Properties of Tool Steels — Toughness

Properties of Tool Steels — Toughness

Toughness, as considered for tooling materials, is the relative resistance of a material to breakage, chipping, or cracking under impact or stress. Toughness may be thought of as the opposite of brittleness. Toughness testing is not as standardized as hardness testing. It may be difficult to correlate the results of different test methods. Common toughness tests include various impact tests and bend fracture tests.

In impact testing, a small sample is held in a fixture and fractured by a moving impacter, such as a calibrated weight on a pendulum. Toughness is reported as the amount of energy, usually measured in foot-pounds or joules, that the sample absorbs before it breaks. Brittle materials will absorb little energy before fracturing. In bend fracture testing, a fixtured sample is subjected to gradually increasing amounts

of pressure, usually side or bending pressure, until it breaks.

Methods of Toughness Testing

Most tool steels are notch-sensitive, meaning that any small notch present in the sample will permit it to fracture at a much lower energy. Solid carbide is even more notch-sensitive than tool steels. Thus, in addition to inherent material properties, the impact resistance of tool components is significantly impaired by notches, undercuts, geometry changes, and other common features of tools and dies.

In service, wear failures are usually preferable to toughness failures (breakage). Breakage failures can be unpredictable, catastrophic, interruptive to production, and perhaps even a safety concern. Conversely, wear failures are usually gradual, and can be anticipated and planned for. Toughness failures may be the result of inadequate material toughness, or a number of other factors, including heat treatment, fabrication (EDM), or a multitude of operating conditions (alignment, feed, etc.) Toughness data is useful to predict which steels may be more or less prone to chipping or breakage than other steels, but toughness data cannot predict the performance life of tools.