Study of welding process of UNS NO8810 thick-walled head
This process not only solves the problem of environmental pollution from the exhaust gas of polysilicon production, but also realizes the recycling of silicon tetrachloride and other by-products, which greatly reduces the production cost. This cold hydrogenation reactor is the core equipment for the cold hydrogenation stage of the 5000 tons/year polysilicon project of a photovoltaic material company in Sichuan. The equipment has two units with vertical fixed structure, DN2000×75×18055mm, the main material is UNS NO8810, the total weight of the equipment is 71,880kg, and the whole equipment needs heat treatment after the production.
Figure.1 Schematic diagram of cold hydrogenation reactor structure
UNS NO8810 in welding, due to S, Si and other impurities in the weld metal segregation, S and Ni form Ni-NiS low melting point eutectic, in the solidification process of the weld metal, this low melting point eutectic in the formation of a liquid film between the grain boundaries, under the action of welding stress to form intergranular cracks. In the welding process, Si and O, etc. in the intergranular boundary to form a brittle silicate film, in the weld metal solidification or after solidification of the high temperature zone, the formation of high-temperature low plasticity cracking. Therefore, S, Si is the most harmful elements in NO8810. NO8810 poor thermal conductivity, especially in the welding of thick-walled materials. Welding heat is not easily dissipated, prone to overheating, resulting in coarse grains, so that the intergranular layer thickened, weakening the intergranular bonding force, but also to make the weld metal liquid solid longer, promoting the formation of thermal cracks.
1. Welding process to determine
UNS NO8810 alloy chemical composition is shown in Table 1.
Table.1 Chemical composition of UNS NO8810
|Nickel, Ni||30 – 35|
|Chromium, Cr||19 – 23|
|Manganese, Mn||1.5 max|
|Silicon, Si||1 max|
|Copper, Cu||0.75 max|
|Aluminum, Al||0.15 – 0.6|
|Titanium, Ti||0.15 – 0.6|
|Carbon, C||0.05 – 0.1|
|Sulfur, S||0.015 max|
Ni content of 30.0% to 35.0%, with a high Ni matrix that can dissolve higher corrosion-resistant elements, making the alloy in the welding, the melt pool metal flow is poor, and the surface is easy to form a refractory oxide film (NiO), making poor penetration, the weld is easy to form debris. Therefore, the selection of welding materials and welding process should be strictly controlled C, S, P and other impurities, and to ensure that the welding material and weld bevel surface clean.
Welding method selection
As the project all equipment head with a thickness of 48mm plate, the cylinder body with a thickness of 75mm plate, is a thick structural parts, and according to the NO8810 alloy welding characteristics, welding, the choice of tungsten arc welding (TIG) welding process.
Welding material determination
According to the chemical composition of the material and the environment of the equipment, the welding wire ERNiCr-3 φ2.4mm was selected.
Because of the poor fluidity of the liquid metal of this alloy, if the bevel form is not suitable, the phenomenon of non-fusion will occur. Therefore, in order to ensure a good fusion of the joint, the bevel should increase the angle and reduce the thickness of the blunt edge, especially for 48mm material, a double U bevel should be opened.
Figure.1 TIG welding double U bevel diagram
Where: b = 2.5 mm; a1 = a2 = 15 °; R1 = R2 = 8 °
2. Welding process evaluation
In accordance with ASME Ⅸ volume – 2010 “welding and brazing assessment standards” and NB/T47014-2011 “pressure equipment welding process assessment” requirements, the UNS NO8810 alloy welding process assessment, test piece size of 300mm × 600mm × 65mm, the best welding process parameters and assessment results.
Table 2 Welding process parameters and assessment results
|Welding material||Specification mm||Welding method||Current A||Voltage V||Welding speed M / h||Post weld state|
|Stretching/MPa||Yield / MPa||Impact J (20 ℃)|
The welding assessment of the test piece for each test, according to the above values, the assessment results are qualified.
3. Head welding process points
- (1) Welded parts should be thoroughly cleared of oxides, oil and dirt on the surface of the weld and wire before assembly, positioning welding and welding.
- (2) The welding process should reduce the welding heat input as much as possible under the condition of guaranteeing the penetration, the deposited metal should be as little as possible, the melting depth should be as small as possible, and the appearance of the weld should be slightly convex; the welding channel should be multi-layer pressure channel welding.
- (3) Nickel alloy weld metal fluidity is not as good as ordinary low-alloy steel, welding is best slightly swing, but its amplitude should not be greater than three times the diameter of the welding rod, so that the viscous weld metal has sufficient time to fill the biting edge.
- (4) TIG welding is generally used when the DC positive connection, welding with high-frequency arc and current can be controlled incremental and attenuation technology, gas flow rate is maintained at 12-15L / min.
4. Welding defects and preventive measures
Most of the common defects in the welding of nickel-based alloys are thermal cracking, as well as porosity, poor fusion, slag and strain aging cracking of precipitation-reinforced alloys.
- (1) To ensure the overall quality of welding, welding as far as possible when using flat welding position, the welding process should always maintain a short arc, in order to better control the molten weld metal.
- (2) Welding crack is the most harmful defects of welded products, the main forms are: weld crack, arc pit crack, weld metal microcrack, heat-affected zone microcrack. Preventive measures are: reasonable design of welded joints and welding sequence, reduce the constraint of the structure; reduce the fusion ratio that reduces the dilution rate; use a small welding current, reduce heat input; weld channel back cooling with water; fill the arc arc pit.
- (3) Porosity and slag is also a nickel-based alloy prone to welding defects. Prevention methods taken: pay attention to pre-weld cleaning (including the base material and welding material), the use of chemical treatment (cleaning with acetone); welding to maintain a stable arc voltage; argon arc welding arc and closing arc prone to porosity and shrinkage, you must pay attention to the quality of operating techniques.
5. Application effects and conclusions
- (1) In accordance with the above welding process of a photovoltaic materials limited company in Sichuan cold hydrogenation reactor device UNS NO8810 material spherical head welding, and in accordance with JB4730 for 100% nondestructive testing, a film pass rate of 99.7%;
- (2) The test plate of the product welded by this welding process was inspected and the mechanical properties and other indicators met the requirements of the standards and drawings, which verified the correctness of the process;
- (3) The use of ERNiCr-3 wire argon arc welding UNS NO8810 thick-walled head, while choosing a reasonable welding heat input, coupled with the use of effective process control, to ensure the quality of the product.
Source: China Head 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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