The Influence of Microstructure on Fatigue Crack Propagation Behavior of Stainless Steel Welds

The influence of microstructure on the fatigue crack propagation behavior of gas metal arc welds in 316L and AL6XN austenitic stainless steels has been investigated. A constant AK (stress intensity range) testing procedure with a stress ratio value of 0.6 was first used to deconvolute stress intensity range and residual stress effects from microstructural effects as the fatigue crack propagated from the base metal into the weld metal. The results of this test demonstrated that the large grain size of the weld metal produced a rough fracture surface with improved fatigue resistance relative to the base metal. The influence of grain size on fatigue resistance was then studied in more detail by generating full fatigue curves over a wide range of delta;K on base metal samples that were heat treated to obtain various uniform grain sizes. Results from fatigue tests conducted on the base metal control samples were consistent with the weld metal results and showed that large grain sizes produced relatively rough fracture surfaces with improved fatigue resistance. The improved fatigue resistance occurred predominately at low stress intensity ranges where the plastic zone size is approximately equal to or less than the grain size. The improved fatigue resistance with increasing grain size was attributed to three main factors, including 1) a tortuous crack path that requires formation of a large surface area for a given length of crack propagation, 2) crack growth out of the Mode I plane, which reduces the stress intensity range available for crack growth, and 3) roughness-induced closure that shields the crack from part of the applied load. Direct crack closure measurements were used to identify the range of AK levels where the third factor was operable. Quantitative estimates of the AK level below which grain size effects are expected to occur are in reasonable agreement with the experimental results.