Five Carburizing Defects and Prevention Methods of Steel Heat Treatment

1. Too high carbon concentration

A.Causes and Hazards

If carburizing heated sharply, the temperature is too high or solid carburizing with a new carburizing agent, or with a strong carburizing agent too much will cause the phenomenon of high carburizing concentration. As the carbon concentration is too high, the bulk carbide or mesh carbide appears on the surface of the workpiece. As a result of this hard brittle structure, the toughness of the carburizing layer decreased sharply. And the formation of high carbon martensite during quenching, grinding crack is easy to appear in the grinding.

B.The method of prevention

(1) can not be heated sharply, need to use appropriate heating temperature, do not make the grain of steel grow up as well. If carburizing grain coarse, after carburizing should be normalized or two quenching treatment to refine grain.

(2) Strictly control the uniformity of furnace temperature, can not fluctuate too much, special attention should be paid to the solid carburizing in the reflection furnace.

(3) when solid carburizing, carburizing agent to new and old ratio. It is best to use 4-7% BaCO3 as infiltration agent, and do not use Na2CO3 as infiltration agent.

2. Too low carbon concentration

A. Causes and hazards

Too many temperature fluctuations or too few osmotic agents can cause insufficient carbon concentration on the surface. The ideal carbon concentration is between 0.9 and 1.0%, less than 0.8%C, parts are easy to wear.

B.The method of prevention

(1) Carburizing temperature is generally used 920-940 ℃, carburizing temperature is too low will cause carbon concentration is too low, and prolong the carburizing time;Carburizing temperature is too high to cause coarse grain size.

(2) The dosage of osmotic agent (BaCO3) should not be less than 4%.

3. After carburizing, the local surface is carbon poor

A. Causes and hazards

Solid carburizing, charcoal particles are too large or mixed with stones and other impurities, or impregnating agent and charcoal mixed uneven, or workpiece contact will cause local no carbon or carbon.Dirt on the workpiece surface can also cause carbon depletion.

B.The method of prevention

(1) solid carburizing agent must be prepared in proportion, stirring evenly.

(2) do not contact the workpiece loading furnace. When solid carburizing, the carburizing agent should be tamped, so as not to make the carburizing collapse and the workpiece contact.

(3) but remove the dirt on the surface.

4. Carburizing intensifies the transition

A. Causes and hazards

Carburizing concentration sudden transition is the surface and the center of the carbon concentration change intensified, not from high to low uniform transition, but a sudden transition. The reason for this defect is that the role of carburizing agent is very strong (such as newly prepared charcoal, old carburizing agent is added very little), and the alloy elements such as Cr, Mn, Mo in steel is to promote the formation of strong carbide, resulting in high surface concentration, low concentration in the center, and no transition layer. This defect causes considerable internal stress inside and outside the surface, resulting in cracks or peeling during quenching or grinding.

B.The method of prevention

Carburizing agent new and old according to the provisions of the ratio system, so that carburizing ease. It is better to use BaCO3 as an osmotic agent, because Na2CO3 is more rapid.

5. Tempering and cracking occur during grinding

A. Causes of Causes

The softening phenomenon of the surface of the carburized layer after grinding is called tempering produced by grinding. This is because the grinding process feed is too fast, the grinding wheel hardness and particle size or speed selection are not appropriate, or the grinding process cooling is not sufficient, are easy to produce such defects. This is because the heat during grinding softens the surface.The wear resistance of parts is reduced when the tempering defect occurs during grinding.

Hexagonal cracks occur on the surface. This is because the surface of the hard grinding wheel by excessive grinding, and caused by fever. It is also related to insufficient tempering of heat treatment and excessive residual internal stress. After acid etching, all defective parts are black and can be distinguished from no defects. This is produced when grinding heat tempering. The cause of the transformation of a horse to a trositic structure. In fact, the cracks are visible to the naked eye after grinding.

B.The method of prevention

(1) after quenching must be fully tempered or repeatedly tempered to eliminate internal stress.

(2) The use of 40~60 particle size of soft or medium alumina wheel, grinding feed is not large.

(3) Open the coolant first during grinding, and pay attention to sufficient cooling in the grinding process.

Five Carburizing Defects and Prevention Methods of Steel Heat Treatment

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