According to different conditions, the metal material is heated to a suitable temperature and kept warm, and then cooled in different ways to change the metallographic structure of the metal material and obtain the required structural properties. This process is often referred to as heat treatment of metallic materials. So, what processes are included in the heat treatment of seamless pipes ?
The heat treatment of metal materials is divided into overall heat treatment, surface heat treatment and chemical heat treatment. The heat treatment of seamless steel pipes generally adopts overall heat treatment.
According to its structure, seamless steel pipes can be divided into one-stage, two-stage and three-stage steel pipes. Among them, two-stage or three-stage roller hearth furnaces are mostly used for bright heat treatment of seamless steel pipes, and are generally called roller hearth bright heat treatment furnaces. One-stage roller hearth furnace is used for continuous heat treatment furnace.
The heat treatment method is specified in the seamless steel pipe standard, and the performance requirements that the seamless steel pipe should meet are specified in the standard. Generally, the finished heat treatment of low carbon steel seamless steel tubes adopts complete annealing or normalization; while the chromium-nickel austenitic stainless steel seamless steel tubes adopts solution treatment.
After the seamless steel pipe is hot-rolled, the non-metallic inclusions (mainly sulfides and oxides, and silicates) in the steel are pressed into thin sheets and delaminated. The residual stress caused by uneven cooling is much larger than the strain caused by load.
Heat treatment of seamless steel pipe:
Carbon steel seamless pipes need to go through basic processes such as heating, heat preservation, and cooling during heat treatment. During these processes, carbon steel pipes (CS pipe) may develop defects. The heat treatment defects of carbon steel seamless steel pipes mainly include unqualified steel pipe structural performance, unqualified size, surface cracks, scratches, severe oxidation, decarburization, overheating or overburning, etc.
The main three processes of carbon steel seamless pipe heat treatment:
1. The first process of heat treatment of seamless steel pipe is heating.
There are two different heating temperatures: one is heating below the critical point Ac1 or Ac3; the other is heating above the critical point Ac1 or Ac3. Under these two heating temperatures, the structural transformation of the steel pipe is completely different. Heating below the critical point Ac1 or Ac3 is mainly to stabilize the structure of the steel and eliminate the internal stress of the steel pipe; heating above Ac1 or Ac3 is to austenitize the steel.
2. The second process of seamless pipe heat treatment is heat preservation.
Its purpose is to make the heating temperature of the steel pipe uniform to obtain a reasonable heating structure.
3. The third process of seamless pipe heat treatment is cooling.
The cooling process is the key process of steel pipe heat treatment, which determines the metallographic structure and mechanical properties of the steel pipe after cooling. In actual production, there are various cooling methods for steel pipes. Commonly used cooling methods include furnace cooling, air cooling, oil cooling, polymer cooling, water cooling, etc.