Stainless steel was invented in 1912 by an English metallurgist named harry brearley while trying to develop an alloy would protect cannon bores from erosion. The first true stainless steel was melted on the 13th August, 1913.
Why is Stainless Steel 'Stainless'
Stainless Steel are iron-based alloys containing a minimum of steels have their 10.5% chromium; this forms a protective self-healing oxide film which is the reason why this group of steels have their characteristic “stainlessness” or corrosion resistance. The ability of the oxide layer to heal itself means that the steel is corrosion resistant, no matter how much of the surface is removed. This is not the case when carbon or low alloy steels are protected from corrosion by metallic coatings such as zinc or cadmium or by organic coatings such as paint.
Classification of Stainless Steels
Although all stainless steels depend on the presence of chromium, other alloying elements are often added to enhance their properties. The categorisation of stainless steels is unusual amongst metals in that it is based upon the nature of their metallurgical structure-the terms used denote the arrangement of the atoms which make up the grains of the steel, and which can be observed when a polished section of the material is viewed at high magnification through a microscope. Depending upon the exact chemical composition of the steel the micro structure may be made up of the stable phases austenite or ferrite, a “duplex” mix of these two, the phase martensite is created when some steels are rapidly quenched from a high temperature, or structure hardened by precipitated micro-constituents.
Ferritic Stainless Steel
These are plain chromium (10.50 to 18%) grades such as grade 430 and 409. Their moderate corrosion resistance and poor fabrication properties are improved in the higher alloyed grades such as grades 434 and 444.
Martensitic Stainless Steel
Martensitic Stainless Steel are also based on the addition of chromium as the major alloying element but with a higher cabon and generally lower chromium content (e.g. 12% in grade 410 and 416) than the ferritic types; grade 431 has a chromium content of about 16% but the microstructure is still martensite despite this high chromium level because this grade also contains 2% nickel.
Austenitic Stainless Steel
This group contains at least 16% chromium and 16% nickel (the basic grade 304 is referred to as 18/ 8) and range through to the high alloy or "super austenitics" such as 904L and 6% molybdenum grades.
Additional elements such as molybdenum, titanium or copper can be added to modify or improve their properties, making them suitable for many critical application involving high temperature as well as corrosion resistance. This group of steels is also suitable for cryogenic applications because the effect of the nickel content in making the steel austenitic avoids the problems of brittleness at low temperatures, which is a characteristic of other types of steel.
Duplex Stainless Steels
Duplex Stainless Steels such as 2304 and 2205 (these designations indicate compositions of 23% chromium, 4% nickel and 22% chromium, 5% nickel but both grades contain further minor alloyed additions) have microstructures comprising a mixture of austenite of austenite and ferrite. Duplex ferritic-austenitic steels combine some of the features of each class: they are resistant to stress corrosion and cracking, albeit not quite as resistant as the ferritic steels; their toughness is superior to that of the ferritic steels but inferior to that of the austenitic steels, and their strength is greater than that of the (annealed) austenitic steels, by a factor of about two. In addition the duplex steels have general corrosion resistances equal to or better than 304 and 316, and in general their pitting corrosion resistances are superior to 316. They suffer reduced toughness below about-50ºC and after exposure above 300ºC, so are used only between these temperatures.
Characteristics of Stainless Steels
The Characteristics of stainless steels can be viewed as compared to the more familier plain carbon "mild" steels. As a generalisation the stainless steels exhibit the following characteristics :-
(Applicable particularly to the austenitic family and to varying degrees to other grades.)
Higher corrosion resistance
Lower alloyed grades resisit corrosion in atnospheric and pure water enviroments, while high-alloyed grades can resisit corrosion in most acids, alkaline solutions and chlorine bearing enviroments.
Fire and heat resistance
Stainless steel resists scaling and retains strength at high tempratures.
Strength to weight advantage
The work-hardening property of austenitic grades that results in a significant strengthning of the material from cold-working alone and he high strength duplex grades allow reduced material thickness over conventional grades and therefore cost savings.
Toughness and impact resistance
The austenitic microstructure of the 300 series provides high toughness, from elevated tempratures to far below freezing, making these steels particularly suited to cryogenic applications.
Ease of Fabrications
Modern steel-making techniques mean that stainless steel can be cut, welded, formed, machined, and fabricated as readily as traditional steel.
The easy cleaning ability of stainless steel makes it the first choice for strict hygiene conditions such as hospitals, restaurants, kitchens, abattoirs and other food processing plants.
The bright, easy to maintain surface of stainless steel provides a modern and attractive appearance.
Low maintenance cost
Stainless steel does not need additional systems to protect the base metal as the metal itself will last. Stainless steel products complete their service life. There is less concern about disposal since this material is 100% recyclable.
Long Term Value
When the total life cycle costs are consiered, stainless steel is often the least expensive option as the material can withstand the action of enviroment and requires less maintenance. It also is a completely recyclable material.