If Q235 is the general-purpose structural steel of China, then 45# is the workhorse of medium-diameter (M16-M24) and machined Class 8.8 fasteners. You’ll encounter it constantly — on quotes for bolts that are too large for ML35, in heat treatment specifications for machined parts, and on mill certificates from every major steel producer.
But what exactly is 45# steel? How strong is it after heat treatment? And why is it the go-to choice for millions of 8.8-grade fasteners in the M16 to M24 range?
What Does 45# Mean?
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45 = Average carbon content in hundredths of a percent (0.01%)
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# = Chinese shorthand for “grade” (often omitted in technical documents)
So 45# steel contains approximately 0.45% carbon. It is classified as a high-quality carbon structural steel under Chinese standard GB/T 699. International equivalents include:
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ASTM/SAE: 1045
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JIS: S45C
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DIN/EN: C45
Chemical Composition
The table below shows the chemical composition ranges for 45# steel based on GB/T 699. Note that actual values vary by mill and heat—always refer to the mill certificate.
| Element | Symbol | Composition Range | Notes |
|---|---|---|---|
| Carbon | C | 0.42 – 0.50% | Core strengthening element; defines the grade |
| Silicon | Si | 0.17 – 0.37% | Improves strength and deoxidation |
| Manganese | Mn | 0.50 – 0.80% | Enhances hardenability and toughness |
| Phosphorus | P | ≤ 0.035% | Impurity; strictly controlled |
| Sulfur | S | ≤ 0.035% | Impurity; strictly controlled |
| Chromium | Cr | ≤ 0.25% | Residual element, not intentionally added |
| Nickel | Ni | ≤ 0.25% | Residual element, not intentionally added |
| Copper | Cu | ≤ 0.25% | Residual element, not intentionally added |
Mechanical Properties
For 45# steel in the as-supplied condition (typically hot-rolled or annealed), GB/T 699 specifies:
| Property | Value |
|---|---|
| Tensile Strength (σb) | ≥ 600 MPa |
| Yield Strength (σs) | ≥ 355 MPa |
| Elongation (δ5) | ≥ 16% |
| Reduction of Area (ψ) | ≥ 40% |
| Impact Energy (Aku2) | ≥ 39 J |
| Hardness (as-rolled) | ≤ 229 HB |
| Hardness (annealed) | ≤ 197 HB |
Hardness After Heat Treatment
45# steel is almost always heat-treated (quenched and tempered) for fastener applications. Typical hardness values:
| Condition | Hardness Range |
|---|---|
| As-quenched (before tempering) | HRC 56-59 (can reach HRC 62) |
| Quenched and tempered (typical) | HRC 22-34 |
| Quenched and tempered (higher strength) | HRC 42-46 |
Summary: 45# at a Glance
| Property | Value |
|---|---|
| Standard | GB/T 699 |
| International Equivalents | SAE 1045, JIS S45C, DIN C45 |
| Carbon Range | 0.42-0.50% |
| Yield Strength (min) | 355 MPa (as-supplied) |
| Tensile Strength (min) | 600 MPa (as-supplied) |
| After Heat Treatment | Achieves Class 8.8 requirements |
| Key Limitation | Low hardenability—not for large diameters |
| Fastener Use | 8.8-grade bolts (≤M16), machined fasteners |
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