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

1. Overview

The frequency of damage of induction hardened gears during use is high, especially the surface hardened gears such as the second shaft gear of the shearer rocker arm and the sun gear with a modulus of 8-10, which are often damaged by pitting, peeling and tooth breaking.

After the analysis, there is no hardened layer below the pitch circle of the gear teeth or the tooth center and tooth root are not quenched, that is, the heat penetration depth of the gear teeth during surface quenching does not reach the tooth center or tooth root.

Although the depth of the hardened layer can be controlled theoretically by adjusting the electrical parameters of the induction heating equipment, due to too many factors affecting the depth of the hardened layer, it is impossible to control it more accurately in actual production.

Up to now, the quality control of hardening process is limited to random random random inspection and damage test, which is time-consuming and costly.

Especially for the characteristics of our company’s multi varieties and small batch production, we need to find efficient testing methods.

The nondestructive testing method has the advantages of non-destructive testing, high testing ratio, high efficiency and low cost.

QNET type equipment, the second generation new product of multi-channel non-destructive measurement system for hardened layer depth manufactured by IZFP, Institute of non destructive testing, Fraunhofer Institute, Germany, is used to test the heat penetration depth and effective hardened layer depth of gears with modules 6, 8, 9 and 10 respectively by orthogonal test, and the comparison test is carried out by physical and chemical anatomy.

2. Theoretical basis of test

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

The gear material is generally 18Cr2Ni4WA or 20Cr2Ni4A, which is an essentially fine grain steel. After induction hardening, the induction heating layer recrystallizes.

Because the induction heating speed is fast, the structure formed on the surface is finer, smaller and denser, and it is difficult for ordinary ultrasonic waves (1-5MHz) to pass through the induction layer.

At the same time, the induction hardening gear is carburized.

Because there are fine granular carbides in the infiltrated layer, they play the role of pinning in the induction hardening, making the grains on the surface of the infiltrated layer more fine. However, the dense surface hardening layer and the heat penetration depth are almost transparent to the 20MHz ultrasonic wave.

When the 20MHz ultrasonic wave occurs at the interface between the hardened layer and the heat penetration depth, the backscattered echo occurs respectively (see 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 (°).

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

Relevant data and measured data show that for gear teeth with modulus ≤ 10, as long as the gear teeth can be heated to the austenitic temperature range and after air cooling, the core hardness is ≥ 36.0HRC.

Therefore, the heat penetration depth of induction hardened gear wheels can reach the core of the gear teeth, and the core hardness can meet the technical requirements.

3. Detection scheme

In view of the actual situation of the surface quenched gear sample, the following inspection scheme is specially formulated:

(1) Select the induction hardening layer thickness detector imported from Germany with high resolution.

(2) Select and set reasonable LN and UN values.

(3) See Table 1 for the technical requirements of four kinds 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) Select high-frequency (20MHz) angle probe and straight probe to detect from the tooth surface and tooth top respectively.

(5) The effective hardened layer depth and heat penetration depth of gear samples were tested by microhardness method and metallographic macro analysis method respectively.

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

4. Test results and analysis

The multi-channel non-destructive measurement system P3213QNET of Fraunhofer IZFP company is used to test five kinds of gears, and the narrow screen wedge block at the tooth top, the wedge block at the tooth plane and the wedge block at the tooth root are selected to carry out non-destructive testing and physical and chemical testing on the tooth top, the tooth surface and the tooth root.

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) In terms of thermal penetration depth detection, the values measured by non-destructive testing method and anatomical analysis method are consistent.

(2) In the indirect non-destructive testing of the hardness of the tooth center, the non-destructive testing method is consistent with the value measured by the anatomical analysis method.

(3) Effective hardened layer depth detection:

① The probe can touch the samples at the pitch circle of the gear teeth (m = 8, m = 9).

The values measured by the nondestructive testing method are consistent with those measured by the anatomical analysis method.

② The probe cannot touch the pitch circle, and the data deviation is large.

(4) Through the comparative analysis of nondestructive testing and physical and chemical testing, the non-destructive testing method can be used to detect the heat penetration depth, surface hardened layer depth and core hardness of surface hardened gears.

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