Everything You Need To Know About Stainless Steel Machining
The typical workshop comprises many machines, sometimes in the hundreds. The versatility of these machines varies a lot. Some are for specified jobs while others can handle many jobs. In any particular job, the user will switch from one machine and tool to the other. They will end up using dozens of tools by the time the job is completed. One machine that suits these dynamics of a shop is the electrical discharge machining (EDM) machine.
Some users call it the spark machining tool. Using spark generation to machine the workpiece, this innovative machining process is available as sinker EDM and wire EDM machines.
Each of these EDM machining processes performs metal removal differently. Therefore, the processes have their place in the industry. Despite the application differences, the fundamentals of the processes are the same. They use thermal energy to machine even the hardest metal with impeccable precision.
Before we look at each of these EDM machining methods in detail, it would be consistent to understand the EDM machining process more clearly.
What Is Stainless Steel?
Stainless steel material is an iron alloy that doesn’t rust or corrode easily. It has a chromium content of at least 10.5%, an aspect that brings about its corrosion resistance. Other elements that may be present in stainless steel include carbon, nickel, nitrogen, copper, silicon, molybdenum, and manganese. The exact composition will vary depending on the desired qualities.
There are over 100 stainless steel alloys, each with unique properties and applications. These different grades of stainless steel are categorized into four major groups namely:
Austenitic stainless steels
Ferritic stainless steels
Martensitic stainless steels
Duplex stainless steels
Before we delve into the different types of stainless steel, let’s explore the machinability of this material.
Can Stainless Steel Be Machined?
Yes, stainless steel is a machinable material. In fact, various machining methods are employed in the manufacture of stainless steel parts. As long as you understand which processes work best and the properties of stainless steel you should have no problem successfully machining this material.
Machinability refers to the ease with which a material can be cut or shaped. It’s important to remember that different stainless steel grades have different machinability ratings. For example, type 316 steel has a machinability of around 60% while grade 304 steel has a machinability rating of 70%. This means machining type 316 steel requires more effort and a higher cutting force compared to grade 304 steel. You may also need to reduce the cutting speed for less machinable materials.
Types of Stainless Steel
There are several types of stainless steel, each with unique chemical, mechanical, and physical properties. Each family of stainless steel requires different types of heat treatment for specific applications.
Here are the common stainless steel types and their properties.
Austenitic Stainless Steels:
Austenitic stainless steels were first discovered between 1912 and 1914 and they remain the most popular type of stainless steels. Their name is derived from their austenite or face-centered cubic (FCC) crystal structure. This structure contributes to the steel’s unique characteristics but renders it non-magnetic.
Machining austenitic stainless steel is relatively easy due to its exceptional formability, weldability, and high corrosion resistance. Further, this material is less likely to experience work hardening.
Austenitic Stainless Steels Types:
Type 304
Type 316
Alloy 20 (Carpenter 20)
Type 321H
Type 309S
Properties of Austenitic Stainless Steel:
Chromium content: 16% – 20%
Carbon content: less than 0.08%
Nickel content: 6% -22%
Is it corrosion-resistant? Yes
Is it magnetic? No
Is it ductile? Yes
Is it tough? Yes
Is it heat-treatable? No
Ferritic Stainless Steels
Ferritic stainless steels, also known as 400 series stainless steels, are more affordable compared to austenitic steels. They have a crystalline structure which gives them their magnetic properties. However, these stainless steel alloys are more susceptible to work hardening and have lower ductility. Their application in machining stems from desirable qualities such as corrosion resistance and good low thermal conductivity too.
Ferritic Stainless Steels Types:
Type 405
Type 409L
Type 410L
Type 430
Type 439
Type 447
Properties of Ferritic Stainless Steel
Chromium content: 10.5% – 30%
Carbon content: less than 0.1%
Nickel content: Typically nickel-free
Is it corrosion-resistant? Yes
Is it magnetic? Yes
Is it ductile? Has moderate ductility
Is it tough? Has moderate toughness
Is it heat-treatable? No
Martensitic Stainless Steels
Martensitic stainless steel, or Martensite steel, has been around since 1913. It has a relatively high carbon content which results in exceptional hardness. Aging and heating treatments enhance the strength and hardness of martensitic stainless steels.
Martensitic Stainless Steels Types
X12Cr13
X20Cr30
X50CrMoV15
X17CrNi16-2
Properties of Martensitic Stainless Steels
Chromium content: 11.5% – 18%
Carbon content: 1.2%
Nickel content: Typically nickel-free, sometimes around 3%
Is it corrosion-resistant? Yes
Is it magnetic? Yes, although some grades are non-magnetic
Is it ductile? Yes
Is it tough? Ranges from low to high for untempered and tempered alloys respectively
Is it heat-treatable? Yes
Stainless Steel Machining Processes
Stainless steel machining is a subtractive manufacturing process that involves removing material from the workpiece to produce the desired shapes and parts. Machining can be done manually in a process that requires constant operator supervision. However, most machine shops prefer to automate their operations through CNC machining. This technology is faster, less susceptible to errors, and capable of machining tight tolerances.
Here are the most commonly employed stainless steel machining processes.
Milling
Milling is the most common stainless steel machining method and it’s suitable for a wide variety of shapes, features, and surface finishes. During milling operations, the workpiece remains stationary and is secured to the worktable. Conversely, the cutting tool rotates at a high speed. The interaction between the cutting and the workpiece removes any unwanted material. Milling machines can create highly precise parts and a smooth cutting edge. For the best milling results, ensure that you’re working with the right tool.
Turning
Turning is a machining process that uses a stationary cutting tool to remove material from a rotating workpiece. When these two components come into contact, the tool bit removes excess material from the outer diameter of the rotating stainless steel workpiece. Turning is typically used to produce cylindrical, axially symmetrical parts such as shafts, pins, and bushings. Due to its high tolerances and quality surface finishes, turning is often used to add precision rotational features to pre-shaped stainless steel parts.
Drilling
Drilling is a cutting process that creates holes in the stainless steel workpiece. It involves using a drill bit that is pressed against the material and spun at high speeds to cut a precise, circular hole. Different drill bits create different-sized holes for various applications. Drilling plays an important role in stainless steel machining. First, it facilitates assembly by making holes for screws, bolts, and rivets. Secondly, this process provides pathways for components such as cables or even fluids. Lastly, drilling may be done on stainless steel parts for aesthetic purposes.
Threading
Threading is a machining process that creates helical edges, known as threads, on the workpiece. Threads can either be internal (situated on the inner surface of a hole) or external (located on the outer surface of a workpiece.) This feature provides a low-cost means of fastening and connecting separate parts. With stainless steel components, threading is typically conducted on pipes and tubes.
Laser Cutting
Laser cutting is a non-contact form of machining that uses a high-energy, focused laser beam to cut stainless steel into the desired shapes. The high-powered laser melts the material on the target area creating a cut edge. Laser cutting is more suited to relatively thin sheets of stainless steel, providing remarkable precision and cutting speed. Further, this process generates minimal waste, reduces your tooling requirements, and produces overall better results.
Grinding
Grinding is a post-processing procedure that uses a rotating abrasive wheel to shave off layers of material from a workpiece. This machining process removes a small amount of material and is typically used to enhance the form and surface finish of the stainless steel workpiece. Grinding can remove common surface defects such as burrs creating an overall smoother and uniform surface.
EDM (Electrical Discharge Machining)
Electrical discharge machining (EDM) is another contactless machining method that uses electrical discharges to vaporize material from the stainless steel workpiece. It, therefore, relies on material erosion to create the desired shapes and details. EDM is a high-precision manufacturing technique that can achieve fine surface finishes even on delicate and intricate parts. While this process is suitable for hard metals, it tends to be limited by the workpiece thickness.
Stainless Steel Machining Applications
Stainless steel machined parts offer strength, durability, and corrosion and wear resistance. It’s, therefore, no surprise that stainless steel machining has been adopted in most industries. Some of its application examples include;
Aerospace
The aerospace industry calls for high-strength components that can resist corrosion and high temperatures. Stainless steel machining is, therefore, used in the production of engine parts, hydraulic system components, turbine blades, and other crucial aircraft systems.
Medical
The medical sector employs highly precise stainless steel machining processes to manufacture orthopedic implants, surgical tools, diagnostic equipment, and medical furniture.
Automotive
Stainless steel machining offers durability and resilience in the automotive industry. Its applications include gear shafts, engine components, chassis reinforcements, and other automotive parts.
Marine
In the marine environment, parts are exposed to seawater increasing the risk of corrosion. Stainless steel components are, therefore, highly beneficial in this industry. They are used for shipbuilding, engine parts, propeller systems, and more.
Food Processing
In addition to corrosion resistance, stainless steel is a sanitary material making it ideal for food processing applications. It’s used in the manufacture of cutting and slicing blades, storage tanks, mixer equipment, and conveyor belts.
Tips for Stainless Steel Precision Machining
Here are some pro tips to help you achieve the best stainless steel machining results.
Opt for High-Quality Materials
Stainless steel machining results directly depend on the quality of the workpiece material. As we’ve already discussed, there are more than 100 grades of stainless steel. Within each grade, there are also various quality alternatives. While choosing a high-quality material may increase the initial cost, it helps cut costs down the line and saves you the hassle of damaged tools.
Tool Material is Critical
Machinists can choose from a wide range of tool materials, each with its own pros and cons. The right tool material will vary depending on the specific stainless steel alloy and the project requirements. High-speed steel (HSS) and cemented carbide tools are some of the most popular cutting tools for machining stainless steels.
As the name implies, high-speed steels are known for their ability to withstand high speeds. However, carbide tooling is designed to handle even higher cutting speeds and creates a better surface finish. Therefore, it’s suitable for large-scale production runs and high cutting speeds. That being said, high-speed steel tools are relatively more affordable.
Choose Sharp and Precise Tools
To achieve a flawless finish, you have to use sharp tooling. Blunt tools are more prone to shattering due to the extra strain they experience and may also damage the workpiece. However, tool wear is a normal part of any machining process. You, therefore, need to replace any worn-out cutting tool regularly to maintain consistency and precision. Machinists can improve tool life by opting for moderate cutting speeds. Higher speeds may generate excessive heat which damages the tools and materials.
Work Hardening
Stainless steels are susceptible to work hardening which refers to the unintentional hardening of the workpiece during machining. This phenomenon is caused by excessive heat generated by friction between the cutting tool and the stainless steel workpiece. Work hardening increases the difficulty of stainless steel machining. Fortunately, you can use coolant to maintain optimal temperatures during machining operations.
Use Quality Lubricants
Lubrication is a crucial part of successful machining operations and it serves three key functions. For starters, it reduces the friction between the stainless steel workpiece and the cutting tool leading to reduced tool wear. It also helps with temperature regulation and, therefore, minimizes the risk of work hardening and other overheating-related issues. Finally, lubricants facilitate the removal of stainless steel residues created during machining processes.
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.