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About the clearance of cemented carbide holes

Porosity

Porosity has always been an important indicator for judging the quality of cemented carbide tools. The presence of pores reduces the strength of the alloy and other performance properties. The size of the pores is an important criterion for the performance of cemented carbide. The porosity is evaluated by comparing standard pictures and microscopic pictures under 100 times magnification. Common pores are mainly divided into type A pores, type B pores and type C pores.

1.1 Reasons for the formation of type A pores

There are clear boundaries on the unetched grinding disc, and the maximum size of the pores on the grinding surface is determined as the pore size. Generally, pores less than or equal to 10 μm are defined as type A pores. The polished surface of the sample is magnified by 100 times and compared with the standard picture. It is divided into four grades A02, A04, A06, A08 and no pores found are called A00.

Figure 1 is a standard picture of type A porosity at 100 times, and Figure 2 is a photograph of type A porosity in alloys at a magnification of 100 times. Generally, the A02 type porosity in cemented carbide is a normal phenomenon in the industry. Because the porosity in the alloy is a relative concept, the characteristics of the carbide materials prepared by powder metallurgy determine that the alloy will not reach 100% relative density during the sintering process, accompanied by a certain amount of porosity.

*In view of the current lack of information on resistant strains, it is impossible to define other types other than sensitive. If the MIC value of the strain is not within the sensitive range, it should be sent to a reference laboratory for further testing.

1.2 Reasons for the formation of type B pores

Generally, the pores less than or equal to 25 μm are defined as type B pores. The polished surface of the sample is magnified 100 times and compared with the standard picture. It is divided into four levels B02, B04, B06, B08 and no pores found are called B00.

The main reasons for the formation of type B pores are (1) sintering process: including underfiring, overfiring and improper sintering process; (2) the compacted density of the green compact is low; (3) the raw material has high impurity elements, and the material is oxidized and dirty; (4) Inhomogeneous mixing of forming agents; (5) The impurity content of oxides in the raw materials is too high, and they are oxidized or reduced into circular or oblique pores. The larger cavities are called bubbling, which is the main oxide It is (TiO2, SO2, Al2O3, MgO, Co2O3, WO2, WO3, etc.); (6) Uneven sintering shrinkage; (7) Mixing material: including cobalt flakes and cemented carbide grinding ball debris.

1.3 Reasons for the formation of type C pores

In cemented carbide woodworking tools, graphite is uniformly distributed on the entire grinding plate in dots or appears as small holes aggregated. If the alloy is carburized from the outside, graphite inclusions may appear on the edge of the grinding plate.

Graphite inclusions, also known as “free carbon”, are actually a kind of pores. The international standards are divided into four levels: C02, C04, C06, and C08. No free carbon is found and called C00.

Under a microscope with a magnification of 100 times, after inspecting the entire grinding surface of the unetched grinding disc, select the field of view with the most graphite inclusions and compare it with the standard micrograph of known graphite content to evaluate the graphite inclusion content of the grinding disc, and report Graphite inclusion level.