The nuts and bolts of metallurgical testing 20 January 2021

Holding structures together has never been more important to us all.  If we look around us, wherever we are in the world, in whichever work or home setting, the number of nuts, bolts and studs will be impossible to count.

These fixtures couple things together and if they fail, they are the weak link in the assembly. Verifying that they are fit for function is critical. If expert verification is not conducted, these products could cause catastrophic health and safety issues. For instance, failed parts have caused oil spills and sunk ships, with devastating environmental, human and economic impact.  

Fastener + Fixing Magazine has therefore teamed up with UK-based Rotech Laboratories, a leading UKAS accredited materials testing laboratory, to explore the significance of fastener testing, how it is carried out and its importance to many sectors of industry.

Rotech is part of the wider Rubery Owen Group and RO Materials Testing Division. Materials testing has been an integral part of business operations since around 1918, due to demands placed on its broad manufacturing base to ensure compliance and validation of goods – against the proliferation of international material, as well as engineering standards becoming essential to its development.

John Cross, general manager at Rotech Laboratories, comments: “Significant development of materials is continually taking place because fasteners are required to be exposed to new and demanding working environments, with each one having to be suitable for purpose and the operational situation. The product must be verified because there are significant legal, safety, economic and environmental implications, and breach failures, if the right materials are not used for the right applications.”

“A core element of our scope of operations is expertise in fastener testing. We are able to test, verify and validate fasteners using mechanical, corrosion, chemical analysis, and metallurgical testing, against many aspects of the specifications and standards. However, it is the importance of the metallurgical assessment of the parts that is so often underappreciated. This is usually because the test observations are not clearly understood by all and the quantitative implications of these results cannot always be easily or directly translated into how the product may react or perform in service.”

John continues: “Of critical importance to the functionality of a fastener, in any application, is that the part is made from an appropriate material needed for a particular application, and that the material has been fully heat treated to give the structure and properties needed to withstand the demands placed upon it. Heat treatment can not only give the correct structure and properties needed but can also, if done incorrectly, introduce internal and surface flaws that could compromise the properties of the material and potentially cause failure out in the field. For example, the presence of surface carburisation/decarburisation or the presence of untempered martensitic bands throughout the internal structure, may not necessarily always be detected by additional mechanical testing.”

“Part of the role metallurgical analysis plays is not only product validation against required standards, to determine the appropriateness of a particular metal for a given application, but also for investigation of failed parts. Metallurgical analysis allows us to determine the mechanism that has caused a metal component to fail and how to prevent it happening again in the future. This is obviously of great importance considering potential product recalls, assembly strip downs, possible legal implications and compromised health and safety criteria.”

Given the importance of the metallurgical aspects of a component, how is this assessed? John Cross explains: “It can be done by a series of methods but usually involves the sectioning, mounting, polishing and grinding of a material. It may also involve etching of the prepared section to reveal its microstructure – depending on the aim of the examination. The assessment is then conducted using either an optical or electron scanning microscope to indicate its structural condition.”

As part of its scope of testing, Rotech’s team of experienced metallurgists, technologists, and technicians, will assess the parts for conformance. This may include fractographic, macrostructural and microstructural examinations of fasteners, as well as structural determinations such as extent of decarburisation, material grain size, cleanliness assessments (presence of non-metallic inclusions), austenite spacing and volume fractions in duplex materials, as well as assessment of hardened surface layers such as case hardening. 

John Cross adds: “Decarburisation assessment in carbon steels, for bolting applications, is a fundamental test carried out widely as part of many international fastener specifications. Decarburisation is the process by which, at higher temperatures in the presence of a decarburising gas, carbon in the surface material is depleted resulting in a softening of the surface layers. Decarburisation is detrimental to the wear life and fatigue life of steel heat treated components and as such should be limited or eliminated altogether.”

“More specialised materials used in fastener parts, such as nickel based alloys (for example the superalloy range including alloys 625 and 718), can be susceptible to the formation of detrimental phases within their microstructure due to certain thermocyclic conditions or poor heat treatment processes. This is where the experience and knowledge of the trained metallurgist comes into play in assessing the microstructure and identifying anything unfavourable, which may cause failure of the part or compromise service life.”

“Examples are the presence of acicular (delta) phase in alloy 718 or the presence of continuous grain boundary networks in alloy 625, both of which may give rise to catastrophic failure in certain service conditions. Again, these unfavourable metallurgical structures are not always detected by mechanical testing.”

Many fastener applications not only require the material to be heat treated, to provide the strength and properties needed for use, but they also require the parts to undergo surface treatments to aid wear resistance, prolong life, aid functionality and provide corrosion resistance. In many situations these additional treatment processes also require metallurgical verification to ensure the fastener’s performance would not be compromised due to failure to meet specifications.

Metallurgical assessment and investigation of fastener parts is essential in providing manufacturers with the confidence they need in selling a product to an international market, assured that it is fit for purpose and eliminates any potential legal concerns, damage to reputation, market position or costly field failures due to non-conforming parts.  

 “We are able to assess if there will be any issues at the design stage. For instance, we verify components into aircraft engines, such as test engines where the cost to strip the engine down is breathtaking if they fail. We verify the components to ensure that this doesn’t happen.”

What are the challenges of metallurgical fastener testing? “We are always aware the stakes are high, so our laboratory must always maintain a high-level of technical expertise to be able to analyse results.  Preparation of each product, from start to finish is also very important, so we have the highest standards throughout the process. As we are UKAS accredited, this means that full traceability must be maintained.”

“This is just as important when we are asked to look at a product from the back end, when it has failed, and we look at the fractured face. Our results are fed back to the customer, with an analysis of what we believe has caused the failure.  They then have to act on that analysis and advice. This report could include information on a design or material change, poor quality material, deficient design, environmental issues, external corrosions, heat treatment failures, or inappropriate heat treatment processes.”

“We regularly see situations where corners have been cut, which can mean issues with furnaces, heat treatments, as well as cheaper or inadequate materials being used in cost cutting measures. Sometimes people just do not understand the implications and consequences of their actions. This is especially prevalent when components have been subcontracted and control of the product is passed on. Examples would be, trying to get more product in and overloading the furnace or cutting back on furnace soak times. If the furnace has not been maintained, seals can fail, furnace integrity is compromised, and this causes issues with the product.”

Keeping our structures together and ensuring that there are no weak links in the assembly, looking after our environment, our people, and business revenues, are the rewards for making sure that our fasteners are fit for purpose.  It is a considerable responsibility.

For more information about Rotech Laboratories go to www.rotechlabs.co.uk or speak to Rotech’s experts on +44 (0)121 505 4050, quoting Fastener + Fixing Magazine. 

Content Director

Will Lowry Content Director t: +44 (0) 1727 743 888

Biog

Will joined Fastener + Fixing Magazine in 2007 and over the last 12 years has experienced every facet of the fastener sector - interviewing key figures within the industry and visiting leading companies and exhibitions around the globe.

Will manages the content strategy across all platforms and is the guardian for the high editorial standards that the Magazine is renowned.