It has been said that the tool materials of one generation become the engineering materials of the next one. This observation is certainly true of tungsten carbide. It is a material that has been around for almost 100 years, and it’s one with which General Carbide is highly familiar. After all, we created The Designer’s Guide to Tungsten Carbide. Many people throughout the industry have told us it’s their go-to source when they need a refresher, or when they need to educate others about this versatile material, and here are some highlights:
At the most basic level tungsten carbide (WC – the “W” based on the German-derived term wolfram, another name for the material and also its periodic table symbol) is an alternative for tool steels in many cutting tool applications and has been developed into an engineering material used to resist the harshest environments of corrosion, high temperature, impact, high compressive loads, deformation and severe abrasion.
Tungsten carbide competes with advanced ceramics in the wear parts arena, but just as tungsten carbide has not totally displaced tool steels, advanced ceramics can’t replace tungsten carbide. That’s because tungsten carbide exhibits superior toughness given its high hardness, which has enabled it to enjoy tremendous growth as a tooling and engineering material.
Tungsten carbide, also referred to as cemented carbide, is a composite material manufactured by a process called powder metallurgy. Tungsten carbide powder, generally ranging in proportion between 70%-97% of total weight, is mixed with a binder metal, usually cobalt or nickel, compacted in a die and then sintered in a furnace.
The term “cemented” refers to the tungsten carbide particles being captured in the metallic binder material and “cemented” together, forming a metallurgical bond between the tungsten carbide particles and the binder (WC-Co), in the sintering process.
A Metallurgy Primer
If permanent deformation of a material at failure is small, the material is considered brittle. If plastic deformation is large, the material is considered ductile. Carbide is classified as a brittle material because it exhibits little or no plastic deformation preceding the initiation of a crack and total failure. Without the presence of the metallic binder, tungsten carbide could be considered a ceramic material, much the same as silicon carbide or aluminum oxide.
The definition of a ceramic material is the marriage of a metal to a non-metal, for example, silicon (metal) carbide (carbon, nonmetal), aluminum (metal) oxide (oxygen, non-metal), or silicon nitride. A cermet is a composite material composed of ceramic (cer) and metallic (met) materials. It is the addition of the metallic binder, i.e., cobalt or nickel that makes the cemented carbide (WC-Co) a cermet and differentiates it from truly brittle materials, that is, the ceramic family of materials.
Cemented carbide is the preferred material for parts that must withstand all forms of wear (including sliding abrasion, erosion, corrosion/wear and metal-to-metal galling) and exhibit a high degree of toughness. It exhibits high compressive strength, resists deflection, and retains its hardness values at high temperatures, a physical property especially useful in metal-cutting applications. It provides long life in applications where other materials would not last or would fail prematurely.
How Tungsten Carbide Came to Be
Since the late 1800’s when a French chemist, Henri Moissan, first synthesized it, tungsten carbide has been known as one of the hardest substances in existence, approaching diamond in this respect. In fact, he was seeking to produce man-made diamonds, but WC was the result. Since large solid pieces could not be produced, cast compositions containing tungsten carbide were tried, but were too brittle and porous for use as an engineered material.
Search for a substitute to replace the diamond dies employed in drawing tungsten wire for electric-lamp filaments led the Osram Lamp Works of Berlin to an interesting discovery. Karl Schroeter and Heinrich Baumhauer found that hard carbide, bonded or sintered together with a metal such as cobalt, was not only hard but acquired enough toughness to suggest its use as a cutting tool.
Although various carbide manufacturers use different manufacturing processes, the final product is obtained by compacting the powder formulation by some technique and sintering the constituents into a solid mass in which cobalt, or a similar metal, bonds or cements the particles of carbide together. Rigorous control is necessary throughout the manufacturing process since the quality of the final product can be greatly affected by seemingly insignificant factors.
Purity, quantity, and particle size of the powdered materials must be closely watched. Mixing, milling, pressing, pre-sintering, and sintering techniques are among the factors influencing the characteristics of the finished material. Carbide should be uniform in structure and grain size, free from porosity, and of maximum density, strength, and hardness. Modifications can be achieved by using various carbides and bonding materials, by varying the proportions of carbide to the cementing matrix, and by regulating the carbide particle size.
The Manufacturing Process
As noted earlier, cemented carbide is made by a powder metallurgy process. The compaction process is performed under very high pressure in a mechanical press as shown in Figure I-1 or in an isostatic chamber to form a part with the consistency of blackboard chalk. A small amount of wax (paraffin) is added to increase the green strength and help in handling the compacted shape. In this “green” state, it can be formed or shaped by conventional methods such as turning, milling, grinding, and drilling
The formed and shaped carbide is then sintered (placed in a vacuum furnace at a high temperature). During the sintering process, the carbide may shrink as much as 20% linearly, or nearly 48% by volume (Figures I-3 and I-4).
For an “as-sintered” part, it is considered an industry standard to be able to hold a tolerance of ± 0.8% of the dimension or ± 0.005”, whichever is greater. Tighter tolerances can be held on smaller pressed parts. After sintering, cemented carbide has achieved its full density and hardness. It can then be fabricated by diamond wheel grinding or electrical discharge machining (EDM) techniques.
Tungsten carbide has many desirable physical properties, including high:
- Transverse Rupture Strength
- Compressive Strength
- Mechanical Strength
- Wear Resistance
- Modulus of Elasticity
- Impact Strength
- Fatigue Strength
Thermal properties include strong coefficient of thermal expansion and conductivity. Although the material is an excellent conductor of heat, its thermal conductivity factor is about one-third that of copper.
Electrical and magnetic properties include good electrical conductivity, which makes it conducive to the use of EDM (electrical discharge machining) as a fabrication tool, and low magnetic permeability, which can be useful in applications requiring strong abrasion resistance such as electronics.
Tungsten carbide particles are corrosion-resistant although binder material is susceptible to leaching in the presence of a strong acid or alkali solution. When corrosion or wear is a key design requirement, specially alloyed tungsten carbide nickel (WC-Ni) grades are the best choice.
When it comes to component design, stress is the primary consideration to ensure that the component can withstand the operating load without fracturing.
The amount of stress the part experiences and the amount of deflection that occurs are the main factors to consider when designing any component. Based on the application, other factors of which to be mindful are:
- Impact/Shock Resistance
Cemented carbide is a brittle material, so little plastic deformation will occur before cracking and ultimate failure of a part. Therefore, yield strength and rupture strength need to be identical to each other, or extremely close at a minimum.
As for safety, only a strong knowledge of design and a thorough understanding of material strength, combined with application engineering experience, can result in a formulation that will perform under required conditions.
Cemented carbide has a range of fracture values caused by micro-voids inherent in brittle materials. That feature makes it necessary to evaluate failure stress differently than one does with other materials. Stress value at fracture can vary widely with size, stress state (tensile, bending, torsion), shape and type of loading.
Transverse rupture strength, or bending strength, is the most common way to determine the mechanical strength of cemented carbide.
Avoiding stress concentrations should be a major consideration when designing a cemented carbide part because a minor modification in shape can reduce stress concentration considerably. Ways to avoid stress concentration include using the largest possible radius when transitioning from one diameter to another in roundtool parts.
Another word of wisdom is to never allow internal sharp corners and, if possible, state on the print the expected radius. For external surfaces, add this note on the technical drawing: Break all sharp edges.
Determining the relationship among transverse rupture strength, tensile strength and torsional strength is essential if the manufactured part is to meet performance requirements. Although tensile strength is the weakest mechanical property of all brittle materials, including cemented carbide, using proper design techniques to take advantage of cemented carbide’s high compressive strength can overcome that weakness.
At elevated temperatures, cemented carbide will retain most of its strength. Its exceptionally high hot hardness is a property readily taken advantage of in metal cutting applications. As temperatures approach 1000°F (538°C), oxidation will occur as a powder layer or flakes on the carbide surface. Above that temperature, conditions are too severe for cemented carbide to be used. Fortunately, most industrial applications don’t reach that temperature extreme.
Regarding corrosion, using cemented carbide for wear resistant components such as seal rings, flow control devices, nozzles and bearings is common.
When it comes to selecting a carbide grade, the best approach is to test the grade under the conditions for which it will be used.
Attaching & Assembling
To take advantage of cemented carbide’s unique properties, including wear resistance, compressive strength and hardness while minimizing effects of the material’s inherent brittleness, it’s advisable from both metallurgical and economic perspectives to join the carbide to a tougher material, such as steel or non-ferrous alloys.
Attachment techniques include brazing, industrial adhesives, interference/shrink fit assembly and mechanical fastening. Although welding is a popular joining technique, it’s not effective for joining carbide to steel due to oxidation-related issues.
Vacuum technologies, including electron-beam welding, have been successful in welding carbide to steel, or two carbide components to each other. “Friction welding” can be used for applications involving small cross-sectional joints such as a carbide tip being attached onto a steel band saw for a metal cutting band saw application.
Brazing and epoxy adhesive techniques are common but have limitations due to their sensitivities to various operating temperatures and the presence of corrosive substances in the production environment, such as sulfur, phosphorus and halides.
For industrial applications, brazing methods include furnace, vacuum, induction, torch and resistance.
Types of brazing compounds for bonding cemented carbide to steel include silver-copper (silver solder alloys), low temperature tin/lead/zinc alloy solders and high temperature copper brazing filler composition.
When deciding which compound to use, consideration must be given to the temperature range of the application and the bonding temperature of the material when specifying a braze compound for a specific job.
Low temperature solders cause less thermal strain after bonding but have low mechanical strength and won’t stand up to operating conditions above room temperature.
Medium temperature brazing – silver solder braze compounds – are the most common brazing alloys for bonding cemented carbide to steel. Available in rod form and as pre-cut shims or strips, they typically contain other elements to help wet the carbide surface.
High temperature brazing – straight copper – retains almost all its strength up to temperatures of 1000°F (538°C). Beyond that temperature, most cemented carbides begin to oxidize.
Copper also makes a good braze for bonding, but at 2100°F (1149°C), the bonding temperature required for using copper, most common steels will experience excessive grain growth and become brittle or weak. Some high-speed steels and air hardening steels, as well as silicon-manganese steels, can withstand those high temperatures.
Other high temperature brazing alloys are composed primarily of nickel and flow at temperatures from approximately 1820°F (993°C) to 1925°F (1052°C).
Brazing cemented carbide to hardened steel presents its own challenges because cemented carbide is unable to withstand a liquid quench which is typically required for hardened steel. Thus, a brazed assembly can not be subjected to rapid cooling without experiencing severe cracking.
As an alternative, a nickel base air hardening steel rather than a chromium base steel is best because the chromium oxide that forms on the steel is difficult to flux away. Additionally, nickel steel has higher toughness and can better relax stresses generated during brazing operations.
High-speed steel can be used for components if hardened and brazened with a low temperature silver solder at a maximum temperature of 1200°F (649°C). The material will retain most of its hardness at that temperature, making it suitable for many applications.
Industrial adhesives are an ideal choice for joining irregularly shaped surfaces which may be problematic for brazing. When adhesives and mechanical fasteners are used together, they form a stronger bond than when used separately.
Interference/shrink fit assembly is a widely used and highly reliable method of mounting round sections of carbide into steel to use as an interference fit. The high compressive strength of carbide makes it ideal for the compressive loading encountered with shrink fits, and the tensile strength of steel is ideally suited to withstand hoop stresses encountered with this method.
Mechanical fastening is a proven method for fastening cemented carbide to steel. Specific techniques include clamping, wedging, dovetailing, screw mounting and draw rod mounting.
Cemented carbide parts can be finished to the desired shape, size, flatness and surface finish by diamond wheel grinding or by diamond lapping and polishing. Another option is EDM (electrical discharge machining).
Diamond wheel grinding removes desirable portions of material from a part by subjecting it to repeated, overlapping contact with a rotating diamond wheel. During the grinding process, the rotating diamond wheel contacts the work piece, so tips of the exposed diamond particles barely touch the surface to be ground.
The process is repeated until the desired amount of material is removed from the part.
When grinding, it’s necessary to avoid thermal shock caused by sudden changes in temperature, such as a loss of coolant flow. To minimize overheating and cracks, wet grinding is preferable to dry grinding. In fact, many grinding fluids have been developed especially for carbide grinding. Such fluids do not contain chemicals that will leach or degrade the carbide structure, yet they provide adequate heat dissipation.
EDM is the process by which a part is machined using the erosive properties of electrical discharges. The two basic types of EDM are wire and probe (die sinker). Wire EDM is used primarily for shapes cut through a select part or assembly.
During EDM, a series of non-stationary timed electrical pulses remove material from the workpiece. The electrode, workpiece and dielectric are all held by the machine tool. A power supply controls the timing and intensity of the electrical discharges, as well as the movement of the electrode in relation to the workpiece.
EDM is best suited to parts with very thin walls, small internal radii, high depot to diameter ratios or that are small and hard to hold while machining.
If wire EDM will be sued to cut the part, it is advisable to let the carbide supplier know so grade formulation can be adjusted to provide maximum resistance to cracking.
Designing parts made with tungsten carbide has its challenges, but they are far outweighed by the application benefits.
We wrote the book on tungsten carbide tooling.