Tuesday, 22 March 2011

Why is the grain size important?

The story of Tungsten Carbide (WC) powder metallurgy, and particularly that of the hardmetal industry, is depicted by a progressively widening range of available grain sizes; whilst simultaneously narrowing the grain size distribution for each grade of WC powder.

The most significant motivation for this broadening of the range of available WC grades is the properties of WC hard metals such as hardness, strength, toughness, thermal conductivity and abrasion resistance can be extensively varied by grain size. At the start of the hardmetal industry the range of available WC grain sizes ranged from 2.0 to 5.0 microns, the grain sizes of WC powders now used in hard metals range from 0.5 microns to 50 microns, or even 150 microns for some very special applications.

The majority of grades we machine are made with standard size grains varying between 1 and 3 microns in size. Using larger grains of 2 - 6 microns will greatly increase the strength and toughness of the material because the larger grains interlock better. The tradeoff is that larger grain materials do not offer as much resistance to wear as finer grain sized materials. Sub-micron materials that vary between 0.4 and 1.0 micron grain size are harder than standard grain materials with the same cobalt content. The sub-micron grains are much more uniform in size and hence give improved hardness as well as increased carbide strength. However, as specs show the transverse rupture strength is perhaps 20% improved on 15% sub micron compared to 15% fine grain material but this can give a false impression as sub-micron carbide is not as resistant to impact and may chip more easily.

There are currently four carbide grades offered for die and wear part function. The first is the typical grade with a 1.5 micron to 17 micron grain size of tungsten carbide. The typical carbide grade is chosen where light, medium, or heavy shock loads are faced. It is commonly used in settings such as airless paint spray nozzles, valve seats, pump seals, cold heading fastener dies, cold impact extrusion punches, wire drawing and seaming rolls.

A submicron carbide is also selected where light, medium, or heavy shock loads occur, but when a fine, keen cutting edge is also required.
If the size of the grain is too large and the percentage of binder is too higher, then the carbide will deform under pressure. One of the major advantages of carbide is its proficiency to cope under pressure or compressive force. If it is too soft it loses that ability.

Neither binder percentage or grain size alone determines how a grade will perform.

Wednesday, 9 March 2011

Tungsten Carbide - What impact does the binder have?

The binder in most grades of Tungsten Carbide is cobalt. The other binder used is nickel. The binder is added as a percentage by weight varying from 3% to 30%. The amount of binder used is a very important factor in determining the properties of each grade. As a rule of thumb the lower the cobalt content the harder the material will become. However variation in grain size and additives can upset this rule.

Tungsten Carbide permutations have a unique variety of properties, high compressive strength, hardness and resistance to wear, as well as a capability to endure shock and impact. Typical applications are seal rings, valves, linear bearings and blades.

Traditional Tungsten Carbides, with Cobalt binder, have limited corrosion resistance, making them unsuitable for applications in which the wear parts are functioning under both severe abrasive and corrosive conditions.

Tungsten Carbide binder

Friday, 4 March 2011

How many grades of carbide are available?

We use Tungsten Carbide every day in a variety of products, for use in a number of different applications throughout many industries. It is an incredibly versatile and useful material due to its unique properties

There are over a dozen different grades of Tungsten Carbide. When evaluating or finding equivalents of Tungsten Carbide grades the important criteria is to specify two of three factors; binder content, hardness or grain size.

When choosing between the numerous distinct grades, it is important to understand what properties are required for the application. For example grade CD-630 is extremely hard wearing, but only able to cope with low impact, so it is perfect blades and seal rings. Whereas CD-70 is designed to endure high impact environments and is therefore perfect for manufacturing header dies.

The Tungsten Carbide properties chart, on our website, shows basic data for each grade manufactured. All specifications are designed and engineered for a purpose and rigid controls are kept throughout production processes to ensure adherence to grade engineering.

Quality control properties such as hardness, density, and minimum transverse rupture strength were determined from tests made on each batch of powder before it is used in the manufacturing process. Other properties such as Young's modulus of Elasticity, Poisson's ratio, Coefficient of Thermal Expansion, Thermal Conductivity and Electrical Conductivity are used by engineers for design calculations. Properties such as compressive strength, grain size and abrasion resistance give the designer additional information about the suitability of the grade for the part being designed

For more information, please contact us today