When welding the NGW113-560 planetary reducer frame made of cast iron, the preheating temperature has a significant regulatory effect on heat input. Increasing the preheating temperature can effectively reduce the cooling rate and reduce heat input requirements, thereby inhibiting the formation of cracks. This is because cast iron has poor thermal conductivity and low plasticity. It is easy to produce thermal stress cracks due to large local temperature differences during the welding process. Therefore, reasonable preheating can improve the thermal field distribution and reduce reliance on high heat input.
1. How does preheating temperature affect heat input and cooling behavior?
1. Reduce the temperature gradient and slow down the cooling rate
When the cast iron parts are not preheated, the initial temperature of the base metal is close to room temperature (about 20°C), and the welding arc is instantly heated to more than 1200°C, forming a huge temperature gradient, resulting in rapid cooling of the weld area, and the formation of martensite and white structures.
When preheating to 300~350℃, the base metal is in a high temperature state as a whole, and the cooling curve after welding is gentle. The cooling rate in the 1200℃→800℃ interval can be reduced from >20℃/s to <10℃/s, significantly reducing the hardening tendency.
2. Reduce the required external heat input
Preheating is equivalent to 'advanced energy storage', making it easier for the base material to reach the fusion temperature. Therefore, under the same penetration depth requirements, lower welding current and voltage can be used, that is, the external heating input is reduced (Q=U×I×60v×1000Q=v×1000U×I×60).
Practice shows that after preheating to 300°C, the heat input can be controlled within 10~15 kJ/cm to still ensure good fusion and avoid grain coarsening and deformation problems caused by high heat input.
3. Improve the tissue stability of the heat affected zone (HAZ)
High preheating temperature promotes graphite precipitation, inhibits the formation of cementite (Fe₃C), and reduces the thickness of the white layer;
At the same time, the peak value of post-weld residual stress is reduced and the risk of cold cracking is reduced.
1. Recommended preheating temperature : 300~350℃ (general safe range for cast iron welding). The heating range is 150mm around the weld. Use an electric heating blanket or flame to heat evenly, and monitor it in real time with an infrared thermometer.
2. Matching heat input control:
Use nickel-based welding rods (such as Z508, Z401), diameter Φ4.0mm;
Welding current 130~160A, voltage 24~26V, welding speed controlled at 4~6m/h;
Actual heat input should be ≤15 kJ/cm to avoid overheating.
3. Process coordination measures:
Jump welding in sections (each section ≤50mm), hammer immediately after welding to release stress;
After welding, cover with asbestos cloth to slow cooling to prevent rapid cooling cracking.
