Mechanism-Based Modeling for Low Cycle Fatigue of Cast Austenitic Steel
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ODUCTION
AUSTENITIC stainless steels are widely used to cast hot-end components of exhaust systems of internal combustion engines (ICE), especially when the gas temperature reaches above 900 C. Under such severe operating conditions, low cycle fatigue (LCF) and thermomechanical fatigue (TMF) are inevitably the major failure modes concerned in the component design. Life prediction for LCF and TMF of austenitic stainless steels is particularly challenging, because the damage process involves multiple deformation mechanisms such as plasticity, dynamic strain aging (DSA) from solute drag effect,[1] and creep, which result in a cyclically non-stable behavior in combination with oxidation.[2–4] Many conventional fatigue analysis methods have been proposed such as strain-based Coffin–Manson relation[5] and energy-based Morrow equation,[6] but they do not delineate these mechanisms explicitly.
XIJIA WU is with the National Research Council Canada, Ottawa, ON, K1A 0R6, Canada. Contact e-mail: [email protected] GUANGCHUN QUAN is with Wescast Industries Inc., Savannah Oaks Dr., Brantford, ON, N3T 5V7, Canada, and also with Tenneco Automotive Operating Company Inc., 3901 Willis Rd, Grass Lake, MI, 49240. CLAYTON SLOSS is with Wescast Industries Inc. Manuscript submitted December 12, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
In order to delineate the contributions of material-intrinsic (deformation mechanisms) and extrinsic (environmental effects) factors in LCF and TMF, it is advantageous to use mechanism-based constitutive laws to evaluate the mechanism strain responsible for the respective damage, instead of the lump-sum viscoplastic strain obtained with the unified constitutive theory.[7] The reason is the following, as commonly understood by physical metallurgy: fatigue damage is caused by alternating plasticity via irreversible slip via dislocation glide[8–11]; while creep by dislocation climb via vacancy flow releases dislocations piled-up at obstacles in glide,[12,13] which tends to ease the fatigue damage; and on the other hand, vacancy flow and grain boundary sliding promotes cavitation and void growth along grain boundaries,[14,15] which lead to the formation of internally distributed damage. These different mechanisms result in either transgranular or intergranular failure. Apparently, the unified viscoplastic strain is not suitable for correlation with failure modes since multiple failure modes may occur with a single-value inelastic strain, depending on whether it is accumulated by plasticity or creep. The ICFT has been successfully applied to ductile cast iron for the above purposes.[16,17] In this study, the mechanism-based approach—ICFT—is used to analyze the cyclic behavior of 1.4848 cast austenitic steel. LCF experiments were conducted on this alloy at different strain rates from 2 9 104 to 2 9 102 s1 in the temperature range from room
temperature (RT) to 1173 K (900 C). The deformation and damage mechanisms involved are discussed in relation to the observed behavior. Life of LCF is evaluated with
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