2 Welding Processes of Inconel 625 Welded Pipe

Inconel 625 is a solid solution strengthened nickel-based deformed superalloy with molybdenum and niobium as the main strengthening element. It has excellent corrosion resistance and oxidation resistance, and has good tensile properties and fatigue properties from low temperature to 980 ℃. And it has good stress corrosion resistance in chloride atmosphere. Therefore, it can be widely used in the manufacture of aero-engine parts, aerospace structural parts and chemical equipment. In order to keep the mechanical properties of the Inconel 625 alloy steel pipe from being damaged after welding, a more targeted welding process must be used during the welding process.

1. Metallographic structure of Inconel 625 alloy:

Inconel 625 has a single-phase structure and is a face-centered cubic lattice structure. When kept at about 650 °C for a long enough time, carbon particles and unstable quaternary phases will be precipitated and transformed into stable Ni3 (Nb, Ti) orthorhombic lattice phase. The molybdenum and niobium components in the nickel-chromium matrix after solid solution strengthening will improve the mechanical properties of the material, but the plasticity will decrease.

2. Weldability of Inconel 625 alloy:

1) In the Inconel 625 alloy, the high content of Cr and Mo provides the alloy with better resistance to pitting corrosion and fission corrosion. Even at high temperature, it still has the characteristics of corrosion resistance;

2) In Inconel 625 alloy, the content of Cr is as high as 22.2%, which improves the strength of the alloy. A certain amount of Mo also improves the creep strength of the alloy. At the same time, a small amount of Al and Ti reduce the age hardening effect of the alloy to a lower level. The degree of welding improves the weldability of the alloy;

3) In Inconel 625 alloy, due to the existence of a certain amount of S and P, the segregation coefficient of S and P is the largest among various elements. At the same time, S and P can also form a variety of low-melting compounds or eutectic in steel, and, Because Ni is an element that strongly stabilizes the γ phase, it reduces the solubility of S and increases the possibility of low-melting eutectic formation, which is conducive to the formation of hot cracks;

4) The coefficient of linear expansion of Inconel 625 alloy is relatively large, but the thermal conductivity is relatively small, resulting in uneven heating and cooling of the alloy, so that the amount of plastic strain that occurs between grains in the alloy at high temperature is also large. The larger the amount of plastic strain, the easier it is to crack. ;

5) Inconel 625 alloy is a nickel-based alloy, the solid-liquid phase temperature distance is small, the solidification speed is fast, and it is easy for the oxygen or water vapor in the liquid molten pool to overflow and form pores;

3. Welding process of Inconel 625 alloy:

According to the physical properties and weldability of Inconel625 alloy, in order to retain the corrosion resistance and other excellent characteristics of Inconel625 alloy, and according to the different wall thickness, choose argon tungsten arc welding (GTAW) or argon electric welding (GTAW+SMAW) Welding method, and made the following targeted measures for its weldability:

1) Preparation before welding:

● environment and artifact group pair

Soldering should be carried out in a clean and dry environment, and the temperature should be above 15°C. It is strictly forbidden to perform welding in the air containing iron and other dust. The workpiece can be clamped and paired after the surface has been cleaned. It is forbidden to directly touch the weldment by hand during the process of clamping and pairing.

● Cutting and groove processing form

The groove angle of the welding base metal of the nickel-based alloy should be larger, the thickness of the blunt edge should be thinner, and the root gap should not be too small. The bevel angle is slightly larger, 60°~70°, and the butt gap is slightly wider (about 2.5~3.5mm).

● Cleaning and welding protection before welding

Before welding, it is necessary to ensure that: first, the surface of the base metal and the surface of the welding wire must be clean; second, the high temperature molten pool and the high temperature weld bead must be effectively protected by argon gas. These two points are important conditions for obtaining high-quality welded joints.

2) Precautions during the welding process:

● Control the welding heat input. In order to make the small weld and the base metal large due to overheating or too long heating time, multi-layer multi-pass welding is adopted, and the thickness of each layer is not more than 4-5mm, and small current and high speed are selected. For welding, the welding bead is staggered by 10mm between the bead and the welding bead, and there is a certain time interval after each welding pass, and the welding is continued after the interlayer temperature drops to 60 ℃ ~ 150 ℃;

●When welding the initial bead, the welding wire is slowly fed from the middle of the weld, and the root of the groove is observed. When circular fusion holes appear on both sides of the groove, it means that the root has been penetrated. , the deposited metal needs to be polished clean before welding;

●Welding slag and debris between the welding bead and the welding bead must be removed before continuing welding. If the welding seam cannot be completely welded due to the delay, when re-welding is performed, the welding seam must be re-welded according to the process requirements. Continue welding after the surface is treated;

●Use the scratching method to strike the arc inside the groove, and do not strike the arc outside the groove, so as to avoid the contamination of the base metal and the welding scar causing cracks and pores; use short arc welding to reduce spatter and pores, and cover with crescent-shaped strips. Surface welding, evenly swing the electrode to about 1mm from the groove surface, and use the reflow arc ending method or the circular arc ending method to terminate the arc to avoid pores and cracks in the arc crater. When the argon arc welding receives the arc, wait for the arc crater to cool Then remove the protective gas;

3) Precautions after welding:

● After welding, the surface of the weld is inspected, and the excess height of the weld is controlled below 1-1.5mm. Too high excess height will cause the internal stress of the weld to concentrate on the position of the weld toe, which will easily cause cracks at the weld toe. When it is too high, use a grinder to reduce the excess height;

●Small welding current and uniform welding technique can reduce undercut defects, but when undercut occurs in the weld, repair welding and grinding should be carried out according to the depth of the undercut to achieve a smooth transition;

●After the welding is completed, pay attention to the protection of the weld to avoid rapid cooling and contamination of the weld by rainwater or other substances when the weld is not completely cooled to normal temperature;

● After the hydraulic test of the pipeline, chemically clean the pipeline again, so that the weld metal has good corrosion resistance;

4. Welding equipment for Inconel625 alloy:

Because the welding method of argon electric welding is selected, in order to reduce the labor intensity of the staff, an inverter welding machine (model: ZX7-315S/ST) that can satisfy both argon arc and hand arc welding is selected. DC forward connection is selected for (GTAW), and DC reverse connection is selected for electrode arc welding (SMAW).

The inverter welding machine has excellent IGBT inverter technology, the power supply used by the welding machine has stable steep drop (or constant current) external characteristics, and has a variety of argon arc operation modes, high frequency arc ignition, strong welding penetration, With electrode arc welding function, stable performance, strong anti-interference ability, energy saving;

5. Welding materials of Inconel625 alloy:

Due to the selection of the welding method of tungsten gas tungsten arc welding (GTAW) or argon electric welding (GTAW+SMAW), according to various factors, the selection of welding wire: ERNiCrMo-3, electrode: ENiCrMo-3, these two welding materials welding,

1) The chemical composition of the welding material: According to the chemical composition of the welding material and the chemical composition of the base metal, ERNiCrMo-3 welding wire and ENiCrMo-3 electrode have similar composition to the base metal, and the content of Mn or Si elements is higher than that of the base metal. The material is slightly larger, these two elements play a positive role in the deoxidation or desulfurization of the molten pool metal, which helps to reduce the tendency of pores and cracks. Ni, Cr, Co, Mo and other elements also play a role in the alloying elements in the molten pool metal. To supplement, reduce the adverse effects caused by the dilution of base metal alloying elements;

2) Mechanical properties of welding consumables: ENiCrMo-3 electrode and ERNiCrMo-3 welding wire have the same level of strength as the base metal, and the tensile strength is greater than 760MPa, and because the base metal is sensitive to thermal cracking, it is necessary to choose crack resistance. Good low hydrogen electrode, ENiCrMo-3 electrode and ERNiCrMo-3 electrode all meet the requirements;

3) The use environment and performance of the weldment: Inconel 625 alloy steel pipe transports oxygen and is in a corrosive working environment. It is necessary to select corrosion-resistant welding materials. ENiCrMo-3 welding rod and ERNiCrMo-3 welding wire belong to nickel-based welding materials. The content of Cr element is relatively large, which can supplement Cr element for the deposited metal, so that the corrosion resistance of Inconel625 alloy can be continued in the weld metal;

4) ENiCrMo-3 welding rod and ERNiCrMo-3 welding wire must be selected with guaranteed quality and good welding material quality, which can reduce rework in welding operations and reduce construction costs to a certain extent;

6. Summary

1) Gas tungsten arc welding (GTAW) or argon electric welding (GTAW+SMAW) is an efficient and high-quality welding method for welding nickel-based alloys such as Inconel625 alloy;

2) When welding Inconel625 alloy, it is necessary to fully understand its weldability, and according to its use characteristics, formulate a reasonable and targeted welding process for welding;

3) A good welding process must be carefully implemented in the welding operation to ensure the welding quality of the weld;

As an excellent alloy material, Inconel625 alloy has great advantages in corrosion resistance and high temperature characteristics. It is gradually being widely used in important links in the petrochemical industry. The welding technology of this alloy is also becoming more and more mature. progress has contributed to the building blocks.