Mastering Stainless Steel Machining: Techniques and Tips for Success

Overview of Stainless Steel

Stainless steel is a recyclable, corrosion-resistant metal alloy with several unique properties that stand out from other metals. It is one of the many classes of steel with varying compositions that result in varying corrosion resistance levels and mechanical properties.

Chromium and iron are primary elements of stainless steel. Nickel, Manganese, and molybdenum are other elements that make up stainless steel. However, molybdenum and titanium enhance stainless steel’s strength and resistance to corrosion, while nickel improves its physical properties. Manufacturers often utilize stainless steel for its strength, aesthetic appeal, and durability in wet or corrosive environments.

Advantages of Stainless Steel

Stainless steel is a widely embraced machining material choice for various applications for many reasons. Here are some of them:

Mechanical Durability

Stainless steel alloys are widely used for durable and precise parts due to their relatively high tensile strength and toughness. Stainless steel parts often maintain structural integrity and resilience when exposed to high temperature and stress environments like automotive or aerospace.

Increased Resistance to Corrosion

Stainless steels are popular for machined parts because they resist corrosion and chemical deterioration. It is an ideal material for environments with extreme conditions or extended periods of seawater or chemical exposure.

Excellent surface Finish and Cleanability

The different stainless steel grades offer a clean, smooth finish after machining, reducing the need for secondary post-processing or surface treatment, which can increase production costs. Machined stainless steel parts have a non-porous surface that is also easy to clean and friendly with sterilization processes.

Extensive Material Option       

Stainless steels are of various grades, each tailored to meet the requirements of different applications and machining processes. Alloys of stainless steel offer flexibility since they are suitable for everything from medical parts and components to automotive parts and kitchenware.

Limitations of Stainless Steels

Although stainless steel is a popular choice when it comes to machining, it possesses inevitable setbacks, including:

Cost

Stainless steel alloys cost more than aluminum or carbon steel. As such, the material cost may be a significant setback if it doesn’t fit within your budget range. Similarly, stainless steel machining costs may be high since the material can be abrasive, causing the tool to wear out quickly without the appropriate tool material and coatings.

Work Hardening

Certain stainless steel, like austenitic stainless steel, can become harder to work on because they are susceptible to hardening during machining. Consequently, it leads to rapid tool wear and can affect the quality of the machined stainless steel parts if the machinists are not well-versed in stainless steel machining.

Low Thermal Conductivity

Different stainless steel grades exhibit lower thermal conductivity, resulting in heat buildup at the cutting area. It causes the tool to wear out faster and the workpiece to deform quickly. Moreover, exposing stainless steel to high temperatures causes sensitization without proper cutting parameters and adequate cooling.

Types of Stainless Steel

Stainless steel can be categorized into different classes. Each grade or class has a varying chemical composition, which determines its mechanical and corrosion resistance properties. While there are more than 100 stainless steel grades used in machining, they are divided into five classes, including:

Austenitic Stainless Steels

Austenitic stainless steels are the commonly used stainless steel variety and were first discovered between 1912 and 1914. The name implies their austenite or face-centered cubic crystal structure, making them inherently non-magnetic and cannot be hardened by heat treatment. In addition, these steels have very high ductility, toughness, corrosion resistance, and welding ability.

Elements such as Chromium (16 to 20%), Nickel (8 to 22%), and Manganese are the main elements that give austenitic stainless steels these characteristics. Austenitic stainless steel is divided into AISI 200 and AISI 300. Even though austenitic stainless steel is easily shaped into desired forms, manufacturers sometimes add molybdenum to increase corrosion resistance.

Typical applications of the 200 stainless steel grades include household products like washing machines, dishwashers, and water tanks. Manufacturers use the 300 stainless steel grades to create aerospace components, mining industry tools, pharmaceutical products, storage equipment, and cutlery.

Typical Austenitic Stainless Steel Examples Include:

• Alloy 20 (Carpenter 20)
• Type 304
• Type 316
• Type 309S
• Type 321H

Ferritic Stainless Steels

Ferritic stainless steels are an economical alternative to austenitic steels, also known as 400 series stainless steels. The discovery of these steels dates back to 1912, but they were popularly used in different applications in the 1980s. Ferritic stainless steels contain 10.5 to 30 % chromium, less than 0.08% carbon content, and are usually nickel-free.

These steel alloys exhibit magnetic properties thanks to their crystalline structure. Other combinations of properties include average toughness, low welding ability, and high resistance to stress corrosion cracking. Unlike other classes of stainless steels, ferritic stainless steels lack reflective physical properties, exhibit moderate ductility, and are prone to work hardening. Typical applications of the ferritic steels include automotive parts, industrial tools, and kitchenware.

Typical Ferritic Stainless Steel Examples Include:

• Type 405
• Type 409L
• Type 410L
• Type 430
• Type 439
• Type 447

Martensitic Stainless Steels

Martensitic stainless steels possess 12 to 17 % chromium, 0 to 4% nickel, and sometimes traceable amounts of molybdenum and carbon. These steels are named after Adolf Martens and are popular for their exceptionally high hardness. Martensitic grades are mostly magnetic, heat-treatable, and highly resistant to corrosion.

Manufacturers apply aging and heat treatments to martensite steels to enhance hardness and tempering. Depending on the application, martensitic stainless steels can contain low-carbon or high-carbon quality. Typical uses of martensitic stainless steel include cutlery, dental equipment, firearms, surgical instruments, ball bearings, bumpers, and door beams.

Typical Martensitic Stainless Steel Examples Include:

• X12Cr13
• X20Cr30
• X50CrMoV15
• X17CrNi16 – 2

Duplex Stainless Steels

Duplex stainless steels have high chromium content with about 18 to 30% chromium, 1 to 9.5% nickel content, and 5% or less molybdenum content. They are heat-treatable and often magnetic because of their ferritic structure. Ferritic stainless steels also exhibit excellent properties such as good to very high corrosion resistance, high toughness, resistance to stress corrosion cracking, and high weldability when they contain Nitrogen.

This category of stainless steel is called a duplex because it possesses a two-phase microstructure. As such, they exhibit combined attributes of austenitic and ferritic stainless steels. Duplex stainless steel comprises three primary grades: standard duplex, super duplex, and lean duplex alloys. Typical applications of duplex stainless steels include piping systems, heat exchangers, reactors, tubes, shells, and condensers.

Typical Duplex Stainless Steel Examples Include:

• X2CrNiN22 – 2
• X2CrCuNiN23 – 2 – 2
• X2CrNiMoSi18 – 5 – 3
• X2CrMnNiMoN21 – 5 – 3
• X2CrNiMoCuN25 – 6 – 3
• X2CrNiCuN23 – 4

Precipitation Hardening Stainless Steel

PH stainless steels are alloys with traceable amounts of elements, including aluminum, titanium, copper, or phosphorus. Manufacturers often meticulously curate the highly variable properties of precipitation-hardening stainless steels. Further, PH stainless steels usually undergo age-hardening treatment after their formation. As such, they often exhibit 3 or 4 times austenitic stainless steel’s yield strength. PH-hardened steels are used in applications with extremely high strength requirements, such as the chemistry industry, marine construction, nuclear plants, and aircraft. The most popular type used in machining is the 17 – 4 PH Steel.

Can Stainless Steel be Machined Easily?

Stainless steel is a generally machinable material compatible with various metalworking processes. However, most stainless steel grades exhibit excellent machinability ratings, and some grades, such as those with higher alloy content like 316 or duplex grades, can pose inevitable setbacks during machining. Difficulties often arise during stainless steel machining due to the overheating encountered. Besides, overheating can compromise the corrosion resistance properties of machined stainless steel parts.

Examples of Hard-to-Machine Stainless Steel

Although various steel grades can show a certain degree of hindrance during machining, some exhibit more complications than others. Examples of some stainless steel that are difficult to machine include:

• High Carbon Steel: These steels exhibit high hardness and strength, which makes them extremely difficult to machine. Also, high-carbon steels contain carbide-grade elements, which cause these steels to wear cutting tools out faster.
• Low Carbon Steel: Low-carbon steels can be hard to machine because they are incredibly soft. The high softness of these steels causes the chips to adhere to the cutting tool during stainless steel machining, resulting in a shortened tool life.
• 304 Stainless Steel: It is one of the difficult-to-machine stainless steel alloys, although it exhibits good weldability. It is prone to rapid work hardening and stress corrosion cracking when used in applications above 60°C.
• 316 Stainless Steel: This stainless steel alloy exhibits about 60% machinability rating, demanding higher cutting forces and specialized cutting tools to achieve the desired form. As such, manufacturers only utilize 316 stainless steel for parts in the absence of an alternative.

Standard Processes of Machining Stainless Steel Alloys

Machinists process various stainless steel materials through different machining procedures to achieve specific outcomes. We’ll explore the specifics of each process in this section:

Milling

Milling is a renowned machining method machinists commonly embrace when working with stainless steel alloys. It is primarily popular for its precision and ability to handle highly complex designs, a variety of features, shapes, and surface finishes.

The blank remains stationary and secured to the worktable during milling operations while the cutting tool revolves at high speed as it engages it. It remains one of the most popular techniques in stainless steel machining because it meets strict requirements in terms of parts’ durability and quality.

Grinding

This post-processing process uses a revolving abrasive wheel to remove layers of material from a stainless steel blank. This machining technique is typically used to improve the surface finish and form of the machined stainless steel part. Machinists often utilize the grinding procedure to eliminate common surface defects like burrs forming an overall smoother and uniform surface finish.

Turning

This is the ideal technique machinists utilize in machining top-quality, precise cylindrical, axially symmetrical stainless steel parts, including shafts, bushings, and pins, which are core parts in different mechanical and industrial applications. Turning involves the use of a stationary cutter to remove material from the external diameter of a rotating stainless steel blank. Machinists generally utilize turning to add precision rotational features to pre-shaped stainless steel components since it provides high tolerances and quality surface finishes.

Drilling

Drilling is a standard machining method for making precisely sized holes in the stainless steel workpiece. It involves using drill bits of specific sizes pressed against the material and rotated at high speeds to create precise, circular holes. Machinists employ different drill bits to make holes of various sizes for different applications. The drilling process plays a pivotal role in machining stainless steel parts. Besides offering aesthetic purposes, drilling facilitates assembly by creating holes for fasteners like rivets, screws, and bolts. Additionally, it makes a pathway for components, including fluids or cables.

Threading

It is a machining method that can make helical edges on the workpiece. These helical edges, known as threads, can be external (created on a workpiece’s outer surface) or internal (located within the inner surface of a hole). Threading provides an economical means of connecting or assembling separate stainless steel components such as tubes, pipes, bolts, and nuts.

Electrical Discharge Machining (EDM)

It is a high-precision and alternative non-contact machining process that vaporizes material from the stainless steel workpiece with electrical discharges. The EDM technique relies on material erosion to add specific features and achieve the desired shapes. Although the electrical discharge machining technique is ideal for hard metals, it can also create fine surface finishes on intricate and delicate stainless steel components.

Laser Cutting

This non-contact machining process employs a high-energy, focused laser beam to remove stainless steel into the preferred form. The high-powered laser beam melts the target area on the stainless steel to make a cut. Laser cutting is ideal for machining relatively thin stainless steel sheets because it can provide remarkable cutting speed and precision. More so, laser cutting offers overall better outcomes, reduces your tooling requirements, and generates minimal waste.

Expert Tips for Precision Stainless Steel Machining

Various helpful considerations can help ease the machining of various stainless steel types, particularly because this material can present more machining challenges. Here are some of the things to look out for:

Tool Choice and Material Selection

Tool selection and material choice are key considerations in stainless steel CNC machining. This is because stainless steel requires meticulous thought and planning when choosing the ideal cutting tools to prevent complications. Machining experts recommend variable helix tools for machining stainless steel due to the potential issues surrounding excessive vibration/chatter.

A cutting tool can withstand the thermal and mechanical stresses of machining stainless steel only when made from high-quality materials. Common tool materials for machining stainless steel include cemented carbides or high-speed steel (HSS). Cemented carbide tools are the ideal choice for cutting stainless steel material when the CNC machine runs at higher speeds and feeds due to their ability to handle large-scale production runs and provide superior surface finish.

Apply Quality Lubricants

Lubricant application is core to the success of stainless steel parts machining since it can combat the effects of high feed rates and deep cuts. Lubrication helps regulate temperature and prevent friction, overcoming the influence of work hardening and low thermal conductivity, especially when machining austenitic stainless steel. Moreover, cutting fluids help prevent the occurrence of tinting colors on the part’s surface due to overheating and facilitates smooth removal of stainless steel residues.

Choose High-Quality Materials

Since there are over 100 stainless steel grades, machining experts recommend using the material with the best quality to guarantee the desired results. There is a range of quality alternatives within each grade discussed earlier. You might want to consider spending a little extra to acquire quality materials to cut costs down the line and prevent tool breakage hassle.

Machine with Sharp Tools

Cutting tools with sharp cutting edges should be a priority when machining stainless because it helps to ensure precision manufacturing of parts with flawless finish. Using blunt tools can subject the stainless steel material to damage. Hence, you should re-sharpen the tools quickly as soon as you detect any signs of deterioration in the quality of cuts to maintain precision and consistency.

Rigid Tooling

When machining stainless steel, it is pivotal to maintain extremely tight tool connections and machine setup to avoid issues such as tool chatter. Further, ensure the machine bed doesn’t vibrate excessively after fitting the workpiece to avoid deflection.

Conclusion

The goal of stainless steel machining is to create precise and durable machined parts by optimizing tool paths to reduce rework and tool wear. With the right tools and machining techniques and an adequate understanding of the different stainless steel grades, you can produce perfectly machined parts with the desired properties.

AT-Machining is a premier stainless steel machining service provider with good years of experience and cutting-edge CNC machinery. Our certified and skilled professionals can help determine the most suitable stainless steel grades for your parts based on the requirements of your industry or applications, guaranteeing reliable and efficient production. Don’t hesitate to contact us today to discuss the details of your next project and how you can help elevate your stainless steel products with our skilled machinists!