20 questions about cutting

Steel plate cutting is a process where a part of the plate is separated from the rest. There are two distinct methods: thermal (hot) and mechanical (cold).
The most common cutting methods used for steel plate are:

Thermal:

Cold:

There are many ways and methods of steel cutting. They can basically be divided into two methods: Thermal cutting and Cold cutting. Thermal cutting methods use thermal energy obtained by combustion reaction, electric arc or light rays. Cold cutting methods use mechanical energy for cutting. Typical examples of thermal cutting methods are oxy-fuel (or flame) cutting, plasma cutting and laser cutting. Typical examples of cold cutting methods are waterjet cutting, shearing, sawing, punching and machining.

The most common thermal cutting methods used for steel plate are: Oxy-fuel (or flame), plasma and laser cutting.

There is no single best method. The most commonly applied compromise is probably oxy-fuel cutting. Factors to consider are plate thickness, required tolerances, cut edge quality, plate hardness, heat influence, cost, available equipment. SSAB Tech Support can give detailed advice. Indicative choice of cutting method for different plate thickness ranges:

When choosing a cutting method, it is good to know that a number of methods can be used for a particular situation. When choosing the cutting method, you should consider the thickness of the plate, cut edge quality and costs. For more information or assistance in choosing the right cutting method for your business’s needs, contact SSAB Tech Support. Indicative choice of cutting method for different plate thickness ranges:

Oxy-fuel cutting is the oldest and most commonly used steel cutting method. It is used for cutting materials 3-300 mm thick. A burning gas jet or flame (acetylene or propane) is used to heat the steel to its ignition temperature (approx. 900°C). Oxygen is mixed into the fuel flame to enhance the melting and burning process. The gas jet oxidizes and blows away the liquid steel and oxidation products, producing a hole. The flame motion at a proper speed forms a continuous gap between the plate parts to be separated.

Plasma cutting is a thermal cutting process where an electrical arc ionizes a gas jet to form a plasma at temperatures exceeding 20 000°C. The steel is melted, oxidized and blown away. The tool is moved along the intended cutting line at a proper speed forming a gap line between the plate parts to be separated.

Laser cutting is a thermal process where a high-power laser beam is used to cut, melt or burn a hole in a steel sheet or plate. The beam motion will produce a line gap, separating the sheet or plate into two.

Waterjet or abrasive waterjet (AWJ) cutting is a cutting method where a high-pressure water jet carrying a fine-grained abrasive erodes a hole in the steel. The waterjet motion will produce a line gap, separating the steel plate into two.

The pros:

The cons:

Preheating steel before cutting increases the temperature in the vicinity of the cut during the actual process. This will reduce the risk for hydrogen-induced cracking in particular for high-strength/hard steel. The maximum temperature allowed greatly depends on the steel grade. Too high temperature may destroy the steel properties.

The purpose of preheating is normally to reduce the risk for hydrogen-induced cracking when applying thermal cutting methods. This risk grows with increasing plate thickness and hardness of the steel plates to be cut. This makes preheating an important factor to consider when cutting Hardox^® and most Strenx^® grades. Detailed recommendations can be found in the SSAB brochures found at SSAB Download center.

Postheating can be applied directly after thermal cutting in particular for high-strength and hard steels. The purpose is to reduce residual stresses and to make hydrogen diffuse out of the cut pieces, thereby reducing the risk for hydrogen-induced cracking. A furnace or a gas flame can be used for post-heating. Using a flame calls for careful temperature control to prevent undesired changes in steel properties.

The single most important factor controlling cutting speed is plate thickness. SSAB steel grades can be cut as any commercial steel grade. The cutting machine suppliers give proper recommendations.

A hydrogen-induced crack is a delayed crack phenomena that might occur during cutting of steel using thermal methods. The cutting process will contribute with some hydrogen, with temperatures stimulating diffusion of hydrogen and residual tensile stresses. The concentration of hydrogen in the steel, close to the cut, will increase. In hard steel in particular, the conditions necessary for hydrogen-induced cracking are now present. Since diffusion of hydrogen in the steel needs some time, cracking can be delayed for some days.

You can minimize the risk for hydrogen-induced cracks in cut edges of Hardox^® and Strenx^® grades by following SSAB’s cutting recommendations. The recommendations consist of additional processes such as preheating, post-heating and slow cooling. Each additional process has recommended parameters that can be found in the SSAB cutting recommendation. (link). 

Oxy-fuel cutting: Make sure the relation between cutting speed and cutting oxygen pressure is correct. A high speed requires a high pressure and vice versa. To obtain satisfactory cut edge quality, the cutting speed must be maintained within a certain range. Check the recommended cutting speed provided by the manufacturer.

Plasma cutting: The recommended speed provided by the manufacturer often works well with new consumables, when the consumables get more worn, it often helps to reduce the cutting speed slightly. The voltage during plasma cutting will affect the shape of the kerf. If the kerf is very narrow at the bottom and wide at the top, the voltage should be decreased. The same situation will occur if the travel speed is to high. If the kerf is very wide at the bottom, the voltage could be increased.

Laser cutting: The most important factors for a good cut edge quality during laser cutting is surface cleanliness and temperature of the material. Remove paint or corrosion prior to laser cutting, try to nest the cutting sequence so the heat is distributed over the entire plate.

Waterjet cutting: A low cutting speed will result in a better cut edge quality, make sure that the nozzle is in good condition when high quality is required.

A dirty, painted or corroded plate surface is detrimental for the cut edge quality, although this depends on the method.
For oxy-fuel and plasma cutting, the surface condition has a limited influence, during oxy-fuel cutting the speed might need to be reduced slightly if the surface is painted or corroded.
For laser cutting, surface quality is very important and a very clean (naked) plate surface near the intended cutting line is needed. 

The Heat Affected Zone (HAZ) is the area of base material which is not melted but has had its microstructure and properties altered by the intensive heat of cutting operations.

The size of the HAZ depends on steel grade, the plate thickness and heat input. Higher heat input will provide wider HAZ. According to that wider HAZ will be when cutting with Oxy-fuel cutting method comparing to laser cutting which will provide narrower HAZ.