Stainless steel CNC machining is widely used to creat precision stainless steel components because of its corrosion resistance. There are many grades of stainless steel. Their formability, strength and machinability are different. Grade 304 stainless steel, also known as A2 stainless steel, contains 18% to 20% chromium and 8% to 10% nickel, and grade 316 or A4 stainless steel contains about 16% chromium, 10% nickel and 2% to 3% molybdenum. This means that one of the biggest differences between 304 and 316 stainless steels is the presence of molybdenum in 316 and never added to 304.
The Mo element is added to help it resist the corrosion of chlorides, such as those found in seawater or de-icing salts, which are comparable to titanium and high-nickel alloys. 316 grade stainless steel also contains trace amounts of silicon, carbon and manganese. The presence of molybdenum makes 316 stainless steel known as Marine grade stainless steel. Type 316 stainless steel is more resistant to chemical corrosion. For example, it will resist fatty acids and sulfuric acid at high temperatures. Grade 316 stainless steel can withstand temperatures up to 1600 degrees Fahrenheit, and grade 304 stainless steel is less heat resistant than 316.
Although grade 304 stainless steel is suitable for most other applications, grade 304 stainless steel is cheaper and easier to machine, which is why it is used in wheel covers, electrical housing, and common steel pipes. In fact, 304 stainless steel is the most widely used austenitic stainless steel in the world. It is stronger than “low-carbon” steel, and its heat sensitivity is even lower than that of grade 302 steel. It can withstand the corrosion of most oxidizing acids, making it easy to sterilize, which is why 304 stainless steel is considered ideal for food processing equipment and hot water systems. If not specifically marked, a material test report will be required to know the difference. Items made of 316 stainless steel may be marked inside the circle 6, grade 304 stainless steel is rarely marked in this way.
Which grade of stainless steel is most suitable for CNC machining?
Austenitic stainless steels are generally considered difficult to machine. 300 series stainless steels are easier to machine than 400 series stainless steels. This includes grades 304 and 316. Grade 304 and 316 stainless steels do differ in their machinability. Type 303 stainless steel is probably the easiest 300 series to machine. It is easier to machine than 304. This makes 303 stainless steel the preferred choice for fittings, gears and fasteners with strict tolerances.
304 is easier to machine than 316 stainless steel. 304 grade steel is not only easier to machine with, but also easier to clean, which is one of the reasons why it has so many different finishes, which is why it is used on surfaces visible to the public. Type 316 stainless steel, which is not only difficult to process but also requires special tools for cutting, performs well in resisting pitting but is not as easy to shape, which explains why it is reserved for applications that other types of stainless steel cannot handle.
What type of stainless steel is easier to machine?
Stainless steel grades have a range of allowable other metal contents to maximize machinability. In addition to the machinability differences between the various types of stainless steel, this makes grade 316 steel easier to process than 316B. 316 steel has a machinability rating of 60, while 316B has a machinability rating of 50. 304 and 304L have the same machinability rating of 70. In contrast, alloy 303 is the easiest stainless steel to machine with. It has a machinability rating of 150 and a benchmark machinability rating of 100.
How to improve the machinability of stainless steel?
The addition of sulfur or selenium will also improve the workability of stainless steel, which is more likely to corrode and interfere with welding. The sulfur content of stainless steel 303 is much lower than that of stainless steel 304, and the allowable carbon content of weldable stainless steel is lower. For example, 304 stainless steel contains up to 0.08% carbon, while 304L or weldable 304 stainless steel contains up to 0.03% carbon, which does not really affect its machinability.
How to CNC machine 304 & 316 stainless steels?
When machining 304 and 316 stainless steel components, if you do not want the machine to be subjected to excessive vibration, all machine shops need to use large machines. Small lathes and milling machines can not cut these grades of stainless steel. Either carbide or high speed steel (HSS) tools can be used for any type of stainless steel, and HSS tools are a better choice when working at lower cutting speeds.
All 300 series stainless steels have some degree of work hardening, and grade 316 is more prone to hardening and requires more effort to prevent this from happening. One solution is to use sharp tools and replace them when they start to wear out, working at a slower rate and higher feed than machining 304 stainless steel. When machining 304 stainless steel, it should be processed at a slower speed.
Conclusion
When machining 304 and 316 stainless steels, there are several ways to make 300 series stainless steels easier to machine. Heat treatment can be performed to make the metal easier to work with, for example, normalizing the steel to a temperature above the annealing temperature. Held long enough to produce smaller austenitic grains, which improves the machinability of steels, these types of steels have low thermal conductivity. Care needs to be taken not to overheat the surface, which can also cause distortions that are difficult to repair. Oil lubrication is used to reduce tool wear and cool objects, mineral oil or water-soluble emulsified oil can be used, and carbide tools are used for high speed work.
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