annealed to Q&T
through-hardened
in annealed condition
with preheat
Steel 4140: The Most Popular Alloy Steel for CNC Machining
Steel 4140 is a chromium-molybdenum alloy steel. Engineers call it "chromoly." It is one of the most used alloy steels in the world.
Why? Because it does everything well. It is strong, tough, and hard. It machines well in annealed condition. You can heat treat it to reach 148 ksi tensile strength and HRC 54 hardness. You can weld it with preheat. And it costs much less than stainless steel or titanium.
Steel 4140 contains 0.40% carbon, 0.8–1.1% chromium, and 0.15–0.25% molybdenum. The chromium adds hardenability and wear resistance. The molybdenum adds toughness and prevents temper brittleness. Together, they make 4140 one of the most versatile steels you can buy.
If you need a part that takes a beating — gears, shafts, bolts, fixtures, or tooling — steel 4140 is probably your best bet. It gives you high strength at a fair price.
Steel 4140 Material Properties
Here are the key numbers for steel 4140 in both annealed and heat-treated conditions.
| Property | Annealed | Quenched & Tempered |
|---|---|---|
| Tensile Strength | 95 ksi (655 MPa) | 148 ksi (1,020 MPa) |
| Yield Strength | 60 ksi (414 MPa) | 130 ksi (896 MPa) |
| Hardness | HRB 92 (~HRC 13) | HRC 28–32 (up to HRC 54 max) |
| Elongation | 25.7% | 15.5% |
| Density | 0.284 lb/in³ (7.85 g/cm³) | |
| Thermal Conductivity | 42.6 W/mK | |
| Composition | 0.40% C, 0.8–1.1% Cr, 0.15–0.25% Mo, 0.75–1.0% Mn | |
| Machinability | Good | Moderate (below HRC 35) |
| Weldability | Yes — preheat 400–600°F required | |
Heat Treatment Options for Steel 4140
Heat treatment is what makes 4140 special. You machine it soft, then heat treat it hard. Here are the common options.
1 Quench and Temper (Q&T)
The most common heat treatment for 4140. Heat to 1,525°F, quench in oil, then temper at 400–1,200°F depending on your target hardness. Higher temper temperature gives more toughness but lower hardness. Q&T at HRC 28–32 is the standard for gears and shafts.
2 Through Hardening
Same process as Q&T but tempered to a higher hardness (HRC 40–54). Used for parts that need maximum wear resistance. Cross sections up to about 2.5 inches can be through-hardened. Larger sections may not harden all the way through.
3 Case Hardening (Nitriding)
Nitriding creates a very hard surface (HRC 60+) while the core stays tough. The part does not need quenching, so distortion is minimal. Great for shafts, gears, and pins that need a hard wear surface with a shock-resistant core.
4 Induction Hardening
Heats only the surface with an electromagnetic coil, then quenches. Hardens just the outer layer (0.040–0.120" deep) to HRC 50–58. The core stays soft and tough. Best for shafts, pins, and bearing journals that need surface wear resistance.
5 Stress Relief Annealing
Heat to 1,050–1,200°F and slow cool. Removes residual stresses from machining without changing hardness much. Recommended for parts with tight tolerances that will be finish-machined after rough machining.
Steel 4140 vs Other Steels
Not sure if 4140 is the right steel? Here is how it compares to the steels engineers consider most often.
| Property | Steel 4140 | Steel 1018 | Steel 4340 | SS 304 |
|---|---|---|---|---|
| Tensile (ksi) | 95–148 | 63 | 108–163 | 73 |
| Hardness (HRC) | Up to 54 | HRB 71 (not hardenable) | Up to 57 | HRB 92 (not hardenable) |
| Machinability | Good (annealed) | Good | Fair | Moderate |
| Heat Treatable | Yes | No (case harden only) | Yes | No |
| Weldable | Yes (preheat) | Yes (easy) | Yes (preheat) | Yes (easy) |
| Corrosion | Poor (needs coating) | Poor | Poor | Excellent |
| Toughness | Very good | Good | Excellent | Good |
| Cost per Part | $85–$350 | $65–$200 | $100–$400 | $120–$350 |
| Best For | Gears, shafts, tooling | General purpose, pins | Extreme loads, landing gear | Corrosion resistance |
Key takeaway: Steel 4140 sits in the sweet spot. It is much stronger than 1018, cheaper and easier to machine than 4340, and far tougher than stainless 304. If you need a part that is strong, hard, tough, and affordable, 4140 is almost always the right choice. Use 4340 only when you need extreme impact resistance. Use stainless 304 or 316 when you need corrosion resistance. Use titanium Ti-6Al-4V when you need to save weight.
Need Steel 4140 Parts?
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Upload CAD for Instant QuoteIndustries That Use Steel 4140
Steel 4140 shows up everywhere that parts need to be tough and hard. Here are the top industries.
Automotive & Motorsport
Gears, axle shafts, crankshafts, connecting rods, and drivetrain components. Q&T 4140 handles the torque and shock loads in transmissions and differentials. Lower cost than 4340 for most applications.
Oil & Gas
Drill collars, subs, valve bodies, wellhead equipment, and downhole tools. 4140 handles the pressure, vibration, and H2S exposure in drilling operations. Heat treated to HRC 22–32 per NACE MR0175.
Defense & Military
Weapon system components, vehicle brackets, breech blocks, and firing pins. 4140 offers the toughness and hardness needed for high-impact defense parts. Full material traceability on every order.
Tooling & Fixturing
Jig plates, collets, mandrels, mold bases, and machine fixtures. 4140 holds its shape under clamping loads and resists wear from repeated use. Pre-hardened 4140 at HRC 28–32 is ready to use without heat treatment.
Industrial Equipment
Power transmission shafts, couplings, heavy-duty pins, sprockets, and spindles. 4140 handles the constant loads and vibrations in industrial machinery. Induction-hardened bearing journals last for years.
Aerospace
Non-critical structural parts, ground support equipment, tooling, and test fixtures. 4140 offers good strength at a fraction of the cost of titanium Ti-6Al-4V or steel 4340 for parts where weight is not critical.
Design Tips for CNC Machining Steel 4140
Steel 4140 machines well, but a few design choices save time and money.
1 Machine Before Heat Treatment
Always machine 4140 in annealed condition when possible. Annealed 4140 cuts fast and clean. After heat treatment to HRC 28–32, it is still machinable but slower. Above HRC 40, only finish grinding works well. Machine close to final size, heat treat, then finish-grind critical surfaces.
2 Allow for Heat Treat Distortion
Heat treatment can warp parts by 0.001–0.003 inches depending on shape and cross section. Leave 0.005–0.010" stock on critical features for finish machining after heat treatment. Symmetrical designs distort less than asymmetrical ones.
3 Avoid Sharp Corners in Hardened Parts
Sharp inside corners are stress risers in heat-treated 4140. They can crack during quenching. Add a 0.015" radius minimum on all inside corners. Larger radii (0.030"+) are better for parts that will see impact loads.
4 Specify Hardness Range, Not Just Max
Call out a hardness range (e.g., HRC 28–32) instead of a single number. This gives the heat treater room to hit the target. A range of 4 HRC points is standard. Tighter ranges increase cost and lead time.
5 Add Corrosion Protection
Steel 4140 rusts. It needs a protective finish for any outdoor or humid environment. Black oxide is the cheapest option. Zinc plating works for outdoor parts. Nickel plating works for chemical exposure. Powder coat works for cosmetic parts.
Surface Finishes for Steel 4140
Steel 4140 needs a finish for corrosion protection. Here are your options.
- As-machined: Clean tool marks, 125 Ra or better. No corrosion protection. Good for indoor parts or parts that will be painted later.
- Black oxide: Thin chemical conversion coating. Adds mild corrosion resistance and a black color. Low cost. Does not change dimensions.
- Zinc plating: Electroplated zinc layer for outdoor corrosion protection. Clear, yellow, or black chromate finish. Adds 0.0002–0.0005" per side.
- Nickel plating: Electroless nickel for uniform coverage and good chemical resistance. Adds 0.0002–0.001" per side. Can be hardened to HRC 65+ by baking.
- Hard chrome plating: Very hard (HRC 68–72) wear surface for shafts, cylinders, and sliding parts. Adds 0.001–0.010" per side. Ground to final size after plating.
- Powder coat: Durable paint finish in any RAL color. For outdoor or cosmetic parts. Adds 0.002–0.004" per side.
- Parkerizing: Phosphate coating per mil-spec. Dark gray matte finish. Good base for oil or paint. Common for defense applications.
- Bead blasted: Uniform matte texture. Hides tool marks. Often done before coating for better adhesion.
Tolerances for CNC Machined Steel 4140
Steel 4140 holds tight tolerances in both annealed and pre-hardened conditions. Here is what RivCut can hold.
- General features: ±0.005" (±0.127 mm)
- Precision features: ±0.001" (±0.025 mm)
- Critical features: ±0.0005" (±0.013 mm) on ground surfaces
- Surface finish: 32–125 Ra standard, 16 Ra or better on request
- Flatness: 0.001" per inch on ground surfaces
- Thread tolerances: Class 2A/2B standard, Class 3A/3B on request
Important: Heat treatment can cause distortion of 0.001–0.003 inches. If your part needs tight tolerances AND heat treatment, plan for finish machining or grinding after heat treatment. We account for this in our DFM review.
See Your Exact Price for Steel 4140 Parts
Upload your CAD file and get real pricing with heat treatment, finish, and lead time options. Steel 4140 parts typically cost $85–$350.
Upload CAD for Instant QuoteWhy RivCut for Steel 4140 Machining
Steel 4140 is common, but machining it right takes experience. Here is what makes RivCut different.
- Annealed and pre-hardened stock. We keep both annealed and pre-hardened (HRC 28–32) 4140 in stock. No waiting for material. Your part goes into production the same day.
- Heat treatment coordination. We manage the entire process — rough machine, send to heat treatment, finish machine critical features. You get one quote, one order, one shipment.
- Post-heat-treat machining. We finish-grind and finish-machine critical features after heat treatment to hit your tolerances. No need to coordinate between a machine shop and a heat treater.
- 100% made in USA. Every steel 4140 part is machined in our facility in Union City, California. No overseas outsourcing. Full traceability.
- Free DFM review. We catch heat-treat distortion risks, stress risers, and tolerance issues before we cut metal. Most shops charge for this kind of review.
- Material certs on every order. We provide mill test reports (MTRs), certificates of conformance, and heat treatment reports. Required for oil and gas and defense applications.
The RivCut Guarantee
If your steel 4140 parts do not match your drawing, we remake them at no charge. Every dimension on your print is a promise we make. We back that promise with calibrated inspection equipment and a team that takes tolerance seriously.
Related Tools & Articles
Steel 4140 CNC Machining: Common Questions
Related Resources
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