Structure and properties of a microduplex maraging steel
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MICRODUPLEX structure in a steel is a fine-scale two-phase mixture of austenite (y) and martensite or ferrite (0'). This type of microstructure can be established in alloys of suitable composition through either thermal processing or thermomechanical treatment. The microduplex structure is intriguing from the perspective of alloy design since it may offer a means for improving the toughness of high strength steels. A suitable distribution of relatively soft austenite in the ferrite matrix may serve to blunt a propagating crack, hence providing a higher resistance to fracture. Alternately, the stability of the austenite may be adjusted so that it transforms to martensite during deformation, hence incorporating the TRIP (Transformation Induced Plasticity) mechanism. Prior research on the strength-toughness properties of microduplex steels has yielded mixed results. Gibson, et al,! and Floreen, et al2 achieved a microduplex structure in an Fe-26Cr-6.5Ni steel through therrnomechanical processing. The alloy had a good combination of mechanical properties at room temperature, and exhibited superplastic behavior at high temperature. The microduplex structure seemed effective in lowering the ductile-brittle transition temperature (DBTT) of the alloy. More recently, Jin -Hwang, and Morrts" found that the introduction of a small amount of retained austenite (3 ~ 5 pct) into a fine-grained Fe8Ni-2Mn-0.25Ti alloy led to a substantial decrease in the DBTT. However, the austenite addition did not impart a detectable increase in toughness when the alloy was tested above the DBTT. A similar result was obtained with a fine-grained Fe-12Ni-0.25Ti alloy:" the introduction of austenite did not improve the fracture toughness of alloys tested above the DBTT. A someS. lIN, D. HUANG, and 1. W. MORRIS, lr. are Assistant Research Engineer, Postgraduate Research Engineer and Associate Professor, respectively, Department of Materials Science and Engineering, University of California, Berkeley, CA 94720. Manuscript submitted June 5, 1975. METALLURGICAL TRANSACTIONS A
what different conclusion was reached by Antolovich, et al,5 who studied a 300 grade maraging steel with a microduplex structure achieved through repeated thermal cycling. They found a significant increase in ductility and toughness associated with the retention of a significant fraction of austenite (20 to 80 pct) in the microduplex structure. However, this increase in toughness was accompanied by a loss in alloy yield strength. Since the toughness of a steel is normally a sharply decreasing function of its yield strength, it is difficult to assess the beneficial effect of austenite without considering the strength variation. The purposes of this research were to investigate the structure and mechanical behavior of high strength mtcroduplex steel and to clarify the role of retained austenite on the fracture toughness. A commercial 250 grade 18Ni maraging steel was chosen since it is easily processed to a microduplex structure while retaining high strength (yield strength ~ 1
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