The Naming and Numbering Of
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THE NAMING AND NUMBERING OF STAINLESS STEELS The variety of numbering and naming systems for stainless steels is based on the fact that the alloys were developed by companies all over the world. Harold M. Cobb* Consultant Kennett Square, Pennsylvania he stainless steel industry had its begin- nings around the year 1912, when metal- lurgists in three countries quite acciden- Back in 1935, Allegheny Ludlum proposed building a stainless steel car to Ford tally performed a bit of alchemy when Motor Co. The idea took shape in the form of this 1936 Ford Deluxe Sedan. That Tthey turned iron into alloys that did not rust. The- car became the centerpiece of a campaign to show the public the wonders of stain- less steel. Image courtesy Allegheny Technologies Inc. oretical studies were conducted on iron-chromium and iron-chromium-nickel corrosion-resistant al- “Stainless Knife.” This is the first known use of loys in Europe between 1904 and 1911. However, the word “stainless” to describe the alloy. Also in it is generally accepted that the commercial de- 1914, the steel was being sold for the production velopment of stainless steels began with the work of aircraft engine valves under the name “Firth’s of Harry Brearley at the Brown Firth Research Lab- Aeroplane Steel” (FAS). In fact, the entire output oratories in Sheffield, England, and with the work of FAS was destined for this purpose throughout of Eduard Maurer and Benno Strauss at the Krupp the 1914-1918 war. In 1917, the Firth-Brearley Laboratories in Essen, Germany, both in 1912. Stainless Steel Syndicate was formed to foster the Christian Dantsizen of the General Electric Re- worldwide production of stainless cutlery steel. search Laboratory at Schenectady, New York, was the developer in 1911 of a third class of alloys that The austenitic alloys became known as “stainless irons.” The Krupp alloy that was developed in 1912 The Brown Firth martensitic iron-chromium was austenitic and had a chemical composition alloy, the Krupp austenitic iron-chromium nickel that was virtually the 18 % chromium, 8% nickel alloy, and the Dantsizen stainless iron alloy are composition of “18-8” or what became known as described in this article, which also discusses the Type 304 stainless steel. Krupp named the alloy development of the names and the systems of “V.2.A” and referred to the material as a corrosion numbering the alloys that are now generally called resistant iron-chromium-nickel alloy. The alloy “stainless steels” in English, “Nichtrostende was a new metal very different from steel. It could Stähle” in German, and “aciers inoxydable” in not be hardened by heat treatment, it had a dif- French. ferent crystal structure, it had vastly different physical properties, it remained ductile at sub- The martensitic alloys zero temperatures, it was nonmagnetic unless The Brown Firth alloy produced in 1912 was an work hardened, and it did not rust. Krupp mar- alloy steel with approximately 13% chromium keted the alloy under the trade name Nirosta KA2, and 0.24% carbon, a composition falling within with Nirosta being an abbreviation of Nichtros- the limits of what became known in America as tende Stähle (non-rusting steel). Type 420 stainless steel. By 1914, the alloy was being made into table knives by George Ibberson The ferritic alloys & Co., a Sheffield cutler. The blades were stamped Dantsizen’s alloy contained 14 to 16% with the manufacturer’s name and the words chromium and 0.07 to 0.15% carbon. This ferritic *Life Member of ASM International alloy was only slightly hardenable by heat treat- ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2007 39 Staybrite (Thos. Firth & Sons, Ltd.), V.2.A. (Krupp Works), Jessop Hi-Gloss (Jessop Steel Co.), For many and Sterling Nirosta KA-2 S (Firth Sterling Steel years the Co.). By 1923, the Firth Sterling Steel Company at term McKeesport, Pennsylvania, was marketing three “stainless types of chromium stainless steels. These were called Stainless Steel Annealed, Stainless Steel steel” was Particularly Annealed and Stainless Iron. They applied only were “sold on Brand and Performance and not to to those Carbon Content or Chemical Analysis.” Four years later, Firth Sterling changed its marketing alloys practice, offering five types B Type A, Type T, Type that were B, Type M and Type H. For each type the carbon hardenable and chromium contents were listed. Type A, for example, was listed as having 0.35% carbon and by heat 13.5% chromium, a composition that meets the treatment, requirements of what became generally known the as Type 420 stainless steel. By the early 1930’s, almost 100 companies in martensitic the United States produced or fabricated stainless alloys. steels, marketing them with almost 1000 trade names, such as Defistain (Rustless Iron Company), Allegheny Metal B (Allegheny Steel Co.), Indus- trial No. 35 (Industrial Steels Co.), Ship Brand Or- The parabolic mirror made of ThyssenKrupp Nirosta stain- less steel at the Almeria, Spain, research facility converts ange Label (Webb Wire Works), and Corrosion solar energy into electricity. Image courtesy ThyssenKrupp. Resisting Circle L (Lebanon Steel Foundry). However, some companies broke away from ment and was at that time called stainless iron be- this practice of giving their products mysterious cause of its relative softness and ductility. The names and started using meaningful names such alloy was initially used for leading-in wires for as Armco 18-8, USS 19-9, and Enduro 20-10, where incandescent lamps. It is interesting to note also the numbers indicated the approximate percent- that there was once a Rustless Iron and Steel Com- ages of chromium and nickel in the alloys. Al- pany that was established in Baltimore in 1920. though AISI numbers had been developed, they The classic book written by the English metallur- were not yet in general use. gist, J. H. G. Monypenny in 1926, was entitled Modern producers continue to name their al- Stainless Iron and Steel. The terms “stainless iron” loys in various ways, including the custom of and “rustless iron” referred to certain iron- using established standard numbers, such AISI, chromium-carbon alloys that were predominantly UNS, DIN, or EN numbers, following their ferritic and not hardenable by heat treatment, or company names or trade names, as in the case only slightly hardenable. However, the terms were of AL 410S, AK Steel 41003, Cronifer 3718, and never adequately defined and are no longer in Outokumpu 4372. general use. For many years the term “stainless steel” was AISI standards applied only to those alloys that were hardenable No standardization of the chemical composi- by heat treatment, the martensitic alloys. This was tions of alloys produced in the stainless steel in- logical since the martensitic alloys had all of the dustry was established until the early 1930’s. The characteristics of alloy steel except that they did American Iron and Steel Institute (AISI) had de- not rust. veloped the now-familiar four-digit numbering system for carbon and alloy steels. They decided “Stainless steel” as a generic name on a three-digit numbering system for stainless When Ernest E. Thum, editor of the American steels. However, it is important to note that AISI Society for Metals’ Metal Progress, published the classed the 4 to 6% chromium steels as stainless first American book on iron-chromium-carbon steels, because these alloys showed a resistance and iron-chromium-nickel alloys in 1933, he called to sulfides four to ten times that of ordinary steel. it The Book of Stainless Steels. This book became a Due to this classification, the dictionary defini- classic in the stainless steel industry and undoubt- tion of stainless steel was “an alloy of iron and edly paved the way for the acceptance of “stain- at least four percent chromium.” less steel” as a generic name for all of these high- The AISI numbering system for stainless steels chromium, stain-resisting alloys. consisted of a 300 series for chromium-nickel austenitic alloys, a 400 series for high-chromium Trade names ferritic and martensitic alloys, and a 500 series for Some of the earliest trade names, around the 4-6% chromium alloys. Examples in each series year 1915, for various stainless steel alloys were include 304, 410, and 501. Anka (Brown, Bayley’s Steel Works, Ltd.), Forty-six AISI numbers and their correspon- 40 ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2007 ding chemical compositions were published in designating alloys standardized by AISI prior to 1932, twenty-eight of which are still in use. Many 1960. In fact, AISI requested that the usage of new AISI numbers were added that were prima- “AISI” should be discontinued, but that was easier rily modifications, using suffix letters, such as said than done. The AISI numbers still serve as 304L, 304H, 304N, and 420F. AISI 329 was the only the primary stainless numbering system in many duplex (austenitic-ferritic) alloy to have been as- countries including Canada, Mexico, Brazil, South signed an AISI number. Africa, India, Australia, New Zealand, and in a In the 1960’s, AISI established the 200 series modified form in the British, Japanese, and Ko- of numbers for the high-manganese austenitic al- rean numbering systems. In addition, the AISI loys and a 600 series to cover proprietary alloys numbers are widely recognized throughout the such as Allegheny Ludlum’s A286 alloy. This alloy world and are often referenced or adopted by was designated as Grade 660, and Armco’s 17- stainless steel producers. 4PH was designated as Grade 630. Although these 600 numbers still appear in ASTM specifications SAE standards and elsewhere, they were never officially adopted The Society of Automotive Engineers intro- by AISI.