It’s hard to imagine a world without it, but how is stainless steel produced? Tony Wheatly explores.
Metallurgists have defined stainless steel as a steel alloy containing at least 10% chromium, by mass it is distinguished from chromium steel by the lower carbon content.
Interestingly it was in 1821 that Frenchman Pierre Berthier first recognised the corrosion resisting properties of iron-chromium alloys and the first practical use he imagined was cutlery; today it’s difficult to picture a table setting without stainless steel.
The problem was that until Hans Goldschmidt of Germany developed the aluminothermic (thermite) process for producing carbon-free chromium, it was not possible to produce alloys with the combination of high chromium and low carbon, that have proved so indispensable in the manufacture of items as diverse as jewellery, automotive exhausts, architectural features and cryogenic storage tanks.
Stainless steel has been selected for the production of thousands of products because of it’s relatively low cost, high strength, resistance to corrosion and staining, low maintenance, and attractive lustre.
In addition, it’s surface has antibacterial properties, can be pressed, bent, machined, welded and even cast into a myriad of different forms and sizes.
The very strong affinity of chromium atoms for oxygen, and the similar size of the oxide molecules, allows a thin, invisible and passive film to form on the surface of steel alloys containing at least 11% chromium when exposed to air.
Although only a few atoms thick, this stable layer of oxide molecules packed on the surface protects it from further corrosion and blocks corrosion from penetrating the metal’s internal structure. Unless contact with oxygen is prevented, any damage to the oxide-layered surface from cutting or scratching is quickly self-repaired by the spontaneous passivation process.
There are unfortunately certain conditions that will compromise the corrosion resistance of stainless steels, by interfering with the stability of the surface oxide layer. In low-oxygen or poor circulation environments, iron atoms will be inadequately protected due to the incomplete formation of chromium oxides.
Contamination with substances that have a stronger affinity for chromium, like chlorides and carbon, will also cause staining and may lead to pitting corrosion – where the iron atoms are left unprotected by the preferential formation of chromium chloride or chromium carbide.
Exposure to high temperature can also destroy the corrosion resistance of stainless steels and this must be carefully controlled when alloys are welded. If stainless steel is heated to a temperature in the range 500 degrees to 800 degrees Centigrade for any reasonable time, there is a risk that atoms will react with any carbon present in the steel thus forming carbides along the grain boundaries.
Stainless steel components that experience tensile stress, either through structural loading or from residual stress from welding fabrication, can be susceptible to cracking when exposed to corrosive elements. This is called Stress Corrosion Cracking and austenitic stainless steel alloys are particularly susceptible requiring careful stress-relieving treatment before assembly.
Types of stainless steel
Online encyclopedia Wikipedia explains that five types or categories of stainless are identified by their predominant crystal structure:
Austenitic or 300 series, stainless steels comprise over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient nickel and/or manganese to retain an austenitic structure at all temperatures from the cryogenic region to the melting point of the alloy.
A typical composition of 18% chromium and 10% nickel (18/10 stainless), is often used in flatware. Super-austenitic stainless steels, such as alloy AL-6XN and 254SMO, exhibit great resistance to chloride pitting and crevice corrosion, due to high molybdenum content (>6%) and nitrogen additions, and the higher nickel content ensures better resistance to stress-corrosion cracking versus the 300 series.
Low carbon versions of the Austenitic Stainless Steel (below 0.03%), for example 316L or 304L, are used to reduce the sensitisation effect.
Ferritic stainless steels are highly corrosion-resistant, but less durable than austenitic grades. They contain between 10.5% and 27% chromium and very little nickel, if any, but some types can contain lead. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo-2Ni.
Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machineable, and can be hardened by heat treatment.
Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), nickel (0-<2%), and carbon (about 0.1-1%) (giving it more hardness but making the material a bit more brittle). It is quenched and magnetic.
Precipitation-hardening martensitic stainless steels have corrosion resistance comparable to austenitic varieties, but can be precipitation hardened to even higher strengths than the other martensitic grades. The most common, 17-4PH, contains about 17% chromium and 4% nickel.
There is a rising trend in defense budgets to opt for an ultra-high-strength stainless steel when possible in new projects; the Lockheed-Martin Joint Strike Fighter is the first aircraft to use a precipitation-hardening stainless steel in its airframe.
Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50/50 mix, although in commercial alloys, the mix may be 40/60 respectively.
Duplex steels have improved strength over austenitic stainless steels and also improved resistance to localised corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels
In 1954, Praxair Inc. invented a process called Argon Oxygen Decarburisation (AOD) as an economical supplement to the electric arc furnace (EAF) method of making stainless steel and today, over 75% of the world’s stainless steel is made using the AOD process.
AOD steelmaking requires a shorter operating time and lower temperature than EAF methods and also increases the availability of the EAF for melting purposes.
Unrefined steel is transferred from the EAF into a separate converter vessel. A mixture of argon and oxygen is blown from the bottom of the vessel through the molten steel. Cleaning agents are added to the vessel along with these gases to eliminate impurities, while the oxygen combines with carbon in the unrefined steel to reduce the carbon level to the required 0.1% to 0.4% range.
Argon dilutes the carbon monoxide formed during the decarburisation and therefore lowers the partial pressure of carbon monoxide which encourages its formation at lower temperature (1600°C). Economic benefits of the AOD process include shorter heating times and lower heating costs, combined with less refractory wear.
Other process enhancements employ nitrogen, carbon dioxide or argon gases to stir molten steel to improve slag-metal mixing and thereby increase the yield, reduce carbon and oxygen content, reduce aluminium consumption, lower power consumption, and homogenise bath temperature and alloy composition.
Oxygen applications include burning carbon monoxide produced in the Basic Oxygen
Furnace and air enrichment to increase productivity of re-heating furnaces and improve fuel efficiency, while reducing NOx, CO2 and SO2 emissions.
Market trends for stainless steel
Analysts predict that 2009 will be an unprecedented third consecutive year of declining global stainless steel output.
The trend began in 2007, when a 2% decline in output from the 2006 peak of 28.4 mmt was witnessed, and a further 3-4 % decline is expected for 2008 based on third quarter production cuts in Europe, the US and Asia.
Surging prices encouraged demand for nickel free substitute alloys during 2007 and after the nickel price bubble burst halting production growth, has left mills holding stocks of high priced nickel bearing steels and the risk of huge de-valuation losses.
A fundamental ferrous metal
Highlighting how integral stainless steel is throughout our everyday lives and functions, Sjef Roymans, Editor-in-Chief of Stainless Steel World publication, explains, “There is hardly a material thinkable that has found its way to so many applications in such a short period of time as stainless steel. Only developed in the first decade of the 20th century, stainless steels are irreplaceable in the world today.”
“Although kitchen sinks, refrigerators and other consumer goods are probably the most visible applications it is the industrial application where stainless steel has its biggest impact on our daily lives. Some chemicals couldn’t be produced without the use of corrosion resistant vessels and piping systems. Oil & gas production would often be close to impossible or very costly without the successful application of stainless steel.”