What Is Carbon Steel?
Carbon steel is the simplest form of steel. It is made of iron and carbon with very small amounts of manganese and silicon. No extra metals are added on purpose. The carbon content is what controls the hardness and strength.
Carbon steel is sorted into three groups based on how much carbon it contains:
- Low carbon (mild): 0.05% to 0.25% carbon. Example: 1018 steel. Soft, easy to weld, easy to machine. Tensile strength around 64,000 psi.
- Medium carbon: 0.25% to 0.60% carbon. Example: 1045 steel. Stronger, can be heat treated. Tensile strength around 91,000 psi.
- High carbon: 0.60% to 1.00% carbon. Example: 1095 steel. Very hard, used for springs and blades. Difficult to weld.
The biggest advantages of carbon steel are low cost and easy machining. For parts that do not need extreme strength, carbon steel is often the best choice.
What Is Alloy Steel?
Alloy steel starts with the same iron and carbon base. But it has other metals added on purpose. These alloy elements include chromium (Cr), molybdenum (Mo), nickel (Ni) and vanadium (V). Each element does a specific job:
- Chromium: Increases hardness and wear resistance. Makes the steel respond better to heat treatment.
- Molybdenum: Boosts high-temperature strength and prevents temper brittleness.
- Nickel: Increases toughness and impact resistance, especially in cold temperatures.
- Vanadium: Refines grain structure and improves fatigue resistance.
These additions make alloy steel stronger, tougher and more versatile than plain carbon steel. The tradeoff is higher cost and sometimes harder machining.
The 4-digit AISI/SAE numbering system tells you what is in the steel. The first two digits show the alloy group (10xx = carbon, 41xx = Cr-Mo, 43xx = Ni-Cr-Mo). The last two digits show the carbon content in hundredths of a percent.
Side-by-Side Comparison Table
| Property | 1018 (Carbon) | 1045 (Carbon) | 4140 (Alloy) | 4340 (Alloy) | 8620 (Alloy) |
|---|---|---|---|---|---|
| Carbon Content | 0.18% | 0.45% | 0.40% | 0.40% | 0.20% |
| Key Alloy Elements | None | None | Cr, Mo | Ni, Cr, Mo | Ni, Cr, Mo |
| Tensile Strength (psi) | 64,000 | 91,000 | 95,000 - 148,000 | 145,000 - 160,000 | 90,000 (core) |
| Hardness (HRC, heat treated) | N/A (case harden only) | 50 - 55 | 28 - 40 | 38 - 45 | 58 - 62 (case) |
| Machinability Rating | 78% (excellent) | 57% (good) | 66% (good) | 50% (moderate) | 66% (good) |
| Weldability | Excellent | Good (preheat needed) | Fair (preheat + PWHT) | Poor (preheat + PWHT) | Good (preheat needed) |
| Cost (per lb, bar stock) | $1.50 - $2.50 | $1.75 - $3.00 | $2.50 - $4.00 | $4.00 - $6.00 | $2.50 - $3.50 |
| Best For | Brackets, pins, fixtures | Shafts, bolts, studs | Gears, axles, tooling | Landing gear, crankshafts | Gears, camshafts, pinions |
Grades Breakdown
1018 Steel (Low Carbon)
1018 is the workhorse of mild steel. It machines fast, welds easily and costs very little. The tensile strength is about 64,000 psi. It is not strong enough for heavy loads, but it is perfect for brackets, spacers, dowel pins and fixture plates. Many machine shops keep 1018 round bar and flat bar in stock at all times.
1045 Steel (Medium Carbon)
1045 has more carbon (0.45%) which makes it stronger and harder. It can be through-hardened by quench and temper to about 50 to 55 HRC. Tensile strength reaches 91,000 psi in the annealed state. It is a good choice for shafts, bolts, studs and axles that need moderate strength without the cost of alloy steel.
4140 Steel (Chromium-Molybdenum Alloy)
4140 is the most popular alloy steel for CNC machining. The chromium and molybdenum give it excellent strength after heat treatment. Tensile strength ranges from 95,000 to 148,000 psi depending on the heat treat condition. It is used for gears, axle shafts, hydraulic cylinders and tooling. Most shops order 4140 in the pre-hardened (28-32 HRC) condition to skip the heat treatment step.
4340 Steel (Nickel-Chromium-Molybdenum Alloy)
4340 is the strongest standard alloy steel. The nickel addition gives it excellent toughness and fatigue resistance. Heat treated, it reaches 145,000 to 160,000 psi tensile strength. Aerospace companies use it for landing gear, crankshafts and high-stress structural parts. It costs more and machines slower than 4140, so only use it when you need the extra strength.
8620 Steel (Case-Hardening Alloy)
8620 is designed for case hardening (carburizing). The low carbon core stays tough while the carburized surface gets extremely hard at 58 to 62 HRC. This gives you the best of both worlds: a hard wear surface with a shock-absorbing core. Gears, pinions, camshafts and bearing races are the classic 8620 applications.
Heat Treatment
Heat treatment is the biggest reason engineers pick alloy steel over carbon steel. Here is how each type responds:
- 1018: Cannot be through-hardened. Can only be case hardened (carburized) to add a thin hard shell. The core stays soft at about 15 HRC.
- 1045: Can be through-hardened by quench and temper. Reaches 50 to 55 HRC. Good for moderate-strength parts.
- 4140: Excellent heat treatment response. Quench and temper to 28 to 40 HRC for most applications. Can go higher for special tooling.
- 4340: Best heat treatment response of any common steel. Reaches 38 to 45 HRC with superb toughness. Through-hardens in thick sections up to 4 inches.
- 8620: Designed for case hardening. Carburize at 1,700 degrees F, then quench. Surface reaches 58 to 62 HRC while the core stays at 25 to 35 HRC.
Machinability
Machinability measures how easy a metal is to cut. Higher numbers mean faster cutting, better surface finish and longer tool life. The baseline is 1212 free-machining steel at 100%.
1018 rates at about 78%. It cuts clean and fast. Most shops can hit 125 Ra surface finish without slowing down. 4140 in the annealed state rates at 66%, which is still good. Pre-hardened 4140 (28-32 HRC) is a bit tougher on tools but still very machinable. 4340 at 50% is the hardest to cut in this group. It needs slower speeds, sharper tools and more coolant.
As a general rule, carbon steels machine 20 to 30 percent faster than alloy steels. This means lower CNC machining costs per part. If you do not need the strength of alloy steel, use carbon steel and save money.
When machining pre-hardened 4140, use coated carbide inserts (TiAlN or AlCrN coating) and flood coolant. Keep surface speed at 300 to 400 SFM. This prevents work hardening and gives you a good surface finish.
Cost Comparison
Carbon steel is about 20 to 30 percent cheaper than alloy steel. Here is why:
- Raw material: 1018 bar stock runs $1.50 to $2.50 per pound. 4140 runs $2.50 to $4.00 per pound. 4340 costs $4.00 to $6.00 per pound.
- Machining time: Carbon steel cuts faster, which means less machine time and lower labor cost per part.
- Heat treatment: Most carbon steel parts do not need heat treatment. Alloy steel parts often do, adding $5 to $20 per part.
- Tool wear: Carbon steel is easier on cutting tools. Alloy steel wears tools faster, raising the tooling cost per part.
Total part cost for a typical bracket made from 1018 might be $25. The same bracket from 4140 might cost $35 to $40. Only use alloy steel when the strength or heat treatment is truly needed.
Applications by Industry
- Aerospace: 4340 for landing gear, actuator rods and high-stress fittings. 4140 for hydraulic manifolds and structural brackets.
- Automotive: 4140 for axle shafts and crankshafts. 8620 for transmission gears and ring gears. 1045 for drive shafts.
- Defense: 4340 for gun barrels and receiver housings. 4140 for armor-piercing components and mount brackets.
- Industrial: 1018 for fixture plates, spacers and jig components. 1045 for machine shafts and coupling hubs.
- Robotics: 4140 for precision gears and actuator housings. 1018 for base plates and non-structural frames.
How to Choose the Right Steel
Ask yourself these five questions:
- What is the load? If the part sees high stress or cyclic fatigue, pick alloy steel (4140 or 4340). For low-stress parts, carbon steel (1018 or 1045) works fine.
- Do you need heat treatment? If you need through-hardening above 35 HRC, you need alloy steel. If you only need case hardening, 8620 or even 1018 can work.
- Is weldability important? Carbon steels like 1018 weld with no special steps. Alloy steels need preheat and post-weld heat treatment.
- What is the budget? If cost is the top concern and strength requirements are modest, use 1018 or 1045.
- What surface finish do you need? Read our surface finish guide to match the right steel to your finish requirements.
At RivCut, we machine all of these steel grades every week. Upload your CAD file and tell us your strength and budget requirements. We will recommend the best grade for your project.
Frequently Asked Questions
What is the difference between alloy steel and carbon steel?
Carbon steel contains mostly iron and carbon with very small amounts of other elements. Alloy steel has larger amounts of chromium, molybdenum, nickel, or vanadium added on purpose to improve strength, toughness, or heat treatment response.
Is 4140 alloy steel or carbon steel?
4140 is an alloy steel. It contains about 1% chromium and 0.2% molybdenum in addition to 0.40% carbon. These alloy elements make it much stronger and more heat-treatable than plain carbon steel.
Which is cheaper, alloy steel or carbon steel?
Carbon steel is about 20 to 30 percent cheaper than alloy steel. For example, 1018 bar stock costs roughly $1.50 to $2.50 per pound, while 4140 costs about $2.50 to $4.00 per pound.
Can you weld carbon steel to alloy steel?
Yes, but it requires care. Alloy steels like 4140 and 4340 need preheat and post-weld heat treatment to avoid cracking. Carbon steels like 1018 weld easily with no special steps.
What is the strongest common CNC steel?
4340 alloy steel is the strongest common CNC steel. Heat treated, it reaches 145,000 to 160,000 psi tensile strength. It is widely used for gears, shafts and landing gear in aerospace.
Is 1018 steel good for CNC machining?
Yes. 1018 is one of the easiest steels to machine. It cuts fast, produces clean surface finishes and is very affordable. It is great for brackets, pins and fixtures that do not need high strength.
What is 8620 steel used for?
8620 is a case-hardening alloy steel. After carburizing, the surface gets very hard (58 to 62 HRC) while the core stays tough. It is used for gears, pinions, camshafts and bearing races.
Does alloy steel rust?
Yes. Standard alloy steels like 4140 and 4340 do rust. They are not stainless. You need a protective finish like black oxide, zinc plating, or powder coat to prevent corrosion.
Can you heat treat carbon steel?
It depends on the carbon content. Low carbon steel like 1018 (0.18% C) does not harden well by quenching. Medium carbon steel like 1045 (0.45% C) can be through-hardened to about 55 HRC.
