Influence of Alloying Elements on the Toughness of Low Alloy Martensitic High Strength Steels

摘要

The study examined the effects of various elements on the notch bend fracture toughness and Charpy impact behavior of a 0.35%C, 3%Ni, Cr-Mo-V martensitic steel having a room temperature yield strength of approximately 160-180 ksi. A classical approach was used in the design of alloys which permitted a direct evaluation of single element effects rather than interactions. The elements C, Mn, Si, Cr, and Mo raised both the notch bend fracture mode transition temperature and the Charpy V-notch transition temperature (100% fibrosity criterion). In amounts above that required for deoxidation and grain refinement aluminum degraded the transition temperature and toughness slightly. A minimum toughness occurred at a vanadium content of 0.1%. Over the entire range of compositions examined (1.26 - 6.23%) nickel decreased the transition temperature and improved toughness at the lower test temperatures. Charpy shelf energy, C sub v (max), and fracture toughness K sub max (at 75F), did not correlate well with work hardening exponent (n). Good agreement was obtained however when these parameters were plotted versus true fracture strain. At -321F, toughness was essentially fracture strain independent suggesting that a critical strain criterion based on fracture strain is valid only when fracture is fibrous. A comparison was made of measured K sub 1c calculated from tensile data. The results indicated that the increased contribution on non-ductile fracture which accompanies increases in strength and/or decreases in test temperature in low alloy steels can lead to large errors in the predicted toughness. (Author)