Rugby School Thailand Design & Technology Senior School

3.6 Metals

Syllabus

• demonstrate a working knowledge of the following metals:

– ferrous metals (cast iron, mild steel, stainless steel, high speed steel [HSS] and carbon steels)

– non-ferrous metals (aluminium, duralumin and other common casting alloys, copper and its alloys, zinc, lead and tin)

• understand how the following processes can change the molecular structure of a material making it more or less suitable for the task it has to perform:

– work hardening

– annealing all metals

– case hardening of mild steel

– hardening and tempering tool steel (HCS)

Ferrous and Non-ferrous metals

Ferrous metals

A metal containing iron and carbon. This classification of metal are magnetic and will rust.

e.g. Pig Iron, Cast Iron, medium carbon steel, high carbon steel, mild steel

Non-ferrous metals

A metal that does not contain iron and so will not rust

e.g. Aluminium, copper, zinc, titanium, silver and gold

Alloys

A combination of metals (two or more or two or more elements including one metal) to enhance their properties (similar to composites)

Ferrous: Stainless Steel

Non-ferrous: bronze, brass, duralumin, pewter

Metals are made up of crystals that contain atoms and molecules.

Metals have been used for thousands of years in the form of Copper, Bronze and Iron

Metal Facts

Metals have good strength in both compression and tension, due to the very strong metallic bonds holding the atoms together.

These metallic bonds also allow free electrons to be shared amongst molecules making metals good conductors of heat and electricity.

All metals are extracted from Ores which are found in the earth.

As a result, they are considered a finite material (one day they will run out).

Ores need to be processed in order to extract the metal from the other minerals, rocks and soil.

25% of the Earth’s crust is made up of metal ores with Aluminium (Bauxite) being the most common, followed by Iron.

Gold is the only metal that can be found in a usable form, while the rest of the metals have to be processed.

The metals produced do not always reflect the name of the ore:

Metal materials such as Aluminum and Steel are considered more modern developments.

Steel is derived from Iron Ore which is one of the easiest to separate from its ore, using a blast furnace to melt the ore, leaving the waste (slag) to float to the top.

As you should know by now - Metals fall into two main categories:

those containing Iron (Fe) known as Ferrous metals
those that do not contain Iron known as Non-ferrous metals.

Iron is technically the only ferrous metal but, with the addition of other elements, it can be made into other metals.

When Iron is separated from its ore it is known as Pig Iron - This is a very high carbon Iron (3.5-4.5%) and is therefore very brittle and not useful as a building material.

It is often transferred within a foundry while still molten to be made into Steel, otherwise it is cast into ingots for transportation to be processed elsewhere.

Ingot

Iron is a naturally occuring element.

Steel is an alloy of Iron and carbon which does not occur naturally.

Steel has better tensile strength and is significantly tougher than Iron.

Wrought Iron (0.04-0.08% Carbon) was traditionally used before the introduction of Steel as a building material for bridges, fencing, gates and structures as large as the Eiffel Tower - It is no longer in commercial production and any reference to it likely means it is simply steel worked (wrought) by hand.

Eiffel Tower. Made from Wrought Iron

Ferrous metals

Cast Iron

Cast Iron is Pig Iron that has been liquified and poured into a mould (cast), and as such is has up to 3.5% carbon as well as Silica.

It is brittle unless it is heat treated to make it malleable (softer / can be worked). Cast iron is used to make anvils and engine casings.

steel

With the addition of small amounts of carbon, iron is made stronger and is classed as Steel.

As the carbon amount is increased so too are the hardness properties of the Steel.

Larger amounts of carbon (over 2%) render the iron brittle.

Mild Steel

Mild Steel is used for the majority of metal construction work, from girders in buildings to the casing on your fridge, from the body of a car to the legs on a table. It is Iron plus 0.15 to 0.3% carbon.

Medium Carbon Steel

The addition of 0.3-0.7% carbon produces Medium Carbon Steel which is used for springs and general gardening tools. It is also referred to as Tool Steel.

High carbon steel

High Carbon Steel (0.7-1.7% Carbon) is also referred to as High Speed Steel (HSS) and is used for tools, cutting and drilling bits, blades etc.

These can also be considered Alloys.

One major disadvantage of using Iron is the fact that it oxidises. While all metals are prone to oxidization, in Iron it can be very destructive to the metal. Iron Oxide is better known as rust.

Unless treated, Iron and Steel will oxidise quickly forming a layer of Iron Oxide which can easily be brushed away and prevented by coating in a layer of oil.

If left untreated the oxide will begin to erode the surface of the metal and, although it can still be removed, it will leave noticeable marks in the metal. Eventually the oxide, or rust, will erode the metal away completely to an orange / brown powder.

rust

Mr Kennedy Oils the tools and machines in Dt regularly to avoid oxidisation (rust)

Non-ferrous metals

As its name implies, these metals contain no Iron therefore they cannot rust but they have other properties which make them more suitable for other applications.

Non-ferrous metals also include precious metals such as Gold, Silver and Platinum.

Despite being the most abundant ore on earth, the processing of Bauxite into Aluminium is difficult and requires a great deal of energy. This explains why it is often 3-4 times more expensive than steel. Aluminium has a good strength to weight ratio. This means that given it is relatively light in weight yet it is also quite strong.

Aluminium

Aluminium is a third of the weight of steel yet it is roughly half as strong. This is a good example of ‘strength to weight ratio’; you may lose half the strength, but you lose two thirds of the weight of a similar size piece of material.

As it does not corrode, Aluminium is often used to replace steel in outdoor applications and, due to being lightweight, is also used for portability in sports and camping applications.

It is also an excellent conductor of heat so it is used in cooking and for heat sinks and electrical casings (iPod, iPad, Macbook etc.) to help dissipate the heat. It melts at 660C

Copper

Copper is equally difficult and expensive to produce and is becoming very expensive to buy. Like Aluminium, it requires an electrolytic process and a similar process is used for Tin and Zinc.

Copper is an excellent conductor of both heat and electricity which is why it is still used for electrical circuits, plumbing and for electric and data cables.

For those that don’t have fiber optic broadband, you still rely on Copper for bringing the internet into your home

It melts at 1083C.

Tin

Tin is rarely used in its pure state, but applications include food wrapping / foil (now replaced with aluminium) and coating for steel plate in the production of food cans. It is also mixed with Lead to make Solder for electrical circuits.

It melts at 232C.

Lead

Lead is a very soft but heavy material which has excellent resistance to chemicals and can block a variety of wavelengths from passing through such as X rays.

It is used to line the containers for radioactive material as well as flashing in buildings where the roof meets the wall.

It melts at 330 C.

Zinc

Zinc is used primarily for coating steels, which is known as galvanizing. This is done for steel support beams, buckets, wheelbarrows and bins for use outdoors. It can also be die cast to produce high-detail products such as lock mechanisms and small gears.

It melts at 419C.

titanium

Titanium is also classed as a modern material!

It has the highest strength-to-weight ratio of metals and is a very clean material making it suitable for medical and surgical applications.

It is also used in performance sports equipment and in spectacle frames.

It melts at 1675C.

You should also be familiar with a range of precious metals used primarily in jewelry, due to their high cost.

Silver melts at 960 degrees C.

Gold is used in electroplating in electronics

Gold melts at 1063 degrees C.

Platinum melts at 1755 degrees C.

alloys

Alloys are a ‘mixture of metals combined to improve their properties’ such as

changing the melting point (enhanced thermal properties)
colour
Increasing strength
Improving the resistance to corrosion

ferrous alloys

Stainless steel

The main Ferrous Alloy is Stainless Steel which is a mixture of Chromium, Nickel and Magnesium added to Steel.

It has a high resistance to corrosion, is very clean and hygienic and is tough and wear resistant making it particularly suitable for kitchenware.

Properties of Stainless Steel:

Hygenic
Non-corrosive
Outstanding surface finish
Excellent toughness
Can be formes using a variety of processes
*difficult to coldwork

tool steel

Also known as die Steel is a high carbon steel with the addition of Chromium and Tungsten. The steel is available in various grades depending on the metal content which varies depending on the application for the steel, but it is hard, tough and durable.

Due to the addition of other metals, tool steel can also be classed as an alloy.

High carbon steel

High Carbon Steel (0.7-1.7% Carbon) is also referred to as High Speed Steel (HSS) and is used for tools, cutting and drilling bits, blades etc.

These can also be considered a Ferous metal!

alloyed with tungsten (7%) or molybdenum, together with percentages of chromium, vanadium and cobalt.

non ferrous alloys

Brass

Brass is a non-ferrous alloy; a mixture of Copper and Zinc (35%). It is used for valves and taps, ornamental fittings, boat fittings and keys as well as the pins on a plug as it has good conductivity but is stronger than pure Copper.

Properties of Brass

Versatile production
recyclable
Good machinability
good corrosion resistance
good hotworking properties
good combination of strength and ductility

Has been traditionally associated with music "brass band"

Connectors

Outdoor padlocks

Inserts

Duralumin

Duralumin is an Aluminium alloy containing 4% Copper, 1% Manganese and 0.1% Magnesium. It is stronger than Aluminium yet lighter and is therefore ideal for use in the aerospace industry.

Bronze

Bronze is a mixture of Copper and Tin (10%). It is used to make coins, statues and bearings. Due to the high Copper content it can be very expensive!

Pewter

Pewter is made mainly from Tin with a small amount of Copper and Antimony. It is often used to make decorative items such as tankards picture frames, models and metal gifts. It is now ‘Lead free’ and has a relatively low cost with a very low melting point around 200 degrees C.

Designer case study:

Watch from minute 3:19 to see discussion about Marc Newson's Lockheed lounge chair.

Marc Newsons Lockheed lounge

The lockheed lounge is one of australian designer Marc Newson's most famous designs.

The main shape is fiberglass and is clad in riveted aluminum sheet.

The feet of its 3 legs are coated in rubber.

The Lockheed Lounge is named after the aircraft manufacturer.

The aluminum panels that make up the surface of the chair were blind riveted to the hand sculpted fiberglass body.

Metal Enhancement

There are a wide range of metals, each with their own working properties such as toughness, hardness, malleability etc. This means that not all enhancements are necessary for every metal. For example, Ferrous metals require a finish or coating to prevent them from rusting while Non-ferrous metals do not.

Work Hardening

When a metal is ‘cold worked’ the crystals within the metal are distorted and changed leading to improved tensile strength and hardness. This is known as work hardening. The affects of work hardening can be removed by annealing.

Annealing

Annealing is used to make work hardened metal easier to work by making it less brittle and more ductile (the metal can be more easily stretched or bent to shape). The metal is heated then slowly cooled. In industry, the process is carried out in a specific temperature-controlled furnace. Copper is a material that can be heated up and then left to cool slowly. The copper then becomes much softer and can be 'beaten' or manipulated into shapes. This can happen many times until the desired shape is realised.

Case Hardening

Case hardening is a process used for hardening the surface of Steels with less than 0.4 % carbon. This makes the surface of the steel more resistant to indentation and abrasion - in other words Tougher. There are two stages:

Carburising which changes the chemical composition of the surface of low carbon steel so it can absorb more carbon. This is often done in a box with carbon powder where it is heated to around 950C. The longer it is ‘baked’, the thicker the carbon layer.
Quenching where the hot metal is quenched (dropped quickly into water) in water. This fast cooling seals the hard surface without affecting the inner core.

Hardening

Hardening is the process of heating medium or high carbon steels to alter the structure, holding them at this temperature for a given time then quenching them in water, oil or salt baths. The hardening and subsequent quenching greatly increase the hardness property but also increase the brittleness - brittleness is how easily a material can shatter.

Tempering

Tempering is a heat treatment for medium (MCS) and high carbon steels (HCS) that is carried out after hardening to reduce some of the excess hardness and increase the toughness. This reduction in hardness results in greater ductility and reduces the brittleness of the metal. To temper a metal it is heated to below the critical point for a given time then air cooled.

EXAM QUESTION

Quiz

Quiz 2

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