Mössbauer Characteristics of ε, χ, and θ Iron Carbides

Abstract

The Mössbauer effect in ⁵⁷Fe has been used to study carbide phases appearing during the tempering process of an alloy steel containing 1.33 wt.% Si, 1.13% wt. Mn, and 0.38 Wt.% C. Carbides were chemically extracted from the samples which had been isothermally tempered for 2 h at various temperatures. Carbides obtained from samples tempered at 300° and 400°C were identified as ε and χ phases, respectively. The ε carbide has an isomer shift (I.S.) of 0.18 mm/sec relative to α iron and an effective hyperfine field (H_n) of 162 kOe at 300°K. The χ carbide has an I.S. of 0.28 mm/sec relative to α iron and H_n of 179 kOe at 300°K. Carbides precipitated at and above 500°C were found to have the orthormbic cementite structure, with a composition of (Fe_1−xMn_x)₃C. As the tempering temperature increases from 500° to 700°C the Curie temperature changes from 440° to 270°K, and x increases from 0.02 to 0.10. Curie points and extrapolated values of H_n(0) were found for all the carbides in question. The early stages of tempering in which transition carbides appear are explained as being controlled by the elastic strain. The elastic binding energy between an interstitial carbon atom and a substitutional silicon atom can then be estimated. Other phases which were found in the chemically extracted samples after tempering the alloy at a low temperature, were identified as oxyhydeoxides and iron gel. In a sample tempered at 700°C the carbide was detected in the steel matrix and compared with the same carbide after chemical extraction.