While the metallurgic term “carbide grades” refers specifically to tungsten carbide (WC) sintered with cobalt, the same term has a broader meaning in machining: sintered tungsten carbide combined with coatings and other treatments. For example, two turning inserts made of the same carbide material but with a different coating or post-treatment are considered different grades. Carbide and coating combinations lack classification standardization, however, so different cutting tool suppliers use different names and classification methods for their grade charts. This can make grade comparison difficult for the end-user, an especially trying problem given that carbide grade suitability for a given application dramatically affects possible cutting conditions and tool life.
To navigate this maze, users must first understand what constitutes a carbide grade and how each element influences different aspects of machining.
What is the Substrate?
The substrate is the bare material of the cutting insert or solid tool underneath the coatings and post-treatments. It is typically made up of between 80- and 95-percent WC. To give the substrate the desired properties, material manufacturers add various alloying elements. The main alloying element is cobalt (Co) — higher levels of cobalt lead to greater toughness, while lower levels of cobalt lead to greater hardness. Very hard substrates can reach a hardness of 1800 HV and provide excellent wear resistance, but are very brittle and are only suitable for very stable conditions. Very tough substrates have a hardness of around 1300 HV. These substrates can machine only at lower cutting speeds and wear out faster, but have better resistance to interrupted cuts and unfavorable conditions.
The right balance between hardness and toughness is the most crucial factor when selecting a grade for a particular application. Picking a grade that is too hard can lead to micro breakages along the cutting edge or even catastrophic failure. At the same time, a grade that is too tough will wear out fast or require decreasing the cutting speed, thus reducing productivity. Table 1 provides some basic guidelines for selecting the correct hardness: