Page 40 - MetalForming February 2015

Tooling by Design
By Peter Ulintz
Stamping with Carbide Tooling
Rockwell A
Rockwell C
91.8-92.8
79.5-81.5
91.5-92.5
79.0-81.0
90.5-91.5
77.0-79.0
90.2-91.2
76.5-79.5
89.8-90.8
75.6-77.6
89.0-90.0
74.0-76.0
88.5-89.5
73.0-75.0
88.0-89.0
72.0-74.0
87.5-88.5
71.0-73.0
87.0-88.0
71.0-72.0
86.0-87.0
69.0-71.0
83.0-84.5
63.0-66.0
81.5-83.0
61.0-63.0
Tungsten carbide ( WC), also called cemented carbide, is a compos- ite material manufactured by a process called powder metallurgy (PM). The process mixes tungsten-carbide powder with a binder, usually cobalt or nickel. The mixture is compacted in a die and then sintered in a furnace to convert the powder into a solid mass by using heat and pressure.
The term “cemented” refers to the tungsten-carbide particles being mixed with the metallic binder matrix and cementing them together via sinter- ing. The carbide industry commonly refers to this material simply as carbide, but the terms solid carbide, tungsten carbide and cemented carbide are used interchangeably.
Carbide materials find use in metal- stamping dies developed for long pro- duction runs. They exhibit high com- pressive strength, resist deflection and retain their hardness values at high tem- peratures, a physical property especially useful in high-speed cutting, punching and forming. Some processes may only be possible with tungsten-carbide punch- es, such as perforating small-diameter holes in hard, tough materials.
Beneficial Properties of Carbide
Hardness likely is the most benefi-
Peter Ulintz has worked in the metal stamping and tool and die indus- tries since 1978. He has been employed with the Anchor Manufacturing Group in Cleveland, OH, since 1989. His back- ground includes tool and die making, tool engi- neering, process engi-
neering, engineering management and product development. Peter speaks regularly at PMA semi- nars and conferences. He is also vice president of the North American Deep Drawing Research Group. Peter Ulintz
pete.ulintz@toolingbydesign.com www.toolingbydesign.com
Fig. 1—Hardness Conversion Chart (HRA to HRC). Source: General Carbide Corp.
cial property of cemented carbide for metal-stamping processes. It is the most important physical property when it comes to abrasion resistance. However, hardness alone does not dic- tate the success of a carbide grade in a particular wear application; other sig- nificant factors include cobalt content and grain size.
Hardness values for cemented car- bide usually are expressed in terms of Rockwell A (HRA) or Vickers values. Traditional tool steels, measured in a similar fashion, are expressed using the Rockwell C (HRC) scale.
Fig. 1 depicts the approximate con- version from HRA to HRC. Note that a traditional tool steel heattreated and tempered to 62 HRC still remains rela- tively soft when compared to a 6-per- cent cobalt-grade carbide with a hard- ness value of 92 HRA.
Compressive strength is another important attribute of cemented car- bide. Ductile materials, when over- loaded in compression, may plastical- ly deform (bulge or swell) without fracturing. Brittle materials under sim- ilar conditions tend to fail catastroph-
ically (sudden breakage).
Cemented carbide exhibits very high
compressive strength when compared to most other materials. Fig. 2 illus- trates how the compressive strength of cemented carbide increases with decreasing binder content and decreas- ing grain size.
When adhesive wear poses prob- lems—during severe draw forming and ironing operations, for example—the addition of tantalum can prove bene- ficial. Tantalum carbides ( TaC) reduce interface friction between the tooling because the tantalum works as a built- in lubricant within the microstructure, serving as an anti-galling agent.
Design Considerations with Carbide
To ensure success with carbide tools, stampers and toolmakers must elimi- nate stress concentrations. Any stress riser located at a point of high stress— a sharp corner radius for example— likely will become a crack-initiation site. A crack would propagate rapidly under continued stress and lead to pre- mature failure. Ductile materials are not as susceptible to stress risers since they can plastically deform at the sites of localized stress and may not fail immediately.
A minor modification in the shape of the tool can considerably reduce the stress concentration. For example, using the largest possible radius when transitioning from one diameter to another can minimize the stress con- centration in round punches.
For mounting round sections of car- bide into a steel holder, stampers often employ 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 the steel holder proves ideal for withstanding the tensile hoop stress encountered with this method.
38 MetalForming/February 2015
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