The Influence of Microstructure on Fatigue Crack Propagation Behavior of Stainless Steel Welds
Fatigue tests on controlled samples exhibited a correlation between large grain size and improved crack resistance
BY C. S. KUSKO, J. N. DUPONT, AND A. R. MARDER
ABSTRACT. The influence of microstruc- ture on the fatigue crack propagation be- havior of gas metal arc welds in 316L and AL6XN austenitic stainless steels has been investigated. A constant DK (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 microstruc- tural effects as the fatigue crack propa- gated from the base metal into the weld metal. The results of this test demon- strated 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 DK 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 fa- tigue resistance. The improved fatigue re- sistance 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 for- mation of a large surface area for a given length of crack propagation, 2) crack growth out of the Mode I plane, which re- duces 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 iden- tify the range of DK levels where the third
C. S. KUSKO is Research Assistant; J. N. DUPONT is Associate Professor and Directo , Joining and Laser Processing Laboratory; and A. R. MARDER is Professo , Department of Materi- als Science and Engineering, Lehigh Universit , Bethlehem, Pa.
factor was operable. Quantitative esti- mates of the DK level below which grain size effects are expected to occur are in reasonable agreement with the experi- mental results.
Stainless steel alloys are used in many applications that are exposed to cyclic loading conditions. In these applications, detailed knowledge of the fatigue crack growth behavior is important for estab- lishing allowable stresses and flaw sizes. In addition, many components are fabricated by welding, so knowledge of the fatigue behavior of the weld is also important.
Although data exist on the fatigue crack growth behavior of stainless steel al- loys and their welds (Refs. 1–7), relatively little work has been conducted to deter- mine the influence of weld microstructure on fatigue crack growth in detail. Results obtained to date have shown that the pres- ence of d-ferrite can influence the nature of the crack propagation path, but this has no significant effect on the actual crack growth rates (Refs. 1–3). It has also been observed that the weld metal often ex- hibits better fatigue resistance (i.e., lower crack growth rates) compared to the base metal (Ref. 8); however, the reasons for this are not yet clear
Most fatigue testing is conducted using standard DK-increasing tests. While such tests are useful for obtaining the direct re- lation between crack growth rate (da/dN)
Fatigue Crack GMAW Austenitic Stainless Steels Fatigue Resistance 316L Stainless Steel AL6XN Stainless Steel
and stress intensity range (DK), it is diffi- cult to understand the role of microstruc- ture on fatigue resistance. For example, in a standard fatigue test conducted on a weld sample, the DK level is varied as the crack propagates from the base metal into the weld. In this condition, crack growth rates will change due to varying DK, vary- ing residual stress level, and/or changes in microstructure. Thus, with all three fac- tors changing simultaneously, it is difficult to determine the role of weld metal mi- crostructure in detail. An alternative ap- proach to this problem is to use a constant DK test (Refs. 9–11). With this approach, a computer-controlled testing algorithm is used that is capable of reducing the ap- plied loads as the crack grows from the base metal into the weld metal so that DK remains constant. In addition, a stress ratio, R (R = ratio of minimum-to-maxi- mum stress), is used that is high enough to overcome residual stress effects. At low R values, the crack may enter into a region in which the compressive residual stress is higher than the minimum applied stress. Under this condition, the crack will re- main closed during a portion of the stress cycle, which reduces the applied DK to some lower, effective DK level and causes a reduction in crack growth rate. In order to overcome this effect, higher R values must be used in combination with a method for directly detecting crack clo- sure conditions so that it is ensured the fa- tigue crack is always open. With this con- stant DK/high R approach, any effects of microstructure on fatigue resistance will readily be signaled by a change in the mea- sured da/dN as the crack propagates across various microstructural zones, thus providing a sensitive method for deconvo- luting microstructure effects from resid- ual stress and stress intensity range effects.
In a companion article (Ref. 12), the fatigue crack propagation behavior of stainless steel gas metal arc welds was in- vestigated using a conventional DK-in- creasing testing procedure. A series of stress ratios from 0.10 to 0.80 was investi-