9NI steel is used in cryogenic conditions ferrite of low temperature steel, after adequate heat treatment, the impact toughness in 77K can greatly improve the. Heat treatment of 9NI steel of the existing three basic norms, namely normalizing + normalizing + tempering (NNT), quenching and tempering (QT), quenching and quenching + tempering (IHT).
Normalizing + normalizing + tempering (NNT)
The first normalized steel 9NI is 900 ℃ air cooling, heat preservation time according to the plate thickness, is about 2.4min/mm, but must guarantee the preservation time is greater than 15min, second times of normalizing at 790 ℃ in air cooling, heat preservation time and the first requirement, tempering at 550 - 580 ℃ air cooling or water cooling, heat insulation time and normalized processing requirements. Normalization is to refine austenite grain, austenite grain finer, heat treatment of 9NI steel after high strength, ductility, impact toughness is also higher. But if the normalizing temperature is too high, or keep long time at high temperature, the austenite grain growth of steel, will significantly reduce the impact toughness of the steel and the crack propagation energy and improve the ductile brittle transition temperature. Therefore, first normalizing temperature is higher than Ac 3 or Ac m, its purpose is to refine the grain. Second times of normalizing temperature slightly low, in order to make the phase transformation, obtain lath martensite structure, tempering is to obtain alpha phase and a small amount of carbon rich, nickel austenite
Quenching and tempering treatment (QT) and treatment of dual phase (IHT)
When quenching and tempering, quenching of 9NI steel but is generally 800 - 900 ℃, holding time enough to make it complete austenitizing, at 565 - 635 ℃ tempering, tempering time calculated according to 1.2min/mm, but at least more than 15min. Dual phase steel 9NI is processed in the quenched and tempered basis, in before tempering (alpha + beta) processing two phase region, the purpose is to make the organization more diffuse, uniform distribution. Microstructure of 9NI steel after quenching for low carbon lath martensite, after tempering get the matrix microstructure is tempered martensite and part of the rotary austenite. 9NI steel after (alpha + beta) processing two phase zone, its impact toughness than -196 ℃ tempering heat treatment increased by 0.5 - 1 times, or even better than that of 1Cr18Ni8Ti stainless steel, 2 times the intensity is 1Cr18Ni8Ti stainless steel, temper brittleness resistance is greatly improved.
In the existing three kinds of heat treatment, the double normalizing heat treatment of 9NI steel (NNT) and its low-temperature toughness is poor, quenching and tempering heat treatment, (alpha + beta) in dual phase region of low temperature toughness of treatment was the best.