Heat treatment is an important link in machinery manufacturing. The quality of heat treatment is directly related to the inherent quality and performance of products or parts. Many factors affect the quality of heat treatment in production. To ensure that the product quality meets the requirements of national standards or industry standards, all heat treatment parts start from the raw materials entering the factory, and each heat treatment process must be strictly inspected. Product quality problems cannot be directly transferred to the next process, to ensure product quality. In addition, it is not enough for a competent inspector in heat treatment production to inspect and check the quality of the heat-treated workpieces according to the technical requirements. The more important task is to be a good staff officer. In the production process of heat treatment, it is first necessary to see whether the operator strictly implements the process regulations and whether the process parameters are correct. In the process of quality inspection, if a quality problem is found, the operator should analyze the cause of the quality problem and find a solution to the problem. Various factors that may affect the quality of heat treatment are controlled to ensure the production of qualified products with good quality, reliable performance, and customer satisfaction.
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Contents of heat treatment quality inspection
1. Pre-heat treatment
The purpose of pre-heat treatment is to improve the structure and softening of raw materials, to facilitate machining, eliminate stress, and obtain the ideal heat-treated original structure. For some large pieces, the pre-heat treatment is also the final heat treatment, and the pre-heat treatment generally adopts normalizing and annealing.
1) Diffusion annealing of steel castings Since the grains are easily coarsened by heating for a long time at a high temperature, complete annealing or normalizing should be performed again after annealing to refine the grains.
2) Complete annealing of structural steel is generally used to improve the structure, refine grains, reduce hardness, and eliminate the stress of medium and low carbon steel castings, welded parts, hot rolling, and hot forgings.
3) Isothermal annealing of alloy structural steel It is mainly used for annealing of 42CrMo and other steels.
4) Spheroidizing annealing of tool steel The purpose of spheroidizing annealing is to improve machinability and cold deformation performance.
5) Stress relief annealing The purpose of stress relief annealing is to eliminate the internal stress of steel castings, welded parts, and machined parts, and reduce deformation and cracking in the subsequent process.
6) Recrystallization annealing The purpose of recrystallization annealing is to eliminate the cold work hardening of the workpiece.
7) Normalizing The purpose of normalizing is to improve the structure and refine the grains. It can be used as a pre-heat treatment or as final heat treatment.
The structures obtained by the above annealing and normalizing are all pearlite. In the quality inspection, the focus is on the inspection of the process parameters, that is, in the process of annealing and normalizing, the flow inspection of the implementation of the process parameters is the first and foremost. Carbon depth, and annealed normalized items, ribbons, reticulated carbides, etc.
2. Judgment of annealing and normalizing defects
1) Too high hardness The hardness of medium carbon steel is too high, often because the heating temperature is too high and the cooling rate is too fast during annealing. High carbon steel is mostly low isothermal temperature, insufficient holding time, and so on. If the above problems occur, the hardness can be reduced by re-annealing according to the correct process parameters.
2) Reticular structure This type of structure mostly occurs in hypereutectoid and hypereutectoid steel, reticulated ferrite in hypoeutectoid steel, and reticulated carbide in hypereutectoid steel, because the heating temperature is too high, the cooling rate is too slow, which can be eliminated by normalizing. Inspection according to the specified standards.
3) Decarburization When annealing or normalizing, it is carried out in an air furnace. When the workpiece is heated without gas protection, decarburization occurs due to oxidation of the metal surface.
4) Graphite carbon is obtained from the decomposition of carbides, mainly caused by excessive heating temperature and long holding time. After the appearance of graphite carbon in the steel, problems such as low quenching hardness, soft spot, low strength, high brittleness, and gray-black fracture will be found. When such graphite carbon appears, the workpiece can only be scrapped.
3. Final heat treatment
The quality inspection of the final heat treatment in production usually includes quenching, surface quenching, and inspection after tempering.
1) Deformation. The quenching deformation shall be checked as required. If the deformation exceeds the specification, it should be straightened again. If it cannot be straightened for some reason, and the deformation exceeds the machining allowance, it can be repaired. The method is to put the workpiece in a soft state. After straightening, quenching and tempering can meet the requirements again. Generally, the deformation of the workpiece after quenching and tempering is not more than 2/3 to 1/2 allowance.
2) Cracking. No cracks are allowed on the surface of any workpiece, so the heat-treated parts must be 100% inspected, and the stress concentration parts, sharp corners, keyways, thin-walled holes, thick-thin junctions, protrusions, depressions, etc. should be checked.
3) Overheating and overburning. Overheating and overburning are detected with a metallographic microscope. After the workpiece is quenched, there must be no coarse acicular martensite overheating structure and grain boundary oxidation overburning structure, because overheating and overburning will cause the strength to decrease, the brittleness to increase, and it is easy to crack.
4) Oxidation and decarburization. For workpieces with small machining allowances, oxidation and decarburization should be controlled more strictly. For cutting tools and abrasives, decarburization is not allowed. When severe oxidation and decarburization are found in quenched parts, the heating temperature must be too high or The holding time is too long, so an overheating test must be carried out at the same time.
5) Soft point. Soft spots will cause wear and fatigue damage to the workpiece, so no soft spots are allowed. Most of them are caused by improper heating and cooling or uneven organization of raw materials, band-like structure and residual decarburization layer, etc. After soft spots appear, it should be timely. Carry out repair processing.
6) Insufficient hardness. Usually, when the workpiece is quenched, the heating temperature is too high, too much-retained austenite will cause the hardness to decrease, the heating temperature is too low or the holding time is insufficient, and the quenching cooling speed is not enough, improper operation, etc. will cause insufficient quenching hardness. In the above cases, only repair processing is required.
7) Salt bath furnace. Workpieces with high and medium frequency and flame quenching are not allowed to have burned.
The surface of the parts after the final heat treatment is not allowed to have defects such as corrosion, bumps, shrinkage, damage, and so on.
Items and methods of heat treatment quality inspection.
The technical requirements of heat-treated parts are different, the heat-treatment processes used are also different, and the items and methods for quality inspection are also different. The quality inspection items and methods commonly used in heat treatment production are as follows.
1. Inspection of chemical composition.
1) Spark identification method. Inspectors and heat treatment workers who have experience in heat treatment production can identify the chemical composition of part materials by observing the spark characteristics generated when the material is ground by a grinding wheel.
2) Spectral analysis. The wavelength and intensity of the spectral lines of different elements can be measured and recorded with a spectrometer, and the elements and contents contained in the material can be obtained by comparing the spectral line table.
3) Chemical analysis. The chemical analysis method in the laboratory can accurately analyze the content of all elements in the metal material, and this method is most commonly used in the factory.
4) Analysis of the chemical composition of the micro area. Micro-area chemical composition analysis methods include electron probe x-ray analysis, Auger electron spectroscopy analysis, ion probe analysis, and other methods.
2. Macrostructure inspection and fracture analysis.
1) Macroscopic inspection method. The macro-experience of steel commonly used acid etching corrosion methods, including hot acid etching inspection, cold acid etching inspection, electrolytic acid etching, and other methods.
2) Fracture analysis. The analysis includes macro fracture analysis and microfracture analysis.
3. Microstructure analysis.
1) Identification of fiber structure after heat treatment of steel.
2) Microscopic defect inspection of steel.
3) Inspection of non-metallic inclusions in the steel.
4) Determination of layer depth of chemical heat treatment.
5) Microstructure inspection of gray cast iron.
6) Microstructure analysis of commonly used non-ferrous metals.
4. Mechanical properties test.
1) Hardness inspection of heat-treated parts.
2) Mechanical properties test of heat-treated parts.
5. Nondestructive testing.
1) Detection of internal defects.
2) Surface defect detection.
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