When welding stainless steels the process and procedures must be selected in consideration of the alloying elements of the two general types of steels: straight chromium, and chromium-nickel.

Straight Chromium Stainless Steel- These steels, especially those containing 18% chromium or more, are subject to a rapid grain growth when heated to a high temperature, and do not respond satisfactorily to heat treatment. They can be softened to a certain extent, provided proper control is maintained after welding by annealing for eight hours or so at about 790°C (1450°F). This may or may not be not be satisfactory, as much depends on the actual welding. As a rule, numerous small beads will produce the best results when followed by annealing.

These alloys, when welded, have very little ductility; the welds are likely to crack on deformation, or bending. Therefore they are not recommended when the product will be subjected to movement or shock at room temperature. However, if a little heat is applied, or the operating temperature is about 95°C (200°F) or more, the welds will be much tougher, and at 200 or 260°C (400 or 500”F), some bending will occur before breaking. These factors should be considered before welding straight chromium steels.

Chromium-Nickel Stainless Steels- The chromium nickel group is highly recommended for welding. These metals, being of an austenitic nature, are extremely tough and ductile in the as-welded condition. A straight chromium weld will probably snap as soon as it is bent, but a chromium-nickel weld will bend back flat on itself with no sign of fracture.

Chromium-nickel alloys can be welded with any of the commonly used processes, such as gas metal arc welding (GMAW), shielded metal arc welding (SMAW) or gas tungsten arc welding (GTAW). Forge welding is not recommended because scale is formed on the surface and prevents proper fusion.

In addition to being very fluid in the molten state, the 18-8 type has a high thermal expansion, about 60% more than that of carbon steel; a low heat conductivity (about 1/3 to 1/2 that of carbon steel), and a lower melting point. These same characteristics apply to the straight chromium type, except that the coefficient of expansion is about 10% less than that of carbon steel. These factors should be considered in the design of any welded equipment to prevent difficulties which might arise due to strains, or warpage.

Carbide Precipitation- While welds in alloys of the chromium-nickel group are far more satisfactory when using the standpoint of physical tests, they do, under certain conditions, exhibit a tendency toward “weld decay,” or lack of corrosion resistance. When an 18-8 stainless steel with more than 0.08% carbon is heated between 540 and 800°C (1000 and 1500°F) and cooled slowly, excess carbon is precipitated, or segregated out of solution, and deposited along the grain boundaries in the form of carbides. These carbides are less resistant to corrosion than the iron-chromium-nickel alloy, with the result that wherever they are present, more rapid attack will occur when exposed to corrosive conditions.

In making a weld, the metal deposited and the joint itself are heated to the melting or fusing temperature, which is around 1475°C (2690”F), and the body of the work remains cold. Hence, there will be a zone near the weld and parallel to it which will be heated between 540 and 800°C (1000 and 1500°F), and in which area carbides will be precipitated. This region may be wide or narrow, near or some distance apart the weld, depending on the type of joint and method of welding, which determines the total amount of heat applied. If welding is rapid, the zone will be narrow and close; if welding is slow, it will be wide and further away. This carbide can be put into solution again by heating to a temperature about 480°C (900°F) or higher, and cooling rapidly through the critical range. Air cooling will be sufficiently rapid if the weldment is thin, 1.6 mm (1/16 in.) or less, but a water quench is advised if the weldment is thick. If the material contains less than 0.08% carbon, such as a modified Type 302, this carbide segregation will be practically negligible, simply because there is not much carbon present and the small amount available remains in solid solution in the alloy itself. This carbide precipitation will not seriously affect the physical properties until it becomes quite extensive, but it will reduce the corrosion resistance considerably, if present even in small quantities. For this reason, only a modified Type 302 is recommended for welded equipment which is to be subjected to highly corrosive attack and which cannot be conditioned after welding. It is also recommended for equipment operating at elevated temperatures, such as 540°C (1000°F) or higher. While reducing the carbon content to below 0.07% will practically eliminate precipitation of carbides during the short time of welding, it will not necessarily stop this condition in equipment operating continuously between 540 to 815°C (1000 to 1500°F). Additions of such alloys as niobium, titanium, or molybdenum to the low carbon stainless steel will further reduce this tendency. Where only heating is the factor, niobium or titanium is satisfactory. If corrosion resistance is of most importance, then molybdenum is preferred. This intergranular corrosion is characteristic of the chromium-nickel alloys of higher alloy content as well as those containing only 18-8, provided the carbon is over 0.08. While corrosion will occur under highly corrosive conditions such as would be produced by an acid attack commonly found in the chemical industries, it should not be assumed that low-carbon alloys are essential for all welded products. Alloys with medium carbon content have proven entirely satisfactory in manufacturing other products, such as food handling apparatus, dairy equipment, architectural trim, or heat-resisting units. Hence, unless the service environment is severely corrosive, the regular 18-8 type will be found to be satisfactory.

Arc Welding

Arc welding produces highly satisfactory results on stainless steels. Direct current, electrode positive (DCEP) should be used, the same as when welding the non-ferrous metals such as bronze, aluminum, or copper, and similar to the practice followed when welding carbon steel with heavy flux-coated electrodes. While direct current electrode positive (DCEP) will generally give best results, it cannot be considered a hard-and-fast rule. In some instances, especially when heavy plates were involved, direct current electrode negative (DCEN) produced better fusion and penetration.

Plate Preparation- Scarfing the edges is not necessary on plate up to 3.2 mm (1/8 in.) thickness. For 4.8 mm (3116 in.), if only one bead is to be laid starting at one side, it is advisable to scarf the edges on a 45o angle to within 1.6 to 2.4 mm (1116 to 3/32 in.) of the bottom. With 6 mm (114 in.) or heavier, it is best to use two or more beads, scarfing starting at either one or both sides and leaving about 2.4 mm (3/32 in.) unbeveled at either the bottom or center, as the case may be. The 18-8 stainless has a high coefficient of expansion, about 60% greater than mild steel. In setting up any job, allowance must be made for this expansion. If automatic arc welding is used, the edges should be clamped parallel in the same way as carbon steel, with extra allowance made only when movement is calculated. If a ring is to be welded to a flat circular sheet and a corner weld used, the sheet will bulge at the center due to contraction around the outside on cooling. For this reason, it is more important than with steel to turn a 25 or 50 mm (1 or 2 in.) flange around the sheet and then butt weld the ring or shell to it; this permits the weld to move slightly without producing a buckle. For the same reason, it is advisable to have proper fixtures for holding the work in place while welding to prevent localized strains pulling at the joints and drawing them out of line. This is almost sure to happen if an attempt is made to weld a curved seam without jig or fixture support.

Welding Current- The 18-8 alloy can be welded with a lower welding current than required for steel, because this alloy has lower heat conductivity and a lower melting point than steel. These characteristics tend to keep the heat of the arc localized at the point of contact rather than allowing it to travel rapidly back into the plate, so less heat is required for the same size plate and wire than is ordinarily used. For example, if 110 to 120 amperes were used with 3.2 mm (1/8 in.) steel wire, only about 90 to 100 amperes would be needed for 18-8 stainless steel. Stainless will penetrate much better than steel because it is very fluid when molten, while ordinary carbon steel tends to be more viscous and sluggish.

Flux Coating on Electrodes- Chromium and nickel are the chief elements in the stainless steel alloys; the balance is iron. These alloys are highly resistant to heat, that is, they will not scale appreciably at high temperatures as long as they remain in solid form, but will oxidize as soon as molten if exposed to the air. The iron and nickel will remain practically unaffected, but chromium will oxidize rapidly, so it is necessary to prevent air contact with the molten metal and to protect it.  In shielded metal arc welding (SMAW), this is accomplished by applying a flux coating on the outside of the electrode which will fuse along with the wire. This protects the metal while going through the arc and covers over the deposited metal, excluding any air until the weld has solidified. If the type of flux coating does not afford the required protection, an imperfect or badly oxidized weld will result.

In addition to protecting the metal, the flux coating should also have a stabilizing effect to assist in maintaining a steady arc. As the weld cools, this slag covering will crack off to a large extent, due to the difference in contraction rates between it and the metal. However, if a weld of more than one bead is to be made, all slag should be removed with an air-operated cleaning tool, or by a similar method, to guard against slag which might be entrapped by further layers. The flux has a low melting point, and any small particles remaining will generally be fused and floated to the surface by the heat of the next beads, but this does not always happen. This cleaning procedure will produce welds which will not show any blowholes, gas pockets or slag inclusions on a ground and polished specimen.

When welding stainless steels, the welding rod should have higher chromium and nickel content than the plate to be welded, to compensate for alloying elements lost across the arc. This will provide similar corrosion, physical and chemical characteristics between the two.

In the straight chromium field, the alloy containing 18% chromium is the most common. This type requires the same procedure in welding as the chromium-nickel variety, the differences being that less warpage is likely to occur, due to its lower expansion, and the welds will be hard and comparatively brittle, due to its martensitic structure. In the lower chromium alloys, for example, 12%chromium, the welds can be toughened by annealing, but in the higher alloys with 18% or more chromium, they do not respond satisfactorily to annealing or heat treating. However, if a proper welding procedure is followed, they can be softened to some extent by annealing for eight hours or so at 790°C (1450°F).These alloys are so brittle at room temperature in the as-welded state that they will snap at the slightest deformation.

Discoloration- The high temperature employed in welding, whether on chromium steel or chromium-nickel steel, will discolor the metal for a short distance on each side of the weld. This is an oxide and is only a surface condition; that is, the oxide on the surface does not affect the metal beneath it. The discoloration can be removed easily by some form of pickling, or by grinding and polishing with abrasive wheels and grits. After grinding and polishing, the metal underneath will be in the same condition as before welding. If this oxide is not removed and the surface becomes wet and dry, a blue to brown color change resembling iron rust, occurs along these areas.  This is also a surface condition only.

Oxyfuel Gas Welding

Oxyfuel gas welding with acetylene can be used on stainless steel, especially in the lighter gauges, such as 18 gauge or thinner. Gas welding, of course, is slower than the electric arc method and therefore apt to produce considerably more buckling and warping.

Neutral Flame- A neutral flame should be used for welding stainless steel; the flame should be as small as possible, supplying only sufficient heat to produce good fusion. Any excess heat will simply aggravate buckling.

Flux- Although the neutral flame will protect the upper side of the weld, it will have no effect on the underside. It is necessary, therefore, to apply a flux along the underside near the edges. The flux may also be applied on the top of the weld as well as on the bottom, or on the wire itself. However, it has been found that the best results are obtained by applying it only to the underside, using as a filler rod a bare wire with the same analysis as the plate.

The flux is generally easiest to apply if it is mixed with water and made into a paste about the consistency of molasses. After applying the paste, it should be allowed to dry long enough to permit it to become fairly solid before welding. As soon as the heat is applied, this flux will fuse, forming a sort of molded cover for the bead and protecting it on the underside. This will produce a smooth, neat-appearing bead; without the flux it will be rough and irregular, and will generally present a burned or bad appearance.

Resistance Welding

Stainless steels are particularly adapted to resistance welding because the inherently high electrical resistance is a fixed property of the steel and is a constant. Stainless steels present a clean surface, oxide and scale free, and unlike plate stock, there is no zinc or lead coating. This tends to reduce the contact resistance. Contact resistance varies with the pressure, the condition of the electrodes, and the condition of the surfaces of the materials to be welded. The inherent resistance of the steel itself is high, so that this proportion of the total resistance is higher than in other weldable materials. Thus, the variable portions of the total resistance are reduced to a minimum and welding control is greatly simplified.

The capacity of the welding machine required to make a weld in stainless steel is likewise materially reduced. This is due to the high resistance of the metal and its low heat conductivity. Low heat conductivity prevents too rapid a dissipation of heat and allows a greater proportion of the heat to go to the weld.

Spot Welding- Spot welding, in principle, is produced by holding two sheets in close contact between two copper electrodes, and passing a low-voltage, high current through the circuit for a short period of time. Fusion immediately takes place between the two sheets, while the excess heat on the outside surfaces is rapidly carried away by water-cooled electrode. See RESISTANCE WELDING.

While the total heat applied will be determined by adjusting the welding control, the area of the electrode points should be maintained as constant as possible. Any increase in area will tend to reduce the heat per unit area, resulting in an improperly or poorly fused joint. A decrease in area will increase the unit heat and will usually burn a hole entirely or partly through the sheet to be welded, other factors remaining constant.

The pressure exerted by the electrodes is generally produced by the compression of helical springs, and can be adjusted by a lock nut on a shaft through the center of the spring. Variable pressures will also affect the quality of the weld. Too much pressure will reduce the resistance of the joint and tend to decrease the heat generated. The pressure generally determines the amount of upset displacement directly following the fusing period, producing an indentation on each side of the welded sheets. In addition to these variables, the time of current flow is of great importance. Too long a period gives the same result as too much heat. Too short a period will produce no weld.

It is evident, therefore, that spot welding depends on the following four variables:

(1) Current

(2) Diameter of electrode contact points

(3) Pressure (controlled by spring or pneumatic pressure)

(4) Length of time the current is allowed to flow. If both electrodes are the same diameter, a depression will occur on both sides. While not serious when a pickle finish is used, the depression can be objectionable on a polished surface. This depression can be reduced by placing a copper block about 23 mm (1/2 in.) thick and 50 mm (2 in.) square between the electrode and the polished side, thus putting the major depression on the underside. An aluminum block 3.2 to 6.4 mm (1/8 to 1/4in.) thick works well in some cases, but due to the lower melting point of aluminum, will tend to pit if a slight arc is drawn. This procedure will reduce the depression but will not eliminate it entirely, because the depression is due to shrinkage of the molten metal in the center of the weld, which pulls the base material of both surfaces. If the work is to be polished, the remaining indentation will have to be ground out.

Spot welding, like any other type of welding requiring a high temperature, will cause a blue oxide to be formed on the surface which will change to a brown color resembling rust if exposed to the weather or moist conditions. This is only a surface condition, affecting the original oxide only. If the welds are to be exposed to the atmosphere, they should be cleaned with acid, as in pickling. In the ground and polished state, spot welds have withstood several hundred hours of salt spray without the least sign of attack.

Shot Welding- Shot welding is also a form of spot welding, but uses a higher voltage and shorter time, which produces less heat on the surface. It tends to confine the heat more completely to the junction of the two sections being welded, with the result that there is less oxide or discoloration on the surface than that produced with spot welding.

Seam Welding- Seam welding is similar to spot welding in principle. Instead of using two electrodes in making one weld at a time, rollers are substituted for electrodes and the work is fed between these, and a continual series of intermittent welds is produced. Various machines employ different methods of producing this intermittent effect but in nearly all cases the resulting weld is a series of overlapping spot welds, as can be noted by a stitch effect on the surface. The adjustment or manipulation is similar to that for spot welding. It should be remembered, however, that the range for welding the chromium and chromium-nickel stainless steels is considerably narrower than for common steel and because of this, a closer adjustment is of vital importance.

Flash Welding- Stainless steel can be flash welded much like ordinary steel products, provided certain conditions noted previously are observed. In flash welding, the two sheets or bars to be joined have their edges held in clamps. The current is turned on, the edges brought together and a certain amount flashed off, during which time the temperature of the metal is rising to welding heat. At the proper time, the current is shut off and the two edges squeezed together, or upset, producing a burr along the outside which, when ground or chipped off, shows a solid weld beneath.

In producing the actual upset, the first stage should be very rapid, followed by a slower movement than with steel. This will prevent the dropping away of the very fluid, nearly molten metal, and the slower and final movement will allow the metal to upset evenly instead of crawling irregularly one side to the other. When conditions are right, this joint will be solid. It is necessary, therefore, that all stages are automatically controlled. The two edges along the joint should be as uniform and straight as possible in order to start heating or flashing along the entire section simultaneously. This will prevent overheating or loss of metal at any one point, as would be the case if contact were made at one end of the joint much earlier than at the other. The grips should also be in good mechanical condition to prevent climbing, especially with thin sections. In general, as in spot welding, less heat or shorter time will be required than with common steel of the same section.

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