The Role of Retained Austenite in Tempered Martensite Embrittlement of 4340 and 300-M Steels Investigated through Rapid Tempering

Tempered martensite embrittlement (TME) is investigated in two medium carbon, high strength steels, 4340 (low silicon) and 300-M (high silicon), via rapid (1, 10, or 100 s) and conventional (3600 s) tempering. Rapid tempering of 4340 diminishes the depth of the TME toughness trough, where improvements in impact toughness correspond to the suppression of retained austenite decomposition. In 300-M, retained austenite decomposition is suppressed to an even greater extent by rapid tempering. While toughness improves overall after rapid tempering, TME severity remains consistent in 300-M across the tempering conditions examined. Through interrupted tensile tests, it was found that the 300-M conditions that exhibit TME are associated with mechanically unstable retained austenite. Unstable retained austenite is shown to mechanically transform early in the deformation process, presumably resulting in fresh martensite adjacent to interlath cementite that ultimately contributes to TME. The present results emphasize the role of both the thermal decomposition and mechanical transformation of retained austenite in the manifestation of TME.

Rapid Thermal Processing to Enhance Steel Toughness

Quenching and Tempering (Q&T) has been utilized for decades to alter steel mechanical properties, particularly strength and toughness. While tempering typically increases toughness, a well-established phenomenon called tempered martensite embrittlement (TME) is known to occur during conventional Q&T. Here we show that short-time, rapid tempering can overcome TME to produce unprecedented property combinations that cannot be attained by conventional Q&T. Toughness is enhanced over 43% at a strength level of 1.7 GPa and strength is improved over 0.5 GPa at an impact toughness of 30 J. We also show that hardness and the tempering parameter (TP), developed by Holloman and Jaffe in 1945 and ubiquitous within the field, is insufficient for characterizing measured strengths, toughnesses, and microstructural conditions after rapid processing. Rapid tempering by energy-saving manufacturing processes like induction heating creates the opportunity for new Q&T steels for energy, defense, and transportation applications.

A Novel Heat Treatment Line for Processing of Tailored Small Batch Steels

This study describes design and construction of a novel flexible heat treatment line for processing customer-oriented small batch steels. The induction heater (600 kW) developed is suitable for the sheet thickness in the range 3.2 30 mm and the width of 85 1250 mm. Sheets are fed using an electrical motor (1.5 kW) and a chain drive, the speed being in the range 0.3 7 m/min, depending on the power and the sheet dimensions. At this study, 4.5 (WR-1) and 10 mm (WR-2) thick wear resistant steels were tempered at different peak temperatures to compare the effect of rapid tempering on mechanical properties. Results showed that the heat treatment line is capable of producing tempered steel grades with adequate properties at industrial product rate. For example, 4.5 thick WR-1 tempered at 550 oC provided a yield strength (YS) over 1000 MPa with minimum bending radius of 6 mm (in the delivered condition YS = 1605 MPa and Rmin = 12). Tempering of WR-1 at 700 oC provided YS of 762 MPa and Rmin of 1 mm. Results were similar between two test materials, but the enhancement in bendability was slightly more effective with the thinner sheet.