Normalizing is to heat the steel pipe (such as seamless pipe and welded steel tube) to 30~50 degrees above Ac3 or Accm, and cool it at a cooling rate slightly higher than annealing after heat preservation. Annealing is to heat the steel pipe to a temperature of Ac3+30~50 degrees or Ac1+30~50 degrees or below Ac1, and generally cool slowly with the furnace temperature.
The purpose of normalizing and annealing:
1. Reduce hardness, increase plasticity, improve cutting and pressure processing performance;
2. Refine grains, improve mechanical properties, and prepare for the next process;
3. Eliminate internal stress caused by cold and hot processing.
Annealing and normalizing belong to the same type of heat treatment. In actual production, the selection of annealing and normalizing is mainly considered from the following three aspects.
1. Considering machinability
The cutting performance of steel pipes, including hardness, chip brittleness, surface roughness and wear on tools, etc. Generally speaking, the hardness of the steel pipe is in the range of 170-230HBW, and the cutting performance is better. If the hardness is too high, it is not only difficult to process, but also makes the tool wear out quickly. If the hardness is too low, it is easy to cause sticking and entanglement of chips during cutting, which reduces the life of the tool, and the cutting surface is rough. In general production, it is more appropriate to use normalizing as a preparatory heat treatment for low and medium carbon structural steel pipes, and annealing (spheroidizing annealing) is better for high carbon structural steel pipes. For alloy pipes, the hardness of steel pipes is increased due to the alloy elements contained, so in most cases, annealing is often used for alloy steel pipes above medium carbon.
2. From the consideration of performance
If the performance requirements of the steel pipe are not too high, normalizing can be used as the final heat treatment. However, if the size of the pipe is large or the shape is complex, normalizing will produce a large residual force or deformation, cracking, and annealing should be selected at this time. The requirements for mechanical properties are high, and quenching + tempering is required for final heat treatment of pipes. From the perspective of reducing the tendency of deformation and cracking, annealing should be selected for preparatory heat treatment.
3. Economic considerations
Normalizing is shorter than annealing production cycle, and easy to operate. Putting it under possible conditions, especially in mass production, it should be given priority to replace annealing with normalizing.
The purpose of normalizing and annealing:
1. Reduce hardness, increase plasticity, improve cutting and pressure processing performance;
2. Refine grains, improve mechanical properties, and prepare for the next process;
3. Eliminate internal stress caused by cold and hot processing.
Annealing and normalizing belong to the same type of heat treatment. In actual production, the selection of annealing and normalizing is mainly considered from the following three aspects.
1. Considering machinability
The cutting performance of steel pipes, including hardness, chip brittleness, surface roughness and wear on tools, etc. Generally speaking, the hardness of the steel pipe is in the range of 170-230HBW, and the cutting performance is better. If the hardness is too high, it is not only difficult to process, but also makes the tool wear out quickly. If the hardness is too low, it is easy to cause sticking and entanglement of chips during cutting, which reduces the life of the tool, and the cutting surface is rough. In general production, it is more appropriate to use normalizing as a preparatory heat treatment for low and medium carbon structural steel pipes, and annealing (spheroidizing annealing) is better for high carbon structural steel pipes. For alloy pipes, the hardness of steel pipes is increased due to the alloy elements contained, so in most cases, annealing is often used for alloy steel pipes above medium carbon.
2. From the consideration of performance
If the performance requirements of the steel pipe are not too high, normalizing can be used as the final heat treatment. However, if the size of the pipe is large or the shape is complex, normalizing will produce a large residual force or deformation, cracking, and annealing should be selected at this time. The requirements for mechanical properties are high, and quenching + tempering is required for final heat treatment of pipes. From the perspective of reducing the tendency of deformation and cracking, annealing should be selected for preparatory heat treatment.
3. Economic considerations
Normalizing is shorter than annealing production cycle, and easy to operate. Putting it under possible conditions, especially in mass production, it should be given priority to replace annealing with normalizing.
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