Laser Cutting of Stainless Steel

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General information about stainless steel

Stainless steel alloys contain at least 12 % chromium as an alloying addition. The surface of polished stainless steel has a mirror finish, long service life, and self-healing properties. Its high corrosion resistance comes from an oxide film made of insoluble oxides of the alloying elements; the film spontaneously regenerates in contact with atmospheric oxygen. This coating restores its integrity by itself whenever it is damaged.

The following alloying elements are used: chromium, nickel, silicon, manganese, molybdenum, tungsten, niobium, boron, copper, vanadium, titanium, etc. The percentage of additives and the alloying process define the physical, mechanical, and chemical properties of the steel.

Anti-corrosion properties are imparted to ferrous metals by chromium and nickel additions — these give steel its light colour and shine. A nickel-bearing alloy prevents iron from reacting with water in the presence of oxygen, while chromium content above 12 % instantly forms a uniform protective film on the surface, blocking the chemical reaction between oxygen molecules and iron atoms.

Classification of alloy steels

By degree of alloying:

low-alloy steel — alloying elements up to 2.5 %;
medium-alloy steel — alloying elements from 2.5 to 10 %;
high-alloy steel — alloying elements from 10 to 50 %.

By properties:

normal and high strength;
cold-resistant;
heat-resistant (heat-stable);
resistant to atmospheric and seawater corrosion;
hardenable by thermal and thermochemical treatment, etc.

By application:

Structural alloy steels. Used to manufacture welded structures.
Engineering steels. Used to produce machinery components, parts, and housings.
Tool steels. Used to produce tools.

Difficulties in machining stainless steel

The difficulties are linked to alloy properties:

the high content of alloying additions can cause slagging of the cut surface;
refractory oxides form in the heating zone, which hampers laser propagation along the cut line and therefore increases energy consumption;
high-chromium and chromium-nickel steels have low flow properties, which also complicates the cutting process.

The stainless steel cutting process

The process takes place in stages:

heating;
melting;
gradual vaporisation of the elements produced by the breakdown of the material.

The laser beam is a heat source where a highly concentrated gas is gathered at very high temperature. The beam has a cross-section of 10–20 µm and a power density of 100 MW/cm². On such a small area this energy is more than enough to melt the material instantly. Thanks to the thermophysical process the steel separates, and the metal structure changes only in the zone of contact.

Features of stainless steel

This steel is essentially an iron mass mixed with chromium. Depending on the manufacturer, nickel and other alloying compounds are added to refine the properties.

Stainless steel has very high corrosion resistance and a long service life (several decades). Its properties remain unchanged even after years of operation. Stainless steel is widely used for sheets, mesh, pipelines, etc.

Main alloying additions

Nickel. Present in austenitic stainless steel grades; it affects energy coupling and heat exchange, limiting the thickness that can be cut at a given laser power.
Chromium. Ferrite-forming element. Used both as a stand-alone alloying agent and in combination with others. Its addition widens the solidification temperature range and increases strength and hardness without changing ductility. Already 1 % improves mechanical properties; raising chromium to 5 % increases heat resistance, while acid-resistant and heat-resistant alloys contain even higher chromium percentages, up to 28 %.
Silicon. Ferrite-forming element. Does not affect viscous properties but raises tensile and yield strength, magnetic permeability, and electrical conductivity. Improves ductility, acid resistance, and strength.
Manganese. Austenite-forming element; improves hardenability and raises the fluidity threshold of the metal. Increases resistance to abrasion and impact.
Molybdenum. Significantly raises hardness, strength, and hardenability. Its highest concentrations are found in heat-resistant and high-speed steels; in structural grades its content usually does not exceed 0.4 %.
Tungsten. Carbide-forming addition that increases strength and hardness. Added to high-speed tool alloys at up to 18 %, it optimises hot strength and impact resistance.
Niobium. Strong carbide-former. Added to stainless alloys to minimise intergranular corrosion, and to manganese steels to reduce temper brittleness.
Boron. Increases hardenability. The best alternative to costly molybdenum and nickel.
Copper. Its addition raises yield strength, ductility, and corrosion resistance. In shipbuilding, it effectively combats fouling of submerged hulls by algae and barnacles.
Vanadium. Carbide-forming agent that raises strength and toughness. Vanadium-bearing alloys show excellent impact resistance and stress inertness but are very expensive.
Titanium. Binding carbon into stable carbides, it refines austenite grain and reduces susceptibility to intergranular corrosion. Raises acid resistance and, together with other carbide-formers, promotes self-hardening of the steel.

Laser cutting of stainless steel with inert and oxygen gas

Laser cutting with inert and truly inert gas is the most common process for cutting stainless steel. Oxygen laser cutting is also used where edge oxidation is not critical. Below we discuss laser cutting of stainless steel with inert (nitrogen) and reactive (oxygen) assist gases, as well as workplace safety considerations for both processes.

In laser cutting with inert gas (also called laser fusion cutting), the laser beam is the only heat source and a high-pressure inert gas jet provides the mechanical force to expel the melt.

Stainless steels have a relatively low thermal conductivity, which allows them to be cut at relatively high speed because the energy stays at the cutting front rather than dissipating into the material ahead of the kerf.

Nitrogen is the most commonly used assist gas for this cutting technique because of its low cost and low chemical reactivity compared with truly inert gases such as argon and helium.

Nitrogen cutting produces high-quality cut edges, and the cutting speed is usually higher than with oxygen.

Adhesion at the lower edge of the material, caused by the high viscosity of the molten material, can be a problem in nitrogen cutting, but it is usually solved by using very high assist gas pressure. High-pressure nitrogen is used when cut edge quality matters more than cutting speed.

Nitrogen is the preferred gas for cutting stainless steel, high-alloy steels, aluminum and nickel alloys; higher gas pressure is needed to remove molten material from the kerf. The high gas pressure provides additional mechanical force to blow molten material out of the cut. When high-pressure nitrogen cutting is used on stainless steel, a bright, oxide-free edge is obtained, but the processing speed is lower than with argon or helium.

The main issue with inert gas cutting is the formation of burrs of deposited material on the underside of the kerf. The problem is solved by optimising the key process parameters: nozzle diameter, focus position, and gas pressure.

Nitrogen pressure lies in the range of 10–20 bar; pressure increases with material thickness. Nitrogen gas purity must be above 99.8 %.