The SAE four-digit system for carbon steels encodes the alloy family and the approximate carbon content. Understanding this system tells you immediately what to expect from any 10xx grade steel.[1]
| Digit position | What it encodes | 1018 | 1045 |
|---|---|---|---|
| First digit (1) | Alloy family: 1 = plain carbon steel (no significant alloying beyond C and Mn) | 1 = plain carbon | 1 = plain carbon |
| Second digit (0) | Modification: 0 = no special modification (vs 1 = resulfurized, 2 = resulfurized + rephosphorized, etc.) | 0 = standard | 0 = standard |
| Last two digits | Approximate carbon content in hundredths of a percent | 18 = 0.18% carbon(actual: 0.15–0.20%) | 45 = 0.45% carbon(actual: 0.43–0.50%) |
This is where the practical difference matters most. 1018 is almost always supplied as cold drawn bar. 1045 precision shafting is supplied as TG&P (turned, ground, and polished). These are fundamentally different manufacturing processes with different outcomes.[2]
| Property | 1018 Cold Drawn | 1045 TG&P |
|---|---|---|
| How it's made | Hot rolled bar pulled through a die at room temperature | Hot rolled bar turned on a lathe, then centerless ground to size and polished |
| Diameter tolerance | ±0.002" to ±0.004" (typical) | +0.000" / -0.001" (bearing slip fit) |
| Surface finish | Bright, smooth (good) | Mirror-polished (excellent, 16 Ra or better) |
| Straightness | Good (commercial straight) | Exceptional (precision straightened) |
| Concentricity | Standard | Superior (centerless ground) |
| Residual stress | High — can warp during machining | Low — turned, not drawn through die |
| Bearing fit (as-received)? | No — must be turned to tolerance | Yes — produced undersize for standard bearings |
TG&P bar is produced undersize of nominal, which lends favorably to fitting standard bearings. The ASTM tolerances below apply to carbon, alloy, and stainless TG&P round bar. Many suppliers also offer tighter "precision" tolerances. Note that all tolerances are minus only — the bar is never oversize.[2]
| Diameter range | ASTM tolerance | Typical precision tolerance |
|---|---|---|
| 1-1/2" and under | +0.000" / -0.001" | -0.0005" to -0.0015" |
| Over 1-1/2" to under 2" | +0.000" / -0.0015" | -0.0005" to -0.0020" |
| 2" to under 2-1/2" | +0.000" / -0.0015" | -0.0005" to -0.0020" |
| 2-1/2" to 3" incl. | +0.000" / -0.0020" | -0.0005" to -0.0025" |
| Over 3" to 4" incl. | +0.000" / -0.0030" | -0.0005" to -0.0035" |
| Over 4" to 6" incl. | +0.000" / -0.0040" | -0.0005" to -0.0045" |
| Over 6" to 10" incl. | +0.000" / -0.0050" | +0.000" to -0.0050" |
This is the most important section for engineers designing shafts and specifying material on drawings. When you order 1045 TG&P, the bar arrives with a bearing-quality surface (+0.000" / -0.001"), exceptional straightness, and a polished finish. This means you don't need to turn the entire OD of the shaft — you only need to machine the features.[2]
1018 in cold drawn bar condition. 1045 in TG&P condition. These are the forms you will most commonly order from distributors.[1][2][3]
Toggle between properties. 4140 prehardened included as the alloy steel reference for when carbon steel isn't enough.[1][3]
Carbon content determines what types of heat treatment are possible. This is one of the most important practical differences between 1018 and 1045.[1]
| Heat treatment | 1018 (0.18% C) | 1045 (0.45% C) |
|---|---|---|
| Through hardening (Q&T) | Not practical — too low carbon to achieve useful core hardness | Yes — can reach 50–55 HRC. Q&T to 28–35 HRC for balanced strength and toughness. |
| Induction hardening | Not practical — requires ≥0.40% C for effective case depth | Ideal — 0.45% C produces a hard, wear-resistant surface (55+ HRC) with a tough core. Standard for shafts, gears, and wear surfaces. |
| Case hardening (carburize) | Yes — the standard application. Diffuses carbon into the surface to create a hard case (58–62 HRC) over a tough, low-carbon core. | Possible but rarely done — already has enough carbon for induction hardening, which is faster and cheaper. |
| Flame hardening | Not practical | Yes — localized hardening of bearing journals, gear teeth, and wear surfaces. |
1018 cold drawn and 1045 TG&P in their standard supply conditions. 1045 Q&T and 4140 PH included as the next steps up in strength.[1][2][3]
| Property | 1018 CD | 1045 TG&P | 1045 Q&T (ref) | 4140 PH (ref) |
|---|---|---|---|---|
| Carbon content | 0.15–0.20% | 0.43–0.50% | 0.43–0.50% | 0.38–0.43% |
| Tensile strength | 64 ksi440 MPa | 100 ksi690 MPa | 130 ksi896 MPa | 130 ksi896 MPa |
| Yield strength | 54 ksi370 MPa | 85 ksi586 MPa | 110 ksi758 MPa | 100 ksi690 MPa |
| Elongation | 15% | 12% | 10% | 18% |
| Hardness | 126 HB | 223 HB | 28–35 HRC | 28–32 HRC |
| Diameter tolerance | ±0.002–0.004" | +0.000" / -0.001" | — | ±0.002–0.004" |
| Surface finish | Bright (cold drawn) | Polished (16 Ra or better) | — | Bright (cold drawn) |
| Straightness | Commercial | Precision (minimal TIR) | — | Commercial |
| Residual stress | High (cold worked) | Low (turned, not drawn) | — | High (cold worked) |
| Weldability | Excellent | Limited (preheat required) | Limited | Good (preheat) |
| Through hardenable? | No | Yes (Q&T to 50+ HRC) | Yes | Yes |
| Induction hardenable? | No | Yes (ideal) | Yes | Yes |
| Case hardenable? | Yes (carburize) | Rarely needed | N/A | N/A |
| Bearing-ready as-received? | No | Yes | No | No |
| Relative bar cost | $ | $$ | $$+HT | $$ |
Our team at C&W manufactures shafts from 1045 TG&P daily. We can advise on whether TG&P stock will reduce your cycle time, coordinate induction hardening with certified vendors, and provide complete cert packages. Submit your shaft drawing for a quote.
