Engineering Steels (4140, 4340, 300M, EN26)
Engineering alloy steels like 4140, 4340, 300M, and EN26 are the workhorses of high-stress mechanical components. These chromium-molybdenum and nickel-chromium-molybdenum steels are designed for applications where fatigue life, impact toughness, and through-hardening capability are critical. These grades are most commonly used for shafts, axles, gears, stub axles, spindles, and drivetrain components in automotive, motorsport, mining, and industrial machinery. They are supplied as round bar or forging blanks and CNC machined to final dimensions, then heat treated (quenched and tempered) to achieve the required mechanical properties. The key difference between these grades is their hardenability (how thick a section can be through-hardened) and toughness at high strength levels. 4140 is the general-purpose workhorse, 4340 and EN26 step up for larger sections and higher stress, and 300M is the premium choice for the most demanding applications.
Key Properties
| Density | 7.85 g/cm³ |
| Tensile Strength | 850–2100 MPa (Q&T, varies by grade) |
| Yield Strength | 700–1700 MPa (Q&T, varies by grade) |
| Elongation | 10–18% (Q&T) |
| Melting Point | ~1,430°C |
| Magnetic | Yes |
| Hardenable | Yes — all grades quench and temper |
Grades
| Property | 4140 (AISI 4140 / AS 1444-4140) | 4340 (AISI 4340 / AS 1444-4340) | EN26 (BS 826M40 / 4340 equivalent) | 300M (AISI 4340 modified) |
|---|---|---|---|---|
| Tensile Strength | 850–1000 MPa | 1000–1200 MPa | 1000–1150 MPa | 1900–2100 MPa |
| Yield Strength | 700–850 MPa | 860–1000 MPa | 850–950 MPa | 1550–1700 MPa |
| Elongation | 15–18% | 12–15% | 13–16% | 10–12% |
| Max Hardened Section | ~50mm dia | ~100mm dia | ~100mm dia | ~150mm dia |
| Typical Use | Shafts, gears, axles, bolts, studs | Heavy-duty shafts, landing gear, axles | Heavy shafts, gears, con rods, mining | Aircraft landing gear, race axles, F1 |
4140 (AISI 4140 / AS 1444-4140)
The general-purpose engineering alloy steel. Chromium-molybdenum (1% Cr, 0.2% Mo) gives good hardenability for sections up to ~50mm diameter. Excellent balance of strength, toughness, and machinability. The most readily available and cost-effective of the engineering steels. Widely used for shafts, gears, axles, connecting rods, and general high-stress components. Machine in the annealed or normalised condition, then quench and temper to final properties.
4340 (AISI 4340 / AS 1444-4340)
A nickel-chromium-molybdenum steel (1.8% Ni, 0.8% Cr, 0.25% Mo) with significantly better hardenability and toughness than 4140. The nickel content allows through-hardening of much larger sections — up to ~100mm diameter. Higher impact toughness at equivalent strength levels makes it the standard for heavy-duty shafts, aircraft landing gear, power transmission components, and any application where 4140 isn’t quite strong enough or the section is too large for 4140 to through-harden.
EN26 (BS 826M40 / 4340 equivalent)
EN26 is essentially the British/Australian designation for a 4340-class steel (2.5% Ni, 0.65% Cr, 0.55% Mo). Very commonly specified in Australian engineering — if a drawing calls for EN26, it’s asking for a 4340-equivalent. Excellent hardenability and toughness for large-section shafts, gears, connecting rods, and heavy machinery components. Often specified in mining, agricultural, and industrial equipment where reliability is critical.
300M (AISI 4340 modified)
300M is a vacuum-melted modification of 4340 with added silicon (1.6% Si) and vanadium. This produces dramatically higher strength while maintaining good toughness — nearly double the tensile strength of standard 4340. Used in the most demanding aerospace and motorsport applications: aircraft landing gear, Formula 1 and V8 Supercars drivetrain components, high-performance axle shafts, and critical fasteners. Expensive and requires careful heat treatment, but nothing else matches its combination of ultra-high strength and fracture toughness.
Available Thicknesses
Round bar: 20mm to 300mm+ diameter. Available in hot rolled, cold drawn, or bright (ground) finish. Forging blanks to suit. Stock availability varies by grade — 4140 is readily available, 4340/EN26 may require ordering, 300M is specialist supply.
Fabrication Notes
Surface Finishes
Hard chrome plating (bearing and seal surfaces), nitriding (surface hardness without distortion), black oxide (mild corrosion protection), zinc plating, phosphate coating, raw with light oil.
When to Choose Engineering Steels (4140, 4340, 300M, EN26)
Choose 4140 for general-purpose shafts, gears, and components up to ~50mm section where high strength is needed. Step up to 4340 or EN26 for larger sections (50–100mm), higher toughness requirements, or when the drawing specifies it. Choose 300M only for the most extreme applications where ultra-high strength with good toughness is essential — typically aerospace and top-tier motorsport.
Limitations
All grades require heat treatment to achieve their full potential — in the as-supplied (normalised) condition they are strong but not exceptional. Heat treatment adds cost and lead time. Welding is problematic after heat treatment. 4340, EN26, and especially 300M are more expensive and less readily available than 4140. 300M requires vacuum melting and specialist heat treatment. Larger diameters of 4140 may not through-harden — use 4340 or EN26 for sections over 50mm.
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