Sulfide Stability, Void Nucleation and the Toughness of Ultra High Strength Steels
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SULFIDE STABILITY, VOID NUCLEATION AND THE TOUGHNESS OF ULTRA HIGH STRENGTH STEELS W. M.Garrison, Jr. J. L.Maloney Department of Metallurgical Engineering and Materials Science Carnegie Mellon University Pittsburgh, PA 15213 1. Introduction
The upper shelf fracture toughness of ultra high strength steels is dependent on both the microstructure, which is determined by composition and heat treatment, and on the inclusions present in the steel. The inclusions In ultra high strength steels are typically oxides and sulfides [1]. In most ultra high strength steels the sulfides are manganese sulfides, although depending on the composition of the steel and the melt practice used, other sulfides are found, such as chromium sulfide, calcium sulfide and lanthanum oxy-sulfide [2]. If the inclusions can be regarded as pre-existing voids then the inclusion volume fraction and spacing appear to be sufficient to characterize the inclusion population from the standpoint of fracture toughness [3,4]. The purpose of this paper is to discuss results which show sulfur can be gettered as particles which are much more resistant to void nucleation than manganese sulfides and that this increased resistance to void nucleation can result in vastly improved upper shelf fracture toughness. In particular, when HY180 steel contains manganese sulfides the fracture toughness is about 250 MPa-F-m but when the sulfur is gettered as particles containing titanium, carbon and sulfur the fracture toughness of HY180 steel will approach 550 MPafj-i. These particles, believed to be titanium carbosulfides, are much more resistant to void nucleation than manganese sulfides and this increased resistance to void nucleation appears to be the reason for the improved fracture toughness. 11.Initial Results
The ultra high strength steel HY1 80 has a nominal composition (in wt%) of O.1OC/IONi/8Co/2Cr/1Mo and is normally heat treated by austenitizing, quenching and tempering for 5 hours at 510 0C. Speich et al. [5] have shown that after this tempering treatment the intralath carbides are fine M2 C carbides where M is chromium and molybdenum. Speich et al. [5] argued convincingly that the excellent toughness of this alloy after this tempering treatment is due, at least in part, to the absence of coarse intralath cementite. However, while the microstructure of this alloy given the above tempering treatment is inherently resistant to fracture, heats of this alloy can vary markedly in toughness. In particular one heat of HY1 80 steel, produced some years ago, has an upper shelf fracture toughness substantially higher than the toughness of other heats of this steel produced at the same time. The unusual toughness of this heat had never been explained. This work began [6] by comparing the mechanical properties and microstructure of that heat to those of another heat made at the same time and by the same process. The chemistries of the two heats are given in Table 1. The chemistries of the two heats are similar except that heat 1, the heat characterised by an unusually hig
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