In the production process of seamless pipes, sometimes there is insufficient hardness after melting, which is a common defect in the process of heat treatment and melting. Insufficient hardness sometimes shows that the hardness value of the whole steel pipe is low, sometimes the local hardness is not enough or soft spots are generated. There are many reasons for insufficient hardness during melting, which are closely related to the inherent metallurgical defects of the material, improper material selection, wrong material; poor structural manufacturability in design, heating process, cooling medium, cooling method and tempering temperature. Summarizing some practical reasons, please summarize the common possible factors:

1. Raw material problem

(1) Improper selection of raw materials or wrong materials. Parts made of high carbon steel or medium carbon steel should be misused as low carbon steel; parts made of alloy tool steel should be borrowed from ordinary high carbon steel.

(2) The microstructure of the raw material is uneven. Such as carbide segregation or aggregation phenomenon, ferrite is distributed in large blocks, graphitic carbon, serious Widmanderin or banded structure, etc. appear.

2. Heating process problems

(1) The heating temperature of the melting fire is low, and the insufficient holding time is also the reason for the insufficient hardness after the melting fire. Such as hypoeutectoid steel, when the heating temperature is between AC3 and AC1, because the ferrite is not completely dissolved in the austenite, the uniform martensite cannot be obtained after the fire, which affects the hardness of the workpiece.

(2) The heating temperature of the melting fire is too high, and the holding time is too long. For tool steel, when the heating temperature of the steel is too high, a large amount of carbide dissolves in austenite, which greatly increases the stability of austenite and reduces the transformation point of martensite, so a large amount of carbide remains in the workpiece after dissolution and fire. Retained austenite reduces the hardness of the workpiece after melting.

(3) When heated by melting fire, the surface of the workpiece is decarburized, so that the surface hardness is insufficient. During metallographic analysis, there are ferrite and low-carbon martensite on the surface. When the surface decarburization layer is removed, the hardness reaches the requirement. When the workpiece is heated in a general box furnace without protection or poor protection, or in a salt bath furnace with poor deoxidation, oxidative decarburization will occur.

3. Cooling process problems

(1) Improper choice of melting medium.

(2) The fire-dissolving medium is too old

(3) The temperature of the melting medium is too high, and soft spots are prone to occur.

(4) Improper control of cooling time, mainly in the following two cases: when the two-liquid solution is fired, the residence time of the parts in the water is too short.Or after taking it out of the water, when it stays in the air for too long and then transfers it to the oil to cool the grading and dissolving fire, the residence time in the grading cooling medium is too long, and the bainite transformation occurs, so the hardness is insufficient.

(5) During the cooling process, improper operation will also produce soft spots. The workpiece does not move properly in the melting medium, resulting in the formation of a vapor film in local areas, reducing the cooling rate, and obtaining a uniform martensite structure, resulting in a low local hardness of the workpiece