Abstract
Low-alloy 42CrMo4 steels were studied by 57Fe Mössbauer spectroscopy (MS), X-ray diffractometry (XRD), and Energy Dispersive X-ray Spectroscopy (EDS) measurements. The investigations were performed on metallographic samples, which were subjected to a series of successive grinding and polishing with a progressively finer grit. Conversion X-ray Mössbauer spectroscopy (CXMS) was used to determine the occurrence of austenite in steel samples. It is a unique method detecting the austenite content very sensitively. Six samples with different surface preparation were investigated, starting with 4.8% of austenite on an as-cut sample, and a large decrease in the retained austenite to 2.6% was observed after the first grinding of a hardened cut sample. Additionally, an unexpectedly large decrease in the austenite content to 2.3% was found due to the final polishing. A second time applied successive grinding and polishing of all samples resulted in identical austenite content determined by CXMS of approx. 5%, which proved the applicability of the CXMS method. Generally, the result calls attention to the importance of preparation of metallurgical samples by grinding and polishing where the results can vary significantly on the level of surface processing.
Highlights
Accepted: 24 December 2021Mechanical processing of the steel samples prepared for metallography analysis may influence the surface properties which may have consequences for the bulk sample characterization
A question arises, how do these modified surfaces differ from the bulk, and how reliable are our measurements by Conversion X-ray Mössbauer spectroscopy (CXMS), as the conversion X-rays cannot be effectively detected from a surface deeper than 20 μm, which is commonly considered as a bulk material
We focused on the determination of the percentage amount of retained austenite in the steel as a consequence of successive grinding and polishing
Summary
Accepted: 24 December 2021Mechanical processing of the steel samples prepared for metallography analysis may influence the surface properties which may have consequences for the bulk sample characterization. The influence arises from two main contributors that are undesirable: (1) heat treatment and (2) mechanical deformation. Both of these contributors can induce a noticeable amount of phase transformations and/or other modifications. For phase transformations occurring in low-alloyed steels, it is well known that austenite can transform into martensite/ferrite as a consequence of mechanically induced stress, strain or plastic deformation because austenite is the metastable phase at low temperatures [1]. Finishing the surfaces by mechanical polishing can lead to other unintentional induced effects. We should note that a steel surface modification by grinding and polishing in a controlled
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