by Christa Kettlewell and Sal Deluca
Mastering both machine and material are a big part of success
Machining stainless steel requires reviewing a number of aspects prior to beginning work in the machine shop. Not only should cutting tool specialists and coolant specialists be consulted, but machine capabilities need to be factored in as well.
First, verify the correct tooling components are being used: cutting tool geometries, substrates, and coatings, type of coolant and coolant pressure among others. With all that done, machining stainless still comes with many unique challenges because it’s a difficult material to machine.
Stainless steel comes in varying grades based on specific properties, and these grades are split into groupings based upon metallurgical qualities. Outlined below are the different families of stainless steel:
A common material, austenitic steel is identified as the Type 300 series; grades 304 and 316 are most common. While austenitic stainless steel cannot be effectively heat treated, it can be hardened through cold working—the process of changing the shape without the use of heat. Corrosion resistance, low magnetism and good formability are characteristics associated with this family of stainless.
Part of the Type 400 series, ferritic stainless steels are characterized by their corrosion resistance, strong ductility and magnetism and are typically iron-chromium alloys. This family can also be altered through cold working rather than thermal hardening methods.
Similar to ferritic stainless, martensitic are also iron-chromium alloys within the Type 400 series. However, this grade can be hardened by heat treatment. Other characteristics include magnetism, good ductility and corrosion resistance.
Precipitation-hardened (PH)Precipitation hardening gives this stainless steel more strength and corrosion resistance. It’s similar to martensitic stainless in terms of chemical makeup.
With a composition of nickel, molybdenum and higher chromium levels, duplex stainless steels combine features of ferritic and austenitic stainless, yet this family demonstrates greater strength and high localized corrosion resistance.
Whether machining valve choke bodies for the offshore oil industry (410 stainless), pump covers for the food processing industry (316 stainless), bushings for the aerospace industry (17-4 stainless) or pumps for the water and wastewater industry (304 stainless), knowing and understanding the varying grades and properties will help overcome challenges when they arise.
One of the greatest challenges of machining stainless steel is chip control. Alloying elements cause stainless steel to be partially heat resistant, which makes forming and vacuating a chip tough. In typical steel cutting applications, heat transfers into the formed metal chip. When machining stainless, alloys prevent this heat transfer, leading to higher cutting temperatures and increased tool deterioration.
One way to combat these challenges is understanding your machine’s condition. Is the spindle rigid? Is the alignment reasonable or near zero runout on a lathe? Ultimately, more torque and horsepower are required to drill stainless than typical steel or aluminum materials.
Another resolution is using a more aggressive geometry to force the chip to form. In austenitic stainless, it is best to use a geometry with a higher rake angle to produce a more manageable chip. When working with PH stainless, increasing the rake angle causes the cutting edge to weaken, reducing tool life.
All in all, stainless steel is not a material that can machined straightaway—every aspect of the process must be reviewed. Not only must machinists understand the different grades of stainless and their properties, but they also need a good understanding of how their machine will interact with stainless. SMT
Christa Kettlewell is a lead management content creation coordinator and Sal Deluca is a product Manager at Allied Machine and Engineering.