Selecting the wrong material option may or may not lead to instant failure. It may lead to slow failure — unforeseen material corrosion, fatigue cracks forming over time, the eventual failure of a weld. Most of these issues arise from the specification decision of a person who didn’t understand the material’s behavior in service.
Steel Grades and When They Apply
In the UK construction industry, structural steels are usually specified to BS EN 10025. Grades of S235, S275 and S355 are the most commonly used, where the number indicates the minimum yield strength, measured in MPa. The most commonly used steel in the UK commercial and industrial environment is S355, as it has a better strength-to-weight ratio than S275, and at current UK stockist prices, the better strength-to-weight ratio means S355 offers better value for money. S235 is used in secondary steelwork when the deflection of the steel, rather than the strength, is of primary concern.
Sub-grade also matters, and in most specifications, is left out completely. An S355 J0 has a minimum Charpy impact value of 27 joules at 0 degree Celsius. An S355 J2 has the same value at -20 degree Celsius. For external steelwork located in Northern England and Scotland, where wintertime ambient air temperatures often fall below 0 degree Celsius, it would be acceptable to specify S355 J0, however specifying S355 J2 would be a better choice for only a marginal cost. It would also eliminate a risk of brittle fracture in the cold.
Stainless steel grades can be divided into three families: ferritic, austenitic, and duplex. Grade 304 (1.4301) covers most light commercial applications, such as catering equipment and architectural fixings. However, 304 will pit with chloride exposure in coastal areas, around swimming pools, and in food processing areas that use salt-based cleaning solutions. Grade 316 (1.4401) has molybdenum added and offers far better resistance to chloride. In the offshore, or more aggressive environments, duplex grades such as 2205 (1.4462) have greater resistance to corrosion, strength, and are much harder to weld with more strict control of the heat input.
Tool steel is a separate category of steel from structural and stainless. H13, D2, and M2 are examples and are used to make cutting tools, dies, and moulds instead of used to build structures. Depending on the grade, hardness, wear resistance, and toughness can be traded.
Titanium Grades Across Industries
Some of the defining characteristics of titanium such as its high strength-to-weight ratio and corrosion resistance, and its biocompatibility are well known. However, it is much less known how significantly the grades vary, and how much the wrong choice can affect both performance and cost.
Commercially pure titanium is categorized into four grades based on strength. Grade 1 is the softest and easiest to form. It is used in chemical processing, heat exchangers, and architectural cladding. Grade 1 is used when corrosion resistance is more critical than strength. Grade 2 is the most widely used grade in the UK, and is used in many general industrial applications. Grades 3 and 4 are less commonly used. Grade 4 is close to the strength of some alloy grades, but has lower ductility.
Most of the titanium used in aerospace is Grade 5, Ti-6Al-4V, and is the titanium alloy most engineers are referring to. Compared to Grade 2, Ti-6Al-4V has lower density and higher yield strength. Grade 5 is used in many aerospace applications, such as aircraft fan blades and fasteners, and in many titanium pressure vessels. Grade 5 is also used in many orthopedic implants, such as hip stems, knee implants, and orthopedic fixation plates. The combination of biocompatibility and strength of Grade 5 titanium makes it very difficult to improve upon titanium in load bearing implants.
Grade 23 is a more refined Grade 5 titanium, which has improved fracture toughness and fatigue performance. Because of this, Grade 23 titanium is often preferred over Grade 5 titanium for long-term implants. If a specification calls for Ti-6Al-4V ELI, this is referring to Grade 23.
Grade 9 is a titanium alloy at an intermediate level of strength and purity, used primarily in tubing and frames when moderate strength and high formability are required. Compared with Grade 5, it has better weldability, and is less sensitive to work hardening.
When considering the procurement of titanium alloys in the UK, it is worth noting that while certified aerospace and medical alloys with full traceability are not hard to find, specialized stockholders may take 8 12 weeks to provide non-standard dimensions and grades. For safety-critical applications, the risk of purchasing from a stockholder who does not provide a material test certificate to BS EN 10204 3.1 is unjustifiable.
Aluminium Alloys
For most structural applications and general fabrication in the UK, the 6000 series of alloys (primarily 6061 and 6082) dominate. Both are weldable and precipitation-hardenable, and 6082 is the more common alloy in UK and European standards. 6061 is more common in US and aerospace specifications. The alloys are similar in most mechanical respects and the designations are also comparable. The designations also indicate the temper of the alloy. T6 represents T6 is the peak strength temper. In contrast, T4, represents the naturally aged temper of the alloy, in which the alloy has better formability but at lower strength.
In the 7000 series, zinc becomes the main alloy. 7075 is the go-to alloy for most aerospace structures. It is much stronger than 6082, and as a substitution alloy, 7075 is used in aircraft structures, and is desirable where the whole structure is strong and light. 7075 is costly and difficult to weld, and is prone to stress corrosion cracking in some tempers. For typical fabrication, 7075 is almost never the correct alloy. However, there are some applications of 7075 outside aerospace, such as high-performance bike parts, tool plates, and parts of firearms. These applications of 7075 are justified by the strength of the alloy in relation to the price and difficulty of fabrication.
Material Certifications
EN 10204 describes the types of inspection documents associated with metallic products and their inspections. The commonly seen documents are 2.2 (testing reports that are based on nonspecific inspections), 3.1 (inspecting certificates that are issued by the quality department of the manufacturer), and 3.2 (inspecting certificates that are issued by the manufacturer and a third party independent of the manufacturer). For most structural applications 3.1 is the minimum that should be provided. In aerospace, medical, nuclear, and pressure vessel works, the standard is 3.2, and it is usually required by contract.
If a certificate does not explicitly state EN 10204, it is not necessarily a completely worthless document, but it is worth checking what the document does certify. Some manufacturers create documents that look like mill certificates but do not confirm compliance with the EN 10204 standards. This is important when material properties are in dispute and a regulating body is reviewing your company’s supply chain.
Positive Material Identification – PMI testing – is the practical counterpart to certification. Handheld XRF analysers enable the quick and easy identification of material composition. On projects where a material mix-up may cause safety issues, PMI provides an on-site verification step beyond receipt and accompanying documentation. This is especially the case with nuclear and petrochemical projects and is becoming the common practice in aerospace MRO.