1. Flame Shape

Flame shape is related to the nozzle structure of the burner. To achieve uniform temperature in the heating zone of the workpiece, multi-head nozzles are usually adopted to obtain reasonable temperature distribution on the quenching surface and determine the optimal form of flame shape.

To obtain the required quenched microstructure and properties, the ratio of oxygen to acetylene must be strictly controlled. Practical production experience shows that the most commonly used ratio of oxygen to acetylene is 1.15～1.25, which is the optimal mixing range.

2. Distance Between Nozzle and Workpiece Surface

According to the flame structure, the reducing zone has the highest temperature (generally, the maximum temperature reaches up to 3200℃ at the position 2～4mm away from the tip of the flame core). Therefore, the distance between the nozzle and the workpiece surface should be controlled, generally kept at 6～8mm. Too large distance will lead to insufficient heating temperature, while too small distance will cause overheating.

3. Relative Moving Speed Between Nozzle and Workpiece

The relative moving speed between the nozzle and the workpiece is adjusted according to different depths of the hardened layer. If a deeper hardened layer is required, the relative moving speed should be lower; if a shallower hardened layer is required, the relative moving speed should be higher. Generally, the relative moving speed ranges from 50～300mm/min. When the depth of the hardened layer is 2～5mm, the relative moving speed can be set at 80～200mm/min.

4. Distance Between Water Sprayer and Flame

If the distance between the water sprayer and the flame is too short, water may spray onto the flame and easily cause the flame to extinguish, which affects normal heating; if the distance is too long, the heating temperature of the workpiece will be insufficient, so it is necessary to extend the heating time to reach the required temperature. Generally, the distance between the water sprayer and the flame is 15～20mm.

5. Cooling Water Temperature

The cooling water temperature should not be too high, otherwise the workpiece can not be hardened properly. Meanwhile, it should not be too low to avoid quenching cracks, so the temperature is generally controlled at 15～20℃. For carbon steel or medium alloy steel with a carbon content above 0.5%, precooling with compressed air is usually applied before water cooling to prevent cracking.