Introduction of commonly used methods for measuring the depth of decarburized layer

Date：2022-11-04 14:30:00Views：1233

Decarburization is the phenomenon that all or part of the carbon on the steel surface is lost due to the oxidation atmosphere during the heating or insulation process of various hot working procedures. The depth of decarburized layer refers to the distance from the surface of decarburized layer to the matrix of decarburized layer where the difference in metallographic structure can no longer be distinguished. The following is a brief analysis of the decarburization layer depth testing method for your reference.

Decarburization:

Decarburization is a heat treatment defect of steel.

Causes problems:

It affects the mechanical and thermal fatigue properties of the parts.

Characterization of decarburization degree:

The depth of decarburized layer can be directly used to characterize the degree of decarburization on the surface of materials and parts. Therefore, the measurement of the depth of decarburized layer on the surface of steel and parts has always been the focus of inspection personnel and quality control personnel.

Decarburization of steel surface greatly reduces the surface hardness, tensile strength, wear resistance and fatigue limit of steel. Therefore, the decarburization layer is specified in the relevant standards of tool steel, bearing steel, spring steel, etc. Decarburization defects are not allowed for important mechanical parts. Therefore, the decarburization layer of parts must be removed during processing.

Test method:

1. Metallography

Metallographic method is to observe the change of the sample structure with the change of carbon content from the surface to the core under the optical microscope. This method is applicable to steel grades with annealed or normalized structure (ferrite+pearlite), as well as products in quenching and tempering, quenching+tempering, rolling or forging conditions.

2. Hardness method

The decarburized layer measured by hardness method can be divided into microhardness method and Rockwell hardness method.

2.1 Microhardness method Microhardness method This method is used to measure the distribution ladder of microhardness values on the cross section of the specimen in the direction perpendicular to the surface. This method is only applicable to steel grades with deep decarburized layer but small thickness compared with the quenching zone. Microhardness method is not accurate for low carbon steel.

2.2 Rockwell hardness method - directly measure on the surface of the sample when using a Rockwell hardness tester. For products that are not allowed to have decarburized layer, the measurement shall be conducted directly on the original product surface of the sample, and for samples that are allowed to have decarburized layer, the measurement shall be conducted on the surface with the decarburized layer removed. The Rockwell hardness method is used to determine the Rockwell hardness value HRC according to GB/T 230.1-2009. This method is only used to determine whether the product is qualified.

3. Determination of carbon content

3.1 Chemical analysis method - use chemical analysis method to measure the carbon content of metal chips peeled layer by layer to determine the depth of decarburization layer. The test chips with a depth of 0.1mm shall be peeled layer by layer parallel to the surface of the sample by mechanical processing method, and the oxide film shall be removed in advance; Collect the metal test chips peeled from each layer and measure the carbon content according to GB/T 20126-2006. The distance from the surface to the point where the specified value is reached is the depth of decarburization layer.

3.2 Spectral analysis method - grind the plane sample layer by layer, with the interval of each layer of 0.1mm, and conduct the spectral determination of carbon on each layer. Try not to overlap the layer by layer spectral spark discharge areas. The depth of decarburization layer is measured from the surface to the point where the carbon content reaches the specified value. Generally, it is only applicable to plane specimens with proper size.

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