Because there are no international standards that define 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 same 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 tool suppliers use different designations and classification methods in their class tables. This can make it difficult for the end user to compare grades, which is a particularly difficult problem given that the suitability of a carbide grade for a given application can significantly affect likely cutting conditions and tool life.
To navigate this maze, users must first understand what carbide 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 base material the desired properties, material manufacturers add various alloying elements to it. The main alloying element is cobalt (Co). Higher levels of cobalt provide greater toughness and lower levels of cobalt increase 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. Choosing a grade that is too hard can result in microcracks along 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 stiffness:
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 carbon layers of titanium nitride, aluminum oxide and titanium nitride. 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 methods that smooth the top layer and reduce friction, which reduces heat generation. The price difference is usually very small and in most cases it is recommended to prefer the treated variety.
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 recommended operating ranges for grades based on a “range of use” expressed as a three-character alphanumeric combination, such as P05-P20.
The first letter indicates the ISO material group. Each material group is assigned a letter and a corresponding color.
The next two numbers represent the relative hardness of grades from 05 to 45 in increments of 5. 05 applications require a very hard grade for favorable and stable conditions. 45 Applications requiring very tough alloys 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, grades marked P10-P20 in two catalogs from different suppliers may have different hardness.
A grade marked P10-P20 in a turning class table may have a different hardness than a grade marked P10-P20 in a milling class table, even in the same catalog. This difference boils down to the fact that favorable conditions vary from application to application. 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 use in turning operations of varying complexity, which may be listed in the catalog of a cutting tool supplier. 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 stamps, there is no official standard for brand names. 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 have made their own adjustments to the system, and it would be wise to be aware of these changes.
More than any other application, alloys play a vital role in turning operations. Because of this, a turned profile will have the largest selection of grades when checking any supplier’s catalogue.
The wide range of turning grades is the result of a wide range of turning operations. Everything falls into this category, from continuous cutting (where the cutting edge is in constant contact with the workpiece and does not experience shock, but generates a lot of heat) to interrupted cutting (which generates strong shocks).
The wide range of turning grades also covers a large number of diameters in production, from 1/8″ (3 mm) 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.
Large suppliers often offer separate series of grades for each material group. In each series, grades range from hard materials suitable for interrupted machining to those suitable for continuous machining.
When milling, the range of grades offered is smaller. Due to the predominantly intermittent nature of the application, cutters require tough grades with high toughness. 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. Thus, 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 rather than a cut.
Therefore, the grades used for parting off 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 similarities to turning, vendors with a large selection of grooving inserts often offer a greater variety of grades for certain material groups and conditions.
When drilling, the cutting speed in the center of the drill is always zero, and 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: Mar-30-2023