Chapter 2 Determination of Surface Hardened Layer Depth
Through chemical heat treatment (carburizing quenching, carbonitriding quenching, nitriding, etc.) or surface heat treatment (induction quenching, flame quenching, electron beam quenching, etc.), as well as metal infiltration and non-metal infiltration, the surface of metal parts can be treated to a certain depth. Hardened layer to suit service requirements. On the premise of determining the definition of the hardened layer, the hardness gradient method can be used to measure the depth of the hardened layer. When there is a clear interface between the hardened layer and the matrix, the metallographic method can also be used.
For the preparation of specimens, the general requirements for the preparation of metallographic specimens should be followed (see Chapter 5, Section 5.1). The sample is required to be mounted to protect the surface edge of the sample from being chamfered; the sample is required to be eroded normally for metallographic testing or to understand the hardness layer distribution when using the hardness method. When measuring hardness, it is recommended to do it in the polished state.
2.1 General rules for determination of surface hardened layer depth
The determination of the depth of the hardened layer obtained under specific process conditions should be carried out in accordance with the requirements of relevant testing standards and technical protocols.
2.1.1 Sample selection and sample preparation
The samples for measuring the depth of the surface hardened layer should be taken from the parts after the specific process is completed. The area and quantity taken should be specified in the process documents, and the samples shipped with the furnace can also be selected according to the regulations.
The depth of the surface hardened layer should be measured on a cross-section perpendicular to the hardened layer. The test surface should include the entire hardened layer and part of the core structure, and should be as close as possible to the axial direction of the part or the area specified in the technical agreement.
For thin hardened layer samples, wedge-shaped specimens can be used, as shown in Figure 2-1. Step specimens can also be used when determining the limits of surface hardened layer depth. The height of each step of the step sample is 0.05mm or 0.10mm, and its surface and each step surface must be ground and processed, as shown in Figure 2-2.
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2.1.2 Determination of hardness gradient
The hardness method is used to determine the depth of the surface hardened layer, which is based on the hardness gradient curve from the surface to the inside.
The hardness gradient is usually measured using the Vickers hardness tester, but the Knoop hardness tester can also be used under the conditions agreed by both parties to the agreement. The applicable range of test force can be 0.9807~9.807N (0.1~1kgf).
The hardness gradient is measured at two or more locations upon agreement between the parties to the agreement. The measurement results at each location should be drawn as a hardness change curve relative to the distance from the surface.
The position of hardness measurement is shown in the table method in Figure 2-3. Within the width W of 1.5mm, the Vickers hardness is measured on one or more parallel lines perpendicular to the surface. The distance between these parallel lines should comply with the Vickers hardness test standard requirements. The distance (Δd) between each two adjacent indentations should not be less than the normal times of the diagonal of the indentation. The distance difference (d2-d1) between the centers of successive indentations and the surface should not exceed 0.1mm. The accuracy of measuring the distance from the center of the indentation to the surface should be within ± 25 μm.
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The center-to-surface distance of the first indentation (d1) on the surface should be at least 2.5 times the diagonal of the Vickers hardness indentation.
Based on the measured data, the curve shown in Figure 2-4 can be drawn, and the corresponding hardened layer depth can be obtained based on the definitions of various hardened layer depths.
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2.2 Determination of depth of carburized or carbonitrided quenched hardened layer of steel parts
The depth of the carburized or carbonitrided quenching hardened layer of steel parts (greater than 0.3mm) is generally measured using the method specified in GB/T 9450-2005 "Determination and Verification of the Depth of the Carburized and Quenched Hardened Layer of Steel Parts". This method It is also the only arbitration method.
2.2.1 Definition of depth of carburizing and quenching hardened layer
GB/T 9450-2005 stipulates that the vertical distance from the surface of the part to the point where the Vickers hardness value is 550HV1 is the depth of the carburizing and quenching hardened layer. This 550HV1 is called the limit hardness value. This definition applies to the workpiece that has been carburized or carbonitrided and quenched, and the hardness value at the depth of the quenching hardened layer from the surface (defined as the center) is less than 450HV. When the point is higher than 450HV, a specific value greater than 550HV (with 25HV as the first level) should be selected as the limit hardness. If the core hardness value at this location is 470HV, the limit hardness value should be set as 575HV. If there is another technical agreement, the provisions of the agreement will be implemented.
2.2.2 Determination of hardened layer depth by hardness method
Use a load of 0.9807~9.807N (commonly used: 0.9807~2.942N) to measure the hardness gradient curve from the surface to the inside according to the method introduced in Section 2.1. Generally, two or more hardness curves should be measured at positions agreed upon by both parties. Determine the distance from the workpiece surface where the hardness value is a defined value (such as 550HV or the corresponding Knoop hardness value) based on each curve, as shown in the table in Figure 2-4. When the difference between each value is less than or equal to 0.1mm, the average value is taken as the depth of the hardened layer. If the difference is greater than 0.1mm, the test should be repeated.
Under the condition that the depth of the carburizing and quenching hardened layer has been roughly determined, the interpolation method can be used for verification. At the position where the distance from the surface is less than the estimated hardened layer depth d1 and greater than the estimated hardened layer depth d2
(The " of d2- d1 should not exceed 0.3mm"). Measure at least 5 hardness values (average value) on each of them. The pictures are respectively. The hardened layer depth (CHD) can be obtained according to the interpolation formula (2-1):
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In the formula, d1——the distance from the surface which is less than the depth of the hardened layer;
d2 ———The distance from the surface that is greater than the depth of the hardened layer;
Picture - the arithmetic mean of the hardness measurement values at d1 and d2, as shown in Figure 2-5;
Picture - the limit hardness value (usually 550HVI).
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2.2.3 Method for expressing the results of carburizing and quenching hardened layer depth measurement
According to the hardness test conditions, the depth of the carburizing and quenching hardened layer is expressed as follows:
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2.2.4 Metallographic method to determine the depth of carburizing and quenching hardened layer
When measuring the depth of the hardened layer using the metallographic method, it is recommended to use a metallographic microscope in which the depth of the hardened layer occupies 1/3~2/3 of the field of view.
The metallographic method is generally measured on the metallographic test surface of the normal cross-section of the carburized sample under slow cooling (equilibrium state) conditions. There is no strict and unified industry standard for the specific calculation method of the depth of the carburized and quenched hardened layer. It is generally based on the raw material. (Carbon steel, alloy steel) and technology can be divided into 3 methods, see Table 2-1.
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While ensuring that the workpiece has a sufficiently deep high-carbon zone, it is also generally required that the carbon concentration gradient in the transition zone of the carburized layer should not be too large to slow down the stress mutation between the carburized layer and the matrix. Therefore, it is generally required that the sum of the hypereutectoid layer and the eutectoid layer accounts for approximately 50% to 75% of the total permeability layer.
2.2.5 Determination of shallow carburized hardened layer depth
For workpieces with a carburized (carbonitriding) hardened layer depth below 0.3mm, the above method is not applicable, but GB/T 9451-2005 "Determination of total hardened layer depth or effective hardened layer depth of thin surface layers of steel parts" should be used. prescribed method. This standard stipulates that the distance from the hardened layer measured in the vertical direction of the workpiece surface to the specified microhardness value is called the effective hardened layer depth, and the distance measured from the hardened layer without significant change in the microhardness value is called the total hardened layer depth.
The specific measurement method and expression method are the same as GB/T 9450-2005, but the load used for Vickers hardness test should generally be 1.97N (0.2kgf) and 2.94N (0.3kgf).
2.3 Determination of nitrided layer depth of steel parts
The depth determination of nitriding (nitrocarburizing) layer of steel parts is generally carried out in accordance with GB/T 11354-2005 "Steel Parts - Nitriding Layer Depth Determination and Metallographic Structure Inspection". Specific measurement methods include hardness method, metallographic method and fracture method. Among them, the hardness method is the arbitration method.
2.3.1 Definition of nitriding layer depth
The depth of the nitriding (nitrocarburizing) layer is the sum of the surface compound layer and the diffusion layer.
Defined by hardness: The vertical distance measured from the surface of the sample to a point 50HV higher than the Vickers hardness value of the substrate is the depth of the nitriding layer.
For parts where the hardness of the nitrided layer of carbon steel or low-carbon low-alloy steel changes very gently, the depth of the nitrided layer can be defined as the distance from the surface along the vertical surface direction where the Vickers hardness is 30HV higher than the Vickers hardness of the matrix. If there is another technical agreement, the provisions of the agreement will be implemented.
2.3.2 Determination of nitriding layer depth by hardness method
The hardness method is used to measure the depth of the nitriding layer. Vickers hardness is used according to the standard (Knoop hardness can also be used). The test force is 0.9807~9.807N, and 0.9807~2.942N is usually used.
The depth of the nitriding layer is based on the base hardness of the part. It is required that the average value (accurate to 10 digits) of the 3-point hardness values measured at 3 times the depth of the nitriding layer from the surface is the measured base hardness.
