Nowadays, there are many reference data on the ball-bearing steels pertaining to the melting, refining, forging, hot-working and heat-treatment, but few reports on the raw material for melting.
In the SKF ball-bearing steels, the sponge iron and returned scrap have been used as the raw materials, while the iron sand, the commercial and returned scrap have been used generally in Japanese ball-bearing steels.
Therefore, it is very interesting to ascertain what the sponge iron has the effect on an improvement in the durability of ball-bearing steels.
In this report, the austenitizing behaviour and durability of ball-bearing steels, made from the sponge iron in a basic electric arc furnace, were studied by means of chemical anaalysis, point-counting, lineal analysis, the Rockwell hardness test, static torsion test and thrusttype life test.
The results obtained were as follows:
(1) The minor impurities like copper, molibdenum and tin were decreased considerably in steels, in which more than 50% of the sponge iron was charged in the ratio of more than 50% of the raw material.
(2) As the charged ratio of the sponge iron was increased, the contents of stringer-type and oxide-type non-metallic inclusions showed a rising tendency, but that of Al2O3-type non-metallic inclusions showed no change.
Total content of non-metallic inclusions was increased with the enhancement of the charged sponge iron.
(3) In each specimen the mean diameters of the spherodized carbide were between 0.84μ and 1.04μ, and this size appeared to be larger considerably than that of the SKF and the New Departure ball-bearings.
(4) The behaviour of carbide solution into austenite showed no difference in each specimen except No.2, of which carbide did not go to solution easily at an austenitizing temperature of 840° on account of 0.2% vanadium addition.
(5) Three groups were divided by the results of life test. Especially the best group was No.3 (50% sponge iron+50% returned scrap) and No.6 (all sponge iron), the second group was No.2 (50% scrap+30% returned scrap+20% iron sand+0.2% vanadium), No.4, (50% iron sand+50% sponge iron) and No.5 (25% returned scrap+75% sponge iron), and the last group was No.1 (50% scrap+30% returned scrap+20% iron sand), which showed the commercial charging ratio of the raw materials.