--- title: "Alloy Steel Machining | 4140, 4340, 4130 Chromoly | OpenSpindle" description: "Get quotes for custom alloy steel parts from independent machine shops. 4140, 4340, 4130 chromoly and 8620 machined, turned, and heat-treated. Upload a drawing." canonical: https://openspindle.com/materials/alloy-steel --- # Custom alloy steel parts from independent machine shops Alloy steels like 4140 and 4340 add chromium and molybdenum to plain carbon steel for far higher strength and fatigue life, and they can be heat treated to a target hardness. Upload a drawing and hear back from shops that machine and treat chromoly every day. ## Alloy steel shops on OpenSpindle Real shops that machine, turn, and heat-treat alloy steel. See their equipment and get a competitive quote for your project. [Browse all alloy steel shops](https://openspindle.com/manufacturers?material=4140%2C4340%2C4130) ## Alloy steel grades shops run most The grade sets strength, hardenability, and how it responds to heat treatment. These are the ones you will see quoted. ### 4140 The workhorse chromoly. Strong, tough, and readily heat treated to a target hardness. Machines well in the annealed or pre-hard condition and is stocked everywhere in bar and plate. **Best for:** Shafts, gears, tooling, structural and pressure parts ### 4340 A nickel-chromium-molybdenum steel with higher strength and much better fatigue and impact toughness than 4140, especially in thick sections. Costs more. **Best for:** Highly stressed shafts, landing gear, gears ### 4130 chromoly A lower-carbon chromoly prized for weldability and toughness at moderate strength. The tubing alloy for roll cages, frames, and aerospace structure. **Best for:** Welded tube frames, roll cages, brackets ### 8620 A case-hardening alloy steel. Left soft and tough in the core, then carburized to a hard wear surface. The standard for gears and bearing races. **Best for:** Gears, pins, bearing races, wear parts ### Pre-hard (4140 PH) Supplied already heat treated to roughly 28-32 HRC so parts are strong as-machined with no post-heat-treat distortion or scale. **Best for:** Mold bases, fixtures, parts needing no post-treat ## Material Properties Representative values for 4140 in the quenched-and-tempered condition. Exact numbers depend on grade and heat treatment. | Property | Value | | --- | --- | | Density | 7.85 g/cm³ (0.284 lb/in³) | | Tensile strength (4140 Q&T) | ~150 ksi (1030 MPa) | | Yield strength (4140 Q&T) | ~130 ksi (900 MPa) | | Hardness range (heat treated) | ~28-55 HRC depending on temper | | Max service temp | ~400 °C (750 °F) before tempering back | | Machinability | Good annealed; harder after heat treat | | Corrosion resistance | Poor; will rust, needs oil, coating, or plating | | Weldability | 4130 good; 4140/4340 need preheat and post-weld heat | Alloy steels are usually machined soft, then heat treated to final hardness. Note the target hardness and whether the shop should coordinate treatment. ## Stock Forms - **Round bar** -- Turned shafts, pins, and cylindrical parts. - **Plate & block** -- Milled and 5-axis parts machined from solid. - **Tube** -- DOM and seamless tube for frames and structure. ## Processes That Run This Material - [CNC Turning](https://openspindle.com/capabilities/cnc-turning.md) -- Turned shafts, pins, and fittings from round bar. - [CNC Machining](https://openspindle.com/capabilities/cnc-machining.md) -- Milled prismatic parts, tooling, and blocks. - [CNC Machining · 5-Axis](https://openspindle.com/capabilities/5-axis-machining.md) -- Complex geometry and gears in a single setup. - [Tube Bending](https://openspindle.com/capabilities/tube-bending.md) -- Bent 4130 tube for frames and roll structure. - [Welding](https://openspindle.com/capabilities/welding.md) -- Welded 4130 tube assemblies with proper preheat. ## Finishes Alloy steel rusts, so most parts get a coating or plating unless they run in oil. - **Black oxide** -- A thin conversion coating that adds mild corrosion resistance and a black finish without changing dimensions. - **Zinc / nickel plating** -- Electroplated corrosion protection for fasteners and exposed parts. Nickel is harder and more durable. - **Nitriding** -- A surface hardening treatment that adds wear and fatigue resistance with minimal distortion, common on shafts and gears. - **Powder coat / paint** -- A durable colored coating for structural and cosmetic parts. Mask threads and bearing fits. - **Oil / as-machined** -- Bare parts shipped oiled for short-term rust protection, fine for parts that run in a lubricated assembly. Specify the target hardness and the finish or plating on your drawing, and mask any bearing fits or threads. ## Applications - Automotive & motorsport - Aerospace - Energy / oil & gas - Industrial / OEM - Robotics & automation - Tooling & dies Alloy steel is the default when a part needs strength, fatigue life, and hardness at reasonable cost. ## How It Compares How alloy steel compares to the other metals shops quote most. ### [Carbon Steel](https://openspindle.com/materials/carbon-steel.md) - Alloy steel is stronger, tougher, and hardens deeper thanks to its chromium and molybdenum - Plain carbon steel (1018, 1045) is cheaper and fine for lower-stress parts - Both rust and need a coating; alloy steel justifies its cost on stressed or heat-treated parts **Verdict:** Choose alloy steel for stressed, fatigue-loaded, or heat-treated parts. Choose carbon steel for general low-stress work at lowest cost. ### [Stainless Steel](https://openspindle.com/materials/stainless-steel.md) - Stainless resists corrosion without a coating; alloy steel must be plated or oiled - Alloy steel reaches higher strength and hardness per dollar and machines more predictably - Stainless is the choice for hygiene and wet or chemical environments **Verdict:** Choose alloy steel for maximum strength at low cost in dry or coated service. Choose stainless when corrosion resistance is the priority. ## Frequently Asked Questions ### What is the difference between 4140 and 4340? 4140 is the all-around chromoly: strong, tough, readily heat treated, and inexpensive. 4340 adds nickel for higher strength and much better fatigue and impact toughness, especially in thick sections, at a higher price. Use 4140 for most shafts and tooling and 4340 for the most highly stressed parts like landing gear and heavy shafts. ### Can alloy steel be heat treated to a specific hardness? Yes. Alloy steels are quenched and tempered to a target hardness, typically given in HRC. Parts are usually machined soft and then heat treated, or bought pre-hard (about 28-32 HRC) to skip post-treat distortion. Note the target hardness on your drawing and whether the shop should coordinate treatment. ### Is chromoly the same as alloy steel? Chromoly (chromium-molybdenum) is a common family of alloy steels, of which 4130, 4140, and 4340 are the best known. When people say chromoly they usually mean 4130 tube or 4140 bar. All are low-alloy steels that heat treat well. ### Does alloy steel rust? Yes. Alloy steels have little corrosion resistance and will rust without protection, so most parts are black-oxided, plated, nitrided, painted, or run in oil. If your part is exposed to moisture, specify a finish; if corrosion resistance is critical, consider stainless instead. ### Can 4130 be welded? 4130 is the most weldable of the common chromoly grades and is used for welded tube frames and roll cages, though it benefits from preheat and controlled cooling. 4140 and 4340 can be welded but need preheat and post-weld heat treatment to avoid cracking. Note welded joints on your drawing. ### What files should I send for an alloy steel part? STEP, IGES, X_T, DWG, DXF, or a dimensioned PDF. Include the grade (for example 4140), the target hardness and heat-treat condition, critical dimensions and tolerances, any finish or plating, and quantity. ## Related Materials - [Carbon Steel](https://openspindle.com/materials/carbon-steel.md) - [Stainless Steel](https://openspindle.com/materials/stainless-steel.md) - [Aluminum](https://openspindle.com/materials/aluminum.md) - [Titanium](https://openspindle.com/materials/titanium.md) - [Inconel](https://openspindle.com/materials/inconel.md) - [Bronze](https://openspindle.com/materials/bronze.md) ## Get alloy steel part quotes Upload a drawing once and compare quotes from independent shops that machine and heat-treat 4140, 4340, and 4130.