A Fe-Cr-C steel based on gradient scale precipitation hardening: Hardening and toughening mechanism of multistage heat treatment

• Published in: Journal of alloys and compounds 2023-06, Vol.946, p.169355, Article 169355
• Main Authors: Su, Shengrui, Song, Renbo, Quan, Shuyi, Li, Jiakang, Wang, Yongjin, Cai, Changhong, Wen, Erding
• Format: Article
• Language: English
• Subjects: Fe-Cr-C steel; Impact toughness; Martensite; Multistage heat treatment; Precipitation hardening; Retained austenite
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2023.169355

In order to eliminate the common carbide networks in Fe-Cr-C tool steel, a low-cost Fe-Cr-C steel with high hardness and high toughness was designed. Correspondingly, a multistage heat treatment process is also designed. Multistage heat treatment includes homogenization, spheroidizing annealing, quenching and low temperature tempering. Compared to AISI D2 steel (58–62HRC, 24.3–27.9 J/cm2), the designed steel maintains high hardness (60.5HRC) and higher unnotched impact toughness (46.2 J/cm2). The hardening and toughening mechanisms of the designed Fe-Cr-C steel were studied by mechanical properties testing, multi-scale characterization and thermodynamic calculation. Experimental results show that the complete austenite phase region of Fe-Cr-C steel is the key factor to eliminate carbide networks by homogenization treatment. The gradient scale M7C3 produced by the spheroidizing annealing and quenching enhances the hardening effect through refining prior austenite grains size, promoting martensite nucleation, and limiting martensite growth. During the low temperature tempering, volume fraction reduction of retained austenite weakens the transformation induced plasticity (TRIP) effect and softens martensite, which is beneficial to improve the toughness. Finally, the decomposition of twinning martensite caused by nanoscale θ-Fe3C precipitation and the precipitation hardening of nanoscale θ-Fe3C play important role in maintaining hardness and improving toughness. •A novel low-cost Fe-Cr-C steel and its multi-stage heat treatment were designed to achieve high hardness and toughness.•The carbide networks are completely eliminated, and the carbides with a diffuse and gradient scale are obtained.•After tempering, the weaken of TRIP effect and martensite softening are important reasons for the increase of toughness.•The decomposition of martensite mechanical twins due to the precipitation behavior of θ-Fe3C is also improve the toughness.