Page 16 - MetalForming-Mar-2018-issue
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time the tooling has with the workpiece by the same factor. This reduces the opportunity for the punch to dissipate heat into the work material, which affords some additional cooling.
Because small punches have much less ability to dissipate heat, they are more prone to over-heat. This can result in a loss of hardness, reduced wear resistance and dimensional insta- bility. High-speed or powdered-metal (PM) tool steels such as M4 and PM- 10V, tempered at temperatures greater than 1000 deg. F, exhibit temper resist- ance superior to A2 and D2 in high- speed applications.
Another tool-steel solution may involve cemented carbide. “Cemented” refers to the tungsten-carbide particles mixed with a metallic binder matrix and cemented together in a sintering process. The carbide industry com- monly refers to this material as carbide, but uses the terms solid carbide, tung- sten carbide and cemented carbide interchangeably.
Hardness is likely the most benefi- cial property of cemented carbide for metal-stamping processes. The phys- ical property is most important when it comes to abrasion resistance.
Hardness values for cemented car-
Hardness Conversion for General Carbide
bide usually are expressed in terms of Rockwell A (HRA) or Vickers values. Traditional tool steels, measured in a similar fashion, are expressed in terms of Rockwell C (HRC).
The Hardness Conversion for Gen- eral Carbine chart depicts the approx- imate conversion from HRA to HRC. Note that a traditional tool steel heat- treated and tempered to 62 HRC remains relatively soft when compared to a 6-percent cobalt grade carbide with a harness value of 92 HRA.
Carbide tooling also requires special care and consideration during tool con- struction and maintenance.
Component failure can occur when using ram-type electrodes in electri- cal-discharge machining (EDM) if the electrical amperage power is too high. Thermal micro-fracturing or heat checks can occur, causing crack initi- ation, notch propagation and prema- ture tool/part wear.
Fracturing or breaking of carbide tools also can occur when using a wire EDM. This damage usually occurs due to improper processing of the carbide material. Specifying EDM-quality material from the carbide manufac- turer helps to avoid this.
Carbide-compatible coolants are required when grinding carbide to ensure proper cooling, lubrication and corrosion protection of the tool. Cor- rosion of cemented carbide, usually referred to as leaching, is the removal of cobalt from the matrix. Without the cobalt binder to hold the tungsten car- bides in place, grain dislodgement occurs, expediting wear on the carbide edge or surface. The result is loss of dimensional stability, erosion of the cutting edge and tool surface, and, ulti- mately, premature tool failure.
Do you want to learn more about high-speed stamping processes? Then plan to attend PMA’s High-Speed Stamping seminar in Cleveland, OH, May 16-17. Visit www.pma.org to reg- ister or contact Marianne Sichi at msichi@pma.org for information.
MF
Tooling by Design
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
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
14 MetalForming/March 2018
www.metalformingmagazine.com
