Effect of forging sequence and heat treatment on microstructure of high-duty power-plant shaft made of Cr-Mo ultra-high strength steel

Abstract

The paper presents results of modeling and testing of a heavy weight part made of Cr-Mo, which was V-modified ultra-high strength steel grade AISI 4140, processed through a novel open-die forging schedule and two alternative routes of two-stage heat treatment cycles designed to meet requirements of high-duty components for energy sector. Taking advantage of unconventional forging conditions based on assumption of large feed and reduction ratios and the modification of chemical composition better control of austenite grain was achieved to minimize abnormal grain growth and/or strain uniformity problems. With aid of Finite Element Modeling of multi-stage sequence of upsetting and cogging strain distribution was optimized so as to minimize strain fluctuations on the length to range 2.2?2.7,and correlated with microstructure produced at every major stage on the large cross-sections of the shaft. Designed with aid of finite element method processing cycles was verified in full-scale physical modeling with use of 16 ton forging ingot, including two alternative quenching strategies: oil vs. water spray and air. Examination of material in as-forged, normalized and heat-treated condition was carried out to observe response of hot-worked material and the effect of cooling conditions on microstructure during final heat treatment. As observed, employing large feed ratios on cogging and varied cooling allowed suppress detrimental effect of inevitable abnormal grain growth which resulted in 1-2 ASTM grain in as-forged conditions to reach 6 ASTM in normalized and 8/9 ASTM after quenching in oil and water spray, respectively, which allows producing after tempering, correspondingly, 44?48 and 85?122 J/cm2 V-notch impact strength in the critical area of the forged shaft.