Heat treatment process

The residual stress value ( kg/mm2) 880 - after carburizing 900 salt bath heating, 260 degrees - isothermal 40 minutes 65 880 - after carburizing 900 degrees of salt bath quenching and isothermal 90 minutes - 260 degrees After carburizing 880-18 900 salt bath heating, 260 degrees 40 minutes, isothermal tempering 90 minutes - 260 degrees 38 can be seen from table 1 the test results of isothermal quenching of low temperature tempering process usually is higher than that in quenching surface residual compressive stress. Even in low temperature tempering after isothermal quenching, the surface residual compressive stress, also higher than low temperature tempering after quenching. So you can come to a conclusion that, the isothermal quenching after carburizing than usual carburizing and quenching and low temperature tempering to obtain higher surface residual compressive stress, from the surface residual compressive stress on fatigue resistance point of view, the beneficial effects of carburizing and isothermal quenching process is one of the effective methods to improve the fatigue strength of carburized parts. Why carburizing and quenching process can obtain the surface residual compressive stress? Why carburizing isothermal quenching can obtain larger surface residual compressive stress? Its main reason has two: one reason is that the surface of high carbon martensite volume is greater than the heart of low carbon martensite volume, surface volume expansion after quenching, and the core of low carbon martensite volume expansion is small, restricting the free expansion of surface, surface caused by tensile stress state of compression of the core. And another more important reason is the high carbon supercooled austenite changes to the martensitic transformation start temperature ( Ms） It is on low carbon content, supercooled austenite to martensite transformation start temperature ( Ms） Low. That is in the process of hardening is often the core first the heart volume expansion, caused martensitic transformation and reinforcement, and at the end of the surface is cooled to its corresponding martensite begins to shift point ( Ms） , it is still in the state of supercooled austenite, has good plasticity, not volume expansion of ecg of martensitic transformation serious suppression effect. Declining to make the surface temperature in the temperature of the quenching cooling down to the place, Ms） Martensitic transformation point below, surface, surface caused by the expansion of the volume. But the core now already into martensite and reinforcement, so the core of surface volume expansion will have a great suppression effect, make the surface residual compressive stress. And during the isothermal quenching after carburizing, when the temperature of the isothermal began in carburized layer of martensitic transformation temperature ( Ms） Above, the core of the martensitic transformation temperature ( Ms） Point under proper temperature isothermal quenching, more than the continuous cooling quenching can guarantee the order of the shift of the characteristics of ( Guarantee the surface martensitic transformation only produced in the cooling process after isothermal) 。 Of course after carburizing of isothermal temperature of the isothermal quenching and isothermal time on the size of the surface residual stress has a great influence. Someone has 35 simn2mov sample after carburizing steel at 260 ℃ and 320 ℃ isothermal surface residual stress tested after 40 minutes, the results such as table 2. From table 2 shows in 260 ℃ isothermal action than in the 320 ℃ isothermal surface residual stress were more than twice as high, visible surface residual stress state of carburizing isothermal temperature of the isothermal quenching is very sensitive. Isothermal temperature not only has influence to the surface residual compressive stress state, and isothermal time has a certain influence. Someone of 35 simn2v steel at 310 ℃ isothermal 2 minutes, 10 minutes, 90 minutes of residual stress tested. The residual compressive stress was 2 minutes 20 kg/mm, for - 10 minutes 60 kg/mm, for - 60 minutes 80 kg/mm, 60 minutes and then extend the isothermal time residual stress changes little. From the above discussion shows that the martensitic transformation of carburized layer and the core of the sequence has important influence on the size of the surface residual stress. Isothermal quenching after carburizing universal significance to further improve the fatigue life of parts. In addition can reduce surface martensite transition temperature ( Ms） Points of surface chemical heat treatment, such as carburizing, nitriding, cyanide, etc all have provided the condition for surface residual compressive stress caused by, nitride - such as the high carbon steel - Quenching process, due to the surface, it reduces the surface with the increase of nitrogen content in martensite begins to shift point ( Ms） After quenching, and won the high surface residual compressive stress fatigue life was improved. And as cyanide process than carburizing has higher fatigue strength and service life, is also due to the increase of nitrogen content can obtain higher than carburized surface residual compressive stress. In addition, from a reasonable distribution of the surface residual compressive stress point of view, a single surface strengthening technology is not easy to obtain ideal surface residual compressive stress distribution, and the surface of the composite reinforcement technology can effectively improve the surface residual stress distribution. Like the carburizing and quenching residual stress in the surface compressive stress is low, the maximum compressive stress appears in certain depth from surface, thick and residual stress. After nitriding surface residual compressive stress is very high, but the residual compressive stress layer is very thin, to fell sharply. If the carburizing - - Nitride composite reinforcement technology, can get a more reasonable stress distribution state. So the surface strengthening technology, such as carburizing - - Nitriding and carburizing - - The direction of the high frequency quenching and so on, are worthy.