Welding stainless steels and nickel alloys is all about cleanliness and choosing the right filler metal. These guidelines are intended as a step-by-step aid to the successful welding of stainless steels and nickel alloys.
STEP 1: SELECTION OF FILLER METAL ALLOY FOR WELDING PROCESS
When both base metals are the same, use the base metal alloy as a guide. For example, if joining 316L to 316L, use 316L filler metal. Past experience may show preferential corrosion in the weld, in which case, moving up in alloy content may be required. Careful consideration regarding how far to move up is necessary, so as not to over-alloy causing galvanic corrosion.
For dissimilar joint welding (example; Stainless Steel to Carbon Steel)
Consideration: Failure can occur as a result of low alloy mixtures if the incorrect filler metal is selected, or if dilution rates are too high. The most common failure mode is cracking but weld embrittlement is also possible. Proper alloy selection and welding technique are therefore crucial for a successful weld:
For dissimilar stainless to stainless or nickel to nickel joints see the dissimilar materials joining guide. Generally, best practice is to use the filler metal designed for the higher alloyed of the two. For example, if joining 304L to 316L base metals, use 316L filler metal. When joining stainless steel to nickel base alloys always use nickel base filler metals.
– DO NOT use low alloy electrodes to join low alloy to stainless steel. Brittle welds will result in this practice.
– DO NOT use lower alloyed stainless steel filler wire to join low alloy to stainless steel. Brittle welds will result in this practice due to martensite formation.
– DO USE over-alloyed grades such as 309 and 312 types, which are designed specifically for this purpose.
– DO NOT use stainless steel filler metals for joining stainless steel to nickel base alloys as there is a very high risk for centerline cracking. This is due to dilution out of the nickel side of the joint. Higher nickel in the stainless weld deposit creates an imbalance in the composition increasing the sensitivity to cracking.
STEP 2: SELECTION OF WELDING PARAMETERS FOR THE WELDING PROCESS
Welding parameters should be selected to achieve as low a heat input as practical to minimize distortion. Thermal distortion can be high enough to overstrain the base materials causing stress cracking.
Heat Input = (Amps x Volts x 60)/Travel Speed.
Lower amperage or voltage gives lower heat input.
Faster travel speed, such as stringer beads compared to weaving, give lower heat input.
Adjust amperage or voltage to optimize:
Use welding technique with short arc lengths to minimize burn off of alloying elements.
– Arc stability
– Penetration (lower voltages tend to give lower penetration)
– Spatter (use either lower wire feed or higher voltage)
– Undercut (higher voltage tends to increase undercut.
– Alternatively, decrease travel speed to allow the molten weld pool to fill in the undercut)
– Dilution (lower penetration gives lower dilution)
STEP 3: PROPER JOINT PREPARATION CONTAMINATION
MOISTURE AND BASE METAL TEMPERATURE
Remove condensation. Allow weldments stored outdoors to warm to ambient temperature to avoid condensation. Check for moisture contamination of shielding gases.
PLASMA CUTTING
Finish grind to clean metal, joints prepared by plasma cutting or processes using nitrogen or air in the plasma. Nitriding of the joint can result which can cause rusting in the heat affected zone of the finished joint. Use uncontaminated abrasives designed for stainless steels.
ANTICIPATE DISTORTION
Stainless steels have a rate of thermal expansion 50% greater than carbon steels. Nickel alloys expand to a lesser degree. Use frequent tacks, or skip welding to reduce stresses. Minimize weaving techniques which result in slower travel speeds and higher heat input. Stringer beads are most desired when welding on stainless steel or nickel base alloys.
NARROW GAPS
Avoid narrow gaps. The root gap should, at a minimum, be equal to the diameter of the electrode. This is particularly important when welding duplex stainless steels and nickel base alloys, which tend to have poor weld flow characteristics, resulting in lack of fusion or undercut. Remove or eliminate all possible sources of contamination including corrosion by products: dirt, oil, grease, scale, paints, and marking inks which may contain chlorides.
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If anti-spatter agents are used, use such materials specifically designed for stainless steels. Beware of oils in compressed air if used to cool or dry weld joints.
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Note that degreasing can add contaminants that will compromise welding as well as create dangerous poisonous gases.
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Do not mix stainless steel and carbon steel fabrications to avoid iron contamination. Iron particles serve to initiate localized corrosion.
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STEP 4: POST-WELD CLEANING
This is a very important step. The purpose of post weld cleaning is to ensure a properly formed chrome oxide film on the surface for optimum corrosion resistance: the smoother the finish, the higher the corrosion resistance. The heat from welding is capable of depleting chrome at the surface which can result in corrosion. To avoid rust, it is very important to remove the chrome depleted zone by chemical or mechanical post weld cleaning. Use of stainless steel brushes and other tools are highly recommended to avoid impinging iron particles into the surface which will cause rust.
CLEANING METHODS
ELECTROLYTIC POLISHING
This is the best method but it is slow and expensive.
PICKLING
Nitric and Hydrofluoric Acid. Along with a smooth surface, this method yields optimum corrosion resistance, and removes surface blemishes. Avoid over-pickling which results in a coarse surface. Note that pickling by-products are to be properly neutralized and disposed of, in compliance with local environmental regulations. A pickled weldment is at the same time passivated. Passivation solutions are not as effective as pickling solutions for
removing contamination.
GRINDING
Corrosion resistance is dependent on the fineness of the surface.
MECHANICAL POLISHING
Almost as effective to electrolytic polishing depending on the grit used: the finer the surface, the better the corrosion resistance
BRUSHING
This is a suitable method as long as uncontaminated stainless steel brushes are used.
SAND/SHOT BLASTING
Use uncontaminated media. Avoid over-blasting which can result in a coarse finish.
SPECIAL CONSIDERATIONS FOR NICKEL AND SUPERAUSTENITIC ALLOYS
Standard 300 series weld deposits contain a level of ferrite which aids in the suppression of microcracks. Microcracks can propagate into continuous cracks which are normally observed in the center of the weld. Microcracking is normally caused by low melting liquid films in the grain boundaries of the solidifying weld, in combination with a high thermal expansion rate. Ferrite serves to provide more grain boundary area thus diluting the amount of low melting intermetallics. Since nickel and super-austenitic alloys do not contain ferrite , they are more susceptible to cracking. In order to lower the risk of cracking, the following can be useful:
JOINT DESIGN
Due to the higher nickel content, weld pool flow tends to be more sluggish. To prevent lack of fusion, it is recommended to use wider joint angles and larger root openings than commonly used in stainless steels.
HEAT INPUT
The lower the heat input, the less susceptibility to cracking. Use of smaller diameter consumables which use lower current is beneficial. Typically a maximum heat input of 25 KJ/inch (1 KJ/mm) is preferred.
BEADSHAPE
Concave bead contour should be avoided. Flat to slightly convex weldbeads are preferred.
INTERPASS TEMPERATURE
When welding alloys which do not contain ferrite, a lower interpass temperature is preferred which lowers thermal stresses. A maximum interpass temperature of 300°F (150°C) is recommended.
SPECIAL CONSIDERATIONS FOR DUPLEX STAINLESS STEELS
Duplex alloys are quite different from standard stainless steels. They contain roughly 50% each of ferrite and austenite. If not properly welded, this class of alloy can be susceptible to formation of embrittled phases or formation of precipitates which are susceptible to pitting. By recognizing this, and properly following recommended procedures, mechanically sound and corrosion resistant fabrications are easily accomplished.
Generally speaking, the following parameters are required to be followed:
JOINT DESIGN
Due to the sluggish nature of ferritic materials, weld pool flow tends to be sluggish. To prevent lack of fusion it is recommended to use wider joint angles and larger root openings than commonly used in stainless steels.
SHIELDING AND BACKING GAS SELECTION
Due to the nature of ferritic materials, weld pool flow is sluggish. This can be compensated for by the proper shielding gas selection, which can also benefit the proper austenite and ferrite balance. The selection of backing gas can have a beneficial affect on the corrosion resistance.
HEAT INPUT
In order to achieve the optimum ferrite to austenite ratio, the heat input must be properly controlled. The recommended heat input range is dependent on the grade of duplex stainless steel being fabricated. See
INTERPASS TEMPERATURE
Duplex alloys have specific interpass temperatures recommended, in order to prevent formation of brittle intermetallic phases. The proper interpass temperature is dependent on the grade being welded and the base metal thickness.
WELDING OF FERRITIC STEELS
Ferritic stainless steel alloys, by their nature, tend to weld sluggishly due to their poor flow characteristics.
WELD OVERLAY
For many industrial applications, it is necessary to contain relatively high pressures conforming to various pressure vessel codes such as ASME. At the same time, corrosion protection is required to extend the life of the vessel. A common solution is to fabricate the vessel with a high strength, low alloy steel, and weld clad the container, with various higher alloy materials, utilizing various processes. Common processes used can be MIG, TIG, SMAW, and SAW using bare wire or wire and flux combinations. In the last several decades, utilization of Strip Electrodes has become more and more common in either a submerged arc or electroslag process. Generally, it is necessary to apply the first layer with an over-alloyed welding consumable to achieve a mechanically sound weld deposit. Subsequent layers can be achieved using a filler metal with the final deposit chemistry desired.
Source: http://smt.sandvik.com/en/
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