PLASMA ARC CUTTING

Plasma is typically an ionized gas. Plasma is considered to be a distinct state of matter, apart from gases, because of its unique properties. Ionized refers to presence of one or more free electrons, which are not bound to an atom or molecule. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields.

Plasma cutting is a process that is used to cut steel and other metals (or sometimes other materials) using a plasma torch. In this process, an inert gas (Argon) is blown at high speed out of a nozzle and at the same time an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas to plasma. The plasma is sufficiently hot to melt the metal being cut and moves sufficiently fast to blow molten metal away from the cut. Plasma can also be used for plasma arc welding and other applications.

The Arc type 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 pilot arc has a return electrical path built into the torch head. The pilot arc will maintain until it is brought into proximity of the work piece where it ignites the main plasma cutting arc. Plasma arcs are extremely hot and are in the range of 15,000 degrees Celsius.

Oxy fuel cuts by burning, or oxidizing, the metal it is severing. It is therefore limited to steel and other ferrous metals which support the oxidizing process. Metals like aluminium and stainless steel form an oxide that inhibits further oxidization, making conventional oxyfuel cutting impossible. Plasma cutting, however, does not rely on oxidation to work, and thus it can cut aluminium, stainless and any other conductive material. While different gasses can be used for plasma cutting, most people today use compressed air for the plasma gas. In most shops, compressed air is readily available, and thus plasma does not require fuel gas and compressed oxygen for operation.

Plasma cutting is typically easier for the novice to master, and on thinner materials, plasma cutting is much faster than oxyfuel cutting. However, for heavy sections of steel (1inch and greater), oxyfuel is still preferred since oxyfuel is typically faster and, for heavier plate applications, very high capacity power supplies are required for plasma cutting applications.

PRINCIPLE OF PLASMA ARC CUTTING

This process uses a concentrated electrical arc which melts the material through a high-temperature plasma beam. All conductive materials can be cut. Plasma cutting units with cutting currents from 20 to 1000 amperes to cut plates with inert gas, 5 to 160 mm thicknesses. Plasma gases are compressed air, nitrogen, oxygen or argon/ hydrogen to cut mild and high alloy steels, aluminium, copper and other metals and alloys.

The plasma arc process has always been seen as an alternative to the oxy-fuel process. In this part of the series the process fundamentals are described with emphasis being placed on the operating features and the advantages of the many process variants.

PRINCIPLE OF PLASMA ARC CUTTING

The plasma is additionally tied up by a water-cooled nozzle. With this energy densities up to 2x106 W/cm2 inside of the plasma beam can be achieved. Because of the high temperature the plasma expands and flows with supersonic velocity speed to the work piece (anode). Inside the plasma arc temperatures of 30000 degree Celsius can arise, that realize in connection with the high kinetic energy of the plasma beam and depending on the material thickness very high cutting speeds on all electrically conductive materials. The term for advisable state of plasma arc is called stability of arc too. The stability of arc is keeping the plasma jet in desired form.

a) Shape of Plasma Torch,

b) Streaming Jet,

c) Water.

We must monitor these parameters

Temperature and electrical conducting,

Density of plasma jet,

Diameter of plasma beam,

Degree of the plasma beam focusing in output from nozzle.

For the cutting process first of all a pilot arc ignition by high voltage between nozzle and cathode takes place. This low- energy pilot arc prepares by ionization in parts the way between plasma torch and work piece. When the pilot arc touches the work piece (flying cutting, flying piercing), the main arc will start by an automatic increase in power

The basic principle is that the arc formed between the electrode and the work piece is constricted by a fine bore, copper nozzle. This increases the temperature and velocity of the plasma emanating from the nozzle. The temperature of the plasma is in excess of 20 000°C and the velocity can approach the speed of sound. When used for cutting, the plasma gas flow is increased so that the deeply penetrating plasma jet cuts through the material and molten material is removed in the efflux plasma.

The process differs from the oxy-fuel process in that the plasma process operates by using the arc to melt the metal whereas in the oxy-fuel process, the oxygen oxidizes the metal and the heat from the exothermic reaction melts the metal. Thus, unlike the oxy-fuel process, the plasma process can be applied to cutting metals which form refractory oxides such as stainless steel, aluminium, cast iron and non-ferrous alloys.

The power source required for the plasma arc process must have a drooping characteristic and a high voltage. Although the operating voltage to sustain the plasma is typically 100 to 160V, the open circuit voltage needed to initiate the arc can be up to 400V DC. On initiation, the pilot arc is formed within the body of the torch between the electrode and the nozzle. For cutting, the arc must be transferred to the work piece in the so-called 'transferred' arc mode.

The electrode has a negative polarity and the work piece a positive polarity so that the majority of the arc energy (approximately two thirds) is used for cutting.

In the conventional system using a tungsten electrode, the plasma is inert, formed using either argon, argon-H2 or nitrogen. However, as described in Process variants, oxidizing gases, such as air or oxygen can be used but the electrode must be copper with hafnium. The plasma gas flow is critical and must be set according to the current level and the nozzle bore diameter. If the gas flow is too low for the current level, or the current level too high for the nozzle bore diameter, the arc will break down forming two arcs in series, electrode to nozzle and nozzle to work piece.

The effect of ‘double arcing’ is usually catastrophic with the nozzle melting. The quality of the plasma cut edge is similar to that achieved with the oxy fuel process.

However, as the plasma process cuts by melting, a characteristic feature is the greater degree of melting towards the top of the metal resulting in top edge rounding, poor edge squareness or a bevel on the cut edge. As these limitations are associated with the degree of constriction of the arc, several torch designs are available to improve arc constriction to produce more uniform heating at the top and bottom of the cut.

The process variants have principally been designed to improve cut quality and arc stability, reduce the noise and fume or to increase cutting speed. The inert or uncreative plasma forming gas (argon or nitrogen) can be replaced with air but this requires a special electrode of hafnium or zirconium mounted in a copper holder, by shearing . The air can also replace water for cooling the torch.

The advantage of an air plasma torch is that it uses air instead of expensive gases. It should be noted that although the electrode and nozzle are the only consumables, hafnium tipped electrodes can be expensive compared with tungsten electrodes. This relatively new process differs from conventional, dry plasma cutting in that water is injected around the arc. The net result is greatly improved cut quality on virtually all metals, including mild steel. Today, because of advances in equipment design and improvement in cut quality, previously unheard of applications, such as multiple torches cutting of mild steel, are becoming common place.

Shielding and Cutting Gases for Plasma Cutting

Inert gases such as argon, helium, and nitrogen (except at elevated temperatures) are used with tungsten electrodes. Air may be used for the cutting gas when special electrodes made from water-cooled copper with inserts of metals such as hafnium are used. Recently, PAC units shielded by compressed air have been developed to cut thin-gauge materials.

Almost all plasma cutting of mild steel is done with one of three gas types:

1. Nitrogen with carbon dioxide shielding or water injection (mechanized)

2. Nitrogen-oxygen or air

3. Argon-hydrogen and nitrogen-hydrogen mixtures

The first two have become standard for high-speed mechanized applications. Argonhydrogen and nitrogen-hydrogen (20 to 35 percent hydrogen) are occasionally used for manual cutting, but the formation of dross, a tenacious deposit of resolidified metal attached at the bottom of the cut, is a problem with the argon blend.

A possible explanation for the heavier, more tenacious dross formed with argon is the greater surface tension of the molten metal. The surface tension of liquid steel is 30 percent higher in an argon atmosphere than in one of nitrogen.

Air cutting gives a dross similar to that formed in a nitrogen atmosphere. The plasma jet tends to remove more metal from the upper part of the work piece than from the lower part. This results in nonparallel cut surfaces that are generally wider at the top than at the bottom. The use of argon-hydrogen, because of its uniform heat pattern or the injection of water into the torch nozzle (mechanized only), can produce cuts that are square on one side and bevelled on the other side. For base metal over 3 inches thick, argon-hydrogen is frequently used without water injection.

Plasma Gas Selection

Air Plasma

1. Mostly used on ferrous or carbon based materials to obtain good quality a faster cutting speeds.

2. Only clan, dry air is recommended to use as plasma gas. Any oil or moisture in the air supply will substantially reduce torch parts life.

3. Air Plasma is normally used with air secondary.

Nitrogen Plasma

1. Can be used in place of air plasma with air secondary.

2. Provides much better parts life than air

3. Provides better cut quality on non-ferrous materials such as stainless steel and aluminium.

4. A good clean welding grade nitrogen should be used.

Argon/Hydrogen Plasma

1. A 65% argon/35% hydrogen mixture should be used.

2. Recommended use on 19mm and thicker stainless steel. Recommended for 12mm and thicker non-ferrous material. Ar/H2 is not normally used for thinner non-ferrous material because less expensive gases can achieve similar cut quality.

3. Provides faster cutting speeds and high cut quality on thicker material to offset the higher cost of the gas.

4. Poor quality on ferrous materials.

Oxygen Plasma

1. Oxygen is recommended for cutting ferrous metals.

2. Provides faster cutting speeds.

3. Provides very smooth finishes and minimizes nitride build-up on cut surface (nitride build-up can cause difficulties in producing high quality welds if not removed).

Secondary Gas Selection for Plasma Cutting

Air Secondary

1. Air secondary is normally used when operating with air plasma and occasionally with nitrogen plasma.

2. Inexpensive - reduces operating costs

3. Improves cut quality on some ferrous materials

CO2 Secondary

1. CO2 secondary is used with nitrogen or Ar/H2 plasma.

2. Provides good cooling and maximizes torch parts life.

3. Usable on any ferrous or non-ferrous material

4. May reduce smoke when used with Ar/H2 plasma.

PLASMA CUTTING CAPABILITY

Plasma is an effective means of cutting thin and thick materials alike. Hand held torches can usually cut up to 2 in (48 mm) thick steel plate, and stronger computercontrolled 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. Since plasma cutters produce a very hot and much localized cone to cut with they are extremely useful for cutting sheet metal in curved or angled shapes.

Plasma Arc Cutter was utilized to perform Stainless Steel (316 L) material cutting. The system and the process are the important elements when utilizing plasma arc cutting. It is important to know current plasma arc cutting research areas to plan the direction of this work so that this work would contribute information that will be useful in future.

Plasma Arc Setup

SYSTEM

Plasma arc cutting can increase the speed and efficiency of both sheet and plate metal cutting operations. Manufacturers of transportation and agricultural equipment, heavy machinery, aircraft components, air handling equipment, and many other products have discovered its benefits. Basically Plasma Arc Cutter comprises of 8 major parts such as air compressor, AC plug, power supply, plasma torch, ground clamp, electrode, nozzle and workpiece.

Plasma Arc Cutter System

ARC STARTING CIRCUIT

The arc starting circuit is a high frequency generator circuit that produces an AC voltage of 5,000 to 10,000 volts at approximately 2 megahertz. This voltage is used tocreate a high intensity arc inside the torch to ionize the gas, thereby producing the plasma.

PROCESS

The basic plasma arc cutting system consists of a power supply, an arc starting circuit and a torch. These system components provide the electrical energy, ionization capability and process control that is necessary to produce high quality, highly productive cuts on a variety of different materials.

The power supply is a constant current DC power source. The open circuit voltage is typically in the range of 240 to 400 VDC. The output current (amperage) of the power supply determines the speed and cut thickness capability of the system. The main function of the power supply is to provide the correct energy to maintain the plasma arc after ionization.

The arc starting circuit is a high frequency generator circuit that produces an AC voltage of 5,000 to 10,000 volts at approximately 2 megahertz. This voltage is used to create a high intensity arc inside the torch to ionize the gas, thereby producing the plasma.

The Torch serves as the holder for the consumable nozzle and electrode, and provides cooling (either gas or water) to these parts. The nozzle and electrode constrict and maintain the plasma jet.

PLASMA TORCH

The Plasma cutting process is used with either a handheld torch or a mechanically mounted torch. There are several types and sizes of each, depending on the thickness of metal to be cut. Some torches can be dragged along in direct contact with the work piece, while others require that a standoff be maintained between the tip of the torch and work piece.

Mechanized torches can be mounted either on a tractor or a on a computer-controlled cutting machine or robot. Usually a standoff is maintained between the torch tip and work piece for best-cut quality. The standoff distance must be maintained with fairly close tolerances to achieve uniform results. Some mechanised torches are equipped with an automatic standoff controlling device to maintain a fixed distance between the torch and work piece. In other cases mechanical followers are used to accomplish this.

PAC torches operate at extremely high temperatures, and various parts of the torch must be considered to be consumable. The tip and electrode are the most vulnerable to wear during cutting, and cutting performance usually deteriorates as they wear. The timely replacement of consumable parts is required to achieve good quality cuts. Modern plasma torches have self-aligning and self-adjusting consumable parts. As long as they are assembled in accordance with the manufacturer’s instructions, the torch should require no further adjustment for proper operation.

Other torch parts such as shield cups, insulators, seals etc may also require periodic inspection and replacement if they are worn or damaged.

Torch Designs

The Single Flow Torch has only a flow of air for cutting. This is because its use is limited to lower amperage, thin gauge sheet metal cutting applications. It does not need a shielding gas flow to cool the torch because of the low amperage output required for cutting thin gauge sheet metal. The Dual Flow Torch has a flow of gas or air for the cutting plasma and shielding gas flow for the torch cooling. This is used for cutting thicker materials, which require higher amperages.

Torch Stand Off

"Torch stand-off" is the distance the outer face of the torch tip or constricting orifice nozzle is to the base metal surface. This standoff distance will be determined by the thickness of material being cut and the amperage required. Low heat build-up while cutting with less than 40 amperes may allow dragging the torch tip on the material. If a high build-up of heat is expected, a standoff distance of 1/16" to 1/8" will be required. This is easily accomplished with a Miller ICE torch with a "Drag Shield".

The "Drag Shield" works with the flow dynamics of the torch to provide better cooling of the consumable parts for longer parts life. This permits the operator to drag the torch on the work piece while cutting at full output, which increases operator comfort and makes template cutting easier.

Torch Consumables

The plasma torch is designed to generate and focus the plasma cutting arc. In either hand held or machine torches, the same parts are used: an electrode to carry the current form the power source, a swirl ring to spin the compressed air, a tip that constricts and focuses the cutting arc, and a shield and retaining ring to protect the torch.

ELECTRODE

The purpose of the electrode is to provide a path for the electricity from the power source and generate the cutting arc. The electrode is typically made of copper with an insert made of hafnium. The Hafnium alloyed electrodes have good wear life when clean, dry compressed air or nitrogen is used (although, electrode consumption may be greater with air plasma than with nitrogen).

SWIRL RING

The swirl ring is designed to spin the cutting gas in a vortex. The swirlring is made of a high temperature plastic with angled holes that cause the gas to spin.

Spinning the gas centres the arc on the electrode and helps to control and constrict the arc as it passes through the tip. The swirl ring on Miller plasma cutting equipment causes the gas to swirl in a clockwise direction.

TIP

The purpose of the torch tip is to constrict and focus the plasma arc. Constricting the arc increases the energy density and velocity. The tips are made of copper, with a specifically sized hole or orifice in the centre of the tip. Tips are sized according to the amperage rating of the torch that they are to be used in