Optimization Design of Quenching and Tempering Parameters for Crankshaft Based on Response Surface Methodology.

Existing optimization research on the crankshaft heat treatment process is mostly based on one-sided considerations, and less consideration is given to the matching of multiple process parameters, leading to irrational designs of heat treatment. To address this problem, this work investigates the influence mechanisms of cooling speed, tempering temperature, and holding time on the performance evaluation indexes of the straightness, residual stress, and martensite content of a crankshaft based on the response surface method. The results showed that the order of influence of these three different process parameters on the performance evaluation index was cooling speed > holding time > tempering temperature, and the order of influence on the performance evaluation indexes under multifactorial process parameters was cooling speed-holding time > cooling speed-tempering temperature > holding time-tempering temperature. The optimal process parameters were a cooling speed of 1.4 times the cooling oil, a tempering temperature of 555 °C, and a holding time of 6 h, with the straightness of the crankshaft reduced by 9.9%, the surface stress increased by 6.7%, and the martensitic content increased by 7.2% after the process optimization. This work can provide new clues for optimizing the heat treatment process parameters of crankshafts.

Simultaneous Enhancement of Strength and Sulfide Stress Cracking Resistance of Hot-Rolled Pressure Vessel Steel Q345 via a Quenching and Tempering Treatment.

Sulfide stress cracking (SSC) failure is a main concern for the pressure vessel steel Q345 used in harsh sour oil and gas environments containing hydrogen sulfide (H2S). Methods used to improve the strength of steel usually decrease their SSC resistance. In this work, a quenching and tempering (Q&T) processing method is proposed to provide higher strength combined with better SSC resistance for hot-rolled Q345 pressure vessel steel. Compared to the initial hot-rolled plates having a yield strength (YS) of ~372 MPa, the Q&T counterparts had a YS of ~463 MPa, achieving a remarkable improvement in the strength level. Meanwhile, there was a resulting SSC failure in the initial hot-rolled plates, which was not present in the Q&T counterparts. The SSC failure was not only determined by the strength. The carbon-rich zone, residual stress, and sensitive hardness in the banded structure largely determined the susceptibility to SSC failure. The mechanism of the property amelioration might be ascribed to microstructural modification by the Q&T processing. This work provides an approach to develop improved strength grades of SSC-resistant pressure vessel steels.