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“Steel” yourself for this in-depth CNC materials guide

In sectors that require high-performance, durable parts, such as the automotive, aerospace, and robotics industries, steel and aluminum tend to be the materials of choice for many applications. However, steel and aluminum have very different physical characteristics and price points, so it’s very important to determine which is better-suited for a given part.

One of those determining factors is typically the manufacturing method, which in these industries is often CNC machining. When machining a metal part, there are several key reasons why steel may be a more appropriate material choice than aluminum. While aluminum is incredibly lightweight, steel is much stronger. What’s more, aluminum tends to be much more expensive.

However, the choice doesn’t just come down to aluminum versus steel. Steel is actually a family of materials, and it’s important to carefully evaluate different steels — and their respective advantages and limitations — during the material selection process. Here are four of the most commonly machined types of steel, and what product teams should know about each one.

1. 4140 steel

4140 steel is used across numerous industries because of its high fatigue strength, overall toughness, and resistance to abrasion and impact. The chemical composition of 4140 makes it particularly strong, tough, ductile, and hardenable. In addition, the chromium and molybdenum contribute to corrosion resistance, and the manganese and carbon content further increase hardenability while promoting deoxidization. It’s considered highly versatile and readily machinable, and can be used for many general purpose steel parts.

4140 steel is much tougher than plain carbon steels, so it requires more force for effective forming. In addition, when machining 4140 steel, annealing is more important than with simple, low-carbon steels. While it is often quench-hardened, 4140 is not easily welded and will require pre- and post-weld thermal treatment to prevent cracking, as well as being mindful of using a compatible filler material.

4140 demands a slightly higher material cost than non-alloy, pure carbon steel, as well as a substantially higher machining cost. However, because it’s so strong and durable, it’s considered highly cost-effective.

Mechanical specifications: 

  • Heat treatment: Oil quenched, 540°C temper, 25mm round
  • Ultimate Tensile Strength: 1140 MPa
  • Elongation at Break: 18%
  • Modulus of Elasticity: 205 GPa
  • Hardness: 35 HRC

2. 1018 steel

1018 steel is a general-purpose carbon steel composed of iron, carbon, manganese, phosphorus, and sulfur. 1018 steel has excellent weldability, but it is not as hard as some other steels.

The primary benefit of 1018 steel is its workability. It’s very easy to form, forge, weld, and hot work, which makes it favorable for parts that require severe bends or other fine features. As such, 1018 steel is often used for applications such as shafts, spindles, pins, or rods. It’s also considered a good choice for carburized parts.

Though 1018 steel can be welded, welding isn’t recommended when the steel has been carbonitrided and carburized. This is a limitation that manufacturing teams should be aware of when working with 1018 steel. Because of the material’s relatively soft texture, it can also be difficult to finish.

1018 steel’s machinability, strength, and precision come with a high price tag. It’s more expensive than some comparable steels, but is much better-suited for CNC machining than these similar steels.

Mechanical specifications: 

  • Cold-drawn
  • Ultimate Tensile Strength: 440 MPa
  • Elongation at Break:15%
  • Modulus of Elasticity: 200 GPa
  • Hardness: 71 HRB

3. 1045 steel

1045 steel is composed of iron, carbon, silicone, manganese, and sulfur or phosphorus. It is a medium carbon steel designed for strength and hardness. Its size accuracy, straightness, and concentricity make it especially well-suited for high-speed applications. It’s frequently used to make bolts, studs, gears, axles, and shafts.

1045 is often selected for its relative strength and impact resistance. 1045 steel measures 54 on the Izod impact test. It’s also highly machinable and can be more easily welded than high carbon or high alloy steels, whether in the cold-drawn or normalized condition.

However, since 1045 is a steel of medium tensile strength and medium hardenability, it is not suitable for applications that require a very high-strength material. For applications that require more strength and wear resistance than mild steels, but less strength than the hardest steels, 1045 may be a good option.

1045 steel can be flame- or induction-hardened in both the rolled and normalized conditions. However, it does not respond favorably to nitriding, as it lacks the requisite alloying elements.

1045 steel is more expensive and offers greater strength than 1018 steel, but much of the same weldability and machinability, contributing to its higher cost.

Mechanical specifications: 

  • Cold-drawn
  • Ultimate Tensile Strength: 625 MPa
  • Elongation at Break:12%
  • Modulus of Elasticity: 206 GPa
  • Hardness: 88 HRB

4. 4130 steel

Considered a low carbon steel, 4130 steel is composed of iron, carbon, chromium, manganese, molybdenum, phosphorus, silicon, and sulphur. 4130 is an exceptionally tough steel that’s relatively easy to heat-treat and highly workable.

4130 steel offers a modulus of elasticity that is higher than most other steels of similar ultimate strength, which enables it to withstand high stresses. This quality makes 4130 steel especially favorable as a structural steel, used for various construction applications. For all of these reasons, 4130 is frequently used in the creation of manufacturing equipment, rock crushing machinery, and resistance welding products. 4130 steel is well-suited for a variety of heat treatment processes.

While 4130 steel is highly heat-treatable, it can be challenging to weld. Its high hardenability makes it challenging to select a compatible filler metal that will have comparable strength. An understanding of the properties of 4130 and any filler metal’s properties is necessary to successfully weld this steel.

4130 steel is a relatively low-cost steel, both in terms of raw material cost and cutting cost. It is, in many ways, comparable to aircraft-grade stainless steel in its physical properties, but comes at a much lower price tag, making it a desirable option for many product teams.

Mechanical specifications:

  • Water-quenched 855°C, 450°C temper, 25mm round
  • Ultimate Tensile Strength: 993 MPa
  • Elongation at Break:18.5%
  • Modulus of Elasticity: 205 GPa
  • Hardness: 31 HRC

Get the most out of any steel with the help of a manufacturing partner

CNC machining is a highly versatile manufacturing process that allows for the speedy creation of strong and functional parts made out of countless materials, including steels. However, determining the best steel — or other metal or plastic — for a given project can be challenging and time-consuming, particularly when faced with so many options.

That’s where a seasoned manufacturing partner like Fast Radius can help. Our team of designers, engineers, and project managers has years of experience helping customers streamline and optimize the entire manufacturing process — from design, prototyping, and material selection to production and fulfillment. We’re well-equipped to help product teams of all shapes and sizes select the CNC machining material that’s right for their unique requirements and price point. Contact us today to get started.

For more information about CNC machining and expert tips for material selection, check out our related articles in the Fast Radius learning center.

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