FERRITIC STAINLESS STEELS
Y- shape, rectangular brush, The root of unbacked or double-V-groove
welds must be thoroughly cleaned before welding the second side. This root surface might require grinding, chipping, or gouging to expose sound weld metal and ensure complete fusion by the second or back weld.
The covering employed for stainless steel electrodes and the resulting slag contain fluor- ides to flux chrome and nickel oxides from the weld metal. In some service environments, these fluorides become highly corrosive to the stainless steel, accelerate crevice and pitting corrosion, and lead to stress-corrosion cracking. All slag must be carefully removed from all stainless steel surfaces, including the root of the weld. If the weldment design will not permit access to the back side of the weld, shielded metal arc welding should not beused to make the rootpass. Instead, gas metal arc, gas tungsten arc, or plasma arc welding should be used.
Occasionally, a weld is undercut along one or both sides of the joint. It is more prevalent when welding in the vertical and overhead posi- tions where the molten weld pool is too large to control and flows to the lowest point as it solid- ifies. Undercutting can be caused by excessive welding current, too large an electrode, or im- proper weaving of the electrode.
As mentioned previously, moisture in the covering of the electrodes can result in porosity in the weld. The electrodes must be dry, and stored properly to avoid absorption of moisture from the atmosphere. Improper cleaning of the base metal can also result in porous welds.
Gas Metal Arc Welding
When gas metal arc welding with adequate inert gas shielding, there is very little loss in alloying elements during transfer of metal across the arc. Reactive elements, like titanium, can betransferred across the arc. Therefore,
stainless steel filler metal can be used with this process. Transfer efficiencies of over 95 percent are commonly reported with argon shielding. Standard sizes of wire electrodes range from 0.020 to over 0.125 in. diameter.
Using argon shielding gas and direct cur- rent, electrode positive, metal transfer from the electrode to the work is in minute droplets, or spray transfer, provided the current density is above a transition value and the arc voltage is in the 26V to 33V range. Below this minimum, the transfer will be in the form of large drops, or globular transfer, that results in excessive spatter and arc instability. The threshold current value for stable spray transfer is about amperes for 0.062 in. electrode.
Metal transfer can also be accomplished by short-circuiting and pulsed spray These variations of GMAW operate at lower effective currents, and at an arc voltage typically between 18 and which can extend the application of GMAW to include 0.010 in. thick material. Welds can be made with low distortion because the heat input is lower than with spray transfer.
Shielding gases used to weld austenitic stainless steels include argon, argon-oxygen, gon-helium, and helium-argon-carbon dioxide.
Argon-oxygen mixtures result in some oxidation in the weld pool that promotes better wetting action and improved arc stability than is obtained with pure argon. Argon-I percent oxygen is very popular for welding with spray transfer.
argon mixtures with 2 to 3 percent carbondioxide additive are frequently used to weld stainless steels with short-circuiting transfer. Pure carbon dioxide causes a large loss of silicon and man- ganese, and can lead to an increase in carbon content in the low-carbon grades of stainless steel. Carbon absorption could the cor- rosion resistance of welds, Accordingly, is not recommended for welding stainless steel,
Helium additions to argon widen the
I Austenitic Steels 113
Typical groove joint designs for austenitic stainless steel
Root Root Groove
design Joint Welding process' Thickness, in. opening, in. face, in. degrees angle, Square-groove, SMAW 0.04-0.13
one pass GTAW 0.04-0.13 0-0.08
GMAW 0.08-0.16
two-pass GTAW 0.12-0.25
SAW 0.15-0.32 0
SMAW 0.12-0.50 0.06-0.12
0.06-0.08
GTAW 0.25-0.63 9-0.02
GMAW 0.15-0.50 0-0.08 0.06-0.12 60
SAW 0.06-0.16
Single-U-groove GMAW 0-0.08 0.08-0.
SMAW 0.04-0.13 60
GMAW 0.5-1.25 0.08-0.12
Square-groove, SMAW
GMAW 0.12-0.32
GTAW
SAW
Double-U-groove SMAW over 1.25 0.08-0.12
a. metal welding
GTAW-gas tungsten arc welding GMAW-gas metal arc welding
arc welding
b. For welding in the horizontal position, the lower member should be beveled only 10 to 15 degrees, and the top member 45 to 55 degrees.
c. Groove to 0.31 in.
tration profile of the weld bead. The penetration with argon is aligned with the electrode axis, and incomplete fusion is possible at the root of a fight butt joint if the torch is misaligned. A mixture of 50 Ar
-
50 He provides a wider pen- etration profile, and eliminates the problem.When welding at speeds above 30 ipm, a trailing shield might be required to protect the weld metal.
Flux Cored Arc Welding
, Austenitic stainless steels are joined using flux cored electrodes described previously (see Table 2.4). They are designed for gas shielding with or argon-2 percent oxygen and without gas shielding (see Table 2.5). Direct current, electrode positive is used for welding with these
The chemical composition for each type of electrode allows adequate latitude for the man- ufacturer to control the Ferrite Number of the undiluted deposit. With the EXXXT-1 classifi- cations using carbon dioxide shielding, there is someminor loss of oxidizable elements and some pickup of carbon. With the EXXXT-2 classifi- cations using argon-oxygen shielding, there is some minor loss of oxidizable elemenfs. With the EXXXT-3 classifications that are used with- out external shielding, there is some minor loss of oxidizable elements and a pickup of nitrogen, which may range from quite low to over 0.20 percent. Low welding currents coupled with arc lengths (high arc voltages) should avoided because they result in excessive nitrogen pickup and excessive loss in the ferrite content of the weld.
electrodes. The
Copyright by the American Welding Society
Thu 1997
114 STAINLESS STEELS
and are normally
ferrite controlled. When used with the recom- mended shielding gases and with reasonable and conventional welding currents and arc lengths, they produce weld metal with a typical ferrite level of 4 to 14
Gas Welding
Gas tungsten arc welding (GTAW) is well suited for joining austenitic stainless steels in all welding positions and most How- ever, it is more costly than the consumable elec- trode welding processes because deposition rates
Fig. weld design
a root edge - -
are lower.
Austenitic stainless steels are welded with this process using direct current, electrode neg- ative. The power source should have a drooping volt-ampere characteristic or constant current output.
Argon, helium, and mixtures of the gases are used for shielding the austenitic stain- less steels with this process. Argon is generally for manual welding because it is easy to start the arc, to control the molten weld pool, and to maintain good gas coverage over the weld.
is also preferred for welding of thin sheet because the heat input is than with helium for a given welding current. Helium can be used to advantage with thick sections for deeper pen- etration. Helium is also used for mechanized welding to take advantage of high welding speeds with less likelihood of undercut and a better bead shape. However, the arc is difficult to start in helium.
Inadequate gas shielding can result in nitro- gen contamination of the weld metal. This, in turn, influences the of weld metal compositions.
The process is well suited to weld the first pass in welds when the back side of the joint is inaccessible for abacking weld, and com- plete joint is required. Although not always necessary, a consumable insert can be used, or the root edge can be flared as shown in Fig. 2.13, to provide filler metal for the first pass, Acceptable unacceptable root surface con- tours are illustrated in Fig. 2.14. A
surface defect caused by of the weld- is show in Fig. 2.15.
Plasma Arc
Plasma welding is readily applied for thin sections of austenitic stainless steels using the melt-in technique. The plasma has a charactedstically long length, and is
pass made a (A) acceptable root
surface, concave root
to variations in arc length.
tion of the weld is unlikely because the tungsten electrode is contained within the welding torch.
Skuare-groove joints can sometimes be welded thicknesses from 0.090 to 0.250 in. without filler wire addition using the keyhole technique.
High welding speeds are attained on contin- uously formed stainless steel tubing, and high speeds and underbeads on
Steels I 115
welds in plate. Circumferential pipe joints can be welded without backing rings or and with reduced filler metal requirements.
Submerged Welding
Submerged arc welding (SAW) is com- monly used to join most austenitic stainless steels to themselves, and in cladding or overlaying car- bon and low alloy steel with austenitic stainless steel. Welded joints and weld cladding that are sound, serviceable, uniform in quality, and ex- cellent in appearance can be made with this pro- cess, provided qualified materials and proce- dures are used.
'Qpical problems that encountered with are traceable to high large weld beads, and low cooling and solidification rates.
These factors tend to increase segregation of alloying elements and coarseness of micro- Large weld beads, for instance, can lead to increased crack sensitivity, particularly in weld metals that are fully austenitic. Cracking of this type appears to related to impurities in the weld metal and to the solidification pattern of the bead.
Elements such as silicon, sulfur, and phos- phorus are, especially harmful when present in excess of specification limits. Carbon and man- ganese, on the other hand, are beneficial within limits. Silicon transfer from into the weld metal can be appreciably higher in submerged
arc welds than it is for welds made by other of silicon can be
to the weld metal as result of
slag-metal reactions, and it either into so- lution or, more likely, forms inclusions in the weld metal. Sulfur phosphorus also be from the flux and, through dilution, from the base metal. Many SAW fluxes are for- mulated match specific heats of electrodes so that weld metal a specified chemistry.
range.
Welding Procedures. Alternating current and direct power supplies commonly used for submerged arc welding, although direct current is preferred, particularly for welding thin material. Many joint designs and welding condi- tions used for carbon steel are useful as a first approximation for stainless steel, with the excep- tion that the welding cunent for stainless steel should be about 20 percent lower than that nor- mally used far a similar weld in carbon The higher electrical resistivity and slightly lower melting temperature of austenitic stainless steel results in a deposition rate that is 20 to 30 percent higher than that of carbon steel, under otherwise identical conditions. Table 2.15 gives typical welding conditions for double-V-groove welds in stainless steel plate.
The electrical resistance of austenitic less steel makes the electrode extension beyond the contact tip more critical than with carbon steel. Resistance heating the electrode before it enters the arc can appreciably affect the deposi- tion rate. Because of its higher resistivity and lower melting temperature, the melting rate of a stainless steel electrode is approximately 30 per- cent higher than that of a carbon steel electrode under the same welding conditions. This should be considered when selecting the electrode size for a particular application.
Dilution. Control can be important factor in submerged arc welding of austenitic stainless steel. Base metal dilution with this process can vary from than to as much as percent, exceeding that of all other consumable electrode welding processes. It usu- ally is desirable, and sometimes mandatory, to hold the weld metal composition to within nar- row limits; control of dilution is necessary to accomplish this.
About 4 to percent of delta is desired in the weld metal to control hot shortness.
Excessive ferrite can lead to during elevated temperature
Base metal dilution below 40 percent is
..
Copyright by the American Welding Society Inc Thu Jul 03 1997
Table 2.15
for submerged double-V-groove pints in stainless steel plate
First Weld' Second Weld'
Plate Root Welding Travel Electrode Welding Travel
in. A V
diam., current, Voltage, speed
thickness, face, diam., current, Voltage. speed
in.
.
in. in. A V0.25 0.188 525 30 20 0.188 575 32 24
0.50 0.25 0.188 700 18 0.188 33 18
0.625 0.25 700 33 16 0.250 35 12
0.25 0.250 700 33 15 0.250 950 35 12
0.815 0.31 715 33 0.250 1025 35
a. Groove angle-90 degrees
.
Austenitic Stainless Steels 117 commonly required to produce sound welds in
austenitic stainless steel. The effect of dilution is of special concern in single and double pass welds, and in the root pass of multiple-pass welds because of the compositional differences between the base metal and the filler metal. In these cases, small changes in penetration and bead shape, and hence in dilution, can produce significant changes in the composition and prop- erties of the deposit.
Electrodes. Standard types of austenitic stainless steel electrodes used for submerged arc welding are listed in Table 2.3. With proper com- position control, all types except fully austenitic and can be used effectively for submerged arc welding. Fully austenitic stainless steels tend to be hot short, and sub- merged arc welding of these is not recommended.
Flux. Fluxes used for submerged-arc weld- ing of austenitic stainless steel are metallurgi- cally neutral or basic in their effect on the weld metal. There are no standard specifications for stainless steel fluxes. Selection remains on a pro- prietary basis, and consultation with the facturer or supplier is recommended.
marily used for submerged arc welding carbon steel are not suitable for welding stainless steel because of loss of chromium to the slag and pickup of manganese and silicon in weld metal from the flux.
Neutral, fused flux for welding stainless steel permits some oxidation of chromium by high temperature slag-metal reactions, but sound welds can be deposited. Basic bonded fluxes, developed specifically for stainless steels, are reinforced with alloying elements by the manu-
facturer to control the weld metal composition.
A of alloying elements can be added with bonded flux. Carbon, chromium, cobalt, columbium, coppcr, manganese, molybdenum, nickel, tungsten, and vanadium are the most common additions. Bonded SAW flux can absorb moisture when exposed to damp and humid con- ditions. Excessive moisture in the flux can cause porosity, worm holes, and surface imperfections in the weld bead. Fluxes that have been inade- quately baked may behave similarly. Damp flux needs to be according to the manufac- turers recommendations. Fluxes should be prop erly handled and stored to prevent contamination.
Mechanical Properties
Austenitic stainless steels are often used be- cause of their excellent strength and oxidation resistance at elevated temperatures. Table 2.16 lists stress-rupture strengths for several at temperatures of to Although these data are for the base metals, weld metals of sim- ilar compositions will show similar performance, as indicated in Table 2.17. For long term service, up to hour design for some applications, austenitic stainless steel welds tend to be weaker and lower in ductility than the corresponding base metals.
Exposure to elevated temperatures can have significant effects on properties of austcnitic -stainless steel weldments. The exact effect is dependent upon the chemical composition of the weId metal, the ferrite content, the temperature, and the time at temperature.
The presence of intermetallic phases in
Table 216
Typical stress-rupture strengths of austenitic stainless steels
Stress, ksi
Copyright by the American Welding Society
Thu 03 1997
AWS
** 0784265
Table
stress-rupture properties of austenitic stainless steel weld metal Stress,
h failure Test temperature,
h failure Test temperature,
Weld metal 1 1200 1200
E308 30 23 15 22 16 10
E309 16
E310 31 21 12 21 14 8
E316 17 10
E341 48 31 26 40 29 19
E16-8-2 25 19
tenitic stainless steel weld metal significantly decreases toughness. The presence of coarse car- bides does not appear to have so significant an effect. The effects time exposure at ele- vated temperature have been evaluated. Welds made with E308 covered electrodes, with as-welded ferrite levels ranging from 2 to 15 FN, were aged for times up to hours at After hours, the ferrite in welds that orig- inally had an of 15 had decreased to about 3 and the 2FN welds had decreased to less than 0.5 These reductions in measured resulted primarily from the transformation of fer- rite to sigma in the higher ferrite welds and from carbide precipitation during ferrite to austenite transformation in the low ferrite welds. Charpy impact tests of aged specimens revealed that the impact strength dropped off rapidly with aging time for the higher ferrite welds because of in- creased sigma phase. However, for the lower ferrite welds, where sigma phase formation was not the primary transformation, impact strength decreased only slightly with increased aging time. These results are illustrated Fig. 2.16.
Exposure at lower temperatures does not have so significant an effect on impact properties.
Both intermetallic phases and coarse car- bides in austenitic stainless steel weld metal tend to decrease ductility under creep conditions.
Welds made with E308 covered electrodes having different ferrite contents have been rupture tested for times up to hours. For all ferrite levels, elongation decreased rapidly to values below 5 percent as the ap- proach hours as shown in Fig. 2.17. Rup- ture failures in long-time tests were found to propagate along austenite-to-sigma phase bound-
aries in the higher ferrite welds, and along aus- tenite-to-carbide boundaries in the lower ferrite welds. Therefore, both transformations appear to be equally detrimental creep ductility. In gen- eral, creep-rupture life decreases as the ferrite content of the weld metal increases. However, when the inherenf scatter in creep data is consid- ered, these decreases may not always be significant.
The toughness of austenitic stainless steels at subzero temperatures is of considerable inter- est to producers of cryogenic equipment. Table shows the impact strengths of weld metal deposited from several types of austenitic stain- less steel covered electrodes. In the as-deposited condition, these weld metals usually contain some delta ferrite, which decreases low temper- ature toughness. If a postweld stress-relieving or a stabilizing heat treatment in the range of 1200 to is given to such weld metal, the delta ferrite can transform to intermetallic phases or form carbides. These heat treatments can reduce the low-temperature toughness of the weld metal, as the data in the Table indicate.