Chemistry and Metallurgy in the Investment Casting Industry

by Rick Price, Process Control Supervisor, Spokane Precision Castings Division of Spokane Industries

I've always been fascinated with metals. When I was a young child, I used to stoke up the campfire as a Boy Scout just to watch aluminum cans melt. All I knew about metals at that time was that aluminum and steel were different. As I worked different jobs at Spokane Industries I learned about different alloys, chemistry and metallurgy and how those elements affect the steel. Though I am not a metallurgist by trade, I have acquired a great depth of knowledge through my thirst for learning and a variety of avenues of study and research afforded me through my employment at Spokane Industries.

SPOKANE INDUSTRIES, through its Spokane Steel Castings, Spokane Metal Products and Spokane Precision Castings operations, has been providing customers around the world with cost effective solutions to their steel castings and industrial fabrication needs since 1952. Today, SPOKANE INDUSTRIES is a modern diversified company with plant and manufacturing facilities occupying more than 240,000 square feet. The company serves a large number of customers in many industries -- from aerospace, construction and agriculture, to transportation, aggregate processing, and manufacturing.

Alloy selection is a key factor when designing an investment casting. Strength, ductility, toughness, hardness, machinability, weldability, wear resistance and corrosion resistance are just a few of the design criteria that must be taken into consideration. In ferrous alloys for example, carbon plays an important role in determining strength and hardness. Other elements, such as molybdenum, chromium and nickel effect through hardness, or hardenability.

Chromium imparts corrosion and oxidation resistance in stainless steel when present in amounts greater than about 11%. Once the proper combination of various elements and weight percentages is determined, most alloys are then heat treated to attain the final properties. Castings are generally subjected to a high temperature soak, quenched at a pre-determined rate and then tempered at an intermediate temperature.

The chemistry of every heat of metal poured at Spokane Precision Castings is analyzed to insure that we meet customer specifications. Metal samples are taken at various stages throughout each heat and sparked in an optical emission spectrometer. During this process, each element emits a unique wavelength of light that is diffracted through a grating and directed to individual photomultiplier tubes. The measured light intensity is translated into a weight percentage for over a dozen elements. Each element has a range it must fall within for the metal to be in specification and to meet certain requirements.

Spokane Precision Castings runs this process on both a preliminary as well as a final sample to ensure consistency. We check chemistry, make alloy additions, pour the heat, and then test another button after the chemistry has been adjusted as the final chemistry for that heat. Our company is in the process of testing a new product to take a sample out of the stream of molten metal as it is being poured into the casting. This will improve our final chemistry analysis and results.

Currently, Spokane Precision Castings is working on creating a range of castings that would be made out of a cobalt-based alloy. Cobalt has many different properties than an iron-based alloy i.e. higher corrosion resistance, good wear properties, and is often used as medical implants.

As the company progresses, we want to improve Spokane Industries Quality Assurance efficiencies to transfer the knowledge of the Quality Assurance personnel to the technicians on the production floor. This constant push to resolve problems at the lowest level helps to rectify in process issues as they occur. We currently are implementing written and visual instructions to facilitate this transfer of knowledge and to augment our ongoing training and quest for continuous improvement.

Jeff Kuntz, Production Manager at Spokane Precision Castings Division of Spokane Industries states "to make a quality investment casting takes a great understanding of materials, a solid grasp on how those materials react with one another, a broad array of process controls and a quest for quality. Making a thousand castings as opposed to a single casting is key in understanding the investment casting industry. The investment casting process is very effective at producing consistent quality over a large run of parts. That being said, it is only true when the organization recognizes the dependency on tight process and quality controls in order to meet all customer demands as we do here at Spokane Precision Castings."