Common Problems and Solutions of Annealing Seamless Steel Tubes of Special Steel Materials and Alloys

How can a workpiece obtain a fine wave structure with excellent performance?

Annealing treatment will soften steel, while quenching treatment will harden steel. By contrast, if normalized treatment is applied, the layered structure can be obtained, which can effectively improve the machinability and wear resistance of steel, and has the advantages of no cracks, less deformation and convenient operation. However, normalization is a difficult heat treatment technology, because it uses air cooling, which will be affected by many factors, such as different cooling effects in summer and winter, different workpiece sizes on air cooling rate, and even wind blowing will affect the cooling rate. Therefore, various methods should be used to maintain uniformity in normalization treatment, such as sunshade, curtain, potholes, fans, etc.

Difference between normalization treatment and annealing treatment

Normalization treatment is a heat treatment procedure of heating to 40~60℃ above A3 point or Acm point for a period of time to make the steel structure become uniform austenite structure, and then cooling to room temperature in static air. For hypoeutectoid steel, the purpose of grain refinement can be achieved and it has good strength and toughness; For hypereutectoid steel, it can prevent the formation of network precipitation on austenite grain boundary and reduce the toughness of the material. The main purpose of complete annealing treatment is to soften steel and improve the machinability of steel. The heat treatment procedure is to heat to 20~30℃ above point A3 (hypoeutectoid steel) or 30~50℃ above point A1 for a period of time, so that after the formation of complete austenite structure (or austenite plus austenite carbon structure), the wave body metamorphosis will fully occur at 50℃ below point A1 to obtain softened steel. In addition, stress relief annealing is heating at 450~650℃ below the transformation point for a period of time and then slowly cooling to room temperature, which can eliminate the residual stress generated in the steel during cutting, stamping, casting and welding.

How to eliminate residual stress of workpiece.

Stress relief annealing is heating at 450~650℃ below the transformation point for a period of time and then slowly cooling to room temperature, which can eliminate the residual stress generated in the steel during cutting, stamping, casting and welding. For carbon steel, the reference heating temperature is 625 25℃; For alloy steel, the reference heating temperature is 700 25℃. The holding time will also be different. For carbon steel, the holding time is 1 hour per 25mm thickness; For alloy steel, the holding time is 2 hours per 25mm thickness, and the cooling rate is below 275℃/ hour after 25mm.

How to prevent heating deformation.

To prevent heating deformation, it is best to heat slowly and carry out preheating treatment. When selecting preheating temperature for general steel, the preheating temperature can be selected according to the following criteria:
(1)The preheating temperature is below the transformation point, for example, about 650~700℃ for ordinary steel and about 800~850℃ for high speed steel.
(2)Take about 500℃ as preheating temperature.
(3)Two-stage preheating, first preheating at about 500℃, after fully preheating for a period of time, raising the preheating temperature to below A1 abnormal point.
(4)Three-stage preheating. For large steel products with high alloy content, such as high-speed steel, the third stage preheating is sometimes required at 1000~1050℃.
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