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CNC Machining Steel: Everything You Need to Know

CNC Machining Steel

Steel is one of the best and most commonly used materials for precision machined parts.

There’s an endless variety of uses for CNC machined steel parts, and they’re essential for many different industries.

Spex uses many different steel alloys to machine millions of precision parts. The cost efficiency and strength of steel make it the ideal choice for CNC machined parts. This article will give you an overview of how different steel alloys are used, and how a variety of industries use steel parts and components.

The pros and cons of CNC machining steel parts

Steel and aluminum alloys are the two most commonly used for CNC machined parts. And there are 100s of different alloys of steel and aluminum.

Steel is a low-cost and widely available metal. You can find steel bars in just about any shape or size that you might need. It’s used for everything from 1mm pins to 40 pound engine blocks.

Overall, steel alloys are highly versatile, but like any metal, it’s not practical for every application. 

Here’s a quick overview of the pros and cons of using steel for CNC machined parts:

Pros

  • Excellent machinability
  • Cost efficient 
  • Easy to weld
  • High strength and durability

Cons

  • Poor corrosion resistance
  • Lower strength-to-weight ratio

The many different types of steel

Just about every metal part you see is an alloy. That means the material is a mixture of multiple elements. On their own, metallic elements like iron, carbon, aluminum, and copper aren’t very useful. But when different elements are mixed together to form an alloy, they become very useful. 

There are thousands of different metal alloys, and hundreds of steel alloys. The vast majority of steel alloys are 90-95% iron. Stainless steel alloys have less iron—about 65-70%.

You can identify the alloy with a 4-digit number label given by the American Iron & Steel Institute (AISI). The first number is the classification of the alloy: 

  • 1XXX = Carbon steel
  • 2XXX = Nickel steels
  • 3XXX = Nickel chromium steels
  • 4XXX = Molybdenum steels
  • 5XXX = Chromium steels

The second number indicates the percentage of the main secondary material. For example, 1018 steel is ~99% iron, and 12L14 steel is 97% iron and added lead and sulfur. 

The last two numbers indicate the amount of carbon in the alloy. 1018 has 0.18% carbon, and 4340 has 0.40% carbon. 

AISI 2330 steel is a nickel steel that has ~3% nickel, and 0.3% carbon.

Here’s what you need to know about the different groups of steel alloys.

Carbon steel

Carbon steel is the most commonly used type of steel alloy. It’s used in about 90% of applications. Adding carbon makes the alloy harder, stronger, and more difficult to machine. And the amount of carbon added varies from low carbon alloys to high carbon. Low carbon steel alloys range from 0.05% to 0.3% carbon. The lower carbon content makes these alloys softer and more malleable. 

High carbon steel has 0.6% to 1.0% carbon content. These alloys are strong and used for springs, hand tools, and high-strength wires.

Common carbon steel alloys: 1137, 11L41, 1215, 12L14

Mild steel

Mild steel is flexible and can be easily machined and shaped. Unlike higher carbon steels, mild steel is easy to shape, drill, and cut to precise specifications. It has good resistance against breakage and good impact resistance.

Common mild steel alloys: 1008, 1018

Alloy steel

Alloy steels add other elements to improve the physical properties of the metal. Nickel, copper, chromium, and other elements are added to improve the durability, corrosion resistance, and appearance of the parts.

Common alloy steels: 4130, 4140, 8620

Stainless steel

Stainless steel is much different than other steel alloys. Stainless steel is much more expensive, but also has much better corrosion resistance and can be used in environments where steel isn’t practical.

Stainless steel alloys are about 60-70% iron, and a mixture of chromium and nickel.

Common stainless steel alloys: 303, 304, 316, 316L

Other types of steel

Tool steel

Tool steel is a separate category of steel alloys, specifically used for tools (as the name hints). Tool steel has great hardness that’s resistant to deforming, and can maintain a sharp edge in high temperatures.

The most common tool steel that we use for machined parts is air hardening grade. This steel has high chromium content and good machinability.

Common tool steel alloys: A2, A286 

High speed steel

High speed steel alloys are used for cutting tools, like saw blades and drill bits. This alloy group is similar to high carbon steels, but maintains its strength better at high temperatures.

Common high speed steel alloys: M2, M35, M42

What grade of steel is best for CNC machining?

With all of the different alloys, it can feel impossible to figure out which one is the best for your parts. There are many different factors that influence which steel alloy is best. 

Even though there are 100s of different steel alloys, only about 10-12 different alloys are used for the majority of CNC machined parts. 

Carbon steel alloys with added lead (usually ~0.2%) have the best machinability. These alloys are also considered free-machining steel alloys. 

12L14 steel is the most popular free-machining alloy steel. This metal is ideal for complex part designs and tight tolerances. The lead and sulfur allows 12L14 to be machined quickly and efficiently, and makes a smooth finish easily achievable. 12L14 steel is often used as the standard machining cost factor that other materials are compared to.

4130 and 4140 alloy steel are also popular alloys for machined parts. These alloys are more expensive to machine, but they’re stronger and more resistant to wear. These alloys are commonly used for automotive, military, and aircraft parts. They have great fatigue resistance, which makes them ideal for engine and transmission parts that need to maintain their strength in high stress environments.

How much does it cost to CNC machine steel?

There are many factors that influence the cost of machining steel parts. In general, steel is one of the less expensive materials to machine. 

The part design, order quantity, and specific alloy of steel have a big impact on the machining costs. 

It’s less expensive to machine softer metals like aluminum and brass, but the raw materials are more expensive than steel. Harder metals like stainless steel and titanium are more expensive raw materials, and more expensive to machine.

Simplifying part design, ordering higher quantities of parts, and choosing a less expensive alloy will lower machining costs.

How thin can steel be machined?

Thin walls require extra care when CNC machining steel parts. Generally, minimum thickness is 0.5mm for metal parts. Machining parts with thin walls increases machining costs and time, so if possible, it’s recommended to increase the thickness part walls to 0.8mm.

Industries that use steel parts

Industries using Steel Parts

Steel alloys are the most common type of metal used in the world. Everything from skyscrapers to motorcycles use steel parts. Of course, steel has its disadvantages, so it’s not used in every industry. 

Steel parts are most commonly used in the automotive, industrial, construction, and manufacturing industries. You’ll find a variety of precision machined steel parts in nearly every car, truck, bulldozer, and crane. Most manufacturing equipment also has a ton of different steel parts. Not every steel part needs to have precise measurements, which means they’re not all manufactured with CNC machining. 

Overall, steel is a highly versatile material that’s used to make millions of different parts and components. 

The medical and aerospace industries tend to avoid using steel to avoid rust. Steel parts are also heavy compared to aluminum or titanium, so it’s not used when weight reduction is a priority.

Secondary processes for steel parts

The main disadvantage of steel parts is their weak corrosion resistance. You’ve probably seen how steel can rust in just a few days. In an environment where there’s high humidity, you’ll see bare steel rust in 3-5 days. If you zoomed in with a microscope, you could watch the steel rust and corrode immediately. 

There are secondary processes that are used to prevent corrosion, and increase the durability of steel. 

These processes include plating, heat treating, and powder coating.

Plating

Plating steel parts allows you to use the advantages of steel: lower cost, higher strength, good machinability, etc. as well as improve the corrosion resistance of the parts. 

Steel parts are often plated with zinc, chromium, or nickel. The thin layer of plating adds visual appeal, and helps avoid rust and corrosion in harsher environments. Plating steel parts does come at a higher cost, but it’s still cheaper than buying stainless steel or titanium. 

Heat treating

Heat treating is a secondary process used to make steel parts harder and more durable. The steel parts are heated up to a certain temperature, and then cooled at a specific rate that slightly changes the physical properties of the metal.

Different heat treatments can affect the steel’s hardness, resistance to wear, tensile strength, and malleability. 

Most heat treated steel parts are hardened or quenched. After the parts are hardened, there is another heat treatment process called tempering. This process makes the hardened parts less likely to crack. 

Lastly, a process called annealing. This process reverses the effects of heat treatments.

Powder coating

Powder coating is a dry paint process that adds a decorative and durable coating to parts. After the parts are machined, they’re sprayed with a powder that attaches to the metal. Then the parts are heated or baked in an oven to cure the coating.

Powder coating offers a strong layer of protection, and the thickness can be adjusted to add more or less. Powder coating is a very environmentally friendly form of finishing. You can also adjust the look and color of powder, making it more or less glossy, and more or less textured.

Spex is an ISO 9001:2015 certified company. Organizations use this standard to demonstrate the ability to consistently provide products and services that meet customer and regulatory requirements.

We incorporate world-class excellence in every step of the process, in our ongoing efforts to ensure your success.

Reach out to our team to get a quote for CNC machined steel parts for your next project. 

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