Because there are no international standards defining carbide grades or applications, users must rely on their own judgment and basic knowledge to be successful. #base
While the metallurgical term “carbide grade” refers specifically to tungsten carbide (WC) sintered with cobalt, the term has a broader meaning in machining: cemented tungsten carbide in combination with coatings and other treatments. For example, two turning inserts made from the same carbide material but with different coatings or post-treatment are considered different grades. However, there is no standardization in the classification of carbide and coating combinations, so different cutting tool suppliers use different designations and classification methods in their grade tables. This can make it difficult for the end user to compare grades, a particularly tricky issue since the suitability of a carbide grade for a given application can greatly affect likely cutting conditions and tool life.
To navigate this maze, the user must first understand what a carbide grade is made of and how each element affects different aspects of machining.
The backing is the bare material of the cutting insert or solid tool under coating and post-treatment. It usually consists of 80-95% WC. To give the substrate the desired properties, material manufacturers add various alloying elements to it. The main alloying element is cobalt (Co) – higher cobalt content results in greater toughness, while lower cobalt content increases hardness. Very hard substrates can reach 1800 HV and provide excellent wear resistance, but they are very brittle and only suitable for very stable conditions. The very strong substrate has a hardness of about 1300 HV. These substrates can only be machined at lower cutting speeds, they wear faster, but they are more resistant to interrupted cuts and adverse conditions.
The right balance between hardness and toughness is the most important factor when choosing an alloy for a particular application. Selecting a grade that is too hard can result in micro-breakage of the cutting edge or even catastrophic failure. At the same time, grades that are too hard wear out quickly or require a reduction in cutting speed, which reduces productivity. Table 1 provides some basic guidelines for choosing the right durometer:
Most modern carbide inserts and carbide tools are coated with a thin film (3 to 20 microns or 0.0001 to 0.0007 inches). The coating usually consists of layers of titanium nitride, aluminum oxide and titanium carbonitride. This coating increases hardness and creates a thermal barrier between the cutout and the substrate.
Even though it only gained popularity about a decade ago, adding an extra post-coating treatment has become the industry standard. These treatments are usually sandblasting or other polishing techniques that smoothen the top layer and reduce friction, thereby reducing heat generation. The price difference is usually small and in most cases post-processing is recommended for variety selection.
To select the correct carbide grade for a particular application, refer to the supplier’s catalog or website for instructions. While there is no formal international standard, most vendors use charts to describe the recommended operating range of grades based on “scope” expressed as a three-letter/number combination, such as P05-P20.
The first letter indicates the material group according to the ISO standard. Each material group is assigned a letter and a corresponding color.
The next two numbers represent the grade’s relative hardness level, ranging from 05 to 45 in increments of 5. 05 applications require a very hard grade suitable for favorable and stable conditions. 45 An application requiring a very tough grade suitable for harsh and unstable conditions.
Again, there is no standard for these values, so they should be interpreted as relative values in the particular grading table in which they appear. For example, a grade marked P10-P20 in two catalogs from different suppliers may have different hardness.
Even in the same catalog, a grade marked P10-P20 in the turning grade table may have a different hardness than a grade marked P10-P20 in the milling grade table. This difference comes down to different favorable conditions for different applications. Turning operations are best done with very hard grades, but when milling, favorable conditions require some strength due to the intermittent nature.
Table 3 provides a hypothetical table of alloys and their uses in various complex turning operations that might be listed in a cutting tool supplier’s catalog. In this example, class A is recommended for all turning conditions, but not for heavy interrupted cutting, while class D is recommended for heavy interrupted turning and other very unfavorable conditions. Tools such as MachiningDoctor.com’s Grades Finder can search for grades using this notation.
Just as there is no official standard for the scope of a class, there is no official standard for class designation. However, most of the major carbide insert suppliers follow the general guidelines for their grade designations. “Classic” names are in the six-character format BBSSNN, where:
The above explanation is correct in many cases. But since this is not an ISO/ANSI standard, some vendors make their own adjustments to the system, and it’s wise to be aware of these changes.
Grades play a vital role in turning applications more than any other application. Therefore, when browsing any supplier’s catalog, the turning part will have the largest selection of grades.
This wide range of turning grades is the result of a wide range of turning operations. This category ranges from continuous cutting (where the cutting edge is constantly engaging with the workpiece and is not impacted, but generates a lot of heat) to interrupted cutting (where strong impacts occur).
A wide range of turning grades is also associated with different diameters in production, from 1/8″ (3mm) for swiss type machines to 100″ for heavy industrial use. Because cutting speed is also dependent on diameter, different grades are required that are optimized for low or high cutting speeds.
Major suppliers often offer separate series grades for each material group. The grades in each series range from hard materials for interrupted cutting to hard materials for continuous cutting.
When milling, the range of grades offered is smaller. Due to the intermittent nature of the application, milling tools require tough grades with high impact resistance. For the same reason, the coating must be thin, otherwise it will not withstand impact.
Most suppliers will mill different material groups with rigid backings and different coatings.
When parting or grooving, grade selection is limited due to cutting speed factors. That is, the diameter becomes smaller as the cut approaches the center. Therefore, the cutting speed is gradually reduced. When cutting towards the center, the speed eventually reaches zero at the end of the cut, and the operation becomes a shear instead of a cut.
Thus, parting quality must be compatible with a wide range of cutting speeds, and the substrate must be strong enough to withstand shear at the end of the operation.
Shallow grooves are an exception to other types. Because of the similarity to turning, suppliers with a wide selection of grooving inserts often offer a wider range of grades for certain material groups and conditions.
When drilling, the cutting speed in the center of the drill is always zero, while the cutting speed at the periphery depends on the diameter of the drill and the speed of rotation of the spindle. Grades optimized for high cutting speeds are not suitable and should not be used. Most vendors offer only a few varieties.
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Post time: Aug-21-2023