Effect of high-temperature surface hardening of metallic materials on their dimensional stability

Surface hardening thermo-chemical treatments provide metallic alloys with very hard corrosion and wear resistant coatings. As the layers obtained are so thin that dissuade from any machining operation after the treatment, it is useful to evaluate possible dimensional changes. This work studies the dimensional stability of alloy Ti6A14V after gas nitriding at 1100 °C and of superalloy MA956 after surface oxidation at the same temperature. For reference purposes, an analysis is also made of the deformations that appear on steel after conventional pack carburising treatment. In this study flat test specimens of 3 mm thickness were used, each of which had central drilled holes surrounded by six smaller outlying drilled holes. A calculation program allows the determination of changes in the drilled hole diameters and in the position of their centres as a consequence of the treatment.