Common defects and prevention methods of carburization in tubular furnaces

Local carbon deficiency on the surface after carburization:
Causes and hazards: During solid carburization, excessive charcoal particles or impurities such as stones, uneven mixing of carburizing agents and charcoal, or contact with the workpiece can cause local carbon deficiency or carbon deficiency. Dirt on the surface of the workpiece can also cause carbon deficiency.
Prevention methods:
The workpiece loaded into the furnace must not come into contact. When solid carburizing, the carburizing agent should be compacted to prevent the carburizing from collapsing and causing the workpiece to come into contact.
Solid carburizing agent must be prepared in proportion and stirred evenly.
But remove the dirt on the surface.
Carbon concentration too high:
Causes and hazards: If the carburizing process is heated rapidly, the temperature is too high, or if a new carburizing agent is used during solid carburizing, or if too many strong carburizing agents are used, it can cause the phenomenon of high carburizing concentration. As the carbon concentration becomes too high, blocky and coarse carbides or network carbides appear on the surface of the workpiece. Due to the formation of this hard and brittle structure, the toughness of the carburized layer sharply decreases. In addition, high carbon martensite is formed during quenching, and grinding cracks are easy to appear during grinding.
Prevention methods:
It is not advisable to heat rapidly, but to use an appropriate heating temperature to prevent the grain growth of the steel. If the grains are coarse during carburization, they should be refined by normalizing or two quenching treatments after carburization.
The furnace temperature uniformity shall be strictly controlled and shall not fluctuate too much. Special attention shall be paid to solid carburization in reverberatory furnace.
When solid carburizing, the carburizing agent should be used in a new and old ratio. It is best to use 4-7% BaCO3 as the penetrating agent, and Na2CO3 is not used as the penetrating agent.
Low carbon concentration:
Causes and hazards: Large temperature fluctuations or insufficient penetration agents can cause insufficient surface carbon concentration. The optimal carbon concentration for a tubular furnace is between 0.9-1.0%, below 0.8% C, and parts are prone to wear and tear.
Prevention methods:
Method of prevention: The new and old carburizing agents should be prepared according to the prescribed ratio, so as to ease the carburizing process. It is better to use BaCO3 as a carburizing agent because Na2CO3 is more rapid.
The carburizing temperature is generally 920-940 ℃. If the carburizing temperature is too low, it will cause the carbon concentration to be too low and prolong the carburizing time; Excessive carburization temperature can cause grain coarsening.
The dosage of penetrating agent (BaCO3) should not be less than 4%.

Tempering and cracking during grinding:
Reason for occurrence: The phenomenon of softening on the surface of the carburized layer after grinding is called tempering caused by grinding. This is because the feed rate during grinding is too fast, the hardness and particle size or speed of the grinding wheel are improperly selected, and insufficient cooling during the grinding process is prone to such defects. This is because the heat generated during grinding softens the surface. When tempering defects occur during grinding, the wear resistance of the part decreases.
Hexagonal cracks appear on the surface. Mainly due to excessive grinding on the surface of a hard grinding wheel, resulting in heat generation. It is also related to insufficient tempering during heat treatment and excessive residual internal stress. After acid etching, any defective parts appear black and can be distinguished from those without defects. This is tempering caused by heat generated during grinding. The reason for the transformation of horse body into troostite tissue. Cracks can be seen with the naked eye after grinding.
Prevention methods:
Using a soft or medium alumina grinding wheel with a particle size of 40-60, the grinding feed rate should not be too high.
After quenching, it is necessary to undergo sufficient tempering or multiple tempering to eliminate internal stress.
When grinding, first turn on the coolant and pay attention to sufficient cooling during the grinding process.

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