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Suggestions for Welding Stainless Steel (back to top)
Stainless steels were primarily developed to render corrosion resistance. There are certain other requirements that must be met in every stainless application. They may include corrosion resistance in a particular medium, avoidance of contamination of product, resistance to oxidation and carburization at elevated temperatures as well as the ability to provide requisite mechanical strength. There are several grades of stainless steels which can be broadly grouped into 300 Series, 400 Series and others. 300 Series stainless steels contain iron, chromium, nickel and carbon as well as principal ingredients. 400 Series stainless steels contain iron, chromium and carbon as principal ingredients. Not all 400 Series are weldable.
Weldable 400 Series stainless steels are also called straight chromium steels since their major alloying element is chromium. The 400 Series can be divided into ferritic grades and martensitic grades. Each grade calls for different preheat and interpass welding temperatures. The martensitic grades contain chromium from 11-14% and are air hardenable unless modified with an addition of aluminum, titanium, columbium or carbon levels below 0.1%. These modified grades and the higher chromium grades up to 30% have markedly decreased hardenability and are called ferritic stainless steels.
The second group of stainless steels are 300 Series. These grades are very popular in the fabrication industry, as they can withstand a variety of corrosion media. The chromium content of these steels range from 16% to 30%, and the nickel content from 5% to 35%. These are called austenitic steels, as the micro-structure of these grades is predominantly austenite. Nonetheless, there is some ferrite in several grades. The other grades which do not contain any ferrite are called fully austenitic grades. A small amount of ferrite is necessary to stop cracking during solidification of welds. However, in certain media, ferrite causes corrosion, and the only choice for such media is to opt for fully austenitic grades. Fully austenitic grades give rise to micro-fissuring during welding, which could be eliminated by choosing low heat input processes along with restricted low melting constituents in the weld metal.
In addition to 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 specific purposes which will be discussed under their respective item description in the following pages. However, duplex and super duplex stainless steels call for special mention.
Welding Requirements back to top
To weld stainless 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 back to top
The 300 Series is comprised of two types of material: those which contain ferrite and austenite; and those which contain only austenite.
None of the above require any preheat or interpass temperature or post weld heat treatment. However, heating up to 150 degrees F before welding is advisable to evaporate any condensed moisture in the joint. The stainless steels which do not contain any ferrite are called fully austenitic steels. These materials are prone to develop micro-fissures during welding. Formation of micro-fissures could be avoided by selecting the low heat input process of welding such as TIG or shielded metal arc with up to 1/8" diameter electrodes. The consumables selected for welding of these materials should be able to deposit weld metal with low levels of impurities and low melting constituents. Welding of austenitic stainless steels with more than 10% ferrite should be done with low interpass temperature in order to avoid temper embrittlement, which could occur between 800 degrees F and 1100 degrees F. Some grades, such as 309L, 309LSi and 312, which contain higher ferrite are used for welding of dissimilar metals, in which cause the resulting ferrite in the weld deposit, after dilution from the base materials, should be taken into consideration. If the ferrite after dilution is too low--say less than 2FN or less--there could be a problem of microfissuring in the welds. If the resulting ferrite is too high, such welds undergo faster embrittlement and it is advisable to limit such welds to one or two layers.
Welding of 400 Series Stainless Steels back to top
Welding of most of the 400 Series stainless steels call for maintaining preheat and interpass temperatures, and in some cases post-weld heating to avoid formation of brittle structure called martensite.
Techalloy 405, 409Cb and 430 grades which are ferritic do not require preheat, but it is advisable to heat to 200 degrees F to avoid possible formation of martensite. Techalloy 420 is a martensitic grade, and is extremely sensitive to air hardening, and should be preheated and weld above 600 degrees F. and subjected to post-weld heating at 500 degrees F for one hour.
Welding of Duplex and Super Duplex Stainless Steels back to top
Duplex and super duplex stainless steels were developed to combine the best properties of austenitic and ferritic steels. They have higher yield strength, 65 Ksi (450 N /mm2), and higher tensile strength, 100 Ksi (69 N / mm2), compared to 300 Series stainless steels. These steels are resistant to corrosion as well as to stress corrosion cracking and pitting from hydrocarbon compounds.
Filler metals to weld duplex and super duplex stainless steels will have similar chemical composition to that of parent metal except that the nickel is higher by 3% to 4%. Higher nickel is required to reduce ferrite in order to obtain optimum mechanical properties.
Duplex and super duplex stainless steels are sensitive to embrittlement around 900 degrees F 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 welding is essential to avoid formation of intermetallic phases. Heat input in the range of 15-60 KJ / inch is recommended for welding.
Duplex stainless steels typically have a pitting index between 35 and 38, and super duplexes typically have a pitting index above 40. Pitting index is calculated with the following formula:
PITTING INDEX = %Cr + 3.3(% Mo) + 16(%N)
Process of Welding back to top
Influence of welding processes and parameters also are to be considered for welding of stainless steels. The major welding processes are:
- Shielded metal are welding (SMAW)
- Submerged arc welding (SAW)
- TIG welding
- MIG welding
SMAW Welding back to top
In shielded metal arc welding, the consumable used for welding is a coated electrode. The coating flux contains various minerals in order to impart different characteristics to welding. Some principal functions of the flux are:
- To ionize the arc atmosphere and improve metal transfer.
- To generate shielding gases, and thus protect the molten weld metal from atmospheric oxidation.
- To provide slag coverage to the molten weld metal.
- To provide deoxidants to react with dissolved oxygen in the weld metal and protect alloying elements.
- To provide alloying elements to the weld.
- To make a clean slag-metal separation on solidification.
The electrode should be transferred to a holding oven when the package is opened to stop them from absorbing moisture from the atmosphere.
SAW Welding back to top
In submerged arc welding, the flux is separately fed into the joint where the consumable wire establishes an arc beneath the flux. In the heat generated from the arc, the wire as well as some part of the flux melts. As the welding head moves on along the joint, slag and metal separate by virtue of difference in their specific gravities, and on solidification, the weld metal makes the joint, and the slag will be chipped off. The functions of the flux are similar to those in shielded metal arc welding. Heat input is high in SAW, leading to higher productivity.
TIG Welding back to top
In TIG welding, the arc is struck between the work piece and the non-consumable tungsten electrode. The consumable wire is melted in the arc atmosphere and the inert-gases like Argon or Helium or their mixture are used as shielding gases. TIG is extremely suited to join thin sheets, tubes and making root pass welding in pipes, since the heat input in this process is minimal. TIG welds do not cause any undercuts or excessive penetration and the distortion is lowest compares to any other welding process. TIG welds offer superior quality, but result in low productivity.
MIG Welding back to top
Gas-Metal-Arc welding is generally called MIG (Metal Inert Gas) welding. In this process the consumable wire travels through a nozzle and tip before it makes an arc with the work piece. The arc atmosphere is shielded by gases like:
- 100% argon
- 99% argon with 1% oxygen
- 97% argon with 3% carbon dioxide
MIG welding is a high-productivity process. MIG welding doesn't need expensive machinery, and the welding machines are easily transportable, making this process very popular on construction sites. In MIG welding, shielding gas, welding parameters, and the consumable assume an important role. Shielding gases are chosen taking quality, cost and operability into consideration.
In the case of welding with flux cored wires, 100% CO2 and 75% Argon + 25% CO2 are used as shielding gases.
Control of Distortion Due to Welding back to top
Two factors contribute to distortion:
- The thermal coefficient of expansion of austenitic stainless steels is very high compared to that of mild steels.
- The conductivity of heat of stainless steels is much less than that of mild steels.
Due to the combination of above factors, stainless steels undergo distortion, which must be controlled by using suitable jigs, fixtures and balanced heat input during welding.
Estimation of Delta Ferrite in Austenitic Stainless Steel back to top
There are three methods of estimating ferrite in stainless steels:
- By measuring with instruments like Magna-Gauge, which work on the principal of measuring the magnetic strength.
- By calculating from the chemical composition with the help of diagrams developed by Schaeffler, Delong and Welding Research Council.
- By metallographic methods.
Of the above, the first two items are popular, while the third approach is laborious and time-consuming. Ferrite can be measured from an undiluted weld metal employing a calibrated instrument. Ferrite can also be estimated from the chemical composition of undiluted weld metal using multiple regression charts. Measured ferrite and estimated ferrite could differ to a certain extent.
The weld parameters, thermal experience, and the size, shape and orientation of ferrite could influence the accuracy of measurements.
AWS - ER2209 back to top
A duplex stainless steel wire used to weld 2205 grade. High resistance to stress corrosion cracking and pitting with higher tensile and yield strength.
AWS - ER308/308H back to top
For welding 201, 202, 301, 302, 304, 305 and 308 stainless. Good resistance to general corrosion.
AWS - ER308L back to top
For welding 301, 302, 304, 304L, 305, 308, 321 and 247 stainless; for transition welds in clad steels. Low carbon maintains stability from intergranular corrosion due to carbide precipitation.
AWS - ER308L Si back to top
High silicon version of 308L; used for similar applications. Silicon content improves arc stability and bead appearance. Produces exceptionally smooth fillet welds and flatter butt welds.
AWS - ER309 back to top
For welding 309 stainless grades and 442 for some applications and stainless clad sheets; for joining stainless to mild steel; for stainless overlay work.
AWS - ER309L back to top
Joining ferritic to austenitic steels for working temperatures to about 570 degrees F; overlaying on mild or low alloy steels. Techalloy 309LCb also available.
AWS - ER309L Si back to top
Recommended for welding 309 base material and 309 to lower alloys and mild steels. Excellent contour of the weld, minimizing the need for grinding.
AWS - ER309L Mo back to top
For welding equipment subjected to severe corrosive environments such as paper mill machinery.
For overlay cladding or buttering layer on mild/low alloy steels.
AWS - ER310 back to top
For 310 stainless; dissimilar metals including high carbon, armor, stainless clad and air hardening steels (405, 410, 430).
AWS - ER312 back to top
Welding stainless to mild, high strength and high yield steels; 304 clad and dissimilar steels; joining abrasion-resistant steels.
AWS - ER316 back to top
For welding 316 stainless, especially for high temperature service. Molybdenum provides increased creep resistance at elevated temperatures. For high corrosion resistance (sulfite liquors) and chemicals.
AWS - ER316L back to top
For welding 316L and 318 stainless; low carbon version of 316 provides stability from intergranular corrosion due to carbide precipitation.
AWS - ER316L Si back to top
For MIG welding of 316, 316L, 18/8 Mo and 16-8-2 grades. Elevated silicon content improves weld metal flow.
AWS - ER317L back to top
For welding 317, 318, 316, 316L, 18/8 Mo and 16-8-2 grades. Resistant to corrosion in most organic/inorganic acids and pitting in chloride-bearing solutions.
AWS - ER320 back to top
For welding Carpenter 20Cb-3 stainless; offers superior resistance to corrosion.
AWS - ER320LR back to top
Similar chemical composition to 320 except Cb and Mn are controlled and C, Si, P and S are reduced to eliminate micro fissuring and hot cracking in welds.
AWS - ER330 back to top
To weld 330 stainless, cast and wrought material of similar analysis. Excellent strength: exc. heat and scale resistance to 1800 degrees F.
AWS - ER347 back to top
For welding 321 and 347 stainless where maximum corrosion resistance is required ; also for 301, 302, 304, 304L and 308.
AWS - ER385 back to top
For joining of base materials of similar composition (904L) including ASTM B 625, B 674 and B 677. Good resistance to stress corrosion cracking, pitting and crevice corrosion.
AWS - ER409Cb back to top
This is a ferritic stainless stabilized with Columbium (Niobium). It can be used for the joining of type 409 stainless and is used extensively in the automotive industry.
AWS - ER410 back to top
For welding 403, 405, 410 and 416 stainless; overlaying carbon steels for corrosion, erosion and abrasion resistance. Corrosion resistant to atmosphere, fresh water and mild acids.
AWS - ER410 NiMo back to top
For welding and repair of 410 and 410 NiMo castings. Extra low carbon; provides better crack resistance and ductility than type 410 filler metal.
AWS - ER420 back to top
For welding 420 stainless. Ideal for overlaying where higher hardness provides excellent abrasion, erosion and corrosion resistance. Used for wear-resistant purposes.
AWS - ER430 back to top
Used for welding of 430 stainless steels and overlay cladding on mild and low alloy steels. 430 is a ferritic, non-hardenable material, but when welded on mild and low alloy steels, the welds become
hard due to dilution.
AWS - ER630 back to top
This precipitation hardening alloy is used for welding 17-4PH to itself and similar alloys.