High carbon concentration
- Causes and hazards
- If the carburization is heated sharply, the temperature is too high, or a new carburizing agent is used in the case of solid carburizing, or a strong infiltration agent is too much, the carburization concentration is too high. As the carbon concentration is too high, massive coarse carbides or reticulated carbides appear on the surface of the workpiece. Due to the formation of such a hard and brittle structure, the toughness of the carburized layer is drastically lowered. And high carbon martensite is formed during quenching, and grinding cracks are likely to occur during grinding.
- Methods of prevention
1 can not be heated sharply, it is necessary to use appropriate heating temperature, so that the grain growth of steel is not good. If the grains are coarse during carburizing, the grains should be refined by normalizing or twice quenching after carburizing.
2 Strictly control the uniformity of the furnace temperature, and it should not fluctuate too much. Special attention should be paid to the solid carburization in the reverberatory furnace.
3 When carburizing solids, the carburizing agent should be used in new and old ratios. Preferably, the infiltration agent uses 4-7% BaCO3, and Na2CO3 is not used as the infiltration agent.
Carbon concentration is too low
- Causes and hazards
High temperature fluctuations or too little infiltration agent can cause insufficient carbon concentration on the surface. The most desirable carbon concentration is between 0.9 and 1.0%, below 0.8% C, and the parts are prone to wear.
- Methods of prevention
1 The carburizing temperature is generally 920-940 °C. If the carburizing temperature is too low, the carbon concentration will be too low and the carburizing time will be prolonged; if the carburizing temperature is too high, the grain will be coarse.
2 The amount of infiltration agent (BaCO3) should not be less than 4%.
Local carbon deficiency after carburizing
- Causes and hazards
When the solid is carburized, the charcoal particles are too large or contain impurities such as stones, or the infiltration agent is not uniformly mixed with the charcoal, or the workpiece is contacted to cause local carbon-free or carbon-poor. Contaminants on the surface of the workpiece can also cause carbon depletion.
- Methods of prevention
1 Solid carburizing agent must be prepared in proportion and stirred evenly.
2 Do not touch the workpiece of the furnace. When the solid carburizing, the carburizing agent should be compacted, so that the carburizing is not caused to collide and the workpiece is in contact.
3 But except for the dirt on the surface.
Carbon concentration is too low
- Causes and hazards
High temperature fluctuations or too little infiltration agent can cause insufficient carbon concentration on the surface. The most desirable carbon concentration is between 0.9 and 1.0%, below 0.8% C, and the parts are prone to wear.
- Methods of prevention
1 The carburizing temperature is generally 920-940 °C. If the carburizing temperature is too low, the carbon concentration will be too low and the carburizing time will be prolonged; if the carburizing temperature is too high, the grain will be coarse.
2 The amount of infiltration agent (BaCO3) should not be less than 4%.
Local carbon deficiency after carburizing
- Causes and hazards
When the solid is carburized, the charcoal particles are too large or contain impurities such as stones, or the infiltration agent is not uniformly mixed with the charcoal, or the workpiece is contacted to cause local carbon-free or carbon-poor. Contaminants on the surface of the workpiece can also cause carbon depletion.
- Methods of prevention
1 Solid carburizing agent must be prepared in proportion and stirred evenly.
2 Do not touch the workpiece of the furnace. When the solid carburizing, the carburizing agent should be compacted, so that the carburizing is not caused to collide and the workpiece is in contact.
3 But except for the dirt on the surface.
Carburizing concentration increases transition
- Causes and hazards
The sudden transition of carburizing concentration is a change in the carbon concentration between the surface and the center, not a uniform transition from high to low, but a sudden transition. The reason for this defect is that the carburizing agent acts very strongly (such as newly prepared charcoal, the old carburizing agent is added little), and the alloying elements such as Cr, Mn, Mo in the steel promote the formation of carbides, resulting in The surface has a high concentration, a low concentration at the center, and no transition layer. The occurrence of this defect causes considerable internal stress in the watch, causing cracks or peeling during the quenching process or during the grinding process.
- Methods of prevention
The carburizing agent is old and new according to the specified ratio to make the carburization moderate. It is better to use BaCO3 as the infiltration agent because Na2CO3 is relatively sharp.
Tempering and cracking during grinding
- Cause
The surface of the carburized layer is softened by grinding, which is called tempering caused by grinding. This is due to the fact that the machining feed rate during grinding is too fast, the hardness of the grinding wheel and the particle size or the rotational speed are improperly selected, or the cooling during the grinding process is insufficient, and such defects are easily generated. This is because the heat during grinding causes the surface to soften. When the tempering defect occurs during grinding, the wear resistance of the part is lowered.
The surface produces a hexagonal crack. This is because the surface of the hard grinding wheel is excessively ground and heated. It is also related to insufficient heat treatment and tempering, and excessive residual internal stress. After etching with acid, any defective part is black, which can be distinguished from the defect. This is the heat tempering that occurs during grinding. The reason why the horse body is transformed into a troostite structure. In fact, the crack can be seen by the naked eye after grinding.
- Methods of prevention
1 After quenching, it must be fully tempered or tempered several times to eliminate internal stress.
2 The soft or medium-sized alumina grinding wheel with 40~60 grain size is used, but the grinding feed is not too large.
3 Turn on the coolant first during grinding and pay attention to adequate cooling during the grinding process.
