Abstract

This experimental program investigated the effects of cryogenic treatment on the mechanical properties and microstructures of AISI 4340 steel and aluminum alloy 7075. Mechanical tests including fatigue resistance, impact energy and hardness were carried out on both alloys, after they had undergone various heat treating conditions and the results were compared. Experimental results showed that the hardness and fatigue resistance of the cryogenically treated 4340 steel, in general, were a little higher than those of the conventionally treated steel. However, the toughness of the cryogenically treated steel was lower when compared to that of the conventionally treated steel. The quantitative assessment of retained austenite using neutron diffraction technique was also explored. It is concluded that the transformation of retained austenite to martensite is the determining factor which improved hardness and fatigue resistance of the cryogenically treated 4340 steel. In aluminum alloy 7075, the results indicated that after cryogenic processing the fatigue resistance was lower than for conventionally solution and precipitation heat-treated samples. However, the hardness and toughness of the cryogenically treated samples were slightly higher in comparison to the conventional T6 treated specimens. The quantitative optical and electron microscopy has shown an increase in second phases after cryogenic treatment which was in agreement with x-ray diffraction results and electron dispersive spectroscopy (EDS)