We all know that 316L is a grade of stainless steel, AISI 316L is corresponding American grade, and sus 316L is corresponding Japanese grade. Standard grade in my country is 022Cr17Ni12Mo2 (new standard), and old grade is 00Cr17Ni14Mo2, which means that it mainly contains Cr, Ni, and Mo number indicates approximate percentage.

 

So do you know difference between 316 and 316L?
First look at chemical composition of material

 

We can see that carbon content of two is different, carbon content of 316L is lower and has better corrosion resistance. "L" here means Low.

Since most of sites are stainless steel pipes with different sizes, according to welding characteristics of stainless steel, heat input is reduced as much as possible, so two methods of manual arc welding and argon arc welding are used.
For pipe diameters greater than 159 mm, argon arc welding is used for bottoming, and manual arc welding is used to cover surface. All pipe diameters less than 159 mm are welded with argon arc welding. Welding machine adopts WS7-400 inverter arc welding machine with dual-purpose manual arc welding and argon arc welding.

Austenitic stainless steel is a steel with special performance. In order to satisfy joints with same performance, welding materials should be selected according to principle of "equal composition". At the same time, to enhance joints’ resistance to thermal cracking and intergranular corrosion, so that a small amount of ferrite appears in the joint, H00Cr19Ni12Mo2 argon arc welding wire and manual arc welding electrode CHSO22 are selected as filler material. Composition is shown in Table 1 and Table 2.

 

Outstanding feature of austenitic stainless steel is that it is sensitive to overheating, so low-current, fast welding is used, inter-layer temperature must be strictly controlled during multi-layer welding, so that inter-layer temperature is less than 60 ℃. See Table 3 for specific parameters.

 

Groove form adopts a V-shaped groove. Due to smaller welding current and small penetration, blunt edge of groove is smaller than that of carbon steel, about 0-0.5mm, groove angle is larger than that of carbon steel, about 65°-700°, form is shown in Figure 1.

 

Due to large thermal expansion coefficient of stainless steel, large welding stress is generated during welding, and strict tack welding is required. For pipes with d≦Φ89mm, two-point positioning is adopted, for d=Φ89-Φ219mm, three-point positioning is adopted, and for d≧219mm, four-point positioning is adopted; length of positioning weld is 6-8mm.

①Welding machine adopts DC reverse connection for manual arc welding, and direct current connection for argon arc welding;
②Before welding, welding wire should be brushed off with stainless steel wire to remove oxide scale on the surface, and cleaned with acetone; welding rod should be dried at 200-250℃ for 1 hour, and used as needed;
③Before welding, clean oil stains within 25mm on both sides of groove of workpiece, and clean 25mm on both sides of groove with acetone;
④When argon arc welding, nozzle diameter is Φ2mm, tungsten pole is cerium tungsten pole, specification is Φ2.0mm;
⑤When welding stainless steel by argon arc welding, back side must be filled with argon gas to protect back surface. Adopt method of partially filling argon in the pipeline, flow rate is 5-14L/min, and front argon flow rate is 12-13L/min.
Note
①Thickness of weld seam should be as thin as possible during bottom welding, and it should be well fused with the root. When arc is closed, it should be in a gentle slope shape. If there is arc shrinkage, use a polisher to grind it away. Arc must be ignited and extinguished in the groove, and crater should be filled when arc is extinguished to prevent crater from cracking.
②Because stainless steel is austenitic stainless steel, in order to prevent carbide precipitation sensitization and intergranular corrosion, interlayer temperature and cooling rate after welding should be strictly controlled, interlayer temperature during welding is required to be controlled below 60℃, and it must be immediately after welding. Water cooling must be used immediately after welding, and segmented welding should be used. Specific segmentation method is shown in Figure 2. This symmetrical and dispersed welding sequence can increase cooling rate of joint and reduce welding stress.

 

①In visual inspection, there are no defects such as porosity, weld bead, dent and undercut, and shape is good.
②Tensile and bending tests were performed on specimens, all mechanical properties met requirements, and no defects such as lack of fusion and cracks were found.
③Macroscopic metallographic inspection found that weld bead is well fused, and penetration depth is 1-1.5mm. Microscopic metallographic inspection, base material and heat-affected zone are all austenitic structure, weld metal is austenitic ten-ferrite (4%) structure, which fully meets requirements of resistance to intergranular corrosion and embrittlement. On-site construction of coal chemical company guarantees quality of welding project.