Hybrid structure of white layer in high carbon steel \u2013 Formation mechanism and its properties

Rumana Hossain,Evelien Witteveen,Rian Dippenaar,B Gangadhara Prusty,Amborish Banerjee,Bill Joe,Veena Sahajwalla,Farshid Pahlevani

doi:10.1038/s41598-017-13749-7

Rumana Hossain, Evelien Witteveen + Show 6 more

Open Access

https://doi.org/10.1038/s41598-017-13749-7

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Abstract

This study identifies for the first time, the hybrid structure of the white layer in high carbon steel and describes its formation mechanism and properties. The so-called ‘white layer’ in steel forms during high strain rate deformation and appears featureless under optical microscopy. While many researchers have investigated the formation of the white layer, there has been no definitive study, nor is there sufficient evidence to fully explain the formation, structure and properties of the layer. In this study, the formation, morphology and mechanical properties of the white layer was determined following impact testing, using a combination of optical and SE- microscopy, HR-EBSD, TKD and TEM as well as nano-indentation hardness measurements and FE modelling. The phase transformation and recrystallization within and near the white layer was also investigated. The microstructure of the steel in the white layer consisted of nano-sized grains of martensite. A very thin layer of austenite with nano sized grains was identified within the white layer by HR-EBSD techniques, the presence of which is attributed to a thermally-induced reverse phase transformation. Overall, the combination of phase transformations, strain hardening and grain refinement led to a hybrid structure and an increase in hardness of the white layer.

Highlights

Steel that has been subjected to high strain rates and/or impact deformation often displays a thin, so-called ‘white layer’ that appears featureless in optical microscopy
The structure of the white layer and adjacent area in high carbon steel seems to consist of different layers of untempered martensite, over-tempered martensite and bulk material[11]
The microstructural response of these steels to severe plastic deformation down to the nano-metre level, we used a variety of characterization techniques including optical microscopy, secondary electron microscope (SEM), X-ray diffraction (XRD), high resolution electron backscattering diffraction (HR EBSD), transmission Kikuchi diffraction (TKD), transmission electron microscopy (TEM) and focused ion beam (FIB) sectioning

Summary

Introduction

Steel that has been subjected to high strain rates and/or impact deformation often displays a thin, so-called ‘white layer’ that appears featureless in optical microscopy. Base/bulk material adjacent to the white layer appears to be stress induced martensite as indicated by the red arrow (Fig. 4a).

Results

Conclusion