Quenching heat treatment of workpiece

The cause of the uneven hardness in quenching heat treatment of workpieces

In order to meet the requirements of mechanical properties of products, more than 90% of steel manufacturers will heat treat them, most of which need to be tempered, such as drill pipe joints, directional joints, screw drilling components, etc. Quenching and tempering is a complex process in the field of metal heat treatment, that is, quenching + high temperature tempering. The purpose is to improve the comprehensive mechanical properties of metal materials and meet the design requirements of product performance. However, during the inspection of the quenched and tempered workpieces, it is found that the hardness of the workpieces processed under the same working condition is often uneven. Some workpieces need to be reworked every heat. After a long time of tracking observation, it is found that there are problems in the quenching process.

According to the theoretical analysis of heat treatment and quenching, the cooling process of workpiece in quenching medium is generally divided into three stages: steam film stage, boiling stage and convection stage. When the workpiece is just quenched into the medium, a layer of steam film is formed on the surface of the workpiece, which insulates the workpiece and the surrounding cooling medium. The steam film is a bad conductor of heat. At this stage, the cooling speed of the workpiece is slow, while in the boiling stage, the thickness of the steam film is continuously reduced until it is cracked. The medium is in direct contact with the workpiece, causing intense boiling, escaping bubbles and taking away large amount of gas The quantity of heat is the stage with high cooling rate and also the stage of real quenching in sense. In the process of heat treatment and quenching, in order to shorten the residence time of the workpiece in the steam film, in the design of the quenching tank, in order to improve the flow speed of the quenching medium, the uniform medium temperature, and the cooling capacity of the quenching medium, a stirring device is added.

However, the use effect of the mixing device is not ideal. The specific process is as follows: after the workpiece is discharged from the furnace, use the crane to lift the material basket to place the workpiece, and then put it into the quenching tank. The low-temperature medium at the bottom of the quenching tank is sucked in by the mixing pump (with a head of 40m), which is discharged from the output pipe and directly impacts on the material basket. In the quenching process, the basket can only move up and down after it is put into the quenching medium. Because the workpiece facing the output tube always contacts the fast flowing medium and the low-temperature medium at the bottom of the groove, the quenching effect is better. On the contrary, the flow speed of the medium slows down due to the obstruction of the basket, which greatly reduces the cooling speed of the workpiece, especially when the single quenching amount is large In order to reduce the cooling speed of the workpiece, the quenching of the same batch is uneven.