Why Does the 316 Stainless Steel Sheath Wall Thickness Below 1.0 Millimeter Require Special Welding Procedures for Flange Attachment in Electric Immersion Heaters?

One of the most crucial procedures in the production of electric immersion heaters for manufacturing engineers and quality assurance professionals is the attachment of mounting flanges to the 316 stainless steel sheath. A faulty weld can allow intrusion of process fluid, electrical short circuits, or full heater separation. Welding processes for normal wall thicknesses of more than 1.5 mm are well established. Welding of thin-walled 316 sheaths below 1.0 mm provides distinct problems. The low thermal mass of the thin wall leads to fast heat build-up and burn-through, excess grain formation and loss of corrosion resistance. This paper describes the measurement of the welding problems of thin wall 316 sheaths and the determination of necessary procedural alterations to assure dependable flange attachments.

Thermal Management Issues of Thin-Wall 316 Welding
To weld a 316 sheath to a mounting flange the base metal is melted using heat to generate a fusion zone. For a thick-walled sheath (> 1.6 mm) the heat sink of the surrounding material removes heat from the weld pool and prevents an excessive temperature rise. For a thin-walled sheath of less than 1.0 mm the effect of heat sink is negligible. The same welding current that would create a controlled weld pool in a 1.6 mm wall will melt through a 0.9 mm wall in milliseconds. 316 stainless steel has a thermal diffusivity of roughly 4 mm2/s at normal temperature. The heat conduction through the 0.9 mm thickness takes only 0.02 seconds. This means that unless the heat input is closely regulated the rear side of the sheath achieves melting point practically immediately after the arc is started. Burnthrough creates a hole in the sheath exposing the MgO insulation and resistance wire. Even without total burn-through, significant heat input will cause grain coarsening in the heat-affected zone. The grain size for thin wall sections can be from ASTM 7-8 (20-30 microns) up to ASTM 2-3 (100-150 microns). Chromium carbides preferentially precipitate at coarse grain boundaries which diminish the mechanical strength and, more critically, the corrosion resistance of 316 in chloride conditions.

Welding procedures for thin wall 316 sheaths changes needed
To properly weld a 0.8–1.0 mm 316 sheath to a mounting flange five modifications to the typical gas tungsten arc welding process are required. The welding current should be lowered 40–60% first as compared to the method of 1.6 mm wall thickness. Typical currents for 1.6mm 316 are 60-80 amperes; for 0.9mm the range is reduced to 25-35 amperes. Second, you need a pulsed current waveform. A background current of 20–30% is used, and the pulses are at 50–100 Hz, allowing the weld pool to cool slightly between pulses so that heat does not build up. Third, the travel speed should be increased to 200-300 mm per minute as compared to 100-150 mm per minute for thick walls. With higher speed less heat is put in per unit length. Fourth, the use of a tungsten electrode of smaller diameter, 1.0 mm or 1.6 mm rather than the typical 2.4 mm, increases arc stability at low currents. Fifth and most important, a backing gas barrier must be put to the interior of the sheath. During welding, the passage of argon into the interior of the sheath avoids oxidation of the rear side and offers some cooling. Without supporting gas, the inner surface is exposed and extensively oxidised, leaving a rough, chromium-depleted layer that is prone to corrosion.

Minimum Wall Thickness With Different Welding Processes
The following table indicates minimum acceptable sheath wall thicknesses for 316 for the various flange attachment weld processes and associated procedure changes for each. Values are for a full penetration weld for mechanical strength.

Welding Process Minimum 316 Sheath Wall Thickness for Consistent Welding Required Procedure ChangesRelative Cost | Standard Weld Quality
GTAW (standard), manual 1.2 mm1.2 mm None; not advised below this Good at 1.2 mm Low
Manual GTAW (pulsed) 0.9 mm Back gas Lower amps Pulsed currentModerate; requires expert operator Low to Moderate
Automated GTAW (pulsed) 0.8 mmProcedures to be fully qualified:Medium Good with correct setup
Laser welding 0.6 mm Accurate fit-up, no gap, beam alignment importantLow heat inputExcellent;
Electron beam welding 0.5 mm Vacuum chamber needed Excellent; no oxidationVery high 
Resistance welding (projection) 0.8 mm Projection of flanges specially designed Good for small diameters Low to medium
Brazing ( not welding ) 0.5 mm Silver or nickel based filler metal Moderate strength ; not for high pressure Low
Most industrial electric immersion heater applications are best welded with sheaths with wall thicknesses between 0.9 mm and 1.2 mm. Automated pulsed GTAW with back gas is the most common process. For <0.9 mm, laser welding is the most reliable, although the capital equipment cost is much higher. For flanged heaters, manufacturers without laser welding capabilities shall provide a minimum wall thickness of 1.0 mm to assure weld reliability.

Thin-Wall Welds: Inspection and Qualification Requirements
More detailed examination is needed for thin-wall 316 sheaths than for thick-wall welds since there is less room for error. For sheaths below 1.2 mm, visual inspection is not sufficient. Radiographic or dye penetrant inspection shall be undertaken on a sampling basis. Frequency shall be based on production volume and criticality of the application. For sheaths smaller than 1.0 mm, 100% dye penetrant examination is advised. Acceptance standards for thin-wall welds should permit some little underfill (up to 10% of wall thickness), but no cracks, porosity > 0.2 mm diameter, or burn-through. All thin-wall welded assemblies designed for use in pressurised operation shall also be subjected to a helium leak test. A good weld has a leak rate of less than 1 x 10-7 mbar·L/sec. If the heater is to be used in a corrosive atmosphere, then a ferrite test of the weld metal is recommended. Ferrite content > 5% suggests possibility for selective corrosion in chloride service. For best corrosion resistance, weld metal ferrite concentration for 316 to 316 welds should be in the range of 2% to 5%. In specifying thin-wall sheaths, engineers should request weld method qualification records from the manufacturer showing cross-section micrographs of the penetration profile, grain size of the heat-affected zone, and no evidence of burn-through. If a supplier is hesitant to supply these documents, they should not be trusted to provide thin-wall heaters. The expense of requalifying a welding method is minimal compared to the cost of field failures due to weld cracks or leaks.