Influence of Aluminum Alloying and Heating Rate on Austenite Formation in Low Carbon-Manganese Steels

Abstract

This investigation focuses on the austenite formation process during continuous heating, over a wide range of heating rates (0.05 to 20 K/s), in three low carbon-manganese steels alloyed with different levels of aluminum (0.02, 0.48, and 0.94, wt pct Al). High resolution dilatometry, combined with metallographic observations, was used to determine the starting (Ac 1 ) and finishing (Ac 3 ) temperatures of this transformation. It is shown that both the aluminum content and the applied heating rate have a strong influence on this process. During fast heating (>1 K/s), the pearlite phase present in the initial microstructure remains almost unaffected up to temperature Ac 1 . On the contrary, during slow heating, cementite lamellas inside pearlite partially dissolve, this dissolution effect being more pronounced for the lower carbon and higher aluminum content steels. The changes in the initial microstructure during slow heating affect the austenite nucleation and growth processes. Furthermore, in the aluminum alloyed steels, slow heating conditions shift the Ac 3 temperature to higher values. This shift is suggested to be due to aluminum partitioning from austenite to ferrite, which stabilizes ferrite and delays its transformation to higher temperatures. Thermodynamic calculations carried out with MTDATA software seem to support some of the experimental observations carried out under very low heating conditions close to equilibrium (0.05 K/s).

Demonstration of elemental partitioning during austenite formation in low-carbon aluminium alloyed steel

“…1a and b). Due to the low carbon and high Al content of the steel, the hardenability of austenite at temperatures close to Ac 3 is extremely low, resulting in the unavoidable formation of ferrite, even at cooling rates greater than 300 ºC/s (confirmed through dilatometry experiments [14]). Consequently, for this water-quenched specimen, the fine grained ferrite is presumed to be formed on quenching austenite grains formed at temperature and the large ferrite is presumed to be untransformed ferrite consistent with the volume fraction of ferrite and austenite at 970 °C based on dilatometry data.…”

“…The initial microstructure of the steel is ferritic with some small pearlite islands. For more details regarding the processing, microstructure and sample preparation readers are referred to other publications [11][12][13][14]. A high resolution dilatometer (Adamel Lhomargy DT1000) was employed to determine the critical transformation temperatures for the end of the austenite formation process (Ac 3 ); samples of 12 mm in length and 2 mm in diameter were continuously heated at 0.05 °C/s and 7 °C/s.…”

A review on the laser welding of coated 22MnB5 press-hardened steel and its impact on the production of tailor-welded blanks

“…According to Martin et al [62] aluminium is a strong ferrite stabiliser that shrinks the single-phase austenite region while promoting ferrite formation that consequently leads to the growth of the two-phase ferrite-austenite region on the Fe-Al phase diagram. In fact, it has been reported that an Al-content of greater than 1.2 wt-% completely inhibits the formation of the austenite phase [34].…”

Performance Evaluation and Application of the Laser Welded Joint with Filler Wire for Al-Si Coated Press-Hardened Steels

“…During laser welding, the Al-Si coating could be melted into the weld pool. As studied by Martin et al [23], the increase in the Al content could stabilize the ferrite phase, which consequently caused the formation of the bulk ferrite phase along the fusion line.…”