Iron and Carbon Steels in CNC Machining: Grades, Properties, Machinability & Use Cases

  • 2025.07.11
  • Bolg
Table of Contents

CNC machining relies heavily on materials that strike a balance between strength, machinability, and cost-effectiveness. As an option among others, iron and carbon steels are some of the most common options in the manufacturing industry. They are used as the basis for making car parts, as well as support and farm implements.

This in-depth guide provides details on the types, properties, and grades of iron and carbon steels used in CNC machining. Interesting tips that one can learn as an engineer, a machinist, or a procurement specialist are discussed here. So, we can provide you the informed material decisions.

4041 Steel

How Would You Define Iron and Carbon Steels?

What are Ferrous Alloys?

Metallic substances in which the major component of ferrous alloys is iron. Such alloys are carbon steel, alloy steel, stainless steel, and cast iron. These include carbon steels, which are the most frequently utilized and abundant due to their features. These features can be their noble strength, ductility, availability, and their costs. They are easily cast, cut, and welded and therefore extremely important in skyscrapers and bridges, engine blocks, and tools.

The carbon levels of steels define the nature of the material when subjected to various conditions, like when machined, welded, and heat-treated. Due to this flexibility, carbon steels are the base material in manufacturing and the CNC machining industry.

Carbon in Steel

The most powerful alloying element of steel is carbon. Materials' mechanical properties depend on a change in carbon content that can be minor. But it affects the material characteristics.

  • Low carbon implies greater ductility and formability.
  • Higher carbon content leads to greater hardness and strength, though at the expense of ductility and machinability.

The way carbon reacts with the crystal lattice of iron, engineering a hardening effect in solids known as solid-solution strengthening, and also with the appearance of microstructures such as pearlite, ferrite, and martensite during cooling and heat treatment.

In CNC machining, the carbon percentage is useful when choosing the appropriate tooling, cutting parameters, and post-processing such as quenching or tempering.

Carbon in Steel

Different Types of Iron and Carbon Steels We Use in CNC Machining

Here are the different types of iron and carbon steels we can use in CNC machining;

1. Plain Carbon Steels

Plain carbon steels are mostly compounds of iron and carbon with a small percentage of manganese, sulfur, and phosphorus. Their carbon classification and price make them the most viable choice when it comes to general fabrication. They are the material of choice in CNC machining in parts that do not need much wear resistance or specific mechanical properties.

Examples: 1020, 1045

2. Alloy steels

Alloy steels also add another element of alloying (chromium, molybdenum, vanadium, nickel) in order to improve the properties of carbon steel. These are added to enhance toughness, resist fatigue, resist corrosion, and harden.

Alloy steels may frequently demand carbide tooling, and also more exotic coolants when machined using CNC, owing to their elevated hardness and diminished machinability in contrast to plain carbon steels.

Examples: 4130 (chrome-moly), 4140, 4340

3. Free-Machining Steels

In free-machining steels, elements, such as sulfur, lead, selenium, or phosphorus, are added, specifically to enhance machinability. These inserts make shorter chips in machining, less wear on tools, and enable faster cutting speed, at least in automated machining or Large-volume CNC machining.

They cannot be used in structural applications owing to their lower mechanical properties, but they are also perfect in fittings, connectors, fasteners, and bushings.

Examples 1215, 12L14

4. Structural steels

Structural steels are crafted concentrating on strength in load-bearing, weldability, and formability, and not the hardness or wear resistance. They find application in buildings, in the framing of machinery, and in ordinary constructions. Structural steels tend to conform to such standards as ASTM A36 or EN S235.

The machine performs adequately in a majority of CNC applications but might not possess as fine machinability as that claimed by free-cutting grades.

Examples: A3 6, ST37-2, S235JR

5. Tool Steels (Other Category)

Tool steels are hard, high-carbon steels that are specially designed to create tools, dies, and elements to sustain wear. They normally abound in tungsten, vanadium, and molybdenum, among others.

Such steels may be very hard to cut in the hardened state, but will commonly be CNC-machined in an annealed condition, followed by heat-treatment to the required hardness.

Examples: O1, D2, A2, H13

6 Spring Steels (Another category)

Another group of medium and high carbon steels is the spring steels, which have very good yield strength and fatigue resistance properties and are easily set back to their original shape after deformation.

They are cold-shaped and are usually heat-treated. In CNC machining of steels in springs, great concern must be paid to deflection of the tools and residual stresses.

Examples: 1075, 1095, 5160

Classification by Carbon Content

The following table gives a brief overview of the carbon content in different steel alloys;

Type of SteelCarbon ContentKey FeaturesCommon Uses
Ultra-Low Carbon Steel≤ 0.03%Extremely ductile, corrosion-resistant, often interstitial-freeAutomotive body panels, deep drawing applications
Low Carbon Steel≤ 0.25%Soft, ductile, highly weldable, easy to machineStructural components, pipes, and automotive frames
Medium Carbon Steel0.25% – 0.60%Balanced strength and ductility can be heat-treatedShafts, axles, crankshafts, train wheels
High Carbon Steel0.60% – 1.0%Hard and strong, less weldable, prone to brittlenessSprings, blades, cutting tools
Ultra-High Carbon Steel> 1.0%Extremely hard, very brittle, not weldableKnives, wear-resistant tools, punches, dies

In-depth Overview of Major Steel Grades

The prerequisites of using iron and carbon steels in the CNC machining process run in numerous grades, having different compositions and properties depending on the industrial applications. Some of the most commonly machined grades and their explanation in terms of characteristics, machinability, and applications are as follows.

Overview of Major Steel Grades

1. 1020 / C20 / 1.0402 Low Carbon Steel

One of the low-carbon steels is the 1020 steel, which is highly weldable, formable, and has moderate strength. Being soft and easy to machine, it is used in components that do not require much strength or hardness (carbon content approximates 0.20 %). It is good for turning operations as well as milling processes, and it can be carburized to provide surface hardness. Typical uses are: shafts, axles, spindles, and general-purpose machine components.

2. 1025 / C25 / 1.0405 - Mild Steel Added Strength

1025 steel has good machinability, good weldability, and a small increase in tensile strength, being close to sitting above 1020 in carbon content. It finds application where improved mechanical performance is desired but is not so critical that forming and cutting capabilities are affected. This balance of strength and machinability is found in such components as light gears, linkages, and forged automotive components.

3. 1045 / C45 / 1.1191 Medium Carbon Steel

1045 is a general-purpose medium carbon steel containing about 0.45 percent carbon, which has an excellent tensile strength, good hardness, and resistance to impact. It is quite suitable for CNC work that needs surface strength and wear resistance, and with heat treatment. It is more difficult to cut when compared to low-carbon steels, but drills and turns without difficulty, provided adequate tooling is used. It can be commonly found in bolts, connecting rods, and crankshafts.

4. 1215/C12/ 1.0715 Free-Cutting Steel

Steel 1215 is specially designed for high-speed machining, to which sulfur is added in order to enhance chip formation and minimize the wear of the tool. It cannot be welded because it contains sulfur, but it works perfectly on CNC lathes during the process of mass production. It creates a fine surface finish under small tool pressure, hence excellent for precision machining components like bushings, hydraulic fittings, couplings, and fasteners.

5. A36 / S235JR / ST37-2 Structural Steel

A36 and its equivalents: Constructions and general fabricating structural carbon steels. They are of moderate strength and have superior weldability; thus, they find application in large and welded assemblies. Between free-cutting steels and most headed steels, they are not easily machined, but they are good enough under the CNC machining operations, such as milling and drilling, with the provision of proper feed rates and the tooling. It is used as support frames, brackets, and base plates, among other applications, in load-bearing structures.

6. 4130 – Chromium-Molybdenum Low Alloy Steel 

4130 is a low-alloy steel, which is manufactured with chromium and molybdenum, and its strength-to-weight ratio and fatigue resistance are high. It is easily machined in the normalized form and is easily heat-treated. In aerospace components, roll cages, bicycle frames, and high-stress machinery components, it is particularly favoured because of its toughness and weldability. 4130 CNC machining needs sharp tools and steadiness in the cutting velocity to avoid work hardening.

7. 4140- Heat Treatable- Alloy Steel

4140 steel is a high wear resistance and overall tough steel, which makes it one of the most popular alloy steels in CNC machining. It is comprised of chromium and molybdenum, which give it resistance even in cases of extreme load. It becomes very hard, after heat treatment, with good machinability on carbide tooling. It can commonly be found in heavy-use gears, shafts, dies, and tooling parts where precision and strength are both needed.

Mechanical and Chemical Properties Comparison Table

GradeCarbon (%)Yield Strength (MPa)Tensile Strength (MPa)Hardness (HB)Machinability (%)Heat Treatment
10200.20350–410410–480~12665%Yes
10450.45570–650600–750~170–23060%Yes
12150.09325–400415–450~150100%No
41400.40655–830850–1000~250–30055%Yes

CNC Machinability of Iron and Carbon Steels

So, let’s discuss this in detail;

CNC-Aerospace-Parts-Manufacturing

1. Turning, Milling, and Drilling Behavior

Low carbon steels are good to machine, but they produce long and stringy chips. Medium and high carbon steels are harder to cut, and some are equipped with better finishes.

Alloyed steels such as 4140 require better control of speeds and feeds. Chatter or lack of dimension accuracy can be a result of poor tooling.

2. Surface Finish and Chip Formation

Free-cutting steels chip to a manageable size, which enhances efficiency. Higher carbon steel may have a smooth finish provided that they are machined accordingly.

Misplaced chip control may spoil the part and block the tools. Consistency requires chip breakers and optimal cutting angles.

3. Tool wear and tooling needs

The steels, such as 4140/4340, are hard steels that need carbide or coated tools. Free-machining steels lower wear and make tool life longer and operations quicker.

Tool degradation in harder steels is accelerated by high cutting temperatures. Frequent observation of the tools and applying coolants enhances the life of the tools.

4. Free-Cutting Elements Effects

Sulfur will help break chips, and it also helps lead finish better, but lead usage is diminished because of environmental issues. These are machining-enhancing additives.

They can, however, slightly decrease the ductility and hardness. The grades with free-cutting characteristics are most applicable in the high-speed, low-load parts.

Heating & Hardening of Consideration

Heat treatment is a very vital consideration in the manipulation of the mechanical properties of iron as well as carbon steels. It has the potential to cause big changes in machinability, toughness, hardness, and wear-resistance; thus, a clear knowledge of the various processes and their effects on particular grades of steel is crucial when handling CNC operations.

Process for making Aluminum alloy parts

1. Normalization, Annealing, & Quenching

  • Normalizing consists of heating a steel beyond its critical temperature and cooling it in air. This smooths the grain and removes the internal stresses, producing a better and less extreme hardness. It enhances processing, and it is generally applied before CNC operations.
  • Annealing involves slowly cooling the material in the furnace after heating, softening the material. It increases ductility and is suitable before heavy machining processes, notably among high-carbon steel such as 1045.
  • Quenching is to cool down quickly through water or oil. It makes steels (and, in particular, those with medium to high carbon content (such as 4140 and 4340 steel)) harder and stronger, but introduces brittleness that must be eliminated by tempering.

2. Hardenability of Grades

  • Hardenability can be defined as how deeply a material can be hardened in quenching. The steels do not all react the same:
  • High alloy steels such as 4340 and 4140 have good through-hardening characteristics and do not lose strength in large cross-sections. These are perfect in such parts as gears and shafts.
  • Lower carbon steels such as 1020 and A36, by contrast, show little increment in hardness due to quenching, and are more appropriate as surface hardened or non-hardened structural components.

3. Case vs through hardening

Where carbon is low, such as with low carbon steels (e.g., 1020), case hardening (i.e., carburizing, nitriding) can also be employed, where the surface is hardened only and wear is resisted, and the core can remain ductile. It is perfect to be used in parts such as camshafts or pins.

It can be used with medium carbon (e.g., 1045) and high carbon (e.g., 4140) steels, which are both capable of being hardened to a uniform depth to produce high strength and wear resistance. This applies to components that are regularly stressed and loaded.

4. Pre-Heated and Post-Heated Machining

In most CNC manufacturing, heat treatment is applied after rough machining to prevent tool wear due to a hardened surface.

Finishing passes and close-tolerance operations are done after heat treatment to guarantee that there is dimensional accuracy and aim at attaining surface integrity. Machining after heat treatment may involve the use of carbide machining tools and lower operating speeds because the material is now harder.

Weldability of Iron and Carbon Steel Alloys

The carbon and alloy steels have big differences in terms of weldability. It is fundamental to learn more about the differences, as this can be the primer in planning the welding procedures and the post-weld CNC machining procedures.

Weldability of Iron and Carbon Steel Alloys

1. Mild Steel Welding

Other steels, such as 1020 and A36, are very weldable with little chance of cracking and distortion. They can easily be fused, since they have low carbon content, and this does not require special precautions.

These grades are well suited to welded parts such as frames and supports, and then they may be CNC finished with holes for brackets or mounting surfaces.

2. Preheat Requirements of Alloy Steels

These include alloy steels, which are quite strong but sensitive to heat when being welded, like the 4140 and 4340. To counter this, they demand that they be preheated to an acceptable temperature before any welding takes place. Preheating enables the temperature gradient to be minimized and prevents thermal shock and cracking. Another point is that matching filler metals that complement the base material to achieve a strong, durable weld should be used, too. The process of heat treatment usually takes place after welding is carried out in order to normalize the structure, as well as to recover toughness.

3. Post-Weld Machining Tips

Final scope of work. After welding, the work may have to be machined to remove distortion or return the parts to tight tolerances. To make post-weld machining best, the part should be cooled down and normalized. Any irregularities due to either the weld beads or warp can be taken off by any finishing processes, such as surface grinding, facing, or boring. Where accuracy is important, it is also usual to machine before welding a weld-prepped joint, and then machine after to obtain an accurate surface.

Selecting the Appropriate Iron and Carbon Steel Alloy

The choice of the most suitable grade of steel to be machined by CNC depends on a compromise between mechanical aspects, machinability, price, and service conditions.

1. Strength Requirement-based Selection

In structural or low-stress uses in general, the strength is good enough and cost-effective with grades such as A36 and 1045.

When a component will be subjected to high stress, impact effects, or is prone to fatigue, then 4140 and 4340 are better choices since they can be heat-treated with high tensile strength and toughness.

2. Cost and Availability

Steels that are low in carbon, like 1020 and A36, are some of the cheapest and readily accessible materials that can be used in CNC machining, and thus most industries use them as a default option.

The high alloy steels, such as 4340, are more costly but satisfy the circumstances that demand high performance.

Application of Iron and Carbon Steels in Industry

  • Construction: A36, S235JR: Forms, raftering, plate.
  • Automotive: 4140, 1045, Shafts, axles, gears.
  • Machinery: 4340, 1025: Hydraulic fixtures, hydraulic parts.
  • Farming Machinery: 5140, A 36: Tool sets, parts.
  • Oil and gas: 4130, 4340: strong fittings, pipes.

Pros and Cons of Iron and Carbon Steel Alloys

Here are some pros and cons of iron and carbon steel grades;

ProsCons
Wide range of gradesSusceptible to rust
Excellent strength and durabilityTougher grades are harder to machine
Cost-effective and recyclableRequires coatings for corrosion areas
Easy to heat treat for performanceCan warp during welding or hardening
Readily available globallyThe weight is higher than that of aluminum

Tips on Machining Iron and Carbon Steels

So, must follow these tips;

  • Carbide Tool and Insert Usage: Alloy and hardened steels are obligatory.
  • Best Cutting Parameters: Lower cutting speed, higher feed for difficult steels.
  • Grease and coolants use: Heat dissipation and chip control. Apply generously.
  • Best Practices in Heat: Treated Steel 11.4: Work with hard setups, no chatter.
  • Preventing Warping and Residual Tension: Stress relief from roughing. Symmetrical patterns and balanced structure.

Conclusion

Iron and carbon steels play a foundational role in CNC machining, offering an excellent balance of strength, machinability, and cost-effectiveness. From easy-to-machine mild steels to tougher alloy steels used in automotive and construction, these materials support a wide range of applications. Their compatibility with turning, milling, and drilling—along with high weldability and the ability to tailor hardness through heat treatment—makes them suitable for both precision components and heavy-duty parts. At ApexRapid, we provide expert CNC machining services for a variety of carbon and alloy steel grades, helping clients optimize performance and production reliability by selecting the right material based on carbon content, hardness, and chip control.

FAQs

Q1: What is the best steel classification to use with high precision CNC turning?

A: 1215 steel is very machineable and is ideal for high precision turning.

Q2: Do you weld 4140 steel?

A: Yes, but it has to be preheated and post-weld heat-treated to avoid cracking.

Q3: What is the most economical carbon steel to use in structural applications?

A: A36 or S235JR are cheap and are widely used in the fabrication of structures.

Q4: What is the impact of heat treatment on CNC machining?

A: It enhances wear resistance and hardness, and it can raise tool wear and lower machinability.

Q5: Which steel grade is suitable for gears and shafts?

A: 1045 or 4340 by strength and wear resistance requirement.

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