Principles and equipment

(See also pp. 323–5 for general safety precautions)

For oxy-acetylene welding, the oxygen is supplied from steel cylinders and the acetylene either from cylinders or from an acetylene generator which can be of the medium-pressure or low-pressure type.

With cylinder gas, the pressure is reduced to 0.06 N/mm2 or under, according to the work, by means of a pressure-reducing valve and the acetylene is passed to the blowpipe where it is mixed with oxygen in approximately equal proportions, and finally passed into the nozzle or tip to be burnt.

The medium-pressure acetylene generator delivers gas to any desired pressure up to a maximum of 0.06 N/mm2 (0.6 bar) in the same way as cylinder gas.

The low-pressure generator produces gas at a pressure of only a few millimetres water column, necessitating the use of ‘injector’ blowpipes where the high-pressure oxygen injects or sucks the acetylene into the blowpipe. In some cases, as for example if the supply pipes are small for the volume of gas to be carried or if it is desirable to use the equal pressure type of blowpipe, a booster is fitted to the low-pressure generator and the gas pressure increased to 0.06 N/mm2.

Without Home Office approval, the maximum pressure at which acetylene may be used in England is 0.06 N/mm2. With approval, the pressure may be increased to 0.15 N/mm2, but this is rare and applies only in special cases.

For a fire to occur we need oxygen, fuel and heat.

Oxygen is present in the atmosphere and in the compressed form contained in cylinders used for oxy-acetylene or oxy-propane cutting.

Fuel may be any combustible substance such as petrol, diesel oil, paper, cleaning materials, rubber or litter etc. – anything that will burn.

Heat may be supplied by discarded cigarette ends, match sticks and, in the welding shop, by sparks from the arc or bad connections such as those at the earth or return terminal to the set. The arc is an intense source of heat and, together with the sparking, will rapidly cause ignition of any combustible solids or liquids in the immediate vicinity.

If any one of these above causes is removed, the fire will be contained. For example the oxygen supply from the atmosphere may be contained by smothering the fire with a blanket or dry sand or by turning off the oxygen supply if it is from a cylinder. The heat can be controlled by ceasing to use the arc and switching off all power supplies to prevent further arcing at poor contacts.

The electric arc

An electric arc is formed when an electric current passes between two electrodes separated by a short distance from each other. In arc welding (we will first consider direct-current welding) one electrode is the welding rod or wire, while the other is the metal to be welded (we will call this the plate). The electrode and plate are connected to the supply, one to the + ve pole and one to the – ve pole, and we will discuss later the difference which occurs when the electrode is connected to – ve or +ve pole. The arc is started by momentarily touching the electrode on to the plate and then withdrawing it to about 3 to 4 mm from the plate. When the electrode touches the plate, a current flows, and as it is withdrawn from the plate the current continues to flow in the form of a ‘spark’ across the very small gap first formed. This causes the air gap to become ionized or made conducting, and as a result the current is able to flow across the gap, even when it is quite wide, in the form of an arc. The electrode must always be touched on to the plate before the arc can be started, since the smallest air gap will not conduct a current (at the voltages used in welding) unless the air gap is first ionized or made conducting.

Heat

Solids, liquids and gases: atomic structure

Substances such as copper, iron, oxygen and argon which cannot be broken down into any simpler substances are called elements; there are at the present time over 100 known elements. A substance which can be broken down into two or more elements is known as a compound.

An atom is the smallest particle of an element which can take part in a chemical reaction. It consists of a number of negatively charged particles termed electrons surrounding a massive positively charged centre termed the nucleus. Since like electric charges repel and unlike charges attract, the electrons experience an attraction due to the positive charge on the nucleus. Chemical compounds are composed of atoms, the nature of the compound depending upon the number, nature and arrangement of the atoms.

A molecule is the smallest part of a substance which can exist in the free state and yet exhibit all the properties of the substance. Molecules of elements such as copper, iron and aluminium contain only one atom and are monatomic. Molecules of oxygen, nitrogen and hydrogen contain two atoms and are diatomic. A molecule of a compound such as carbon dioxide contains three atoms and complicated compounds contain many atoms.

An atom is made up of three elementary particles: (1) protons, (2) electrons, (3) neutrons.

The proton is a positively charged particle and its charge is equal and opposite to the charge on an electron.

Production and properties of iron and steel

Before proceeding to a study of iron and steel it will be well to understand how they are produced.

Iron is found in the natural form as iron ores. These ores are of four main types:

1. (1) Haematite, red or brown Fe2O3, containing 40–60% iron.

2. (2) Magnetite or magnetic oxide of iron, Fe3O4, containing up to 70% iron.

3. (3) Limonite, a hydrated ore, Fe2O3·3H2O, containing 20–50% iron.

4. (4) Siderite, a carbonate, FeCO3, with iron content 20–30%.

Limonite and siderite are termed lean ores since they are so low in iron. The ore found in England in Lincolnshire, Northamptonshire, Leicestershire and Oxfordshire is one of low iron content and is generally obtained by opencast working.

Iron ore, as mined, contains appreciable amounts of earthy waste material known as gangue, and if this were fed into the furnace with the ore, more fuel would be consumed to heat it up and it would reduce the furnace capacity. Ores are washed, or magnetically separated in the case of the magnetic ores, to remove much of this waste material. They are roasted or calcined to drive off the moisture and carbon dioxide and to remove some of the sulphur by oxidation to sulphur dioxide, and crushed to bring the lumps to a more uniform size.

Notes on the laser

The cost of the CO2 lasers has mitigated against their general acceptance by industry, except for heavier work. They can perform welding, cutting, drilling, spot welding and marking operations quickly and efficiently, with very little distortion, due to the narrow HAZ, but it is still only the larger firms that use them where there is much repetition work. We will discuss the following types of laser here: solid state, such as the Nd–YAG, and gas, such as the CO2 axial flow and the CO2 transverse or cross flow.

The Nd-YAG laser

The Nd-YAG laser is of neodymium and yttrium-aluminium garnet (the last word indicating that its crystals are cubic). This laser is relatively light and portable, and its use is greatly increased by the fact that the laser beam can be conducted along a fibre-optic cable to a gun head, and can thus reach awkward places. It is used for marking hardened parts and no matter how thin, these are clearly marked using a low-power laser (Fig. A14.1). The drilling of thin sheet and stainless steel is performed with a very small focussed beam; the width of the beam determines the hole diameter.

Plastics

Plastics are replacing steel and other metals used in construction. They are now used in the home, in gas engineering, farming, sports equipment, and in the automobile industry etc. Equipment made of plastic is lighter than that made of steel, and since welding and fabrication is relatively easy and the cost of repair lower, it will be well to consider some of the processes involved.

Plastics are derived mainly from oil, coal and wood. Polystyrene and polyvinylchloride (PVC) are examples of those derived from oil and coal while the cellulose types (such as cellulose acetate and ethylcellulose) are produced from wood. Plastics consist of larger molecules built up into chains arranged randomly, from single molecules (monomers) by a process of polymerization, assisted by a catalyst. (A catalyst is a substance that accelerates a chemical reaction but undergoes no permanent change during the reaction.) If differing monomers are used, copolymers are produced from which the characteristics of the original monomers cannot be obtained.

There are two main types of plastic: (1) thermo-setting and (2) thermoplastic. A thermo-setting plastic is one that softens when first heated and is then moulded into the required form. Upon cooling it sets hard and gains its full strength. Upon reheating it does not again soften and thus cannot be reshaped. An example of a thermo-setting plastic is that made from phenol and formaldehyde, which with an acid catalyst gives a soluble and fusible resin used in varnishes, and with an alkaline catalyst gives an infusible and insoluble resin (Bakelite).