CRITERIA FOR SUCCESS
Prominent in the steel casting’s success were the following manufacturing engineering elements integrated into the cast- ing design for overall specification compliance: • Conversion from carbon or low alloy steel (both of which suffer from very poor fluid life and limited ability to form thin cast walls) to a martensitic high alloy steel. 17-4 PH in the H1100 aged condition was selected for strength, toughness, wear resistance and fluidity in its liquid state.
• Choice of the investment casting process for the 17-4 PH alloy, poured into a hot mold with a metal delivery system designed to provide pouring pressure and mold atmosphere venting suf- ficient to enable cast walls 0.08 in. (2 mm) thick.
• Integration of the metal delivery geometry with 17-4 PH solidification gradients to provide properly placed sources of liquid to feed solidification shrinkage at the required Grade B and C integrity levels.
• Provision for hot isostatic pressing (HIP) of the rough casting to assure the Grade B and C integrity levels were achieved in the first prototype iteration.
• Designing the solidification gradients in the metal delivery system to preclude the need for HIP in eventual production castings.
• Further integration of the metal delivery system with “tie- bars,” which are braces designed to resist warping and twisting of a thin, rangy casting shape during solidification and heat treatment.
• Combining the tie bar geometry with the geometric dimensioning and tolerancing (GD&T) zones required for dimensional compliance.
• Evaluation of high alloy steel contraction rates in the investment shell mold tooling design to make a best estimate for centering critical as-cast dimensions in the required GD&T zones.
• Use of rapid prototyping/rapid tooling technologies to create the investment shell mold tooling via stereo- lithography (SLA) to save the lead time required for a traditional wax die and avoid the additional dimen- sional centering variable of wax contraction.
• Creation of a dimensional checking/straightening fix- ture directly from the as-cast solid model via selective laser sintering of nylon powder. The resulting nylon fixture is tough enough to withstand processing of prototype quantities.
42 | MODERN CASTING February 2014
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