Alloy Steel
CNC Machining

4140 alloy steel (also known as 42CrMo or SCM440) is a chromium-molybdenum alloy steel with excellent strength, toughness, and fatigue resistance. It is heat-treatable to a wide hardness range (HRC 20–54). In CNC machining it is widely used for gears, shafts, spindles, bolts, hydraulic tool bodies, dies, and any load-bearing component that requires a combination of high strength and good machinability.

4140 is a chromium-molybdenum alloy steel offering good strength and toughness at a competitive cost — the most common alloy steel in general industrial machining. 4340 adds nickel (1.65–2.00%) to the Cr-Mo base, producing significantly higher hardenability, deeper through-hardening on large cross-sections, and better fatigue strength at elevated loads. 4340 is specified where 4140 cannot achieve the required through-hardness on thick or large parts — common in aerospace forgings, large-section shafts, and high-fatigue structural parts.

42CrMo is the Chinese GB/T standard designation for a Cr-Mo alloy steel that is nearly equivalent to AISI 4140. The '42' refers to the nominal carbon content of approximately 0.42%, 'Cr' indicates chromium, and 'Mo' indicates molybdenum. SCM440 is the Japanese JIS standard equivalent. All three — 4140, 42CrMo, and SCM440 — are functionally interchangeable for most machining applications, though minor composition differences mean exact equivalency depends on the specific property requirements.

Yes, with limitations. Alloy steels like 4140 and 4340 in annealed or pre-hardened condition (HRC 28–36) are regularly CNC machined. Parts that require final hardness above HRC 45 are typically machined soft first, then heat-treated, then finish-ground on critical dimensions. We coordinate the full sequence — rough machining, heat treatment, and post-HT precision grinding — as a single-source managed process.

Common surface treatments for alloy steel CNC parts include: black oxide (minimal dimensional change, light rust protection), electroless nickel (uniform corrosion and wear resistance), hard chrome (wear-critical shaft surfaces), zinc plating (general corrosion protection for brackets and fasteners), and nitriding or case hardening as post-heat-treatment finishing for wear surfaces. We manage black oxide, nickel, zinc, and chrome as coordinated finishes.

Standard CNC machining tolerance for alloy steel is ±0.01 mm. For precision bore fits (H7, H6) and shaft diameters (h6, g6), we hold these tolerances routinely in production. After heat treatment and grinding, we achieve ±0.005 mm and tighter on critical journal diameters and bearing seat features. CMM inspection reports are available for all precision alloy steel orders.

8620 is a low-alloy nickel-chromium-molybdenum case-hardening steel. It has a soft, tough core and can be case-hardened (carburized) to a hard outer surface (HRC 58–62) while retaining a ductile core. It is used for gears, pinions, camshafts, and drive components where a hard wear-resistant surface combined with a tough impact-resistant core is required — applications where through-hardening alone would make the part too brittle.

Yes. Mill certificates with heat number, chemical composition, and mechanical properties are available for all alloy steel orders. For aerospace and defense orders, we supply third-party material test reports and can source to specific material standards (AMS 6382 for 4140, AMS 6414 for 4340, etc.) on request.