Eight items of stainless steel welding pay attention to nine major issues!

Eight precautions for stainless steel welding

1. Chromium stainless steel has certain corrosion resistance (oxidizing acid, organic acid, cavitation), heat resistance and wear resistance. It is 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 hardenability 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 electrode 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 (do not repeat the drying multiple times, otherwise the coating will easily crack and peel) to prevent the welding rod 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.

Nine major problems of stainless steel welding

1. What are 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. like:

〈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 are prone to 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 line 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, reducing residence time of 450-850℃

<5> Argon gas protection on the back of the 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 of dissimilar steel welded joints between austenitic stainless steel and carbon steel and low-alloy steel, 25-13 series welding wires (309, 309L) and welding rods (Austenitic 312, Austrian 307, etc.) must be used as the deposited metal of the weld. 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. black. The pickling passivation method can remove the black skin and restore the original surface color of stainless steel.

8. Why does solid stainless steel welding wire 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? Don’t need a 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.

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