Early Martensitic Transformation in a 0.74C–1.15Mn–1.08Cr High Carbon Steel

Abstract

The martensitic transformation in a high carbon steel was studied by a new experimental approach focusing on the nucleation and growth as well as the variant pairing of the early-formed martensite. A mixed microstructure with tempered early-formed martensite and fresh later-formed martensite was achieved by a heat treatment with an isothermal hold below the martensite start temperature. In-situ high-energy X-ray diffraction showed no further transformation of austenite to ferrite/martensite during the isothermal hold. The tempered early-formed martensite was characterized with a combination of light optical microscopy and local tetragonality determination by electron backscatter diffraction. The characterization allowed qualitative as well as quantitative analysis of the tempered early-formed martensite with regard to the prior austenite grain boundaries (PAGB) and variant pairing. The early-formed martensite was shown to grow predominantly along the PAGBs and clustering was observed indicating an autocatalytic nucleation mechanism. The variant pairing of the early-formed martensite had a stronger plate character compared to the later-formed martensite.

Evolution of Martensite Tetragonality in High-Carbon Steels Revealed by In Situ High-Energy X-Ray Diffraction

“…This increase in carbon content in the interface would locally decrease Ms resulting in a slower transformation, which is even more impactful since clear indications for autocatalysis have been found in the C74 steel. [51] The largest difference in the final phase fraction between Rietveld and dilatometry is observed for 0.5 C/s (C54), Figures 5(a) and (c). This difference can be explained by the stronger autotempering with loss of tetragonality (Figure 3(a)) and the resulting volume loss, which is not corrected for in the dilatometer phase fraction determination.…”

The Crystal Structure of As-quenched Fe–C Martensite

“…e.g. [7][8][9] This might be described as dilute tetragonality as not all the carbon atoms are concentrated on the specific sub-lattice. This conclusion has also been reached after Rietveld refinement but sometimes in surprising situations.…”

“…At first sight it would appear to be an ideal tool for interpreting Fe-C martensitic structures, and has been applied in many cases for this purpose, (e.g. [6][7][8][9] but there are complications that render its applicability doubtful.…”

Geometry of Butterfly Martensite in Fe-18Ni-0.7Cr-0.5C Alloy

“…They have proposed a model in which butterfly-pairing is formed in the early stage of the transformation. Kohne et al 7) investigated the change in preferential variant pairing tendency upon the progress of the transformation in high-carbon steels. They reported that V1/V16 pairs are preferentially formed in the early stage of the transformation.…”

“…A characteristic martensitic microstructure is formed when crystallographically equivalent martensitic variants are connected with each other [2][3][4][5][6] . The combinations of these martensitic variants are not random, but specific variant pairs are connected at high frequencies 4,5,[7][8][9][10][11][12] . In this paper, such junction of two variants is called variant pairing.…”