Influence of deformation-induced martensite on fatigue crack propagation in 304-type steels
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INTRODUCTION
II.
EXPERIMENTAL PROCEDURE
MANY common
A. Materials
Z. MEI, Research Engineer, and J.W. MORRIS, Jr., Professor, are with the Center for Advanced Materials, Lawrence Berkeley Laboratory, and the Department of Materials Science and Mineral Engineering, University of California at Berkeley, Berkeley, CA 94720. Manuscript submitted November 28, 1989.
The materials used in this study were commercial grade AISI 304L and 304LN stainless steels. Their chemical compositions are listed in Table I. They differ primarily in nitrogen content, which is higher in 304LN. Increasing nitrogen raises the yield strength at low temperature (Table II) and stabilizes the austenite phase. Plates of 304L stainless steel were processed in two different ways. The basic material was annealed at 1050 ~ for 1 hour, followed by a water quench to create a homogeneous austenite phase. Some of the plates were then rolled 13 pet at liquid nitrogen temperature (LNT) to form a two-phase mixture of austenite and martensite. Plates of 304LN were used in the as-received (annealed and quenched) condition. The average grain sizes of 304L and 304LN were 100 and 70 tzm, respectively. Optical micrographs of the annealed 304L and cold-rolled 304L are shown in Figure 1. X-ray diffraction tests confirmed that the annealed 304L and as-received 304LN were essentially pure austenite (y), while the cold-rolled 304L was about 50 pet austenite and 50 pct martensite ( a ' ) with a small admixture of the hexagonal close-packed e-martensite phase. The tensile properties of the annealed and as-received 304LN were measured and are listed in Table II. I~:] The martensite start temperatures on cooling (Ms) and deformation (Ma) were estimated from the empirical formulas given in References 13 and 14 and are: for 304LN, Ms < 0 K, Md < 255 Kand, for 304L, M, < 38 K, Ma < 299 K. The thermal stability of the annealed 304L steel was confirmed by soaking in liquid helium for more than 2 hours; no a ' or e martensite was detected by X-ray diffraction. The volume fractions of martensite developed during tensile strain at room temperature (RT) and LNT were measured as a function of strain by X-ray diffraction. [~21 The results are plotted in Figure 2. Despite the similarity of the computed Md temperatures, the austenite phase in 304L is very much less stable on mechanical deformation than that in 304LN.
austenitic stainless steels are mechanically metastable at low temperature and undergo spontaneous transformations when subjected to sufficient stress or strain. The martensitic transformation causes a shape deformation that is evidenced by surface relief effects ti] and a volume change that is dependent on the composition and is ~ 2 pct in 304-type stainless steels, t2,3) The strain that occurs ahead of the crack tip during fatigue crack growth in a metastable material induces a partial transformation to martensite, which alters both the microstructure and the stress state at the crack tip, and should, therefore, change the fatigue crack growth rate. It is necessar
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