Gear Manufacturing Process-gear Hardness Testing And Metallographic Analysis

After the gear has completed the heat treatment process (such as carburizing and quenching, induction hardening or nitriding), its surface hardness, core hardness and microstructure distribution directly determine its fatigue strength, wear resistance and overall life. In order to ensure that the heat treatment process meets the design requirements, a comprehensive quality assessment must be conducted through hardness testing and metallographic analysis.

The purpose of gear heat treatment inspection

The essence of heat treatment is to improve the mechanical properties of gears through microstructure transformation. In order to verify whether the heat treatment quality is qualified, the following three points should be confirmed:

Whether the surface hardness meets the design requirements (such as 58~62 HRC)

Whether the depth of the effective hardening layer is sufficient

Whether there are adverse structures such as martensite, residual austenite and reticular cementite in the metallographic structure

Analysis of gear hardness testing method

Brinell hardness test

Scope of application: gear core hardness testing, heat treatment before rough testing

Principle: steel ball or hard alloy ball is pressed into the workpiece with a large load, and the diameter of the indentation is measured

Recommended standards: GB/T 231.1, ISO 6506

Vickers hardness test（Vickers Hardness, HV）

Scope of application: gear surface hardness and effective hardened layer depth detection

Principle: The pyramid diamond indenter is used to press the material with a small load, and the hardness value is calculated by measuring the diagonal length

Recommended standards: GB/T 4340.1, ISO 6507

Definition and evaluation of effective hardening layer depth (CHD)

definition

The effective hardened layer depth (Case Hardening Depth, CHD) is the depth at which the hardness drops to the base hardness +50HV or to 550HV (the usual value).

Measurement steps:

The gear is cut along the center line of the tooth thickness after quenching

Sand and polish with fine grit sandpaper and polishing cloth

Make 5 to 10 equal spacing Vickers indentations on the section

Record the indentation hardness and draw the "depth-hardness" curve

The position that determines the decrease to 550HV is CHD

Metallographic analysis method

Metallographic analysis can further verify whether the heat treatment structure is qualified, and is an important means to determine the performance.

Detection process:

Sampling: select the root, tip and center of the gear for sampling

Polishing: use sandpaper (400~3000#) and polishing paste in turn

Observation: The morphology of martensite, carbide and crack were observed by optical metallographic microscope

Standard reference and quality determination basis

It is recommended to combine JB/T 9051, JB/T 6057 and specific customer drawing technical specifications for implementation.

Common defects and risk response of heat treatment quality

Heat treatment is the critical step in endowing gears with high performance, while hardness testing and metallographic analysis serve as the "health check tools" to verify the success of this process. Through Brinell, Vickers hardness tests, and metallographic analysis, not only can we determine whether the heat treatment of gears meets standards, but also provide first-hand data support for subsequent failure analysis and process improvement.