II. MARCO TEÓRICO
2.2. Bases teóricas
2.2.1. Bonito (Sarda chiliensis)
Five different arc welding processes are generally used with heat resisting alloys. The most common, in North America, is Gas Metal Arc Welding (GMAW), formerly known as MIG (Metal Inert Gas), using spooled bare wire filler. Next in popularity is Shielded Metal Arc Welding (SMAW), or just plain “stick” welding, with covered electrodes. The least volume of work is done by Gas Tungsten Arc Welding (GTAW), formerly called TIG (Tungsten Inert Gas) and originally trade named Heliarc. Two other methods are Plasma Arc Welding (PAW) and Submerged Arc Welding (SAW). In addition resistance welding, particularly cross wire resistance welding, is often used in heat resistant alloy fabrication.
There are two basic types of welding machines, Constant Current, and Constant Potential. A constant current machine is used for GTAW (TIG) and SMAW (stick) welding. Practically speaking it won’t work for GMAW (MIG) welding. The dial on a Constant Current machine reads in amperes, and the current is regulated by this dial. Constant Potential (voltage) machines are used for GMAW (MIG) welding. They don’t work well with covered electrodes (SMAW). The dial regulates voltage, and is marked with numbers in the 20-40 range.
12-9 Gas Metal Arc Welding
though 0.035” (0.89 mm) and 0.0625” (1.59 mm) are also stocked, typically on 25-30 pound (11-14 kg) spools. Wire is fed continuously through a hollow cable to the welding gun, where it makes electrical contact. The arc between weld wire and workpiece melts the metal.
Molten weld filler transfers as either a spray of fine drops, or as larger globs. The metal is protected from oxidation by a continuous flow of inert shielding gas, usually argon, through the weld torch and around the wire. Current is always Electrode Positive (DCRP, direct current reverse polarity).
The GMAW process is fast and well suited to high volume work. It can be automated, as for welding long tubes. Welding with relatively high current, about 190-220 amperes for 0.045”
(1.14 mm) wire, and argon shielding is used for the spray-arc transfer mode. In this mode, molten weld metal crosses the arc to the work as a fine spray. At lower current, roughly 100 amperes for 0.035” (0.89 mm) wire, with 75% argon 25% helium shielding, the molten weld metal transfers as large, individual drops. This is known as short-arc, or short-circuiting arc, welding, characterized by a noisy arc and low heat input.
Choice of shielding gas is important. First, do not use oxygen additions to the gas when welding nickel alloys and NEVER use 75% argon 25% carbon dioxide for GMAW welding either stainless or nickel alloys. Oxygen above 2% starts burning out major alloying elements. CO2 above 5% adds carbon to the low carbon stainless grades. Although very small amounts of CO2 may be used in argon, at above 15% CO2 in argon the arc transfer mode is no longer spray, but rather a hot globular transfer with a great deal of spatter.
For spray-arc welding the most common gas is 100% argon. To improve bead contour and reduce arc wander, respectively, 10 to 20% helium and a very small amount of CO2 may be added to the argon. RA 602 CA requires an addition of nitrogen to the shielding gas, to prevent hot cracking. One suggested mixture is 90%Ar 5%He 5%N2. The patented gas developed in Germany specifically for GMAW RA 602 CA is. A mix of 75%Ar 25%He is also used, although the transfer mode will then not quite be a true spray-arc. For short-circuiting arc transfer 75% Ar 25% He is used, as is the commonly available 90%He 7 1/2% Ar 2 1/2%
CO2.
Because the welding wire must be pushed through a cable, ranging from 10 to 15 foot (3 to 4 1/2m) long, there may be feeding problems. The result can be a tangle of wire known, appropriately, as a “bird’s nest”. This shuts down the operation until the welder clears it.
The care with which the filler metal is wound on the spool affects how smoothly the wire feeds. While the manufacturer is often blamed for feeding problems, more often than not proper attention to machine set up will ensure freedom from “bird’s nests”.
12-10 WELDING, gas metal arc, continued
Smooth feeding depends on the cast and helix of the spooled wire. Both AWS A5.9 for stainless, and A5.14 for nickel alloy wire require cast and helix of wire on 12 inch (300mm) spools to be4 “such that a specimen long enough to produce a single loop, when cut from the spool and laid unrestrained on a flat surface, will do the following:
The following discussion is based on information from Ron Stahura, AvestaPolarit Welding Products, Inc. Many heat resistant alloy weld wires are much higher in strength than stainless wire (e.g., ER308 or ER316L), and therefore require more care to feed smoothly.
When tangling, or bird’s nest, occurs the first thing we suggest is to examine machine set-up.
Does this problem occur on more than one machine? How long is the cable—the longer the cable, the more tension in the feed rolls. Are the feed rolls, inlet guide and outlet guide all clean? Incidentally, V groove rolls are used with solid stainless/nickel alloy wire, U groove for copper or aluminum and serrated rolls for flux cored wire.
Use minimal pressure on the feed rolls—more is not better. A rule of thumb is to hold the wire between the fingers as it enters the feed rolls. If you can hold it back, there is not enough pressure. Adjust the pressure until you just can not hold the wire, then give it another half turn beyond that.
For 0.045 inch (1.14mm) wire, use a 1/16 inch (1.6mm) conduit, instead of a 0.045”/1.14mm conduit. The oversize conduit won’t hurt, and will give more room for the wire to flex.
A heavy duty contact tip is preferred instead of a standard contact tip. When spray-arc welding the tip runs hot, and the wire may swell into the tip and jam it. The heavy duty tip simply has more copper, and can handle more heat.
Flux Cored Arc Welding
FCAW is similar to GMAW except that the wire used is tubular, with flux and metal alloy powders inside. Because this wire contains its own flux, gas shielding may be 75% Argon 25% CO2, even with nickel alloys!
The advantage of flux cored wire is that welding is easier than when solid wire is used, and the arc is “softer”. As a result there is greater overall productivity with flux cored wire. Flux cored wire is sensitive to moisture pick-up, and should be left in its sealed plastic bag until ready to use.
12-11 Flux Cored Arc Welding, continued
Shielded Metal Arc Welding
Covered welding electrodes consist of an alloy core wire and a flux coating. The core
wire is usually, but not always, about the same composition as the base metal. Often, however, various alloy additions are made in the coating itself, so that the weld bead chemistry will not be the same as the chemistry of the core wire itself.
In the case of RA330-80-15 or -16, and RA330-04-15 covered electrodes, a 35%Ni 15%Cr AWS E330 core wire is used. The additional carbon, manganese and chromium required in the weld deposit are added to the flux coating. During welding, these additions melt in and adjust the chemistry of the weld bead to the specified composition. RA333-70-16 electrodes do use RA333 core wire. The electrode coating does four basic jobs:
1. Provides a gas that shields the metal crossing the arc from oxidation 2. Produces a molten slag which further protects the molten weld bead
from oxidation, affects out-of-position weldability, and controls the bead shape 3. Adds more alloying elements, such as manganese, carbon or chromium 4. Promotes electrical conductivity across the arc and helps to stabilize the arc,
important when alternating current (AC) is used
12-12 Shielded Metal Arc Welding, continued
There are three types of coatings used on Rolled Alloys electrodes. Coating type is designated by “-15”, “-16”, or “-17” after the alloy number.
DC lime-type coatings are designated -15. RA330-04-15 and RA330-80-15 both have DC (Direct Current) lime coatings. This means that these electrodes can ONLY be used with direct current. Normally the current is reverse polarity (DCRP, or Electrode Positive). That is, the electrode is positive and the workpiece is the negative electrical pole of the circuit, electrons are emitted from the work and go toward the electrode.
If the welder attempts to use a DC electrode with an AC (alternating current) setting on the welding machine, the electrode simply won’t run. He will not be able to keep the arc going.
This is very basic knowledge, but every couple of years someone complains that RA330-04-15 “won’t run”. Well, it will indeed run on DC current, but not on AC. That is, not unless that AC current is turned up so high that the whole electrode glows red and the coating spalls off.
The AC/DC titania coated electrodes are designated -16. RA333-70-16 and RA330-80-16 both have AC/DC coatings. These electrodes may be used with alternating current (AC).
They have compounds of potassium and titanium in the coating which stabilize the arc. This means it will not extinguish itself as the current reverses direction (and goes to zero) 60 times a second on normal 60 cycle current (50 cycle in Europe).
AC/DC electrodes may also be used with direct current, DC. In fact, they run better when using DC. Weld repair with RA333-70-16 covered electrodes is best accomplished using direct current, reverse polarity (DCRP).
The more recent coating designation is -17, which also operates on alternating current, as well as on direct current. RA 253 MA-17 is currently the only electrode we stock with this coating.
The slag from the electrode coating is extremely corrosive at elevated temperatures. After welding, all traces of this slag must be removed, prior to using the fabrication at elevated temperature. Otherwise the slag destroys the protective chromium oxide scale on the metal.
Under oxidizing conditions this simply results in excessive loss of metal to oxidation. In a carburizing atmosphere small traces of slag will cause local carburization to proceed rapidly.
In any reducing atmosphere the fluoride flux will scavenge enough sulphur from the atmosphere5, even a very low-sulphur atmosphere, to cause sulphidation attack of the base metal.
12-13 Gas Tungsten Arc Welding
unmelted. The argon shielding gas, which protects both the hot tungsten electrode and the molten weld puddle, is brought in through a nozzle or gas cup which is around the electrode.
This process used to be called TIG (Tungsten Inert Gas), and was originally patented as Heliarc ®, a name still used occasionally.
For both stainless and nickel alloys the current used is DCSP, direct current straight polarity.
The work is electrically positive and the tungsten electrode is the negative electrical pole.
The electrode is usually thoriated tungsten, that is, tungsten metal with 1 or 2% thorium oxide added to improve the emissivity of electrons. Rare earth oxides are also used. For aluminum welding the electrode is pure tungsten, used with AC (alternating current).
Shielding gas must be pure argon or helium. Argon is used for manual welding. A helium addition may be used for automated welding, where a hotter arc is preferred. No oxygen or carbon dioxide can be tolerated or the tungsten electrode would literally burn up. For some corrosion alloys, such as AL-6XN® or RA2205, up to 4% nitrogen is added. This may cause some erosion of the tungsten electrode but improves weld bead properties in these particular alloys.
In the case of RA 602 CA, it is necessary to add 2 to 2 ½% nitrogen to the argon shielding.
This is to resist hot cracking.
The arc between the tungsten electrode and the work is what melts the workpiece. The weld filler metal is fed by hand into the molten puddle. GTAW weld wire for heat & corrosion resistant alloys is sold as 36” (914 mm) straight lengths of bare wire, in 10 pound (4 1/2 kg) tubes.
The welder has the most control when using gas tungsten arc, and this process makes the best quality weld, but it is relatively slow. It may be automated for volume production. In automatic GTAW the wire is fed into the joint from a spool of wire, just like GMAW wire. For faster welding speed helium is added to the argon shielding gas. GTAW is often used to make the root pass in pipes or whenever the joint can only be made from one side. The rest of the weld may be built up with either GMAW or SMAW, both of which are faster.
Remember--the core wire of RA330-04-15 covered electrodes is AWS ER330, and not RA330-04 chemistry. Welders sometimes knock the coating off an electrode and use the core wire as GTAW filler. Do not do this with RA330-04-15 or the RA330-80 electrodes. This AWS ER330 will make a crack-sensitive weld, without the benefit of the alloying elements which were in the coating.
12-14 Gas Tungsten Arc Welding, continued
Atmospheric contamination, as from strong winds or too long an arc length, is a potential cause of porosity. Look at work to tip distance, shielding gas flow rates, cup size and consider the use of a gas lens. When using a 2—4% nitrogen addition for welding the corrosion alloys, the shielding gas will be just that much more sensitive to atmospheric contamination.
Minimize the arc length, no more than 1/4 to 3/8 inch (6-9.5mm). The longer the arc length, the greater the opportunity to entrain air into the shielding gas. Gas cup size depends upon what diameter tungsten electrode is being used. A 3/32” (2.4mm) electrode should use anywhere from a No. 6 to No. 8 cup (9.5-12.7mm cup dia), No. 7 (11mm) being about right.
An 1/8 inch (3.2mm) electrode requires a No. 8 (12.7mm) cup. Consider using a gas lens, a wire screen which serves to reduce turbulence of the shielding gas flow. It is this turbulence which causes air to get mixed in with the argon shielding gas.
Gas Metal Arc (MIG) Gas Tungsten Arc (TIG)
Plasma Arc Welding
The plasma arc torch is roughly analogous to a GTAW torch. It generates intense heat in a very narrow zone, and has been used to weld RA330 without added filler (with GTAW this would be extremely difficult). PAW is an excellent welding process for heat resisting alloys.
12-15 Submerged Arc Welding
Submerged arc uses a spool of weld wire, much like GMAW. Instead of shielding gas, a hopper feeds granulated flux into the arc to shield the arc and molten weld puddle. While it is possible to use 0.045” (1.14 mm) dia. Wire, larger sizes such as 1/16 or 3/32” (1.6 or 2.4 mm) are generally preferred. For nickel alloys such as RA330 a strongly basic flux must be used, such as Avesta Flux 805 or Böhler-Thyssen’s RECORD NiCrW. Absolutely do not use acid fluxes or any flux meant for stainless steel. Heat input must be as low as possible. For this reason 1/8” (3.2 mm) wire is not suggested for submerged arc welding the nickel heat resistant alloys.
SAW is a process naturally inclined to high heat input, but this heat must be kept to a minimum to avoid centerbead cracking in fully austenitic alloys.
Resistance Welding6
Spot and seam welding parameters for heat resistant alloys will differ from those used with stainlesses such as 304L or 316L, and markedly from those used for carbon steel. Heat resistance alloys may have twice the yield strength of stainless and considerably higher electrical resistivity. Electrode force, welding current and time, and electrode tip contours may all need to be modified accordingly.
12-16 Resistance Welding, continued
A restricted-dome electrode is suggested for spot welding. Average dome radius may be 3 inch (76 mm) for material up to 11 gage (3mm). For a larger nugget size in material 16 to 11 gage (1.6 to 3mm) a 5 to 8 inch (127 to 203mm) radius dome is sometimes preferred.
In seam welding heat time should be adjusted to ensure that the wheel maintains pressure until the weld nugget has solidified, to avoid porosity and cracking. Likewise cool time should be sufficient that welded areas are not remelted.
The metal must be clean and free of all grease, or a sound weld cannot be made.
References
1. Thaddeus B. Massalski, Editor-in-Chief, Binary Alloy Phase Diagrams, Volume 1, ISBN 0-87170-262- American Society for Metals, Metals Park, Ohio, U.S.A., 1986
2. Avesta handbook for the welding of stainless steel, Inf. 8901, Avesta Welding AB, S-74401 Avesta, Sweden 1989
3. Berthold Lundqvist, SANDVIK Welding Handbook, Sandvik publication 0,34 E, Sandvik AB, Sandviken, Sweden June, 1977
4. Specification for Nickel and Nickel-Alloy Bare Welding Electrodes and Rods, ANSI/AWS A5.14/A5.14M-97, ISBN 0-87171-543-0, American Welding Society, Miami, Florida, U.S.A.
5. G. R. Pease, Corrosion of Nickel-Chromium-Iron Alloys by Welding Slags, Welding Journal Research Supplement, September, 1956
6. Resistance Welding Manual, 4th Edition, ISBN 0-09624382-0-0, Resistance Welder Manufacturers’
Association, 1900 Arch Street, Philadelphia, Pennsylvania 19103 U.S.A. 1989
The best general reference we know for welding this class of materials is: R. J. Castro & J.J.
de Cadenet, Welding Metallurgy of Stainless and Heat-resisting Steels, ISBN 0 521 20431 3, Cambridge University Press, 1975. First published, in French, as: Métallurgie du soudage des aciers inoxydables et 127esistant à chaud, by Dunod, Paris, 1968.
12-17
Suggested Weld Fillers
Base Metal Preferred Alternates bare wire covered electrodes
RA330® RA330-04 RA330-04-15 RA333® , RA82
- - RA330-80-15 RA333-70-16
RA333 RA333 RA333-70-16 ERNiCrWMo-1
RA 602 CATM S 6025 6225 Al ERNiCrCoMo-1 (lacks
(SG-, EL-NiCr25FeAlY) oxidation resistance)
RA601 RA333 RA333-70-16 - -
601 6225 Al
RA600 82 182 RA330-04
RA 353 MA® RA 353 MA RA 353 MA - -
RA 253 MA® RA 253 MA RA 253 MA-17 RA333, RA333-70-16
RA800H/AT RA333 RA333-70-16 ERNiCrCoMo-1
556 - - RA330-04, ENiCrFe-2
RA309 ER309 E309-16 RA330-04*
RA310 ER310 E310-15 RA330-04*
RA446 ER309 E309-16 E312-16
ER310 E310-15
HK, HT, HU RA330-80-15 DC lime is the preferred 35% nickel rod for cast heat resistant alloys. Alternates RA333-70-16, RA330-04-15
General: Do choose the weld filler for its performance under the expected service conditions, as well as for weldability issues.
Do not use—any stainless weld filler on nickel alloys (e.g., RA330, RA333, RA600, RA601, RA 353 MA, RA800H/AT). Dilution by nickel will eliminate ferrite, and the welds will crack. It is better not to use alloy X (ERNiCrMo-2, ENiCrMo-2) weld fillers on RA333 base metal. The X weld bead may be subject to catastrophic oxidation at the higher service temperatures where RA333 is commonly used.
*Where sulphidation is an issue, do not use high nickel fillers such as RA330-04 12-18
Dissimilar Metal Joints, Weld Filler Guidelines
Considerations in selecting a filler metal for a dissimilar metal weld joint include the expected service conditions at the joint, relative thermal expansion coefficients of the three metals involved, and freedom from weld metal hot cracking. The final selection should be approved by the end user and weld procedures qualified by the fabricator.
Note: The carbon steel joint must be ground to bright metal. A “mill finish” is not acceptable. All rust, blue-black hot rolling scale and paint must be removed before welding with any stainless or nickel alloy weld wires. These alloys lack the deoxidation characteristics of carbon steel weld wires.
A617 (ERNiCrCoMo-1) lacks the oxidation resistance of RA 602 CA
BThe weldability of RA333 weld filler used on RA 602 CA has not yet been determined
CThese high nickel fillers are not suggested for sulfur bearing environments.
12-19
12-20
BRAZING and SOLDERING
Heat resistant alloys are normally assembled by welding. Brazing is used on occasion to attach cooling coils or thermocouples. The age hardening aerospace grades, by contrast, are commonly joined with nickel-silicon-boron braze fillers.
SOLDERING
Copper cooling coils may be lead-tin soldered to heat resistant alloys. Somewhat better strength may be obtained by using a tin base solder. These may be alloyed with about 2% of
Copper cooling coils may be lead-tin soldered to heat resistant alloys. Somewhat better strength may be obtained by using a tin base solder. These may be alloyed with about 2% of