Steel Grade Table
| SAE series | Family | Typical members |
|---|---|---|
| 1xxx | Carbon steels (Mn 1.00% max) | 1018 mild, 1045 medium, 1095 spring |
| 11xx | Resulfurized (free machining) | 1117, 1141, 1144 |
| 12xx | Resulfurized + rephosphorized | 1215, 12L14 (lead added) |
| 13xx | Manganese 1.75% | 1340 |
| 15xx | Plain carbon, Mn 1.00-1.65% | 1526 |
| 40xx / 44xx | Molybdenum | 4023, 4419 |
| 41xx | Chromium-molybdenum | 4130, 4140 |
| 43xx / 47xx | Nickel-chromium-molybdenum | 4340 |
| 51xx / 61xx | Chromium / chromium-vanadium | 5140, 6150 |
| 86xx / 87xx / 88xx | Nickel-chromium-molybdenum (triple alloy) | 8620, 8640 |
| 92xx | Silicon-manganese | 9255 |
| Grade | Read it as | Everyday translation |
|---|---|---|
| 1018 | Plain carbon, 0.18% C | Welds and bends without drama; brackets and cold-drawn bar |
| 1045 | Plain carbon, 0.45% C | The general shafting steel you can heat-treat |
| 1095 | Plain carbon, 0.95% C | Spring and blade territory, cracks if welded carelessly |
| 4140 | Cr-Mo, 0.40% C | The machinery default: dies, axles, fixture plates |
| 8620 | Ni-Cr-Mo, 0.20% C | Carburizing workhorse: hard skin, tough core, gears |
| 12L14 | Resulfurized + lead, 0.14% C | Screw-machine legend, machineability over strength |
| 304 / 316 | Three-digit stainless: series serial, NOT carbon | 18-8 kitchen steel / 316 adds Mo for salt |
A four-digit SAE/AISI number is a recipe written as a code: the first digit names the main alloying family, the second digit flags the modifying element, and the last two digits give the carbon content in hundredths of a percent by weight. A 1045 bar is plain carbon steel at 0.45 percent carbon; 4140 is chromium-molybdenum steel at 0.40 percent; 12L14 is a resulfurized, rephosphorized free-machining steel with lead added. Read the code and the steel's whole personality - weldability versus strength, cost versus hardenability - falls out before you open a datasheet.
The system has two deliberate traps. First, in most light-industrial work the labels are interchangeable shorthand: a mill certificate may read 4140, AISI 4140 or SAE 4140, and buyers accept all three because the SAE and AISI systems were merged in practice even though the organizations never fully formalized it. Second, stainless steel abandons the scheme entirely - a three-digit system where the last two digits are a series serial number, not carbon content - so a 304 is not a 0.04-carbon steel no matter how logical that reads.
How to use
- Decode before you price: the last two digits are carbon in basis points - 1018 (0.18% C) welds beautifully and bends without cracking, 1045 (0.45% C) is the general shafting steel you can heat-treat, and 1095 (0.95% C) is spring and blade territory. If a drawing says 1050 and the yard stocks 1045, you are half a carbon point apart - usually an acceptable swap after a call to the engineer, never a silent one.
- Match the family to the failure you fear: 41xx chromium-molybdenum (4130, 4140) for toughness and wear at moderate cost, 86xx nickel-chromium-molybdenum (8620) for carburized gears that need a hard skin over a soft core, 11xx and 12xx resulfurized grades (12L14) when the part lives on a screw machine and dies by thread-cutting time, plain 1xxx when you need to weld without preheat drama.
- Check the suffixes and the certification wording: an H at the end (4140H) guarantees hardenability in a band rather than a composition window, and L means lead was added for machinability. On paperwork, accept AISI 4140 or SAE 4140 as the same bar stock, but insist the certificate carries heat numbers - the four-digit code names a recipe, the heat number proves the batch.
Frequently asked questions
Is a grade 8 bolt made of 8xxx steel?
No - the collision of names is a coincidence and a famous one. A SAE J429 grade 8 bolt is a medium-carbon alloy steel quenched and tempered to a minimum tensile strength of 150,000 psi, and the 'grade' is a strength class marked on the head, not an alloy family; typical grade 8 fasteners are made from 4140-or-similar Cr-Mo steels. The 8xxx in the steel numbering system is the nickel-chromium-molybdenum family (8620 being the carburizing workhorse). The two systems grade different things: one rates a finished part's strength, the other names a melt's recipe.
What is the difference between 1018 and 1045?
Eight hundredths of a percent of carbon - and everything that follows from it. 1018, at 0.18 percent carbon, is the mild steel of brackets, weldments and cold-drawn bar: it bends, welds and drills with almost no precautions, and cannot be hardened much beyond as-rolled strength. 1045, at 0.45 percent, is the entry point of heat-treatable steel: flame-harden the teeth of a sprocket, quench and temper a shaft, and it carries roughly double the tensile strength of 1018 - but weld it carelessly and the heat-affected zone cracks. The choice is a straight trade: 1018 when you join, 1045 when you harden.
Why is 4140 the default alloy steel?
Because chromium and molybdenum buy the two properties plain carbon lacks - deep, even hardenability and abrasion resistance - at a surcharge that stays affordable in bar quantities. 4140 oil-quenches to a useful through-hardened state in sections that would leave 1045 soft at the core, machines reasonably in the annealed state, and responds to nitriding for wear parts. That versatility made it the machinery industry's default: axle shafts, dies, fixture plates, drills. The paperwork reflects the same looseness the market does - suppliers, mills and certs write 4140, AISI 4140 and SAE 4140 interchangeably, and in light-industrial practice all three name the same bar.
Why doesn't 304 stainless mean 0.04 percent carbon?
Because stainless runs on a three-digit system built for a different question: which crystal structure and which alloy family. The 200 series is austenitic chromium-nickel-manganese (cheap, manganese replacing some nickel), the 300 series austenitic chromium-nickel - 304 is the classic 18-percent-chromium, 8-percent-nickel kitchen steel, 316 adds molybdenum for chloride resistance - and the 400 series ferritic and martensitic. In none of these do the trailing digits encode carbon; when carbon matters on stainless it gets its own suffix, as in 304L for the low-carbon weldable version. The four-digit decoding on this page stops at the stainless door on purpose.