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HOW TO TROUBLESHOOT HOLE-
MAKING IN STAINLESS STEEL Provided by Allied Machine & Engineering
Unlike machining other materials, machining stainless steel requires review of a myriad of aspects prior to be- ginning work in the machine shop. Not only should cut- ting tool specialists and coolant specialists be consulted, but machine capabilities should be addressed as well. Fur- thermore, one must verify that the correct tooling compo- nents are being used: cutting tool geometries, substrates, and coatings, type of coolant and coolant pressure among others. Still though, machining stainless comes with many unique challenges because of its low machinability—a ma- chinability rating that needs to be overcome to utilize the many benefits of stainless steel.
Stainless steel is offered in varying grades based on specific properties. These grades are also split into group- ings based upon metallurgical qualities. Outlined below are the different families of stainless steel.
• Austenitic – A rather common material, austenitic steel is identified as the Type 300 series; grades 304 and 316 are the most accessible. While austenitic stainless steel can- not 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 also characteristics associated with this family of stainless.
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• Ferritic – As part of the Type 400 series, ferritic stainless steels are characterized by their corro- sion resistance, strong ductility and magnetism and are typically iron-chromium alloys. This family can be altered through cold work- ing rather than thermal hardening methods.
• Martensitic – Similar to ferritic stainless, martensitic are also iron-chromium alloys within the Type 400 series; however, this grade is able to be hardened by heat treatment unlike the ferritic grade. Other characteristics include mag- netism, good ductility and corro- sion resistance.
• Precipitation-hardened (PH) – Through the precipitation harden- ing process, precipitation-hardened stainless steel attains more strength in addition to greater corrosion re- sistance. Additionally, it is similar to martensitic stainless in terms of
chemical makeup.
• Duplex – With a composition made up of nickel, mo-
lybdenum 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 pro- cessing industry (316 stainless steel), bushings for the aero- space industry (17-4 stainless steel) or pumps for the water and wastewater industry (304 stainless steel), knowing and understanding the varying grades and properties of stainless steel will enable machinists to effectively utilize stainless steel and overcome its challenges when they arise.
One of the greatest challenges of machining stainless steel is chip control. Alloying elements such as nickel cause stainless steel to be partially heat resistant, which results in difficulty forming a chip and, thus, poor chip evacuation. In typical steel cutting applications, heat transfers into the formed metal chip. When machining stainless, the heat resis- tant nickel alloys prevent this heat transfer.
Simply stated, the nature of the material and its high amount of elasticity make it difficult to achieve chip forma- tion and induce quite a bit of wear on the cutting tool.
CNC WEST April/May 2021