Brinell hardness testing supports a whole industrial ecosystem spanning multiple sectors
Across Britain’s heaviest engineering sectors the Brinell hardness test remains indispensable. It runs from North Sea wellheads, through high-speed, inter-city locomotive wheels, via submarine hulls to steering and suspension components. Why the Brinell test? Read on!
Brinell testing for foundries and forges
In the modern form of the Brinell test (regulated by the international Standards ASTM E10 and ISO 6506) a tungsten carbide ball – typically 10mm in diameter for ferrous metals – is pressed into the surface of a component under a controlled load, most commonly 3,000 kgf for steel and held for ten to fifteen seconds. The diameter of the resulting indentation is then optically measured and converted, via the Brinell Equation, into a hardness value; a numerical figure that is suffixed ‘HBW’ for Hardness Brinell Wolfram (Wolfram being an alternative name for tungsten carbide).
What distinguishes Brinell from the alternative hardness testing methods – chiefly the Vickers and Rockwell tests – is the size of that indentation. Vickers uses a small diamond pyramid (think inverted pyramid); Rockwell measures indentation depth, rather than diameter, with most indentations being less than 1mm across. Both can produce a precise reading of a single, small point on a material. The much larger ball and heavier load of a Brinell tester instead spread the indentation across many individual grains of metal at once, producing what metallurgists describe as a “mechanical average” of hardness over that area. For a very small or thin part this is a disadvantage: the large indentation could actually damage the material but for large, coarse-grained, or what might loosely be called ‘structurally uneven’ material ie forgings and, above all, castings, where the internal grain structure is not uniform, there is a risk, however small, that a small indenter could land on an area of the material that is unrepresentative of the whole and thus return an erroneous hardness value. It is this suitability for castings and forgings that explains why Brinell hardness testing is found so often in heavy industry.
Oil and gas
The oil and gas sector uses Brinell testing on multiple components including wellheads, subsea valves, and drilling equipment. Components manufactured to API 6A, the American Petroleum Institute’s specification for wellhead and ‘Christmas tree’ equipment (the Christmas tree is so-called because it’s a tapering pipe array, with a central vertical ‘trunk’ pipe and ‘branches’ coming out at the sides), must, where they will be exposed to fluids containing hydrogen sulphide, also comply with NACE (National Association of Corrosion Engineers) MR0175/ISO 15156, which sets a maximum hardness for carbon and low-alloy steel pressure-containing parts of 22 HRC (that is 22 on the Rockwell ‘C’ scale), equivalent to approximately 237 HBW, because steel hardened beyond this threshold becomes susceptible to sulphide stress cracking, a brittle failure mode caused by hydrogen absorption from hydrogen sulphide in the fluids mentioned.
NACE MR0175 actually names Vickers hardness (HV10) as its formal reference method for material qualification, with Rockwell C or Brinell permitted as accepted alternatives. What makes the Brinell alternative permission so helpful is that a portable Brinell tester can be used to measure hardness directly on a flange or valve body in the workshop or in the field, without needing to remove a sample to a laboratory for Vickers testing. This portability, combined with Brinell’s suitability for the forgings from which so many oil and gas components are made, is why it appears in quality documentation.
Rail
The same rationale applies in rail engineering. British and European standards for railway wheels, principally BS EN 13262, specify Brinell hardness testing (via the ‘companion’ standard BS EN ISO 6506) as part of the qualifying process for the steel of wheels and tracks, alongside tensile, Charpy impact, and ultrasonic testing. The Eschede derailment, the worst train disaster in modern German history, was the result of a fracture in a single, excessively hard wheel.
Foundries and forges
Because cast billets and forgings are inherently coarser-grained than rolled bar or machined stock, Brinell is the method most consistently recommended in industry technical guidance for this class of component, for exactly the ‘averaging’ reason set out above.
Defence
The defence-nuclear supply chain draws on a similar pool of skills, equipment and accreditation built up to serve oil and gas. Abbey Forged Products, based in Sheffield, states on its own website that its in-house, UKAS-accredited testing facility carries out “hardness testing incorporating Brinell to BS EN ISO 6506-1 & ASTM E10,” alongside Rockwell and Vickers testing. Having built its reputation supplying forgings into the oil and gas sector, the company went on to gain the accreditations needed to become a supplier to the Ministry of Defence’s Astute and Dreadnought submarine programmes.
Sheffield Forgemasters is a not dissimilar case. The company’s own documentation confirm it has supplied cast and forged steel components for the Trafalgar, Vanguard and Astute submarine classes, and that hardness testing sits within its own UKAS-accredited testing scope. It was this critical role that led to the company being nationalised in 2021.
And, of course, there is the example of William Cook Stanhope / Cook Defence Products which performs Brinell hardness tests on every link, for every tank track in use by the British Army’s tracked vehicles; ample evidence that Brinell testing’s role in UK defence extends beyond the submarine programme.
Automotive / commercial vehicles
Nissan, Ford, Rolls Royce, Caterpillar and JCB (to name but five) all employ Brinell testing. It is almost certainly the case that if you examine an engine block made by these manufacturers you will find a Brinell indentation on it somewhere! Pailton Engineering, the Midlands manufacturer of steering and suspension components is another Brinell user, as can be seen in one of our case studies.
An ‘ecosystem’
So, Brinell testing connects the industrial ecosystem of cast and forged steel. No test devised since Brinell gives a more reliable result on forgings and castings, wherever they might be destined – be it for a wellhead, a railway locomotive, a suspension arm or a submarine hull. And because a portable Brinell tester can go where more delicate laboratory equipment cannot, the test has an invaluable role to play in the field.
