NDT for Quenched Gear: Heat Penetration Depth, Surface Hardening Layer Depth and Core Hardness

1. Overview

The frequency of damage to induction-hardened gears during use is high, particularly for surface-hardened gears such as the second shaft gear of a shearer rocker arm and sun gears with a modulus of 8-10. These gears often experience damage in the form of pitting, peeling, and tooth breaking.

After analyzing, we found that there is no hardened layer below the pitch circle of the gear teeth and the tooth center and tooth root are not quenched, meaning that the heat penetration depth during surface quenching does not reach these areas.

While we can theoretically control the depth of the hardened layer by adjusting the electrical parameters of the induction heating equipment, it is difficult to achieve accurate control in practice due to various factors that can affect the depth.

Current quality control of the hardening process relies on random inspections and damage tests, which are both time-consuming and costly. Given our company’s focus on producing a variety of small batch products, we need to find more efficient testing methods.

Nondestructive testing has the benefits of being non-destructive, having a high testing ratio, being highly efficient, and being cost-effective.

To test the heat penetration depth and effective hardened layer depth of gears with modules 6, 8, 9, and 10, we use the QNET type equipment, a second-generation multi-channel non-destructive measurement system for hardened layer depth manufactured by the IZFP, Institute of Non-Destructive Testing, and the Fraunhofer Institute in Germany. Orthogonal tests and comparison tests using physical and chemical anatomy are performed.

2. Theoretical basis of test

The heat treatment process of induction hardening gear is: carburizing → quenching and tempering → surface hardening.

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The gear material used is typically 18Cr2Ni4WA or 20Cr2Ni4A, which is a type of fine-grain steel. After induction hardening, the induction heating layer undergoes recrystallization.

The fast induction heating process results in a finer, smaller, and denser structure on the surface, which makes it difficult for ordinary ultrasonic waves (1-5 MHz) to penetrate the induction layer.

Additionally, the gear undergoes carburization during induction hardening. Fine granular carbides in the infiltrated layer help to stabilize the structure by pinning the grains on the surface of the infiltrated layer, making them even finer. However, the dense surface hardening layer and the heat penetration depth are transparent to 20 MHz ultrasonic waves.

When the 20 MHz ultrasonic wave reaches the interface between the hardened layer and the heat penetration depth, backscattered echoes occur, as shown in Fig. 1 and Fig. 2.

Fig. 1 ultrasonic backscatter detection method

Fig. 2 ultrasonic backscatter echo

The heat penetration depth and the hardened layer depth of the surface hardened gear can be obtained by using the following formula.

Rht=(vtcos β)/ two


  • Rht — hardened layer or heat penetration depth (mm);
  • V — sound velocity in the tested material (mm / s);
  • t — time (s) of acoustic wave from workpiece surface to interface;
  • β — Ultrasonic refraction angle of hardened layer (°).

Currently, the gear tooth materials 18Cr2Ni4WA and 20Cr2Ni4A used in production are martensitic steels.

Data and measurements indicate that for gear teeth with a modulus of 10 or less, as long as the gear teeth are heated to the austenitic temperature range and then air-cooled, the core hardness will be at least 36.0 HRC.

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Therefore, the heat penetration depth of induction-hardened gears can reach the core of the gear teeth, ensuring that the core hardness meets the technical requirements.

3. Detection scheme

Given the current situation of the surface-quenched gear sample, we have developed the following inspection plan:

(1) Use a high-resolution induction hardening layer thickness detector imported from Germany.

(2) Choose and establish appropriate lower and upper limit values (LN and UN).

(3) Refer to Table 1 for the technical requirements of four types of surface-hardened, carburized gears with cutting modules of 6, 8, 9, and 10.

Table 1 quenching technical requirements for test gear samples

Gear number:ModulusTexture of materialPenetration depth of tooth surface / mmTooth core hardness (HRC)Matrix hardness (HRC)

(4) Choose a high-frequency (20 MHz) angle probe and a straight probe to detect the tooth surface and tooth top, respectively.

(5) The depth of the effective hardened layer and the heat penetration depth of gear samples were determined through microhardness testing and metallographic macro analysis, respectively.

(6) The accuracy and reliability of the NDT data were assessed by comparing the NDT method with conventional microanalysis and metallography techniques.

4. Test results and analysis

The Fraunhofer IZFP company’s multi-channel non-destructive measurement system, P3213QNET, was used to test five different gear types. Nondestructive testing and physical and chemical testing were performed on the tooth top, tooth surface, and tooth root using narrow screen wedge blocks at the tooth top, tooth plane, and tooth root, respectively.

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The results are shown in Table 2.

Table 2 Comparison of test results of two methods

Gear number:










Distance between tooth crest and tooth Center / mm





NDT results

Effective hardened layer of tooth surface pitch circle / mm





Heat penetration depth / mm





Tooth core hardness (HRC)





Physical and chemical test results

Effective hardened layer of tooth surface pitch circle / mm





Heat penetration depth / mm





Tooth core hardness (HRC)





According to the test results, the following conclusions can be drawn:

(1) The values obtained from non-destructive testing and anatomical analysis methods for thermal penetration depth detection are in agreement.

(2) The results of indirect non-destructive testing of tooth center hardness are consistent with those obtained through anatomical analysis.

(3) For effective hardened layer depth detection:

① When the probe can reach the pitch circle of gear teeth (m = 8, m = 9), the values obtained from non-destructive testing are in agreement with those from anatomical analysis.

② When the probe cannot reach the pitch circle, the data deviation is significant.

(4) Through a comparison of non-destructive testing and physical and chemical testing, the non-destructive testing method can be used to determine the heat penetration depth, surface hardened layer depth, and surface hardened gear core hardness.

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