9 major problems of stainless steel welding
9 major problems of stainless steel welding
1. What is stainless steel and stainless acid-resistant steel?
Answer: The content of the main element "chromium" in metal materials (other elements such as nickel and molybdenum also need to be added)
It can make the steel in a passivated state and has the characteristics of stainless steel. Acid-resistant steel refers to steel that resists corrosion in strong corrosive media such as acid, alkali, and salt.
2. What is austenitic stainless steel? What are the commonly used grades?
Answer: Austenitic stainless steel is the most widely used and has the most varieties. 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 there a certain technological difficulty in welding stainless steel?
Answer: The main process difficulty is:
〈1〉 Stainless steel materials have strong thermal sensitivity, and the residence time in the temperature zone of 450-850℃ is slightly longer, and the corrosion resistance of welds and heat-affected areas is seriously reduced.
〈2〉 Hot cracks easily occur.
〈3〉Poor protection, serious high temperature oxidation. 〈4〉The linear expansion coefficient is large, resulting in large welding deformation.
4. Why should effective process measures be taken for welding austenitic stainless steel?
Answer: The general process measures include:
<1> We must strictly select welding materials based on the chemical composition of the base metal.
〈2〉Small current, fast welding; small wire energy, reduce heat input.
<3> Thin-diameter welding wire, welding rod, non-oscillating, multi-layer and multi-pass welding.
〈4〉Forced cooling of welding seam and heat-affected zone reduces the residence time of 450-850℃.
〈5〉Argon gas protection on the back of TIG weld.
<6> Welds in contact with corrosive media are finally welded.
〈7〉Passivation treatment of welding seam and heat-affected zone.
5. Why should we use 25-13 series welding wires and electrodes for welding of austenitic stainless steel, carbon steel, and low-alloy steel (dissimilar steel welding)?
Answer: For welding joints of dissimilar steels connected with austenitic stainless steel, carbon steel and low alloy steel, the weld deposited metal must use 25-13 series welding wires (309, 309L) and electrodes (Austenitic 312, Austrian 307, etc.). If other stainless steel welding consumables are used, a martensite structure will be produced on the fusion line of carbon steel and low alloy steel, which will cause cold cracks.
6. Why use 98%Ar+2%O2 shielding gas for solid stainless steel welding wire?
Answer: When using solid stainless steel wire for MIG welding, if pure argon gas protection is used, the surface tension of the molten pool will be large, and the weld will be poorly formed, showing a "humpback" weld shape. Add 1-2% oxygen to reduce the surface tension of the molten pool, and the weld seam is formed smooth and beautiful.
7. Why is the surface of the solid stainless steel wire MIG welded seam black?
Answer: The MIG welding speed of solid stainless steel wire is relatively fast (30-60cm/min). The shielding gas nozzle has run to the front molten pool area. The welding seam is still in the state of red hot and high temperature. It is oxidized by the air and oxides are formed on the surface. black. The pickling passivation method can remove the black skin and restore the original surface color of stainless steel.
8. Why do solid stainless steel welding wires need to use pulsed power supply to achieve jet transition and spatter-free welding?
Answer: In MIG welding with solid stainless steel welding wire, φ1.2 welding wire can realize jet transfer only when the current I≥260-280A; droplets smaller than this value are short-circuit transfer, with large splashes, and generally cannot be used. Only by using a pulsed MIG power supply with a pulse current greater than 300A, can the pulse droplet transition under a welding current of 80-260A be realized without spatter welding.
9. Why is the flux-cored stainless steel welding wire protected by CO2 gas? Do not use pulsed power supply?
Answer: Currently commonly used flux-cored stainless steel welding wire (such as 308, 309, etc.), the welding flux formula in the welding wire is developed according to the welding chemical metallurgical reaction under the protection of CO2 gas, so it cannot be used for MAG or MIG welding; Pulsed arc welding power source.
Eight precautions for stainless steel welding
1. Chromium stainless steel has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance and wear resistance. Usually used for power station, chemical, petroleum and other equipment materials. The weldability of chromium stainless steel is poor, so attention should be paid to the welding process and heat treatment conditions.
2. Chromium 13 stainless steel has greater hardening after welding and is prone to cracks. If the same type of chromium stainless steel electrode (G202, G207) is used for welding, preheating above 300°C and slow cooling treatment at about 700°C after welding must be carried out. If the weldment cannot be subjected to post-weld heat treatment, chromium-nickel stainless steel electrodes (A107, A207) should be used.
3. Chromium 17 stainless steel, in order to improve the corrosion resistance and weldability, appropriate amount of stability elements Ti, Nb, Mo, etc. are added appropriately, and the weldability is better than that of chromium 13 stainless steel. When using the same type of chromium stainless steel electrode (G302, G307), it should be preheated above 200°C and tempered at about 800°C after welding. If the weldment cannot be heat treated, chromium-nickel stainless steel electrodes (A107, A207) should be used.
4. When chromium-nickel stainless steel is welded, carbides are precipitated after repeated heating, which reduces corrosion resistance and mechanical properties.
5. Chromium-nickel stainless steel electrodes have good corrosion resistance and oxidation resistance, and are widely used in chemical, fertilizer, petroleum, and medical machinery manufacturing.
6. The chromium-nickel stainless steel coating has titanium calcium type and low hydrogen type. Titanium-calcium type can be used for AC and DC, but the penetration depth is relatively shallow during AC welding, and it is easy to be red, so use DC power as much as possible. Diameter 4.0 and below can be used for all-position weldments, and 5.0 and above are used for flat welding and flat fillet welding.
7. The welding rod should be kept dry when in use. The calcium-titanium type should be dried at 150°C for 1 hour, and the low-hydrogen type should be dried at 200-250°C for 1 hour. Skin sticky 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 corrosion between the eyes due to heating, the welding current should not be too large, about 20% less than carbon steel electrode, the arc should not be too long, the inter-layer cooling is fast, and the narrow welding bead is better.
Points to note when cutting stainless steel sheet
The use of CNC plasma cutting machine and laser CNC cutting machine equipment to realize the processing of stainless steel sheet is the current mainstream processing method. As a low-priced material, the requirements for welding performance are also different for different uses. A type of tableware generally does not require welding performance, and even includes some pot companies. However, most products require good welding performance of raw materials, such as Class II tableware, thermos cups, steel pipes, water heaters, and water dispensers.
Sheet cutting is mainly used in advertising, sheet metal and other industries. Generally, the main processing material is stainless steel, and the processing thickness is generally below 3.0mm. When choosing a cutting machine, CNC plasma cutting machine is generally used for cutting. This cutting machine was chosen mainly because of its fast cutting speed, low cutting cost, and more importantly, there will be no burning edge problem. If a CNC flame cutting machine is used to cut thin plates, there will be burning edge problems, and cutting stainless steel It is impossible to use a flame cutting machine for plates. CNC plasma cutting is superior to CNC flame cutting in terms of cutting quality and cutting benefits in the processing industry. It can cut various metals with different working gases, especially for non-ferrous metal sheets. However, as a thermal processing equipment, the CNC plasma cutting machine will inevitably be thermally deformed during the cutting process. Studies have shown that choosing different cutting processes in the cutting process can effectively improve the amount of deformation.
Through the study of the deformation law and the influence range of the workpiece cut by the CNC plasma cutting machine, it is found that the appropriate plate leveling treatment is carried out before the cutting, and then the plate is fixed reasonably, which can effectively prevent the workpiece from moving during the cutting process. , This can prevent or reduce the occurrence of thermal deformation; at the same time, when compiling the cutting program, choose a reasonable cutting process, so that the maximum size surface of the workpiece can be separated from the mother board at the end; when cutting slender or special-shaped parts, The use of two-piece paired cutting and other control methods can also effectively prevent and reduce the thermal deformation of the cut workpiece.
The CNC plasma cutting machine can cut various metals with different working gases, especially when cutting non-ferrous metals with a better cutting effect. However, as a thermal processing equipment, thermal deformation accidents may also occur during the cutting process, so in order to be able to To effectively avoid thermal deformation accidents, different cutting processes should be selected for cutting during the cutting process.
Of course, in the actual processing and production, the CNC plasma cutting machine for the processing and use of stainless steel sheet also has many details worthy of our attention. Below we will explain the issue of the arc voltage control height of the cutting torch of the CNC plasma cutting machine:
1. The automatic slitting and board edge recognition function allows cutting from the edge of the workpiece, which improves the utilization of materials and reduces waste. At the same time, it can effectively protect the cutting torch and cutting nozzle, thereby increasing their service life.
2. Through the keyboard, set the tracking arc voltage reference value. During the cutting process, the arc voltage setting value is adjustable, so that the tracking height during the cutting process can be adjusted to facilitate better cutting quality. Initial positioning function: After the cutting nozzle at the end of the cutting torch is in contact with the workpiece, the cutting torch is accurately raised to the position of the arcing height and the height of the cutting torch within a millimeter range.
3. With linear bearing lifting mechanism, once the force of the cutting torch against 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 lowering actions.
4. Arc voltage automatic tracking function: through the arc voltage sensor, the cutting torch height is always constant during the whole cutting process, even if the workpiece is uneven, the cutting can still be carried out. The constant cutting torch height ensures a high level of cutting quality .
1. What is stainless steel and stainless acid-resistant steel?
Answer: The content of the main element "chromium" in metal materials (other elements such as nickel and molybdenum also need to be added)
It can make the steel in a passivated state and has the characteristics of stainless steel. Acid-resistant steel refers to steel that resists corrosion in strong corrosive media such as acid, alkali, and salt.
2. What is austenitic stainless steel? What are the commonly used grades?
Answer: Austenitic stainless steel is the most widely used and has the most varieties. 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 there a certain technological difficulty in welding stainless steel?
Answer: The main process difficulty is:
〈1〉 Stainless steel materials have strong thermal sensitivity, and the residence time in the temperature zone of 450-850℃ is slightly longer, and the corrosion resistance of welds and heat-affected areas is seriously reduced.
〈2〉 Hot cracks easily occur.
〈3〉Poor protection, serious high temperature oxidation. 〈4〉The linear expansion coefficient is large, resulting in large welding deformation.
4. Why should effective process measures be taken for welding austenitic stainless steel?
Answer: The general process measures include:
<1> We must strictly select welding materials based on the chemical composition of the base metal.
〈2〉Small current, fast welding; small wire energy, reduce heat input.
<3> Thin-diameter welding wire, welding rod, non-oscillating, multi-layer and multi-pass welding.
〈4〉Forced cooling of welding seam and heat-affected zone reduces the residence time of 450-850℃.
〈5〉Argon gas protection on the back of TIG weld.
<6> Welds in contact with corrosive media are finally welded.
〈7〉Passivation treatment of welding seam and heat-affected zone.
5. Why should we use 25-13 series welding wires and electrodes for welding of austenitic stainless steel, carbon steel, and low-alloy steel (dissimilar steel welding)?
Answer: For welding joints of dissimilar steels connected with austenitic stainless steel, carbon steel and low alloy steel, the weld deposited metal must use 25-13 series welding wires (309, 309L) and electrodes (Austenitic 312, Austrian 307, etc.). If other stainless steel welding consumables are used, a martensite structure will be produced on the fusion line of carbon steel and low alloy steel, which will cause cold cracks.
6. Why use 98%Ar+2%O2 shielding gas for solid stainless steel welding wire?
Answer: When using solid stainless steel wire for MIG welding, if pure argon gas protection is used, the surface tension of the molten pool will be large, and the weld will be poorly formed, showing a "humpback" weld shape. Add 1-2% oxygen to reduce the surface tension of the molten pool, and the weld seam is formed smooth and beautiful.
7. Why is the surface of the solid stainless steel wire MIG welded seam black?
Answer: The MIG welding speed of solid stainless steel wire is relatively fast (30-60cm/min). The shielding gas nozzle has run to the front molten pool area. The welding seam is still in the state of red hot and high temperature. It is oxidized by the air and oxides are formed on the surface. black. The pickling passivation method can remove the black skin and restore the original surface color of stainless steel.
8. Why do solid stainless steel welding wires need to use pulsed power supply to achieve jet transition and spatter-free welding?
Answer: In MIG welding with solid stainless steel welding wire, φ1.2 welding wire can realize jet transfer only when the current I≥260-280A; droplets smaller than this value are short-circuit transfer, with large splashes, and generally cannot be used. Only by using a pulsed MIG power supply with a pulse current greater than 300A, can the pulse droplet transition under a welding current of 80-260A be realized without spatter welding.
9. Why is the flux-cored stainless steel welding wire protected by CO2 gas? Do not use pulsed power supply?
Answer: Currently commonly used flux-cored stainless steel welding wire (such as 308, 309, etc.), the welding flux formula in the welding wire is developed according to the welding chemical metallurgical reaction under the protection of CO2 gas, so it cannot be used for MAG or MIG welding; Pulsed arc welding power source.
Eight precautions for stainless steel welding
1. Chromium stainless steel has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance and wear resistance. Usually used for power station, chemical, petroleum and other equipment materials. The weldability of chromium stainless steel is poor, so attention should be paid to the welding process and heat treatment conditions.
2. Chromium 13 stainless steel has greater hardening after welding and is prone to cracks. If the same type of chromium stainless steel electrode (G202, G207) is used for welding, preheating above 300°C and slow cooling treatment at about 700°C after welding must be carried out. If the weldment cannot be subjected to post-weld heat treatment, chromium-nickel stainless steel electrodes (A107, A207) should be used.
3. Chromium 17 stainless steel, in order to improve the corrosion resistance and weldability, appropriate amount of stability elements Ti, Nb, Mo, etc. are added appropriately, and the weldability is better than that of chromium 13 stainless steel. When using the same type of chromium stainless steel electrode (G302, G307), it should be preheated above 200°C and tempered at about 800°C after welding. If the weldment cannot be heat treated, chromium-nickel stainless steel electrodes (A107, A207) should be used.
4. When chromium-nickel stainless steel is welded, carbides are precipitated after repeated heating, which reduces corrosion resistance and mechanical properties.
5. Chromium-nickel stainless steel electrodes have good corrosion resistance and oxidation resistance, and are widely used in chemical, fertilizer, petroleum, and medical machinery manufacturing.
6. The chromium-nickel stainless steel coating has titanium calcium type and low hydrogen type. Titanium-calcium type can be used for AC and DC, but the penetration depth is relatively shallow during AC welding, and it is easy to be red, so use DC power as much as possible. Diameter 4.0 and below can be used for all-position weldments, and 5.0 and above are used for flat welding and flat fillet welding.
7. The welding rod should be kept dry when in use. The calcium-titanium type should be dried at 150°C for 1 hour, and the low-hydrogen type should be dried at 200-250°C for 1 hour. Skin sticky 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 corrosion between the eyes due to heating, the welding current should not be too large, about 20% less than carbon steel electrode, the arc should not be too long, the inter-layer cooling is fast, and the narrow welding bead is better.
Points to note when cutting stainless steel sheet
The use of CNC plasma cutting machine and laser CNC cutting machine equipment to realize the processing of stainless steel sheet is the current mainstream processing method. As a low-priced material, the requirements for welding performance are also different for different uses. A type of tableware generally does not require welding performance, and even includes some pot companies. However, most products require good welding performance of raw materials, such as Class II tableware, thermos cups, steel pipes, water heaters, and water dispensers.
Sheet cutting is mainly used in advertising, sheet metal and other industries. Generally, the main processing material is stainless steel, and the processing thickness is generally below 3.0mm. When choosing a cutting machine, CNC plasma cutting machine is generally used for cutting. This cutting machine was chosen mainly because of its fast cutting speed, low cutting cost, and more importantly, there will be no burning edge problem. If a CNC flame cutting machine is used to cut thin plates, there will be burning edge problems, and cutting stainless steel It is impossible to use a flame cutting machine for plates. CNC plasma cutting is superior to CNC flame cutting in terms of cutting quality and cutting benefits in the processing industry. It can cut various metals with different working gases, especially for non-ferrous metal sheets. However, as a thermal processing equipment, the CNC plasma cutting machine will inevitably be thermally deformed during the cutting process. Studies have shown that choosing different cutting processes in the cutting process can effectively improve the amount of deformation.
Through the study of the deformation law and the influence range of the workpiece cut by the CNC plasma cutting machine, it is found that the appropriate plate leveling treatment is carried out before the cutting, and then the plate is fixed reasonably, which can effectively prevent the workpiece from moving during the cutting process. , This can prevent or reduce the occurrence of thermal deformation; at the same time, when compiling the cutting program, choose a reasonable cutting process, so that the maximum size surface of the workpiece can be separated from the mother board at the end; when cutting slender or special-shaped parts, The use of two-piece paired cutting and other control methods can also effectively prevent and reduce the thermal deformation of the cut workpiece.
The CNC plasma cutting machine can cut various metals with different working gases, especially when cutting non-ferrous metals with a better cutting effect. However, as a thermal processing equipment, thermal deformation accidents may also occur during the cutting process, so in order to be able to To effectively avoid thermal deformation accidents, different cutting processes should be selected for cutting during the cutting process.
Of course, in the actual processing and production, the CNC plasma cutting machine for the processing and use of stainless steel sheet also has many details worthy of our attention. Below we will explain the issue of the arc voltage control height of the cutting torch of the CNC plasma cutting machine:
1. The automatic slitting and board edge recognition function allows cutting from the edge of the workpiece, which improves the utilization of materials and reduces waste. At the same time, it can effectively protect the cutting torch and cutting nozzle, thereby increasing their service life.
2. Through the keyboard, set the tracking arc voltage reference value. During the cutting process, the arc voltage setting value is adjustable, so that the tracking height during the cutting process can be adjusted to facilitate better cutting quality. Initial positioning function: After the cutting nozzle at the end of the cutting torch is in contact with the workpiece, the cutting torch is accurately raised to the position of the arcing height and the height of the cutting torch within a millimeter range.
3. With linear bearing lifting mechanism, once the force of the cutting torch against 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 lowering actions.
4. Arc voltage automatic tracking function: through the arc voltage sensor, the cutting torch height is always constant during the whole cutting process, even if the workpiece is uneven, the cutting can still be carried out. The constant cutting torch height ensures a high level of cutting quality .
