Stainless Steel Technical Guide

Definition and Origin of Stainless Steels

Definition

Stainless steels are defined as iron base alloys containing relatively low carbon and a minimum of about 11 percent chromium (some put the minimum at 10 percent and others at 12 percent). Most commercial grades are higher than 11 percent chromium and contain up to about 0.08 percent carbon. Some may go to 0.2 percent carbon or even to 1 percent carbon. For increased corrosion resistance or for manufacturing requirements in specific applications, chromium may be increased and other elements such as nickel or molybdenum may be added as required. The effects of alloying elements and impurities in stainless steels and high-strength, heat-resisting alloys are shown in the section “Effects of Alloying Elements and Impurities in Stainless Steels and High Strength Heat-Resisting Alloys” on page 32. Stainless steels are sometimes called stain-resisting steels, since the term stainless may suggest non-staining. However, stainless is a correct designation; it does not mean non-staining in all environments, but less staining or less resistant to corrosive attack when compared with steels containing under about 11 percent chromium. When the total alloy content exceeds about 50 percent, the designation “heat-resisting” is more applicable than “stainless.”

Origin

There is an old story about stainless steel being discovered shortly after World War I by a European scrap dealer. After noticing one shiny cannon barrel in a pile of rusty scrap cannon barrels, he had it analyzed and found its chromium level to be quite high. While that event may have occurred, the claim to original “discovery” is considered fictional. Somebody must have known something to produce a superior artillery piece.

Another legend has it that stainless steels were discovered by Harry Brearley, an English metallurgist. In the early 1900s he made a series of experimental steels with 6–16 percent chromium for gun barrels. In a discarded heap he observed that some barrels had not rusted like the rest of the heap.

During the century preceding World War I, extensive research with chromium alloy steels was conducted in Europe, England and the U.S. Unfortunately, many of the early metallurgists studied alloys of unfavorable composition, e.g., chromium too low or too high and carbon too high (likely due to difficulty in obtaining low carbon-level alloys.) To make matters worse, early experiments were limited to exposure in sulfuric acid, completely overlooking the significance of resistance to attack by nitric acid.

“Discovery” of stainless steels having compositions within the ranges of the three major classes recognized today (martensitic, ferritic and austenitic) occurred progressively during the productive period of about 1905–1915 in three distinct directions: constitution, corrosion resistance and industrial applications. In 1905–1906, Leon Alexandre Guillet (France) was the first to produce and explore stainless steels metallurgically and mechanically in compositions falling within the three major classes. However, he did not discover the phenomenon of passivity, the remarkable resistance of stainless steels to most corrosive chemical solutions, which would readily attack ordinary steels. Passivity is a relative term, since a stainless steel may be passive, e.g., inert (corrode at very low rates) in one medium, and active (corrode at high rates) in another.

From 1908–1910, the research of Phillip Monnartz (Germany) disclosed the ‘stainlessness’ of stainless steel as a function of passivity. If any one person can be given the credit for discovery of the stainless phenomenon of the steels described as stainless, it should be Monnartz. He noted the significance of the minimum level of about 11 percent chromium, the need for low carbon, the fact that carbon could be stabilized, and the contribution of molybdenum in enhancing corrosion resistance.

As it pertains to industrial usefulness or commercial applications, history records that Benno Strauss and Eduard Maurer promoted austenitic grades in Germany in the 1909–1912 time frame. In the U.S. (1911- 1914), Frederick Becket and Christian Dantsizen were conducting work with ferritic grades. In addition, Harry Brearley introduced cutlery steels (Type 420, martensitic) in England (1913–1916).

In summary, we can say that the discovery of stainless steels followed the cumulative efforts of many investigators during the century preceding 1915, with full revelation occurring in the productive decade of discovery between 1905 and 1915. Within another 10–15 years, stainless steels were well recognized and widely in use in a large variety of applications.

Since the 1905–1915 glory years of discovery, many new grades of stainless steel have been developed. The American Iron and Steel Institute (AISI) now lists 60 standard types. Many proprietary grades are also in use commercially. In recent years, the precipitation hardening (PH) stainless steels have emerged as the fourth class. More recently, duplex stainless steels, containing approximately 50 percent ferrite and 50 percent austenite, are being promoted for aggressive environments where resistance to stress corrosion cracking is of paramount importance.