Cutting technology for thick sheet metal parts has changed over the past 30 years. Thermal cutting processes such as oxyfuel cutting have often been replaced by plasma cutting (see image cover) and laser cutting (see image above).
The various thermal cutting techniques produce cutting edges that differ in roughness and the presence of more or less sharp edges. At the surface, the action of heat changes both the steel structure and steel composition. Oxide layers of different stability form on the cutting edges. Some of these oxides are impossible to remove in the standard pretreatment series at the galvanizing plant. Each of these aspects can individually or in conjunction with each other affect the structure, thickness and appearance of the zinc layer.
In hot-dip galvanizing, the course of the iron-zinc reaction depends primarily on the silicon and phosphorus contents in the steel. These contents change at the cutting edges due to the action of heat in absolute terms as well as in relation to each other. It is known from literature that during oxy-fuel cutting the silicon disappears to a depth of about 30 – 40 μm due to combustion. It may be assumed that at higher cutting temperatures, such as with plasma cutting, the heat-affected zone will be larger.
When, in the case of reactive steels with a (silicon and phosphorus) content greater than 0.09%, the silicon disappears from the cutting zone, the cutting edge may have a thinner zinc layer after galvanizing than on the unaffected parts. In laser cutting, another phenomenon occurs in which the edges of the cutting edges are so sharp that the zinc layer may jump off even with a slight mechanical load.
Each of the different thermal cutting techniques cause different effects on the steel composition and steel structure. In addition, the gas composition and purity of oxygen also affect the cut made. The EN-ISO 1461 indicates that cut edges created by cut burning, laser cutting and plasma cutting have an altered steel composition and structure in the zone on and around the cut surface, so that the minimum coating thickness may be more difficult to obtain and the formed coating may show reduced cohesion or adhesion. To obtain a reliable (uniform) zinc coating, the heat-affected part should be removed by grinding and sharp edges should be removed during fabrication.