The material grade for the valve sealing surface is denoted by the uppercase letter “Y” – what is its composition?

Tungsten carbide and cobalt, along with small amounts of additives; tungsten carbide accounts for about 80–94%, cobalt for 6–20%, while additives such as VC, CrC, TaC, etc. usually make up less than 1%.

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Cemented carbide is produced by adding an appropriate amount of cobalt to tungsten carbide, and then compressing and sintering it using powder metallurgy methods. Therefore, it is fundamentally different from tool steel in terms of composition and manufacturing method, which results in various differences in properties. 1) There is no directionality in cemented carbide. Cemented carbide is produced by pressing and sintering powders; since no casting process is used, there is no difference in density between the surface layer and the interior, which eliminates any differences in local mechanical properties that could arise from such density differences. 2) There are no heat treatment issues with cemented carbide. Apart from the effect of thermal stress during heating and cooling, it can be assumed that its mechanical properties do not change as a result of these processes (for example, even after being heated to 1000°C and then cooled, the mechanical properties remain unchanged). Therefore, for the machining of such cemented carbides, pre-machining cannot be carried out on the softened strips; that is, the pre-machining of cemented carbides must be done before sintering. After sintering, it is impossible to machine them using other simple methods except diamond tools or electrical machining techniques. The mechanical properties of cemented carbide are primarily determined by the cobalt content and the particle size of tungsten carbide. 3) Cemented carbide has a high Young’s modulus, 2.5 to 3 times that of tool steel, and a low Poisson’s ratio; the Poisson’s ratio for cemented carbide is 0.21 to 0.24, while that for tool steel is around 0.30. Therefore, under the effect of processing stress, the change in the inner diameter size of cemented carbide molds is much smaller than that of steel molds, allowing for the production of products with dimensions very close to those of the mold. 4) Cemented carbide has high compressive strength. Compressive strength is related to the cobalt content; cemented carbides with low cobalt levels can achieve a strength of over 6000 Mpa, which is nearly twice that of tool steel. Therefore, it is advantageous to use this material for the working parts of cold extrusion dies that are subjected to high unit pressures. 5) Cemented carbide has a low coefficient of thermal expansion. Compared to the thermal expansion coefficient of tool steel, which is 11×10-6/K, that of cemented carbide is 5–7×10-6/K, which is lower. This should be taken into consideration during mold design and manufacturing. 6) High thermal conductivity. The thermal conductivity of cemented carbide is three times higher than that of tool steel. This prevents the sintering problem between the material to be processed and the mold. 7) The elastic and plastic deformation amounts are very small. This is an advantage for molds, but it also presents a disadvantage. It is highly sensitive to concentration in the mold, which can easily cause damage to the mold. This is a major issue in the design and use of cemented carbide molds. 8) High hardness and good wear resistance. This is the most popular feature of cemented carbide. Although its hardness is similar to, or even lower than, that of tool steel, its wear resistance is better due to the stability of its hardness, which does not change; as a result, it has a stronger ability to resist adhesion and thermal sticking. The cemented carbides used for molds in China are primarily of the tungsten-cobalt type, with the cobalt content varying between 6% and 25%, as shown in Table 8-48. Generally speaking, materials with low cobalt content have greater wear resistance, while those with high cobalt content exhibit better impact resistance; therefore, the tungsten carbide particles should be finer when the cobalt content is low, and coarser when the cobalt content is high.

May I ask: if the valve core is made of 304 or 316 stainless steel, can it still achieve the mechanical properties of cemented carbide Y? Is that possible?

The hardness of stainless steel won’t be sufficient, and 316 is even softer than 304

Question: For some valve sealing surface materials, the letter H is used to denote them. What material does this H represent?

H represents Cr13 series stainless steel; for detailed information, refer to JB/T308-2004

Hello, H stands for hard seal. Usually, the valve body is made of WCB; hard seal is indicated by H. If the valve body is made of stainless steel, hard seal is indicated by W.

I used to think that if the code for the sealing surface material of a stainless steel valve was W, then the sealing surface material was identical to the material of the valve body. May I ask: Are my views above correct? Thank you!