Selecting The Right Grade For Laser Cutting

Laser cutting of steel has become increasingly widespread in Australia in the last few years, and the successful use of this technology depends at least partly on selecting the right grade of steel to be cut.

News Item: 17 October 2002

Laser cutting offers the advantages of speed, precision and quality of cut over the more conventional forms of cutting. In addition to the ability to cut steel, laser cutting machines can also be used to cut a wide variety of other materials such as plastic and wood. However, these advantages are offset by restrictions on the thickness of steel that can be cut and the financial outlay for installing a laser cutting machine.

When using laser cutters for cutting steel, there are additional demands on the selection of the appropriate steel grade, compared to using other steel processing technologies. Selection of the incorrect grade may lead to a poor quality of cut or even the inability to cut the steel.

Industry experience has shown that two properties have a large impact on both the ability to cut steel and the quality of the cut. These properties are surface quality and flatness.

Surface quality

Steels suitable for laser cutting need to have a thin, tightly adherent surface scale. The adherence of the scale, in particular, tends to have a major impact on the ability of a laser cutter to actually cut the steel. Experience has shown that steels with loose or flaked scale will generally stop the laser cutting process.

Overall, surface cleanliness is critical to successful laser cutting. Rust presents much the same problems as mill scale, and other surface contaminants such as grease, stencil markings or paint will also interfere with the process.

Flatness

The impact of plate flatness on laser cutting is due to the change in distance between the plate and the cutting head. Laser cutting involves focusing the beam onto the plate surface, and even small changes in the distance between the laser head and the plate, as may occur when the plate is not flat, may impact on the ability of the laser to cut. The flatness requirements of laser cutters exceed those of other cutting processes and are significantly more stringent than the current flatness standards in AS/NZS 1365.

Although surface quality and flatness are the two properties that have the most significant impact on laser cutting, it is also important to consider the end use requirements of the steel grade. As with any other application, the steel selected must meet the specified strength, toughness or forming requirements of the relevant standard.

Grade characteristics

The XLER® steel Products Technical Library has previously addressed grade selection in general terms. Here we will look at the grade characteristics that are specific to selecting steel for laser cutting, and at some special grades available for this purpose.

Coil plate has less surface scale than plate from the Plate Mill (also referred to as pattern plate), and as a result it tends to be better for laser cutting. Also, in the thinner gauges ( < 8 mm) the AS/NZS 1594-HA250 grade achieves the flatness required for laser cutting and would therefore be the preferred option.

However, at the top end of the coil plate thickness range (8-12 mm) there may on occasion be issues with the flatness of the plate, due to the uncoiling of the coil to cut into plate lengths. In this thickness range most customers find the flatness of AS/NZS 1594-HA250 is satisfactory, but we have already noted that laser cutting is especially sensitive to flatness.

As a result, BlueScope Steel has worked with selected customers to develop two new grades of pattern plate specifically suited to laser cutting. Sold as Lasercut 250 and Lasercut 350, they address both the flatness issues of coil plate and the surface quality issues of pattern plate for plate in this thickness range. Unlike coil plate, they are also available in thicknesses greater than 12 mm.

In applications where flatness is critical, BlueScope Steel's Technical Service Metallurgists should be contacted to discuss what levels of flatness can be achieved and what options may be available in producing plate to meet requirements - Contact Us . Also, it should be noted that the two standards AS/NZS 3678 and AS/NZS 1594 have different tolerances for thickness, width, length and flatness. They also have different requirements for surface quality and structural integrity, which are detailed in the respective standards. Where any of these attributes is critical, the AS/NZS 1365 standard should be consulted.

Laser grades

As mentioned, BlueScope Steel has developed two new steel grades specifically for use in the laser cutting industry. Through a series of trials conducted at W. H. Williams & Sons in Sydney and Molnar Engineering in Adelaide, BlueScope Steel has developed and to comply with all the requirements of the AS/NZS 3678 standard. These grades are equivalent in terms of strength to the traditional AS/NZS 3678-250 and 350 grades respectively, but with modifications to the steel chemistry that significantly improve the laser cutting ability.

Development work carried out with our customers has led to the development of these grades with low silicon and carbon levels. These chemistry changes to our standard plate grades produce steel that has a more tightly adhered scale layer, resulting in a superior performance during laser cutting. Full details of the laser grade chemistries, and a comparison with the standard chemistries, are shown in the table. Although surface quality and flatness have the most significant impact on laser cutting, it is also important to consider the end use.

Table 1. A Comparison of the Typical Standard and "Lasercut" Chemistries

	C	P	Mn	Si	S	Al
AS/NZS 3678-250 XLERPLATE®	0.15	0.02	0.65	0.15	0.013	0.025
Lasercut 250 XLERPLATE®	0.09	0.02	1.30	< 0.05	0.013	0.035
AS/NZS 3678-350 XLERPLATE®	0.15	0.02	1.20	0.30	0.013	0.03
Lasercut 350 XLERPLATE®	0.15	0.02	1.15	< 0.05	0.013	0.035