GB756364A - Improvements in or relating to steel shot for use in blast cleaning, peening, polishing and the like - Google Patents

Abstract

Steel shot is made by melting iron or steel in a furnace and maintaining it in an oxidized state throughout the heat, boiling the molten bath to remove gases and metallic oxide inclusions, keeping the silicon content below 0,10 per cent until the final silicon alloying, de-oxidizing and alloying the bath to 1,15 to 1,40 per cent carbon, 1,00 to 1,50 per cent manganese, 0,40 to 0,90 per cent silicon, 0 to 5 per cent chromium, 0 to 0,4 per cent vanadium, 0 to 2,5 per cent nickel and 0 to 1,0 per cent copper, the remainder being iron and the usual impurities, pouring the metal, disintegrating it into particles, initially quenching the particles in a mild quench medium and then heat-treating the particles to produce a tempered martensite micro-structure. The preferred method of performing the invention is as follows: steel scrap with a low sulphur and phosphorus content is melted in a high temperature, e.g. electric arc, furnace and then ore (iron oxide) is added in an amount approximately 1 per cent by weight of the total charge. The silicon and manganese contents are reduced respectively to below 0,10 per cent and 0,20 per cent and the bath then boils, after which the slag composition is controlled by the addition, in the case of basic furnaces, of ferro-manganese, mill scale, burned lime or fluorspar, and in the case of acid furnaces, ferromanganese, mill-scale, silica-sand or lime. At the pouring temperature additions are made to alloy and deoxidize the bath; ferrosilicon and ferro-manganese are added as well as a strong deoxidizing agent such as calcium-silicon-manganese, aluminium, titanium or vanadium. The molten metal is preferably disintegrated as described in Specification 638,826, but may also be performed by other methods such as directing a solid stream of water from a nozzle against the falling stream of metal, and the disintegrated steel particles are caught in a bath of water, preferably at 120 DEG to 160 DEG F., which serves to initially quench the shot. The entire disintegrating apparatus may be surrounded by an inert gas such as nitrogen. The shot is given the tempered martensite micro-structure by heating at 1650 DEG to 1750 DEG F. in a rotary retort-type furnace fed with raw natural gas or other carburizing type gas, quenching in warm water or other mild quench medium such as oil or a soluble oil-water mixture and finally tempering at 900 DEG to 1050 DEG F.