Why do we Test Steel?
Quality control is critical in steelmaking. An iron or steel component that isn’t strong enough could fail with disastrous consequences. For example in 1879 the Tay Bridge near Dundee collapsed when high winds caused cast iron elements in the bridge to fail. A train was crossing the bridge at the time and all 75 people on board were killed when it fell into the Tay. An enquiry into the disaster concluded that the cross-bracing on the bridge was too weak, allowing the bridge to shift and break the crucial cast iron lugs that held the bridge together.
In the days before accurate testing equipment was developed engineers usually ‘over-engineered’ structures, leaving a large margin of safety. However, this limited what they could do.
Steelmakers offered steels of different types from which engineers could choose which to buy. However, testing allowed the steelworks to control the process from start to finish and actually make steel to the precise specifications of their clients.
The Scottish engineer David Kirkaldy (1820-1897) pioneered the testing of materials. He was brought in to do tests after the Tay Bridge Disaster. Kirkaldy designed his own tensile testing machine in the 1860s. The machine used a hydraulic ram to apply force to samples of iron and steel. The machine is still in Kirkaldy’s former test shop in London, now a museum.
By the time large-scale steel manufacture started in Scotland in 1881 it was usual for malleable ironworks and steelworks to have access to a ‘test shop’. This was a building filled with equipment that would test the mechanical properties of the metal, ie how far it could be pushed, pulled, bent and stretched. There would also be a laboratory where scientists would examine the chemical properties and microscopic structure of the steel.
Different Types of Steel
The quality and behaviour under stress of a piece of steel is affected by:
- How it was made or refined
- Its chemical composition (eg the amount of carbon in the steel)
- How it has been worked (eg in a rolling mill)
- Any heat treatment it has undergone
Steel can be formed into shape by casting the molten steel straight into a mould but this is not possible for very small components or for larger ones that need to be strong. Working the steel into shape (known as ‘forging’) improves the suitability of the steel by refining its grain structure and closing up any cavities.
Steel can be worked ‘hot’ or ‘cold’. Hot working is when the steel is above the temperature range of 700 to 900 degrees Celsius where as cold working is when the steel is worked at a temperature below that range (so not really cold!). Heating the steel makes it more malleable and possible to work without weakening it.
There are three ways to mechanically work steel:
- Hammering was the earliest method. Before the invention of the steam engine hammers were often powered by water. A Scot, James Nasmyth played an important role in the development of the steam hammer although the concept had been developed earlier by other engineers including James Watt. Nasmyth developed his hammer as a way to forge the huge shaft that was needed IK Brunel’s new steam ship, the Great Britain. In the event, the development of screw propulsion led Brunel to do away with paddle wheels for his new ship.
- The forge press was invented in 1861 as a more controlled alternative to hammering. It was made possible by the development of hydraulic power transmission. The press can reduce steel more quickly than the hammer and with less shock.
- However, the most common method of shaping steel was by rolling it in a ‘rolling mill’, sometimes known as a ‘steel mill’. A rolling mill was a way to reduce steel down from thick cast ingots to much thinner plates, strips and bars by continually passing the steel between rollers with a decreasing gap between them.
Steelworks had their own laboratories and ‘test house’. The test house contained a fantastic array of machinery designed to push and pull samples of steel to destruction. This ensured the qualities of a batch of steel could be carefully controlled to make sure it was suitable for the intended purpose.
The two most important tests were of the ‘tensile strength’ and ‘hardness’ of a piece of steel. These two factors are related so that sometimes only a hardness test would be done to gain a quick idea of the strength of a batch of steel.
The Pulling Test
One of the most important mechanical tests that would be done on a sample of steel was to pull it apart in a tensile testing machine. This machine worked a little like a beam scale, with the beam tilting as one end was pulled down. As the other end raised, it stretched the piece of steel that was being tested.
The test piece would usually be cut from the steel as it came out of the rolling mill. This cutting could introduce stresses and weakness to the outside surfaces so the test piece would then be turned in a lathe to create a straight length of rod.
Steel has a certain amount of elasticity which is why it can be used to make springs. This means that the steel sample can be safely stretched to a certain point without weakening it. When the steel’s elastic limit is exceeded then the microscopic grain structure of the metal starts to change and the steel suddenly lengthens. The test piece has now been weakened and as the machine continues to stretch it a point is reached where the steel fails and the test piece is broken.
The two pieces are then carefully fitted back together so that the length of the test piece at the moment that it snapped can be measured.
The Brinell Hardness Test
This test involves pushing a 1cm hardened steel ball into the surface of a sample of steel to see how resistant it is to pressure.
The reason a ball was used was because the further the ball was pressed into the steel sample the wider the diameter of the dent it left. The diameter of the dent could be measured and calculated against the weight applied to give a number for the hardness of the steel. The pressure in kilogrammes was divided by the area of the impression to give the hardness number.
The museum collection contains a selection of testing equipment removed from the Test House at Glengarnock Steel Works in Ayrshire. Glengarnock had been taken over by Colville’s during the First World War. When the steel industry was nationalised it became part of the British Steel Corporation.
The machines from Glengarnock include a tensile testing machine originally used at Mossend Steel Works in Bellshill. Mossend started life as a malleable iron works before merging with the Summerlee Iron Company in the late 1800s. The two companies separated again in 1905. A new state-of-the-art steelworks was built at Mossend during the First World War but stopped making steel in 1931.
After the Second World War new forms of non-destructive testing were developed, including radiography, magnetic particle inspection and ultrasonic testing. These had the advantage that they could be used to test the actual product to be used without destroying it.
The Steelworks Laboratory
Colville’s Research Lab
After the Second World War new industries demanded new kinds of steel. Major growth areas were the oil and nuclear industries. At the same time new ways of making steel were being developed, such as continuous casting (known to British Steel employees as ConCast) where steel was tapped from the furnace and slowly cooled and rolled in one continuous, controlled process.
Colville’s established a research laboratory in Motherwell to develop new steels and new steelmaking techniques.