4340 is the nickel-chromium-molybdenum alloy steel that design engineers pick when a part has to carry heavy loads, absorb impact, and keep working under repeated cyclic stress. If you are looking for a shop that supplies OEM CNC machined 4340 steel in China, this page tells you what the grade is, how the same material is named under different national standards, which parts are most often machined from it, and how a full CNC machining program for 4340 runs from a single prototype to high-volume production.
We machine custom parts from 4340 on a daily basis: round bar, forged blanks, and flame-cut plate, in the annealed, normalized, or quenched-and-tempered condition depending on what the drawing calls for. CNC machined 4340 steel in China is no longer a sourcing compromise. Between the material cost, the machining labor, the heat treatment, and the finishing operations, the total landed cost is usually well below what the same part costs in Europe or North America, and the engineering communication is direct. Send us a drawing, and you deal with the people who actually plan and run the jobs, not a trading desk.
What 4340 steel is and why it is specified
AISI 4340 belongs to the family of low-alloy high-strength steels hardened by nickel, chromium, and molybdenum in combination. Nickel is the toughness element, chromium adds hardenability and strength, and molybdenum refines the grain structure and reduces temper embrittlement. The result is a steel that reaches high tensile strength after oil quenching and tempering, while keeping impact toughness that plain carbon steels at the same strength level simply cannot match.
Typical chemical composition of 4340 (AISI/SAE limits):
| C | Si | Mn | P | S | Cr | Mo | Ni |
| 0.38–0.43% | 0.15–0.35% | 0.60–0.80% | max 0.035% | max 0.040% | 0.70–0.90% | 0.20–0.30% | 1.65–2.00% |
The three properties that make 4340 a workhorse grade:
Deep hardenability. The combined chromium-nickel-molybdenum content lets the steel harden fully through thick sections. A shaft 100 mm or more in diameter can still develop a martensitic structure through the core on an oil quench, which is why 4340 is used where heavy cross-sections have to be strong all the way through, not just at the surface.
Strength and toughness together. Quenched and tempered 4340 typically delivers tensile strength in the range of 930–1,400 MPa depending on the tempering temperature, with yield strength around 800–1,200 MPa, and elongation of roughly 10–18%. Low-temperature toughness is good, which is why the grade appears in aerospace and oilfield applications where parts see cold and impact at the same time.
Fatigue resistance. With proper heat treatment and shot peening, machined 4340 components hold up well under cyclic loading. Crankshafts, drive shafts, and gearbox shafts are specified in 4340 precisely because the fatigue strength stays respectable even after years of service.
From a machining point of view, 4340 behaves well in the annealed or normalized condition. Carbide tooling with rigid setups and steady coolant gives clean results on turning, milling, and drilling. The steel hardens sharply after heat treatment, so the common route is to rough and semi-finish in the soft condition, heat treat, then finish grind the critical diameters and bores. Gear teeth are normally cut before the final hardening pass and ground afterward when the tolerance demands it.
4340 equivalents under different national standards
Because 4340 is specified all over the world, the same chemistry family appears under different designations in different national standards. This matters when you source CNC machined 4340 steel in China: a Chinese drawing will usually call out 40CrNiMoA to GB/T 3077, while an American drawing says AISI 4340 and a European drawing says 1.6582. They are equivalent grades, not identical chemistries, and a machining shop that knows the differences can advise you on which substitute to use and whether any adjustment is needed.
| Country / Region | Standard | Designation | Notes |
|---|---|---|---|
| United States | ASTM A29 / A322 / A331, AISI/SAE | 4340 | The original designation; UNS G43400. The reference grade for the whole family. |
| China | GB/T 3077 | 40CrNiMoA | The standard Chinese equivalent. The suffix “A” marks premium quality with tighter limits and guaranteed mechanical properties; this is the grade Chinese aerospace and defense drawings specify. |
| Germany / Europe | DIN EN 10083-3 | 1.6582 (34CrNiMo6) | The most common European substitute for 4340. Slightly different carbon/nickel balance but the same strength-toughness character and the same heat treatment response. |
| Europe (alternative) | DIN EN | 1.6511 (36CrNiMo4) | Sometimes offered as a leaner near-equivalent; check the drawing requirements before substituting, because the nickel content sits at the lower end. |
| Japan | JIS G4108 | SNCM439 | Japanese equivalent, widely used in Japanese construction and mining machinery; composition ranges are close to AISI 4340. |
| United Kingdom | BS 970 | 817M40 (EN24) | The long-standing British equivalent; EN24 in the quenched-and-tempered condition is a direct stand-in for 4340 in most applications. |
| Russia | GOST 4543 | 40KhN2MA (40ХН2МА) | Russian designation for the same Ni-Cr-Mo family, used in heavy machinery and oilfield equipment. |
In practice the differences between these designations are small: the element ranges shift slightly from one standard to the next, the sulfur and phosphorus limits differ, and the Chinese “A” grade carries the premium-quality clause. Heat treatment response is essentially the same, so a part machined from 40CrNiMoA in China will meet a drawing written to AISI 4340 or EN 1.6582, provided the machinist checks the specified mechanical property requirements rather than assuming the grade names are interchangeable. When we quote a job we state which standard the material will be supplied to, so there is no ambiguity on the shop floor or at your end.
Mechanical properties after heat treatment
4340 is almost always delivered to the machine shop in the annealed or normalized condition, machined, then hardened and tempered to the specified strength range. The heat treatment route for a typical 4340 component: austenitize around 840–860 °C, quench in oil, then temper between 200 °C and 600 °C depending on the target hardness. Lower tempering temperatures give hardness above 50 HRC with maximum strength, higher tempering temperatures give roughly 28–35 HRC with the best toughness and ductility.
| Condition | Tensile strength | Yield strength (0.2%) | Elongation | Hardness |
|---|---|---|---|---|
| Annealed / normalized (as supplied for machining) | 650–850 MPa | 470–680 MPa | 18–25% | Typically 200–280 HBW depending on mill practice |
| Quenched and tempered | 930–1,400 MPa | 800–1,200 MPa | 10–18% | Approx. 28–55 HRC depending on tempering temperature |
| High-strength condition (low temper) | Up to 1,600–2,000 MPa in fully hardened thin sections | 1,400–1,800 MPa | 10–15% | 50–55 HRC |
Two additional treatments are worth knowing about when you plan a 4340 part:
Induction hardening. For parts that need a hard wear surface on top of a tough core, such as axle shafts and gear shafts, the bearing journals and spline sections can be induction hardened locally after the core has been quenched and tempered. This keeps the core tough while the loaded surface reaches high hardness.
Nitriding. 4340 responds very well to nitriding. A nitrided case typically reaches well over 55 HRC equivalent, with excellent wear resistance and improved fatigue strength, and because nitriding is a low-temperature process, distortion is minimal compared with through hardening. Nitrided 4340 spindles and valve stems are a common combination.
When we machine CNC 4340 steel parts, the heat treatment step is normally handled in our own contracted furnace network with hardness testing after tempering, so the drawing requirement is verified before the finishing grind starts, not discovered later at your incoming inspection.
Common CNC machined 4340 steel parts and the industries that use them
There is no single industry that owns 4340, which is exactly why CNC machined 4340 steel in China is such a steady sourcing category. The grade appears wherever a component carries high loads, sees impact, or rotates under cyclic stress. Here is the breakdown of the parts we machine most often, by part family and by industry.
| Part family | Typical parts | Industries and applications |
|---|---|---|
| Gears and gearbox components | Spur gears, helical gears, spiral bevel gears, pinion gears, ring gears, gear shafts, splined gears | Mining machinery, construction equipment, off-highway vehicles, industrial gearboxes, wind turbine drives |
| Shafts and axles | Drive shafts, output shafts, splined shafts, crankshafts, camshafts, axle shafts, torsion bars, half shafts | Automotive, heavy trucks, agricultural machinery, marine propulsion, material handling |
| Rotors and spindles | Machine tool spindles, motor shafts, pump shafts, rotor shafts, mixer shafts | Machine tools, power generation, pumps and compressors, food and chemical processing |
| Connecting rods and linkage | Connecting rods, tie rods, link arms, rocker arms, clevis pins | Diesel engines, reciprocating compressors, excavators, articulated loaders |
| High-strength fasteners and hardware | High-tensile bolts, studs, pins, dowels, threaded rods | Structural steelwork, oil and gas piping, heavy equipment assembly |
| Hubs, couplings and flanges | Gear coupling hubs, flange couplings, drive flanges, yokes, universal joint crosses, bearing housings | Transmission systems, rolling mills, conveyors, marine drives |
| Aerospace and defense components | Landing gear components, actuator shafts, helicopter rotor spindles, high-stress fittings and brackets | Aircraft landing gear, flight control actuators, rotorcraft, defense equipment |
| Oil and gas components | Drill collars, tool joints, valve stems, wellhead components, mud pump shafts | Drilling, well completion, production equipment, downhole tools |
| Mining and quarry parts | Bucket pins, crusher shafts, screen shafts, wear shafts, idler shafts | Surface mining, crushing and screening plants, quarry operations |
| Motorsport and performance parts | Half shafts, suspension arms, uprights, connecting rods, hub carriers | Racing cars, performance vehicles, prototype development |
To make the keyword picture concrete: CNC machined 4340 steel gears are our most requested item after shafts. A typical order for CNC 4340 steel shafts runs from one prototype shaft to five hundred production shafts. CNC machined 4340 steel pinions, CNC 4340 steel axle shafts, CNC 4340 steel connecting rods, CNC 4340 steel spindles, and CNC 4340 steel couplings all come through the shop on a regular schedule, so the tooling, the programs, and the process know-how are already in place for any new drawing in these families.
From a single piece to mass production
The most common question we get from overseas buyers is whether a Chinese CNC shop will bother with a one-off prototype. The answer is yes, and it matters more than the part itself: a prototype confirms the material behavior, the heat treatment result, the tolerance achievement, and the finish, before you commit to a production quantity. Our batch range for CNC machined 4340 steel in China covers the full spectrum:
Single-piece and prototype work. One part, machined from 4340 bar or a supplied forging, with full drawing review before cutting. Prototype lead times are short because the job runs on flexible CNC machines rather than dedicated tooling. We report back on anything in the drawing that is difficult or unnecessary to hold, so you can correct the design before it becomes an expensive habit in production.
Small batches. Runs of 5 to 50 pieces are handled on the same flexible machines with CAM programs saved for repeat orders. Small-batch work is where material condition, tool selection, and cutting parameters get locked in.
Medium and large batches. From 50 to several thousand pieces, we move to dedicated fixtures, optimized tool paths, and in-process inspection at defined intervals. Repeatability between batches is the priority here: the same program, the same fixtures, the same subcontract heat treatment line, so the second order matches the first.
Mass production. For annual requirements in the thousands of pieces, we run scheduled production with raw material stocking, agreed lead times, and packing to your specification. This is where the cost advantage of machining 4340 in China really shows, because the material cost, machining labor, heat treatment, and finishing are all handled inside one supply chain with no handoffs between middlemen.
The machining processes available for 4340 parts in our shop include CNC turning and milling (3-axis, 4-axis, and 5-axis), drilling and tapping, gear hobbing, spline milling and broaching, keyway cutting, internal and external thread milling, cylindrical and surface grinding, and EDM for complex internal features. Secondary operations are managed in-house or through long-standing subcontractors: normalizing and annealing, oil quenching and tempering, induction hardening, nitriding, shot peening, balancing, phosphating, black oxide, zinc and nickel plating, and protective painting for export.
Working with an OEM partner for CNC machined 4340 steel in China
Getting a useful quotation is straightforward. Send us the following and we will come back with a process route, a price, and a delivery date:
The drawing. PDF, DWG, or STEP. If you only have a sketch or an existing part, we can work from that and confirm the critical dimensions with you before cutting.
The material specification. State the grade and standard, for example AISI 4340, GB/T 3077 40CrNiMoA, EN 1.6582, or JIS SNCM439. If you have no preference, we will recommend the grade that is most economical for your quantity and application, usually 40CrNiMoA or 4340 round bar for machined components.
The heat treatment and hardness requirement. This is the specification that most people forget, and it is the one that most affects performance. Tell us the target hardness in HRC or HBW, the required strength range, or simply the application, and we will set the temper accordingly.
Tolerances and surface finish. Standard machining tolerances are around IT7–IT8, and grinding gets you to IT5–IT6 on diameters and bores. Surface roughness down to Ra 0.4 µm is achievable with grinding; Ra 0.8 µm is routine from turning. If your drawing does not state these, we quote to sensible defaults and tell you what they are.
Quantity and delivery target. Single piece, trial batch, or annual volume. We quote per piece with the quantity break clearly shown, so you can see exactly what the volume buys you.
On the quality side, every production run is checked against the drawing: first article inspection, dimensional reports, hardness verification after heat treatment, and surface roughness measurement on the critical faces. Parts are individually checked before packing, and the packing itself is export-grade: rust protection, wooden cases or pallets for heavy parts, and full shipping documentation for sea or air freight.
Why OEM buyers choose CNC machined 4340 steel in China
The reasons are the same ones that apply across the whole alloy steel machining category, but they are stronger for 4340 specifically because the material and its heat treatment are so established in China:
One supplier for the whole route. Material sourcing, CNC machining, heat treatment, grinding, surface treatment, and packing sit in one program. You send one drawing and receive one finished part, with no risk of a middleman losing the specification between steps.
Engineers who understand the grade. 40CrNiMoA has been a standard grade in Chinese heavy industry for decades. The people quoting your job know how it cuts, how it moves in heat treatment, and where to leave grinding allowance, which shows up in the quality of the finished part.
Cost that scales with quantity. A single prototype of a CNC machined 4340 steel gear or shaft is already competitive, and the per-piece price drops steadily as the quantity rises, because the setup cost is amortized and the machining time is optimized. For annual volumes in the thousands, the total cost including freight is typically a fraction of the domestic European or American price for the same part.
Flexible batch sizes. Most overseas buyers start with one or two prototypes, test them in service, then scale. We are structured for exactly that pattern and keep the program files so the third order looks like the first.
Direct communication. English-speaking project engineers handle the correspondence, and the drawing and specification do the talking. No one else in the chain reinterprets your requirements.
If you are evaluating suppliers for CNC machined 4340 steel in China, the fastest way to test us is a small job: send a drawing, get a quote, machine a sample, and put it in service. The material behavior, the heat treatment result, and the machining quality are all visible in the first part, and the volume pricing is visible in the first quotation.
Send your drawing today for a quotation on CNC machined 4340 steel in China, from a single prototype shaft or gear to a full production program.
We provide custom machined parts, 5-axis linkage CNC machining, and the design, production, and assembly of machinery and equipment.