Nine problems of stainless steel welding
Nine problems of stainless steel welding
1. What are stainless steel and stainless acid resistant steel?
Answer: the content of “chromium” is the main element added in metal materials (other elements such as nickel, molybdenum, etc. also need to be added)
It can make the steel in passivation state and has stainless property. Acid resistant steel refers to the corrosion resistant steel in acid, alkali, salt and other strong corrosive media.
2. What is austenitic stainless steel? What are the commonly used brands?
A: austenitic stainless steel is the most widely used and has the most varieties. For example:
- Series 18-8: 0Cr19Ni9 (304) 0cr18ni8 (308);
- Series 18-12: 00cr18ni12mo2ti (316L);
- Series 〈 3 〉 25-13: 0cr25ni13 (309);
- Series 25-20: 0Cr25Ni20, etc.
3. Why is it difficult to weld stainless steel?
A: the main technical difficulties are:
- (1) stainless steel has strong thermal sensitivity, and the residence time is slightly longer in the temperature range of 450-850 ℃. The corrosion resistance of weld and heat affected zone is seriously reduced.
- (2) hot cracking is easy to occur.
- (3) poor protection and high temperature oxidation.
- (4) large coefficient of linear expansion results in large welding deformation.
4. Why should effective process measures be taken for welding austenitic stainless steel?
A: general process measures include:
- (1) strictly select welding materials according to the chemical composition of base metal.
- (2) small current, fast welding; Small line energy, reduce heat input.
- (3) fine diameter welding wire and electrode, no swing, multi-layer and multi pass welding.
- (4) forced cooling of weld and heat affected zone to reduce residence time of 450-850 ℃.
- (5) the back of TIG weld is protected by argon.
- (6) final welding of welds in contact with corrosive medium.
- (7) passivation treatment of weld and heat affected zone.
5. Why should 25-13 series welding wires and electrodes be used for austenitic stainless steel, carbon steel and low alloy steel welding (dissimilar steel welding)?
A: for welding dissimilar steel welded joint connecting austenitic stainless steel with carbon steel and low alloy steel, 25-13 series welding wire (309, 309l) and welding rod (AO 312, Ao 307, etc.) must be used for weld deposit metal. If other stainless steel welding materials are used, martensite structure will be produced on the fusion line of carbon steel and low alloy steel, and cold cracks will be produced.
6. Why is 98% Ar + 2% O2 shielding gas used for solid stainless steel welding wire?
Answer: when MIG welding with solid stainless steel welding wire, if pure argon gas is used for protection, the surface tension of molten pool is large, the weld formation is poor, and the shape of “hunchback” weld is shown. By adding 1-2% oxygen, the surface tension of the molten pool is reduced, and the weld is smooth and beautiful.
7. Why is the surface of MIG weld of solid stainless steel welding wire blackened?
A: solid stainless steel wire MIG welding speed is fast (30-60cm / min), the protective gas nozzle has run to the front end of the pool area, the weld is still red hot and high temperature state, oxidized by air, surface oxide, weld black. The black skin can be removed by pickling and passivation, and the original surface color of stainless steel can be restored.
8. Why does solid stainless steel welding wire need pulsed power supply to realize jet transition and spatter free welding?
Answer: when MIG welding with solid stainless steel wire, φ 1.2 welding wire, when the current I ≥ 260-280a, the jet transition can be realized; When the value is less than this, the droplet is short-circuit transfer, and the spatter is large, so it can not be used generally. Only with pulse
Only when the pulse current is greater than 300A, can the pulsed droplet transfer and spatter free welding be realized under 80-260a welding current.
9. Why is flux cored stainless steel wire shielded with CO2 gas? No power supply with pulse?
Answer: the flux cored stainless steel welding wire (such as 308, 309, etc.) is commonly used at present. The flux formula in the welding wire is developed according to the chemical metallurgical reaction of welding under the protection of CO2 gas, so it can not be used for mag or MIG welding; It is not allowed to use arc welding power source with pulse.
Eight points for attention in stainless steel welding
- 1. Chromium stainless steel has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance and wear resistance. Usually used in power plant, chemical industry, petroleum and other equipment and materials. The weldability of chromium stainless steel is poor, so the welding process and heat treatment conditions should be paid attention to.
- 2. Cr13 Stainless steel has great hardenability after welding and is easy to crack. If the same type of chromium stainless steel electrode (g202, G207) is used for welding, preheating above 300 ℃ and slow cooling treatment about 700 ℃ after welding must be carried out. If post weld heat treatment cannot be carried out on weldments, chromium nickel stainless steel electrodes (A107, A207) shall be selected.
- 3. The weldability of Cr 17 stainless steel is better than that of Cr 13 stainless steel by adding some stable elements such as Ti, Nb and Mo to improve the corrosion resistance and weldability. When using the same type of chromium stainless steel electrode (G302, G307), it should be preheated above 200 ℃ and tempered about 800 ℃ after welding. If the weldment cannot be heat treated, chromium nickel stainless steel electrodes (A107, A207) shall be used.
- 4. During the welding of Cr Ni stainless steel, carbide precipitates after repeated heating, which reduces the corrosion resistance and mechanical properties.
- 5. Chromium nickel stainless steel electrode has good corrosion resistance and oxidation resistance, which is widely used in chemical industry, chemical fertilizer, petroleum, medical machinery manufacturing.
- 6. The coating of Cr Ni stainless steel consists of Ti CA type and low hydrogen type. Titanium calcium type can be used for AC and DC welding, but the penetration of AC welding is shallow, and it is easy to turn red, so DC power supply should be used as far as possible. Diameter 4.0 and below can be used for all position weldment, 5.0 and above can be used for flat welding and flat fillet welding.
- 7. The welding rod should be kept dry when it is used. The titanium calcium type should be dried at 150 ℃ for 1 hour, and the low hydrogen type should be dried at 200-250 ℃ for 1 hour (it can not be repeatedly dried, otherwise the coating is easy to crack and peel off), so as to prevent the coating from sticking oil and other dirt, so as not to increase the carbon content of the weld and affect the quality of the weldment.
- 8. In order to prevent interocular corrosion caused by heating, the welding current should not be too large, about 20% less than that of carbon steel electrode, the arc should not be too long, the interlayer should be cooled quickly, and the narrow weld bead is suitable.
Points for attention in cutting stainless steel sheet
Using CNC plasma cutting machine and laser CNC cutting machine to realize the processing of stainless steel sheet is the mainstream processing method at present. As a kind of material with low price, its different uses have different requirements for welding performance. A class of tableware generally does not require welding performance, even including some pot enterprises. But the vast majority of products need raw materials, good welding performance, such as class II tableware, thermos cup, steel pipe, water heater, water dispenser, etc.
Thin plate cutting is mainly used in advertising, sheet metal and other industries. Generally, the main material is stainless steel, and the processing thickness is generally less than 3.0 mm. When choosing the cutting machine, the NC plasma cutting machine is generally used for cutting. All these cutting machines are mainly selected because of their fast cutting speed, low cutting cost, and more importantly, there will be no edge burning problem. If the CNC flame cutting machine is selected to cut thin plates, there will be edge burning problem and cutting stainless steel plates using flame cutting machine can not be completed. Numerical control plasma cutting is superior to numerical control flame cutting in terms of cutting quality and cutting efficiency in the processing industry. It can cut all kinds of metals with different working gases, especially for non-ferrous metal sheet. But numerical control plasma cutting machine as a hot processing equipment, in the cutting process will inevitably occur thermal deformation. The research shows that the deformation can be effectively improved by choosing different cutting processes in the cutting process.
Through the research on the deformation law and influence scope of the workpiece cut by CNC plasma cutting machine, it is found that before cutting, the appropriate plate leveling treatment should be carried out, and then the plate should be fixed reasonably, which can effectively prevent the workpiece from moving in the cutting process, so as to prevent or reduce the number of thermal deformation; At the same time, when programming the cutting program, choosing a reasonable cutting process can make the maximum dimension surface of the workpiece separate from the motherboard at last; When cutting slender parts or special-shaped parts, the control methods such as two-piece pair cutting can also effectively prevent and reduce the thermal deformation of the cutting workpiece.
Numerical control plasma cutting machine with different working gas can cut all kinds of metals, especially when cutting non-ferrous metal thin, its cutting effect is better, but as a kind of hot processing equipment, thermal deformation accidents will also occur in the cutting process, so in order to effectively avoid thermal deformation accidents, different cutting processes should be selected in the cutting process.
Of course, in the actual processing and production, there are many details worthy of our attention for the use of CNC plasma cutting machine for stainless steel sheet processing. Below, we will explain the arc voltage control height of the cutting torch of CNC plasma cutting machine:
- 1. Automatic slotting and plate edge recognition functions allow cutting from the edge of the workpiece, improve the utilization rate of materials, reduce waste, and effectively protect the cutting torch and cutting nozzle, so as to improve their service life.
- 2. Through the keyboard, set the reference value of tracking arc voltage. In the cutting process, the setting value of arc voltage is adjustable, so as to adjust the tracking height in the cutting process, so as to get better cutting quality. Initial positioning function: after the cutting nozzle at the end of the cutting torch contacts with the workpiece, the cutting torch can be accurately lifted to the position of arc starting height and cutting torch height within the range of millimeter.
- 3. With the linear bearing lifting mechanism, once the force from the cutting gun to the workpiece exceeds the range, the lifting mechanism will slip and will not damage the machine. Through the keyboard, the lifting mechanism can be manually adjusted to complete the lifting and descending action.
- 4. Arc voltage automatic tracking function: through the arc voltage sensor, the height of the cutting torch is always constant in the whole process of cutting. Even if the height of the workpiece is not normal, the cutting can still be carried out. The constant height of the cutting torch ensures a high level of cutting quality.
Source: China Steel Pipes Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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