Matters need to attention in quality inspection of heat treatment

Heat treatment is an important link in mechanical manufacturing. The quality of heat treatment is directly related to the internal quality and performance of products or parts.There are the matters need to attention in quality inspection of heat treatment.

1.Pre-heat treatment

The purpose of pre-heat treatment is to improve the structure and softening of raw materials, so as to facilitate mechanical processing, eliminate stress and obtain the ideal original structure of heat treatment. For some bulk pre-heat treatment is also the final heat treatment, pre-heat treatment is generally used normalizing and annealing.

Heat treatment of pipe

Heat treatment of pipe

1) diffusion annealing of cast steel. Because the grain is easy to thicken after heating for a long time at high temperature, the grain should be refined by complete annealing or normalizing after annealing.

2) complete annealing of structural steel. It is generally used for improving structure, refining grain, reducing hardness and eliminating stress of middle and low carbon steel castings, welding parts, hot rolling and hot forging parts.

3) isothermal annealing of alloy structural steel. Mainly used for annealing 42CrMo steel.

4) spheroidizing annealing of tool steels. The purpose of spheroidizing annealing is to improve the machining performance and cold deformation performance.

5) stress relieving annealing. The purpose of de-stress annealing is to eliminate the internal stress of steel casting, welding and machining parts and to reduce the deformation and cracking in the process.

6) recrystallization annealing. The purpose of recrystallization annealing is to eliminate cold work hardening of the workpiece.

7) normalize. The purpose of normalizing is to improve the structure and refine the grain.

The structures obtained by annealing and normalizing above are pearlite. In the quality inspection, the emphasis is to do the inspection of the process parameters, that is, in the process of annealing and normalizing, do flow inspection of the execution of the process parameters, which is the primary, after the end of the process, the main test hardness, metallographic structure, decarbonization depth, and annealing normalizing items, ribbon, network carbide.

Heated pipe fitting forming in Haihao Factory

Heated pipe fitting forming in Haihao Factory

2.Determination of annealing and normalizing defects

1) the hardness of medium carbon steel is too high, and the heating temperature is too high and the cooling speed is too fast. High carbon steel is mostly low isothermal temperature, insufficient insulation time and so on. The hardness can be reduced by re-annealing according to the correct process parameters.

This kind of structure is mostly found in subeutectoid and hypereutectoid steel. In subeutectoid steel, there is reticular ferrite, and in hypereutectoid steel, there is reticular carbide. The reason is that the heating temperature is too high and the cooling speed is too slow. Inspection shall be conducted according to the prescribed standards.

3) decarburization when annealing or normalizing, in the air furnace, the workpiece without gas protection heating, due to the metal surface oxidation and decarburization.

4) graphite carbon graphite carbon is obtained by the decomposition of carbides, mainly caused by too high heating temperature and too long insulation time. After the occurrence of graphite carbon in steel, it will be found that the quenching hardness is low, soft point, low strength, brittleness, fracture is black and so on.

There are many factors that affect the quality of heat treatment in production. In order to ensure that the product quality meets the requirements stipulated by the national standards or industrial standards, all the heat treatment parts of Haihao Group are inspected strictly after each heat treatment process, starting from the raw materials entering the factory. Product quality problems cannot be directly transferred to the next process, so as to ensure product