Heat treatment of forgings is an important link in machinery manufacturing. The quality of heat treatment is directly related to the intrinsic quality and performance of products or parts. There are many factors affecting the quality of heat treatment in the production. In order to ensure that the quality of forgings meets the requirements of the national or industry standards, all the heat treatment forgings start from the raw materials into the factory, and strict inspection must be carried out after each heat treatment process. Product quality problems can not be directly transferred to the next process, so as to ensure product quality. In addition, in heat treatment production, it is not enough for a competent inspector to conduct quality inspection and check on the forgings after heat treatment according to the technical requirements. The more important task is to be a good adviser. In the process of heat treatment, it is necessary to see whether the operator strictly implements the process rules and whether the process parameters are correct. In the process of quality inspection if quality problems are found to help the operator to analyze the causes of quality problems, find out the solution to the problem. All kinds of factors which may affect the quality of heat treatment are controlled to ensure the production of qualified products with good quality, reliable performance and customer satisfaction.

Content of heat treatment quality inspection 

(1) Pre-heat treatment of forging

The purpose of preheat treatment of forgings is to improve the microstructure and softening of raw materials, so as to facilitate mechanical processing, eliminate stress and obtain the ideal original microstructure of heat treatment. Pre-heat treatment for some large parts is also the final heat treatment, pre-heat treatment is generally used normalizing and annealing.

1) The diffusion annealing of steel castings is easy to coarsene because the grains are heated at high temperature for a long time. After annealing, complete annealing or normalizing should be carried out again to refine the grains.

2) Complete annealing of structural steel is generally used to improve microstructure, refine grain, reduce hardness and eliminate stress of medium and low carbon steel castings, welding parts, hot rolling and hot forgings.

3) Isothermal annealing of alloy structural steel is mainly used for annealing of 42CrMo steel.

4) Spheroidizing annealing of tool steel The purpose of spheroidizing annealing is to improve the cutting performance and cold deformation performance.

5) Stress relief annealing The purpose of stress relief annealing is to eliminate the internal stress of steel castings, welding parts and machined parts, and reduce the deformation and cracking of the post-process.

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

7) Normalizing the purpose of normalizing is to improve the structure and refine the grain, which can be used as a pre-heat treatment or as a final heat treatment.

The structures obtained by annealing and normalizing are pearlite. In the quality inspection, the focus is to do the inspection of process parameters, that is, in the process of annealing and normalizing, do flow check the execution of process parameters, which is the first, at the end of the process mainly test hardness, metallographic structure, decarbonization depth, and annealing normalizing items, ribbon, mesh carbide and so on.

 (2) The judgment of annealing and normalizing defects

1) The hardness of medium carbon steel is too high, which is often caused by high heating temperature and too fast cooling rate during annealing. High carbon steel is mostly isothermal temperature is low, holding time is insufficient and so on. If the above problems occur, the hardness can be reduced by re-annealing according to the correct process parameters.

2) This kind of organization appears in subeutectoid and hypereutectoid steel, subeutectoid steel network ferrite, hypereutectoid steel network carbide, the reason is that the heating temperature is too high, cooling rate is too slow, can be used to eliminate normalizing. Inspect according to the specified standard.

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

4) Graphite carbon Graphite carbon is produced by the decomposition of carbides, mainly caused by high heating temperature and too long holding time. After the appearance of graphite carbon in steel, it will be found that the quenching hardness is low, soft point, low strength, brittleness, fracture is gray black and other problems, and the workpiece can only be scrapped when the graphite carbon appears.

 (3) Final heat treatment

The quality inspection of the final heat treatment of forgings in production usually includes quenching, surface quenching and tempering.

1) Deformation. Quenching deformation should be checked according to the requirements, such as the deformation exceeds the provisions, should be straightened, such as for some reason can not be straightened, and the deformation exceeds the processing allowance, can be repaired, the method is to quench and temper the workpiece in the soft state straightening to meet the requirements again, the general workpiece after quenching and tempering deformation, not more than 2/3 to 1/2 allowance.

2) Cracking. No cracks are allowed on the surface of any workpiece, so the heat treatment parts must be 100% inspected. Stress concentration areas, sharp corners, keyways, thin wall holes, thick-thin junctions, protrusions and dents, etc., should be emphasized.

3) Overheat and overheat. After quenching, the workpiece is not allowed to have coarse acicular martensite superheated tissue and grain boundary oxidation superheated tissue, because overheating and overburning will cause strength reduction, brittleness increase and easy cracking.

4) Oxidation and decarbonization. Processing allowance of small workpiece, oxidation and decarbonization to control some strict, for cutting tools and abrading tools, not allowed to have decarbonization phenomenon, in the quenching parts found serious oxidation and decarbonization, heating temperature must be too high or holding time is too long, so it must be at the same time for overheating inspection.

5) Soft spots. Soft point will cause workpiece wear and fatigue damage, so there is no soft point, the formation of the reasons for improper heating and cooling or uneven organization of raw materials, the existence of banded organization and residual decarbonization layer, and so on, soft point should be repaired in time.

6) Insufficient hardness. Usually the workpiece quenching heating temperature is too high, too much residual austenite will lead to the reduction of hardness, low heating temperature or insufficient holding time, and quenching cooling speed is not enough, improper operation will result in insufficient quenching hardness. The above situation can only be repaired.

7) Salt bath furnace. High and medium frequency and flame quenching workpiece, no burn phenomenon.

After the final heat treatment of the parts surface shall not have corrosion, bump, shrinkage, damage and other defects.