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In addition, these elements modify the microstructure of the alloy which in turn has a distinct influence on their mechanical properties and weldability. Stainless steels can be broadly classified into five groups as detailed below:
• Austenitic stainless steels which contain 12 - 27% chromium and 7 - 25% nickel
• Ferritic stainless steels which contain 12 - 30% chromium with a carbon content below 0,1%
• Martensitic stainless steels which have a chromium content of between 12 and 18% with 0,15 - 0,30% carbon
• Ferritic-austenitic stainless steels which contain 18 - 25% chromium, 3 - 5% nickel and up to 3% molybdenum
• Martensitic-austenitic steels which have 13 - 16% chromium, 5 - 6% nickel and 1 - 2% molybdenum. The first four of these groups will be discussed in detail below.
Austenitic Stainless Steels
This is by far the largest and most important group in the stainless steel range. These steels, which exhibit a high level of weldability, are available in a wide range of compositions such as the 19/9 AISI 304 types, 25/20 AISI 310 types and 19/12/2 AISI 316 types, which are used for general stainless steel fabrications, elevated temperature applications and resistance to pitting corrosion respectively. As the name implies, the microstructure of austenitic stainless steel consists entirely of fine grains of austenite in the wrought condition. When subjected to welding, however, a secondary ferrite phase is formed on the austenite grain boundaries, in the heat affected zone and in the weld metal. The extent of the formation of this secondary phase is dependent on the composition of the steel or filler material and the heat input during welding.
While delta ferrite formation can have negative effects on the resistance to corrosion and formation of sigma phase at operating temperatures between 500°C and 900°C, delta ferrite in weld metal is necessary to overcome the possibility of hot cracking.
In general, austenitic welding consumables deposit a weldment containing 4 - 12% delta ferrite. For special applications, i.e. when dissimilar steels are welded under conditions of high restraint, austenitic consumables having weld metal delta ferrite contents as high as 40%, may be required. The delta ferrite can be calculated using the procedure given at the end of this section with the aid of the Schaeffler diagram.
The carbon content of austenitic stainless steels is kept at very low levels to overcome any possibility of carbide precipitation, where chromium combines with available carbon in the vicinity of the grain boundaries to produce an area depleted in chromium, which thus becomes susceptible to intergranular corrosion.
The titanium and niobium stabilised AISI 321 and 347 steels together with ELC (extra low carbon) grades are available to further overcome this problem.
Ferritic Stainless Steels
These steels which contain 12 - 30% chromium with carbon content below 0,10% do not exhibit the good weldability of the austenitic types. The steels, which become fully ferritic at high temperatures and undergo rapid grain growth, lead to brittle heat affected zones in the fabricated product. No refinement of this coarse structure is possible without cold working and recrystallisation. In addition, austenite formed at elevated temperatures may form martensite upon transformation, which can cause cracking problems. The brittleness and poor ductility of these materials have limited their applications in the as welded condition.
Ferritic stainless steels are also subject to intergranular corrosion as a result of chromium depletion from carbide precipitation. Titanium and niobium stabilised ferritic steels and steels with extra low interstitials (i.e. C, N) are available to overcome this problem.
As this material has a coefficient of expansion lower than that of carbon manganese steels, warpage and distortion during welding is considerably less. They are magnetic, however, and therefore subject to magnetic arc blow. Ferritic stainless steels cannot be hardened by conventional heat treatment processes.
Martensitic Stainless Steels
Martensitic stainless steels contain between 12 - 18% chromium with 0,15 - 0,30% carbon. As a result of their composition, these steels are capable of air hardening and thus special precautions should be taken during welding to overcome possible cracking. Cold cracking, as a result of hydrogen, which is experienced with alloy steels, can also occur in martensitic stainless steels and thus hydrogen-controlled consumables must be used.
Martensitic steels, because of their lower chromium content and responsiveness to heat treatment, have limited applications for corrosion resistance but are successfully used where their high strength and increased hardness can be utilised, e.g. turbine blades, cutlery, shafts, etc.
As in the case of ferritic stainless steels, the martensitic types have a lower coefficient of expansion than mild steels and are magnetic.