Turn to a Fine Balance Between Hardness and Toughness

At first, hardness and toughness may seem like interchangeable concepts, but they actually exist at opposite ends of a continuum that defines indexable and solid tool performance, particularly when it comes to carbide inserts. For any given application, there is an optimal balance of hardness and toughness – and Seco has worked diligently to offer a range of tools that can meet the diverse needs of manufacturers.

Hardness represents wear resistance, which translates to a tool's ability to withstand byproduct heat during metal cutting. The temperature resistance that correlates with hardness plays a huge role in cutting tool behavior and selection, particularly when temperatures can easily rise above 1,400° F in the cutting zone. Hard carbide can withstand and dissipate those temperature levels and, therefore, handle the high heat generated in continuous cutting. However, high hardness levels can make metal brittle, which explains why heat-resistant carbide tools tend to chip in situations that produce large amounts of pressure or vibration.

At the other end of the continuum, toughness represents pressure and impact resistance, which correlate with a tool's ability to withstand high feed rates, heavy depths of cut and the impact associated with interrupted cuts. In exchange for durability, however, tough carbide becomes vulnerable to heat, which makes it less than ideal for high-speed, continuous cutting.

To find the perfect balance between toughness and hardness, shops can use tool behavior to help find and correct factors that cause shorter tool life or unexpected breakage. For example, inserts that develop large craters in continuous cutting applications show their lack of wear resistance and indicate the need for a harder tool. Conversely, if a hard grade fractures in a continuous cut, wear analysis quickly shows the need for a tougher tool instead. In these instances of tool fracture or breakage, shops should also evaluate the condition of the toolholder along with the stability of the machine tool itself and the part setup.

With the increased focus on hard part turning, shops also need to decide whether to machine before or after a material reaches its hardened state – and make cutting tool selections that match the hardness of their materials. In a "green" state, many materials test at half the hardness they demonstrate after hardening, with an obvious effect on tool selection, life and behavior.

Additionally, case-hardened and through-hardened materials present two very different hardness scenarios. Case hardening creates a hard surface with potentially softer material underneath it, while through hardening produces a uniformly hardened workpiece. In these cases, depth of cut plays a vital role in tool selection because it determines whether the task requires an insert that can cut hard or less-hard material.

Cutting tool manufacturers strive to support their customers' success with innovative tools and thorough support for informed tool selection. Seco Tools continues to develop grades, geometries and coatings that handle new materials with excellent results, and to add new performance options for existing materials. Our Tec Team (tec-team.us@secotools.com) always stands ready to guide customers toward optimal selections for individual and unique cutting tasks.

Dynamic Duo for Aerospace: Rough With Ceramic, Finish With Carbide

By Aaron-Michael Eller and Scott Causey of Seco Tools

  

The superalloys found in turbine blades, jet engines and other critical aerospace components are some of the most difficult-to-machine materials in manufacturing, a fact that has driven the development of advanced cutting tools and materials. However, strict regulations and the materials’ physical characteristics often require the use of standard processes performed with standard carbide. As a result, the aerospace industry can find itself caught between a regulatory rock and cost-effectiveness hard place – unless manufacturers use an approach that brings conventional and advanced tooling together in one process.

Everything from aluminum and boron to yttrium and zirconium can be found in superalloys, which are formulated specifically to maintain the structural integrity of aerospace components exposed to extreme temperatures. As a result, the optimal cutting tools for these materials are usually made of ceramics, which possess outstanding heat resistance that enables cycle times that are up to 10-15 times faster than carbide in turning applications. The round inserts often used for these applications also offer two to three times more cutting edges than the equivalent conventional solution and feature excellent tool life, making it ideal for the demanding aerospace market.

At the top speed of applications using ceramic inserts, however, enough heat is generated that the metal is essentially plasticized, a problem for aerospace components. Because flight-critical parts must meet stringent regulatory standards, the fact that advanced ceramic tooling creates heat-affected zones and a brittle white layer in the material necessitates a different approach for finishing. Instead, manufacturers simply stick with ceramics for roughing and switch to carbide for finishing. But when white layer formation itself isn’t an issue, Seco offers specific PCBN grades for nickel-based materials that can finish at speeds six to eight times faster than the equivalent carbide tool.

When parts absolutely must be finished with carbide, a requirement for working with some companies in the industry, manufacturers can still achieve superior results by pairing that carbide with Jetstream Tooling^®. When roughing with ceramics, air blasting is generally sufficient for chip control, but for the high tolerances and surface finish requirements for flight-critical components, high-pressure coolant is the best way to keep chips and temperature controlled for an optimally finished part.

Of course, in the fast-moving world of the aerospace industry, the next leap forward in materials science may only be a few years or months away. Already, carbon-fiber-reinforced polymers (CFRP) and powdered metals used for additive manufacturing processes are presenting new challenges for aerospace shops. To maximize productivity in the face of untested materials, turn to your tooling supplier, as Seco works closely with its customers as well as partners in the materials industry to find the cutting parameters, geometries, lead angles and grades that can achieve cost-effective results. Find out more at www.secotools.com.