Steel Chemical
Elements Explained.

Carbon is the most important element in steel. It is the primary factor controlling hardness, strength and wear resistance after heat treatment. The higher the carbon content, the greater the potential hardness — but also the lower the toughness, ductility and weldability.

Low carbon (0.10–0.25%) — mild steels like EN3B (070M20). Excellent weldability and ductility. Cannot be significantly hardened by quenching. Used for general fabrication, brackets and low-stress components.

Medium carbon (0.30–0.55%) — engineering grades like EN8 (080M40, 0.36–0.44% C) and EN9 (070M55, 0.50–0.60% C). Can be hardened by quenching and tempering. The backbone of general engineering shafts, bolts and tooling blanks.

High carbon (0.60–1.00%) — bearing steels such as 100Cr6 (0.95–1.10% C). Achieves 60–64 HRC when hardened. Hard, wear-resistant but brittle — only used where the hardness is the functional requirement.

Rule of thumb: carbon content × 100 gives a rough guide to maximum achievable hardness after quenching, in HRC. 0.40% C → approximately 55–57 HRC maximum. In practice, section size and quench medium limit the hardness achieved through the section.

Chromium is one of the most important alloying elements in engineering steel. At levels of 0.70–1.50%, it significantly improves hardenability — allowing larger sections to achieve full hardness when quenched. It also increases wear resistance by forming hard chromium carbides and improves corrosion resistance.

In EN19 (42CrMo4), chromium at 0.90–1.20% combined with molybdenum allows the steel to harden fully in sections up to approximately 65mm when oil quenched. Without chromium, EN8 at 0.40% C can only reliably through-harden sections to about 40mm.

Carbide former: Chromium forms chromium carbides (Cr₇C₃, Cr₂₃C₆) which resist dissolution at hardening temperatures and provide excellent wear resistance. This is why bearing steel 100Cr6 with 1.35–1.65% Cr achieves such high hardness and fatigue life.

At levels above 10.5%, chromium makes steel stainless by forming a passive chromium oxide surface layer. This is a fundamentally different use of the element — standard engineering bar grades use chromium purely for hardenability.

Chromium increases susceptibility to temper embrittlement if the steel is held or slowly cooled through the 375–575°C range — this is why molybdenum is almost always added alongside chromium in alloy steels.

Molybdenum is one of the most potent hardenability-improving elements available — it achieves a large hardenability increase per unit of addition, making it extremely cost-effective despite its relatively high cost per tonne.

Preventing temper embrittlement is arguably its most important role in chromium steels. When chromium steels are tempered or slowly cooled through 375–575°C, impurities segregate to grain boundaries causing embrittlement. Adding 0.15–0.40% Mo suppresses this mechanism, allowing chromium steels to be safely tempered across the full temperature range. This is why virtually all chromium alloy steels also contain molybdenum (EN19 = 42CrMo4).

High-temperature strength: molybdenum significantly improves creep resistance and retained strength at elevated temperatures. This makes Mo-containing steels preferred for components that run warm in service.

In EN40B (31CrMo12), the high chromium (2.80–3.30%) and molybdenum (0.40–0.70%) combination is specifically chosen because chromium nitrides form during nitriding — the chromium is there to respond to the nitriding process, while molybdenum provides core strength.

Nickel is unique among the alloying elements in that it improves toughness without reducing ductility — and it continues to improve toughness even at very low temperatures where other elements make steel brittle. This makes nickel-containing grades essential for cryogenic applications and any service where impact resistance at low temperature is critical.

Hardenability: nickel stabilises austenite and slows its transformation during cooling, allowing hardening through much larger cross-sections. EN24 (34CrNiMo6) with 1.30–1.70% Ni can be fully through-hardened in sections exceeding 100mm — something EN19 (without nickel) cannot achieve.

In case hardening steels, nickel is the primary toughening element for the core. EN36 (655M13, 3.00–3.75% Ni) achieves its outstanding core toughness beneath a hard carburised case because the nickel maintains ductility and impact resistance in the low-carbon core material.

Residual nickel (<0.30%) is present in many grades and generally has a beneficial effect on toughness without being a deliberate alloy addition. Where you see <0.30% Ni as a maximum on a cert, the steel is specifying a residual limit, not an alloy addition.

Vanadium is a powerful grain refiner and carbide former. Even at very small additions (0.05–0.15%), it has a significant effect on grain size during austenitising, producing a finer grain structure that improves both strength and toughness simultaneously — a combination that is otherwise difficult to achieve.

Secondary hardening: vanadium carbides (VC) dissolve in austenite at hardening temperature and reprecipitate as fine carbides during tempering, causing a secondary increase in hardness. This means vanadium steels can be tempered at higher temperatures (gaining toughness) without losing as much hardness as non-vanadium grades.

Vanadium also raises the austenitising temperature required for full hardening — a consideration for heat treaters. If the hardening temperature is too low, vanadium carbides remain undissolved and the full hardenability benefit is not achieved.

Not common in standard engineering bar grades but appears in some tool steels, hot work grades and high-strength low-alloy (HSLA) structural steels. When seen on a cert at > 0.05%, it is a deliberate addition.

Boron is the most potent hardenability-improving element by weight in steel. Additions as small as 0.0005% (5 parts per million) can increase hardenability significantly — equivalent to adding much larger quantities of chromium or molybdenum.

The mechanism is that boron segregates to prior austenite grain boundaries and inhibits the nucleation of ferrite and pearlite during cooling, allowing the transformation to hard martensite to occur at lower cooling rates (in larger sections, or during slower quenches).

Critical requirement: boron only works when the steel is free of dissolved nitrogen. If nitrogen is present, it combines with boron to form boron nitrides, rendering the boron ineffective. Boron-treated steels therefore always contain aluminium or titanium to fix the nitrogen first.

On a mill cert, boron appears as “B” with a value like 0.0015% — it is easy to miss given the tiny numbers. It is common in grades developed for direct quenching after hot rolling, some fastener grades, and agricultural implement steels where hardenability is required without the cost of chromium-molybdenum additions.

Lead is insoluble in steel — it does not dissolve in the iron matrix but instead exists as minute globules dispersed throughout the microstructure. These act as internal lubricants during cutting, reducing friction between the tool and chip. The result is dramatically better surface finish, lower cutting forces, and the ability to run at higher speeds on automatic lathes.

EN1A Pb (230M07Pb) contains 0.20–0.35% Pb, making it the fastest-machining standard engineering steel grade available. Lead additions give roughly a 25–35% improvement in cutting speed over the non-leaded equivalent (EN1A) before tool failure.

Lead has no effect on mechanical properties at the levels used (0.15–0.35%). Hardness, tensile strength and impact values are essentially unchanged. Lead is purely a machinability aid.

Environmental concern: lead is a regulated substance under EU/UK REACH regulations. Many manufacturers now avoid leaded grades for environmental reasons, substituting EN1A (sulphur-only free-cutting) or using calcium/bismuth additions instead. Where REACH compliance is a consideration, check with your customer before specifying Pb grades.

Copper in engineering steel is almost always a residual element from scrap-based steelmaking — copper cannot be removed from steel once present. As steelmaking uses increasing proportions of recycled scrap containing copper-bearing components (electric motors, wiring, etc.), copper levels in bar have gradually risen over decades.

At low levels (<0.30%), copper has a mild beneficial effect on corrosion resistance (similar to the well-known weathering steels that use 0.30–0.50% Cu deliberately) and a small solid-solution strengthening effect. At these levels it is essentially harmless.

Above 0.30%, copper begins to cause problems with hot workability. During hot rolling, copper enriches at the surface as the iron oxidises preferentially, forming a copper-rich layer that penetrates grain boundaries and can cause surface cracking during rolling — known as hot shortness or copper checking. This is why most specifications set a maximum of 0.30% Cu.

Copper is not a functional alloy addition in any of the engineering bar grades stocked by Parkside Steel. When it appears on a cert within normal limits (<0.25%), it is simply a residual from the scrap charge with no practical effect on bar properties.