Why should we consider machinability during annealing and normalizing?

Normalizing is to heat steel pipes (such as seamless pipes and welded steel pipes) to 30 to 50 degrees above Ac3 or Accm, and then cool them at a slightly higher cooling rate than annealing after insulation. Annealing is to heat the steel pipe to a temperature of Ac3+30~50 degrees or Ac1+30~50 degrees or Ac1 or below, and generally cools slowly with the furnace temperature.

The purpose of normalizing and annealing:
1. Reduce hardness, increase plasticity, and improve cutting and pressure processing performance;
2. Refine the grains, improve mechanical properties, and prepare for the next process;
3. Eliminate the 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. Consider machinability
The cutting performance of steel pipes includes hardness, chip brittleness, surface roughness and tool wear. Generally speaking, the hardness of steel pipes is in the range of 170-230HBW and has better cutting performance. If the hardness is too high, it will not only be difficult to process, but also cause the tool to wear quickly. If the hardness is too low, chips will easily stick and entangle during cutting, reducing tool life and making the cutting surface rough. In general production, it is more appropriate to use normalizing as a preliminary heat treatment for low-carbon and medium-carbon structural steel pipes, and it is better to use annealing (spheroidizing annealing) for high-carbon structural steel pipes. For alloy pipes, the hardness of the steel pipe increases due to the presence of alloying elements, so in most cases, alloy steel pipes with medium carbon or above are often annealed.

2. Considering 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 pipe size is large or the shape is complex, normalizing will produce large residual forces or deformation and cracking. At this time, annealing should be selected. The mechanical performance requirements are high, and the final heat treatment of the pipe requires quenching + tempering. From the perspective of reducing the tendency of deformation and cracking, annealing should be selected for preparatory heat treatment.

3. Economic considerations
Normalizing has a shorter production cycle than annealing and is easy to operate. Where possible, especially in mass production, priority should be given to using normalizing instead of annealing.

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