Weldability of austenitic Fe-Mn-Al-C lightweight steels

Abstract

Recently, the interest for lightweight materials has increased due to the issue related to energy efficiency and environmental regulation. As a result, a number of researches are conducted to develop competitive advanced high-strength steels (AHSS) (i.e. dual-phase and transformation-induced plasticity steels). However, there is always a problem of structural stiffness weakening for improving the strength of steels, which causes degradation of crashworthiness. Accordingly, in order to solve such problem, lightweight steel that possesses low density with high strength attracts high attention these days.

Especially, austenitic lightweight steel has remarkable mechanical strengths with high density reduction, and several studies were conducted to investigate the effect of alloying elements on the mechanical strength of austenitic lightweight steel. However, the study about the weldability of austenitic lightweight steel is very insufficient, in spite of its importance. Generally, welding processes involve rapid heating and cooling, so heat affected zone (HAZ), numerous defects, concentrated stress and residual stress are inevitably induced around the welds. Since such phenomena usually produce the unfavorable microstructural evolution, which can cause the local brittleness in the HAZ or solidification cracking in the welds, followed by mechanical property degradation of lightweight steel. Thus, the research about the weldability of austenitic lightweight steels must be conducted to resolve such problems, so related things were investigated in this study.

First of all, hot ductility tests were conducted on austenitic Fe-30Mn-9Al-0.9C steel in order to understand the welding characteristics (cracking resistance) of the heat affected zone (HAZ). During the on-heating thermal cycle, ductility was altered by a decrease in microband induced plasticity (MBIP) (softening) and an increase in dynamic recrystallization (DRX) (softening) as the temperature increased. Specifically, in the range of 800-900℃, ductility was fairly degraded because neither MBIP nor DRX took place. During the on-cooling thermal cycle, ductility behavior was changed by both softening and hardening factors, including formation of brittle (Fe, Mn)3Al intermetallic compounds with grain growth and re-solidified grain boundaries.

In addition, the investigation of cracking in the heat-affected zone (HAZ) of lightweight steel was conducted using Varestraint testing, particularly focusing on the κ-carbide-related behavior. Moreover, the effect of Al (κ-carbide creation promoting element) and Cr (κ-carbide creation suppressing element) addition on crack susceptibility was elucidated to clearly confirm the effect of κ-carbide. Varestraint tests showed that cracks were mainly found in the 750°C to 900°C peak HAZ, 1000 to 1500 μm away from the fusion line. The cracks occurred at the austenite/austenite and austenite/ferrite grain boundaries. The microstructural and hardness evaluation revealed that the cracking along the grain boundaries at the 750°C to 900°C peak HAZ was induced by not only relative weakening of grain boundaries due to grain interior κ-carbide but also by grain boundary embrittlement due to grain boundary κ-carbide. The effect of κ-carbide on the crack susceptibility of HAZ in lightweight Fe-Mn-Al-C steel was further clarified by investigating the cases of Al (κ-carbide creation promoting element) and Cr (κ-carbide creation suppressing element) addition.

Moreover, the effect of alloying elements (i.e. C, Nb, V and Cr) on the hot cracking susceptibility in the fusion zone was investigated. In order to achieve this, Varestraint test was conducted for the steels used in this study, and their solidification cracking susceptibility was checked. According to the result of Varestraint test, the solidification cracking susceptibility was higher in order of high C, Nb/V, and Cr added steels than the case of standard. The primary cause for the increased solidification cracking susceptibility of the lightweight steel consisting various alloying elements was the eutectic phases created at final solidification region of grain boundaries in the weld. The eutectic phase of high C and Cr added steels was M3C/γ eutectic, and that of Nb/V added steel was MC/γ eutectic. Thus, it was confirmed that the alloying elements used in this study have bad effect on the solidification cracking susceptibility.