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.