5.1 Metals, Fe-C Alloys & Steel Classification

5.1 Metals, Fe-C Alloys & Steel Classification

Quick Answer: For IWI-S Welding Technology (WT2.1–2.3), inspectors do not design alloys—but they must read chemistry, microstructure language, and material classification on certificates and WPS/WPQR documents. Know BCC ferrite vs FCC austenite, the main Fe–C phases (ferrite, austenite, cementite, pearlite), how steel is made, and how EN, ASTM, and ISO/TR 15608 labels appear on the shop floor.

Metallurgy underpins almost every weldability decision an inspector later checks: preheat, interpass temperature, consumable matching, hardness limits, and cracking risk. Module WT2 introduces materials science at a level suitable for Standard inspectors—enough to interpret drawings, mill certificates, and procedure ranges without becoming a research metallurgist.

Crystal Structures Overview

Metals solidify into ordered crystal lattices. Three lattices appear constantly in welding metallurgy discussions:

Iron is allotropic: it changes crystal structure with temperature. Pure iron is BCC (α) at ambient temperature, becomes FCC (γ, austenite) on heating through the critical range, and returns toward BCC on cooling if transformation is allowed. Carbon and alloying elements shift these temperatures and change how completely transformation occurs. Welding superimposes rapid, localised heating and cooling, so the same steel can form different microstructures in base metal, HAZ, and weld metal.

Inspectors do not measure lattice type with a pocket tool. They use the concept when a procedure, hardness survey, or cracking discussion refers to “austenitising,” “ferritic microstructure,” or “martensite in the HAZ.” Martensite forms when austenite cools too quickly for diffusion of carbon—typical of high carbon equivalent or high cooling rate joints.

Pure Metals vs Alloys

A pure metal is essentially a single chemical element (for example commercial purity copper or aluminium), though industrial “pure” grades still contain residual impurities. Pure metals often have:

• Relatively low strength compared with their alloys
• High ductility and thermal/electrical conductivity (element-dependent)
• Narrow melting ranges (essentially a melting point)
• Limited hardenability by heat treatment alone

An alloy is a metallic material with deliberate additions of other elements. Steel is an iron–carbon alloy, usually with manganese and controlled residuals, and often with chromium, nickel, molybdenum, vanadium, niobium, or other elements for strength, toughness, corrosion resistance, or elevated-temperature performance. Alloying can:

• Raise strength through solid-solution hardening and precipitation
• Increase hardenability (ability to form martensite or bainite in thicker sections)
• Improve corrosion or oxidation resistance
• Shift transformation temperatures and weldability

For inspectors, the practical distinction is documentation: pure metals and alloys are specified by grade standards, cast/heat numbers, and chemical composition limits. When a WPS lists “S355J2” or “ASTM A516 Grade 70,” it is naming an alloy system with controlled chemistry and mechanical properties—not “just iron.”

Fe–C Diagram Essentials for Inspectors

The iron–carbon equilibrium diagram (often discussed up to about 6.7% C for cementite, with steels usually below ~2% C) is a teaching map of stable phases under slow heating/cooling. Welding is not equilibrium, but the named phases still organise inspection language.

Key phases and constituents

Other terms appear in advanced modules (bainite, martensite, retained austenite). For WT2 foundations, remember:

• Heating a ferritic steel into the austenite field dissolves carbides and can coarsen grains if held hot too long.
• Cooling rate decides whether austenite becomes pearlite/bainite or hard martensite.
• Carbon content strongly affects how hard and crack-sensitive a transformed HAZ can become.

Critical temperatures (A₁, A₃, Acm for hypereutectoid steels) mark the start/finish of austenite formation on heating under near-equilibrium conditions. Welding heat cycles pass through these ranges locally in fractions of a second to a few seconds depending on heat input and thickness—so grain size and transformation products vary across the joint.

Why inspectors care

When a hardness reading in the HAZ exceeds a code or contract limit, the metallurgical story is usually: austenite formed, carbon and alloy content supported hard products, and cooling was fast. When a WPS requires preheat, the same story is being managed proactively. Understanding ferrite–austenite–pearlite/cementite vocabulary lets the inspector follow WPS notes, WPQR metallurgical discussion, and NCR root-cause statements.

Manufacture of Steel (Inspector View)

Steelmaking routes determine chemistry control, residual elements, and cleanliness (inclusions, sulphur, phosphorus). Inspectors see the results on inspection certificates (for example EN 10204 type 3.1) rather than in the melt shop, but a short process picture prevents magical thinking about “what the grade guarantees.”

Typical modern routes:

1. Primary melting — Basic oxygen steelmaking (converter from hot metal) or electric arc furnace (scrap/DRI based).
2. Secondary metallurgy — Ladle treatment for deoxidation, alloying, vacuum degassing, inclusion shape control.
3. Casting — Continuous casting into slabs, blooms, or billets; solidification structure and segregation begin here.
4. Hot working — Rolling or forging to plate, section, pipe, or bar; thermomechanical control can refine grain and raise toughness.
5. Optional heat treatment — Normalising, quenching and tempering, annealing, or stress relieving depending on product standard.

Cast vs heat number on a certificate links the product to a chemical analysis. Matching that heat to cut parts and weld maps is a traceability duty (covered more fully under inspection documentation). Residual copper, tin, or high sulphur can affect hot cracking or lamellar tearing risk; inspectors do not redesign chemistry, but they should not ignore certificate values that fall outside the WPS base-metal range.

Classification Systems Inspectors See on Certificates

Material identity on drawings, BOMs, mill certs, and WPS parent-metal tables uses national and international designation systems. Three families dominate IWI-S practice contexts.

EN designation steels (example: structural steels)

European structural steels often use forms such as S355J2:

• S — structural steel
• 355 — minimum yield strength in MPa for the thinnest thickness band of the standard
• J2 — impact energy/temperature designation (for example 27 J at −20 °C under the product standard rules)

Other EN product standards cover pressure vessel steels, fine-grain steels, quenched and tempered grades, and stainless designations (for example X2CrNi18-9 style names). Inspectors verify that the certificate grade matches the drawing/WPS and that mechanical and impact results meet the ordered standard.

ASTM grades

North American practice frequently uses ASTM specifications such as A36, A516 Grade 70, A106 Grade B, or A333 Grade 6. ASTM documents define chemical limits, tensile requirements, heat treatment, and testing. Grade numbers are not a universal strength code across all ASTM standards—always read the specification named on the certificate, not the number alone.

ISO/TR 15608 material groups (introduction)

ISO/TR 15608 groups metallic materials for welding procedure qualification purposes. Steels fall into groups (commonly discussed as groups 1–11 for ferrous materials in the technical report’s grouping logic), with subgroups based on yield strength, composition, or product type. Grouping allows a WPQR on one representative material to cover a range of similar materials under ISO 15614-type rules (detailed further in section 5.3 and in codes/qualification chapters).

Inspectors should recognise ISO/TR 15608 group numbers when they appear on WPS/WPQR forms and should understand that “same group” is not the same as “identical grade.” Thickness, delivery condition, and impact requirements still matter.

Cross-walking designations

Projects often mix EN plate with ASTM fittings or use dual-certified materials. The inspector’s job is not to invent equivalence, but to confirm that the documented material on the certificate is within the WPS qualified range (standard, grade, group, thickness, and condition). If dual certification is claimed, both specifications’ mandatory tests should be supported by the certificate package as required by the contract.

Linking Chemistry to Weldability (Preview)

Carbon and alloys that raise hardenability also raise cold-cracking risk if hydrogen and restraint are present. Section 5.3 treats carbon equivalent formulas and ISO/TR 15608 groups in depth. For this section, retain the chain:

Certificate chemistry → classification/group → hardenability expectation → WPS controls (preheat, heat input, consumable hydrogen level).

Exam Focus for IWI-S

WTE questions at Standard level often test:

• Which crystal structure ferrite vs austenite has
• What pearlite and cementite are
• Why cooling rate from austenite matters for hardness
• What information EN/ASTM designations and ISO/TR 15608 groups provide
• That mill certificates are the objective evidence of ordered material—not paint marks alone

Exam tip: If a question mentions “soft ductile matrix with lamellar ferrite and cementite,” the constituent is pearlite (or a ferrite–pearlite microstructure), not pure austenite at room temperature in a plain carbon steel.