The possible defects in the heat treatment of 20CrMnTiare as follows:
1. Uneven depth of carburized layer
The depth of carburizing layer on the surface of spiralis not uniform, which results in the discontinuous performance of different parts. The weak area is damaged first, then the whole gear is damaged, which seriously affects the service life of the gear.
Preventive measures: when gas carburizing, pay attention to the full circulation of atmosphere in the furnace, the furnace temperature should be uniform, the oil stain on the tooth surface should be removed, the furnace load should not be too much, the sealing performance of the carburizing furnace is good, the leaking muffle tank should be replaced in time, and the carburizing furnace should be maintained regularly.
2. Low surface hardness after quenching
The low hardness of carburized surface of spiral bevel gear will lead to the decrease of wear resistance and fatigue resistance of gear, which will have a negative impact on the friction and wear resistance of gear surface.
1). the gears with low carbon content on the gear surface shall be treated with proper carburization.
2). select materials with appropriate hardenability and cooling medium with appropriate cooling capacity, and quench for cooling.
3). take measures in advance to reduce the amount of retained austenite after quenching. The carburized gear with too much retained austenite is tempered at 650 ~ 670 ℃ for more than 3 hours, which makes part of carbide precipitate out, so as to reduce the Austenite Stability during reheating quenching and promote the transformation of austenite to martensite.
The carburizing cooling or reheating quenching of gears shall be carried out in a protective atmosphere. The oxide skin of the gears that have been oxidized shall be removed and the surface carburizing shall be carried out before quenching.
4). if the hardness of gear surface is too low due to too high tempering temperature, it should be re quenched and tempered at a suitable temperature.
3. The hardness of spiral bevel gear center is insufficient
The core of spiral bevel gear is required to have certain hardness. The lower the hardness, the lower the yield point of the, the easier to produce the plastic deformation of the center, the lower the anti peeling performance of the hardened layer of the gear surface and the bending fatigue performance of the tooth root.
1). select the cooling medium with good cooling performance for quenching, so as to obtain low carbon martensite structure in the center.
Proper quenching temperature and heating time are selected to obtain uniform austenite in the center, so as to obtain martensitic structure after quenching.
2). choose steel with good hardenability and material as carburizing gear material.
4. Carburizing and quenching gear deformation
The deformation of carburized and quenched gears is mainly caused by the residual stress of parts during machining, the thermal stress and structural stress during heat treatment, and the self weight deformation of parts. There are many factors that affect the deformation of gear, including gear geometry, raw material and metallurgical quality, forging and machining residual stress, charging method, heat treatment process and equipment.
1). heating speed: in general, the higher the quenching temperature is, the larger the deformation of the part is. This is due to the increase of the temperature difference on the section of the part, the increase of the thermal stress, and the growth of the austenite grain due to the high heating temperature, which reduces the resistance of plastic deformation and increases the quenching deformation. In order to reduce the deformation, the lower quenching temperature should be chosen as much as possible.
2). quenching and heat preservation time: the selection of heat preservation time shall not only ensure that the parts are thoroughly burned, and reach the required hardness or other mechanical properties after quenching, but also consider its influence on quenching deformation. In fact, the quenching temperature is increased by prolonging the quenching holding time.
Carburizing and quenching equipment: selecting good carburizing and quenching equipment is an important means to reduce gear deformation. When the well type gas carburizing furnace is used for carburizing and quenching, the temperature control accuracy shall be ± 1 ℃, furnace temperature uniformity shall be ± 7 ℃, and carbon potential uniformity shall be ± 0.05%.