Bonnie and Clyde favored Ford V-8's for getaway cars, as did many other bank robbers of the 1930's. The reason was simple: the Ford V-8 was a quantum leap hotter than the straight-six cylinder engines of other mass manufacturers. These felonious flights were enabled by a major advance in foundry practice: Ford's ability to cast a single-piece V-8 engine block. The one-piece block was both cheaper and better than the two-piece bolted blocks used earlier.
Almost every useful metallic object passes through at least one casting stage. Sand casting is the simplest and most flexible technique. First a model (pattern) is made of the desired object. Special casting sand is packed around the pattern to form a mold. The mold is then split open, the pattern removed, and molten metal poured in to form the desired part. It isn't quite that simple. Intricate passageways (known as sprues, gates, runners, and risers) are needed to ensure free flow of the incoming metal and the escaping air to make a quality part. Casting is a tremendously flexible and popular materials processing technique. But, there are plenty of places to screw up, from the part and mold design, to foundry practice, and even the final finishing and inspection of the part.
Cold storage plants are often large, and at several hundred dollars per cubic foot, easily hold millions of dollars worth of meat. Loss of refrigeration may be disastrous. In this case the refrigeration on a food storage plant had failed, with a large property loss. The culprit was a leaking casting called an ammonia strainer. This device resembled an ordinary plumbers tee joint about the size of my fist. I was given the valve and asked to find out what had caused the leak. There was no need for sophisticated equipment on this case. Hand examination, simple optical metallography, and chemical analysis were sufficient.
The valve was gray cast iron of 3.5 percent carbon and 2.3 percent silicon, which is a very common and readily castable alloy. Microscopic examination of a polished and etched section at about 100-fold magnification revealed the expected microstructure of graphite flakes in a ferrite (nearly pure iron) matrix. None of this told me why the casting was leaking.
I cut and polished a slice from the strainer from near the leak. The part of the slice in Figure 1 shows a crack extending almost all the way across the thickness of the piece. Figure 2 is a close-up of the region to the left in Figure 1.
I wanted to know where the crack went beneath the polished surface so I flexed the slice, which promptly snapped into two parts. Figure 3 shows the mating fracture surfaces. The regions to the top and lower left had never fused and were coated with oxide. The remainder of the surfaces shows the brittle fracture expected of gray cast iron. The ammonia had escaped by working its way along the non-fused surface.
The defect is known as a cold shut, which occurs when two streams of molten metal meet, but fail to unite. Cold shuts occur when the meeting streams of metal are too cold to unite, or when oxide gets incorporated into the casting. I do not know which was the case here.
What was to be done? It would be simplistic to simply scapegoat the foundry. A careful reexamination of the whole casting operation would be in order. Part and mold design should be examined to make sure that the molten metal can completely feed the casting. Pouring practice, in particular melt temperature should also be reconsidered. Leak testing of the finished casting would also be in order, in view of the possibly serious consequences of coolant escape.
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