Iron-based electrode materials for spark restoration of parts

Abstract

The composition of iron-based electrode material for the spark restoration of steel parts is optimized. The material permits depositing coatings twice as thick as those made of sormite. Varying the content of chromium carbide, one can change coating hardness from 3.9 to 6.8 GPa. The wear resistance of coatings is increased by 20 to 35%.Austenitic chromium-nickel steels, stellite, and sormite are used to restore machinery parts with spark deposition. Chromium-nickel steel promotes good deposition resulting in coatings up to 1 mm thick. The steel is austenitic and does not harden (rapid quenching takes place in spark deposition) and thus plastic deformation may occur when a coating is formed. Such coatings have insignificant hardness but are usually used in friction conditions intended for hard, tempered steel.Stellite coatings have good hardness (40-46 HRC) and thickness (to 1 mm), but the base metal for these alloys is cobalt, which is very expensive. The most acceptable material for depositing restoration coatings is sormite (stellite-like material). Its base metal is iron, which contains (wt.%) 1.5-5.5 C, 27-32 Cr, 1-4 Si, 1-2 Mn, 1-5 Ni, 0.1-1 Mo, and 0.2-0.4 W [1]. Sormite is cheaper than stellite, also ensures good deposition (coating thickness to 1 mm), and has high hardness (44-52 HRC).This material has been developed for arc or oxyacetylene deposit welding with flux or gas protection against oxidation. Spark deposition differs from deposit welding by discreteness, large temperature gradients, and no need for oxidation protection in air. Therefore, the composition of electrode material should be adjusted to such conditions. Mangan as an alloy deoxidizer in spark deposition does not give good results (steel G13 is also austenitic and should seemingly promote proper deposition, but this is not actually so and the coating surface is dark because of extensive oxidation). Hence, only silicon should be used as a deoxidizer since it forms protective glassy films [2].Tempering at high temperatures makes steels containing chromium, nickel, and silicon brittle. Sormite is also among such materials. Temper brittleness may be avoided by either rapid quenching or addition of a small amount of Mo (0.2-0.4%) or W (0.5-0.7%). Steel cannot be rapidly quenched in deposit welding since heat is generated continuously and the electrode moves slowly over the surface. Hence, molybdenum or tungsten is introduced into sormite. In spark discharge, heat is generated during 0.2-0.4 µsec, which is only 2-4% of the total treatment time at a pulse frequency of 100 Hz. The rest of the time goes for rapid quenching as metals have high heat conductivity. Temper brittleness is prevented by rapid quenching, and Mo or W is not required in electrode material.