How to choose a steel grade: 6 grade groups and their uses

“Steel” is not one material but a large family whose properties change greatly with carbon content, alloying elements and heat treatment. Two shafts of the same diameter, one in S235JR and the other in 42CrMo4, carry very different loads and cost different amounts to machine. In this article we summarise the questions that lead to the right grade and the six most common groups.

Four questions to ask before choosing

1. What load does the part carry? A constant load, a constantly changing (fatigue) load, or sudden impact?
2. Is there wear on the surface? Contact surfaces such as gear teeth, pins or cams need hardness.
3. How will it be made? Welding, machining, forging or cold forming each favour different grades.
4. Will it be heat-treated? Plans for case hardening, hardening and tempering or surface hardening decide the grade directly.

Once cost and availability are added to the answers to these four questions, the options usually come down to one or two grades.

The six main grade groups

1. Structural steels

Non-alloy steels such as S235JR, S275JR and S355J2 (formerly St37, St44 and St52). They are highly weldable and formable and are used in steel structures, frames, platforms and machine bodies. They are not intended to be hardened by heat treatment.

2. Carbon steels

In steels such as C35, C45 and C60 (SAE 1035–1060), the mechanical properties are set mainly by the carbon content. More carbon means more strength and hardness, but less ductility and weldability. C45 is the most common member of the group: it is used for shafts, keys, gears and general machine parts and can be surface hardened by induction or flame.

3. Case-hardening steels

Low-carbon steels such as 16MnCr5, 20MnCr5 and 20NiCrMo2. Their surface is enriched with carbon and hardened, giving a hard, wear-resistant surface over a tough, shock-absorbing core. Gears, pinions, pins and camshafts are typical applications.

4. Quenched and tempered steels

Medium-carbon, mostly alloyed steels such as 42CrMo4, 34CrNiMo6 and 41Cr4. Hardening followed by tempering gives them high strength and toughness throughout the section. They are used for axles, crankshafts, high-strength bolts and heavily loaded shafts. We explain the difference between case hardening and hardening and tempering in a separate article.

5. Free-cutting steels

Steels with added sulphur and/or lead, such as 11SMnPb30 and 11SMn37 (SAE 12L14, 1215). Chips break short, tool life is longer and parts can be made in volume at high speed on automatic lathes. In return, weldability and toughness are low. Details: What is free-cutting steel?

6. Micro-alloyed steels

Steels containing vanadium, such as 38MnSiVS5 and 30MnSiVS6. With controlled cooling after forging they reach high strength without a separate hardening and tempering step. Because they remove a heat-treatment operation, they save cost in forgings made in volume, such as crankshafts and connecting rods.

How to read grade names

The names are not random; they often summarise the material:

• S355J2 — S: structural steel, 355: minimum yield strength (MPa), J2: 27 J impact energy at −20 °C.
• C45 — C: non-alloy carbon steel, 45: about 0.45% carbon. CK45 / C45E is the special-steel version with tighter limits on sulphur and phosphorus.
• 42CrMo4 — 42: about 0.42% carbon, with Cr and Mo as alloying elements; the 4 is four times the chromium content (about 1% Cr). Material number 1.7225, US equivalent SAE 4140.
• St52 — The old DIN designation; 52 is the minimum tensile strength in kgf/mm². Today's equivalent is the S355 class.

Summary table