Effects of subzero treatments on the mechanical properties of the sae 4140 steel

Abstract

In this study, effects of holding in liquid nitrogen and cryogenic methods applied onto the SAE 4140 steel were empirically investigated. In this context, austempering, holding in liquid nitrogen, the conventional cryogenic process and tempering processes were applied in different orders and combinations. The temperature of the samples to be tempered in liquid nitrogen was decreased from room temperature to -196 °C at the average rate of 1.6 °C/min and held at this temperature for 24 h. The temperature of the samples to undergo the traditional cryogenic process was decreased from room temperature to -140 °C at a constant rate of 2 °C/min and held at this temperature for 24 h. After a microhardness analysis and tensile tests, it was observed that the toughness was increased without the hardness being changed during both cooling processes carried out after austempering processes. The most obvious improvement in terms of toughness was observed on the samples, to which austempering was applied, exibiting an increase of 32.83 %, followed by the samples undergoing the cryogenic process and, lastly, the ones undergoing the tempering process. The largest increase in the tensile strength also occurred in the samples subjected to austempering, exhibiting a 66 N/mm2 improvement. In terms of the properties investigated during this study, it was seen that the process of holding steel in liquid nitrogen could consitute an alternative to the cryogenic process using different rates.