Suggestions for Welding Stainless Steel
Stainlesssteels were primarily developed to render corrosion resistance. There arecertain other requirements that must be met in every stainless application.They may include corrosion resistance in a particular medium, avoidance ofcontamination of product, resistance to oxidation and carbonization at elevatedtemperatures as well as the ability to provide requisite mechanical strength.There are several grades of stainless steels which can be broadly grouped into300 Series, 400 Series and others. 300 Series stainless steels contain iron,chromium, nickel and carbon as well as principal ingredients. 400 Seriesstainless steels contain iron, chromium and carbon as principal ingredients.Not all 400 Series are weldable.
Weldable 400Series stainless steels are also called straight chromium steels since theirmajor alloying element is chromium. The 400 Series can be divided into ferriticgrades and martensitic grades. Each grade calls for different preheat andinterpass welding temperatures. The martensitic grades contain chromium from11-14% and are air hardenable unless modified with an addition of aluminum,titanium, columbium or carbon levels below 0.1%. These modified grades and thehigher chromium grades up to 30% have markedly decreased hardenability and arecalled ferritic stainless steels.
The secondgroup of stainless steels are 300 Series. These grades are very popular in thefabrication industry, as they can withstand a variety of corrosion media. Thechromium content of these steels range from 16% to 30%, and the nickel contentfrom 5% to 35%. These are called austenitic steels, as the micro-structure ofthese grades is predominantly austenite. Nonetheless, there is some ferrite inseveral grades. The other grades which do not contain any ferrite are calledfully austenitic grades. A small amount of ferrite is necessary to stopcracking during solidification of welds. However, in certain media, ferritecauses corrosion, and the only choice for such media is to opt for fullyaustenitic grades. Fully austenitic grades give rise to micro-fissuring duringwelding, which could be eliminated by choosing low heat input processes alongwith restricted low melting constituents in the weld metal.
In additionto the 300 and 400 Series, stainless steels are also classified as 200 Series,505, 505 modified, 630, 2209, 2253, etc. These products are used for specificpurposes which will be discussed under their respective item description in thefollowing pages. However, duplex and super duplex stainless steels call forspecial mention.
Welding Requirements
To weldstainless steels, three factors are to be considered:
The type of stainless steel material that is to be welded.
The process of welding.
The distortion due to welding.
Welding of 300 Series Stainless Steels
The 300Series is comprised of two types of material: those which contain ferrite andaustenite; and those which contain only austenite.
None of theabove require any preheat or interpass temperature or post weld heat treatment.However, heating up to 150 degrees F before welding is advisable to evaporateany condensed moisture in the joint. The stainless steels which do not containany ferrite are called fully austenitic steels. These materials are prone todevelop micro-fissures during welding. Formation of micro-fissures could beavoided by selecting the low heat input process of welding such as TIG orshielded metal arc with up to 1/8" diameter electrodes. The consumablesselected for welding of these materials should be able to deposit weld metalwith low levels of impurities and low melting constituents. Welding ofaustenitic stainless steels with more than 10% ferrite should be done with lowinterpass temperature in order to avoid temper embrittlement, which could occurbetween 800 degrees F and 1100 degrees F. Some grades, such as 309L, 309LSi and312, which contain higher ferrite are used for welding of dissimilar metals, inwhich cause the resulting ferrite in the weld deposit, after dilution from thebase materials, should be taken into consideration. If the ferrite afterdilution is too low--say less than 2FN or less--there could be a problem ofmicrofissuring in the welds. If the resulting ferrite is too high, such weldsundergo faster embrittlement and it is advisable to limit such welds to one ortwo layers.
Welding of 400 Series Stainless Steels
Welding ofmost of the 400 Series stainless steels call for maintaining preheat andinterpass temperatures, and in some cases post-weld heating to avoid formationof brittle structure called martensite.
Techalloy405, 409Cb and 430 grades which are ferritic do not require preheat, but it isadvisable to heat to 200 degrees F to avoid possible formation of martensite.Techalloy 420 is a martensitic grade, and is extremely sensitive to airhardening, and should be preheated and weld above 600 degrees F. and subjectedto post-weld heating at 500 degrees F for one hour.
Welding of Duplex and Super Duplex Stainless Steels
Duplex andsuper duplex stainless steels were developed to combine the best properties ofaustenitic and ferritic steels. They have higher yield strength, 65 Ksi (450 N/mm2), and higher tensile strength, 100 Ksi (69 N / mm2), compared to 300Series stainless steels. These steels are resistant to corrosion as well as tostress corrosion cracking and pitting from hydrocarbon compounds.
Fillermetals to weld duplex and super duplex stainless steels will have similarchemical composition to that of parent metal except that the nickel is higherby 3% to 4%. Higher nickel is required to reduce ferrite in order to obtainoptimum mechanical properties.
Duplex andsuper duplex stainless steels are sensitive to embrittlement around 900 degreesF and could rapidly form brittle inter-metallic phases (such as CHI and SIGMA)between 1300 degrees F and 1500 degrees F. Control of heat input during weldingis essential to avoid formation of intermetallic phases. Heat input in therange of 15-60 KJ / inch is recommended for welding.
Duplexstainless steels typically have a pitting index between 35 and 38, and superduplexes typically have a pitting index above 40. Pitting index is calculatedwith the following formula:
PITTINGINDEX = %Cr 3.3(% Mo) 16(%N)
