Keywords: quenchi, quenching, hardness, heating, holding time, heating temperature, cooling process, microstructure, austenite, ferrite, pearlite, cooling medium, heat treatment, cooling, cooling speed, oil cooling, circulating cooling, scale, rust,

Analysis of reasons for insufficient hardness after quenching

  In the production process, sometimes there will be insufficient hardness after quenching, which is a common defect in the process of heat treatment and quenching. Insufficient hardness sometimes shows that the hardness value of the whole workpiece is low, and sometimes the local hardness is not enough or soft spots are produced. There are many reasons for insufficient hardness during quenching, which are related to the inherent metallurgical defects, improper material selection and wrong material; The poor structure processability in design is closely related to heating process, cooling medium, cooling method and tempering temperature. Once such a situation occurs in production, it must be analyzed and investigated systematically, and the problem must be handled correctly. The following is only an analysis of several common situations in the factory.

1. Raw material problems

  (1) Improper selection or wrong distribution of raw materials. Parts made of high carbon steel or medium carbon steel should be misused as low carbon steel; The parts that should be made of alloy tool steel are misapplied to ordinary high carbon steel. Therefore, the hardness of parts can not be reached after quenching. This situation is common in factories. For example, 20CrMnT is wrongly used for gears made of 40Cr steel; Results When the tooth surface is quenched at high frequency, the hardness is only HRC25~30, and there is no change in the hardness after the second quenching. The final analysis showed that the material was wrong during feeding. Finally, the only way to save this batch of workpieces was to use carburization. On the other hand, when designing and selecting materials, if the section of the workpiece is large (greater than 50mm) or the section thickness of the workpiece is significantly different, if carbon tool steel is used, the hardness at the large section will be low due to the insufficient hardenability of the steel. At this time, the workpiece should be made of steel with high hardenability.

  (2) The microstructure of raw materials is uneven. For example, carbide segregation or aggregation phenomenon, ferrite is distributed in bulk, graphite carbon, severe Widmanstein structure or banded structure appear. If the original structure has the above structure, the hardness value after quenching will be low and uneven. In this case, forging or pre-heat treatment (normalizing or homogenizing annealing) can generally be carried out before quenching to homogenize the structure.

Quenching hardness

2. Heating process problems

  (1) Low quenching heating temperature and insufficient holding time are also the reasons for insufficient hardness after quenching. For example, when the heating temperature of hypoeutectoid steel is between AC3 and AC1, the hardness of the workpiece will be affected because the ferrite is not completely dissolved in the austenite and the uniform martensite cannot be obtained after quenching. Undissolved ferrite can be seen during metallographic analysis. For tool steel,

  Especially for high alloy steel, if the heating is insufficient, there will be untransformed pearlite cells and more undissolved carbides. In actual operation, this situation is often due to the high indication of the instrument or uneven furnace temperature, which makes the actual temperature of the workpiece lower. Sometimes it is also caused by the operator's weak sense of responsibility for work and the short insulation time.

  (2) The quenching heating temperature is too high and the holding time is too long. For tool steel, when the heating temperature of the steel is too high, a large number of carbides are dissolved in the austenite, which greatly increases the stability of the austenite and reduces the initial transformation point of martensite. Therefore, a large number of residual austenite is retained in the quenched workpiece, which reduces the hardness of the quenched workpiece. During metallographic analysis, it can be seen that the undissolved carbide is rare and the residual austenite is much more.

  (3) During quenching and heating, the surface of the workpiece is decarburized, resulting in insufficient surface hardness. During metallographic analysis, there are ferrite and low-carbon martensite on the surface. When the surface decarburization layer is removed, the hardness will meet the requirements. Oxidative decarburization will occur when the workpiece is heated in general box furnace without protection or poor protection, or in salt bath furnace with poor deoxidation.

3. Cooling process problems

  (1) Improper selection of quenching medium. For example, if oil cooling is used for workpieces that should be quenched or quenched with alkali, due to insufficient cooling capacity and too slow cooling speed, pearlite structure transformation will occur in austenite during cooling process, and martensite will not be obtained, resulting in low hardness value of workpieces.

  (2) The quenching medium is too old. For example, the quenching oil used in our factory, during the long-term use, the carbon black and residue make the viscosity rise, resulting in the cooling capacity decline and the oil aging. During the quenching process, the oil continuously undergoes oxidation, polymerization, thermal decomposition, vaporization and other processes, resulting in the deterioration of the oil. In addition, the increase of water in the oil also promotes the aging of the oil into emulsified state, so that the workpiece can not meet the requirements during the quenching. At this time, the original old oil is replaced or filtered to make the quenching hardness reach the standard.

  (3) If the temperature of quenching medium is too high, soft spots are easy to appear. If a large number of parts are continuously quenched and the quenching medium has no circulating cooling system, the temperature of the quenching medium will rise and the cooling capacity will drop, resulting in the phenomenon of hardening.

  (4) The cooling time is improperly controlled, mainly in the following two cases:! During double liquid quenching, the parts stay in the water for too short, or after being taken out of the water, stay in the air for too long and then turn into oil for cooling. Due to the relatively high temperature of the workpiece itself, the phenomenon of self tempering occurs, resulting in low hardness During the stage quenching, the bainite transformation occurs due to the long stay time in the stage cooling medium, thus the hardness is insufficient.

  (5) During the cooling process, improper operation will also produce soft spots. The workpiece does not move properly in the quenching medium, resulting in the formation of steam film in local areas, r

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