High tensile spring steel - is subjected to repeated surface induction heating to obtain extremely small grain size and increase fatigue strength

摘要

The fine grain size of steel decreases progressively from its core to its surface, this structure being obtd. by brief repeated high frequency induction heating of the surface of the steel so through heating is gradually achieved. The steel is then quenched, followed by tempering using high frequency induction heating; and the steel pref. contains 0.3% carbon. Each brief heating cycle pref. raises the surface of the steel to above the Ac3 transformation point and is followed by a pause in which the surface drops below the Ar1 transformation point and the cycle is repeated at least twice so the steel is heated right through, or nearly right through, to the centre of the core. The treatment increases the strength, toughness, and fatigue strength of the steel, so it is suitable for coil springs, torsion bars etc.