Gear Heat Treatment Process

quenching & tempering plant - Wenlio Gear

Gear Machining Process:

Blank → Heating (normalizing) → Gear cutting → Surface hardening (quenching, carburizing, nitriding etc.) → Finishing (grinding)

Wenlio Heat Treatment Center

Founded in 2004, PairGears LTD (Wenlio Gear is a subsidiary of PairGears LTD) run a dedicated heat treatment center that keeps the whole production flow on time and under control. Over the years, with expansion and upgrades, our workshops now run on digital systems.

We design heat-treatment plans based on material, downstream machining, and end-use requirements. As a result, we can match the process more closely to both performance targets and later production needs. Our capabilities cover almost all common metal heat-treatment methods, and we can handle parts up to Ø 6,800 mm. In addition, since 2010, we have expanded our plasma nitriding capacity to meet higher performance demands. With modern equipment, we can process both single parts and large batches efficiently.

Beyond supporting our own bevel gear production, Wenlio Gear also takes stand-alone heat-treatment orders from the global market. So, if you have parts that need reliable heat-treatment support, contact us to discuss your requirements and lead times.

How to choose heat treatment methods?

First, select the material and load/size, then decide on the heat treatment route (whether high surface hardness vs. high core toughness is required).
If high precision and low distortion are required, prioritize ion nitriding or cryogenic processes. If maximum contact strength is required, choose carburizing, quenching, and grinding.
For large diameters and low stresses, normalizing is more economical. For general enhancement, quenching and tempering are recommended. For impact and wear resistance, case hardening or carburizing is recommended.

Gears after Quenching & Tempering

Heat Treatment – Quick Comparison

Process	Typical Tooth/Surface Hardness	Core Toughness	Distortion Risk	Best For
Normalizing	HB 163–217	High	Low	Large/low-load gears, pre-machining
Quenched & Tempered	HB 220–285	Medium–High	Medium	General-purpose strength; small/medium gears
Surface Hardening	HRC 45–55 (surface)	Tough core (pre-treated)	Medium	Impact load; higher surface wear
Carburizing + Quench	HRC 58–63 (case)	Tough core	Higher (finish grind needed)	Heavy-duty / high load & shock
Ion Nitriding	High surface hardness (thin case)	Good (low-temp process)	Low	Precision parts; wear/corrosion resistance

Wenlio large pit-type carburizing furnace

Large Pit-type Carburizing Furnace – Wenlio Gear

Structural Diagram of the Large Pit-type Carburizing Furnace

Furnace shell
The furnace shell has a vertical cylindrical structure and uses 12 mm Q235 steel plate with a welded design. Inside, it combines lightweight insulation bricks, high-strength anti-carburizing refractory bricks, and ceramic fiber modules to improve insulation performance and structural stability. In addition, the system divides the heating elements into three zones. It then uses power regulators with PID control to manage each zone more precisely.
Retort
The retort also has a cylindrical structure and uses high-temperature-resistant stainless steel, SUS310S. To increase strength, the shell adopts a roll-formed corrugated profile. At the same time, the base includes an integrated oil groove for sealing. As the retort expands at high temperature, it can extend freely downward into the oil groove. Therefore, the design helps maintain reliable sealing performance during thermal expansion.

Furnace lid
The sealing flange of the lid adopts a water-cooled design, keeping the rim seal at low temperature to ensure tightness. The lid is equipped with a lifting guide, a pressure-holding valve, an exhaust gas afterburner, a flame detector, a circulation fan, and an automatic lifting and rotation mechanism.
Process inlets
Multiple gas/liquid inlets are provided on the top of the lid for introducing methanol, propane, air, nitrogen, and other media. All media are adjustable via flowmeters and controlled by solenoid valves, with manual bypass circuits available for abnormal conditions.

Temperature and Carbon-Potential Control

Temperature control

Electric heating serves as the furnace heat source. To improve temperature control, the system uses a primary control thermocouple near the effective work zone and equips each heating zone with a tracking thermocouple. It then adjusts zone power output based on both the master and zone thermocouple readings, so the effective chamber maintains uniform temperature. As a result, TUS results show that temperature uniformity within the φ2500 mm × 2700 mm effective space stays within ±5 °C. The figure below presents the measured TUS results for the FPQ250270 furnace.

Original Time and Temperature Curves of TUS

Carbon-potential control
A nitrogen-based atmosphere of nitrogen and methanol supports furnace control. To ensure higher accuracy, the system combines an oxygen probe with an infrared analyzer for dual-loop monitoring. The oxygen probe measures oxygen partial pressure in the furnace, while the infrared analyzer tracks CO in real time. Then, the PT700 carbon controller calculates the carbon potential continuously for precise control. In addition, the infrared analyzer can also measure CO₂ at any time to calibrate the oxygen probe.

Carbon-potential Test Checklist for Specimens

Product Processing

Product: Wind-turbine gear
Material: CrNiMo7-6
Heat-treatment requirements: Surface hardness 58–62 HRC; effective case depth 2.5–3.8 mm.

Gear On-site Production – Wenlio Gear

Final results: Surface hardness 59–60 HRC; retained austenite ≤ 25%; carbide grade 1; effective case depth 3.1–3.3 mm — fully meeting customer requirements.

The large pit-type carburizing furnace can form a complete production line with a quench oil tank, slow-cooling pit, tempering furnace, and other supporting equipment according to user needs. As a result, the system offers high automation, precise temperature and carbon-potential control, strong safety and reliability, and better efficiency with lower energy consumption. In addition, it simplifies operation and maintenance while providing a long service life. Therefore, it supports the green, intelligent, and precise development of heat-treatment equipment and processes for wind-turbine gears.

Energy-saving heat treatment workshop