Machining Stainless Steel

How do you define stainless steel?

Stainless steels are alloys of steel that feature a shiny finish and are highly resistant to corrosion. They are composed of iron with a minimum of 10.5% chromium, along with other alloying elements such as nickel and molybdenum.

When chromium reacts with oxygen, it creates a thin layer of Cr2O3 on the steel’s surface. This layer provides stainless steel with its corrosion resistance by preventing oxygen from reaching the underlying metal. Consequently, it stops rust from penetrating into the material.

Stainless steel comes in over 150 different grades, grouped into five main subcategories.

Austenitic Stainless Steel

Austenitic stainless steel is the most widely used class of stainless steels. Characterized by a high chromium content of up to 20% and nickel levels reaching 15%, this group is known for its superior resistance to corrosion. The significant nickel content, however, makes these alloys pricy and very challenging to machine. Compared to other stainless steels, austenitic grades generally have lower strength and hardness. Most alloys in this category contain less than 0.1% carbon, which increases ductility and therefore requires careful attention to chip control and built-up edge (BUE). Variants marked with an “L” suffix, such as 304L or 316L, have extremely low carbon content, around 0.03%, and are even more difficult to machine (because of extra ductility). Common machining difficulties include high cutting forces, substantial heat generation and BUE, where the workpiece material adheres to the tool’s cutting edge. Notch wear frequently occurs at the depth-of-cut line, and alloys with increased nickel and molybdenum content exhibit poorer machinability.

Best practice:

• Use TiAlN PVD grades or thin-layer CVD grades.
• Don’t use very low cutting speeds, as built-up-edge forms when there isn’t enough heat at the cutting edge.
• Apply good coolant supply directed to the cutting edge.
• Vary the depth of cut to reduce notch wear risk.

Martensitic Stainless Steel

Martensitic stainless steel is the second most common category of stainless alloys. These alloys feature up to 14% chromium and contain only small amounts of nickel. They can undergo heat treatment and hardening, providing greater strength compared to austenitic alloys. However, their corrosion resistance is limited, making them suitable primarily for use in atmospheric environments.