Tuesday, July 22, 2014

Best Practices for Austenitic Stainless Steel (ISO M) Machining

By Don Graham, Manager of Education and Technical Services
Austenitic stainless steel, an iron-chromium-nickel alloy, provides enhanced strength and corrosion resistance for a wide variety of today’s demanding component applications. The benefits of this ISO M material, however, come with a downside. The alloy’s nickel content, which boosts its corrosion-resistance capability, also makes the material harder and therefore more difficult to machine. Fortunately, there are strategies that address this problem and help manufacturers significantly boost productivity in austenitic stainless steel machining operations. 
Milling Stainless Steel
Here are some of the most effective strategies:
Use sharp tools with carbide substrates. Traditionally, machinists assumed that because the austenitic stainless steel alloys were stronger, mechanical cutting forces would be higher, necessitating the application of stronger, negative-geometry tools at reduced cutting parameters. That approach, however, resulted in shorter tool life, longer chips, frequent burrs, unsatisfactory surface roughness and harmful vibration.  In reality, the mechanical cutting forces involved in cutting austenitic stainless steel aren’t much higher than those typical when machining traditional steels. Most of the extra energy consumption required to machine austenitic stainless steels is due to their thermal properties and work hardening characteristics. 
Metalcutting is a deformation process, and when deformation-resistant austenitic stainless steel is machined, the operation generates excessive heat. Evacuating that heat from the cutting zone is of primary importance. Unfortunately, in addition to being resistant to deformation, austenitic stainless steel also has low thermal conductivity. Because of this, the workpiece and chips generated during machining absorb very little heat, so, excess heat transfers directly into the cutting tools to severely shorten their working lives. 
Carbide tool substrates, on the other hand, combat this problem and provide hot hardnesses that endure the elevated temperatures when machining austenitic stainless steel. Paired with sharper cutting edges, carbide substrate tools actually cut the stainless steel – as opposed to deforming it – to reduce the amount of generated heat.
Take large depths-of-cut at aggressive feedrates. With austenitic stainless steel, the bigger the chip the more heat it can carry away from the cutting zone. The most effective way to generate big chips is through large depths-of-cut and aggressive feedrates. Larger depths-of-cut will also reduce the number of cutting passes required to complete a part – an important consideration because austenitic stainless steel tends to progressively strain harden during lengthier machining operations. 
There are practical limitations to these aggressive machining tactics, however. For instance, the power available from the machine tool, as well as the strength of the cutting tool and the workpiece, will ultimately determine how aggressive the cutting parameters can be. In addition, machinists should rethink the finishing process, which traditionally involves multiple passes at smaller depths-of-cut and light feedrates. The most effective strategy is to maximize machining parameters whenever possible, as this can improve tool life and workpiece surface finish.
Use appropriate coolant, applied under high pressure. Due to the problematic thermal properties of austenitic stainless steels, the application of coolant during machining operations is almost always crucial for favorable machining results. The coolant must be of high quality, with at least eight or nine percent oil content in an oil/water emulsion – compared with the three or four percent oil content typical for many machining operations. The higher the pressure at which coolant is delivered to the cutting zone, the better it will do its job. Specialized delivery systems, such as Seco’s Jetstream Tooling, that delivers a high-pressure stream of coolant directly to the cutting zone are even more effective. 
At Seco, we’ve developed several advanced products and strategies that address ISO M austenitic stainless steel machinability – and will continue to do so as the use of these high-performance workpiece materials grows. If you have any questions or would like more information on how our tools and machining strategies for austenitic stainless steels can boost your productivity, please don’t hesitate to contact me.
About the Author
Don is the manager of education and technical services for Seco, responsible for all educational activities for the NAFTA market, new product testing and various other technical functions. Outside of work, he enjoys making maple syrup, restoring antique tractors and farming.