Low carbon steelsThese are also called mild steels. They are the cheapest and most widely used group of steels. Although they are the weakest of thesteels, nevertheless they are stronger than most of the non-ferrous metals and alloys. They can be hot and cold worked and machined with ease.

Unit 1 Metals

Medium carbon steelsThese are harder, tougher, stronger and more costly than the lowcarbon steels. They are less ductile than the low carbon steels andcannot be bent or formed to any great extent in the cold conditionwithout risk of cracking. Greater force is required to bend and form them. Medium carbon steels hot forge well but close temperature control is essential. Two carbon ranges are shown. The lower carbon range can only be toughened by heating and quenching (cooling quickly by dipping in water). They cannot be hardened. The higher carbon range can be hardened and tempered by heating and quenching.

Unit 1 Metals

High carbon steelsThese are harder, stronger and more costly than medium carbon steels. They are also less tough. High carbon steels are available as hot rolled bars and forgings. Cold drawn high carbon steel wire (piano wire) is available in a limited range of sizes. Centreless ground high carbon steel rods (silver steel) are available in a wide range of diameters (inch and metric sizes) in lengths of 333 mm, 1 m and 2 m. High carbon steels can only be bent cold to a limited extent before cracking. They are mostly used for making cutting tools such as files, knives and carpenters’ tools.

Unit 1 Metals

Unit 1 Metals

CopperPure copper is widely used for electrical conductors and switchgearcomponents. It is second only to silver in conductivity but it is muchmore plentiful and very much less costly. Pure copper is too soft andductile for most mechanical applications.For general purpose applications such as roofing, chemical plant,decorative metal work and copper-smithing, tough-pitch copper isused. This contains some copper oxide which makes it stronger,more rigid and less likely to tear when being machined. Because it isnot so highly refined, it is less expensive than high conductivitycopper.There are many other grades of copper for special applications.Copper is also the basis of many important alloys such as brass andbronze, and we will be considering these next. The general propertiesof copper are:• relatively high strength• very ductile so that it is usually cold worked. An annealed(softened) copper wire can be stretched to nearly twice its lengthbefore it snaps• corrosion resistant• second only to silver as a conductor of heat and electricity• easily joined by soldering and brazing. For welding, aphosphorous deoxidized grade of copper must be used.Copper is available as cold-drawn rods, wires and tubes. It is alsoavailable as cold-rolled sheet, strip and plate. Hot worked copper isavailable as extruded sections and hot stampings. It can also be cast.Copper powders are used for making sintered components. It is oneof the few pure metals of use to the engineer as a structural material.

BrassBrass is an alloy of copper and zinc. The properties of a brass alloyand the applications for which you can use it depends upon theamount of zinc present. Most brasses are attacked by sea water. The salt water eats away the zinc (dezincification) and leaves a weak, porous, spongy mass of copper. To prevent this happening, a small amount of tin is added to the alloy. There are two types of brass that can be used at sea or on land near the sea. These are Naval brass and Admiralty brass.Brass is a difficult metal to cast and brass castings tend to becoarse grained and porous. Brass depends upon hot rolling from cast ingots, followed by cold rolling or drawing to give it its mechanical strength. It can also be hot extruded and plumbing fittings are made by hot stamping. Brass machines to a better finish than copper as it is more rigid and less ductile than that metal. Table 1.2 lists sometypical brasses, together with their compositions, properties andapplications.

Tin bronzeAs the name implies, the tin bronzes are alloys of copper and tin.These alloys also have to have a deoxidizing element present to prevent the tin from oxidizing during casting and hot working. If thetin oxidizes the metal becomes hard and ‘scratchy’ and is weakened.The two deoxidizing elements commonly used are: • zinc in the gun-metal alloys.• phosphorus in the phosphor–bronze alloys.Unlike the brass alloys, the bronze alloys are usually used as castings. However low-tin content phosphor–bronze alloys can be extensively cold worked. Tin–bronze alloys are extremely resistant to corrosion and wear and are used for high pressure valve bodies and heavy duty bearings. Table 1.3 lists some typical bronze alloys together with their compositions, properties and applications.

AluminiumAluminium has a density approximately one third that of steel.However it is also very much weaker so its strength/weight ratio isinferior. For stressed components, such as those found in aircraft,aluminium alloys have to be used. These can be as strong as steel andnearly as light as pure aluminium.High purity aluminium is second only to copper as a conductor ofheat and electricity. It is very difficult to join by welding or solderingand aluminium conductors are often terminated by crimping. Despitethese difficulties, it is increasingly used for electrical conductorswhere its light weight and low cost compared with copper is anadvantage. Pure aluminium is resistant to normal atmospheric corrosion but it is unsuitable for marine environments. It is available as wire, rod, cold-rolled sheet and extruded sections for heat sinks.Commercially pure aluminium is not as pure as high purity aluminium and it also contains up to 1% silicon to improve its strength and stiffness. As a result it is not such a good conductor of electricity nor is it so corrosion resistant. It is available as wire, rod, cold-rolled sheet and extruded sections. It is also available as castings and forgings. Being stiffer than high purity aluminium itmachines better with less tendency to tear. It forms non-toxic oxides on its surface which makes it suitable for food processing plant and utensils. It is also used for forged and die-cast small machine parts.