There are hundreds of AISI/SAE steel grades, but a working machinist encounters maybe 15-20 of them regularly. The numbering system is logical once you understand the pattern: the first two digits identify the alloy family, and the last two digits tell you the carbon content. Beyond that, knowing what each alloy element contributes to machinability, hardness, and toughness lets you make smart decisions about speeds, feeds, and tooling without memorizing every spec sheet.
This guide covers the numbering system, the role of major alloying elements, and the grades you are most likely to see in a machine shop — from free-machining 12L14 to aerospace-grade 4340 to tool steels like A2 and D2.
The AISI/SAE Four-Digit System
The AISI/SAE steel designation is a four-digit number where the first two digits identify the alloy family and the last two digits represent the nominal carbon content in hundredths of a percent. A 1045 is a plain carbon steel with 0.45% carbon. A 4140 is a chromium-molybdenum alloy steel with 0.40% carbon. A 8620 is a nickel-chromium-molybdenum steel with 0.20% carbon.
The major alloy families are: 10xx (plain carbon), 11xx (resulfurized / free-machining), 12xx (resulfurized and rephosphorized), 13xx (manganese), 40xx (molybdenum), 41xx (chromium-molybdenum), 43xx (nickel-chromium-molybdenum), 46xx (nickel-molybdenum), 51xx (chromium), 61xx (chromium-vanadium), 86xx (nickel-chromium-molybdenum), and 92xx (silicon-manganese).
Carbon content is the single most important number for predicting machinability and hardenability. Low carbon (under 0.25%) is soft, ductile, and easy to machine but cannot be through-hardened. Medium carbon (0.25-0.55%) can be heat treated to useful hardness levels and is the workhorse range for machinery components. High carbon (over 0.55%) is used for springs, cutting tools, and wear surfaces — it is hard but less ductile and more difficult to machine.
First 2 digits = alloy family
Last 2 digits = carbon content (hundredths of %)
Examples:
1018 = plain carbon, 0.18% C
4140 = chrome-moly, 0.40% C
4340 = nickel-chrome-moly, 0.40% C
8620 = Ni-Cr-Mo, 0.20% C
AISI/SAE Steel Grade Decoder
Decode AISI/SAE steel grades instantly. Alloy composition, machinability rating, heat treatment notes, weldability, and typical applications for 40 common carbon, alloy, and tool steels.
What Each Alloy Element Does
Carbon (C): The primary hardening element. More carbon means higher potential hardness after heat treatment but lower ductility and weldability. Above 0.30% carbon, preheating before welding is usually required.
Chromium (Cr): Increases hardenability (the ability to achieve full hardness throughout the cross-section), wear resistance, and corrosion resistance. At 10.5%+ chromium, the steel becomes stainless. In alloy steels (1-2% Cr), it improves through-hardening and temper resistance.
Molybdenum (Mo): Increases hardenability and high-temperature strength. Prevents temper embrittlement in Cr-Mo steels. The combination of chromium and molybdenum (41xx series) produces steels with excellent through-hardening capability and toughness.
Nickel (Ni): Increases toughness, especially at low temperatures. Improves hardenability without increasing brittleness. The 43xx series (Ni-Cr-Mo) produces the toughest structural steels — 4340 is the benchmark for high-strength, high-toughness applications.
Manganese (Mn): Improves hardenability and tensile strength. Present in almost all steels at 0.3-1.0%. Higher levels (1.0-1.65% in the 13xx series) significantly increase hardenability and work-hardening tendency.
Sulfur (S) and Lead (Pb): Free-machining additives. Sulfur forms manganese sulfide inclusions that act as chip breakers. Lead forms tiny soft inclusions that lubricate the cutting zone. Both dramatically improve machinability at the cost of toughness and weldability.
Carbon: hardness and strength (but less ductility)
Chromium: wear resistance and hardenability
Molybdenum: hardenability and high-temp strength
Nickel: toughness and low-temp performance
Sulfur/Lead: machinability (but poor weldability)
Common Grades and When You See Them
1018 / 1020: Low-carbon general purpose steel. Easy to machine, easy to weld, case-hardenable but not through-hardenable. Used for shafts, pins, brackets, and any non-critical structural component. This is the steel you grab when the print just says "mild steel."
1045: Medium-carbon steel. Can be through-hardened to 50-55 HRC. Good balance of machinability and strength. Common for shafts, gears, axles, and bolts that need more strength than 1018 but do not require alloy steel properties. Machines well in the annealed condition.
4140: The workhorse alloy steel. Chrome-moly with 0.40% carbon. Through-hardens to 54-59 HRC in small cross-sections. Used for shafts, gears, connecting rods, tooling, and any part that needs high strength and moderate toughness. Pre-hardened 4140 (28-32 HRC) is commonly stocked and machines well with carbide tooling.
4340: The premium alloy steel. Adds nickel to the 4140 base for superior toughness and deeper hardenability. Used for aircraft landing gear, heavy-duty shafts, and any application requiring the highest combination of strength and toughness. More expensive and harder to machine than 4140.
12L14 / 1215: Free-machining steels. Lead and sulfur additions make these the easiest steels to machine — they are the baseline for machinability ratings. Used for high-volume screw machine parts, fittings, and any component where machining cost matters more than strength or weldability.
If the part needs to be heat treated above 30 HRC, 1018 cannot do it (low carbon). Switch to 4140.
If the shaft diameter is over 3 inches and needs uniform hardness, 1018 and 1045 may not through-harden. Switch to 4140 or 4340.
If the part sees impact loading, 1045 may be too brittle after hardening. Switch to 4140 (or 4340 for extreme cases).
Machinability Comparison Tool
Compare machinability ratings, SFM ranges, and chip loads for 30+ metals. Filter by material category, operation type, and tooling. Interactive sortable reference.
Stainless and Tool Steel Grades
303: Free-machining stainless. Contains sulfur for improved chip breaking. The easiest stainless to machine. Use for any stainless part that does not need welding and where moderate (not maximum) corrosion resistance is acceptable.
304: The general-purpose austenitic stainless. 18% chromium, 8% nickel. Excellent corrosion resistance and weldability. Machines poorly compared to carbon steel — stringy chips, work hardening, and aggressive tool wear. Run sharp tools at moderate speeds with heavy feed to stay below the work-hardened layer.
316: Similar to 304 with added molybdenum for improved corrosion resistance in chloride environments (marine, chemical). Machines slightly worse than 304. Used when 304 does not provide adequate corrosion performance.
A2 tool steel: Air-hardening tool steel. 5% chromium. Hardened to 58-62 HRC with minimal distortion (air quench, not oil or water). Used for punches, dies, forming tools, and fixture components that need high hardness and dimensional stability through heat treatment.
D2 tool steel: High-chromium (12%), high-carbon tool steel. Hardens to 58-62 HRC with excellent wear resistance. More wear-resistant than A2 but tougher to machine. Used for long-run stamping dies, slitter knives, and any tool that needs maximum wear life.
• Use sharp, positive-rake inserts (not neutral or negative)
• Feed heavy to stay below the work-hardened layer
• Never let the tool dwell or rub — it work-hardens the surface
• Use high-pressure coolant aimed at the cutting zone
• Expect 40-60% of the SFM you use for carbon steel
AISI/SAE Steel Grade Decoder
Decode AISI/SAE steel grades instantly. Alloy composition, machinability rating, heat treatment notes, weldability, and typical applications for 40 common carbon, alloy, and tool steels.