Principle of Flame Straightening
During flame straightening, the steel part is heated locally in a targeted manner up to the plastic range of the steel (heating above the plastic range will result in more warpage and distortion.
Due to local heating, permanent compression occurs as a result of impeded thermal expansion. During cooling, there is a shortening in the workpiece, which leads to the desired change in shape.
How does Flame Straightening work?
When we heat metal in a uniform manner, metal got expanded and increase in size. If we cool it, the metal will get a size reduction again.
Once it is back to the initial temperature (the temperature before we applied the heat), it will back to its original size.
Now, if the part is heated locally, there will be restrictions in expansion and contraction due to adjacent cold metal.
During cooling, the metal will shrink, and knowing which size we need to reduce to warpage, we can apply this principle to rectify distorted welded parts.
The flame straightening process works on:
- Heating the part,
- Prevented expansion by restraining,
- Upsetting (deformation) to get rectification,
- Reduction in length during cooling.
Rules for successful flame straightening
- Mark the area to be straightened.
- Ensure to prevent excessive thermal expasion.
- Select right gas and heating torch.
- Make locally limited heat accumulation.
- Consider shrinkage.
Procedure for Flame Straightening
- Determine the point to be straightened (too long point), because this has to be compressed with the flame.
- Select a suitable straightening torch. The greatest heat build-up is achieved with an oxygen-excess acetylene-oxygen flame.
- Heat the material up to the plastic range Steel, unalloyed, and low-alloy steel and stainless Cr Ni steel 550-700 °C (“dark red heat “, visual check based on temper color appearance), Aluminum and aluminum alloys 150-450 °C (use Thermo color pencil or IR thermometer).
- Localized heat build-up causes compression in the heated area due to the adjacent cold material.
Types of Heating for Flame Straightening
- Localized Point Heating: E.g.: Straightening sheet metal with dents.
- Spread Heating: Eg: straighten bends and weld on one side.
- Wedge shape Heating, E.g.: Align profiles and slats. (main Application: for severe bending, e.g. B. of profiles and slats. The thermal wedge is long and narrow. Starting from the tip to the base, it is heated evenly to the target temperature).
- Oval point Heating: E.g., straightening pipes (Application: on pipes, e.g. B. after welding sockets. The heat is applied in an oval-shaped and heated through and arranged in the longitudinal direction of the pipe axis. Depending on the shape of the component, a combination of these types of heating makes sense).
Flame straightening of high-alloy austenitic steels
In principle, the microstructure of these steels is not changed during flame straightening, apart from possible oxidation of the surface, if the flame straightening temperature is maintained between 550 and 750°C (“dark red heat”).
Any carburization is avoided with a neutral flame, or even better with a flame with excess oxygen. Because of the lower thermal conductivity and the greater thermal expansion capacity, heat build-up and good directivity are achieved quickly.
Smaller burner inserts are therefore used than with construction steel. Abrupt cooling has a positive effect on the material and the corrosion behavior (intergranular corrosion) of these steels.
Flame straightening of hot-dip galvanized components
Hot-dip galvanized components can be straightened without impairing corrosion protection.
The most favorable flame straightening temperature is “dark red heat”. It cannot be seen on hot-dip galvanized components.
The use of the brazing flux type FH 10 makes work easier. Due to its effective temperature, it is a good temperature indicator and at the same time protects the surface from oxidation.
Flame straightening of aluminum and aluminum alloys
A neutral flame or a flame with a slight excess of acetylene is used. Because of the high thermal conductivity, larger burner inserts are used than used with steel.
The thermal expansion of aluminum is twice that of steel. Therefore, in many cases, mechanical aids are used to prevent expansion during heating.
Depending on the alloy, the recommended temperature is between 150 and 450 °C. It can be checked quickly and easily with a Temperature crayon pen.