Critical Assessment 34: Are χ (Hägg), η and ϵ carbides transition-phases relative to cementite in steels?

Abstract

Hägg carbide (χ) is, during the tempering of carbon-rich martensite, referred to as a transition carbide which eventually gives way to cementite. However, there are Fe-C binary phase diagrams estimated using thermodynamic data, that define a low-temperature phase field where a mixture of Hägg carbide and ferrite is more stable than that of cementite and ferrite. In this scenario, it may be cementite which is the transition carbide. Evidence is presented here that the predominance of Hägg carbide over cementite in the circumstances described is unlikely to be correct based on historical and new observations. Literature data are also interpreted to show that η and ε-carbides are best regarded as transition-phases relative to mixtures of cementite and ferrite.

Nanophase iron carbides in fine‐grained rims in CM2 carbonaceous chondrites: Formation of organic material by Fischer–Tropsch catalysis in the solar nebula

“…2014; Bhadeshia et al. 2019). This situation is further exacerbated by the differences in the thermodynamic properties of nanoparticles compared with bulk materials that can change the stability fields of individual phases significantly (Navrotsky 2003; Navrotsky et al.…”

“…Based on the best current thermodynamic properties of Fe carbides (e.g., Bhadeshia et al. 2019), the thermal stability increases in the following sequence: η‐carbide (Fe 2 C) → χ‐carbide (Fe 2.2 C) → θ‐carbide (Fe 3 C ‐ cohenite). However, the presence of cohenite is not consistent with nebular formation conditions.…”

“…However, FTT experiments under solar nebular conditions (5 x 10 ‐4 atm, 473K, H 2 :CO = 250:1), using kamacite as the catalyst, result in the formation of ε‐carbide (Fe 2.4 C) rather than cohenite (Llorca and Casanova 1998), which argues against a nebular origin. But, according to Bhadeshia et al (2019), ε‐carbide (Fe 2.4 C) is probably a metastable transitional phase in the Fe‐C system and hence is metastable. Based on these data, it must be concluded that there are too many uncertainties in the behavior and stability of Fe,Ni carbide nanoparticles to make any definitive statement of the stability of cohenite under nebular conditions, particularly when the precursor phase may well have been nanophase Fe,Ni metal.…”

Effect of Cooling Path on Microstructures and Hardness of Hot-Stamped Steel

“…The tempering of martensite involves five different stages [16][17][18][19][20][21]: redistribution of carbon atoms, the precipitation of ηor ε-carbide, the formation of Hägg carbide, the decomposition of retained austenite, and the formation of cementite and recrystallization of ferrite. As for auto-tempering during a quenching process, the formation of carbides occurs immediately when austenite transforms to martensite.…”

“…Auto-tempering of martensite usually occurs immediately when martensite forms [16]. The tempering of martensite is accompanied by the formation of carbides [16][17][18][19][20][21], which consumes the carbon of the martensite matrix, leading to a decrease in effective carbon content [22] and dislocation density [23,24]. A lot of research has been conducted on the phase transformation of hot-stamped steels as well as the tempering of martensite under isothermal conditions.…”