Induction hardening is a very familiar process for hydraulic cylinder manufacturers. It is usually used in the production process of hydraulic chrome plated rods levers to increase the hardness and toughness of the rods, thereby extending the service life of the hydraulic cylinders.
To effectively control the effect of induction hardening of chrome plated rods, systematic management is required in terms of material selection, process parameter optimization, coating and substrate matching, and process monitoring. The following are key control points and implementation steps:
1. Control the matching of chrome plating layer and substrate
Substrate selection:
Preferably use medium carbon steel (such as 45# steel) or low alloy steel (such as 40Cr) to ensure that the substrate has good hardenability and forms a stable bond with the chrome plating layer.
Chrome plating process optimization:
The plating thickness is controlled at 0.03–0.1mm. Too thick will easily lead to stress concentration during hardening and cause cracking.
Use micro-crack chromium or microporous chromium process to increase the bonding strength between the plating and the substrate and reduce the risk of peeling during hardening.
2. Precise control of induction hardening process parameters
Heating parameters:
Frequency selection: high frequency (100–500 kHz) is suitable for shallow hardening (0.5–2mm), medium frequency (1–10 kHz) is suitable for deep hardening (2–5mm).
Power and time: The optimal power density (usually 2–5 kW/cm²) and heating time are determined through experiments to avoid overheating leading to oxidation of the coating or coarsening of the matrix grains.
Temperature control:
Surface target temperature: 850–950°C (adjusted according to the material), infrared thermometer is required for real-time monitoring to prevent temperature fluctuations.
Avoid local overheating of the chrome layer (exceeding 300°C may cause embrittlement of the chrome layer).
Cooling process:
Cooling medium: Use polymer aqueous solution or compressed air to avoid excessive stress in the coating caused by water quenching.
Cooling rate: Controlled at 100–200°C/s to ensure martensitic transformation while reducing the risk of deformation.
3. Inductor design and process matching
Inductor shape: Profile inductors (such as rings or U-shaped) ensure that the heating area covers the chrome-plated surface evenly and reduces edge overheating.The gap with the workpiece is adjusted to 1–3 mm to optimize the electromagnetic field distribution.
Scanning hardening: For long rods, a continuous scanning process is used to control the moving speed (such as 2–10 mm/s) and match the power to avoid temperature gradients.
4. Process monitoring and quality inspection
Real-time monitoring: Use infrared thermal imager or pyrometer to monitor heating temperature distribution, and automatically adjust power with PLC.Record parameters such as current, frequency, moving speed, and establish a process database.
Hard layer detection: Metallographic analysis: Confirm the depth of the hardened layer (such as 0.8-1.5mm) and structure (fine needle-shaped martensite).
Hardness test: The surface hardness should reach HRC 55-62, and the matrix hardness should transition smoothly with the coating gradient.
Coating adhesion test: Check whether the coating is peeling through a cross-cut test or a thermal shock test (heating to 150-200°C and then cooling with water).
5. Post-treatment and stress control
Low-temperature tempering: Immediately after hardening, temper at 180–220°C × 2–4h to eliminate residual stress and improve the bonding stability of the coating.
Straightening process: Use a multi-point hydraulic straightening machine to correct deformation and avoid mechanical impact damage to the coating.
