Cemented tungsten carbides for tools, dies and wear parts.
Selecting the proper tungsten carbide grade boils down to an educated compromise. The specifier, aware that no metal offers the ultimate in everything desired, must determine what properties are needed most, and then strike the balance that best meets his or her needs.
How much toughness is needed? What about resistance to corrosion and shock? How important is wear and abrasion resistance? What design parameters must be considered? What are the applied loads and stresses? Is edge preparation a consideration?
Then add to this matrix considerations such as tungsten carbide grain size, binder alloy percentage, and the addition of titanium, tantalum carbide, and other alloying elements. All of these variables have to be calculated to achieve the best compromise and result.
Cemented tungsten carbide consists of tungsten carbide particles that are glued or cemented together by a relatively ductile, pure metal cobalt or nickel if corrosion is a consideration. The tungsten carbide acts as "bricks" and the binder material as "mortar."
Key to determining the properties of a tungsten carbide material is the relationship between carbide grain size and binder percentage. The smaller the carbide grain size, the higher the hardness and abrasive resistance, but the lower the resistance to shock. Larger carbide grain size increases toughness and resistance to shock loads, but reduces hardness.
When the binder percentage is lowered, shock resistance declines. Conversely, raising the binder percentage improves shock resistance. This behavioral change occurs because the binder is the ductile constituent in the cemented tungsten carbide.
Three carbide categories
Three categories of carbide grades have been used for tool, die, and wear part applications. The first is the conventional grade, with a 1.0[micro] to 6.0[micro] tungsten carbide grain size. The second is a submicron 0.7[micro] average grain size. The third is an ultrafine submicron 0.5[micro] tungsten carbide grain size. Although all three use a cobalt binder (Fig 1), they also can use a nickel binder.
The conventional carbide grades have been used most frequently where light, medium, or heavy shock loads are encountered. Recent successes have been reported for this grade when substituted for steel:
* Necker dies to make aluminum beverage cans for the container industry--die life increased up to 20 times.
* Poppet valve stems and seats in airless paint spray compressors for the commercial and industrial spray paint industry--25 times longer wear life, with much less production downtime.
* Crush form rolls, used to crush specific forms into an abrasive grinding wheel. The rolls were used for high volume grinding of parts for the automotive, steel and aircraft industries--25 times longer roll wear life.
Other applications where the conventional carbide grade have been used include: rotary mechanical pump seals, electrical fractional motor lamination dies, cold heading fastener dies, back shaving dies and tube mill rolls.
The submicron carbide grades have been used also where light, medium, or heavy shock loads are encountered, but more particularly when a fine, keen cutting edge is also needed.
Typical applications have included:
* Staking and caulking punches used to coin valve seats in automotive antilock braking system compressor pumps. By moving from a conventional to a submicron tungsten carbide grade, one plant increased its quality and quintupled its punch wear life.
* Flex blades used for cutting applications in the paper converting and nonwovens industry. One large manufacturer, changing from a steel blade cutting edge to tungsten carbide, increased edge wear life 20 times.
* Circular slitters used to slit magnetic tape for audio, video and memory tapes. Four large electronics companies gained ten times longer cutting edge life simply by changing from a conventional tungsten carbide grade to a submicron grade. For the preceding applications, the tungsten carbide grain size was held below 1[micro]. The binder materials were varied as required to obtain the desired balance of strength, toughness and wear resistance.
The ultrafine submicron carbide grades have been used where light, medium, or heavy shock loads may be encountered and, in addition, where a strong and exceptionally keen cutting edge is required.
Ultrafine submicron tungsten carbide provides an unusually high fracture toughness for a given hardness. This combination of properties is especially advantageous where thin, sharp sections may be subjected to moderate shock loads.
There is an important relationship between tungsten carbide grain size and binder percentage. As suggested earlier shock loads can be absorbed by a larger grain size and higher binder percentage. However, if the carbide binder percentage is too high (say in the 20% to 25% range) the grade may become too ductile for the application. A condition known as swaging or peening could occur, causing the binder to plastically deform and the die or wear part to fail prematurely.
When the carbide grain size is too small, and the binder percentage does not give the wear part enough ductility, the carbide grade may be too hard and lack enough shock resistance for the application. Fracturing, cracking, or chipping may cause premature tool failure.
Shock, wear, and toughness properties can be balanced for optimum performance by carefully adjusting grain size. Larger grain size and higher binder percentages can absorb shock. To sustain severe impact shock load, therefore, a coarser grain size should be used, with a medium-to-high range of binder percentage. For a less severe bending load, a finer grain size would be more appropriate, with a low-to-medium binder level.
After determining the carbide grain size and percentage of binder, the specific type of binder and other alloying elements need to be considered. The binder systems directly affect the life of the tungsten die or wear part.
If straight abrasive wear is a concern, a cobalt binder system is recommended. If corrosion poses a threat, a nickel binder system is generally more suitable. Nickel-cobalt, nickel-chromium, chromium-cobalt and/or nickel-chromium-molybdenum systems may be even better if corrosion attack is severe.
A cobalt binder should not be used with a tungsten carbide material if there is any possibility of galvanic corrosion. A more corrosion-resistant binder should by used in such cases to minimize the potential for this or other types of corrosion. The right binder can stretch the life of the carbide die or wear part by many times.
Conditions such as galling or hot welding can be remedied by adding tantulum/niobium and titanium carbides to the binder system. At high temperatures, these additions form a tenacious oxide on the working surfaces. The oxide surface reduces friction and heat and increases the hot hardness of the carbide material.
For greatest galling resistance, tantalum carbide of up to 25% can significantly improve tool performance, particularly at temperatures above 1000F (538C).
Tool geometry can have a major effect on tool, die, and wear part life. Carbide has an extremely high hardness (up to Ra 94.5) and a modulus of elasticity of 80,000 ksi (about 2 1/2 times the 30,000 ksi for steel). This combination of high hardness and high stiffness makes carbide "notch sensitive" (Fig 2), thus susceptible to the cracking, chipping, and fracturing known as macrochipping.
Stress raisers in design should be avoided to reduce the possibility of fracturing. Honing sharp edges (Fig 3) of carbide tools can reduce stress-producing flaws and the tendency to crack.
Tungsten carbide is so hard that it exhibits virtually no ductility or plastic deformation prior to fracturing. Due to its extreme hardness, it is sensitive to stress concentrations found at holes, sharp internal angles, and notches. This design should be avoided on surfaces that are loaded in tension.
Sintered tungsten carbides have a thermal expansion coefficient that typically ranges from 4.5 to 8.5 x 10 -6/k from 32F to 1472F (0C to 800C)...less than half that of tool steels. This means that thermal stress and strain can cause a problem, especially when carbide and steel are brazed in an assembly.
Carbide tools, dies, and wear parts should be designed so that loads are transmitted away from weak surfaces to stronger, larger structural areas. Shear angles on punches for progressive stamping dies can reduce fracturing of the carbide and permit use of lower press tonnage.
Sintered tungsten carbide has compressive strength ranging as high as 1000 ksi. That is greater than the compressive strength of almost any other group of materials, metallic or non-metallic.
It is also three to five times stronger in compression than in tension. Thus, it should be loaded in compression whenever possible. Shrink interference fits are highly recommended, with carbide dies prestressed in compression by an outer ring. In general, the uniform application of stresses will reduce the possibility of sudden failure.
How carbide is made
One final note relates to method of manufacture. Carbide is made by means of powder metallurgy which distinctly minimizes, but does not entirely eliminate, the possibility of porosity. However microscopic, porosity can be detrimental to tool life and performance consistency. Except for small amounts, porosity should be avoided.
Internal porosity can be eliminated by Hot Isostatic Pressing (HIP), using Argon gas at 14,000 psi and a temperature of over 2372F (1300C) to densify and eliminate porosity in carbide materials. Therefore, it is advisable to specify "HIP" treated tungsten carbides, especially when they are intended for critical applications.
Many variables and conditions have to be considered in selecting the proper tungsten carbide grade for tool, die, and wear part applications. Every application should be evaluated carefully and individually to optimize the composition most likely to provide the right balance of hardness, wear, strength, toughness and corrosion resistance.
While initial costs of tungsten carbide tooling are likely to exceed those for conventional tooling, the tungsten carbide upgrade can be expected to provide significant end use cost reductions.
Carbides at work
* A major eastern manufacturer used carbide blades to cut photographic film and nearly quadrupled blade service life by switching from a conventional tungsten carbide grade to an ultrafine submicron grade from Crafts Technology. The original blades were good for 384,000 cuts before they required removal for resharpening. The replacement blades produced 1.6 million cuts before resharpening was needed.
* A New England saw manufacturer used a tungsten carbide grade developed for extreme wear environments to increase blanking die parts production by more than five times. Dies made of steel and a conventional carbide grade were producing an average of 40,000 parts over a period of up to 12 months before they had to be removed for resharpening.
When the company made its blanking dies from Crafts Technology's tungsten carbide grade, it boosted average output to 225,000 parts over the same time frame. In addition, the company reduced regrinds from 30 to six per month, with a proportionate reduction in machine downtime.
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|Comment:||Cemented tungsten carbides for tools, dies and wear parts.|
|Publication:||Tooling & Production|
|Article Type:||Industry Overview|
|Date:||Jan 1, 1999|
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