Effects of processing on the transverse fatigue properties of low-sulfur AISI 4140 steel
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I.
INTRODUCTION
IN most structural alloys, fatigue crack initiation is found to occur at a pore or nonmetallic inclusion. In evaluating the effects of inclusions on fatigue behavior, inclusion orientation, shape, size, and type are some of the variables that will influence fatigue crack initiation and, therefore, fatigue life. A. Microstructural Effects on Fatigue
Ransom m found that the elongation of nonmetallic inclusions during forming operations were responsible for lower fatigue properties in the transverse direction. His research indicated that the elimination of nonmetallic inclusions via vacuum remelting techniques resulted in a steel with transverse fatigue properties approaching those of the longitudinal orientation. Ransom concluded that "it would seem plausible that a change of the shape and possibly of the type of inclusions might yield improvements approaching those obtained b y elimination of inclusions entirely. For example, spherical inclusions of small size, uniformly dispersed, should not have as deleterious effects on the transverse fatigue properties as do the larger and elongated types." R a n s o m ' s postulations concerning distribution, shape, size, and type of inclusion are essentially correct and have been examined by other researchers, t2-8~ The shape of sulfide inclusions can be controlled by chemical, thermal, and thennomechanical treatments. Small additions of elements, such as calcium, can effectively harden the manganese sulfide solid solution so that it does not deform during working, t9j Tomita t1~ introduced a decreased hot-rolling reduction treatment SUNNIVA R. COLLINS, Graduate Student, and GARY M. M I C H A L , L T V Steel A s s o c i a t e P r o f e s s o r , are w i t h the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204. Manuscript submitted May 22, 1992. METALLURGICAL TRANSACTIONS A
combined with a short-time isothermal treatment above the Ms temperature to reduce sulfide elongation. According to Kiessling, tS] the type of inclusion most detrimental to the fatigue response of steels is A1203, followed by SiOz, then by TiN, with MnS being the least damaging. The A l e O 3 inclusions are angular, brittle, and hard. Cracks have been observed to nucleate from their sharp edges. The MnS inclusions, on the other hand, are highly deformable. No cracks develop at the MnS inclusion/steel interface during hot-working. Table I shows physical and mechanical properties for A1203 and MnS, with values for AISI 4140 steel for comparison. Brooksbank and Andrews [13"14'15] have shown that internal stresses can be generated around inclusions due to differences in the thermal expansion coefficients of the inclusion and the matrix. These stresses may be either positive in the steel, or a void may form at the interface, depending on whether the coefficient of thermal expansion of the inclusion material is smaller or greater than that of the matrix. Sulfides usually form voids at the inclusion/matrix interface, while oxides cause di
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