The action of MetaLL ife involves proprietary spherical media that, under rigid controls, bombards the surface of the tool steel. Each piece of media striking the surface acts as a tiny hammer, imparting small indentations on the surface. The formation of these indentations yields the surface fibers of the material in tension. Below the surface, the fibers try to restore the surface to its original shape and thereby create a cold worked material highly stressed in compression. The rigid controls allow the overlapping dimples to develop an even layer of residual compressive stress. Tension is usually the stress that produces cracking in steel. The opposing compressive stress layer, which is in the range of 130-140% of the materials effective yield strength, prevents cracks from propagating.

The Charpy V-notch specimen for all surface treatments tested is shown above. This test measures the amount of energy that is needed to break this sample V-notch bar. In general, the impact resistance and fracture toughness or gross cracking resistance are inversely proportional to the hardness. Lower toughness is attributed to high hardness at the surface. It has been found that Charpy values exceeding 15 joules (11 ft.-lbs) are desirable in practical die casting. The low temperature surface treatments tested, such as Nitrotec, ion nitriding, and Melonite (a non-toxic form of Tufftriding) had only a range of 2.5 to 7 ft.- lbs with an average of 3 to 4.4 ft.-lbs. This is significantly lower than the tested conventionally machined and ground notch specimen that exhibited an average of 12.5 ft. Ibs.

As expected all of the MetaLL ife samples for both the E2 and E5 EDM methods, whether or not polished and/or tempered afterwards, showed a higher impact energy than that of the ground surface or low temperature surface treatments. For undetermined reasons there was somewhat less of an improvement on the high intensity (H.l.M) E2 samples that had been tempered (T) and polished (P). Electropolishing also showed an increase in impact strength but not as much as the MetaLL ife samples.

CONCLUSIONS

Previous testing of H-13 materials has shown that dies receiving the MetaLL ifeÒ process have a much greater resistance to heat checking and thermal fatigue This is true whether it be applied to a NEW or USED tool. This test now confirms that,in the case of EDM H-13 material, the resistance to gross cracking is also increased by the application of MetaLL ifeÒ. These average increases in impact energy were found to be anywhere from 23.2% to 39.2% greater than that for a machined and ground surface. The poor thermal fatigue resistance of the ion nitriding and Nitrotec treatments is attributed to the brittleness of the compound zone consisting of FE4N/FE2N3 in the ion nitrided sample and FE3N in the Nitrotec sample. The Melonite sample also had significantly reduced Charpy impact strength.

If the die caster desires resistance to gross cracking and increased resistance to fatigue cracking, MetaLL ifeÒ is the most effective surface treatment to apply to EDM or milled/ground surfaces. In addition, none of the other surface treatments have the ability to close existing cracks in die surfaces to keep them from further propagating. The die caster therefore has two choices for longer die life: he can PREVENT IT NOW- to extend his new tool life. or RESTORE IT LATER- to obtain additional life from dies that are already heat checked.

Click HERE for the H13 Thermal Stress Dip Tank Test.
You may download or print a hard copy of the EDM test along with other brochures.

Some web pages were last modified 07/14/2010
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