Abstract

Reversed austenite transformation behavior plays a significant role in determining the microstructure and mechanical properties of heat affected zones of steels, involving the nucleation and growth of reversed austenite. Confocal Laser Scanning Microscope (CLSM) was used in the present work to in situ observe the reversed austenite transformation by simulating welding thermal cycles for advance 5Mn steels. No thermal inertia was found on cooling process after temperature reached the peak temperature of 1320 °C. Therefore, too large grain was not generated in coarse-grained heat-affected zone (CGHAZ). The pre-existing film retained austenite in base metal and acted as additional favorable nucleation sites for reversed austenite during the thermal cycle. A much great nucleation number led to the finer grain in the fine-grained heat-affected zone (FGHAZ). The continuous cooling transformation for CGHAZ and FGHAZ revealed that the martensite was the main transformed product. Martensite transformation temperature (Tm) was higher in FGHAZ than in CGHAZ. Martensite transformation rate was higher in FGHAZ than in CGHAZ, which is due to the different grain size and assumed atom (Mn and C) segregation. Consequently, the softer martensite was measured in CGHAZ than in FGHAZ. Although 10~11% austenite retained in FGHAZ, the possible Transformation Induced Plasticity (TRIP) effect at −60 °C test temperature may lower the impact toughness to some degree. Therefore, the mean absorbed energy of 31, 39 and 42 J in CGHAZ and 56, 45 and 36 J in FGHAZ were exhibited at the same welding heat input. The more stable retained austenite was speculated to improve impact toughness in heat-affected zone (HAZ). For these 5Mn steels, reversed austenite plays a significant role in affecting impact toughness of heat-affected zones more than grain size.

Highlights

  • Grain size plays a significant role in determining the strength and toughness of materials

  • It is reported that increasing the austenite grain size shifts the continuous cooling transformation diagram to longer reaction time and increases the possibility of martensite formation

  • Large austenite grain size is of particular concern during welding where the heat-affected zone (HAZ) experiences rapid thermal

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Summary

Introduction

Grain size plays a significant role in determining the strength and toughness of materials. Austenite tends to grow thermally and the austenite grain size provides the initial condition for the subsequent phase transformation during cooling and affects the final microstructure and resulting mechanical properties [1,2]. They reduce the austenite growth rate and result in finer grain at room temperature in CGHAZ. Mn enriches the reversed austenite and enhances its stability, improving the cryogenic toughness. In this present work, 5% Mn cryogenic steel was experimented. The effect of different austenite grain size on continuous cooling transformation temperature, and martensite transformation and reversed austenite transformation. The role of microstructure in influencing the cryogenic toughness of heat-affected zone

Experimental
Nucleation and Growth of Reversed Austenite
Reversed
Austenite
Simulated heat-affected zone diagram
Martensite
Martensite Transformation and Retained Austenite
Crystallographic analysis in sample
The and retained
Hardness
Impact Toughness and the Microstructure Effect
Conclusions
Full Text
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