We all know that 316L is a stainless steel material grade. AISI 316L is the corresponding American grade, and sus 316L is the corresponding Japanese grade. my country's standard grade is 022Cr17Ni12Mo2 (new standard), and the old grade is 00Cr17Ni14Mo2, which means it mainly contains Cr, Ni, and Mo. The numbers indicate the approximate percentage.
So do you know the difference between 316 and 316L?
First, take a look at the chemical composition of the material
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We can see that the carbon content of the two is different, and the carbon content of 316L is lower, which has better corrosion resistance. Here, "L" means Low.
Let's make an easy-to-understand analogy. Today, pure vegan cuteness is beautiful. If you have a Shamatte hairstyle, then you are LOW.
When everyone is discussing Shakespeare, you have to talk about Zhao Benshan, then you are LOW.
When your friends all love music, chess, poetry and painting, you just like to play mahjong with the aunties in the community, then you are LOW.
When everyone loves cleanliness and hygiene, you just spit and litter everywhere, then you are LOW.
Low means "low" in English. Now you know the difference between them! Now do you know the difference between 304 and 304L, 317 and 317L?
If you don't know, you are Low
1 Welding method
Since most of the pipelines on site are stainless steel and of different sizes, according to the welding characteristics of stainless steel, the heat input is reduced as much as possible, so manual arc welding and argon arc welding are used.
For pipes with a diameter greater than 159 mm, argon arc welding is used for base laying and manual arc welding for covering. Pipes with a diameter less than 159 mm are all argon arc welding. The welding machine uses a WS7-400 inverter arc welding machine that can be used for manual arc welding/argon arc welding.
2 Welding materials
Austenitic stainless steel is a special performance steel. In order to meet the same performance of the joint, the welding material should be selected according to the principle of "equal composition". At the same time, in order to enhance the resistance of the joint to thermal cracking and intergranular corrosion, a small amount of ferrite is present in the joint. H00Cr19Ni12Mo2 argon arc welding wire and CHSO22 welding rod for manual arc welding are selected as filler materials. Their compositions are shown in Tables 1 and 2.
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3 Welding parameters
The outstanding feature of austenitic stainless steel is that it is sensitive to overheating, so low current and fast welding are used. When multi-layer welding is performed, the interlayer temperature must be strictly controlled to make the interlayer temperature less than 60 ℃. See Table 3 for specific parameters.
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4 Groove form and assembly positioning welding
The groove form adopts a V-shaped groove. Due to the use of a smaller welding current and a smaller penetration depth, the blunt edge of the groove is smaller than that of carbon steel, about 0-0.5mm, and the groove angle is larger than that of carbon steel, about 65°-700°. Its form is shown in Figure 1.
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Since the thermal expansion coefficient of stainless steel is large, large welding stress is generated during welding, and strict positioning welding is required. For pipes with d≦Φ89mm, two-point positioning is used, d=Φ89-Φ219mm is used for three-point positioning, and d≧219mm is used for four-point positioning; the length of the positioning weld is 6-8mm.
5 Welding technical requirements
① The welding machine uses DC reverse connection for manual arc welding and DC positive connection for argon arc welding;
② Before welding, the welding wire should be brushed off the surface oxide scale with a stainless steel wire brush and cleaned with acetone; the welding rod should be dried at 200-250℃ for 1h and used as needed;
③ Before welding, clean the oil and other dirt within 25mm on both sides of the groove of the workpiece, and clean the 25mm range on both sides of the groove with acetone;
④ During argon arc welding, the nozzle diameter is Φ2mm, the tungsten electrode is cerium tungsten electrode, and the specification is Φ2.0mm;
⑤ When welding stainless steel with argon arc welding, the back must be filled with argon gas for protection to ensure the back forming. The method of locally filling argon in the pipeline is adopted, with a flow rate of 5-14L/min, and the front argon flow rate is 12-13L/min.
Note
① During the base welding, the weld thickness should be as thin as possible, and the root should be well fused. When the arc is closed, it should be in a gentle slope shape. If there is an arc shrinkage hole, it should be ground off with a polisher. The arc must be struck and extinguished in the groove, and the arc pit should be filled when the arc is extinguished to prevent arc pit cracks.
② Since the stainless steel is austenitic stainless steel, in order to prevent carbide precipitation sensitization and intergranular corrosion, the interlayer temperature and post-weld cooling rate should be strictly controlled. The interlayer temperature during welding is required to be controlled below 60℃, and water cooling must be performed immediately after welding. At the same time, segmented welding is adopted. The specific segmentation method is shown in Figure 2. This symmetrically dispersed welding sequence can increase the cooling rate of the joint and reduce the welding stress.
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6 Results
① The appearance inspection shows that there are no defects such as pores, weld nodules, depressions and undercuts, and the formation is good.
② The specimens were subjected to tensile and bending tests, and all mechanical performance indicators met the requirements, and no defects such as lack of fusion and cracks were found.
③ Macroscopic metallographic inspection found that the weld was well fused, with a fusion depth of 1-1.5mm. Microscopic metallographic inspection showed that the parent material and heat-affected zone were all austenite structures, and the weld metal was austenite + ferrite (4%) structure, which fully met the requirements of resistance to intergranular corrosion and embrittlement. The quality of the welding project was guaranteed by the on-site construction of the coal chemical company.
