Engineering drawing

The principal method usually adopted in the making of machine drawings is known as orthographic projection.

Suppose the part under construction is shown in Fig 7.1a. This ‘picture’ is known as an isometric view. It is of small use to the engineer, since it is difficult to include on it all the details and dimensions required, especially those on the back of the picture, which is hidden.

Imagine that around the object a box is constructed (O being the corner farthest from the observer) having the sides Ox, Oy, Oz all at right angles to each other. The plane or surface of the box bounded by Ox and Oy is the vertical plane, indicated by VP; that bounded by Oy, Oz is the side vertical plane, SVP, and that bounded by Ox and Oz, the horizontal plane, HP, these three planes being the three sides of the box farthest from the observer. Lines are projected, as shown, onto these planes from the object under consideration, and the view obtained when looking at the object in the direction of the arrow A. The end elevation is the view obtained by projection on to the side vertical plane, while the plan is the view obtained by projection on to the horizontal plane. The arrow B shows the direction in which the object is viewed for the side elevation and C the direction for the plan.

The Science and Practice of Welding was divided into two volumes for the eighth edition: Volume 1, Welding Science and Technology; Volume 2, The Practice of Welding. Volume 1 covers all the basic material on the physics and chemistry of welding, including metallurgy, equilibrium diagrams, testing of welds, drawing and welding symbols and an appendix with tables and conversions. Volume 2 gives a comprehensive survey of the welding methods in use today and the tools involved.

This tenth edition has been brought completely up to date throughout. Volume 1 has a new chapter on the inverter, which has become popular as a power unit because of its reduced weight and size compared with a conventional unit. There is also an up-to-date section on the classification of stainless steels. Volume 2 has a new chapter on welding plastics and new sections on welding duplex stainless steel and air plasma cutting. There are two new appendices (one illustrating the latest plant and equipment, and one on refraction). The appendix on proprietary welding gases has been completely revised.

My thanks are due to each of the following firms who have rendered every technical assistance and have supplied information and photographs as indicated.

1. Air Products Ltd: NDT of welding fabrications and all radiographs.

2. Alcan Wire Ltd: classification and welding of aluminium and its alloys.

3. Alpha Electronics Ltd: instrument technology.

4. Aluminium Federation: classification and properties of aluminium and its alloys.

5. Amersham International: X-ray and gamma-ray testing of welds, and gamma-ray sources.

There has been a great increase in the number of automatic processes designed to speed up welding production. Automatic welding gives high rates of metal deposition because high currents from 400 to 2000 A can be used, compared with the limit of about 600 A with manual arc welding. Automatic arc control gives uniformly good weld quality and finish and the high heat input reduces distortion and the number of runs for a given plate thickness is reduced. Twin welding heads still further reduce welding time, and when used, for example, one on each side of a plate being fillet welded, distortion is reduced. The welding head may be:

1. Fixed, with the work arranged to move beneath it.

2. Mounted on a boom and column which can either be of the positioning type in which the work moves or the boom can traverse at welding speed over the fixed work.

3. Gantry mounted so that it can traverse over the stationary work.

4. Self propelled on a motor-driven carriage.

The processes which have been described previously in this book, namely TIG, MIG and CO2 (gas shielded metal arc) with their modifications, are extensively used fully automatically. Heads are now available which, by changing simple components, enable one item of equipment to be used for MIG (inert gas), CO2 and tubular wire, and submerged arc processes.

Duplex and Super Duplex stainless steels are particularly useful in the petrochemical industry because of their pitting and corrosion resistance. Duplex stainless steels are so called because they have two phases in their microstructure, ferrite and austenite, so they are ferritic-austenitic steels. One of the most common of the Duplex steels has 22% Cr, 5% Ni, 3% Mo and 1% N2, remainder ferrite. The ferrite gives the high tensile strength and the austenite gives the toughness. The latest of these steels is called Super Duplex stainless steel and has a greater resistance to corrosion and a high pitting resistance. A typical example of a Super Duplex steel has 25% Cr, 7% Ni, 4% Mo and 0.15% N2, remainder ferrite. Duplex steels are not sensitive to corrosion cracking but must not be worked above 300°C, as above this embrittlement occurs. The ferritic-austenitic balance is important as a high ferrite content will cause brittleness and a low one will cause a lowering of resistance to stress corrosion. Contents of 30–70 FN (ferrite number) are normal. The balance between ferrite and austenite depends upon the heat treatment and the composition of the filler metal, the heat input when welding and the interpass temperature.

The following British Standards apply to this chapter:

1. BS 6072 Method for penetrant flaw detection

2. BS 6443 Method for magnetic particle flaw detection

3. BS 2600 Radiographic examination of fusion welded butt joints in steel

4. BS 2901 Radiographic examination of fusion welded circumferential butt joints in steel pipes

5. BS 709 Methods of destructive testing fusion welded joints and weld metal in steel

6. BS 3923 Methods for ultrasonic examination of welds. Parts 1 and 2

7. BS 3451 Methods of testing fusion welds in aluminium and aluminium alloys

8. BS 4206 Methods of testing fusion welds in copper and copper alloys

9. See also BS 3863 Glossary of terms used in non-destructive testing

During the process of welding, faults of various types may creep in. Some, such as those dealing with the quality and hardness of the weld metal, are subjects for the chemist and research worker, while others may be due to lack of skill and knowledge of the welder. These, of course, can be overcome by correct training (both theoretical and practical) of the operator.

In order that factors such as fatigue may not affect the work of a skilled welder, it is evidently necessary to have means of inspection and testing of welds, so as to indicate the quality, strength and properties of the joint being made.

Visual inspection, both while the weld is in progress and afterwards, will give an excellent idea of the probable strength of the weld, after some experience has been obtained.

As previously stated, almost all electrodes are produced by the extrusion process. The present day electrode consists of a core wire, centrally placed in a coating, usually silicate bonded (Vol. 1, p. 60). Double coated electrodes are also produced.

A semi-automatic production line may consist of the following:

1. Wire drawing. Non-slip hydraulic machines draw the wire through dies to the correct size, after which the wire is stored on reels.

2. Wire cutting. The wire is then straightened and cut to electrode length, giving burr-free wire rods of any required length which are stacked in bins.

3. The coating. The raw materials of the coatings are kept in raw material silos and from these the materials are sieved and dosed. It is the type and composition of these materials that determines the characteristic of the particular electrode. The dosing installations consist of a number of silos each fitted with a discharge system, dosing control and weighing facilities.

4. Silicate storing consists of tanks that are temperature controlled and from which the silicate is pumped to the mixer.

5. Dry blending and wet mixing. The batch is first thoroughly mixed in the dry state and then a measured quantity of silicate is added from a dosing unit. The wetted batch is then roughly mixed, emptied and removed to the slug press, where it is pressed into slugs ready for extrusion.

6. […]

The Science and Practice of Welding was divided into two volumes for the eighth edition: Volume 1, Welding Science and Technology; Volume 2, The Practice of Welding. Volume 1 covers all the basic material on the physics and chemistry of welding, including metallurgy, equilibrium diagrams, testing of welds, drawing and welding symbols and an appendix with tables and conversions. Volume 2 gives a comprehensive survey of the welding methods in use today and the tools involved.

This tenth edition has been brought completely up to date throughout. Volume 1 has a new chapter on the inverter, which has become popular as a power unit because of its reduced weight and size compared with a conventional unit. There is also an up-to-date section on the classification of stainless steels. Volume 2 has a new chapter on welding plastics and new sections on welding duplex stainless steel and air plasma cutting. There are two new appendices (one illustrating the latest plant and equipment, and one on refraction). The appendix on proprietary welding gases has been completely revised.

My thanks are due to the following firms for their help and cooperation by supplying much technical information and photographs as indicated:

AGA Gas Ltd: Industrial gases

Air Products Ltd, Crewe: special gases and mixtures, welding of aluminium, stainless and heat resistant steels.

Air Products Ltd, Ruabon: welding of aluminium and its alloys, stainless and 9% nickel steels and plasma cutting.

Electrical technology

Sources of electrical power

The principle sources of electrical power of interest to the welder are (1) batteries and accumulators, (2) generators.

Batteries generate electrical energy by chemical action. Primary batteries, such as the Leclanché (used for flash lamps and transistor radios), continue giving out an electric current until the chemicals in them have undergone a change, and then no further current can be given out.

Secondary batteries or accumulators are of two types: (1) the lead–acid, and (2) the nickel–iron alkaline. In the former, for example, there are two sets of plates, one set of lead peroxide and the other set of lead, immersed in dilute sulphuric acid (specific gravity 1.250, i.e. 4 parts of distilled water to 1 part of sulphuric acid). Chemical action enables this combination to supply an electric current, and when a current flows from the battery both the lead peroxide plates and the lead plates are changed into lead sulphate, and when this change is complete the battery can give out no more current. By connecting the battery to a source of electric power, however, and passing a current through the battery in the opposite direction from that in which the cell gives out a current, the lead sulphate is changed back to lead peroxide on one set of plates and to lead on the other set. The battery is then said to be ‘charged’ and is ready to supply current once again.

Metallic alloys

The metals with which a welder may have to deal are often not pure metals but alloys consisting of a parent metal with one or more alloying elements added in various proportions, e.g. mild steel is an alloy mainly of iron with a small amount of carbon, while brasses are alloys of copper containing up to 45% of zinc.

The properties of such alloys vary according to the form in which the alloying element is present. There are several possible forms:

(1) The alloying element may be present in an unchanged form in a state of fine mechanical mixture with the parent metal so that it can be seen as a separate particle or crystal (constituent) under the microscope.

Examples of this are:

1. (a) Carbon present as flakes of graphite in grey cast irons.

2. (b) Silicon present as fine silicon crystals in the aluminium–silicon alloys.

3. (c) Lead present as round particles in free cutting brasses. Alloying elements present in this condition do not generally produce a great increase in strength, but they increase or decrease the hardness, reduce the ductility and improve the machinability of the parent metal.

(2) The alloying element may be present in solid solution in the parent metal, i.e., actually dissolved as salt or sugar dissolves in water, so that under the microscope only one constituent, the solid solution, can be seen, similar in appearance to the parent metal except that the colour may be changed.

The electric arc, used in MMA, MIG-MAG, TIG, and other welding processes, causes visible and invisible fumes to be given off by the melting flux when the arc is burning. These fumes can be harmful if inhaled through the nose and mouth of the welder. The content of the fumes may be either asphyxiating or toxic.

Asphyxiating fumes

Asphyxiating fumes are largely due to the shielding gases used around the arc. They are invisible and produce their effect by displacement of the surrounding air and thus, of its oxygen. (The air which we breathe consists mainly of nitrogen and oxygen, roughly in the ratio of four parts nitrogen to one part oxygen.) Asphyxiation begins when the oxygen content of the air falls below about 18%.

The gases usually used in welding are argon, carbon dioxide, helium and their mixtures. Argon and carbon dioxide are heavier than air (the mixtures are known under trade names). A mixture is selected to give the best welding conditions for a particular process. A simple mixture is the rutile coating for mild steel electrodes in the MMA welding of mild steel. The rutile gives a smooth arc, which is easily managed, but it gives off some hydrogen, which is absorbed by the molten metal and leads to hydrogen cracking under certain conditions of stress.

A weld is indicated on a drawing by (1) a symbol (Fig. A1. 1a) and (2) an arrow connected at an angle to a reference line usually drawn parallel to the bottom of the drawing (Fig. A1. 1b). The side of the joint on which the arrow is placed is known as the ‘arrow side’ to differentiate it from the ‘other side’ (Fig. A1. 1c). If the weld symbol is placed below the reference line, the weld face is on the arrow-side of the joint, while if the symbol is above the reference line the weld face is on the other side of the joint (Fig. A1. 1c). Symbols on both sides of the reference line indicate welds to be made on both sides of the joint while if the symbol is across the reference line the weld is within the plane of the joint. A circle where arrow line meets reference line indicates that it should be a peripheral (all round) weld, while a blacked in flag at this point denotes an ‘on site’ weld (Fig. A1. 1d). Intermittent runs of welding are indicated by figures denoting the welded portions, and figures in brackets the non-welded portions, after the symbol (Fig. A1. 1e). A figure before the symbol for a fillet weld indicates the leg length. If the design throat thickness is to be included, the leg length is prefixed with the letter ‘b’ and the throat thickness with the letter ‘a’.

Inversion is the opposite of rectification (Fig. 5.1). Rectification is the conversion of alternating current into direct current and is achieved using rectifiers or diodes (described on pp. 199–203): four diodes in bridge connection for single phase and six diodes for three-phase input (shown in Fig. 4.44b, c). Inversion converts direct current into alternating current. An inverter is not simply a device like a diode, but a network of many components designed to achieve this conversion (Fig. 5.2).

The great majority of arc welding today uses direct current, and since the mains are invariably alternating current, some method is required for changing from a.c. to d.c. Inversion is only part of what happens within the inverter power source. Many other electronic and electrical conversions and transformations are required to convert the mains supply into one suitable for welding. Figs. 5.3 a, b and c illustrate a typical power source, transformer and inductor of the inverter type, together with one of similar output but of non-inverter design. There is a great reduction in size and weight (and hence portability) resulting from the inverter-based design. There are other advantages discussed later but size and weight are the most obvious.

Transformer and inductor

In a transformer there are two separate coils (primary and secondary), or windings, wound on a laminated iron core, the laminations being coated with insulating varnish; the bolts or rivets fastening the laminations together are mounted on insulating sleeves to decrease the circulating eddy currents further.