The Basic Guide to Stainless Steel Machining

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The Basic Guide to Stainless Steel Machining

Welcome to the stainless steel machining world! If you are new to this form of metal machining and are looking for in-depth coverage, this post is for you. This guide to machining stainless steel covers the various types of stainless steel alongside the different processes for machining. We hope it will add to your understanding of the metal machining process.  

What is Stainless Steel?

Stainless steel CNC Machining Parts

Stainless steel is a strong and versatile iron-based alloy that comprises at least 10.5% chromium. Having chromium also makes this group of alloys highly corrosion-resistant. The secret to this high corrosion resistance is the formation of a thin layer popularly known as the “passive layer.”

For an alloy to qualify as stainless steel, it also has to contain up to 1.2% carbon. Other elements that stainless steel can carry include titanium, molybdenum, nickel, silicon, and nitrogen. Notably, stainless steel is a blend of contains metals and several elements. This is the main reason this metal presents different properties.  

The development of this blend of metals and elements is attributed to the Englishman Harry Brearley in 1913.  Upon this discovery and the new popularity of cutlery from Sheffield, the application of this alloy would soon reach global scales.

Even today, the demand for stainless steel is still increasing – an annual rate of 5% according to some estimates. These alloy grades may be relatively expensive, but they have some attractive characteristics. To add to the alloy’s corrosion resistance and strength, stainless steel grades have a modern appearance and are easy to maintain.

Types of Stainless Steel

There are five categories of these: Austenitic stainless steels, ferritic stainless steels, martensitic stainless steels, duplex stainless steels, and precipitation-hardened stainless steels. Here’s an overview of each.

Austenitic Stainless Steel

Austenitic Stainless Steel

Austenitic stainless steel is easily the most popular class of stainless steel. Its grades are characterized by high nickel and chromium content. This results in metals with excellent mechanical properties and corrosion resistance. Austenitic stainless steels are also known for work hardening.

Austenitic grades mean that the stainless steel has an austenitic microstructure. The cells of the microstructure of austenitic stainless steels are cube-like. Each corner of the cell in austenitic alloys has an atom. Another atom is placed in the middle of the cube. In the crystallographic world, this is a Face Centered Cubic Structure (FCC).

Austenitic Stainless Steel Types

  • 304, 304 L Stainless Steel,
  • 310 Stainless Steel,
  • 316, 316L Stainless Steel,
  • 321 Stainless Steel,
  • 347 Stainless Steel
  • 904L Stainless Steel

Ferritic Stainless Steels

Ferritic Stainless Steels

Ferritic stainless steels contain carbon of up to 0.1% and chromium of between 10.5 and 27%. They can also be called ferritic steels. Although this variety does not heat harden, it is magnetic. The core reason for the development of ferritic stainless steel was a product that has corrosion resistance and oxidation resistance.

Another property of ferritic stainless steel is high ductility. Combining all these properties results in a versatile type of stainless steel. It is applicable in the manufacture of a variety of items including kitchenware and industrial devices.

Although heat-hardening ferritic stainless steel is out of the question, annealing or cold working is possible. For budget-conscious projects, this stainless steel is a good choice.

You will come across the following five groups of ferritic stainless steel:

  • Type 409 / 410L

  • Type 430

  • Type 430Ti, 439, and 441

  • Type 434, 436, and 444

  • Type 446, 445, and 447

Martensitic Stainless Steels

As-machined

When it came to developing stainless steel for commercial use, this was the first type to emerge. The most popular application for the steel was the cutlery. Martensitic stainless steel has a relatively higher carbon content of between 0.1 and 1.2%.

They are comparable to ferritic stainless steels in that they have high chromium content. The lowest is 12% and could reach 18%.

Popular grades in this category are 440C stainless steel and 420 stainless steel. They are abrasion-resistant and applicable to engineering parts respectively.

Martensitic stainless steel continues to gain popularity across industries. The automotive industry is a good example of where the material is used extensively.

Duplex stainless steels

Stainless Steel Materials

When austenitic and ferritic microstructures are combined, the result is duplex stainless steel. The chromium content of steels in this category is as much as 28%. However, the nickel content in the duplex alloys is lower at between 4.5 and 8%.

Since the nickel content is not high enough, a true austenitic structure is not achievable. A new microstructure – duplex – is formed instead.

As you would guess, this type of stainless steel has properties from both ferritic and austenitic stainless steels. They include corrosion resistance, weldability, and formability. Their yield and tensile strength are higher than for the two steels – ferritic and austenitic.

The most popular metal under this category is 2205 stainless steel. It is mostly used in refineries and related equipment.

Precipitation-hardened stainless steels

The precipitation hardening stainless steel (PHSS) class has the properties of austenitic and martensitic stainless steels. The grades are both heat-treatable and corrosion-resistant.

Precipitation hardening is a heat treatment process that enhances the tensile and yield strength of a material. It also promotes machinability and wear resistance.

That explains why precipitation-hardened stainless steel is applicable in aerospace, automotive, oil, and many other industries.

Stainless Steel Machining Processes

Milling

stainless steel milling

Milling is the process of removing portions of a workpiece using a rotating cutter in milling machines. In the conventional form, the process involves a workpiece fixed on a moving table. The rotating cutter removes material from the workpiece as the table moves.

In stainless steel CNC machining, milling, and other process could be problematic. A few considerations and precautions are necessary. A prominent one is the choice of cutting tool due to tool wear challenges.

Several reliable choices of cutting tools exist for this process. For 304 and other regular stainless steel, carbide tooling can help. There are also instances where HSS tools work.

Turning

stainless steel milling

In the turning process, the workpiece rotates around its axis while the tool is stationary. Turning stainless steel can be challenging. The level of difficulty depends on the alloying elements present, the manufacturing process used, and the heat treatment method used.

For effective steel CNC machining projects, the operator must consider a few things:

  1. The cutting force of the CNC lathe must be in the range of 1800 to 2850 N/mm²

  2. The chip of the stainless steel should be short

  3. Adding sulfur to some steels, for instance, 303, may be necessary for enhanced machinability

  4. The Rockwell hardness for martensitic and ferritic stainless steels should be 55 HRC or higher for the turning process to be effective  

Drilling

The hardness of stainless steel is the biggest challenge when it comes to drilling. The typical drill bits and drilling approaches may not succeed in this process. Therefore, make sure you are armed with the right drilling tool, accessories, and skills.

An ideal drill bit for stainless steel would be a coated HSS drill bit. For even harder stainless steel grades, consider investing in cobalt drill bits.

Threading

Threading stainless steel is necessary for parts or components that need fasteners or screws. The process can be performed on the interior or exterior of the workpiece. Tubes and pipes are the most popular types of workpieces for threading.

Akin to the other processes, threading stainless steel is not easy, especially for hardened stainless steel. If there is no balance between the feed rate and the cutting speed, heat will be generated. However, the right technique and tool can make a difference.

Laser Cutting

laser cutting parts

Stainless steel can also be laser cut, a machining process that involves a strong light beam. However, laser cutting for stainless steel is only effective in thin sheets. Someone may argue that cemented carbide tools perform the same job. However, these tools wear a lot. Laser cutting has its disadvantages though. Apart from requiring skilled manpower, this machining process is expensive.

Grinding

If you want to improve the surface finish of stainless steel or improve the accuracy of the part, grinding is a viable technique. This method of machining stainless steel is defined by abrasive wheels that rub against the surface. It creates a smoother surface, removing welding blemishes and other imperfections.

EDM (Electrical Discharge Machining)

For the EDM process, electrical pulses of high frequency and voltage cut the stainless steel by melting. Again, the thickness of the metal can limit the usage of the process.

Surface Finishes For Stainless Steel Machined Parts

Polishing

CNC Machining Surface Finish Polishing

As a follow-up process to machining stainless steel, polishing may be necessary for aesthetic appeal and functionality. The process may be accomplished by hand or machine.

Machine polishing of stainless steel is preferable for larger projects. The requirement for polish remains, but the hands are replaced by a polishing wheel. Once well done, the surface should be devoid of streaks, marks, and excess polish.

Blasting and Pickling

Blasting and pickling are necessary for stainless steel to remove residues from earlier processes. Blasting is effective in cleaning the surface, leaving it spotless and fresh. The machined stainless steel part may also be placed in a pickling bath. This bath helps get rid of impurities such as rust, stains, and coatings.

Brushing

Put a brushed surface finish on the material for that distinctive appearance of parallel marks. A 120–180 grit wheel or belt comes in handy in this process. The surface finish is popular in architecture, sporting equipment, and the automotive industry.

Stainless Steel Machining Applications

For its strength, corrosion resistance, and other desirable properties, stainless steel finds application in the following industries:

  1. Medical – MRI scanners, dental equipment, replacement joints, operation platforms

  2. Automotive – exhaust systems, body frames, shipping containers

  3. Construction – handrails, storage equipment, pressure vessels

  4. Aviation – aircraft frame, engines

  5. Manufacturing – Heat exchangers

  6. Food – cutlery, refrigerators, kitchen countertops, pots

Challenges in Machining Stainless Steels

Given the diversity of stainless steel, producing custom stainless steel parts may not always be simple. You can expect the following challenges when machining stainless steel:

Overheating in Machining Stainless Steel

Stainless steel has relatively poor heat conductivity. It becomes hot quickly during the CNC machining process. Using low-speed tools can be a good solution to the problem of overheating when machining stainless steel. Additionally, you want to minimize the amount of force exerted on the cutting tools during the machining of the stainless steel alloy. This minimizes excessive vibration that may cause overheating.

Difficult Surface Finish

Apart from the CNC machining process lasting long, the surface finish requirements of stainless steel surfaces are demanding. Not many machinists can achieve perfection with this material even with the best efforts on surface finish.

Technical Expertise

Not everybody enjoys machining austenitic stainless steel or any other type of stainless steel. Experienced machinists are required because there are so many factors to consider – speed of the tool, type of drill bit, the feed rate, and so on. Remember also the danger of stress corrosion cracking, which may be introduced during machining stainless steel if the workmanship is poor.

Poor Machinability of Stainless steel alloys

The relatively gummy nature of stainless steel is a big challenge during the custom CNC machining process. Fast hardening makes the problem even more serious because even the tool’s life is at stake. The choice of cutting tools matters a lot because it can mean the difference between machinable stainless steel and one that is not.

In Conclusion

Stainless steel machining offers endless opportunities for professionals and beginners. Its unique properties and availability of a variety of machining processes make it possible to create different stainless steel parts and items. These products can withstand harsh environments, alongside other capabilities.

Whether you are looking for custom CNC machined stainless steel alloys for automotive, aerospace, or construction, our custom stainless steel machined products are ideal for you. Partner with us for even more insights into the world of machining stainless steel and other materials.

Author

Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.

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