A study of the effect of precipitated austenite on the fracture of a ferritic cryogenic steel
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I.
INTRODUCTION
IT is
desirable to use ferritic steels at low temperatures because of their low cost. Ferritic steels also offer a good strength/toughness combination provided that the service temperature is above the ductile-to-brittle transition temperature (/'8). A great deal of research has been done with the goal of decreasing the Ts. The research has followed two paths in the attempt to lower the TB of ferritic Ni steel. The first involves refining the grain size of the steel. ~ This approach has been used to process a 12Ni steel so that it is ductile down to 4 K. 5 The second line of research, and the topic of this paper, involves the addition of precipitated austenite to the ferritic matrix. 6 It is generally agreed that the precipitated austenite must be thermally stable to have a beneficial influence on cryogenic properties. 7 However, several hypotheses have been proposed to explain how the precipitated austenite lowers the/'8. These hypotheses include: Austenite acting as a crack blunter. 8.9.~0This hypothesis is based on the assumption that a crack propagating through the steel would blunt in the ductile fcc austenite. However, Fultz and Morris, 11and Kim and Schwartz 12have shown that the austenite is mechanically unstable and transforms to martensite in front of the crack tip. Recent work by Kim, et al.'3 has shown that the austenite is in fact brittle and could not blunt the crack. Austenite scavenging deleterious elements. TM The austenite acts as a sink for grain boundary pollutants, such as P and S, and the toughness is increased by eliminating intergranular fracture. Crystallographic cleavage impairment m o d e l . 6 The austenite transforms to a variant of martensite that corresponds to the highly strained region in front of the crack tip. It is probable that this new variant of martensite is different from the matrix '5 and thus creates a high angle boundary. Upon brittle fracture the fracture surface would consist of smaller facets due to the presence of the mechanically transformed austenite. This smaller facet size has been seen qualita-
tively but has not been quantitatively measured. By creating more high angle boundaries the initiation of cleavage cracks becomes more difficult and thus lowers the TB much in the same way that refining the grain size accomplishes the same result. This work was designed to investigate the function that austenite has in the fracture of a ferritic steel. This investigation has a threefold purpose: (1) to measure quantitatively the facet size on the brittle fracture surfaces of a ferritic steel that contains austenite and compare these resuits to specimens that contain no austenite, ( 2 ) t o investigate whether the presence of austenite alone will lower the TB, and (3) to determine the effect the tempering treatment has on the yield strength. An alloy of nominal 8Ni-2Mn-0.1Ti steel was chosen for this investigation because previous work 3 on this same alloy showed that an intercritical temper would precipitate out an even distribution of austenite. A low
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