AISI0434 تاثير ريزساختار بر خواص مكانيكي فولاد

THE EFFECTS OF MICROSTRUCTURE ON MECHANICAL PROPERTIES OF 4340 STEEL

Bars of 4340 steel were heat treated to produce steel with different microstructures, i.e. tempered martensite, lower bainite and ferrite-bainite microstructures. Fatigue test results showed that the fatigue limit of steel with a ferrite-martensite microstructure is greater than the fatigue limit of the other two microstructures. The greater fatigue limit of ferrite-martensite steel is related to the initiation of secondary cracks from the original crack and a slower fatigue crack propagation rate in this microstructure, due to the ductile ferrite phase. Calculation of fracture toughness from impact energy showed that ferrite-bainite steel has a greater fracture toughness than the other two microstructures.