In casual conversation, steel gets treated as a single material. In engineering, it never is. The grade selected for a component drives almost every downstream decision: weldability, machinability, heat treatment requirement, NDT method, inspection regime, and ultimately whether the finished part will perform as intended in service.
Specifying the wrong steel grade, or treating two grades as if they were interchangeable because they look the same, is one of the more common causes of fabrication defects, premature failures, and audit findings. Understanding what each steel family is, where it is used, and what it demands from the supply chain is fundamental to getting the project right.
This article works through the main families of steel, what they are typically used for, and what NECIT does to support clients working with each one.
Carbon steels are alloys of iron and carbon with very limited additions of other elements. They are categorised by carbon content.
Low carbon steels, also known as mild steels, sit at 0.05 to 0.25 percent carbon. They are soft, ductile, and readily weldable. They form the bulk of structural steelwork, sheet metal, pipes, tubes, and general fabrication. Most fabricators encounter EN 10025 grades such as S275 and S355 daily.
Medium carbon steels at 0.25 to 0.60 percent carbon offer higher strength and hardness, with reasonable wear resistance. They can be heat treated to alter their properties. Typical applications include shafts, axles, railway tracks, gears, and machine parts.
High carbon steels at 0.60 to 1.50 percent carbon are very hard and strong but with reduced ductility. They are used for springs, cutting tools, knives, blades, and high strength wire. They demand careful heat treatment and present specific welding challenges including hydrogen cracking risk.
Carbon content is not a detail. It governs preheat requirement, filler metal selection, hardness control in the heat affected zone, and post-weld heat treatment decisions. Carbon equivalent calculations are routine in welding procedure development for a reason.
Alloy steels contain deliberate additions of chromium, nickel, molybdenum, vanadium, manganese and other elements. The additions are made to achieve specific property improvements: higher strength, better hardenability, improved wear and abrasion resistance, or enhanced heat and corrosion resistance.
Chromium-molybdenum (Cr-Mo) steels are widely used in pressure equipment, oil and gas process plants, and elevated temperature service. Grades such as 1.25Cr-0.5Mo and 2.25Cr-1Mo will be familiar to anyone working in refinery or petrochemical fabrication. Nickel-chromium (Ni-Cr) steels are used where toughness at low temperature matters, including subsea and cryogenic applications. High strength low alloy (HSLA) steels deliver yield strengths well above standard structural grades while remaining weldable and economical, which is why they appear in bridges, pipelines, offshore structures, and heavy lift equipment.
Alloy steels almost always demand controlled preheat, controlled interpass temperature, and in many cases post-weld heat treatment. They are also subject to Pressure Equipment Directive and Pressure Equipment (Safety) Regulations requirements when used in pressure-containing duty, and to NORSOK or API specifications in oil and gas. Traceability of material certification through to the finished component is non-negotiable.
Stainless steels are defined by a minimum 10.5 percent chromium content, which produces a passive surface oxide that resists corrosion. They are not a single material. They are four distinct families, and confusing them in the field has practical consequences.
Austenitic stainless steels, including 304, 316 and 309, have a face-centred cubic (FCC) structure. They are non-magnetic in the annealed condition, offer high ductility and toughness, and are the most common stainless grades in food, pharmaceutical, chemical, and general process applications. Their main risks are sensitisation during welding, where chromium carbide precipitation depletes the grain boundary and reduces corrosion resistance, and stress corrosion cracking in chloride environments.
Ferritic stainless steels, such as grade 430, have a body-centred cubic (BCC) structure, are magnetic, and offer good corrosion resistance at lower cost than austenitic grades. They have limited weldability due to grain coarsening in the heat affected zone, and they are not normally specified for thick section welded fabrication.
Martensitic stainless steels, including 410 and 420, can be hardened by heat treatment. They are magnetic, high strength, and used for cutlery, surgical instruments, and components requiring both corrosion resistance and high hardness.
Duplex stainless steels, including 2205 and 2507, have a mixed austenitic and ferritic structure. They combine high strength with superior corrosion resistance and are widely used in chemical plants, marine environments, and offshore equipment. Duplex grades demand strict control of welding heat input and ferrite balance. A duplex weld with the wrong ferrite content is technically out of specification regardless of how clean the bead looks.
The point that gets missed in practice: an austenitic stainless steel and a duplex stainless steel are not interchangeable, and treating them as the same material because both are corrosion resistant is a fast
route to failure in service.
Tool steels are formulated for high hardness, wear resistance, and the ability to retain hardness at elevated temperature. They are categorised broadly into high speed steels, hot-work tool steels, and cold-work tool steels.
High speed steels (HSS) contain tungsten, molybdenum, vanadium and cobalt in significant quantities. They retain hardness at high temperature, which is essential for cutting tools running at speed.
Hot-work tool steels resist softening at elevated temperature and are used in forging dies, die casting tooling, and extrusion dies. Cold-work tool steels deliver high hardness and wear resistance at room temperature and are used for dies, punches, and cutting tools that operate without significant heat input.
Tool steels are difficult to weld and often unsuitable for conventional fusion welding. Where joining or repair is required, specialised techniques and tightly controlled procedures are needed. Heat treatment of tool steels is a discipline in its own right, with hardening, tempering, and in many cases multiple temper cycles required to achieve the design hardness
Advanced High-Strength Steels are a relatively recent family developed primarily for the automotive industry, where they enable higher strength with reduced weight.
Dual phase (DP) steels combine ferrite and martensite to deliver high strength with good formability. They are widely used in car body panels. TRIP steels (Transformation Induced Plasticity) offer excellent strength and ductility together, with high energy absorption, making them suitable for crash components. TWIP steels (Twin Induced Plasticity) push ductility even further while retaining very high strength, and are used in complex pressed parts. Martensitic AHSS grades deliver ultra-high strength for safety critical components such as bumpers, door beams, and reinforcements.
AHSS grades demand specific welding strategies. Resistance spot welding parameters that work for mild steel will not give acceptable joint strength on AHSS. Heat affected zone softening is a real concern. The metallurgical complexity that gives these steels their performance also makes them sensitive to processing.
Inspection method selection is not generic, it is tailored to suit the unique properties of the material.
Magnetic particle inspection (MPI) requires a ferromagnetic material. It will not work on austenitic stainless steel, where dye penetrant inspection (DPI) or eddy current is used instead. Radiography acceptance criteria differ for ferritic and austenitic materials. Ultrasonic testing of austenitic welds is significantly more difficult than ferritic welds because of the coarse grain structure and acoustic anisotropy, which is why phased array techniques and tailored procedures are often required. Hardness testing limits in the weld and heat affected zone differ by code and by grade, and the limits matter because they relate directly to in-service performance.
Choosing the wrong NDT method, or applying the right one with the wrong parameters, produces inspection records that look complete but provide no actual assurance. In regulated sectors, that is a finding waiting to happen.
NECIT operates across three divisions, and between them we cover every steel family discussed above.
Our ISO 17024 accredited welding certification scheme qualifies welders and operators across the full range of steel groups under BS EN ISO 9606-1, including unalloyed and low alloy steels, high strength steels, creep resisting steels, and the four stainless steel families.
We witness WPQR test coupons across these material groups, including the heat treatment of those coupons where required. Through our Approved Examination Centre (AEC) programme, we extend the same certification framework to colleges and training providers, ensuring consistency of standard across the supply chain.
Our Vendor Inspection division witnesses fabrication, welding, NDT, heat treatment, and final inspection activities at manufacturing facilities globally. We work across structural steelwork under EN 1090, pressure equipment under PED and the Pressure Equipment (Safety) Regulations, oil and gas equipment under NORSOK and API, rail components, and subsea fabrication. The materials encountered cover the full range from low carbon mild steel to duplex stainless and exotic alloys. The discipline is the same regardless of grade: verify the material certification matches the specification, verify the WPS and WPQR are appropriate to the material, witness the fabrication activities, review the inspection records, and confirm conformance before release.
Our UKAS accredited NDT division applies the appropriate inspection method to the material, not a default method to every job. That includes MPI, DPI, ultrasonic testing, phased array UT (PAUT), eddy current, IRIS ultrasonic for tube inspection, and radiography. Our personnel are certified to BS EN ISO 9712 and applied across all the steel families covered in this article. Method selection, procedure development, and report interpretation all take account of the specific material being inspected.
In addition, the Welding Development Centre (WDC) supports clients on welding procedure development, qualification testing, and metallurgical investigation work that crosses material boundaries, including aluminium alloys alongside the steel families discussed here. The WDC provides the technical capability to take a project from material selection through to qualified welding procedures and validated production routes.
Steel is not one material. It is a family of related but mechanically and metallurgically distinct alloys, each with its own behaviour during fabrication, welding, heat treatment, and service. Treating them as interchangeable produces compliance gaps and, in the worst case, in-service failures.
The grade does not change what good practice looks like – it changes how good practice is delivered. If your project involves any of the steel families covered in this article, and you need certification, vendor inspection, NDT, or technical welding support, NECIT operates across all of them.
