304 vs. 316 Machining Stainless Steel

Stainless steel is widely used due to its corrosion resistance. However, there are many grades of stainless steel. They vary in formability, strength, and machinability. Let’s learn more about the most commonly used grades of stainless steel before discussing the machining of each grade of stainless steel. However, you might ask, what is the difference between 304 vs. 316 stainless steel? 

What is the difference between 304 and 316 stainless steel?

The 304 grade of stainless steel, also called A2 stainless steel, contains 18 to 20 percent chromium and 8 to 10 percent nickel. Grade 316 stainless steel or A4 stainless steel has around 16 percent chromium, 10 percent nickel, and 2 to 3 percent molybdenum. This means one of the biggest differences between 304 and 316 stainless steel is the presence of molybdenum in 316; type 304 never contains it. That element is added to help it resist corrosion by chlorides such as those found in seawater or de-icing salts. It rivals the corrosion resistance of titanium and high nickel alloys. Grade 316 steel also contains trace amounts of silicon, carbon, and manganese.

The presence of molybdenum is why 316 stainless steel is called marine grade stainless steel. That makes it the obvious choice for a floating oil rig and oil refinery piping. Type 316 stainless steel is more chemically resistant, as well. For example, it will resist fatty acids at high temperatures as well as sulfuric acid. Grade 316 stainless steel can resist temperatures up to 1600 degrees Fahrenheit. Grade 304 stainless steel is weak compared to 316.

While grade 304 stainless steel is fine for most other applications, type 304 stainless steel is cheaper and easier to fabricate. That’s why it is used in wheel covers, electrical enclosures, and general steel pipes. In fact, 304 stainless steel is the most widely used austenitic stainless steel in the world. It is stronger than “mild” steels. It is also less heat sensitive than even grade 302 steel. It can withstand corrosion from most oxidizing acids, making it easy to sanitize. This is why 304 stainless steel is considered ideal for food processing equipment and hot water systems.

Note that you can’t see the difference between the two grades of stainless steel. A material test report is necessary to know the difference if they aren’t specially marked. Items made of 316 stainless steel may be marked with a 6 inside of a circle. Grade 304 stainless steel is rarely marked in this way.

What grade of stainless steel is best for machining?

Austenitic stainless steels are generally considered difficult to machine. The 400 series of stainless steel is easier to machine than the 300 series of stainless steel. That includes grade 304 and 316 stainless steel. However, the 304 and 316 grades of stainless steel do differ in their machinability.

Is 304 vs. 316 easier to machine?

Type 303 stainless steel is probably the easiest 300 series to machine. It is much easier to machine than 304. That makes grade 303 steel the first choice for fittings, gears, and fasteners where tight tolerances are a must. However, its relative weakness means it isn’t used as often as grade 304 stainless steel.

Type 304 is more machinable than 316 stainless steel. Not only is grade 304 steel easier to machine but it is easier to clean. That is one reason why it comes in so many different finishes. That is also why it is used on surfaces that are visible to the public. Think car fenders and stainless steel serving trays.

Type 316 stainless steel is not just harder to machine but requires special tools to cut. It performs well against pitting, but it isn’t as formable. That explains why it is reserved for applications other types of stainless steel can’t handle.

What makes some grades of stainless steel easier to machine than others?

We mentioned that stainless steel grades have a range of allowable mineral content. More machinable stainless steel will be formulated to have the extreme ends of the allowable chemical formulation to maximize machinability. That is aside from the differences in machinability between various types of stainless steel.

This makes grade 316 steel more machinable than 316B. Grade 316 steel has a machinability rating of 60, while 316B has a machinability rating of 50. Grade 304 and 304L have the same machinability rating of 70 by American Iron and Steel Institute or AISI standards. For comparison, alloy 303 is the most easily machined stainless steel. It has an AISI machinability rating of 150. The baseline machinability grade is 100.

How can you increase the machinability of stainless steel?

Adding sulfur or selenium increases the machinability of stainless steel, as well. Unfortunately, that also makes it more prone to corrosion and interferes with welding. Stainless steel alloy 303 contains sulfur to well below that of stainless steel 304.

What about carbon? Weldable stainless steel has a lower allowable amount of carbon. For example, 304 stainless steel has a maximum of 0.08 percent carbon, whereas 304L or weldable 304 stainless steel has a maximum of 0.03 percent carbon. This doesn’t really affect its machinability.

How do you machine 304 vs. 316 stainless steel?

Whether you’re machining 304 or 316 steel, you need to use substantial machines. Small lathes and milling machines cannot cut these strong grades of stainless steel. You don’t want machines subject to excessive vibration. For either type of stainless steel, you can use cemented carbide tools or high-speed steel (HSS) tools. Molybdenum HSS tools are the better choice when working at lower cutting speeds.

Machining 304 vs. 316 Stainless Steel – what do you do differently?

Grade 316 is more prone to work hardening, so you need to invest more effort into preventing that. One solution is using sharp tooling and replacing tools the moment they start to wear down. Work at a slower speed and higher feed than if you were machining 304.

To machine 304 stainless steel, you should machine it at slower speeds. Complicating matters is the fact grade 304 steel can work harden (All 300 series stainless steel work hardens to some degree). When you’re machining 304, use deep cuts at a high feed rate to minimize work hardening.

What are some tips for machining 300 series stainless steel?

When debating between the ideal processes for machining 304 vs. 316 stainless steel, there are a few ways to make all 300 series stainless steel more machinable.

Heat treatment may be done to make the metal more machinable. For example, normalizing heats the steel to a temperature higher than the annealing temperature. It is held long enough to cause smaller austenitic grains. This improves the machinability of the steel.

These types of steel have low thermal conductivity. You need to be careful not to let the surfaces overheat. Overheating can also cause distortion that is hard to correct. Use oil to lubricate reduce tool wear and cool things down. You can use either mineral oils or water-soluble emulsifiable oils. The latter is the best choice if you’re working at high speed with carbide tooling.

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