Post‐oxidizing effects on surface characteristics of salt bath nitrocarburized AISI 02 tool steel type

Abstract

The effect of post-oxidizing treatment on the characteristics of modified surface layers produced by salt bath nitrocarburizing on the industrial American Iron and Steel Institute (AISI) 02 tool steel types is investigated. Nitrocarburizing treatment is performed for 6 h and 8 h at 570• C and post-oxidizing treatment for 30, 60 and 90 min at 520 • C, using argon-steam mixture. Formed layers are characterized by their basic properties such as thickness layer, depth, surface hardness and wear resistance. Detailed estimation of the modified metal surface quality, in terms of chemical composition, formed phases, microstructures and diffusion mechanisms are performed by metallographic techniques, EDX, X-ray diffraction, scanning electron microscopy (SEM) and glow discharge optical electron spectroscopy (GDOES). The corrosion resistance was investigated in 0.4 M H 2 SO 4 solutions, using steady-state electrochemical polarization methods. The obtained results revealed the existence of a superficial oxide layer which consists of magnetite (Fe 3 O 4 ) and hematite (Fe 2 O 3 ) and the presence of an ε-phase associated with a small amount of γ -phase. Important improvements in wear, microhardness and corrosion resistance occur after these treatments and it is specifically concluded that the sole application of a nitrocarburizing treatment does not significantly ameliorate the corrosion resistance of the as-received steel. In fact, post-oxidation treatment contributes to increase corrosion resistance by forming a dense magnetite layer and at the same time, it partially covers the compound layer pores.

Investigation of Microstructure, Nanohardness and Corrosion Resistance for Oxi-Nitrocarburized Low Carbon Steel

“…Through the redistribution of nitrogen during the oxidation step, the γ -phase grew and the thickness of the compound layer of the steam-specimen increased. In this manner, the volume fraction of the ε-phase in the compound layer decreased for the steam-specimen [12,13]. Figure 3b,e shows elemental maps of oxygen for each specimen.…”

“…On the other hand, the nanohardness of the oxide layer was 216 Hv, which is much lower than that of the oxide layer of the air-specimen. In general, the hardness of the surface hardened layer decreases with the formation of the oxide layer due to the low hardness of the oxide in comparison to the nitride [4,13]. Furthermore, the nanohardness of the oxide layer in the For the air-specimen, the nanohardness of the oxide layer was 621 Hv, and those of the ε and γ -phases were 793 Hv and 589 Hv, respectively.…”

“…Additionally, the oxidation time for the steam-specimen is also longer than that for the air-specimen. These two factors contributed that the porosity in the ε-phase adjacent to the oxide increased further, and a larger amount of oxygen penetration occurred in the steam-specimen due to the increase in porosity [1,13]. Therefore, the oxide layer could be formed deeper into the steam-specimen.…”

“…During the oxidation process, iron-based nitride like the ε-phase will decompose into Fe and nitrogen gas, and the oxide layer could be grown continuously by supplying Fe from the ε-phase [12]. As the Fe cations migrate from the ε-phase to the oxide layer, a vacancy is generated in the ε-phase adjacent to the oxide layer [13]. In the case of the steam-specimen, the oxygen ratio in the oxidation process was high, which promoted oxide formation.…”

“…On the other hand, the steam-specimen revealed a compound layer with a thickness of approximately 11 μm, in which relatively small and isolated grains of ε-phase existed in the γ'-phase. Generally, the oxidation process provided an annealing effect on the compound layer, which caused the ε-phase to decompose to the γ'-phase and the nitrogen to redistribute [1,12,13]. Through the redistribution of nitrogen during the oxidation step, the γ'-phase grew and the thickness of the compound layer of the steam-specimen increased.…”