QC/T 262-2025 in English

Background of Standard Revision and Technological Evolution

QC/T 262-2025 "Metallographic Inspection of Carburized Gears for Automobiles," as an important industry standard replacing the 1999 version, reflects the comprehensive improvement of the automotive manufacturing industry's requirements for gear heat treatment quality control. This revision took 26 years and fully absorbed advanced metallographic inspection technology and automotive gear manufacturing experience from both home and abroad. The standard was jointly drafted by mainstream domestic automotive companies such as Shanghai Automotive Industry Corporation Limited and Dongfeng Commercial Vehicle Co., Ltd., ensuring the practicality and advancement of the technical requirements.

With the development of the automotive industry towards lightweight and high reliability, higher requirements are placed on the fatigue life and load-bearing capacity of carburized gears. The technical changes in the new version of the standard are mainly concentrated in seven aspects: the improvement of terminology definitions, the refinement of specimen requirements, the expansion of inspection items, the updating of assessment criteria, the addition of core structure assessment, the addition of surface defect detection, and the standardization of instrument requirements. These changes have comprehensively improved the scientific nature and operability of the standard.

Analysis of core term definitions

Based on GB/T 7232, the standard adds four new key term definitions, among which the definition of non-martensitic structure is of great technical significance. Non-martensitic structure refers to the black structure produced after quenching when the workpiece is heated and the oxygen generated in the medium diffuses inward along the grain boundaries of the workpiece surface, causing grain boundary oxidation of alloy elements, resulting in reduced hardenability. This structure will significantly reduce the fatigue strength and wear resistance of gears and is a key indicator for quality control.

The clear definitions of surface hardness and core hardness resolve the measurement position disputes in previous inspections. Surface hardness is defined as the hardness of the surface near the pitch circle in the middle of the tooth width, or the hardness tested at 0.10mm from the surface; the core hardness is clearly measured on the cross section in the middle of the tooth width, where the center line of the tooth intersects the root circle. This precise positioning requirement ensures the comparability and repeatability of the inspection results.

Specifications for Sample Preparation and Inspection Methods

The standard sets strict technical requirements for sample preparation: the sample should be cut from a cross section in the middle of the gear tooth width, including the entire area from the tooth top to the tooth root, and the inspection surface of the tooth profile sample should be on the normal section of the tooth. During the preparation process, it must be ensured that the inspection surface is perpendicular to the surface of the carburized layer, and its structure and hardness must not be changed due to heat.

The inspection method adopts a unified 500x field of view comparison method, and the etchant is specified to be 4% nitric acid alcohol solution. This standardized inspection condition ensures the consistency of the results obtained by different laboratories and different inspectors. The comprehensiveness of the inspection items has also been significantly improved, covering seven key indicators such as martensite, retained austenite, carbide, core structure, surface oxidation depth, surface non-martensitic structure depth and effective hardened layer depth.

Metallographic structure grading system

Martensite grading

Martensite grading is based on the size of the martensite needles, ranging from Grade 1 (hidden and fine martensite) (≤3μm) to Grade 6 (coarse martensite) (>20-30μm). Acceptable grades are Grades 1-5, meaning martensite needles no longer than 20μm. Fine martensite provides a better balance of strength and toughness, helping to improve contact fatigue and bending fatigue strength of gears.

Retained austenite grading

The retained austenite content directly affects the dimensional stability and service performance of gears. The standard stipulates that Grades 1-5 are acceptable, corresponding to a retained austenite content no more than 30%. An appropriate amount of retained austenite (5-15%) can improve the toughness and impact resistance of gears, but excessive retained austenite will transform into martensite during service, causing dimensional changes and stress concentration.

Carbide Grade Assessment

Carbide assessment utilizes a dual-track system, network and granular, with a comprehensive assessment based on the morphology, quantity, size, and distribution of carbides. Grades 1-4 are considered acceptable, requiring carbides to be primarily fine-grained and avoid a continuous network distribution. Network carbides can severely reduce gear toughness and become the source of fatigue cracks.

Core Microstructure Grade Assessment

The newly added core microstructure assessment is a key highlight of this revision. This grade is assessed based on the core microstructure morphology and the size, shape, and quantity of ferrite. Acceptable grades are 1-4, requiring primarily low-carbon martensite, with strictly controlled free ferrite content. Appropriate core hardness (usually HRC 35-45) can provide good support for the carburized layer and avoid crush failure during service.

Surface defect control requirements

The standard has added technical requirements for the depth of internal surface oxidation and the depth of surface non-martensitic structure, both of which are required to be no more than 20μm deep. Internal oxidation is the oxidation of alloy elements caused by the diffusion of oxygen along the grain boundaries during heat treatment, and non-martensitic structure is the area of reduced hardenability caused by grain boundary oxidation. These surface defects will significantly reduce the fatigue strength of gears and must be strictly controlled.

The inspection method stipulates that the depth of internal surface oxidation is measured in the unetched state, and the depth of surface non-martensitic structure is measured after shallow etching, both using metallographic method under a 500x field of view. This differentiated inspection condition can accurately distinguish between two different surface defects.

Testing Instruments and Quality Control

The standard explicitly requires that hardness testers and standard blocks, metallographic microscopes, or related micrometer scales must be calibrated and used within the calibration cycle. This mandatory instrument management requirement ensures the accuracy and reliability of test results.

The effective hardened layer depth test is conducted in accordance with GB/T 9450, while surface hardness and core hardness tests are conducted in accordance with GB/T 4340.1 (Vickers hardness) and GB/T 230.1 (Rockwell hardness), respectively. This reference to national standards ensures the standardization of test methods and mutual recognition of results.

Standardized Inspection Report Requirements

The inspection report must include eight elements: standard number, test name, specimen identification, material brand and process, sampling and testing locations, test results and special instructions, signatures of the inspector and reviewer, and inspection date. This standardized reporting format facilitates quality traceability and technical communication. It is particularly important to note that inspection results should be evaluated based on the worst field of view. If the maximum grade is exceeded, a grade greater than 6 may be used, and intermediate (half-grade) grades are not adopted. This strict evaluation principle ensures rigorous quality control. Implementation Recommendations and Considerations When implementing the new standard, companies should focus on the following aspects: First, establish a comprehensive specimen preparation process to ensure that sampling locations and preparation methods are standardized. Second, upgrade inspection equipment to ensure that metallographic microscopes have a 500x field of view and precise measurement capabilities. Third, strengthen inspection personnel training to ensure they are proficient in the assessment criteria for each organizational level. Fourth, improve the quality control system and incorporate the newly added inspection items into routine inspections. Gear manufacturers are advised to optimize their processes according to the requirements of this standard during the process development phase, particularly to control internal oxidation and the formation of non-martensitic microstructures. Measures such as optimizing the carburizing atmosphere and controlling the quenching cooling rate can effectively improve surface quality and increase the service life and reliability of gears.

The technical requirements of this standard are basically aligned with the quality control standards of advanced domestic and foreign automobile manufacturers. The implementation of this standard will help improve the overall level of automotive gear manufacturing in my country and provide technical support for the transformation and upgrading of the automotive industry.