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In production, insufficient hardness after quenching can sometimes occur; this is a common defect in the quenching process of heat treatment. ”\"Insufficient hardness\" can manifest in two ways: one is a low hardness value across the entire workpiece, and the other is insufficient hardness in certain areas or the presence of soft spots. When a deficiency in hardness occurs, methods such as hardness testing or metallographic analysis are used to determine what type of hardness deficiency it is. Then, the causes are sought in aspects such as raw materials, heating processes, cooling media, cooling methods, and tempering temperatures, in order to find a solution. 1. In terms of raw materials: 1.1 Improper selection of raw materials or using the wrong type of material can lead to problems. Parts that should be made from medium-carbon or high-carbon steel being made instead from low-carbon steel, or parts that should be fabricated from alloy tool steel being made from ordinary high-carbon steel, can all result in insufficient hardness or the presence of weak spots. Example 1: Gears should be made of 45# steel, with a quenched hardness of around 60HRC; however, 25# steel was mistakenly used, resulting in a hardness of around 380HBS. Example 2: Molds should be fabricated from 9Mn2V steel, but T8 steel was used instead. Since it is difficult to distinguish between the sparks produced by 9Mn2V and T8 steel, the quenching process intended for 9Mn2V was applied, with oil cooling being used, which resulted in a hardness of only around 50HRC. The above two situations represent insufficient overall hardness, which can be determined through hardness testing or metallographic testing. Solutions: 1. Appropriate materials should be selected during design; 2. Strengthen material management by conducting chemical analysis on materials before they are stored in inventory, and then categorizing and labeling them, which can effectively prevent the use of incorrect materials. Heat treatment operators should perform spark analysis before starting the process to roughly determine whether the material of the parts meets the requirements specified in the drawings. When the cross-section of the workpiece is large or when there is a significant difference in thickness across the cross-section, using tool steel may result in lower internal hardness in the larger sections due to its poor hardenability. In such cases, alloy steel with better hardenability should be used instead. 1. 2 Uneven microstructure of the raw materials leads to insufficient local hardness or the presence of soft spots. The microstructure may exhibit one of the following conditions: carbide segregation or aggregation; for example, aggregation of ferrite, presence of graphite, or severe bainite structure – all of which can result in insufficient hardness or soft spots. Solution: Perform repeated forging or preliminary heat treatment (such as normalizing or homogenization annealing) before quenching to homogenize the microstructure. 2. In terms of the heating process: 2.1 The quenching heating temperature is too low, and the holding time is insufficient. In the case of hypoeutectoid steels, when the heating temperature lies between Ac3 and Ac1 (for example, the quenching heating temperature for 25# steel is below 860°C), the ferrite does not fully dissolve into austenite; as a result, uniform martensite cannot be formed after quenching, and instead ferrite and martensite remain, which affects the hardness of the workpiece. Metallographic analysis reveals undissolved ferrite (as shown in Figure 3). For high-carbon steels, especially high-alloy steels, insufficient heating or holding time can prevent the transformation of pearlite into austenite, resulting in the failure to obtain martensite. In actual production, the above situations often occur due to deviations in the instrument readings (higher indicated temperatures) or uneven furnace temperature, which results in a lower actual temperature of the workpiece; incorrect estimates of the workpiece’s thickness also lead to an insufficient holding time. Solutions: 1. Control the heating rate carefully to avoid excessive speeds, which can lead to uneven temperatures in the furnace and also result in premature start of the holding time, thereby insufficient holding time. 2. Regularly check whether the temperature indicators are functioning properly and accurately, to prevent situations where the indicators show that the desired temperature has been reached when in fact the actual temperature is still too low. 3. Follow the specifications in the material manual precisely regarding the heating rate and temperature for quenching, to avoid either too low or too high quenching temperatures. 4. Estimate the thickness of the material correctly, especially for irregularly shaped pieces. 2.2 Excessively high quenching heating temperatures and prolonged holding times are problematic for tool steels (such as T8 steel). When the quenching heating temperature is at 780°C, austenite and carbides (Fe3C) are formed; at this temperature, the carbon content in austenite is slightly above 0.77%. Upon cooling, austenite transforms into martensite. If the heating temperature is too high or the holding time is too long, a large amount of carbon in the carbides (Fe3C) will dissolve into the austenite, resulting in an elevated carbon content in the austenite. This also increases its stability significantly, facilitating the transformation of austenite into martensite (AyM). As the temperature begins to drop, a large amount of residual austenite (Ac) remains in the workpiece after quenching; the resulting microstructure is M + Ac. Since residual austenite possesses the properties of austenite, namely low hardness, this leads to a decrease in hardness after quenching. The effect of heating temperature and tempering temperature on the residual austenite content. Solutions to the impact of heating temperature and tempering temperature on the residual austenite content: 1. Strictly control the quenching heating temperature and holding time to prevent excessive carbon from dissolving into austenite (A); controlling the heating temperature is particularly important. 2. Reduce the quenching cooling rate, or use staged quenching, to ensure that the supercooled austenite transforms fully into martensite. 3. Apply cold treatment to facilitate the transformation of residual austenite into martensite. 4. Use high-temperature tempering to reduce residual austenite, although this will result in an increase in hardness. 2. During quenching heating, the surface of the workpiece suffers from decarburization. After quenching 45# steel, metallographic analysis shows that its surface consists of ferrite and low-carbon martensite; however, after removing the decarburized layer from the surface, the hardness meets the required standards. This phenomenon often occurs when heating in a box furnace without proper protection or with inadequate protection, or when using a salt bath with poor deoxidation properties – this leads to oxygen reacting with the carbon atoms in the workpiece to form CO, thereby reducing the carbon content on the surface and resulting in insufficient surface hardness. Solutions: 1. Use an oxidation-free heating furnace with a protective atmosphere, such as one that utilizes an atmosphere created by the pyrolysis of alcohol or methanol; use vacuum heating for quenching; for ordinary box furnaces, pig iron shavings or charcoal can be used to seal the furnace; apply an anti-oxidation coating to the surface of the workpiece; place charcoal inside the furnace; coat the workpiece with a solution of boric acid and alcohol before heating it. 3. Cooling process issues 3.1 Improper selection of quenching medium: When a workpiece that should be quenched in water or a salt bath is instead cooled with oil, the insufficient cooling capacity results in a too slow cooling rate. During this process, austenite transforms into a pearlitic structure (AyP), and martensite (M) does not form (especially in the core part of the workpiece). As a result, the hardness of the workpiece is low. For example, hammers made from T10 material, when quenched in oil, have a hardness of only around 45 HRC; metallographic analysis reveals a troostite structure rather than martensite. Solution: The appropriate cooling medium must be selected based on the material and size of the workpiece. 3.2 The influence of the quenching medium temperature: During water quenching, when a large number of parts are quenched sequentially, the absence of a circulating cooling system leads to an increase in water temperature and a decrease in cooling capacity. The phenomenon of failure to harden will occur. During oil cooling, at the start of quenching, the low temperature and poor fluidity of the oil result in weak cooling capacity, which prevents proper hardening. Solution: When using water quenching, a circulating cooling system should be employed to maintain the water temperature at around 20°C. In the case of oil quenching, it is necessary to heat the oil appropriately, especially at the beginning, so that its temperature reaches above 80°C. This is the principle behind what is referred to as \"cold water and hot oil\" in quenching processes. 3.3 When the quenching medium is outdated, soft spots during quenching can occur easily in case the alkaline (salt) bath contains many impurities or there is insufficient water. Solution: The quenching medium should be replaced in a timely manner, and the moisture content in the alkaline (salt) bath should be controlled. 3.4 Improper control of cooling time: When manufacturing switches made of carbon steel that are complex in structure or have a large cross-sectional area, water quenching or oil cooling is used to prevent deformation and cracking. If the parts stay in water for too short a time, or if they are left in the air for too long after being taken out of water before being placed in oil, the high temperature of the parts themselves, especially the slow cooling rate in the core area, prevents the formation of uniform and complete martensite. Solution: Properly control the water cooling time. When gripping the workpiece with pliers, once no vibration is felt by the hand, immediately transfer it to oil. For molds with large cavities, remove the waste material first to reduce the thickness of the workpiece before carrying out quenching. During gradient quenching, staying in the salt bath for too long leads to bainite transformation, resulting in insufficient hardness. In short, insufficient quenching occurs frequently; operators should conduct a detailed analysis based on different situations to identify the causes and find ways to overcome them.