Periodically, questions arise about shell vs. investment cast links for continuous heat treat belt application. I have been requested to give a brief overview of the metallurgical differences between them. There was a time 20 years ago or so that 3 or 4 foundries were producing investment cast links for belt use. Premature field failures of many of these belts prompted several of the companies that had them to micrograph and analyze the material to understand the cause of these failures. They found that, although the chemistry was in specification, the alloy grain size presented in the micrographs was extremely large with failures occurring along the large grain boundaries. The failure mode for these belts was a fast rate of creep (i.e. stretch) so that the belts had exceeded the useful link length (.100 over original length) in a matter of 8 months, instead of the normal 12 to 24-month period.
Since grain size in heat resistant alloy castings is a product of cooling rate, the casting method was suspect. Investment casting molds (mono-shell ceramic molds) are fired in a curing oven at 1800 degrees Fahrenheit to facilitate a phase change in the ceramic material. The molds are removed from the oven and placed into some type of support device so that they may be quickly poured full of metal. This quick pour-off is necessary because the ceramic mold material is dense and will act as a heat sink with regard to the higher temperature molten metal being poured in. If the mold cools too much, the metal will freeze before the mold is full and the castings will be incompletely formed (i.e. cold shut).
It is necessary to heat the metal to a temperature above 2850 degrees Fahrenheit to achieve a carbon boil. This is required to assure that the metal is free of nitrogen and hydrogen gas prior to pour. Normally, a pour-off temperature of 2950 degrees Fahrenheit is sought to provide sufficient time after tap to pour the molds (i.e. pot life). When 2900 degree Fahrenheit metal is poured into a mold at, say, 1700 degrees Fahrenheit, the mold temperature rises while the metal temperature falls. If equilibrium temperature of the metal and mold are reached at a point of 2500 degrees Fahrenheit or above (this happens for most molds) then the resulting cool down is very slow. 2500 degrees Fahrenheit is at the lower end of the liquidus range for most heat-resistant alloys. This means that the slow cooling rate from liquidus will allow very large grains to form, such as the ones found in the failure micrographs.
To alter this problem, it appears that some of the investment foundries have increased their carbon content above the ACI (Alloy Casting Institute) specification of .75% maximum. This added carbon will lower the freeze point of the alloy and may reduce the grain size somewhat. The higher carbon level, however, will allow the alloy to become brittle after very little time in a furnace atmosphere, secondary carbide precipitation occurs quickly and the links fracture from furnace stress.
Resin bonded shell molds, conversely, are cold when they are poured and are also exothermic after pyrolysis. This allows for rapid cool down through the mushy phase to solidus and produces a fine grain structure with minimal grain boundaries.
The principle reason most belt buyers give for purchasing an investment cast belt is the better dimensional control provided by that process. In truth, the standard tolerance given by investment casters is plus or minus .005 inches per inch. This means that the dimensional accuracy on a four inch long casting is plus or minus .020. Because of the much greater real cost of manufacture of investment castings, few people ever want to machine them to a closer tolerance. A shell-molded link, however, can be machined after casting to a tolerance of plus or minus .005 total and cost less overall to produce than an investment link.
It is probably, for this reason, that most investment foundries have discontinued
their production of link castings via the ceramic mono-shell method and have
opted, instead, to produce resin bonded shell molded links. Shell molded and
machined links provide better metallurgy, dimensional accuracy and lower real
sustainable production cost than any other process now employed for the purpose.