Most sheet metal cutting techniques are based on localized melting of the material. The area between the melt part and the unaffected base metal undergoes chemical and structural modifications, and is called Heat-Affected Zone (HAZ).
It can often be recognized by a series of brightly colored bands, also visible near welds. Colors are caused by surface oxidation, and are an approximate indicator of the temperature reached by the metal.
These colors, also called “heat tint”, depend on four factors:
- Steel chromium content: this metal increases the material resistance to oxidation, therefore colors will be less intense or their formation will be delayed;
- Oxygen level: during welding, usage of protective gas and electrode coating can reduce the coloration because they partly shield metal from oxidation;
- The rougher the surface, the faster it oxidizes, causing darker colors;
- Substances like paint, oil, rust, and even fingerprints can alter heat tint, but do not affect the extension of the heat-affected zone.
In some cases, the heat-affected zone does not cause these colors, or even extend farther than the colored area. On the other hand, the HAZ can sometimes cause discoloration instead of a heat tint.
Causes
The most important factor is thermal diffusivity. Technically speaking, this coefficient depends on thermal conductivity, density, and specific heat of a substance. Materials that show a high thermal diffusivity are able to quickly transfer variations in heat, rather than heat itself.
In other words, the higher the thermal diffusivity of a material, the faster it cools, and HAZ is reduced. Conversely, lower coefficients mean that the energy cannot be drained quickly and the heat-affected zone will be wider. 304A-grade stainless steel, for instance, has a thermal diffusivity of 4.2 mm²/s, much lower than structural steel (11.72 mm²/s).
From the point of view of the production process, the extension of the HAZ depends on three factors: quantity of heat applied, duration of exposure, and affected area. If you provide large amounts of energy, for a long time, and with wider beams, you will get a larger heat-affected zone.
This explains why, regardless of the material being cut, any cutting technique causes a different effect:
- Shearing and water jet cutting do not provoke a HAZ because they do not overheat the sheet metal;
- Laser cutting generates the smaller heat-affected zone among all thermal cutting techniques, because it applies heat on a very small area;
- Plasma cutting generates an intermediate HAZ, because it uses a larger beam. Higher currents allow for a higher cutting speed, reducing the duration of exposure and therefore the width of the heat-affected zone;
- Oxyfuel cutting, because of the intense heat, slow speed, and wide flames, generates the wider HAZ of all thermal cutting systems.
What are the effects?
Heat provided by the welding or cutting process, and consequent fast cooling, result in both chemical and metallurgical alterations. Oxidation is the most noticeable and immediate change, and as we have seen, is also responsible for the brightly colored bands. A light surface nitriding can occur, resulting in an increased hardness and decreased weldability of the metal.
Another common issue is corrosion, derived from stainless steel sensitization. Intense heat causes the precipitation of chromium carbides around the grain boundaries. In these areas, chromium content drops below 10,5% and steel will lose its ability to form a passive film and will no longer be stainless. The result is the so-called intergranular corrosion. In extreme cases, metal will turn black.
High temperature can also provoke hydrogen embrittlement. Gas diffuses through the metal and creates a strong pressure within the lattice, reducing its tensile strength and toughness. Should hydrogen gas not be removed, it can cause spontaneous cracking even 24 hours after heating. From a metallurgical point of view, heat generates localized hardening. In some circumstances, austenitic stainless steel can turn into martensitic, increasing its hardness as well as its brittleness. In other cases, heated metal can become less strong.
How to deal with the HAZ?
The heat-affected zone can alter the sheet metal figures as declared by the rolling mill to a great extent. When forming, the HAZ can make it difficult to manage the bending angle because it becomes impossible to foresee how metal will behave after a heat-intensive cutting process.
One of the affected behaviors is springback. To get consistent angles with varying elastic recovery, it’s useful to rely on an angle control system, either laser- or contact-based. The former are quite common but somehow cumbersome, and they do not operate with small flanges, and with rough or polished surfaces. GPS4 contact systems are more precise and work well also with counterbends or small flanges, but have a slightly reduced angle span.
Imprecise crowning is the other culprit: every cut piece will be different from each other, and the machine frame will be stressed in an unpredictable way. For this reason, it’s important to use a press brake with an ACSG real-time crowning system, able to compensate for the machine deformation. This technology helps obtaining regular angles along the entire length of the profile.
Heat tint caused by oxidation can be removed with fine sandpaper or ground away. This exposes the underlying layer and activates chromium self-passivation, but you risk weakening the part. On the other hand, the only way to remove the entire extension of the heat-affected zone is to machine it away, but at the price of a higher waste of material and time.
About Gasparini
Gasparini offers two main product platforms: press brakes and hydraulic shears. All products are positioned at the high end of their specific market, providing a significant technological content, a robust design, together with an “easy to use” configuration and a wide range of options and accessories, to customize the machine according to your needs. All the staff at Gasparini are dedicated to customer satisfaction, highly valuing the opinion of their partners and end users, since their first aim is to improve their performance and to fulfill market expectations.
For more information, visit www.gasparini.it/en
