More on the many flavors of stainless steel
To the editor:
The recent letter by Jeff Stives on broken turnbuckles ("Stainless steel failures raise voyager's concern," Issue 122, May/June 2002) was interesting. There is a saying in engineering circles regarding the proverbial glass of water. An optimist finds it half full, a pessimist finds it half-empty, but an engineer finds that the glass is twice as big as necessary. In short, things like turnbuckle bodies should be made of the most economical material that does the job. However there appears to be evidence that commonly used stainless steel is not always adequate for these applications.
To understand the nature of stainless steel, it is not necessary to go to engineering school. There are several excellent Internet sites run by stainless-steel trade associations in the United States, England and Australia that explain stainless steel very well. The Australian site, at www.assda.asn.au/marineapps.html, even directly addresses the problem with marine applications of stainless steel.
To summarize, there are basically four different types of stainless steel. They differ by grain structure caused by the various elements alloyed with iron. Two types, ferritic and martensitic, are not useful for marine purposes, lacking sufficient corrosion resistance. They are also magnetic, hence the old marine surveyor trick of ferreting out bad stainless by testing it with a magnet.
The most common type of stainless steel is austenitic, a family of alloys with a high proportion of chromium. Austenitic has a relatively high resistance to surface corrosion in its passive state, which is when sufficient oxygen reaches the surface to form a protective layer of chromium oxide. Types 302 and 304 are commonly used austenitic stainless steels. The addition of small amounts of molybdenum further enhances corrosion resistance. This alloy is commonly known as type 316 and is marketed as marine stainless steel. About 80 percent of all stainless steel used everywhere is either 302, 304 or 316. This makes it easy to get and relatively cheap. All of this is austenitic and has austenitic grain structure.
The problem is that in the presence of warm temperatures, seawater and stress, austenitic stainless steel, including type 316, is subject to stress-corrosion cracking. It can and will crack through and break without warning.
Overbuilding the component is not a remedy. The problem is intrinsic in the austenitic grain structure. Some high-end yacht builders polish all their type-316 stainless steel to a mirror finish. While this might inhibit visible surface corrosion, stress-corrosion cracking is an insidious process that will start where stress loads are the greatest.
I have seen expensive new American-built yachts with poorly aligned backstay chainplates with beveled holes for flathead bolts. These unnecessary bevels remove about 40 percent of the cross section of the chainplate at the point of maximum stress. Even worse are cantilevered, welded backstay attachments that place maximum stress on the end of the weld. These designs play to the inherent weakness in austenitic stainless steel.
Nigel Calder, responding to a question several months ago in Ocean Navigator, suggested several of the new super alloys, such as super-ferritic or super-austenitic, which have great corrosion resistance. While these exotics would no doubt do the job, the problem is that you can't get them in the small quantities necessary for yacht repair, or even yacht manufacture.
The answer, however, is simple. This problem is not unique to the marine industry. It has already been solved by the stainless-steel industry for those industrial applications in which type-316 austenitic stainless steel has proven inadequate because of stress-corrosion cracking. It is clearly described in the Australian website and in the supplier sites linked to that and the other sites.
The answer is the fourth type of stainless steel, duplex stainless steel. This alloy has less chromium than austenitic stainless. Duplex grain structure is half austenitic and half ferritic. It has higher corrosion resistance than type 316, and is highly resistant to stress-corrosion cracking. Best of all, it is available. Bar stock in type-2205 duplex suitable for chainplates is available on the Internet for about twice the cost of type 316. This adds about 15 to 20 percent to the cost of a finished chainplate, but it would remedy the problem of losing a mast.
So why don't yacht designers use duplex stainless for load-bearing components on their boats? Maybe this is the same as asking why shouldn't yacht designers have to be engineers.
Quent Kinderman lives in Edgewater, Md. His account of chainplate failure appeared in Issue no. 107, July/Aug. 2000.