Thermal Deformation And Microstructure Evolution of Thick-walled Welded Steel Pipes

Thick-walled welded pipe is a hard-to-deform precipitation-strengthened nickel-based high-temperature alloy with a composition similar to that of the former Soviet Union's ЭИ929 alloy. Its alloy elements have a high level of solid solution strengthening and precipitation strengthening of the γ' phase. It has excellent oxidation resistance, hot corrosion resistance, and excellent yield strength, tensile strength, and creep strength at high temperatures. It is mainly used in environments with high temperatures, complex stresses, and corrosive media, such as making engine turbine blades. Due to the relatively narrow range of thermal processing parameters of this alloy, when used for hot forging of turbine working blades, the forgings are prone to structural instability cracks and other defects, resulting in a high scrap rate. Therefore, studying the hot deformation behavior of this alloy under different hot deformation conditions is of great significance for obtaining qualified forgings. The researchers analyzed the rheological behavior characteristics of the alloy through data obtained from high-temperature compression experiments of thick-walled welded pipes, established the constitutive equation of thick-walled welded pipes within the range of thermal deformation parameters, and studied the effects of deformation temperature and strain rate on the alloy microstructure.

The raw material used in the experiment is a thick-walled welded pipe hot-rolled bar, and the original structure is mainly composed of equiaxed grains with a grain size of 10 to 30 μm. The bar was processed into a cylindrical sample of Φ8 mm × 12 mm, and shallow grooves for storing high-temperature lubricant were processed at both ends of the sample. An isothermal compression experiment was conducted on a Gleeble-1500 testing machine. The deformation temperatures are 1090, 1120, 1150, and 1180°C, the strain rates are 0.1, 1, 10, and 50 s-1, and the maximum deformation degree is approximately 60%. During the experiment, the testing machine automatically collects and calculates stroke, load, stress, and strain data. After the deformation is completed, the sample is cooled with water, and then the sample is cut longitudinally, ground and polished, and then corroded by CuSO4 (20g) + H2SO4 (5ml) + HCl (50ml) + H20 (100ml) solution, and then observed under a metallographic microscope. Alloy microstructure. The results showed that:

1. When thick-walled welded pipes are deformed under different conditions, as the strain increases, rheological softening occurs. The reason for rheological softening is the dynamic recrystallization of the alloy during thermal deformation. As the strain rate decreases, both the strain and the peak stress when the flow stress reaches its peak decrease.

2. A constitutive equation for high-temperature deformation of thick-walled welded pipes was established. The calculated values of the equation are in good agreement with the experimental values, and the relative errors are both below 8%, indicating that the equation accurately describes the rheological behavior of the alloy during thermal deformation.

3. Deformation temperature has a significant impact on the microstructure of thick-walled welded pipes. As the temperature increases, dynamic recrystallization becomes sufficient, the grain size becomes larger, and the uniformity of the grain structure improves; as the strain rate increases, the grain size first decreases and then increases. When the strain rate is 1s-1, the grain structure is relatively fine.

Horizontal fixed welding of thick-walled stainless steel pipes: Stainless steel pipes are hollow long strips of steel that are widely used as pipelines for transporting fluids, such as oil, natural gas, water, gas, steam, etc. Stainless steel pipes are lighter in weight when they have the same bending and torsional strength. They are widely used in the manufacture of mechanical parts and engineering structures. They are also commonly used to produce various conventional weapons, gun barrels, shells, etc. For steel pipes that require thicker walls to withstand fluid pressure, hydraulic tests must be conducted to check their pressure resistance and that they will not leak, wet, or expand under the specified pressure. Stainless steel pipes are divided into seamless and seamed. Seamless stainless steel pipes are also called stainless steel seamless pipes. They are made of steel ingots or solid tube blanks perforated into capillary tubes, and then hot-rolled, cold-rolled, or cold-drawn. The specifications of seamless steel pipes are expressed in terms of outer diameter × wall thickness in millimeters. Commonly used stainless steel pipes are 1Cr18Ni9Ti. The following uses the 1Cr18Ni9Ti stainless steel pipe with a diameter of 159mm×12mm as an example to introduce its horizontal fixed welding method.

First, welding analysis: 1. Cr18Ni9Ti stainless steel Ф159mm×12mm large pipe horizontal fixed butt joints are mainly used in pipes that require heat and acid resistance in nuclear power equipment and some chemical equipment. Welding is difficult and requires high welding joints. The inner surface is required to be formed, with moderate convexity and no concavity. PT and RT inspections are required after welding. In the past, TIG welding or manual arc welding was used. The former has low efficiency and high cost, while the latter is difficult to guarantee and has low efficiency. To both ensure and improve efficiency, the TIG inner and outer filler wire method is used to weld the bottom layer, and MAG welding filler and cover layers are used to ensure both safety and efficiency. 2. The thermal expansion rate and electrical conductivity of 1Cr18Ni9Ti stainless steel are greatly different from those of carbon steel and low alloy steel, and the fluidity of the molten pool is poor and the forming is poor, especially when welding in all positions. In the past, MAG (Ar+1%~2%O2) welding of stainless steel was generally only used for flat welding and flat fillet welding. During the MAG welding process, the extension length of the welding wire should be less than 10mm, the swing amplitude, frequency, speed and edge dwell time of the welding gun should be properly coordinated, the movements should be coordinated, and the angle of the welding gun should be adjusted at any time to make the edges of the weld surface fused neatly and the shape beautiful to ensure filling and covering layer.

Second, welding method: The material is 1Cr18Ni9Ti, the pipe specification is Ф159mm×12mm, manual tungsten arc welding is used for the base, mixed gas (CO2+Ar) shielded welding for filling and cover welding, vertical horizontal fixed all-position welding.

Third, preparation before welding: 1. Clean the oil and dirt, and grind the groove surface and the surrounding 10mm to give a metallic luster. 2. Check whether the water, electricity, and gas lines are smooth, and the equipment and accessories should be in good condition. 3. Assemble according to the size. The tack welding is fixed by ribs (2 o'clock, 7 o'clock, and 11 o'clock are fixed by ribs). In-groove tack welding can also be used, but pay attention to tack welding.