Carbide Grades

While the metallurgic term “carbide grades” refers specifically to tungsten carbide (WC) sintered with cobalt it’s used for various applications such as Nozzles, Dies, Rollers, Crush Rolls, and Cutting Tools. 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 Grade Selection describes the different carbide tool grades and explains how to select the proper grade for a cutting operation. Carbide grades are classified by two systems. The ANSI C-system lists grades of C1 through C8. The ISO classification system designates carbide grades as P, M, and K, followed by a number that further describes the qualities of the carbide. Carbide grade is often dependent on the type of metal used: tungsten, titanium, or tantalum. Grades have different levels of hardness, toughness, and wear resistance. Coating carbide tools can increase wear resistance and part quality.

What is the substrate?

In general, smaller tungsten carbide grain sizes allow for the manufacture of carbides with a finer microstructure. A prerequisite for this is the prevention of grain growth during the sintering process by adding suitable doping (process of adding some impurity atoms) components in the right amount, adjusted to the cobalt content.  The cobalt content is determined based on the required performance specifications for the carbide. Since the specific surface of a carbide depends reciprocally on its grain size, a finely grained carbide can adsorb more binder than a coarsely grained TC. 

An increase in cobalt content results in increased toughness (TRS) while hardness and wear resistance are reduced. This opposite development of the two desirable parameters, hardness, and toughness can be countered by reducing the carbide grain size. The result is an increased hardness because of the finer grain of the carbide which at the same time permits a high binding metal content as the grain structure offers a large surface area on which to bind:  It also creates a high toughness. Consequently, superfine grain carbide grades offer increased hardness while maintaining toughness.

Some basic guidelines for selecting the correct hardness:

Material

Turning

Milling

 

Continuous

Light Interrupted

Heavy Interrupted

 

Steel

Hard

Medium

Tough

Tough

Stainless Steel

Hard

Medium

Tough

Tough

Aluminum

Hard

Medium-Hard

Medium

Medium

Inconel

Very Hard

Hard

Medium

Tough

Titanium

Very Hard

Hard

Medium

Medium

 

What Are Carbide Coatings?

About 80 percent of the inserts used in machining today are coated cemented carbide grades. These tool inserts earned and maintained that growing market share because of their broad application for removing large amounts of material while they sustain a long tool-life.

The high material removal rates and long life for these tools are achieved through an incomparable balance of wear resistance and toughness. Relatively thin insert coatings -- 0.00004 in. to 0.0007 in. -- protect inserts from the heat, corrosion and abrasion that shorten their lives.

Coating processes: Two principal coating processes are used for indexable inserts to provide cutting edges with fundamentally different properties for machining:

  • Chemical vapor deposition (CVD) which uses a higher temperature and gives thicker coatings.
    • Physical vapor deposition (PVD) uses a lower temperature and gives thinner coatings.

Each of these processes give rise to different tensions in the coating material. Chemical vapor deposition develops tensile stress in the substrate, while physical vapor deposition tends to develop compressive stress. These stresses provide different desirable characteristics for the insert.

The CVD-coated insert typically has a thicker coating and has a high degree of wear resistance and coating adherence. The PVD-coated insert has a thinner coating, high toughness and is more suitable for sharper cutting edges.

CVD is the dominant coating process, partly because it is the only process capable of satisfactorily depositing layers of Al2O3.

Recent developments include CVD processes for depositing TiCN and Al2O3 in ways that lower stress levels and develop fewer tendencies for crack formation in the coated surface of the insert. After the heat of the CVD-coating processes a network of cooling cracks tend to form, resembling a dry riverbed. The resulting tensile stresses, due to the different coating materials involved in the several layers, can negatively reduce the toughness properties of the insert.

Coating Selection Guide:

Materials

Turning (High Vc)

Turning (Low Vc)

Grooving

Milling

P

CVD

PVD

PVD

PVD

M

Thin CVD

PVD

PVD

PVD

K

CVD

PVD

CVD or PVD

PVD or Thin CVD

N

Uncoated

Uncoated

Uncoated

Uncoated

S

Thin PVD

Thin PVD

Thin PVD

PVD

 

Post Treatment:

All coatings have flaws in their surface uniformity caused by coating droplets and non-uniform grain size. Most modern grades add a post-treatment after the coating process. This treatment is usually:

  • Sandblasting or tumbling,
  • Aiming to smoothen the finial coating layer.
  • Reducing the friction and, as a result, also the generated heat.

Carbide Grades Designation:

Just as there is no official standard for grade application ranges, there is no formal standard for grade designations. That said, most major carbide inserts suppliers follow common guidelines in their grade designations. The "classic" designation follows a six-character format BBSSNN, where:

  • BB — Brand Code: Each major supplier has its own letters associated with it.
  • SS — Grade Series Number: Grade series numbers are usually represented by two random digits. A series is usually a group of grades designed for a particular raw material and sharing a common coating type. Some examples of grade series could be:
  • BB85 — CVD grades for turning steel.
  • BB64 — PVD grades for nickel-based alloys.
  • BB23 — CVD grades for milling cast iron.
  • NN — Hardness Level: The last two digits, in most cases, reflect the hardness level of the different grades in a series. The number usually ranges from 05 to 45 according to the same system explained above about the grade charts. For example:
  • BB8505 — A very hard grade for turning steel in stable conditions.
  • BB8540 — A very tough grade for turning steel in heavy interrupted cuts.