First. Hot Deformation and Microstructure Evolution of Thick-Walled Welded Steel Pipes
Thick-walled welded steel pipes are a difficult-to-deform precipitation-strengthened nickel-based superalloy, similar in composition to the former Soviet Union’s EI929 alloy, exhibiting high levels of solid solution strengthening and γ’ phase precipitation strengthening. It possesses excellent oxidation resistance, hot corrosion resistance, and high yield strength, tensile strength, and creep strength at high temperatures. It is mainly used in environments with high temperatures, complex stress, and corrosive media, such as in the manufacture of turbine blades for aero-engines. Due to the relatively narrow range of hot working parameters for this alloy, when used for hot forging of turbine blades, forgings are prone to microstructure instability and cracks, leading to 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. Researchers analyzed the rheological behavior characteristics of this alloy using data obtained from high-temperature compression experiments on thick-walled welded steel pipes, established the constitutive equation for thick-walled welded steel pipes within the range of hot deformation parameters, and studied the influence of deformation temperature and strain rate on the alloy’s microstructure.

The raw material used in the experiment was hot-rolled thick-walled welded steel pipe bars, whose original microstructure mainly consisted of equiaxed grains with a grain size of 10–30 μm. The bars were machined into cylindrical specimens of Φ8 mm × 12 mm, with shallow grooves at both ends for storing high-temperature lubricant. Isothermal compression tests were conducted on a Gleeble-1500 testing machine. The deformation temperatures were 1090, 1120, 1150, and 1180 °C, and the strain rates were 0.1, 1, 10, and 50 s⁻¹, with a maximum deformation of approximately 60%. During the experiment, the testing machine automatically collected and calculated the stroke, load, stress, and strain data. After deformation, the specimens were water-cooled, then longitudinally cut, ground, polished, and then etched with a solution of CuSO₄ (20 g) + H₂SO₄ (5 ml) + HCl (50 ml) + H₂O (100 ml). The alloy microstructure was then observed under a metallographic microscope. Experimental results show that:

1. Under different deformation conditions, thick-walled welded steel pipes exhibit rheological softening with increasing strain. This softening is caused by dynamic recrystallization during hot deformation. As the strain rate decreases, both the strain at which the flow stress reaches its peak and the peak stress decrease.
2. A constitutive equation for high-temperature deformation of thick-walled welded steel pipes was established. The calculated values ​​and experimental values ​​show good agreement, with relative errors all below 8%, indicating that the equation accurately describes the rheological behavior of the alloy during hot deformation.
3. Deformation temperature significantly affects the microstructure of thick-walled welded steel pipes. With increasing temperature, dynamic recrystallization becomes more complete, grain size increases, and the uniformity of the grain structure improves. With increasing strain rate, grain size first decreases and then increases. The grain structure is relatively fine when the strain rate is 1 s⁻¹.

Second, horizontal fixed welding of thick-walled stainless steel pipes. Stainless steel pipes are hollow, long steel materials widely used as pipelines for transporting fluids such as oil, natural gas, water, coal gas, and steam. Stainless steel pipes, while maintaining the same bending and torsional strength, are lighter in weight and widely used in the manufacture of mechanical parts and engineering structures. They are also commonly used in the production of various conventional weapons, gun barrels, and artillery shells. For steel pipes subjected to fluid pressure, thicker steel walls are required, and hydraulic tests are conducted to verify their pressure resistance and ensure that they do not leak, become wetted, or expand under specified pressure. Stainless steel pipes are divided into seamless and welded types. Seamless stainless steel pipes, also known as seamless stainless steel tubes, are made by piercing steel ingots or solid tube blanks to form a rough tube, which is then hot-rolled, cold-rolled, or cold-drawn. The specifications of seamless steel pipes are expressed in millimeters as outer diameter × wall thickness. A commonly used type is 1Cr18Ni9Ti stainless steel pipe. The following section uses a 1Cr18Ni9Ti stainless steel pipe with a diameter of Ф159mm × 12mm as an example to introduce its horizontal fixed welding method.
1. Welding Analysis:
(1) Horizontal fixed butt joints of Cr18Ni9Ti stainless steel Ф159mm×12mm large steel pipes are mainly used in nuclear power equipment and certain chemical equipment where heat and acid resistance are required. Welding is difficult, and the requirements for the welded joints are very high. The inner surface must be well-formed, with moderate convexity and no concavity. Post-weld PT and RT inspections are required. Previously, TIG welding or manual arc welding were used. The former is inefficient and costly, while the latter is difficult to guarantee and inefficient. To ensure both efficiency and reliability, TIG internal and external filler wire welding is used for the root layer, and MAG welding is used for the filler and capping layers, thus ensuring both efficiency and reliability.
(2) The thermal expansion coefficient and electrical conductivity of 1Cr18Ni9Ti stainless steel differ significantly from those of carbon steel and low alloy steel. Furthermore, the molten pool has poor fluidity and poor formability, especially during all-position welding. Previously, MAG (Ar+1%～2%O2) welding of stainless steel was generally only used for flat welding and fillet welding. During MAG welding, the wire extension length should be less than 10mm. The welding torch oscillation amplitude, frequency, speed, and edge dwell time should be appropriately coordinated, with consistent movements. The torch angle should be adjusted as needed to ensure neat and aesthetically pleasing weld edge fusion, guaranteeing the filling and capping layers.

2. Welding Method: The material is 1Cr18Ni9Ti, and the pipe fitting specifications are Ф159mm×12mm. Manual tungsten inert gas (TIG) welding is used for the root pass, followed by mixed gas (CO2+Ar) shielded welding for the fill and capping passes. Vertical horizontal fixed all-position welding is employed.

3. Pre-welding Preparation:
(1) Clean oil and dirt, and grind the bevel surface and the surrounding 10mm area to achieve a metallic luster.
(2) Check that the water, electricity, and gas lines are unobstructed, and that the equipment and accessories are in good condition.
(3) Assemble according to dimensions. Positioning welding uses ribs for fixation (2 points, 7 points, and 11 points are fixed). Internal bevel positioning welding can also be used, but care should be taken with positioning welding.