Manage deformations and thermal effects

All thermal cutting methods apply heat to the material, causing it to expand. While steels generally have similar thermal expansion coefficients, high tensile steels tend to distort more severely than mild steel due to their increased resistance to yielding.

If heating is done in a small area and the expansion of this area is restricted by surrounding material, the material will distort or warp. Steels with lower strength may allow some stresses to be relieved through material yielding, whereas high-strength steels are more resistant to yielding, resulting in more severe deformations. 

When heating is done in a small area and the expansion is restricted by surrounding material, distortion or warping occurs. Steels with lower strength may allow some stresses to be relieved through material yielding, whereas high-strength steels are more resistant to yielding, resulting in more severe deformations.

Thicker materials tend to distort less because their stiffness makes them more resistant to deformation.

How to minimize distortions from thermal cutting

There are several ways to minimize distortions from cutting operations.

• If the material doesn’t require preheating, cutting submerged in water will minimize the distortion.
• For materials requiring preheating or postheating, using submerged cutting followed by postheating is recommended over preheating to minimize distortion.
• Switching to a cutting method that generates less heat can reduce distortion. Oxy-fuel cutting is the most heat-intensive, followed by plasma cutting, and finally laser cutting, which introduces the least amount of heat.
• Make nesting so that parts are cut out at different locations of the plate. Strategically nesting parts on the plate ensures uniform heat distribution and reduces distortion.
• If cutting long slender parts cut both sides of the part simultaneously with to torches. This if of course only possible in certain cases.
• Simultaneously cutting both sides of long, slender parts with two torches can mitigate distortion, although feasible only in specific cases.
• Non-thermal processes like abrasive water jet or sawing can be alternatives to thermal cutting, eliminating heat-induced distortions altogether.

Internal stresses released during thermal cutting

In some cases, high-strength steels may have higher internal or residual stresses compared to lower-strength steels. These stresses can vary depending on the manufacturing process and the material strength.
A steel with a yield stress of 355 MPa cannot have higher internal stresses than approximately 355 MPa, whereas a material with a yield stress of 700 MPa can have approximately 700 MPa in internal stresses. The internal stresses can be relieved during the cutting operation, leading to increased distortion. 

The higher the internal stress, the greater the distortion.

Thermal cutting inevitably leads to the formation of a Heat Affected Zone (HAZ) independent of the steel grade. In the case of quenchable steels, the HAZ can be divided into two distinct zones: a re-quenched zone and a tempered zone (see graph below to the left).

The peak temperature of the material at certain distances from the cut edge is depicted in the graph below to the right.

Zone 1 and 2

Peak temperature from the cut edge

The outermost part of the HAZ, situated approximately 1-2 mm (0.039-0.079 inches) from the cut edge, experiences temperatures exceeding 900 °C (482 °F) during the cutting process. As the cutting torch progresses, heat rapidly disperses into the plate, causing Zone 1 to cool so rapidly that it undergoes re-quenching. 

Consequently, the hardness and strength of this zone are higher than in other parts of the HAZ and the unaffected parent metal. Zone 2, positioned between Zone 1 and the unaffected parent material, is heated to temperatures below 900 °C (1,652 °F) during cutting. The material in Zone 2 undergoes tempering due to the heat generated during cutting operations. Hardness values in this zone vary depending on the steel grade and the efficiency of the cutting process.

Therefore, the maximum preheating temperatures are crucial to consider during cutting operations to ensure the integrity and performance of the materials. Maximum allowable temperatures for SSAB steels can be found in this table.

The smaller the piece that is thermally cut, the greater the risk of the whole component being softened. When small parts are cut, the heat supplied by the cutting torch and by preheating will accumulate in the workpiece. Consequently, the smaller the size of the cut part, the higher the risk of softening.

When oxy-fuel is used for cutting 30 mm (1.181”) or thicker plate, a rule of thumb applies: There is a risk of loss of hardness of the entire component if the distance between two cuts is less than 200 mm (7.874”). However, for thicknesses below 30 mm (1.181”), it is possible to cut smaller parts without loss of hardness.

A convenient method to determine if the piece is too small is to measure the temperature of the part directly after it has been cut out.

The best way of eliminating the risk of softening is to use cold cutting methods, such as abrasive water jet cutting. If thermal cutting must be performed, laser or plasma cutting is preferable to oxy-fuel cutting, as oxy-fuel cutting subjects the workpiece to higher levels of heat compared to plasma or laser cutting.

For oxy-fuel cutting, an effective way of limiting and reducing the extent of the soft zone is to water-cool the plate and the cut surface during the cutting operation. This can be achieved either by submerging the plate in water or by spraying water on the piece during and after cutting. However, even with submersion or water spraying, a soft zone will still develop in the cutting edge within approximately 5-10 mm (0.197 – 0.394”). This occurs because any water is pushed away by the ignition flame and the cutting oxygen jet preventing the water from cooling the cutting front until after softening has occurred. Nevertheless, water cooling can prevent "global" softening of the component.