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The content and method of heat treatment quality inspection

Issuing time:2021-02-26 10:43Author:momo

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 many factors that affect the quality of heat treatment in production. In order to ensure that the product quality meets the requirements of national standards or industry standards, all heat-treated parts start from the raw materials entering the factory, and strict inspections must be carried out after each heat treatment process. Product quality problems cannot be directly transferred to the next process, so as to ensure product quality.


In addition, a qualified inspector is important in heat treatment production. In the heat treatment production process, it is first necessary to see whether the operator strictly implements the process regulations and whether the process parameters are correct. If a quality problem is found during the quality inspection process, the inspector should help the operator analyze the cause of the quality problem and find a way to solve the problem. Various factors that may affect the quality of heat treatment are controlled to ensure that qualified products with high quality, reliable performance and user satisfaction are produced.


A.   The content of heat treatment quality inspection.


a)   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 an ideal heat-treated original structure. For some large parts, the pre-heat treatment is also the final heat treatment, and the pre-heat treatment generally uses normalizing and annealing.


The structure obtained by annealing and normalizing is pearlite. In the quality inspection, the focus is to check the process parameters, that is, to check the execution of the process parameters during the annealing and normalizing process, which is the most important. After the process is over, the main inspections are hardness, metallographic structure, depth of decarburization,and annealing normalizing items, band-shaped, network-shaped carbides, etc.


b)   Judgmentof annealing and normalizing defects.


The hardness of medium carbon steel is too high, and 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. The above problems can be re-annealed according to the correct process parameters to reduce the hardness.


The network structure mostly appears in hypoeutectoid and hypereutectoid steels.Reticulated ferrite appears in hypoeutectoid steel, and reticulated carbide appears in hypereutectoid steel. The reason is that the heating temperature is too high and the cooling speed is too slow, which can be eliminated by normalizing.


When annealing or normalizing, it is carried out in an air furnace. When the workpiece is heated without gas protection, the metal surface is oxidized and decarburized.

Graphite carbon is obtained from the decomposition of carbides, which is mainly caused by excessive heating temperature and long holding time. After graphite carbon appears in the steel, problems such as low quenching hardness, soft point, low strength, high brittleness, and gray-black fracture will be found. When this kind of graphite carbon appears, the workpiece can only be scrapped.


c)   Finally heat treatment. The quality inspection of the final heat treatment in production usually includes the inspection after quenching, surface quenching, andtempering.


It mainly includes: deformation, cracking, overheating and overburning, oxidation and decarburization, soft point, insufficient hardness, salt bath furnace and so on.


B.   Items and methods of heat treatment quality inspection.


The technical requirements of heat-treated parts are different, the heat-treatment process used is 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.


a)   pection of chemical composition.


Spark identification method. Inspectors and heat treatment workers who are experienced in heat treatment production can identify the chemical composition of the material by observing the spark characteristics generated when the material is ground by the grinding wheel.


Spectroscopic analysis. The wavelength and intensity of the spectral lines of different elements can be measured and recorded with a spectrometer, and the elements and content in the material can be obtained by comparing the spectral line table.


Chemical analysis method. Chemical analysis in the laboratory can accurately analyze the content of all elements in metal materials, this method is most commonly used in factories.


Micro area chemical composition analysis. The methods of micro-area chemical composition analysis include electron probe x-ray analysis, auger electronspectroscopy, ion probe analysis and other methods.


b)   Macroscopic tissue inspection and fracture analysis.


Macro inspection method. The macro-experience of steel materials commonly uses acid immersion corrosion methods, including hot acidetching inspection, cold acid etching inspection, electrolytic acid etching andother methods.


Fracture analysis. The analysis includes macro-fracture analysis and micro-fracture analysis.


c)    Microstructure analysis,including the identification of fiber structure after heat treatment of steel, the inspection of steel micro-defects, the inspection of non-metallic inclusions in steel, the determination of layer depth of chemical heat treatment, the organization of gray cast iron and the organization of commonly used non-ferrous metals.


d)   Mechanical performance test, including hardness test and mechanical performance test of heat-treated parts.

e)   Non-destructive testing, including internal defect detection and surface defect detection.


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