Prevent thin and/or flaking coatings from thermally cut edges
How to prevent thin and/or flaking coatings from thermally cut edges?
Steel typically arrives at the galvanizing plant directly from the fabricator where a number of fabrication steps are performed, including cutting. The cutting method is often selected based on the material type, thickness, and dimensional tolerances required during fabrication. Many plate edges are sheared by large machines if the material does not have critical dimensions or if the steel is a low strength. Other material may be thermally cut by flame, plasma, or laser cutting. These thermal cutting techniques introduce a significant amount of energy in the steel, and can alter the microstructure and properties of the steel within the heat affected zone (HAZ):
Flame cutting (oxy-fuel cutting) is a widely used, inexpensive, and versatile method to cut steel. In flame cutting, the exothermic reaction between the oxygen and the steel delivers heat to cut the base steel (3,600 to 6,000+ F) while a high-pressure stream of oxygen performs the actual cutting operation. Flame cutting can only be used on metals whose oxides have a lower melting temperature than the base metal, therefore flame cutting is only suitable for carbon steel and not stainless steel or aluminum. For flame cut steel, the size of the heat affected zone can vary depending on the steel alloy and thickness. For steels such as ASTM A36 and A572, a HAZ ranging from 1/8 1/32 wide can be expected, depending on the thickness of the material and the cutting speed (i.e. a wider HAZ for a greater steel thickness or slower cutting speed). Additionally, the HAZ is typically wider for steels with a high carbon content.
In laser cutting, laser optics and CNC are used to direct a high-power laser beam which heats the steel. As the laser beam melts the base material, a high-pressure gas jet expels the molten material. Laser cutting is suitable for cutting of a variety of materials in addition to steel. Microstructure changes can be evident 130 microns (5.2 mils) from a laser cut edge, and the micro-hardness can be more than two times that of the uncut steel. Laser cutting is likely to provide the cleanest cut with the narrowest dimensional tolerances and cause the least amount of metallurgical change on the cut face.
In plasma cutting, a super-heated and high-speed jet of gas expels the melted base steel which has been heated by the plasma (45,000+ F), leaving a cutting gap. Plasma cutting is suitable for any steel, regardless of surface preparation (rusted, painted, etc.). Microstructure changes can be evident 380 microns (15.2 mils) from a plasma cut edge, and the microhardness can be more than two times that of the uncut steel. Plasma cutting provides a cleaner cut, more dimensional control, and less metallurgical change to the cut edge than flame cutting, but less than laser cutting. However, high-definition plasma cutting can produce results similar to laser cutting.
Each of these thermal cutting techniques results in increased steel hardness near the cut edge, changing the diffusion properties of the steel in the HAZ and occasionally making it difficult to form a galvanized coating over the cut edge. This issue is further complicated when the base steel has a silicon and/or phosphorus content that is very reactive in the galvanizing kettle, causing the development of a thick coating with reduced adhesion. A thick coating which is not well adhered to a thermally cut edge can result in edge-flaking when handled.
So what can be done to prevent thin coatings and edge flaking from thermally cut edges? For single instances where thermally cut edges are identified during inspection prior to galvanizing, it is possible to grind cut edges up to 1/16 inch. However, where a pattern exists, demonstrating the pattern to the fabricator can help to justify grinding these areas prior to arrival at the galvanizing plant, especially for large orders and commonly galvanized fabrications. Additionally, the use of alternative mechanical cutting methods can be discussed with the fabricator such as sawing (rotary, hack, band), grinding, turning (lathe), chiseling, drilling, or shearing.
Some of these cutting techniques produce a clean edge, but if the technique produces a significant amount of energy in the steel during the cutting process; the high-energy cutting techniques can change the local properties of the steel and increase the local steel hardness at the edge of the plate. The increase in hardness can change the diffusion properties of the steel at the edge and make it very difficult to form the galvanized coating on this edge.
This issue is further complicated when the plate steel has a silicon and/or phosphorus content that is very reactive in the galvanizing kettle. The reactive steel grows a thick coating, which in and of itself can be prone to flaking due to the stresses in the coating from the metal contraction differences in the cooling of the plate from the galvanizing temperature to ambient. Thick coatings and high stress in the plate steel can make the coating susceptible to flaking and this is due to the silicon and/or phosphorus content in the plate steel.
When the galvanized coating is thick, and the edge does not grow a well-adhered coating, the edge may show signs of flaking. This can also be propagated by handling the plate steel by forklifts and by contact with the pavement when stored on the lot at the galvanizing facility. These material handling steps can add stress to the already weak coating adherence and cause the flaking to begin at the edge. The edges of plate material should be visually examined for signs of flaking and the areas can be repaired in accordance with ASTM A780.
Communication between the galvanizer, the fabricator and the steel supplier can expose the potential of this problem before it becomes an inspection issue at the job site after the plate has been hot-dip galvanized.
© 2019 American Galvanizers Association. The material provided herein has been developed to provide accurate and authoritative information about after-fabrication hot-dip galvanized steel. This material provides general information only and is not intended as a substitute for competent professional examination and verification as to suitability and applicability. The information provided herein is not intended as a representation or warranty on the part of the AGA. Anyone making use of this information assumes all liability arising from such use.