Strength-Toughness Balance and Hydrogen Embrittlement Susceptibility of a Precipitation-Strengthened Steel Adopted Tempering Process

Two steels with different nickel (Ni) content were investigated to reveal the role of Ni on strength-toughness balance and hydrogen embrittlement susceptibility. Although they were similar in microstructure, i.e., nano-particles were precipitated on martensitic laths, different mechanical behaviors were exhibited. After tempering, the yield strength of 3.5 Ni steel reached a peak at 500 °C, while that of 2.5 Ni steel kept a downward trend, indicating that precipitation strengthening was significant in 3.5 Ni steel. Combined with thermodynamic and kinetic analyses, it was shown that when the Ni content increased, the rich-copper (Cu) precipitation transformation driving force would be enhanced and the reverse of austenite transformation accelerated to improve its stability. Moreover, the increase of Ni content also induced the increase in high-angle grain boundaries (HAGBs), which could inhibit crack propagation. Under the comprehensive effects of strengthening and ductility mechanism, 3.5 Ni steel exhibited excellent cryogenic toughness. Although it was not possible to obtain the ideal balancing of strength-toughness for the steel with lower Ni content, its hydrogen embrittlement susceptibility is satisfying. The results showed that the increase of grain boundary density caused by the grain refinement of 2.5 Ni steel is the key factor for its lower hydrogen embrittlement sensitivity index. Moreover, with the reduction of Ni content, the decrease of HAGBs and the increase in Σ11 boundary were conducive to reducing hydrogen-assisted cracking, while the residual Fe3C in 3.5 Ni steel would deteriorate the hydrogen embrittlement resistance