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Stainless steel welding eight items pay attention to the nine problems!

DataTime:[3/8/2019]
1. Chromium stainless steel has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance and wear resistance. Usually used in power plant, chemical, petroleum and other equipment materials. The chrome stainless steel has poor weldability, and attention should be paid to the welding process and heat treatment conditions.
2. Chromium 13 stainless steel is harder after welding and is prone to cracks. If welding of the same type of chrome stainless steel welding rod (G202, G207) is used, it is necessary to perform preheating at 300 °C or more and slow cooling at around 700 °C after welding. If the weldment cannot be post-weld heat treated, a chrome-nickel stainless steel electrode (A107, A207) should be used.
3. Chromium 17 stainless steel, in order to improve corrosion resistance and weldability, appropriately increase the amount of stability elements Ti, Nb, Mo, etc., the weldability is better than the chromium 13 stainless steel. When the same type of chrome stainless steel electrode (G302, G307) is used, preheating at 200 °C or higher and tempering at around 800 °C after welding should be performed. If the weldment cannot be heat treated, chrome-nickel stainless steel electrodes (A107, A207) should be used.
4. When chrome-nickel stainless steel is welded, it is repeatedly heated to precipitate carbides, which reduces corrosion resistance and mechanical properties.
5. Chrome-nickel stainless steel welding rod has good corrosion resistance and oxidation resistance, and is widely used in chemical, fertilizer, petroleum and medical machinery manufacturing.
6. The chromium-nickel stainless steel coating has a titanium calcium type and a low hydrogen type. Titanium-calcium type can be used for AC and DC, but the AC welding is shallower and easier to redden, so DC power is used as much as possible. 4.0 and below diameters can be used for all-position weldments, 5.0 and above for flat and fillet welds.
7. The electrode should be kept dry when it is used. The titanium calcium type should be dried at 150 °C for 1 hour. The low hydrogen type should be dried at 200-250 °C for 1 hour (do not repeat the drying repeatedly, otherwise the coating is easy to crack and peel off) to prevent the electrode Sticky oil and other dirt, so as not to cause the weld to increase the carbon content and affect the quality of the weldment.
8. In order to prevent the corrosion between the eyes due to heating, the welding current should not be too large, about 20% less than the carbon steel electrode, the arc should not be too long, the layer is fast cold, and the narrow bead is suitable.
Nine problems with stainless steel welding
1. What is stainless steel and stainless acid resistant steel?
Answer: The “chromium” content of the main additive element in the metal material (other elements such as nickel and molybdenum are also required) can make the steel in a passivated state and have stainless steel properties. Acid-resistant steel refers to steel that is resistant to corrosion in highly corrosive media such as acids, bases, and salts.
2. What is austenitic stainless steel? What are the commonly used grades?
A: Austenitic stainless steel is the most widely used and the most diverse. Such as:
<1>18-8 series: 0Cr19Ni9 (304) 0Cr18Ni8(308)<2>18-12 series: 00Cr18Ni12Mo2Ti (316L)<3>25-13 series: 0Cr25Ni13(309)<4>25-20 series: 0Cr25Ni20, etc.
3. Why is it that the welding of stainless steel has a certain degree of technical difficulty?
A: The main process difficulty is:
<1> The stainless steel material has strong heat sensitivity, and the residence time in the temperature range of 450--850 °C is slightly longer, and the corrosion resistance of the weld and heat-affected zone is seriously degraded.
<2> Thermal cracking is likely to occur.
<3> Poor protection and severe high temperature oxidation.
<4> The coefficient of linear expansion is large, resulting in large welding deformation.
4. Why do you need to take effective measures to weld austenitic stainless steel?
A: The general process measures are:
<1> According to the chemical composition of the base metal, the welding material is strictly selected.
<2> Small current. , fast welding; small line energy, reducing heat input.
<3> Fine-diameter welding wire, welding rod, no swing, multi-layer multi-pass welding.
<4> Forced cooling in weld and heat affected zone, reducing 450-850 °C dwell time <5> argon gas protection on the back of TIG weld.
<6> The weld in contact with the corrosive medium is finally welded.
<7> Weld and heat affected zone passivation treatment.
5. Why do austenitic stainless steel and carbon steel and low alloy steel weld (dissimilar steel welding) use 25-13 series welding wire and welding rod?
Answer: Welded joints of welded austenitic stainless steel and carbon steel and low alloy steel. The weld deposit metal must use 25-13 series welding wire (309, 309L) and welding rod (Ao 312, Austrian 307, etc.). If other stainless steel welding consumables are used, martensite structure is generated on the side of the carbon steel and low alloy steel fusion line, which will cause cold cracks.
6. Why do solid stainless steel wire use 98% Ar+2% O2 shielding gas?
Answer: When the solid stainless steel wire MIG is welded, if the pure argon gas is used for protection, the surface tension of the molten pool is large, and the weld bead is poorly formed, which is in the shape of a “humpback” weld. Add 1-2% oxygen to reduce the surface tension of the molten pool, and the weld bead is flat and beautiful.
7. Why is the surface of the solid stainless steel wire MIG weld blackened?
A: The solid stainless steel wire MIG welding speed is fast (30-60cm/min), the protective gas nozzle has been run to the front end molten pool area, the weld is still in red hot and high temperature state, is oxidized by air, the surface generates oxide, and the weld is made. black. The pickling method can remove the black skin and restore the original surface color of the stainless steel.
8. Why does a solid stainless steel wire use a pulsed power supply to achieve a jet transition without spatter soldering?
Answer: When solid stainless steel wire MIG is welded, φ1.2 wire, when the current I≥260-280A, can achieve the jet transition; less than this value, the droplet is a short-circuit transition, the splash is large, generally can not be used. Only with the MIG power supply with pulse, the pulse current is greater than 300A, can achieve the pulse drop transition under 80-260A welding current, no splash welding.
9. Why is the cored stainless steel wire protected by CO2 gas? No need for a pulsed power supply?
Answer: At present, the cored stainless steel welding wire (such as 308, 309, etc.) is used. The flux formulation in the welding wire is developed according to the chemical and metallurgical reaction of welding under the protection of CO2 gas, so it cannot be used for MAG or MIG welding; Pulsed arc welding power supply.
Source: China Stainless Steel Pipes Manufacturer - Yaang Pipe Industry Co., Limited (www.yaang.com)

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