Cutting - the basics

TWI Bulletin, November/December 1993

Stephen Rennie was a Research Engineer in TWI's Arc Welding Department. Having graduated with a Bachelor of Applied Science (Metallurgy) from the Western Australian Institute of Technology, he worked as a metallurgist for Wundowie Foundry and the State Energy Commission of Western Australia (SECWA).

In 1990 he joined TWI, where his main responsibilities were in cutting (primarily plasma, oxy-fuel and abrasive water jet), narrow gap welding and plasma welding.

New materials, both metallic and non-metallic, and escalating costs for hard tooling are among the reasons that manufacturers have explored alternative methods of cutting engineering materials. In this article, Stephen Rennie outlines the concepts of the more commonly-used oxy-fuel, air plasma, and abrasive water jet (AWJ) cutting techniques for newcomers to these methods.

A part from the escalating costs for hard tooling, exacting demands of the engineering industry have called for higher cutting speeds and more accurate production methods. Cutting a component is an important but often neglected part of the production operation. The choice of which cutting process to use depends on the compatibility of a process to cutting a particular material, cutting speed, quality of cut, overhead cost and consumable cost. Other considerations such as health and safety, and operating/cutting environment will vary from one application to the next.

Oxy-fuel, air plasma and AWJ cutting are all compatible with most X-Y or computer numerically controlled (CNC) systems. A programme change and process adjustments are all that is required to accommodate an alternative cutting process. With so many variables in the cutting operation, an understanding of process potentials and limitations is required to select the process best suited to a particular application.

Oxy-fuel

Oxy-fuel cutting is a process where the severing of material is accomplished by chemical reaction of pure oxygen with metal. A preheat flame is first directed onto a spot on the metal which is heated above ignition temperature. At this point, a jet of pure, high pressure oxygen burns through the spot and is removed by the velocity and pressure of the oxygen stream (Fig. 1).

Although oxy-fuel cutting is primarily used on carbon steels, it can also be used for cutting titanium (up to five times the rate of steel). Most metals cannot be cut because either their melting point is at or below the ignition temperature (e.g. Al, Pb), a low heat of reaction makes it difficult to maintain preheat, or because of their high thermal conductivity (e.g. Cu), which also makes it difficult to maintain sufficient local preheat.

Materials such as cast iron and oxidation resistant steels can still be cut using oxy-fuel but may require one of the following methods to perform the cut:

• Torch oscillation;
• Waster plate;
• Wire feed;
• Powder cutting;
• Flux cutting.

Oxy-fuel cutting is one of the most versatile processes. It can be used for manual cutting or in an automated CNC operation. It is capable of cutting from approximately 3mm to more than 300mm. When cutting plate under 25mm, oxy-fuel cutting receives considerable competition from plasma cutting. Above 25mm, oxy-fuel cutting is generally faster than plasma cutting although this depends on the power level of the plasma unit. Figure 2 shows oxy-fuel cutting of 100mm thick steel. High speed nozzles, used with higher oxygen pressures, give increased cutting speeds, especially in machine cutting.

The primary disadvantage of oxy-fuel over processes such as plasma, apart from the lower cutting speeds at thicknesses less than 25mm, is the limited types of material that can be cut.

Air plasma

A pilot arc is first established between the electrode and the nozzle. When this is brought close to the material to be cut, the arc transfers to the workpiece, increases in power, and penetrates the full thickness of the workpiece. Molten metal is blown out by the high flow rate of the gas.

In the past, inert or non-oxidising plasma gases have been used because the tungsten electrode is easily eroded in oxidising environments. The use of zirconium and hafnium electrodes has enabled air to be used as the plasma gas, Fig.3. Zirconium and hafnium form stable oxides and nitrides which protect the electrode surface during cutting. The surface is, however, destroyed during high voltage arc initiation and this means that the consumable life in air plasma cutting is dependent upon the number of starts.

The melting points of zirconium (1852°C) and hafnium (2230°C) are considerably lower than tungsten (3380°C) and as a consequence require more efficient electrode cooling. Operating current must also be kept below approximately 250A to ensure adequate electrode life.

Plasma cutting requires no preheat and, due to the concentrated nature of the arc, produces minimal distortion in the material being cut. Similarly, the heat affected zone (HAZ) of plasma cuts is narrower than that of oxy-fuel cutting. This could be important if the metal is to be welded or finished. The thickness of the HAZ is approximately 0.25mm for austenitic stainless steel, 0.5mm for carbon steels, and between 1.1 and 2.7mm for aluminium.

Air plasma cutting can be used effectively to cut ferrous and non-ferrous materials. For cutting carbon steels, air plasma competes favourably with oxy-fuel predominately in the range of 3-25mm thickness. For cutting of 12mm plate, cutting speeds of approximately 700-800 mm/min could be expected with oxy-fuel. Air plasma cutting could be expected to yield cutting speeds of 2-2.5 m/min using a 150A power source. The thickness above which oxy-fuel cutting is faster than air plasma cutting, for carbon steels, depends on the power of the plasma cutting unit (Fig.4).

There is, however, more to consider than just cutting speed. In some operations capital cost could be a limiting factor. For oxyfuel this is typically 1/20th of that for plasma.

Another limitation of air plasma compared with oxy-fuel is the portability of equipment; air plasma equipment requires a 3-phase power supply, and is therefore less portable.

There is greater potential hazard of fume, noise, and radiation with air plasma than there is with oxy-fuel.

The degree of fume depends on many factors such as arc current, cutting speed, plate material and orifice gas. Local ventilation to remove fume from the work area may be required.

Another way of reducing the level of fume involves using a water table. In this method, the workpiece is totally submerged so that the top surface of the workpiece is approximately 50-75mm below the surface of the water. As the operator cannot see the workpiece during cutting, it is recommended for use with numerically controlled systems. A possibility of hydrogen detonation beneath the work surface exists when cutting aluminium. This is believed to be due to hydrogen released by the interaction of molten aluminium and water.

Plasma cutting is considerably noisier than oxy-fuel cutting. The level of noise depends primarily on current. Hearing protection is usually required when not using a water table which can considerably reduce the noise level and also reduce the level of radiation in its depth. Problems associated with the water table are appreciable heat transfer from the sheet to the water, leading to decreased cutting speed, and increased slag content.

One problem associated with air plasma cutting is porosity formation when welding on plasma cut edges. The tendency to porosity formation, when welding carbon steels, is greater in thicker sections using either MIG or SAW. Gas pocket formation can be completely suppressed by locally heating the edges to 500-600°C for six minutes or more prior to welding.

Gas plasma

In dual gas operation the plasma is created by a conventional tungsten electrode in an inert gas, but a secondary gas shield is introduced around the nozzle (Fig.5). The cutting gas is usually argon, argon-hydrogen or nitrogen, and the secondary gas is chosen to suit the metal being cut, for instance air or oxygen for mild steel to increase cutting speed.

The high definition technique uses a highly constricted plasma arc to produce a superior cut quality. In the torch, the oxygen plasma jet is forced to swirl as it enters the plasma orifice, and a secondary flow of gas is injected downstream of the plasma nozzle.

Abrasive water jet (AWJ)

The principal components of an AWJ cutting system are the high pressure pump powered by an electric motor or internal combustion engine, the abrasive feed system, and the cutting lance with mixing chamber and cutting nozzle (Fig.6). The cutting lance is mounted on the cross carriage of the machine and the mounting point swivels to enable bevel cutting in addition to linear and profile cutting.

After cutting through the workpiece the water jet is contained within the cutting tank which is usually filled with water to a level just below the workpiece. The power remaining in the jet is dissipated by the water in the tank. Alternatively a catcher, tube can be used to avoid the need for the water tank. Such a tube mirrors the movements of the cutting lance and 'catches' the jet as it emerges from the base of the workpiece. For site use, the pumping equipment is usually containerised or mounted on a truck, and catching systems are generally unnecessary. Mechanised cutting should be used wherever possible for improved productivity and safety and because the reaction forces involved make the process generally inappropriate for manual use.

The applications of AWJ cutting are very wide ranging and include metals, glass, ceramics, and composites, both plastics and metal based, in a wide range of thicknesses. Typical cutting speeds for various engineering materials using AWJ and Plasma are compared in the Table.

Table Cutting speeds for AWJ (high pressure and plasma cutting

The most important advantage of the process is its ability to cut materials without heat, thus avoiding the formation of heat affected zones in metals, sometimes associated with microcracking in high strength metals. This also applies to plastics and composite materials where thermal cutting often degrades or chars the edges of the material being cut. Sandwich materials are being used more and more in industry and with large differences in melting points, a non-thermal cutting process is the only alternative.

Today, the AWJ process is used in a wide variety of industries, not only for initial manufacturing, but also for repair and maintenance, and demolition. The very low fire risk associated with the process has led to extensive use in the oil and chemical industries, for modification or removal of process plant, including various types of pressure vessels, tanks and pipelines. Due to the inherent safety of the process a cold work permit is sufficient in sensitive areas. The common problem of plant clad or lined with non-metallic materials, and requiring cutting of both metallic and non-metallic layers, is easily handled by the AWJ process.

AWJ cutting has also been used for decommissioning of offshore structures, for profiling of parts in both composites and titanium alloys in the offshore industry. The process is particularly suitable for cutting Kevlar and similar materials. High quality cut edges are produced which require no subsequent edge finishing.

The process is also used in glass cutting and, in the automotive industry, for trimming plastics components for interior fittings such as dashboards, and also for cutting carpet and sound insulation.

AWJ cutting is not, however, without its problems. Kevlar and some other materials absorb an amount of water. Some mil spec cutting operations do not allow AWJ cutting because it can cause delamination. In most cases, however, Kevlar dries without problem.

AWJ cutting can leave a contamination zone due to abrasive particles being embedded in the work material. This is rarely a problem as the debris deposited on the kerf surface is loose, and a second pass with pure water will rinse it away.