A study has been made of nitriding of interstitial free (IF) steel in the potassium-nitrate salt bath at temperatures ranging from 400 to 650°C. The salt is decomposed to generate nitrogen and oxygen. Nitrogen diffuses into steel, or steel is nitrided, while oxygen reacts on steel surface to form the oxide scale. The oxide scale thickness is much smaller than the nitriding thickness. Most of nitrogen resides in steel as a form of interstitial solid-solution. For nitriding at higher temperatures, nitride precipitates (gЈ-Fe 4 N and zЈ-Fe 2 N) exist mostly in grain boundaries and partly in grains of the steel. The nitrate nitriding gives rise to much larger nitriding depth than other nitriding methods at similar nitriding temperature and time. The nitrate nitriding of steel substantially increase its tensile strength as well as hardness, e.g., an IF steel specimen nitrided at 650°C for 1.5 h shows a tensile strength of 916 MPa, which is 2.2 times higher than that of non-nitrided IF steel specimen, and an elongation of 20 % at 70°C. Severe serrations are observed in flow curves of nitrided steel specimens, mainly due to dynamic strain aging that occurs because of interaction between dissolved nitrogen and moving dislocations. The effective diffusion coefficient of nitrogen D N obtained from the nitriding data, D N ϭD 0 exp(ϪQ/RT ) with D 0 ϭ3.789ϫ10 Ϫ7 m 2 · s Ϫ1 and Qϭ76.62 kJ mol
Ϫ1, is approximately the same as that for diffusion of nitrogen in a-iron. (5) Reaction (2) has been observed at temperatures ranging from 517 to 537°C. 28) In addition, potassium nitrate may also directly decompose into potassium oxide 30,34,35) according to the following reaction (6) or (7) (7) Thus, the thermal decomposition of KNO 3 on heating can liberate nascent nitrogen and the nascent nitrogen can diffuse into the steel coupon. Therefore, there is a possibility of nitriding in the liquid KNO 3 salt bath under ambient pressure.Interstitial-free (IF) steels are ultra low carbon and nitrogen steels manufactured by adding carbide, nitride and sulphide-forming elements such as titanium and aluminum to form precipitates such as TiC, TiN, AlN, and TiS, leading to an elimination of the nitrogen, carbon and sulphur elements from the solid solution like nearly pure iron. For this reason, in order to investigate the effect of solid-solution strengthening by diffusion of nitrogen to the interstitial sites of a-Fe lattice when KNO 3 salt bath is used as nitriding of iron and steels, we chose to use a TiϩAl stabilized IF steel, in which Ti and Al scavenge both carbon and nitrogen.The purpose of this study is to investigate nitriding behavior of IF steel in the KNO 3 salt-bath, with emphasis on solid-solution hardening and nitriding kinetics.
Experimental ProceduresA hot-rolled IF steel sheet of 4 mm in thickness was used as the initial material. The chemical composition of the steel is given in Table 1. The sheet was cold-rolled by 10, 30, and 75 % in thickness under lubrication.Extra pure potassium nitrate (99.0 wt% min. purity) with impurit...
