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Author: Ali Kosari Mehr
To harden martensitic steel, one should deliberately select the strategy for thermal treatment and austenitization such that the enlargement of the grains, decarburization, and presence of the austenitic phase can be prevented [1]. In a reported case in which not only was a primary thermal treatment used, but also the samples were subjected to quench hardening and post-annealing, the hardness of martensitic steel reached the value of 700-800 HV (i.e., a threefold increase) [2].
Moreover, corrosion current hinges upon the amount of the chromium content and the extent to which carbides dissolve into the steel. Hence, to increase the corrosion resistance of steel, one should prevent the precipitation of chromium carbide and facilitate the dissolution of carbides and chromium in the texture. In this regard, conspicuous success in improving the corrosion resistance of martensitic steel has not been reported by these means, and all the efforts have been undertaken not to allow a further decrease in the corrosion resistance [3].
For improvement in mechanical properties and corrosion performance, it should be accommodated that the carbon is not easily dissolved in the steel at temperatures below 650°C and that chromium tends to form carbide. The precipitation of carbides leads to sensitization and a decrease in the amount of chromium in the vicinity of grain boundaries, culminating in the decreased fatigue and corrosion resistance [4]. To avoid the precipitation of carbides, one can use thermal treatment and carburization at low temperatures utilizing plasma; plasma keeps the chromium in a solid solution state and helps the carbon atoms to be incorporated into the surface of the austenitic steel to a depth of few microns [5]. According to the literature, by these means, the hardness and the corrosion resistance have ultimately improved by 500% (i.e., 1000-1200 HV) and 2000%, respectively [5, 6].
by utilizing plasma nitriding, the formation of a solid solution state full of nitrogen – as a result nitrides – is expected, which could improve the mechanical properties and corrosion performance of steel. However, the precipitation of CrN – accordingly a decrease in the contribution of chromium in solid solution structure – is a factor decreasing the corrosion resistance of the steel [7, 8]. In this connection, it has been reported that the hardness and corrosion resistance can be increased by 600% (i.e., 1500 HV) and 200%, respectively [9–11].
In the following article, the hardening and anti-corrosive effects of heat treatment processes will be compared with those of thin-film processes.
