Thermomechanical treatment of steel for high strength
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A thermomechanical treatment with a 90% reduction below the recrystallization temperature of constructional steels with a broad range of stable austenite can develop σb=280 to 300 kg/sq.mm at δ5=6% which by far exceeds the properties of these steels after ordinary or stepped hardening followed by low tempering.
The strength of a steel thus treated increased with carbon up to 0.5% C; at still higher carbon, the tensile strength fell and the fracture was semi-brittle or brittle.
The optimum ratio of strength and ductility after such a treatment was achieved in these steels after tempering at 100°C; at higher tempering, both strength and ductility decreased.
An increase of the deformation from 50 to 90% during a themomechanical treatment in the zone of stable austenite below recrystallization increased the strength considerably.
A 50% reduction in the same conditions improved greatly the ductility but rather little the strength. The advantage of such a deformation is that it can be carried out at relatively high temperatures, even above recrystallization.
Thermomechanical processing increased the brittle strength of steel.
Such processing is accompanied by refinement of the martensite platelets and by a substantial decrease of the areas of coherent scattering (size of blocks). The structural refinement was apparently one of the factors which increased the resistance to plastic deformation as well as the brittle strength.
KeywordsAustenite Tensile Strength Martensite Brittle Recrystallization
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