The heat treatment quality characteristics of automotive gear and shaft parts include effective hardened layer depth, surface and core hardness, surface carbon content, metallographic structure (carbide, retained austenite, internal oxidation) and deformation. The key to ensuring consistency is stable carrier gas, accurate carbon potential control and daily calibration, temperature system accuracy, standardized quenching management and complete traceability records, which are also the focus of the CQI-9 audit.
- Key characteristics: effective hardened layer, hardness, surface carbon, structure, deformation
- Atmosphere: Stable carrier gas + carbon potential closed loop + daily calibration
- Temperature: TUS, SAT, thermocouple management
- Quenching: oil temperature, stirring, transfer time
- Traceability: part to heat curve; meets CQI-9
Why are automotive parts particularly sensitive to heat treatment?
Transmission gears, differential gears, drive shafts, camshafts, etc. are subjected to alternating loads and contact stresses. The failure modes are mainly bending fatigue, contact fatigue (pitting corrosion, spalling) and wear. These properties are highly dependent on the surface structure and residual stress, which are determined by the carburizing and quenching process.
Critical quality characteristics
| Characteristic | Description | Main influencing factors |
|---|---|---|
| Effective hardened layer depth | As to the depth of 550 HV | Temperature, time, carbon potential |
| surface hardness | Typically high hardness range | Surface carbon, quenching, tempering |
| Heart hardness | Ensure toughness and load-bearing | Material hardenability, quenching |
| Surface carbon content | Affects hardness and retained austenite | Diffusion phase carbon potential, alloy factor |
| microstructure | Carbide morphology, retained austenite, internal oxidation | Carbon potential, atmospheric oxidability, quenching |
| deformation | Tooth shape, runout | Furnace loading, quenching, materials |
Process control focus
1. Atmosphere - Use a carrier gas with stable composition (endothermic RX gas or strictly controlled nitrogen-methanol); - The oxygen probe controls the carbon potential in a closed loop and manages the probe life; - Use three-gas analysis, hot wire or fixed carbon sheet to check the carbon potential every day.
2. Temperature - Take TUS and SAT as required; - Establish usage records for thermocouples.
3. Quenching - Regular testing of oil temperature, stirring speed, and medium cooling characteristics; - The transfer time from tapping to quenching is controlled.
4. Loading the furnace - Standardized furnace loading method to reduce deformation and soft spots.
5. Recording and Traceability - The carbon potential, temperature and quenching curve of each heat are saved electronically; - Part batches are bound to batches.
Controlled atmosphere program reference
- integral-quench furnace line: centralized gas supply RX generator + independent carbon potential control for each furnace + automatic line scheduling;
- Mesh belt furnace (fasteners, small parts): double furnace generators ensure continuous gas supply;
- pusher furnace (high volume gear): zoned carbon potential control + online diffusion calculation.
Auto parts projects controlled by Gosi include: delivery of a 40 m³/h double-furnace Endo generator for Zhejiang Jieneng Auto Parts, as well as the transformation of heat treatment atmospheres for Shanghai Natiefu, Zhejiang Heritage Power and other companies (according to official website news).
Frequently asked questions
What is the generally required depth of the carburizing layer of gears?
The effective hardened layer range will be given on the drawings as determined by the gear module, load and design. Generally, the larger the modulus, the deeper the layer depth requirement.
Can auto parts be carburized with low pressure vacuum?
Yes, especially high-end transmission gears. Low-pressure carburizing can avoid internal oxidation and has small deformation when combined with high-pressure gas quenching, but the investment and operating costs are high.
Is there a requirement for internal oxidation depth?
Many automotive parts specifications have upper limit requirements on the depth of internal oxidation (grain boundary oxidation), which need to be met by controlling the oxidation properties of the atmosphere and the process.