The physical properties of many materials can be changed by simply heating and cooling. There are four basic processes used to change the properties of steels, and the various different types of steels respond in different ways to this heat treatment.
• Annealing– The process of heating up to an annealing temperature and allowing to cool as slowly as possible. Annealing leaves the steel in a softer state, making it easier to machine. Annealing temperatures depend on the percentage of carbon in the steel:
mild steel (0.2% carbon) is annealed at 850◦C; medium carbon steel (0.5% carbon) is annealed at 810◦C; and high-carbon steel (1.0 % carbon) is annealed at 780◦C.
• Normalising– Heating as before, but allowing to cool in air at room temperature. Normalising returns the steel to its normal state and undoes the effect of any heat treatment.
• Hardening– Only medium and high-carbon steels can be hardened.
To harden these steels, they must be heated to the annealing temper- ature and cooled quickly, usually by quenching it in water or oil.
Hardening leaves the steel rather brittle and vulnerable to cracking.
• Tempering– The tempering process removes some of the brittle- ness that results from hardening. It allows the steel to retain some hardness and become more tough. The hardened steel is reheated to a precise temperature in the range of 230–320◦C, depending on the use to which the steel is to be put, that is level of toughness required, is then allowed to cool. The actual temperature of the steel being toughened can be judged by the colour of its surface while heating. This is only apparent if it has been polished before heating.
Colour Temperature Use
Pale straw 230◦C Metal-cutting tools
Dark straw 250◦C Punches
Brown 275◦C Chisels
Purple 300◦C Hammer-heads
Blue 320◦C Springs
EXERCISE 5.3
Take the box of eight material samples from the tool store. Examine and record each sample’s properties in the table after conducting simple workshop tests and observations.
From the information gained, you should be able to identify each of the material types.
The eight materials are listed as follows (in alphabetic order) Aluminium Brass Bronze Cast iron Copper High-
carbon steel
Mild steel Stainless steel
134 Identifying and selecting engineering materials
No. Material colour (grey, silver/grey,
yellow, white, golden brown, silver)
Magnetic (Yes / No)
Density (heavy, medium,
light)
Hardness (hard, medium,
soft)
I think this material is:
1.
2.
3.
4.
5.
6.
7.
8.
Write down a simple method of detecting if a piece of steel is mild steel, which contains less than 3%
carbon, or medium/high carbon steel, which contains more than 5%
carbon.
Write down a method you could use to cool a piece of steel that is being annealed so that the cooling rateis as slow as possible.
When sharpening a chisel on a grinding wheel, why is it important to keep the point cool by dipping it into coolant or water at regular intervals?
Witness testimony
The above exercise has been completed satisfactorily by. . . .
Signed. . . .Job title. . . .Date. . . .
Forms of supply of materials
Raw materials can be supplied in many different forms. The choice enables companies to order their materials in a condition that will enable them to undertake their processing more efficiently. This results in savings to be made because the material is bought in the most appropriate form. It is therefore important to know and identify the forms in which materials can be supplied.
Black bar wrought iron gates
• Black rolled bar– Black Bar is supplied in long rectangular, square, hexagonal or round sections that are not smooth or precisely accurate in size. It is a cheap source of steel that is easy to cut.
Black bar is usually selected when large amounts of machining are to be carried out on the workpiece, or the roughness of the unmachined surfaces is unimportant.
Hot rolled roof supports
• Hot rolled sections– Often used for fabrications and in the construction industry, hot rolled sections are available in a wide variety of shapes and sizes. ‘H’, ‘I’ and ‘Channel’ sections are common. Angle bar is probably the most widely used hot rolled section but special sections can be bought, for example, railway lines.
Ship’s hull made from plate
• Plate– Plate is the term used for pieces of hot rolled steels. Plate is not smooth on its external surface, it is between 5 mm and 50 mm thick and up to 5 m wide. Plate is used in shipbuilding and boiler making.
• Bright rolled bar– Bright bar, as it is commonly known, is smooth and accurate in its dimensions. It is supplied in long lengths of constant cross section, usually square, rectangular or round. Bright bar is used if the external dimensions of the bar are similar to those of the finished product. It is not suitable for applications involving a lot of machining because it has internal stresses and is tougher than black bar (see above).
Round section bar shaft
• Sheet– Sheet is supplied in rectangular cut lengths to most compa- nies and is also available in coiled rolls for users of large amounts, for example the motor industry. Sheet has a smooth surface and can be bent, folded or pressed into a wide variety of products. Sheet is cold rolled, resulting in better dimensional accuracy and surface finish than plate. While most sheets are supplied at a thickness of around 1 mm to 2 mm, the thinnest mild steel sheet available is only 0.15 mm thick. Sheets can be supplied coated with zinc (galvanised), tin (tin plate) or plastic (colour coat) surfaces for corrosion resistance at extra cost.
Rolled sheet steel for
motorcar body panels • Castings– For intricate, complicately shaped components, it is common practice to purchase pre-made cast blanks of the component which are then ready for finishing. Sand casting is the most common method of manufacturing large iron items, whereas the die-casting process is used for smaller non-ferrous components that need a smooth surface.
136 Identifying and selecting engineering materials
Cast Iron vice frame (not the jaws)
• Forgings– A forged component is one that has been shaped by hammering while hot. Forging is frequently selected as a manufac- turing process, as it produces components with an enhanced grain structure, resulting in greater toughness and strength. Forging is also a quick method of producing a complicated shape.
Chisel and hammer head
Electrical architrave
• Extrusions– Extrusion is to squeeze a material through a shaped hole. Toothpaste is extruded through the tube nozzle. Malleable metals and plastics can be extruded to make long lengths of regular cross-section strips for applications such as greenhouse frames and uPVC window frames. Pipes and tubes may well be extruded.
Used for many plastic products, e.g. computer
parts
• Mouldings– This is the term used for some plastic shaping processes. Plastics can be die-cast in a similar way to metals. As polymers become plastic when heated, plastic sheets can be shaped around a pattern with air pressure (or a vacuum) to form products with very thin walls, for example chocolate box separators and plastic bottles.
• Wood
(a) Wood can be supplied as rough sawn planks or as machined sections. Machined wood is usually about 3–5 mm smaller than specified, as it is measured before planing. Planed, tongued and grooved, dowel and mouldings are types of machined wood.
(b) Laminated plywood, chipboard, hardboard or fibreboard are all available as wooden sheets. These are supplied as large sheets (2240 mm×1220 mm in size). Veneered coatings can be applied to make the product better looking, stronger or waterproof. A special type of plywood called marine ply is more resistant to water but is expensive.
• Copper-coated circuit board– Electronic circuit boards are made from a Bakelite board that is coated with copper. The electronic circuit is etched onto the copper and the excess is removed in an acid bath, leaving only the copper tracks on the board’s coating.
EXERCISE 5.4
Look around the workshop and find the products listed in the table below.
• For each product, state the probable form of supply to the original manufacturer.
• State the reason why the selected form of supply was chosen for the product.
The machine frame has been filled in for your guidance.
Product Form of supply Reason for using this
form of supply
Machine frame
Casting Cast iron is rigid and
strong
Steel rule
Screwdriver handle Filing cabinet Ladder Vice jaw
Socket screw head Surface plate
Workshop bench or table Pliers
Witness testimony
The above exercise has been completed satisfactorily by. . . .
Signed. . . .Job title. . . .Date. . . .