| A new development of ultra-high strength stainless steel, maraging stainless steel OOCr11Ni11MoTi, were the preferred key structural material of high strength stainless spring wire, using on aviation. Ultra-strength steel always kept serious notch sensitivity, trending to embrittlement. So improving the strength and toughness of ultra-high strength maraging stainless steel which kept a notch was important on optimizing the process. Focusing on this theme, notch effect of maraging stainless steel OOCr11Ni11MoTi were studied on this article. The research contained:First, the forged steel billet on different heat treatment was analyzed, studying the effect of heat treatment on microstructure and properties of steel. And based on description above, the whole tension wire aged at different temperature was analyzed, studying the effect of aging process on microstructure and properties of steel. Then, effect of aging temperature on notched tensile strength of ultra-high strength maraging stainless steel were studied by introducing of adjusting the aging temperature, while the effect of cold deformation. Considering the effect of deformation on improving the strength and toughness of steel, the effect of deformation process before aging on ultra-high strength maraging stainless steel were studied at last. The main results were as follows:Considering the grain refinement and toughness improvement, solid solution at900℃was recommended. For the whole tension wire, brittle fracture was introduced when aging at440℃. Strength decreased sharply when aging at520℃. Reduction of area was high which proved better plasticity when aging at500℃, though strength was a little lower than that of peak aging at480℃. Moreover, aging at500℃was propitious to the precipitation of reverted austenite. Thereby increasing the capacity of resistance to crack nucleation and propagation to a large extent which leaded material to better toughness, while retaining ultra-high strength. So aging at the temperature in a range of500℃could make materials obtain the optimal obdurability.Cold deformation before aging has reinforced the role of the aging dynamics, accelerating the aging process, and the effect of strengthening was obvious. The smooth and notch tension strength of cool drawing were higher than the material in solid solution state. But the effect of strengthening of cold deformation decreased as the aging temperature increased. For material in solid solution state, the smooth tensile strength of aging at500℃when has overaged was slightly lower than that of aging at480℃which was at peak aging temperature, but the notch tension strength was obviously higher. So aging at500℃was recommended for the optimization of material in solid solution state. For material in cold drawing state, though, the smooth tensile strength of aging at480℃approximated to aging at450℃, the notch tensile strength increased significantly. So aging at480℃was recommended for the optimization of material in cold drawing state. These take advantage for enhancing the reliability of safety of the material.No matter which state the steel introduced, at the same process, the peak aging temperature of notch tension was higher than smooth tension. It probably related to the special status and fracture behavior of notch in material. Material in hot rolling has low notch sensitivity after solution. But the effect of hot deformation has been cleared. So strength was relatively low. On the double effects of hot and cold deformation, material in direct cold drawing developed a large increasement on dislocation density. And the grains were more elongated and refined, the interface area increased more, the effect of reinforced aging process was further, which leading a highest strength. Material in cold drawing after solid solution which resulted in the effect of hot rolling clear away let the strength slightly decrease, comparing with direct cold drawing state. But the material in this state obtained better toughness and lower brittle tendency of notch. So the treatment of drawing after solid solution obtained the optimal obdurability. |
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