QC/T 262-2025Metallographic examination for automobile carburized gears (English PDF)

3.1 Surface hardness

The hardness at the surface near the pitch circle in the middle of the tooth width, or the hardness measured at a distance of 0.10 mm from the surface.

3.2 Core hardness

The hardness at the intersection of the tooth centerline and the tooth root circle on the cross-section at the middle of the tooth width.

3.3 Effective hardened depth

The vertical distance from the working surface of the tooth to the specified hardness point in its vertical direction.

3.4 Non-martensitic structure

A black structure generated after quenching when a workpiece is heated, oxygen generated in the medium diffuses inward along the grain boundaries of the workpiece surface, causing oxidation of the alloying elements at the grain boundaries and reducing hardenability.

4.2 When preparing the specimen, the test surface shall be perpendicular to the surface

of the infiltrated layer.

4.3 During specimen preparation, its structure and hardness must not be altered due to heating.

5.1 Metallographic inspection specimens are prepared according to GB/T 13298.A 4%

nitric acid-alcohol solution is used as the etching agent.

5.2 The classification of inspection items and inspection methods are shown in Table 1.

Table 1 -- Inspection items and inspection methods

5.3 The inspection locations are specified as follows.

a) The inspection sites for martensite and retained austenite are both located at the pitch circle of the tooth, 0.05 mm~0.15 mm away from the surface of the diffusion layer;

b) The detection site for carbon compounds is at the tip of the tooth in Figure 1, or according to process requirements;

c) Unless otherwise specified, the surface hardness and effective hardened depth of gears are generally measured at the pitch circle of the tooth surface in Figure 1;

d) The depth of non-martensitic structure on the surface is inspected at the transition surface between the pitch circle and the root of the tooth in Figure 1.Unless otherwise specified, the area at the bottom of the tooth groove may be excluded from the inspection;

e) The core hardness and tissue inspection sites are located at the gear core position in Figure 1.

6.1 General

6.1.1 Metallographic microstructure grading includes the grading of martensite, retained austenite, carbides, core microstructure, surface oxidation depth, and surface non-martensitic structure depth. Grading charts are provided in Annexes A, B, C, D, and E.

6.1.2 The worst field of view shall be selected for the examination and evaluation of metallographic structures. If the highest grade is exceeded, a grade greater than 6 can be used.

6.1.3 The assessment of metallographic structures does not adopt intermediate (half-grade) grades.

6.2 Martensitic grade

The grade is determined based on the size of the martensitic needles. The assessment is conducted according to the grade images in Annex A. The assessment characteristics are explained in Table 2.Grades 1~5 are considered acceptable.

6.3 Residual austenite grade

The grade is determined based on the amount of retained austenite. It is assessed according to the grade images in Annex B. The assessment characteristics are explained in Table 3.Grades 1~5 are considered acceptable.