1. Laser pulsed welding of 304 and 316 Stainless Steel
2. Orbital Tube Welding
3. Passivating Stainless Steel Welds
Pulsed Laser Welding of 304 and 316 Stainless Steels
304 and 316 are commonly laser welded in the pulsed mode. While 304 usually does not have any issues, 316 is prone to cracking when welded in pulsed mode. The pdf document below describes the reasoning for the cracking and methods to avoid it.
Orbital Tube Welding
There are many applications in industry where two tubes of the same size and material are to be joined to form a practically continuous tubing system. The tubes are often small, do not have easy 360 degree access for a manual welding operation, and cannot be rotated. In these special situations, an orbital weld head can be used which is portable and can be latched on to the tube. The weld head encloses the tube-to-tube butt joint and provides good shielding to prevent oxidation. Enclosed weld heads can weld tubes in the range of 1/16” (1.6 mm) to 6” (150 mm) in diameter.
The weld head, which looks like an over-sized pipe wrench, has an internal gear track along which the TIG welding electrode is able to move and produce a hermetic weld. Tube diameter, wall thickness, and roundness have to be tightly controlled and the tube ends should be prepped with facing equipment to ensure squareness and flatness with no burrs and chamfers. The welding process is conventional TIG with weld size and shape a function of electrode tip geometry, welding energy, shielding gas, and part chemistry. Welding of steels is very sensitive to amount of sulfur impurity which can drastically change the weld puddle size and shape (Storm in a tea cup - Fall 2013 Weld Nugget). Shielding gas can be Argon, Helium, or a combination; austenitic stainless steels can benefit from using a mix of Argon with less than 5% Hydrogen to produce a very clean weld. Note that for most materials, the inside of the tube also has to be purged with shielding gas. Welding is done autogenously, i.e. no filler wire, and process parameters are designed to produce full penetration through the wall thickness, a.k.a. CJP or Complete Joint Penetration. With proper procedure development, clean and defect-free welds can be made which do not require any post-weld processing. Orbital Tube welding has found wide application in clean room piping, pharmaceuticals, food, aerospace, and power plant industries.
Passivating Stainless Steel Welds
Unless made in a perfectly shielded environment, stainless steel welds and the HAZ (heat affected zone) usually develop a tint which is essentially surface oxidation. The oxides so formed during welding are not uniform and strongly adhering, and hence do not provide good corrosion resistance. The tint can be anywhere from light tan in color to blues and browns. If a tinted weld has to be used in a corrosive environment, it will have to be passivated. Passivation is the process of developing a very thin layer of inert chromium oxide on the surface; such an oxide layer is only a few atoms thick but provides protection against corrosion. Before the weld and HAZ can be passivated, the oxidized layer on the weld has to be removed, either mechanically or by pickling with a strong acid. After pickling, the surface is treated with a dilute Nitric acid or Citric acid which removes any free iron on the surface and exposes more of the chromium. The exposed chromium then oxidizes when it reacts with air to form a uniform protective oxide layer on the surface. The passivation layer is colorless and the weld retains the original grey color of the stainless steel.