THE EFFECT OF PHASE TRANSFORMATION ON THE FATIGUE CRACK GROWTH RATE OF AUSTENITIC STAINLESS STEELS TESTED IN ARGON AND HYDROGEN ATMOSPHERES

This work is a comparative study of the fatigue crack growth rate (FCGR) of two austenitic stainless steels, AISI 301 and AISI 302. The objective was to determine how differences in the austenitic stabilities ((gamma)(--->)(alpha)') of the two steels would affect their respective FCGR's. Tests were run in argon, hydrogen, and a smaller number in air. In addition to determining the FCGR's, a number of other quantities were also measured using various techniques. The plastic zone size of some specimens was determined by using a microhardness tester and electron channeling data. The residual stress around the crack tip was measured using strain gages. The volume fraction of martensite was determined by measuring the magnetic permeability and by using quantitative metallography. The phase present along the path of the fatigue crack was determined by using glancing incidence electron diffraction.;Results from the work show that the relatively unstable AISI 301 stainless steel has a FCGR approximately 50 percent lower than AISI 302 stainless steel when tested in argon or air at a low mean stress, less than 66 MPa. At higher mean stresses the FCGR's are equal. The plastic zone sizes of AISI 301 specimens are generally smaller than for AISI 302. The cause for the lower FCGR observed in the AISI 301 seems to be the residual compressive stresses that develop around the crack tip as a result of the martensite formation. Testing in hydrogen caused the FCGR of both steels to greatly increase with the AISI 301 being affected to a much larger extent. Glancing incidence electron diffraction showed that the fatigue crack preferentially followed the (alpha)' when tested in hydrogen. This indicates that the (alpha)' is being embrittled and is thereby causing the observed increase in FCGR.