Laser Cutting of Carbon / Mild Steel

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Carbon steel basics

Carbon steel (mild steel / ferrous metal) is an iron–carbon alloy with carbon content up to 2 %. Depending on the carbon content, carbon steel is classified as:

Low-carbon (mild) steel — carbon content below 0.25 %.
Medium-carbon steel — carbon content 0.25–0.6 %.
High-carbon steel — carbon content above 0.6 %.

By rolling method, carbon steel is either hot-rolled or cold-rolled. The main difference is the processing temperature.

Hot-rolled steel is produced from lower-grade raw material, so its price is lower than that of its cold-rolled counterpart. Hot-rolled steel can reach thicknesses above 160 mm.

Cold-rolled steel is typically used for thin-sheet metal structures, 0.4 mm to 6 mm thick. Cold-rolled steel has better quality than hot-rolled, so its price is higher as well.

Material selection

Supplied steel grades:

First grade
Second grade
Third grade
Off-spec / non-conforming

First and second grade are suitable for laser cutting, especially for parts to be painted or for decorative products. Grades differ in alloy composition quality and production process.

For stable operation, the laser cutting operator should request a sample sheet of the required thickness from the manufacturer, with a description of the steel grade and production process. The operator can then evaluate defects, assess risks and, by agreement, select a stable mode for series production while working with this supplier on a permanent basis — or simply change the supplier to avoid problems in serial part cutting.

Material composition

The material you use is critical. Low-grade steel often contains impurities that are highly reactive when heated. Several thermal reactions occur, which can affect scale formation, sudden molten-metal ejections and other cutting defects. If the melt viscosity is high, scale will stick to the material and require extra cleaning. Reducing speed or adding finishing operations is clearly not a plus for part production.

Material quality

If sheets were rolled in violation of the technology and stored improperly, you should expect rust-covered sheets, non-uniform surface, varying thickness across the plane, and porosity in the metal — all of which cause cutting defects.

Gas selection

Oxygen and air are typically used for cutting mild steel and low-alloy steels. Nitrogen is a universal cutting gas, but because of its high consumption it is reserved for special cases.

Oxygen cutting

Produces an exothermic reaction that benefits cutting of carbon steel. However, oxygen cutting results in oxidised cut edges and requires careful process control to minimise scale adhesion, slag, dross, surface roughness and heat-affected zone (HAZ).

Oxygen nozzle pressure typically lies in the range 0.5–5 bar.
Oxygen pressure is reduced as plate thickness increases (to avoid burning) and the nozzle diameter is increased.
High gas purity matters: 1 mm steel can be cut up to 30 % faster using 99.9 % or 99.99 % oxygen compared with standard 99.7 % oxygen.
Maximum material thickness is relatively higher with oxygen cutting than with high-pressure nitrogen cutting.

Compressed air

Best for thin-sheet cutting. Cutting speed is much higher than with oxygen.
For metals 2 mm and thicker, cutting defects are hard to avoid — extra finishing will be required.
Air pressure raised to 5–6 bar is enough to blow molten metal out of the kerf.
Since nearly 80 % of air is nitrogen, compressed-air cutting is essentially fusion cutting.
A downside of compressors is regular service (oil changes) and filters that occasionally fail. After three months of normal operation they can start "spitting": condensate from the receiver flies through the line. If the air path is contaminated once, simply installing clean filters afterwards will not help — you have to clean the path itself by purging it with alcohol.

Beam alignment and check of the optical system and nozzle

At shift start, verify that the laser beam alignment is correct and that the protective glasses are clean. If defects appear, start troubleshooting by checking the beam centring and protective-glass cleanliness. Also remember that for oxygen cutting you should use double nozzles and regularly check the geometry of the orifice.

Cutting defects: scale, burrs, slag, dross

Scale is an undesirable accumulation of waste produced from molten material — a by-product of the cutting process.

The three main causes are low-speed scale, high-speed scale and fine-dispersed scale (slag).

High-speed scale. If cutting speed is too high, the arc starts to lag in the kerf, leaving uncut material on the underside of the plate. Scale begins to accumulate, degrading quality.

Low-speed scale. If cutting speed is too low, the cutter looks for additional material to cut. The arc diameter keeps growing, resulting in a wider kerf at a point where the high-speed plasma component no longer disperses the molten metal. The molten material then accumulates around the plate underside.

Slag (fine-dispersed scale). Forms when re-solidified metals leave deposits on the surface, which later flake off in pieces cut from the metal. In most cases this is due to two factors: speed too high or nozzle pressure too low. Unlike the other two, removing slag takes very little effort and can be done by anyone.

Dross. Solidified droplets of molten metal on the workpiece edge, appearing when speed or other technological parameters of thick-plate cutting are off.

Bead-shaped dross with downward grooves, sticking to structural steel. Cause: focus position too high relative to nominal, or cutting speed too high. Fix: slow down by 10 % or lower the focus position.
Dross with crumbs and pitting on the workpiece. Caused by focus position below nominal combined with excessive oxygen pressure and particle adhesion at high speed. Fix: raise the focus and slow the process by 5–10 %.

Burr is a highly adherent solidified material or solidified oxide slag formed on the bottom side of the cut. Molten materials with high surface tension and low viscosity are harder to remove from the cutting front with assist gas and can cause scale to stick on the underside.

Heat-affected zone (HAZ)

Laser cutting creates a heat-affected zone (HAZ) next to the cut edge. The HAZ is the part of the metal whose structure is influenced by heat without being melted. Microstructural change in the HAZ is one of the characteristics that determine laser-cut quality.

The HAZ produces structural changes that weaken the part in this area, so any metal ejection makes further cutting harder. Solutions: pre-piercing, continuous purging, cutting from the centre outward in different directions, and additional purging.

Material blow-out / piercing pit

Workpiece tear-out at the lead-in side, crater at piercing. Caused by low focus, incorrect parameters (low cutting height, high frequency, oversized nozzle, excessive piercing power).

To avoid this defect:

raise the focus;
use a smaller nozzle;
set lead-in parameters to default;
perform a pre-piercing step first.

Conditions for oxygen cutting of metal

For oxygen cutting of metal the following conditions must be met:

(a) The metal's burning temperature in oxygen must be lower than its melting temperature, otherwise the metal will melt and turn liquid before it starts burning in oxygen.

(b) The metal oxides formed must melt at a temperature lower than the metal's burning temperature and must not be too viscous.

(c) The amount of heat released by the metal burning in oxygen must be large enough to sustain the cutting process. When cutting steel, about 70 % of the preheat heat comes from the metal burning in oxygen and only 30 % from the preheat flame.

(d) The metal's thermal conductivity must not be too high, otherwise intense heat dissipation may interrupt the cutting process.

Effect of steel composition on cutting

Pure iron and low-carbon steels best satisfy the conditions above. Pure iron has an ignition temperature in oxygen of 1050 °C and a melting temperature of 1528 °C. When the steel contains 0.7 % carbon, its ignition temperature in oxygen rises to 1300 °C, which equals the onset of melting for that composition. According to A. N. Shashkov, selective oxidation of iron in oxygen during steel cutting starts at about 1130 °C, and intense carbon burn-off begins at 1300 °C and above.

Besides composition, the ignition temperature is also influenced by the surface state, the size of the metal pieces, and the pressure and velocity of the oxygen stream. A rough surface makes ignition easier. Iron powder can ignite in pure oxygen at 315 °C — far below rolled metal. The surface of a large steel piece ignites at 1200–1300 °C. At a pressure of 25 kgf/cm² and oxygen-flow velocity of 180 m/s, the ignition temperature of carbon steel in oxygen drops to 700–750 °C.