Carbon Steel Grades for CNC Machining: Low, Medium & High Carbon Steel Comparison

Low Carbon/Mild Steel CNC machining: SAE 1018 vs SAE vs 1020, 1215 vs 12L14

SAE 1018 (EN C22 / 1.0402)

SAE 1018 is the most widely used mild steel for CNC machining. It has a relatively soft matrix and excellent weldability, but can’t be through-hardened by quenching. It supports surface hardening via carburizing to achieve a surface hardness while retaining a tough core. It is not suitable for custom functional shafts subject to torque or impact loads.

SAE 1020 (EN C22 / 1.0402)

SAE 1020’s performance close to 1018, with slightly higher carbon content and marginally improved strength. It also features outstanding weldability and supports carburizing for surface hardening. 1018 and 1020 are interchangeable in most applications; strict differentiation is only required for precise strength requirements.

1215 Low Carbon Free-Machining Carbon Steel

1215 is a lead-free low carbon free-machining carbon steel, fully compliant with RoHS requirements. Its elevated sulfur content improves chip breaking performance, making it ideal for high-volume precision turning on Swiss lathes. Drawbacks: poor weldability and can’t be hardened by quenching.

12L14 Leaded Low Carbon Free-Machining Carbon Steel

12L14 is a leaded low carbon free-machining carbon steel. The addition of lead further optimizes machinability, delivering superior surface finish and more consistent chip breaking. It is the preferred material for high-volume production on automatic & Swiss lathes. Drawbacks: contains lead, not compliance with RoHS/REACH; high sulfur content results in poor weldability & can’t be hardened by quenching.

Medium Carbon Steel for CNC machining: SAE 1035 vs SAE 1045 vs SAE 1050

SAE 1035 (EN C35 / 1.0501)

SAE 1035 is a transitional grade between low and medium carbon steel. It can be quenched & tempered. Compared to 1045, it has lower cutting resistance and lower risk of heat treatment distortion, and offers limited weldability under preheating conditions. For parts with medium loads where reduced machining distortion is desired, 1035 is an often-overlooked cost-effective option. Its core limitation is hardenability: it is only suitable for parts with an effective cross-section of ≤15 mm.

SAE 1045 (EN CK45 / 1.1191)

SAE 1045 is the most widely applied medium carbon steel in global machining, and also the most frequently over-specified material. It has a quenched and tempered hardness. Parts with small cross-sections (≤20 mm) achieve acceptable strength after quenching and tempering. Once the effective cross-section exceeds 20 mm, the core is difficult to harden, and only the surface layer achieves tempering effects, leading to significantly reduced fatigue life. Production Best Practice: Rough machine first with 0.3–0.6 mm stock allowance, then perform quenching and tempering, followed by finish machining. If parts are fully machined before tempering, warpage and dimensional deviation are highly likely.

SAE 1050 (EN C50E / 1.0540)

SAE 1050 is a medium-high carbon steel, It has higher strength and wear resistance than 1045. It can also undergo high-frequency induction surface hardening. Two Key Practical Tips:

1. 1050with inherently poor hardenability. It only works for parts with a cross-section of ≤15 mm; thick parts will not achieve sufficient core hardness and can’t be used as a substitute for alloy steel.
2. Increased carbon content significantly deteriorates weldability; welding is generally not recommended. It has acceptable machinability in the annealed state, Cutting resistance rises noticeably after quenching and tempering, accelerating tool wear. The same process rule applies: rough machine with stock allowance before heat treatment.

Free-Machining Plain Carbon Steel for CNC machining: SAE 1144 (EN 1.0726)

SAE 1144 is a high-sulfur free-machining carbon steel. Its elevated sulfur content improves chip control, enabling higher throughput and lower tool wear in high-volume CNC Swiss lathe production. It can be quenched and tempered to achieve medium strength, suitable for plain shafts, small lead screws, and batch-produced fasteners. However, its impact toughness is reduced: it must not be used for impact loads or alternating loads parts. It also has extremely poor weldability, so welding operations should be eliminated at the design stage.

Practical Pain Points & DFM Design Guidelines for Carbon Steel CNC Machining

The Critical Limitation of Carbon Steel Heat Treatment: Poor Hardenability

No matter how high the carbon content insufficient hardenability in thick sections can’t be overcome. Say, a drawings specify 1045/1050 quenched and tempered to HRC 28–32, but for parts with 40 mm wall thickness, only the surface layer meets the hardness requirement, while the core remains soft. During the DFM review phase, we proactively advise customers: for load-bearing rotating parts with a cross-section >20 mm, we recommend evaluating an upgrade to alloy steel to prevent premature part failure.

How to Avoid Built-Up Edge and Residual Stress Deformation During Carbon Steel Precision CNC Machining?

Low carbon grades 1018 & 1020 are soft and highly prone to built-up edge (BUE) at low feed rates, which degrades Ra surface finish. Machining requires tools with properly designed chip breakers and ample cutting fluid. 1035 & 1045 have moderate machinability; 1050 has higher cutting resistance after quenching and tempering, requiring higher-grade tooling. For slender shafts and thin-walled parts, the recommended process route is: rough machining → stress relief annealing → finish machining. Cold-drawn bar stock has good dimensional accuracy but higher internal residual stress, posing higher risk for thin-walled parts.

Rust Prevention, Surface Treatment Defects & Dimension Chain Considerations

All plain carbon steels are not corrosion-resistant and will rust rapidly when left exposed after machining. Available surface treatments include: black oxide, phosphating, zinc plating, nickel plating and etc. electroplated coatings add measurable thickness, which must be factored into the dimension chain for critical fit tolerances. If not, maybe resulting in binding during final assembly. Carbon steel is not an ideal choice for long-term outdoor service; stainless steel is recommended instead.

Plain Carbon Steel CNC Machining Sourcing Pitfall: Equivalent Grades Are Not Direct Drop-In Replacements

For example, BS 080M46 and SAE 1045, BS EN8 and SAE 1050 have similar compositions, but differ in sulfur and phosphorus impurity control across steel mills. If a drawing explicitly specifies an SAE grade without allowing equivalent substitutions, you can’t directly replace it with EN or GB standard materials. Prior written confirmation must be obtained and documented, along with a Material Test Certificate.

Plain Carbon Steel Selection Decision Table for CNC Machining

Sourcing & RFQ Guidelines for Custom Carbon Steel CNC Parts

When inquiring CNC machining carbon steel parts, be sure to clearly specify:

• Full material grade designation
• Whether equivalent grade substitution is allowed
• Heat treatment method and target hardness range
• Surface treatment type; specify coating thickness for electroplating
• Critical tolerance requirements
• Whether Material Test Certificate is required

Raw carbon steel material is low-cost but heat treatment & surface treatment will add lead time and cost. There is a significant price difference between prototype and batch production. Our team includes a DFM review with every quotation, highlighting hardenability risks and deformation risks. For example, if you selects 1050 for a part with excessive wall thickness, we will proactively communicate performance limitations and recommend adjusting the material or modifying the design.

Summaries:

Do not expect carbon steel to handle thick-section heavy-load applications.

• Specify the exact grade, clearly state heat treatment and hardness requirements.
• Explicitly note whether equivalent grade substitution is permitted.
• Factor coating thickness into dimensions for electroplated parts.
• Request MTC material reports for critical parts.

If you are unsure about material, share your part’s cross-section dimensions and load conditions with us for tailored selection recommendations. Or you can reference our CNC materials selection page & click our other blog of  CNC stainless steel guide, CNC machined alloy steel material selection,  Aluminum CNC materials Selection.

If you are prototyping or batch-producing custom non-standard carbon steel parts, send us your STEP/IGS drawings for a free DFM engineering review, material selection recommendations, tolerance assessment, and detailed quotation.

FAQ

1. Can 1045 plain carbon steel directly replace 4140 alloy steel? 

It’s not recommended for direct replace. Cuz 1045 has poor hardenability, It may be evaluated only for low-load applications with cross-section ≤20 mm. It is strictly forbidden to substitute 1045 for thick-section, alternating impact, or high-fatigue applications.

2. SAE 1050 has higher strength than 1045, canused as alloy steel?

 No. 1050 is still a plain carbon steel. Its hardenability poor, delivering higher wear resistance only for thin small parts. For sections over 15 mm, the core remains soft, and it can’t replace alloy steel for heavy-load components.

3. Can SAE 1018 be through-hardened by quenching?

 1018 has very low carbon content and can’t be through-hardened. It can only be surface-hardened via carburizing, with a hard surface and a soft, tough core.

4. What are the usage limitations of SAE 1144 free-machining carbon steel?

1144 has high sulfur content, which reduces impact toughness. It is not suitable for parts subject to impact loads. It also has very poor weldability, so welding should be avoided. It is suitable for high-volume general parts without impact loading.

5. Can carbon steel be CNC finish-machined after quenching and tempering? What is the correct process sequence? 

 Yes. The standard process is: rough machine raw material with 0.3–0.6 mm stock allowance → quenching and tempering heat treatment → finish machining. If parts are fully machined before tempering, they will most likely deform and go out of tolerance.

6. How do you prevent rust on carbon steel parts? What is the difference between black oxide and electroplating?

Plain carbon steel is inherently prone to rust. Black oxide only provides short-term rust protection and adds no measurable coating thickness. Zinc plating and nickel plating offer much better rust protection, but coating thickness that must be accounted into tolerance design. If you wanna custom anti-rust parts, recommend stainless steel.

7. What is the minimum order quantity for carbon steel parts? Can I order just 1 prototype?

We support production from 1 prototype piece up to full OEM batch quantities. If quenching & tempering or carburizing heat treatment is required, we suggest prototype without do it, as the prototype unit cost is much higher than large-batch orders.

8. What is the typical lead time for carbon steel CNC parts?

 7–10 working days for prototype parts (including heat treatment); 10–25 working days for batch orders depending on quantity. Lead time is excluded shipping transport.

9. Can you provide MTC material test certificates and dimensional inspection reports for carbon steel parts?

Yes. We can provide both.

10. How to quickly select between SAE 1035, 1045, and 1050?

Choose 1035 for low distortion and limited weldability; choose 1045 for general medium-load thin parts; choose 1050 for higher wear resistance on very thin parts with no welding requirement. All three grades are limited to small cross-sections, upgrade to alloy steel for large cross-sections.

11. Is A36 a carbon steel?

Yes, ASTM A36（EN S235JR / 1.0038，BS S275JR） is a low carbon structural carbon steel，mostly use for simple stamping, bending, welding & CNC machined structural parts, not recommended for precision cnc machined functional components.