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Hardness Improvement Method of Seamless Steel Pipe

Date:2019-11-23View:371Tags:Hardness Improvement Method of Seamless Steel Pipe
In order to improve the hardness of seamless steel tube, the heat treatment process of quenching seamless steel tube can be used to improve the hardness. After seamless steel pipe processing, in order to improve the performance of seamless steel pipe, it needs to be cooled, and there are five cooling methods of quenching process:
1. Single liquid quenching method of seamless steel pipe. Heat seamless steel pipe or parts for austenitizing, and then quench them into water, oil or other cooling medium, and cool them for a certain time (cooling to the area below pearlitic transformation temperature or martensitic transformation temperature). Because the cooling process of seamless steel tube is completed in a single cooling medium, it is called a single liquid quenching method.

2. Two liquid quenching method for seamless steel pipe. The quenching and cooling process of seamless steel tube are carried out in two kinds of cooling medium (the most common is water and oil). The ideal cooling process is rapid cooling in pearlite transformation region and slow cooling in martensite transformation region. Specifically, it is heated to austenitizing temperature. Firstly, the seamless steel pipe or parts are quenched to the first medium (usually water or brine solution) in the high temperature zone to inhibit the pearlite transformation of undercooled austenite. When cooled to about 100 ° C, remove it quickly. In the second medium (usually oil), the medium cools slowly in the low temperature region. Since martensitic transformation is carried out under mild cooling conditions, it can effectively reduce or prevent deformation and cracking, which is usually called water quenching and oil cooling. This method requires high skill. It is sometimes understood as three media, the first water, the last oil and the last air.

3. seamless steel tube spray quenching method. Large and complex workpieces, especially thick and thin workpieces and seamless steel pipes, in order to avoid uniform and excessive quenching stress for cooling, control the cooling speed of different parts in different stages of cooling process. There are sprays (water or water solution), sprays (compressed air and atomized parts of water in different parts of the parts), gas quenching, etc. their advantages are that they can control different media or different flow rates, and control and regulate the temperature under pressure. Cooling rate of the area; or changing the number and location of different nozzles to achieve uniform cooling. This is the most popular vacuum high pressure gas quenching in mold heat treatment.

4. Classification quenching method of seamless steel pipe. Seamless steel tubes or workpieces heated to austenitizing temperature are quenched in a cooling medium (usually salt bath) with a temperature close to the martensitic transformation temperature, and kept for a period of time, so that the surface and center temperatures of the workpieces gradually become uniform. After taking out the air for cooling, the martensitic transformation is completed at a lower cooling rate. This method can significantly reduce the deformation of seamless steel pipe and improve its toughness, which is one of the commonly used quenching methods for die parts. There are two options for the quenching temperature of seamless steel pipes. One is to take the Markov transformation starting temperature (MS point) of the steel of the workpiece to be processed as 10-30 ° C. The other is to select 80-100 ° C below MS point. The retention time of classification should also be well controlled. If the temperature is too short, the temperature is not uniform enough to achieve the purpose of graded quenching. If the time is too long, non martensitic transformation may occur to reduce the hardness.

5. Tempering method of seamless steel pipe. The seamless steel tube workpiece heated to the austenitizing temperature is quenched in a hot bath at a temperature slightly higher than the MS point of the quenched steel, and the transformation is completed to obtain the lower bainite structure or the mixed structure of the lower bainite and martensite. The purpose of this method is to reduce the effects of deformation and cracking as well as smaller quenching stress. It has the same strength and toughness as tempered martensite.