Rapid Thermal Processing to Enhance Steel Toughness

V K Judge,K D Clarke,J G Speer,A J Clarke,K O Findley

doi:10.1038/s41598-017-18917-3

Abstract

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.

Highlights

Quenching and Tempering (Q&T) has been utilized for decades to alter steel mechanical properties, strength and toughness
Tempered martensite embrittlement leads steel manufacturers and end-users to avoid tempering in the affected time-temperature regime, thereby eliminating certain strength-toughness combinations that would be desirable if suitable heat treatments could be designed
Older work[21,22,23] began to explore rapid tempering within the tempered martensite embrittlement (TME) regime; the current study reveals the novel discovery of improved toughness for a given strength level compared to conventional tempering treatments

Summary

OPEN Rapid Thermal Processing to Enhance Steel Toughness

Quenching and Tempering (Q&T) has been utilized for decades to alter steel mechanical properties, strength and toughness. There is an established decrease in toughness in medium carbon, low-alloyed steels for tempering times of 1 h with increasing temperature between 200 and 400 °C2,4–13 This phenomenon, known as tempered martensite embrittlement (TME), has been attributed to a multitude of mechanisms, including: the thermal and mechanical decomposition of retained austenite[6,12], interlath brittle cementite formation[6], and cementite particle growth[7,13]. The observations of hardness and microstructure (retained austenite) in the present work demonstrate that different tempering mechanisms influence hardness and toughness, and these mechanisms do not obey the same relationship for time-temperature equivalence Such mechanisms include carbon diffusion in (body-centered cubic or tetragonal) martensite, carbon diffusion in (face-centered cubic) austenite, and iron self-diffusion.

Ni Cr Mo Nb

Methods

Findings

Additional Information