Non-Metallic Materials & Joining Metals

Many non-metallic materials are in general use. Their improved properties have resulted in their replacing conventional metals for many applications. The majority are organic, being produced either synthetically or from naturally occurring material. Ceramics are being increasingly considered for marine use particularly where galvanic corrosion is a problem. 

Sintered alpha silicon carbide and other silicon-based ceramics have good strength properties and are inert in seawater. The general term 'plastic' is used to describe many of these non-metallic materials. Plastics are organic materials which can be moulded to shape under the action of heat or heat and pressure. 

There are two main classes, thermoplastic and thermosetting, although some more modern plastics are strictly neither. Thermoplastic materials are softened by heat and can be formed to shape and then set by cooling, e.g. perspex, polyvinyl chloride (PVC) and nylon. Thermosetting materials are usually moulded in a heated state, undergo a chemical change on further heating and then set hard, for example, Bakelite, epoxy resins and polyesters. 

Some general properties of plastic materials are good corrosion resistance, good electrical resistance and good thermal resistance; but they are unsuitable for high temperatures. To improve or alter properties, various additives or fillers are used, such as glass fibre for strength.

Asbestos fibre can improve heat resistance and mica is sometimes added to reduce electrical conductivity. Foamed plastics are formed by the liberation of gas from the actual material, which then expands to form a honeycomb-like structure. 

Such materials have very good sound and heat-insulating properties. Many plastics can be foamed to give low-density materials with a variety of properties, for example, fire extinguishing. Some well-known nonmetallic materials are: 

Must read  ➢ Common Metals And Alloys

Asbestos 

A mineral which will withstand very high temperatures and is unaffected by steam, petrol, paraffin, fuel oils and lubricants. It is used in many forms of jointing or gasket material and in various types of gland packing. It does however present a health hazard in some forms. 

Cotton 

A fibrous material of natural origin which is used as a backing material for rubber in rubber insertion jointing. It is also used in some types of gland packing material. 

Glass-reinforced plastics (GRP) 

A combination of thin fibres of glass in various forms which, when mixed with a resin, will cure (set) to produce a hard material which is strong and chemically inert. It has a variety of uses for general repairs. 

Lignum 

vitae A hardwood which is used for stern bearing lining. It can be lubricated by seawater but is subject to some swelling. 

Nylon 

A synthetic polymer which is chemically inert and resistant to erosion and impingement attack. It is used for orifice plates, valve seats and as a coating for saltwater pipes. 

Polytetrafluorethylene (PTFE) 

A fluoropolymer which is chemically inert and resistant to heat. It has a low coefficient of friction and is widely used as a bearing material. It can be used dry and is employed in sealed bearings. Impregnated with graphite, it is used as a filling material for glands and guide rings.

Polyvinylchloride (PVC) 

A vinyl plastic which is chemically inert and used in rigid form for pipework, ducts, etc. In a plasticised form it is used for sheeting, cable covering and various mouldings. 

Resin 

Resins are hard, brittle substances which are insoluble in water. Strictly speaking, they are added to polymers before curing. The term 'resin' is often incorrectly used to mean any synthetic plastic. Epoxy resins are liquids which can be poured and cured at room temperature. The cured material is unaffected by oils and seawater. It is tough, solid and durable and is used as a chocking material for engines, winches, etc. 

Rubber 

A tree sap solidifies to form a rough, elastic material which is unaffected by water but is attacked by oils and steam. It is used as a jointing material for fresh and seawater pipes and also for water-lubricated bearings. 

When combined with sulphur (vulcanized) it forms a hard material called 'ebonite' which is used for bucket rings (piston rings) in feed pumps. Synthetic rubbers such as neoprene and nitrile rubber are used where resistance to oil, mild chemicals or higher temperatures is required.

Joining metals

Many larger items of engineering equipment are the result of combining or joining together smaller, easily produced items. Various joining methods exist, ranging from mechanical devices, such as rivets or nuts and bolts, to fusion welding of the two parts. 

It is not proposed to discuss riveting, which no longer has any large-scale marine engineering applications, nor will nuts and bolts be mentioned since these are well known in their various forms. 

Brazing and soldering are a means of joining metal items using an alloy (solder) of a lower melting point than the metals to be joined. The liquefied solder is applied to the heated joint and forms a very thin layer of metal which is alloyed to both surfaces. On cooling the two metals are joined by the alloy layer between them. Welding is the fusion of the two metals to be joined to produce a joint which is as strong as the metal itself. 

It is usual to join similar metals by welding. To achieve the high temperature at which fusion can take place, the metal may be heated by a gas torch or an electric arc. With gas welding, a torch burning oxygen and acetylene gas is used and rods of the parent plate material is melted to provide the metal for the joint. 

An electric arc is produced between two metals in an electric circuit when they are separated by a short distance. The metal to be welded forms one electrode in the circuit and the welding rod the other. The electric "arc produced, creates a region of high temperature which melts and enables fusion of the metals to take place. 

A transformer is used to provide a low voltage and the current can be regulated depending upon the metal thickness. The electrode provides the filler material for the joint and is flux-coated to exclude atmospheric gases from the fusion process.