Effects of dynamic strain aging on environment-assisted cracking of low alloy pressure vessel and piping steels

Strain aging occurs in alloys containing solutes that segregate strongly to dislocations. In low-alloy steels (LAS) static strain aging is a process where aging takes place after pre-straining and results in return of Lüders strain. Dynamic strain aging (DSA) is a process where aging is sufficiently rapid to occur during straining and it produces inhomogeneous deformation, serrated yielding. DSA occurs at temperatures of 150-350°C, where stress-strain curves show serrations, being most marked at 250°C depending on strain rate. The mechanism of DSA in LAS is explained based on the interstitial (N, C, H) interactions with dislocations and their immobilization. The important role of the accumulation of vacancies, which are diffusion vehicles for the solute atoms, is also considered in case of EAC. In general, activation energy of DSA in LAS is equal to that of N/C diffusion in ferrite. The effects of DSA of LAS are evaluated based on peaks in UTS, hardness and strain hardening rate in the DSA temperature range and minimum of ductility (A, Z) and temperature of peaks decreases with decreasing strain rate. DSA causes an increase in the ductile-to-brittle transition temperature following plastic deformation in the DSA temperature range, lowering of the ductile fracture resistance (decrease of tearing modulus) at temperatures within the DSA temperature range, as well as ductile crack instabilities (crack jumps) in the DSA temperature range, decreases low-cycle fatigue resistance and the susceptibility of LAS to EAC coincides with DSA behavior, in terms of temperature and strain rate ranges. The present knowledge of DSA on above mentioned properties of LAS is reviewed and DSA susceptibility of some pressure vessel steels is demonstrated by internal friction method and slow-strain rate tensile testing.