When steel forms the backbone of your technology, engineering and construction industries, then you really need to be able to trust your material. The invention (or discovery, whichever you prefer!) of alloy steel grades was a real watershed moment for society. It allowed us to make steels with more strength and heat treatment capabilities.
The trust that you have in your steel comes from knowing exactly where its limits are, and in order to find that out, sometimes we basically have to destroy the steel by pushing right up to and then beyond its capabilities. Only then can we say that a particular grade of steel will be able to tolerate the forces or use that the customer needs it to do. These ‘destructive tests’ are carried out on the parent material or samples of the products. There are also a range of non-destructive tests that the finished product goes through, which we’ll cover separately.
Destructive tests aim to find out how far a steel can be pushed and include:
This aims to plot, on a graph, where certain reactions to tensile stress come into play. The test will not only tell us the Tensile Strength of a steel, but also at what point the steel will start to Elongate and hit the Elastic Limit (also known as the Yield Point). Put simply, they pull a piece of steel until it stretches and snaps. The point where it begins to stretch is the elongation, and as soon as it’s permanently deformed and won’t go back into shape, it’s hit the elastic limit. The points between the elongation and elastic limit are known as Young’s Modulus, which sets the elasticity between point A and point B. All this information gives us the tensile strength.
There are a number of hardness tests:
- Brinell’s Hardness test uses a 1cm hardened steel ball under a press for 15 seconds – the resulting indentation is measured by microscope and translated into a score.
- Vickers Diamond testing uses a pyramid shaped diamond in a similar manner, but is converted into kg/mm2, the latter being the area of the final indentation.
- Rockwell Hardness is determined by the depth that a diamond or hardened steel ball can penetrate the surface under load.
Impact tests measure the resistance to a sudden shock to give its toughness, most usually the Charpy Impact test. The steel is supported at both ends, and hit with a pendulum that swings from a set point. When the steel breaks, they measure the height that the pendulum swings to, in order to see how much energy was absorbed; the lower the swing, the more energy was absorbed. Heat plays a part in how the steel will react, so this may be specified on the results.
These are performed on steel tubes and weld points, to make sure they’re safe. The bend tests involve bending the material to at least 90°, while the flange test runs a flange over the body of the tube at right angles. There are also flattening tests and a drift test, which forces a cone into a steel tube to see how badly it expands or cracks under pressure.
Stainless steel is supposed to be anti-corrosion, but it does have its limits, and customers need to know that their stainless steel will stand up to its intended purpose.
- Weld decay – the tubes are boiled in a copper sulphate (or H2SO4) for 3 days – if they don’t crack when bent to 90°, that’s a pass test.
- Strauss testing is similar to the above weld decay, but the boiling solution must have solid electrolyte copper and the test only takes 15 hours.
- Huey tests boils the material in nitric acid over 5 periods of 48 hours. The steel is weighed each time to see how much of the material has been lost through corrosion.
- Potentiostat tests are complicated, but basically produce polarisation curves which relate to current flow and electrode potentials.
Many times we think of steel – and stainless steel in particular – as completely trustworthy and capable, but it’s easy to forget what it has to go through long before anybody can certify that it’s suitable to use for a specific project! These tests provide valuable insight into how the metals perform and we couldn’t get along without them.