Stainless steel plate plasma cutting process

Although stainless steel plate accounts for a relatively small proportion of world steel consumption, it remains critical to specific manufacturing industries such as energy and food processing markets. At present, the global consumption of stainless steel plate materials is growing.

Technological advances in recent years have significantly improved the quality of plasma cutting of stainless steel plates. These advances span a thickness range that has reached 6.25 inches.

Early plasma cutting consumed high current and cut very slowly. One of the older systems used nitrogen as the plasma gas and a water injection shield to cut 3-inch-thick material at 15 inches per minute (IPM) at 750 amps.

So if you want to cut a 5-inch stainless steel plate, you need to use H35 (35% hydrogen, 65% argon) plasma gas and a coaxial flowing water skirt to reach an astonishing 1000 amps.

Fortunately, today's machines are much more powerful. An expanded range of gas selections and amperages provides plasma system operators with a variety of thickness cutting options. This way, processors can select the optimal amperage for a given thickness and achieve the ideal balance between productivity and cut quality.

Oxygen plasma gas with an air shield provides excellent cut quality across the entire thickness range of light and carbon steel. However, successful cutting of stainless steel plates requires different gases and consumables in different thickness ranges and grades.

Choosing the right shielding gas is the first step to successfully cutting stainless steel plates with plasma. Air used as a cutting gas and shielding gas is a common choice for fast cutting speeds and low cost, but the black surface that tends to cause severe oxidation on the stainless steel plate surface often requires labor-intensive secondary operations to remove it.

Depending on the requirements for the finished stainless steel plate, there are several other options available, as follows:

Protective Gas

Nitrogen plasma gas and shielding (N2/N2) cuts faster and has a smoother cut surface with less oxide, but produces a black cut surface similar to the air/air combination. It can also result in top edge fillets and important angles.

Processors requiring a finer cutting surface, good color and sharp top edges with limited angularity and scum require the use of special gases for best results. The modern N2/H2O process can effectively cut medium and thin stainless steel.

Material Type

The next consideration for successful plasma cutting of stainless steel is the material type. When the process of plasma cutting 304L stainless steel plate is applied to another similar austenitic alloy, such as 316L stainless steel plate, it will produce dross and rough cutting edges. When cutting the 304L stainless steel plate, it is clean and there is no visible residue.

By slightly increasing the cutting speed and shielding pressure, the cutting quality of 316L stainless steel plates is improved. Each material type responds differently to plasma cutting. Many stainless steel options require many different cutting processes for best results.

Dealing with scum and punctures

Another challenge with plasma cutting stainless steel is the viscosity of the molten material. When cutting mild steel with/air, the viscosity of the molten material is much lower than when cutting stainless steel. The hardened slag on the bottom of the stainless steel plate is very easy to remove and usually does not require a secondary operation like grinding. With stainless steel scum, the viscosity is higher, so it can be challenging to remove.

Proper equipment design, gas type, gas settings, cutting speed and cutting height can all help reduce or even eliminate stainless steel dross formation. One way to prevent scum is to rotate the shielding gas in the opposite direction to the plasma gas. This causes the scum to stick to the skeleton of the board instead of slicing.

Perforated stainless steel panels are challenging due to the nature of the molten material. A slag pile is an accumulation of slag on the surface of the plate surrounding a drilled hole, which can accumulate and create a nested layout of components.

When penetrating stainless steel plates thicker than 2 inches, operators often need to adjust the welding gun movement to accommodate the slag pile, or they must pierce, stop and scrape away the zinc slag before it hardens and adheres before making a partial cut.

Using the HD plasma cutting process and nozzle exhaust, the best plasma cutting quality can be achieved on thin stainless steel sheets.

Although the process was first developed to improve the cut quality of mild steel, it is now being applied to thin stainless steel plates. The higher gas volume with the exhaust nozzle increases the pressure to create a tighter constriction on the plasma arc.

This tighter constriction enables the use of smaller nozzle holes and higher energy densities. The exhaust increases nozzle life by constricting the arc, and the higher flow rate cools the nozzle.

This method of plasma cutting thin stainless steel plates is designed to provide sharp tip quality, a bright surface finish and excellent edge angles with reduced angle variation. Advanced torch and consumable technology delivers more consistent cut quality over the life of the consumables.

The development of medium-sized stainless steel plate cutting technology returns to the topic of gas type.

Using H35 offers the advantages of a non-oxidized edge with good cutting quality and trimming effect, but from a production efficiency perspective, the cutting speed is very slow. Nitrogen has the advantage of faster cutting speeds for increased productivity, but the cutting edges are oxidized.

Machine control technology can mix H35 and N2 as plasma gases, significantly increasing cutting speeds while maintaining the ideal silver or gray cutting edge.

It may be necessary to adjust the gas mixture for different materials. Too much N2 will result in a gray to black cut surface and the possibility of some dross buildup. Too much H35 will cause a gold color and form scum on the cut surface.

Modern piercing technology has expanded the range of stainless steel plate piercing and plasma cutting for thick materials well beyond what was previously achievable. The liquid-cooled shield repels molten material that may adhere to the torch shield during the piercing process.

Advances in plasma penetration have also led to different surgical approaches.

For example, production piercing has become a rating that describes a system's ability to penetrate (up and down) a given thickness 300 times with a set of consumables. In the largest perforations, the plasma torch is moving along the plate rather than simply up and down.

This rating uses tightly controlled mobile perforation technology to increase the system's production perforation rating to an unprecedented 4 inches. Testing of this technology against the new maximum penetration provided 50 penetrations at 400 amps and 25 penetrations at 800 amps.