Five types of carburization defects and their prevention methods

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Excessively high carbon concentration ⒈ Causes and hazards If heating is carried out too rapidly during carburizing, if the temperature is too high, or if brand-new carburizing agents are used in solid carburizing, or if excessive amounts of strong carburizing accelerants are employed, all these factors can lead to an excessively high carbon concentration. As the carbon concentration becomes too high, large chunky carbides or networked carbides appear on the surface of the workpiece. The formation of this hard and brittle structure causes a sharp decline in the toughness of the carburized layer. Moreover, high-carbon martensite is formed during quenching, which makes grinding cracks prone to occur during grinding. ⒉ Prevention methods: ① Avoid rapid heating; an appropriate heating temperature should be used to prevent the growth of steel grains. If the grains are coarse during carburizing, normalizing after carburizing or two quenching treatments should be carried out to refine the grains. ②Strict control is required over the uniformity of furnace temperature to prevent excessive fluctuations, which is particularly important when carrying out solid carburizing in a reverberatory furnace. ③During solid carburizing, the carburizing agent should be used in a mixture of new and old materials. It is best to use 4–7% BaCO3 as a permeation promoter; Na2CO3 should not be used as such a promoter. Too low carbon concentration ⒈ Causes and hazards Large temperature fluctuations or too little penetrant can both lead to an insufficient carbon concentration on the surface. The ideal carbon concentration is between 0.9% and 1.0%; below 0.8%C, the parts tend to wear out easily. ⒉ Prevention methods: ① The carburizing temperature is generally set at 920–940°C; if this temperature is too low, the carbon concentration will be too low, and the carburizing time will increase ； Excessively high carburizing temperatures can cause grain coarsening. ② The amount of the impermeabilizer (BaCO3) should not be less than 4%. Local carbon deficiency on the surface after carburizing 1. Causes and hazards: During solid carburizing, oversized charcoal particles or impurities such as stones mixed in with the charcoal, uneven mixing of the carburizing agent with the charcoal, or improper contact with the workpiece can all lead to local areas lacking carbon or having reduced carbon content. Contaminants on the workpiece surface can also cause carbon deficiency. ⒉ Prevention methods: ① Solid carburizing agents must be prepared in the correct proportions and mixed thoroughly. ② Be careful to ensure that the workpieces in the furnace do not come into contact with each other. When performing solid carburizing, the carburizing agent must be compacted to prevent it from collapsing too much and causing the workpiece to make contact. ③ It only removes the surface dirt. Intensified carbon concentration transition ⒈ Causes and hazards The sudden transition in carbon concentration refers to an intensified change in carbon concentration between the surface and the core; it is not a gradual transition from high to low, but rather a sudden one. The reason for this defect is the strong effect of the carburizing agent (such as newly prepared charcoal or very little old carburizing agent used), together with alloying elements such as Cr, Mn, and Mo in the steel, which promote the formation of carbides intensively. This results in a high carbon concentration on the surface and a low concentration at the center, without any transition layer. The occurrence of this defect generates considerable internal stress on the inner and outer surfaces, leading to cracks or spalling during quenching or grinding. ⒉ Prevention method: The carbonizing agent is prepared in the specified ratio of old and new materials to moderate carburization. Using BaCO3 as a permeation promoter is better, because Na2CO3 has a more drastic effect. Tempering and cracks occur during grinding. 1. Causes: The softening of the surface of the carburized layer as a result of grinding is known as tempering induced by grinding. This is due to an overly fast feed rate during grinding, improper selection of the wheel’s hardness and grain size or rotational speed, or insufficient cooling during the grinding process – all of which can lead to such defects. This is because the heat generated during grinding softens the surface. If tempering defects occur during grinding, the wear resistance of the part decreases. Hexagonal cracks appear on the surface. This is because the surface of the hard grinding wheel has been excessively abraded, resulting in heat generation. It is also related to insufficient heat treatment tempering and excessive residual internal stress. After acid etching, any defective areas turn black, allowing them to be distinguished from the defect-free areas. This is tempering caused by heat generated during grinding. This is what causes the matrix to transform into martensitic structure. In fact, the cracks can be seen with the naked eye after grinding. ⒉ Prevention methods: ① After quenching, thorough tempering or multiple temperings must be carried out to eliminate internal stresses. ② Use soft or medium-grit alumina grinding wheels with a particle size of 40–60, and keep the grinding feed rate low. ③ Turn on the coolant before grinding, and ensure adequate cooling throughout the grinding process.