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

Abstract

The martensitic transformation was studied by in situ and ex situ experiments in two high-carbon, 0.54 and 0.74 wt pct C, steels applying three different cooling rates, 15 °C/s, 5 °C/s, and 0.5 °C/s, in the temperature range around Ms, to improve the understanding of the evolution of martensite tetragonality c/a and phase fraction formed during the transformation. The combination of in situ high-energy X-ray diffraction during controlled cooling and spatially resolved tetragonality c/a determination by electron backscatter diffraction pattern matching was used to study the transformation behavior. The cooling rate and the different Ms for the steels had a clear impact on the martensitic transformation with a decrease in average tetragonality due to stronger autotempering for a decreasing cooling rate and higher Ms. A slower cooling rate also resulted in a lower fraction of martensite at room temperature, but with an increase in fraction of autotempered martensite. Additionally, a heterogeneous distribution of martensite tetragonality was observed for all cooling rates.

HRTEM Investigations on the Substructures of the Quenched Pearlite and Martensite

“…As the cooling rate increases, the temperature range between Ms and Mf for a given steel increases. Thus, the earlier martensite transforms into pearlite, and the later martensite remains after quenching. − …”

The Crystal Structure of As-quenched Fe–C Martensite

“…The question is, however, still open to debate. There is some more recent support for this viewpoint from the in-situ neutron diffraction work of Wang et al 19) on a 0.4%C-1%Cr steel and also in-situ synchrotron experiments of Kohne et al 20) The structure after initial cooling below the Ms temperature was identified as tetragonal but this rapidly changed to cubic on subsequent cooling or holding. 4.…”

Behavioural Study of High Carbon Steel Material in Hot and Cold Working Media: A Review

“…Compared to their as-cast state, it has been found that both the matrix and the composite significantly improved their mechanical and tribological characteristics under T6 heat treatment conditions. Ramakoteswara et al [24], to better understand the evolution of martensite tetragonality c/a and phase fraction generated during the transformation, in situ and ex-situ studies were performed on two high-carbon sheets of steel, 0.54 and 0.74 wt pct C, using three different cooling rates: 15 °C/s, 5 °C/s, and 0.5 °C/s. The transformation behaviour was investigated using a combination of in situ high-energy X-ray diffraction under controlled cooling and spatially resolved tetragonality c/a determination by electron backscatter diffraction pattern matching.…”