The art of machining stainless steel
Austenitic and duplex stainless steels have properties that make them difficult to machine compared to carbon steels or ferritic and martensitic stainless steels. The greatest difference is their high ductility and tendency to work harden and form built-up edges on the cutting tool. Moreover, in duplex materials, chip forming is more difficult due to their high strength.
Machinability has been improved in Sanmac® materials without jeopardising properties such as corrosion resistance, mechanical strength or weldability. Non-metallic inclusions are of great significance to the improved machinability. In addition to sulphides, Sanmac® steels contain oxide inclusions that improve chip breaking and reduce tool wear.
Apart from the material and tool, the machining result is influenced by the machine itself, by the cutting data, coolant, and type of operation. To improve the insert’s tool-life with good chip breaking when turning, a generous flow of cutting fluid should be used. The cutting fluid removes heat from the chip and facilitates chip breaking, at the same time giving improved dimensional stability. In the case of internal machining, chip evacuation is improved if cutting fluid is applied through the cutting tool.
In all machining that involves cutting austenitic and duplex stainless
steels, work hardening is more predominant than in carbon steel or ferritic and martensitic stainless steels. The machined surface becomes harder and tool-life decreases.
Chip formation is one of the most crucial aspects of any machining activity. Poorly managed chip breaking can harm the workpiece, the cutting tool and even be a major hazard for the operator. Sanmac® materials have been designed and developed to give better chip breaking properties compared to similar products available on the market.
The chip formation process implies that a fresh metal interface is continually produced between the tool material and the workpiece at varying cutting forces, angles and temperatures. When a cutting edge performs its metal cutting function properly, it deforms part of the workpiece material plastically then ejects it.
Chips normally break off in one of three ways:
- Self-breaking: chips that are an ideal size pose no threat to the machine and will not damage the part, tool holder, or insert
- Breaking against the tool: chips that break against the tool can cause chip hammering and tool breakage
- Breaking against the workpiece: those that break against the workpiece can damage the surface finish
When chips break poorly, bird nesting can occur. When the system produces long, stringy chips, the operator will need to pause the machine and manually clear them from the machining envelope. This increases the cycle time, adding to the cost and resulting in a safety risk. Cut fingers are not unusual.
Bad chip breaking can also affect productivity in other ways. Long chips take up more space in the hopper, which requires it to be emptied more often. When combined with the time lost due to pausing the machine and removing chips, productivity is reduced and per-part costs are higher.