 and m₇=1.3 for the studied austenitic stainless steels. The model is applicable to small fatigue cracks (0.05 - 4 mm) growing in varrying temperature and strain fields and its temperature-independent in the studied range.