Nitrogen strengthening of interstitial-free steel by nitriding in potassium nitrate salt bath

Nitriding of interstitial-free steel in the potassium nitrate salt bath at 592 or 650 °C has been studied by using hardness and tensile testing. Pronounced nitrogen solid-solution-strengthening of the steel has been achieved. After nitriding at 592 °C for 27 h, a fully recrystallized specimen shows a tensile strength of 1066 MPa, which is increased by 263% compared to that of non-nitrided steel, and the hardness increases by up to 443% accompanied by a decrease in ductility. The salt can be decomposed to generate nitrogen, NO, and NO2 gases, which are thought to give rise to nitriding of the steel samples.