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 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 . 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. duced a modification of the AISI stainless steel By 1957 there were 39 AISI numbers for stain- numbers for SAE standards. SAE embedded most less steels, including the 4 to 6 % chromium Types of the AISI three-digit stainless steel numbers into 501 and 502. These 39 alloys are those that have a five digit system in which the two digits”30” often been referred to as “standard stainless steels” preceded the AISI 300 series numbers and “51” whereas all of the remaining alloys have been re- preceded the AISI three digit 400 series numbers. ferred to as “nonstandard stainless steels” simply AISI 304 became SAE 30304 and AISI 410 became because they were not included among those al- SAE 51410. loys having assigned AISI numbers. SAE also developed a series of special EV des- In the mid-1960’s, AISI discontinued the prac- ignations, such as EV-4 and EV-13, for proprietary tice of assigning numbers because of questions high-chromium, heat-resisting, aircraft engine concerning the legality of a trade association being valve steels. in the business of writing standards (i.e., estab- lishing chemical composition limits for stainless ASTM standards steel alloys). However, the AISI numbering system In the 1960’s, the American Society for Testing is still the basic system in the United States for and Materials (ASTM) Committee A-10 on Iron-

ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2007 41 first standard numbering system for steels in Great Britain. Stainless steels were assigned En num- The first bers 56 through 61 with letter suffixes. An alloy three similar to AISI 304, for example, received the des- ignation En 58E. The En numbers, which should digits in not be confused with the modern EN (Euronorm the British Numbers), were superseded around 1960, but they continue to be used and are often seen in ref- system are erence books. the AISI Around 1960, the BSI adopted a new num- numbers, bering system that incorporated the AISI system in most cases, but not all. The first three digits in followed by the British system are the AISI numbers, followed the letter by the letter S, for stainless, and two digits to in- S, for dicate modifications. 304S18 and 410S21 are ex- amples of the British Standard numbers for Types stainless, 304 and 410 stainless steels. Exceptions include and two numbers such as 301S81, which is not equivalent digits to to AISI 301, but is a designation for the 17-7PH alloy; and 284S16, which has a composition sim- indicate ilar to that of AISI 202. modifications. There are also British Aerospace numbers for stainless steels. These include HR numbers such as HR 3, S numbers such as 2S.130, and T num- The Air Force Memorial in Washington, D.C., ranks as bers such as T.69. one of the world’s largest structural applications of stainless steel. It consists of three stainless steel spires reaching 64 me- Deutsches Institut für Normung ters into the sky. Each spire has a 19-mm-thick skin of S31600 In Germany, DIN developed the Werkstoff stainless steel covering a core of reinforced concrete. Image numbering system for steel in 1959, reserving the courtesy Nickel Institute. 1.4000 — 1.4999 series of numbers for stainless Chromium, Iron-Chromium-Nickel and Related steels. The Werkstoff number 1.4301 is for a stain- Alloys introduced a series of thirty-four XM (XM- less steel composition that is similar to Type 304. 1 through XM-34) numbers to designate certain In addition to developing Werkstoff numbers, proprietary stainless steel alloys. This scheme was DIN also developed name designations for each adopted so that the proprietary alloys could be alloy. These consist of chemical symbols and num- included in specifications without using trade bers that indicate the approximate percentages names, which are generally not accepted in ASTM of the elements. The name associated with the specifications. The precipitation hardening alloys 1.4301 designation is X5CrNi18-10. The DIN five- were given XM numbers and Armco’s 15-5PH digit numbering system has been generally alloy, for example, was assigned the designation adopted for the Euronorm EN numbers. Various XM-12. forms of this name designation system have been adopted by the national standards organizations Alloy Institute of most European countries and ISO, as well as The Alloy Casting Institute, a former division by Russia and China. of the Steel Founders Society, developed a system for designating stainless and heat-resistant casting AFNOR (France) alloys. Casting designations within this system In France, a dual designation system consists begin with either the letter C for of steel names and a numerical system. The Stainless steel corrosion-resistant alloys, or H names are based on a mixture of abbreviations in eleven languages for heat-resistant alloys. Typical for chemical symbols and other names. The examples are CA-15 and HK-40. names start with the letter Z. The abbreviations English Stainless steel This is the most common desig- include C for chromium, N for nickel and F for Czech Nerezová ocel nation system for stainless steel free machining. The name for Type 304 stainless steel is Z6CN18.09 and for Type 410F, it is Dutch In roestvast staal casting alloys in the United States. Although the ACI organ- Z30CF13. French Acier inoxydable ization no longer exists, the The numerical system consists of the AISI German Nichtrostende Stäl system is maintained by ASTM number followed by the letter F and two digits, Italian Acciaio inossidabile Subcommittee A01.17 on Steel such as 304F01. Castings. Polish Gatunków stali SIS (Swedish Institute for Standards) Portuguese Aço inox British Standards Institution In 1947, SIS developed a four-digit numbering Russian Nerzhaveiushchei stali During the Second World War, system for metals, reserving the 23xx series for Spanish Acero inoxidable the British Standards Institution stainless steels. For example, the number 2332 was introduced En Numbers(Emer- established for an alloy having a composition sim- Swedish Rostfritt stål gency Numbers) which was the ilar to that of Type 304. 42 ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2007 JIS (Japan Industrial Standards) KS (Republic of Korea) Japan has generally adopted the AISI num- Korea has adopted a numbering system which bering system for stainless steels with some mod- appears to be consistent with the Japanese system ifications. In the Japanese system, SUS 304 corre- for stainless steel. The prefix letters STS are used sponds to Type 304 stainless steel. SUH numbers for corrosion-resisting steels, STR for heat-resisting serve to designate the heat-resistant alloys such alloys, and SSC for stainless steel castings. STS a SUH 310. SCS and SCH numbers are used for 304 corresponds to Type 304. stainless steel castings. GOST (Russia) ISC (People’s Republic of China) Steel names standardized by GOST use chem- China has a dual system of designating stain- ical symbols and abbreviations of chemical sym- less steel similar to that used in the DIN and EN bols. Ch and K are used for chromium, N for systems. It consists of the alpha-numerical ISC nickel and T for titanium. The name for Type 304 system and a steel name system based on chem- is 08Ch18N10. ical symbols and numbers indicating percentages. ISC is the Iron and Steel Code: Unified Num- UNE (Spain) bering System for Iron, Steel and Alloys. This ISC In Spain there are three numbering systems for system resembles the American UNS system in stainless steel. The steel names use chemical sym- many respects. It uses a letter followed by five bols and numbers such as X6CrNi19-10, which digits, and for stainless steels the initial letter is S. also corresponds to the F number F.3804. Some of the entries in the ISC system, such as AISI numbers are also used with the prefix E S31603, S31653 and S41008, are the same as those such as E305. in the UNS system, and in such cases the chem- ical compositions for the alloys are identical in UNI (Italy) each system. The ISC system for stainless steel Steel names standardized by UNI use chemical groups the alloys into five categories: S1xxxx (Fer- symbols and numbers. The name for Type 304 is ritic). S2xxxx (Austenitic-Ferritic), S3xxxx X8CrNi18-10. (Austenitic), S4xxxx (Martensitic), and S5xxxx (Precipitation Hardening). UNS (Unified Numbering System In the steel naming system, 00Cr17Ni13Mo2N for Metals and Alloys) is the designation for the316LN alloy, which is The ASTM/SAE UNS system of numbering S31653 in both the ISC and UNS systems. metals and alloys is widely used for the designa-

ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2007 43 Comparable designations high nickel contents. The UNS designation for alloy 20Cb-3 is N08020. for Type 304 stainless steel For steel casting alloys in the UNS system, a USA 304; 30304; S30400 Jxxxxx series of numbers was established. This J Hungary AoX; 7CrNi 18-9; KO 33 series is based on the total alloy content. For stain- less steel casting alloys, the numbers fall in a European EN 1.4301; X5CrNi 18-10 J9xxxx series. ACI alloy CA-15 has the UNS des- India 04Cr18Ni 11 ignation J91150. The second, third, and fourth International ISO X5CrNi 18-10 digits of J91150 indicate that the alloy has a min- imum alloy content of 11.5%, which happens to Italy X8CrNi 19-10 be the minimum chromium content for this alloy. Australia 304; S30400 Japan SUS 304 ISO (International Standards Organization) Austria X5CrNi 18-10 KKW Around 1960, the International Standards Or- ganization established ISO Committee TC17 on Korea STS 304 Steel, and Subcommittee SC4 on Heat Treated and Brazil E304 Alloy Steel, which developed ISO 683/ XIII, cov- Mexico MT304 ering stainless steel chemical compositions. Type Bulgaria 0Ch18N10 304 stainless steel was designated as Type 11 in that document. In 1987 the Type Numbers for Poland 0H8N stainless steels were changed to the identical steel Canada 304; S30400 names used by DIN. The former Type 11 stainless PRC China 0Cr19Ni9; S30408 steel is now designated by ISO as X5CrNi 18-10. An exception to this rule is that stainless steels for Czech Republic 240 surgical implants are designated by letters of the Rumania T6NiCr180 alphabet, since these alloys are not included in the France Z6CN18-09; 304F01 DIN system. Spain X6CrNi 19-10; F.3504; E-304 In 1980, a group within ISO wanted to develop an International Numbering System for Metals. Germany 1.4301; X5CrNi 18-10 The proposal was to use the basic format of the Sweden 2332 UNS system with an initial letter and five digits. Great Britain En 58E; S30418 The initial letters were the same as those in the Russia 08Ch18N11 UNS system in some cases, such as A for Alu- minum and C for and Copper Alloys. The tion and cataloguing of stainless steels in America letter S, however, was to designate Steel Alloys and abroad. rather than Stainless Steels as in the UNS system. The Unified Numbering System for Metals was It was obvious to the developers of the UNS developed in 1970 by the American Society for system that two systems that looked alike, but in Testing and Materials (ASTM) and the Society fact were different, would lead to a great deal of of Automotive Engineers (SAE) with the cooper- confusion. The ISO numbering system never went ation of the Aluminum Association (AA), the beyond the planning stage. American Iron & Steel Institute (AISI), the Copper Development Association, the Steel Founders So- EN (Europäischen Normen) ciety of America (SFSA), representatives from the Beginning in 1988, a new series of mandatory metals industry, and the U.S. government. European standards (Euronorms) were created The goal was to develop a numbering system to replace national standards, such as British Stan- for metals and alloys that could be applied dards, DIN, Swedish Standards, and French Stan- throughout the metals industry. Furthermore, it dards, throughout 18 countries of Western Eu- was to be a system with sufficient capacity to ac- rope. In 1995, three parts of EN 10088 were commodate the possible development of hun- published. Part 1 listed the chemical compositions dreds of new alloys without the need to add suffix of 83 stainless steels. In addition, EN 10027-1, Part letters as in the case of the AISI numbers for 1, Designation Systems for Steels: Steel Names; carbon, alloy, and stainless steels. and EN 10027-2, Part 2, Designation Systems for The existing numbers in the aluminum, copper, Steels : Numerical System, were also published and steel industries were to become a part of the in 1995. new system. For stainless steels, the system con- The designation system for steel names gener- sists of the letter S followed by five digits. For Type ally adopted the DIN steel names, such as X5CrNi 304 stainless steel, the UNS number is S30400 and 18-10 for AISI 304 stainless steel. EN 1.4301 was for Type 304L the UNS equivalent is S30403. The adopted from DIN Werkstoff number DIN 1.4301, system also permits the numbering of stainless also for the AISI 304 alloy. steel alloys for which there is no AISI designation, such as S35500 for stainless alloy AM 355. For Fer- ralium 255, the UNS designation is S32550. For more information: Harold Cobb, 117 Crosslands The letter N also designates certain stainless al- Drive, Kennett Square, PA 19348; tel: 610/388-1930; loys that contain less than fifty percent iron and [email protected]. 44 ADVANCED MATERIALS & PROCESSES/SEPTEMBER 2007