Good article, 8 major problems and 9 major attentions in stainless steel welding, do you know it? In addition, you should also pay attention to cutting the thin plate, dear

9 major problems in 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 must also be added)

A steel that can make steel in a passivated state and has stainless properties. Acid-resistant steel refers to steel that is resistant to corrosion in strong corrosive media such as acids, alkalis, and salts.

 

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 and so on

 

3. Why is welding stainless steel difficult?

Answer: The main technological difficulties are:

〈1〉The heat sensitivity of stainless steel materials is relatively strong, and the residence time in the temperature range of 450--850 ℃ is slightly longer, and the corrosion resistance of the weld seam and the heat-affected zone is seriously reduced.

<2> Hot cracks are prone to occur.

<3> Poor protection, severe high temperature oxidation. <4> The linear expansion coefficient is large, resulting in large welding deformation.

 

4. Why should effective technological measures be taken for welding austenitic stainless steel?

Answer: The general process measures include:

<1> Strictly select welding materials according to the chemical composition of the base metal.

<2> Small current, fast welding; small line energy, reducing heat input.

<3> Thin-diameter welding wire, welding rod, no swing, multi-layer multi-pass welding.

<4> The welding seam and the heat-affected zone are forced to cool to reduce the residence time at 450-850℃.

<5>Argon gas protection on the back of the TIG weld.

<6> Welds that are in contact with corrosive media are welded last.

<7> Weld seam and heat-affected zone passivation treatment.

 

5. Why should welding wires and electrodes of the 25-13 series be used for the welding of austenitic stainless steel, carbon steel, and low-alloy steel (dissimilar steel welding)?

Answer: When welding dissimilar steel welded joints connecting austenitic stainless steel with carbon steel and low alloy steel, the deposited metal of the weld must use 25-13 series welding wire (309, 309L) and welding rod (Austria 312, Austenitic 307, etc.). If other stainless steel welding consumables are used, martensitic structure will be formed on the fusion line on the side of carbon steel and low alloy steel, which will cause cold cracks.

 

6. Why does solid stainless steel welding wire use 98%Ar+2%O2 shielding gas?

Answer: During MIG welding of solid stainless steel wire, if pure argon gas is used for shielding, the surface tension of the molten pool is high, and the weld is poorly formed, showing a "humpback" weld shape. Add 1-2% oxygen to reduce the surface tension of the molten pool, and the weld is smooth and beautiful.

 

7. Why is the surface of solid stainless steel welding wire MIG weld black?

Answer: MIG welding speed of solid stainless steel welding wire is faster (30-60cm/min), the protective gas nozzle has run to the front molten pool area, the weld seam is still in red hot high temperature state, oxidized by the air, oxides are formed on the surface, and the weld seam hair 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 wires need a pulsed power supply to achieve jet transition and spatter-free welding?

Answer: During MIG welding of solid stainless steel welding wire, φ1.2 welding wire, when the current I ≥ 260-280A, can realize jet transfer; if the value is less than this value, the droplet is a short-circuit transfer, and the spatter is large, so generally it cannot be used. Only use pulsed

The advanced MIG power supply, with a pulse current greater than 300A, can realize the pulse droplet transfer under the welding current of 80-260A and spatter-free welding.

 

9. Why is the flux-cored stainless steel welding wire protected by CO2 gas? No pulsed power supply?

Answer: Currently commonly used flux-cored stainless steel welding wire (such as 308, 309, etc.), the 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 in power plants, chemicals, petroleum and other equipment materials. Chromium stainless steel has poor weldability, so attention should be paid to the welding process and heat treatment conditions.

2. Chromium 13 stainless steel has high 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 at about 700°C after welding must be carried out. If the weldment cannot be heat treated after welding, chrome-nickel stainless steel electrodes (A107, A207) should be used.

3. Chromium 17 stainless steel, in order to improve corrosion resistance and weldability, appropriate amount of stable 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 ℃ and tempered at about 800 ℃ after welding. If the weldments cannot be heat treated, chrome-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. 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 shallow during AC welding, and it is easy to redden, so DC power supply should be used as much as possible. Diameter 4.0 and below can be used for all-position weldments, and diameter 5.0 and above can be used for flat welding and flat fillet welding.

7. The electrode should be kept dry when used. 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 drying many times, otherwise the coating will easily crack and peel off) to prevent electrode contamination. 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 interocular corrosion due to heating, the welding current should not be too large, which is about 20% less than that of carbon steel electrodes, the arc should not be too long, and the interlayers are cooled quickly.

 

Points for attention when cutting stainless steel sheet

The use of 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 low-priced material, different uses have different requirements for welding performance. Class I tableware generally does not require welding performance, even some pot companies. However, most products require good welding performance of raw materials, such as second-class tableware, thermos cups, steel pipes, water heaters, water dispensers, etc.

Thin plate cutting processing is mainly used for cutting materials in industries such as advertising and sheet metal. Generally, the main processed material is stainless steel, and the processing thickness is generally below 3.0mm. When selecting a cutting machine, a CNC plasma cutting machine is generally used for cutting processing. The main reason for choosing this cutting machine is its fast cutting speed, low cutting cost, and more importantly, there will be no edge burning problem. If you choose a CNC flame cutting machine to cut thin plates, there will be edge burning problems, and cutting stainless steel Plates cannot be finished with a flame cutting machine. CNC plasma cutting is superior to CNC flame cutting in terms of cutting quality and cutting efficiency in the processing industry. It can cut various metals with different working gases, especially for non-ferrous metal sheet cutting. However, as a thermal processing equipment, the CNC plasma cutting machine will inevitably undergo thermal deformation during the cutting process. Studies have shown that choosing different cutting processes during the cutting process can effectively improve the amount of deformation.

Through the study of the deformation law and the scope of influence of the workpiece cut by the CNC plasma cutting machine, it is found that the appropriate plate leveling treatment should be carried out before cutting, and then the plate should be fixed reasonably, which can effectively prevent the workpiece from moving during the cutting process. , so as to prevent or reduce the number of occurrences of thermal deformation; at the same time, when programming the cutting program, choose a reasonable cutting process, so that the maximum size surface of the workpiece can be separated from the motherboard at last; when cutting slender or special-shaped parts, Control methods such as two-piece paired cutting can also effectively prevent and reduce thermal deformation of the cutting workpiece.

CNC plasma cutting machine can cut various metals with different working gases, especially when cutting non-ferrous metals, the cutting effect is better, but as a thermal processing equipment, thermal deformation accidents will 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 actual processing and production, there are many details worthy of our attention for the use of CNC plasma cutting machines for processing stainless steel sheets. Below we will explain the issue of arc voltage control height of CNC plasma cutting machines:

1. Automatic slotting and plate edge recognition function, allowing cutting from the edge of the workpiece, improving the utilization rate of materials, reducing waste, and effectively protecting the cutting torch and cutting nozzle, thereby increasing their service life.

2. Through the keyboard, set the reference value of the tracking arc voltage. During the cutting process, the setting value of the arc voltage can be adjusted, so as to adjust the tracking height during the cutting process, so as to obtain better cutting quality. Initial positioning function: After the cutting nozzle at the end of the cutting torch contacts the workpiece, the cutting torch is precisely lifted to the position of the arc starting height and the cutting torch height within the range of millimeters.

3. Use the linear bearing lifting mechanism. Once the force of the torch 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 upward and downward movements.

4. Arc voltage automatic tracking function: through the arc voltage sensor, the height of the cutting torch is always kept 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 .