1. Too high carbon concentration
A. Causes and hazards
If the carburizing is heated sharply, the temperature is too high, or a new carburizing agent is used during solid carburizing, or too much strong catalytic agent is used, the carburizing concentration will be too high. With the high carbon concentration, blocky and coarse carbides or network carbides appear on the surface of the workpiece. Due to the generation of this hard and brittle structure, the toughness of the carburized layer drops sharply. And high-carbon martensite is formed during quenching, which is prone to grinding cracks during grinding.
B. Prevention methods
① Do not heat sharply, and use an appropriate heating temperature to prevent the grains of steel from growing. If the grains are coarse during carburizing, normalizing or double quenching should be performed after carburizing to refine the grains.
② Strictly control the uniformity of the furnace temperature, and do not fluctuate too much. Special attention should be paid to solid carburizing in the reverberatory furnace.
③ During solid carburizing, the carburizing agent should be used in a new and old ratio. It is best to use 4-7% BaCO3 as a catalyst, and not use Na2CO3 as a catalyst.
2. Carbon concentration is too low
A. Causes and hazards
Large temperature fluctuations or too little catalyst will cause insufficient carbon concentration on the surface. The most ideal carbon concentration is between 0.9-1.0%. If it is lower than 0.8%C, parts are prone to wear.
B. Prevention methods
① The carburizing temperature is generally 920-940℃. If the carburizing temperature is too low, the carbon concentration will be too low and the carburizing time will be extended; if the carburizing temperature is too high, the grain size will be coarse.
② The amount of catalyst (BaCO3) should not be less than 4%.
3. Local carbon deficiency on the surface after carburizing
A. Causes and hazards
During solid carburizing, if the charcoal particles are too large or mixed with impurities such as stones, the catalyst is not mixed evenly with the charcoal, or the workpiece is in contact, it will cause local carbonlessness or carbon deficiency. Dirt on the surface of the workpiece can also cause carbon deficiency.
B. Prevention methods
① The solid carburizing agent must be prepared in proportion and stirred evenly.
② Be careful not to have contact between the workpieces loaded in the furnace. When solid carburizing, the carburizing agent must be compacted to prevent the carburizing from collapsing and causing the workpieces to contact.
③ Remove the dirt on the surface.
4. Carburizing concentration aggravation transition
A. Causes and hazards
The sudden transition of carburizing concentration means that the carbon concentration changes between the surface and the center aggravate, not a uniform transition from high to low, but a sudden transition. The reason for this defect is that the carburizing agent has a very strong effect (such as newly prepared charcoal, the old carburizing agent is added very little), and at the same time, the alloying elements such as Cr, Mn, and Mo in the steel promote the formation of carbides, resulting in high concentration on the surface, low concentration in the center, and no transition layer. This defect causes considerable internal stress inside and outside, and cracks or peeling occur during quenching or grinding.
B. Prevention methods
The new and old carburizing agents are prepared in the prescribed ratio to ease carburization. It is better to use BaCO3 as a catalyst because Na2CO3 is more drastic.
5. Tempering and cracks during grinding
A. Causes
The phenomenon of softening the surface of the carburized layer after grinding is called tempering caused by grinding. This is because the feed rate is too fast during grinding, the hardness and particle size of the grinding wheel the speed is improperly selected, or the cooling is insufficient during the grinding process, which makes it easy to produce such defects. This is because the heat during grinding softens the surface. If tempering defects are produced during grinding, the wear resistance of the parts will be reduced.
Hexagonal cracks are produced on the surface. This is because the surface of the hard grinding wheel is over-grinded and heated. It is also related to insufficient heat treatment tempering and excessive residual internal stress. After acid etching, all defective parts are black and can be distinguished from those without defects. This is because the heat tempering is generated during grinding. The martensitic structure is transformed into troostite. The cracks can be seen with the naked eye after grinding.
B. Prevention methods
① After quenching, it must be fully tempered or tempered multiple times to eliminate internal stress.
② Use a soft or medium-quality alumina grinding wheel with a grit of 40~60, and the grinding feed should not be too large.
③ Turn on the coolant first during grinding, and pay attention to sufficient cooling during the grinding process.
Post time: Sep-30-2024