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How to process 304 and 316 stainless steel, what is the difference in processing technology

Stainless steel is widely used for its corrosion resistance. There are many types of stainless steel, which vary in formability, strength and workability. Grade 304 stainless steel, also known as A2 stainless steel, contains 18% to 20% chromium and 8% to 10% nickel.

Grade 316 stainless steel 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 steel is the presence of molybdenum in 316 and no added molybdenum in 304.

This element is added to help it resist chloride corrosion. Grade 316 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 chemical resistant. For example, it resists fatty acids and sulfuric acid at high temperatures. Grade 316 stainless steel can withstand temperatures up to 1600 degrees Fahrenheit. Compared to 316, grade 304 stainless steel is less heat resistant.

What grade of stainless steel is best for machining

Austenitic stainless steels are generally considered difficult to machine. 400 series stainless steel is easier to machine than 300 series stainless steel. This includes levels 304 and 316. The machinability of grades 304 and 316 stainless steel is indeed different. Type 303 stainless steel is probably the easiest 300 series to machine. It is easier to machine than 304. This makes grade 303 steel the first choice for fittings, gears and fasteners with tight tolerances. Its relative weakness means that it is not used as often as grade 304 stainless steel.

304 is easier to machine than 316 stainless steel. Grade 304 steel is not only easier to machine, but also easier to clean, which is one of the reasons it has so many different finishes, which is why it is used for surfaces that are visible to the public. Type 316 stainless steel is not only difficult to machine, but also requires special tools to cut. It performs well in pitting corrosion resistance, but is not as easy to form, which explains why it is reserved for applications that other types of stainless steel cannot handle.

What grade of stainless steel is easier to process

Stainless steel grades have a range of permissible other metal contents to maximize machinability. This makes grade 316 easier to machine than 316B, in addition to the differences in machinability of the various types of stainless steel. 316 steel has a machinability grade of 60, while 316B has a machinability grade of 50, and the 304 and 304L grades have the same machinability grade of 70. In contrast, Alloy 303 is the easiest stainless steel to machine. It has a machinability level of 150 and a benchmark machinability level of 100.

How to improve performance

Adding sulfur or selenium also improves the machinability of stainless steel, but is also more prone to corrosion and interferes with welding. The sulfur content of 303 stainless steel alloys is much lower than that of 304 stainless steel, and the allowable carbon content of weldable stainless steel is lower. For example, 304 stainless steel has a carbon content of up to 0.08%, and 304L or weldable 304 stainless steel has a carbon content of up to 0.03%, which does not really affect its machinability.

How to machine 304 VS 316 stainless steel

Machining 304 steel or 316 steel, do not want the machine to be subjected to excessive vibration, and need to use a large machine. Small lathes and mills cannot cut these grades of stainless steel. For any type of stainless steel, carbide tools or high-speed steel (HSS) tools can be used. HSS tools are a better choice when working at lower cutting speeds.

All 300 series stainless steels have some degree of work hardening, grade 316 hardens more easily and requires more effort to prevent this from happening. One solution is to use a sharper tool, change the tool when it starts to wear out, and work at a slower speed and a higher feed than machining 304. When machining 304 stainless steel, it should be machined at a slower speed. When machining complex 304 steel, make deep cuts at high feed rates to minimize work hardening.

In the ideal process for machining 304 and 316 stainless steels, there are several ways to make all 300 series stainless steels easier to machine. Heat treatment can be done to make the metal easier to work, for example, normalizing heats the steel to a temperature higher than the annealing temperature. Holding it long enough to produce smaller austenite grains improves the machinability of the steel. These types of steel have low thermal conductivity. Be careful not to overheat the surface, which can also cause hard-to-repair deformations. Use oil lubrication to reduce tool wear and cool objects. It is possible to use mineral oil or water-soluble emulsified oil for high-speed work with carbide tools.