Abstract
For the first time, the influence of gas mixture on first damage resistance of a plasma nitrided DIN 18MnCrSiMo6-4 bainitic steel was investigated. Samples were nitrided at 500 °C with three different N2-H2 gas mixtures, containing 5, 24, and 76 vol.% N2. Samples were characterized concerning the resulting roughness, microstructure, compound layer’s phase composition, residual stresses in the diffusion zone, and surface hardness. Tribological ball-on-flat tests were carried out in reciprocal mode using zirconia as ball material for friction coefficient and the compound layer resistance until the first damage. The test results were evaluated statistically by analysis of variance (ANOVA). As the amount of nitrogen in the gas mixture decreases, the ε-Fe2-3(C)N content in the compound layer decreases. A γ’-Fe4N monophasic compound layer was achieved at 5 vol.% N2 gas mixture. The diffusion zone as expected presented compressive residual stresses with the highest values near the surface. In the tribological tests, better results were obtained for 5 and 24 vol.% N2 in the gas mixture as higher amounts of γ’-Fe4N were formed. The 76 vol.% N2 gas mixture led to a brittle behavior, due to the biphasic compound layer (γ’-Fe4N and ε-Fe2-3(C)N) with a predominant content of ε-Fe2-3(C)N.
Highlights
Continuous cooling bainitic steels has an increasing use in industrial processes owing to its excellent combination of yield strength and toughness[1,2,3,4]
The microstructure of the compound layer and of the diffusion zone, as well as the combination with the steel core properties are designed to Materials Research accomplish with the desired final properties[19,20,21]
The importance of studying the influence of different nitrogen composition in the gas mixture is due to the variation of Fe2-3N and γ’-Fe4N formed in the compound layer, and these, in turn, have a direct influence in the first damage resistance
Summary
Continuous cooling bainitic steels has an increasing use in industrial processes owing to its excellent combination of yield strength and toughness[1,2,3,4]. In previous articles the authors investigated the microstructure and residual stresses of plasma nitrided DIN 18MnCrSiMo6-4 continuous cooling bainitic steel[22,23], which showed excellent results concerning case depth, surface hardness, and compressive residual stresses in the diffusion zone. The influence of different plasma nitriding temperatures on the surface modifications and reciprocating dry sliding wear properties of the continuous cooling bainitic steel was investigated[21]. The purpose of the present article is to discuss, for the first time, the influence of different plasma nitriding gas mixtures on the surface modifications and the compound layer resistance until the first damage (microcracks formation or delamination of the surface layer) on reciprocating dry sliding of the continuous cooling bainitic steel. The importance of studying the influence of different nitrogen composition in the gas mixture is due to the variation of Fe2-3N and γ’-Fe4N formed in the compound layer, and these, in turn, have a direct influence in the first damage resistance
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