TITLE: Study the Science and System operation of thermal cutting using Gas Cutting, Plasma Cutting, and Air Arc Gouging process.

OBJECTIVE:

To study the science, systems operations and the fundamentals of gas cutting, plasma cutting and air arc gouging.

INTRODUCTIONS

Oxy - Cutting

Oxy-fuel gas cutting (OFC) is similar to oxy-fuel welding, but the heat source is now used to remove a narrow zone from a metal plate or sheet. This process is suitable particularly for steels. The basic reaction with steel is:

Fe+O→FeO+Heat

3 Fe+2O2→F e3O4+Heat

And

4 Fe+3O2→2F e2O3+Heat

The greatest heat is generated by the second reaction, and it can produce a temperature up to 870 °C. However this temperature is not enough to cut steels, therefore is oxy-fuel cutting process, pre heat is need before start the cutting process. Preheat flames are used to raise the surface or edge of the steel to approximately 1800°F (bright red colour). Pure oxygen is then directed toward the heated area in a fine, high pressure stream. As the steel is oxidized and blown away to form a cavity, the preheat and oxygen stream are moved at constant speed to form a continuous cut. The process generates a kerf which is a gap distance of cutting.

Figure 1: Oxy-fuel cutting orientation

The maximum thickness that can be cut by oxy-fuel cutting depends mainly on the gases used. With oxyacetylene gas, the maximum thickness is about 300 mm and the kerf width range from 1.5 to 10mm with reasonably good control of tolerance.

In oxy-fuel cutting, there are three type of flame. The first type is the carbonizing flame or reducing flame. This flame is produced when acetylene is more than oxygen. The second type of flame is neutral flame. Neutral flame is produced when oxygen and acetylene are in the same amount. The last type is oxidizing flame. The oxidizing flame is produced when oxygen is more than acetylene.

Figure 2: Types of flame

Plasma Cutting

Plasma Cutting is an extension of the Gas Tungsten Arc Welding (GTAW) process. The arc is formed between an electrode (which is usually but not always made of a sintered tungsten) and the work piece. The key difference from GTAW is that in PAW, by positioning the

electrode within the body of the torch, the plasma arc can be separated from the shielding gas envelope. This plasma then forced through a fine-bore copper nozzle which constricts the arc and the plasma exits the orifice at high velocities (approaching the speed of sound) and a temperature approaching 20,000 ‘C. Plasma Arc Welding is advancement over the GTAW process. This process uses a non-consumable tungsten electrode and an arc constricted through a fine-bore copper nozzle. PAW can be used to join all metals that are weld able with GTAW (i.e., most commercial metals and alloys).

Figure 3: Conventional plasma arc cutting process

Plasma cutters work by sending an electric arc through a gas that is passing through a constricted opening. The gas can be shop air, nitrogen, argon, oxygen etc. This elevates the temperature of the gas to the point that it enters a 4th state of matter. They have 3 types in plasma cutting which are solid, liquid, and gas. Scientists call this additional state plasma. As the metal being cut is part of the circuit, the electrical conductivity of the plasma causes the arc to transfer to the work. The restricted opening (nozzle) the gas passes through causes it to squeeze by at a high speed, like air passing through a venture in a carburetor. This high speed gas cuts through the molten metal. The gas is also directed around the perimeter of the cutting area to shield the cut.

In many of today's better plasma cutters, a pilot arc between the electrode and nozzle is used to ionize the gas and initially generate the plasma prior to the arc transfer. For the most commonly known substance, water, these states are ice, water and steam. If add heat energy, the ice will change from a solid to a liquid, and if more heat is added, it will change to a gas (steam). When substantial heat is added to a gas, it will change from gas to plasma, the fourth state of matter. If add more energy to water, it vaporizes and separates into two gases,

hydrogen and oxygen, in the form of steam. By adding even more energy to a gas, we find that its characteristics are modified substantially in terms of temperature and electrical characteristics. This process is called ionization, the creation of free electrons and ions among the gas atoms. When this happens, the gas, which has now become plasma, is electrically conductive because free electrons are available to carry current. Many of the principles that apply to current conduction through metals also apply to plasmas. For example, if the current-carrying cross-section of a metal is reduced, the resistance increases. A higher voltage is needed to force the same amount of electrons through this cross-section and the metal heats up. The same is true for a plasma gas; if more reduce the cross-section, the hotter it gets. In this historic review of the plasma arc process, we will follow the development of a plasma arc with high speed gas flow which is, essentially, the "plasma cutting process."

The torch uses a two cycle approach to producing plasma. First, a high-voltage, low current circuit is used to initialize a very small high intensity spark within the torch body, thereby generating a small pocket of plasma gas. This is referred to as the pilot arc. The now conductive plasma contacts the work piece, which is the anode. The plasma completes the circuit between the electrode and the work piece, and the low voltage, high current now conducts. If the plasma cutter uses a high frequency/high voltage starting circuit, the circuit is usually turned off to avoid excessive consumable wear. The plasma, which is maintained between the work piece and electrode, travels at over 15,000 km/h (over twelve times the speed of sound of the ambient air).

Plasma is an effective means of cutting thin and thick alike. Handheld torches can usually cut up to 1/2 in (13 mm) thick steel plate, and stronger computer-controlled torches can pierce and cut steel up to 12 inches (300 mm) thick. Formerly, plasma cutters could only work on conductive materials, however new technologies allow the plasma ignition arc to be enclosed within the nozzle thus allowing the cutter to be used for non-conductive work pieces.

Power source

The plasma arc is normally operated with a DC, drooping characteristic power source. Because its unique operating features are derived from the special torch arrangement and separate plasma and shielding gas flows, a plasma control console can be added on to a conventional TIG power source.

Special-purpose switched DC power sources are available. By misbalancing the waveform to reduce the duration of electrode positive polarity, the electrode is kept sufficiently cool to maintain a pointed tip and achieve arc stability.

Electrode

The electrode used for the plasma process is tungsten-2%thoria and the plasma nozzle is copper. The electrode tip diameter is not as critical as for TIG and should be maintained at around 30-60 degrees. The plasma nozzle bore diameter is critical and too small a bore diameter for the current level and plasma gas flow rate will lead to excessive nozzle erosion or even melting. It is prudent to use the largest bore diameter for the operating current level.

Plasma and shielding gases

The normal combination of gases is argon for the plasma gas, with argon plus 2 to 5% hydrogen for the shielding gas. Helium can be used for plasma gas but because it is hotter this reduces the current rating of the nozzle. Helium's lower mass can also make the keyhole mode more.

Application

The plasma process is equally suited to manual and automatic applications. It has been used in a variety of operations ranging from high volume welding of strip metal, to precision welding of surgical instruments, to automatic repair of jet engine blades, to the manual welding of kitchen equipment for the food and dairy industry.

The plasma process can gently yet consistently start an arc to the tip of wires or other small components and make repeatable welds with very short weld time periods. This is advantageous when welding small parts components such as needles, wires, light bulb filaments, thermocouples, probes and some surgical instruments.

Applications include Pressure and Electrical Sensors, Bellows, Seals, Cans, Enclosures, Micro switches, Valves, Electronic Components, Motors, Batteries.

Air Arc Gouging

Air carbon arc cutting is a process of removing metal by means of heat generated from a carbon arc. This process employs a carbon/graphite electrode, compressed air and a standard power source. The intense heat arc created between the electrode and a metal work piece cuts and melts the work piece. At the same time, the molten metal is blown away by the compressed air. This process is commonly used for cutting stainless steel, carbon steel, and alloys of copper, brass, aluminium and magnesium.

The main purpose of this process is to remove defective or old welds in order to repair or dismantle equipment. Unlike oxy-fuel cutting, this process avoids the need for oxidation of the metal to be cut. The rate of metal removal is based on the efficiency of air jet in removing molten metal and the melting rate.

In this process, an intense arc is used to form a molten pool on a metal work piece, and compressed air is used for blowing the molten metal thoroughly from the metal surface. It is important that the metal is only gouged or cut along the airflow direction. Moreover, appropriate arc length has to be maintained to completely remove the molten metal.

Figure 4: Air Arc Gouging Process

The main elements in air arc gouging are the power source, compressed air, electrode and gouging torch. Those element been discuss below.

Power source 

Air carbon arc cutting requires a three-phase welding power source with high capacity. A voltage drop is created in the circuit only if the open-circuit voltage is greater than the required arc voltage. The arc voltage required to perform this process should be in the range of 35 to 56 V. Hence, the open-circuit voltage should be at least 60V. However, the actual arc voltage is set based on the type of gouging and an arc length.

Compressed Air 

Compressed air with pressures in the range of 80 to 100 psi can be used. Higher pressures may not remove the molten metals properly.

Electrode 

Three types of electrodes can be used for the air carbon arc cutting process. The most widely used electrodes are DC copper-coated electrodes that are formed by combining carbon-graphite mixture with a binder. These electrodes have groove uniformity, stable arc characteristics and prolonged service life. DC plain electrodes are another type of electrodes, which are formed similar to the coated electrodes with an exception of the copper coating. AC coated electrodes on the other hand are made using a special binder in addition to carbon and graphite. They include copper coating and rare- earth materials for stabilizing the arc while using alternating current.

Gouging Torch 

A swivel head on the torch holds the electrode in one or more air holes. Usually, torches are air-cooled. Sometimes, water-cooled cables are used along with heavy-duty torches for high-current applications.

The electrodes are fixed to the swivel head of the torch various positions that include flat, vertical, horizontal and overhead positions. The air jet on the two sides of the electrode is

turned on before it strikes the arc. During the process, the air jet in the electrode end sweeps all the molten metal. A slight contact of the electrode to the workpiece creates an arc. It should be noted that the electrodes are not drawn back after the ignition of the arc. However, the smoothness of the cut can be achieved by controlling the electrode movement.