Materials Selection - Cast Iron Is Not Cheaper Than Brains

to change. A plastic gear, even though it may serve the same function as a metal gear, if designed properly, should look different than a plastic gear. If we simply try to drop in a plastic material into the same geometry as a part that was originally made from metal we are almost assured of failure.

There are numerous reasons why manufacturers consider material substitutions. Most frequently the driver for material substitution is cost reduction, but sometimes it is a change in supply base, or changes in material availability or manufacturing issues with the original material. The scope of the substitutions can vary from minor (e.g., changing from one alloy to another alloy in the same alloy family) to major (e.g., dramatically changing materials and processing such as changing from an aluminum die casting to a plastic injection molding), with infinite gradations in between.

Obviously, material substitutions of any complexity can be and are regularly accomplished, but in over 30 years I've witnessed many material substitution failures and here are some of the reasons: 1. Predicted cost savings aren't realized. Not all material substitution failures are technical, some are economic. As an example, with wildly increasing alloy costs over the past decade manufacturers sought out lower-alloy content stainless steels to minimize cost. A common substitution was to replace Types 304 or 302 stainless with a lower nickel alternative, Type 201.

For many applications this was an excellent technical solution, Type 201 provided good corrosion protection at significantly reduced cost from the steel mill. However, for manufacturers buying steel from service centers rather than mills, there actually could be cost increases because the service centers did not normally stock Type 201 and charged a premium for obtaining it in small quantities.

2. Critical properties change. Historically for many types of machine parts made out of steel, the critical property in design was strength. When lower weight and/or lower cost aluminum and plastic materials started to appear with equivalent strength to plain carbon steel there were many wholesale changeover programs from steel to these alternatives and many of these changeovers were miserable failures. Those failures usually occurred because with the alternative materials some other property (often stiffness) had not even been a consideration in the steel part design, now governed the integrity of the part. The bottom line is that properly designed parts made from alternative materials may look considerably different than the original part made from the traditional material.

3. We get greedy. As mentioned above, the most common driver for material substitution is cost reduction. With that being the case it is natural for us to consider material substitutions in those applications where the cost savings are the greatest. When making major changes, this may not be the best approach. If you're considering using a technology that is new to you, it might be well to start with an application that is fairly simple, one where you can learn and build on your experience. Early in my career I became an advocate for powder metallurgy technology and started out to apply P/M on several high profile projects. The parts I chose to work on were complex and P/M offered very substantial cost savings over the traditional steel fabrication methods that had been used to make prototype parts. Unfortunately, we ran in continual problems which, due to our inexperience, we didn't know how to solve and the part designs reverted to traditional manufacturing methods in order to meet project schedules. It took many years for me to get design engineers interested in applying P/M again after these spectacular failures. Any subsequent success we had with P/M came from building our experience starting with simpler parts and working our way up the learning curve.

In conclusion, successful material substitutions occur every day, but without attention to detail,