Large block or network carbides appear in the carburized layer
The cause of the defect: the surface carbon concentration is too high
1. Drip carburizing, the drop volume is too large
2. Controlled atmosphere carburizing, too much enriched gas
3. Liquid carburizing, salt bath cyanide content is too high
4. The carburized layer is air-cooled, and the cooling rate is too slow
Countermeasure:
1. Reduce the surface carbon concentration, reduce the amount of droplets during the diffusion period and appropriately increase the humidity during the diffusion period, and also appropriately reduce the amount of droplets during the carburizing period
2. Carburizing agent for reducing solid carburization
3. Reduce the cyanide content of liquid carburizing
4. The room temperature is too high in summer, and the air-cooled parts can be blown to help cooling after infiltration
5. Increase the quenching heating temperature to 50~80ºC and prolong the holding time appropriately
6. Twice quenching or normalizing + quenching, or normalizing + high temperature tempering, and then quenching and tempering
A large amount of retained austenite appears in the seepage layer
Defect causes:
1. Austenite is relatively stable, and the content of carbon and alloying elements in austenite is relatively high
2. The tempering is not timely, and the austenite is thermally stabilized
3. The cooling after tempering is too slow
Countermeasure:
1. The surface carbon concentration should not be too high
2. Reduce the temperature of direct quenching or reheating and quenching, and control the level of ferrite in the core to be less than or equal to level 3
3. Quick cooling after low temperature tempering
4. It can be reheated and quenched, cold treated, or re-quenched after high temperature tempering
Surface decarburization
Defect causes:
1. In the late stage of gas carburization, the carbon potential of the furnace gas is low
2. After solid carburizing, the cooling rate is too slow
3. Air cooling time is too long after carburizing
4. Unprotected cooling in cooling wells
5. Air furnace heating and quenching without protective gas
6. The salt bath furnace is heated and quenched, and the deoxidation of the salt bath is not complete
Countermeasure:
1. Infiltrate in a medium with suitable carbon potential
2. Shot peening after quenching
3. Grinding allowance, larger pieces are allowed to have a certain decarburization layer (≤0.02mm)
The troostite structure (black structure) appears after the carburized layer is quenched
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