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Quenching

Aug 27, 2019

Quenching, there may be many friends do not know this concept! So what does quenching mean? What is the purpose of quenching? Let's let ALLES CNC give you a detailed introduction! I hope the following content can provide some reference for everyone.


cuihuo(alles)


First, the introduction

Quenching is to heat the steel above the critical temperature, keep it for a certain period of time, and then cool it at a temperature greater than the critical cooling rate to obtain an unbalanced structure dominated by martensite (also obtain bainite or maintain single-phase austenite as needed). a heat treatment process. Quenching is the most widely used process in steel heat treatment.

 

Second, the purpose of quenching

The supercooled austenite is transformed into martensite or bainite to obtain martensite or bainite structure, and then tempered at different temperatures to greatly increase the rigidity, hardness, wear resistance and fatigue strength of the steel. And toughness, etc., to meet the different use requirements of various mechanical parts and tools. Special physical and chemical properties such as ferromagnetism and corrosion resistance of certain special steels can also be met by quenching.

The metal workpiece is heated to a suitable temperature for a period of time and then immersed in a quenching medium for rapid cooling of the metal heat treatment process. Commonly used quenching media are brine, water, mineral oil, air, and the like. Quenching can improve the hardness and wear resistance of metal workpieces, so it is widely used in various tools, molds, measuring tools and parts that require surface wear resistance (such as gears, rolls, carburized parts, etc.). Through quenching and tempering at different temperatures, the strength, toughness and fatigue strength of the metal can be greatly improved, and the combination of these properties (comprehensive mechanical properties) can be obtained to meet different application requirements. In addition, quenching can also obtain certain physical and chemical properties of some special properties of steel, such as quenching to make permanent magnet steel strengthen its ferromagnetism, stainless steel to improve its corrosion resistance. The quenching process is mainly used for steel parts. When the commonly used steel is heated above the critical temperature, the original or at room temperature will transform all or most of the microstructure into austenite. The steel is then immersed in water or oil for rapid cooling and the austenite is transformed into martensite. Martensite hardness is the highest compared to other structures in steel. Rapid cooling during quenching causes internal stresses in the workpiece, and when it is large enough, the workpiece will be distorted or even cracked. To do this, you must choose the right cooling method. According to the cooling method, the quenching process is divided into four types: single liquid quenching, double medium quenching, martensite grading quenching and bainite austempering.

 

Third, the process

It includes three stages of heating, heat preservation and cooling. Taking the quenching of steel as an example, the principle of selecting the process parameters in the above three stages is introduced.

Quenching heating temperature

Based on the critical point of phase transformation of steel, fine and uniform austenite grains are formed during heating and quenching, and fine martensite structure is obtained after quenching. The quenching temperature selection principle also applies to most alloy steels, especially low alloy steels. The heating temperature of the hypoeutectoid steel is 30 to 50 ° C above the Ac3 temperature. From the figure, the state of the steel at high temperature is in the single-phase austenite (A) zone, so it is called complete quenching. If the heating temperature of the hypoeutectic steel is higher than Ac1 and lower than Ac3, then some of the pro-eutectoid ferrite is not completely transformed into austenite at high temperature, which is incomplete (or subcritical) quenching. The quenching temperature of the hypereutectoid steel is 30-50 ° C above the Ac1 temperature, which is in the austenite and cementite (A+C) dual phase region. Therefore, the normal quenching of the hypereutectoid steel is still incomplete quenching, and the microstructure of the cementite distributed on the martensite matrix is obtained after quenching. This - tissue state has high hardness and high wear resistance. For hypereutectoid steel, if the heating temperature is too high, the proeutectoid cementite dissolves too much, or even dissolves completely, the austenite grains will grow and the austenite carbon content also increases. After quenching, the coarse martensite structure increases the stress in the quenched state of the steel, increases the microcracks, and increases the deformation and cracking tendency of the parts. Due to the high austenite carbon concentration, the martensite point decreases and the retained austenite amount Increased to reduce the hardness and wear resistance of the workpiece. Refer to the table in the above figure for the quenching temperature of common steel grades. The table shows the heating temperature for quenching common steel grades.

In actual production, the choice of heating temperature should be adjusted according to the specific conditions. For example, the carbon content in the sub-eutectoid steel is the lower limit. When the amount of furnace is large, the upper limit of the temperature can be selected when the depth of the hardened layer of the part is to be increased. If the shape of the workpiece is complicated, the deformation requirement is strict, and the lower temperature limit is adopted.


Quenching insulation

The quenching and holding time is determined by various factors such as the heating method of the equipment, the size of the parts, the composition of the steel, the amount of furnace installed, and the power of the equipment. For the overall quenching, the purpose of the insulation is to make the internal temperature of the workpiece uniform. For all types of quenching, the holding time ultimately depends on obtaining a good quenching heating structure in the area where quenching is required. Heating and heat preservation are important links affecting the quality of quenching. The microstructure state obtained by austenitizing directly affects the performance after quenching. Generally, the austenite grains of steel are controlled at 5 to 8.


Quenching cooling

In order to transform the high-temperature phase of the steel, austenite, into a low-temperature metastable phase, martensite, during cooling, the cooling rate must be greater than the critical cooling rate of the steel. During the cooling process, there is a difference between the surface and the cooling rate of the core. If the difference is large enough, it may cause the portion larger than the critical cooling rate to transform into martensite, and the core smaller than the critical cooling rate cannot be converted into The case of martensite. In order to ensure that the entire section is transformed into martensite, a quenching medium with sufficient cooling capacity is required to ensure a sufficiently high cooling rate in the core of the workpiece. However, the cooling rate is large, and internal stress is caused by uneven expansion and contraction of the inside of the workpiece, which may cause deformation or cracking of the workpiece. Therefore, we must consider the above two contradictory factors, and reasonably choose the quenching medium and cooling method.

In the cooling stage, not only the parts are properly organized, the required performance, but also the dimensional and shape accuracy of the parts is a key part of the quenching process.


The quenched CNC machine tool can be used by customers for 10-20 years, and the CNC machine tool without quenching can hardly achieve this effect. Therefore, the CNC machine tool selects SHADONEG ALLES ENERGY TECHNOLOGY CO., LTD.


More details: www.allescncmachine.com

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