§1What carbon does
Carbon is the master alloying element of steel. A fraction of a percent transforms soft iron into a material that can be hardened, and the amount present is the single strongest predictor of a steel’s behaviour.
More carbon means more strength and hardness attainable, but less ductility and weldability. The bands are worth carrying: low-carbon steel (below about 0.25 % C) is soft, tough and readily welded — structural and sheet steel; medium-carbon (0.25–0.55 %) can be heat-treated to a useful hardness — shafts, gears and axles; high-carbon (above 0.55 %) hardens hard and holds an edge but is brittle and hard to weld — springs, cutting tools and rails. Everything else steel does is built on this carbon backbone, which is why the designation system puts carbon content front and centre.
Contents§2The SAE designation system
The SAE/AISI four-digit number is a compact recipe. The first two digits name the alloy class, and the last two give the carbon content in hundredths of a percent — the “carbon points”.
| Digits | Meaning | Example |
|---|---|---|
| 1st | major alloy class (1 = carbon, 4 = molybdenum group, 3 = nickel-chromium…) | 4140 → alloy |
| 2nd | subclass / approximate alloy content | 41 → Cr-Mo |
| 3rd–4th | carbon in hundredths of a percent | 40 → 0.40 % C |
| So 1018 is a plain-carbon steel with 0.18 % C; 1045 is plain-carbon with 0.45 % C; 4140 is a chromium-molybdenum steel with 0.40 % C. The code is a decodable recipe, not a lookup — read the digits and you know the essentials before opening any datasheet. | ||
§3Plain carbon steels
The 10xx series — iron and carbon with only incidental other elements — are the workhorses, and three grades cover most of what a general shop meets.
1018 (0.18 % C) is the standard mild steel: soft, tough, cheap, weldable, and the usual choice for brackets, frames and general fabrication — it will not harden appreciably by quenching. 1045 (0.45 % C) is the medium-carbon standard: strong enough to be used as-rolled for shafts and can be flame- or induction-hardened at the surface for wear. Higher still, grades around 1080 (0.80 % C) reach the spring and cutting range. The jump from 1018 to 1045 — a quarter of a percent more carbon — is the difference between a steel you weld freely and one you can harden, which is why those two numbers anchor the series.
Contents§4The alloying elements
Beyond carbon, a handful of elements are added deliberately, each for a specific effect. Alloy steels exist to gain hardenability, toughness or heat resistance that carbon alone cannot give.
| Element | Principal effect |
|---|---|
| Chromium (Cr) | hardenability, wear resistance, corrosion resistance (stainless above ~11 %) |
| Molybdenum (Mo) | hardenability, high-temperature strength, resists temper embrittlement |
| Nickel (Ni) | toughness, especially at low temperature |
| Manganese (Mn) | hardenability and strength; deoxidiser (present in all steels) |
| Vanadium (V) | fine grain, strength; forms hard carbides |
| Silicon (Si) | strength and elasticity; deoxidiser (spring steels) |
| The most important effect is hardenability — not how hard a steel gets, but how deeply a quench can harden it. A plain-carbon steel hardens only in a thin skin; add chromium and molybdenum, as in 4140, and a thick section hardens through, which is what makes alloy steels the choice for large, highly-stressed parts. | |
§5Carbon equivalent and weldability
Weldability falls as carbon and alloy rise, and it is captured by a single number — the carbon equivalent — that rolls every element into an effective carbon content.
A weldable structural steel (0.18 % C, 1.2 % Mn) has CE = 0.18 + 1.2/6 = 0.38 — below the ≈ 0.40 threshold, so it welds readily without preheat. By contrast 4140 (0.40 C, 0.85 Mn, 0.95 Cr, 0.20 Mo) gives CE = 0.40 + 0.85/6 + (0.95 + 0.20)/5 = 0.77 — well above 0.40, so welding it risks a hard, cracking-prone zone and demands preheat, controlled cooling and often post-weld tempering. The carbon equivalent is why the same properties that make 4140 a superb hardening steel make it a difficult one to weld.
§6Choosing a grade
Grade selection follows from what the part must do, read against the carbon and alloy content the code reveals.
Start with the duty. If the part is welded or lightly loaded, a low-carbon plain steel such as 1018 is cheapest and simplest. If it must be strong or wear at the surface, a medium-carbon 1045 that can be hardened locally suits. If it is large, highly stressed and must harden through its section, an alloy steel such as 4140 earns its cost through hardenability. If it must resist corrosion, chromium above about 11 % takes you to stainless. The decision chain is duty → carbon band → alloy need → grade, and the four-digit code lets you check any candidate against it in seconds.
Contents§7Quick reference
The working core of the page on one card rack.
Carbon bands
<0.25 % low · 0.25–0.55 med
>0.55 % high
SAE code
1st = class · last two = carbon
1045 → 0.45 % C
Plain steels
1018 weld · 1045 harden
Alloys add
Cr, Mo → hardenability
Ni → toughness
Weldability
CE < 0.40 readily weldable
4140 → 0.77 (preheat)