Preheating steel plates before cutting them has a number of benefits. When you flame cut steel plates the high heat tempers and transforms the surrounding cold steel (The heat affected zone). The Flame cutting can soften this area meaning that the steel no longer has the same properties predicted by its grade and MTC. Additionally there is also a risk of cut edge cracking, where cracks appear in the edge of the cut plate over 48 hours after the cutting has taken place. This is related to hydrogen cracking and is related to the thickness of the plate and the hardness of the steel.
Preheating steel plates helps to avoid these risks by reducing the temperature gradient. Too little preheating and there is more risk of cracking than expected. Too much heating and significant energy costs are incurred.
Dillinger Hutte have produced an easy and useful tool to help you calculate the exact level of preheating steel plates require based on the grade, the chemical composition of the steel and the thickness of the plate.
To use it effectively you need:
- Steel Grade
- Actual MTC with chemical composition
The calculator is only for use with flame cutting (oxyfuel) methods. It doesn’t apply to laser or waterjet cutting.
Instructions for Using the Preheating Steel Plates Calculator
- Enter the steel grade. This sets boundary conditions on the calculation of the preheating temperature of the steel plate. Changing the steel grade does NOT automatically pick the appropriate chemical composition
- Enter the actual plate thickness. As plates get thicker the preheating temperature increases significantly.
- Enter the chemical composition of the steel For most accurate results you should use the MTC for the plate that you are actually cutting, but the calculator should work well with the chemical composition based on the appropriate standard for indicative purposes only
This gives you an output graph similar to the one above where you then read of the plate thickness against the preheating temperature required. Do note that the temperature is in C and the thickness in mm to avoid disastrous mistakes…..
Contact Dr. Tobias Lehnert ([email protected]) if you have any questions or suggestions about how the calculator works
Benefits of Optimising Preheating Temperature
Heating a 25 tonne plate by 130C requires about 1.5 million kJ (assuming a specific heat capacity of 0.49 kJ/kg) or about 450 kWh. If we assume that plate heating efficiency is about the same as a gas grill in your kitchen (40%) then we get a total energy input of about 1125 kWh. If we take UK retail prices for gas of about 4p/kWh then the cost of heating the plate is about EUR55.
For companies doing a large amount of cutting of heavy plate being able to save 5 % or 10% energy costs will become significant over time and help to improve low margins in the fabrication industry
This quick approximation doesn’t take into account a variety of factors such as heat loss, wind chill etc etc and assumes that the plate is a uniform temperature etc.