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Hardness Improvement Method of Seamless Steel Pipe

Date:2019-11-25    keywords:seamless steel pipe,hardness

In order to improve the hardness of the seamless steel pipe, it can be improved by the heat treatment process of the quenched seamless steel pipe. After the seamless steel pipe is processed, in order to make the performance of the seamless steel pipe better, it needs to be cooled, and the quenching process cooling method mainly There are five types:

1. Single-liquid quenching method for seamless steel pipe. Heating seamless steel tubes or parts to austenitizing and then quenching them into water, oil or other cooling medium, and cooling them for a certain period of time (cooling to a region below the pearlite-type transition temperature or martensite transformation temperature) . Since the seamless steel pipe cooling process is completed in a single cooling medium, it is called a single liquid quenching method.


2. Seamless steel pipe two-liquid quenching method. The quenching and cooling process of the seamless steel tube is carried out in two cooling mediums (most commonly water and oil). The cooling process is ideal, which is rapidly cooled in the pearlite transformation region and slowly cooled in the martensite transformation region. Specifically, it will be heated to the austenitizing temperature.The seamless steel pipe or part is first quenched into the first medium (usually water or brine solution) in the high temperature zone to suppress the pearlite transformation of the supercooled austenite. When it is cooled to about 100 °C, it is quickly taken out. In a second medium (usually oil) that is slowly cooled in the low temperature zone. Since the martensite transformation is carried out under milder cooling conditions, it can effectively alleviate or prevent deformation and cracking, which is commonly referred to as water quenching and oil cooling. This method requires a high level of skill. Sometimes understood as three kinds of media, namely the first water, the last oil, and finally the air.


3. Seamless steel pipe spray quenching method. Large-scale complex, especially thick and thin workpieces and seamless steel tubes, in order to avoid excessive quenching stress evenly for cooling, control the cooling rate of different parts of different stages of the cooling process. There are spray (water or aqueous solution), spray (compressed air and water sprayed to different parts of the part by atomization), gas quenching, etc., which have the advantage of controlling different media or different flow rates, and controlling and adjusting the temperature by pressure. The cooling rate of the zone; or changing the number and location of different nozzles allows for even cooling. This is the most popular vacuum high pressure gas quenching in mold heat treatment.


4. Graded quenching method for seamless steel tubes. The seamless steel pipe or workpiece heated to the austenitizing temperature is quenched into a cooling medium (usually a salt bath) whose temperature is near the martensite transformation temperature, and stays for a period of time, so that the surface and the center temperature of the workpiece gradually become uniform After taking out the air cooling, the martensite transformation is completed at a lower cooling rate. This method can significantly reduce the deformation and improve the toughness of the seamless steel pipe, and is one of the commonly used quenching methods for the mold parts. There are two choices for the temperature of the quenching of seamless steel tubes. One is to take the Markov transformation starting temperature (Ms point) of the steel of the workpiece to be processed by 10~30 °C; the other is to select 80~100 °C below the Ms point. The residence time of the classification should also be well mastered. If the temperature is too short, the temperature is not uniform enough to achieve the purpose of classification quenching; if it is too long, non-martensitic transformation may occur to reduce the hardness.


5. Seamless steel pipe austempering method. The seamless steel pipe workpiece heated to the austenitizing temperature is quenched into a hot bath at a temperature slightly higher than the Ms point of the quenched steel grade, and the phase transition is completed to obtain the lower bainite structure or the lower bainite and Martensite mixed tissue. The purpose of this method is to alleviate the effects of deformation and cracking, and small quenching stress. Has similar strength and toughness to tempered martensite.


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