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

The raw materials used in the experiment are hot-rolled bars of thick-walled welded steel pipes. The original structure is mainly composed of equiaxed grains with a grain size of 10 to 30 μm. The bars are processed into cylindrical specimens of Φ8mm×12mm. Shallow grooves for storing high-temperature lubricants are processed at both ends of the specimens. Isothermal compression experiments are carried out on a Gleeble-1500 testing machine. The deformation temperatures are 1090, 1120, 1150, and 1180℃, the strain rates are 0.1, 1, 10, and 50s-1, and the maximum deformation degree is about 60%. During the experiment, the testing machine automatically collects and calculates the stroke, load, stress, and strain data. After deformation, the specimens are water-cooled, and then cut longitudinally, ground, polished, and corroded by CuSO4 (20g) + H2SO4 (5ml) + HCl (50ml) + H20 (100ml) solution. The alloy microstructure is observed under a metallographic microscope. The test results show that:

1. When the thick-walled welded steel pipe is deformed under different conditions, rheological softening occurs as the strain increases. The reason for rheological softening is that the alloy undergoes dynamic recrystallization during the hot deformation process. As the strain rate decreases, the strain and peak stress when the flow stress reaches the peak value both decrease.
2. The constitutive equation for high-temperature deformation of thick-walled welded steel pipe is established. The calculated value of the equation is in good agreement with the experimental value, and the relative error is less than 8%, indicating that the equation accurately describes the rheological behavior of the alloy during hot deformation.
3. The deformation temperature has a significant effect on the microstructure of the thick-walled welded steel pipe. As the temperature increases, the dynamic recrystallization is more sufficient, the grain size becomes larger, and the uniformity of the grain structure increases; 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 pipe: A stainless steel pipe is a hollow long steel strip, which is widely used as a pipeline for conveying fluids, such as oil, natural gas, water, coal gas, steam, etc. Stainless steel pipes are lightweight when their bending and torsional strengths are the same. They are widely used in the manufacture of mechanical parts and engineering structures and are also often used to produce various conventional weapons, gun barrels, shells, etc. For steel pipes that withstand fluid pressure, thicker pipe walls are required, and hydraulic tests must be carried out to test their pressure resistance and that they do not leak, soak 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 tubes through perforation to form rough pipes, and then hot rolled, cold rolled, or cold drawn. The specifications of seamless steel pipes are expressed in millimeters of outer diameter × wall thickness. Commonly used are 1Cr18Ni9Ti stainless steel pipes. The following takes the 1Cr18Ni9Ti stainless steel pipe with a diameter of Ф159mm×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 joint is mainly used in nuclear power equipment and some chemical equipment that require heat and acid resistance. The welding difficulty is high, and the requirements for the welding joint are very high. The inner surface requires forming, moderate convexity, and no concave. PT and RT inspections are required after welding. In the past, TIG welding or manual arc welding were used. The former is inefficient and costly, and the latter is difficult to guarantee and inefficient. To ensure and improve efficiency, TIG internal and external wire welding is used for the bottom layer, and MAG welding is used for filling and covering layers so that efficiency is guaranteed.
2. The thermal expansion coefficient and conductivity of 1Cr18Ni9Ti stainless steel are quite different from those of carbon steel and low alloy steel, and the molten pool has poor fluidity and poor forming, especially in full-position welding. In the past, MAG (Ar+1%~2%O2) welding of stainless steel was generally only used for flat welding and flat corner welding. During the MAG welding process, the wire extension length is less than 10mm, the welding gun swing amplitude, frequency, speed, and edge dwell time are properly coordinated, the movements are coordinated, and the welding gun angle is adjusted at any time to make the weld surface edge fused neatly and beautifully formed to ensure the filling and cover layer.

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

Third, preparation before welding:
1. Clean oil and dirt, and grind the groove surface and the surrounding 10mm to produce metallic luster.
2. Check whether the water, electricity, and gas lines are unobstructed, and the equipment and accessories should be in good condition.
3. Assemble according to size, and use rib plate fixation for positioning welding (2 points, 7 points, and 11 points are rib plate fixation). Positioning welding in the groove can also be used, but pay attention to positioning welding.