The researchers of the State Scientific Centre JSC “RPA “CNIITMASH” (belonging within JSC Atomenergomash, a machine-building division of ROSATOM) and specialists of JSC Tyazhpressmash developed an integrated technology of manufacturing large diameter pipes of steel 16ГС for NPP equipment, capable of increasing the labor productivity, reducing the metal consumption and enhancing the physical and mechanical properties of pipe metal.
Large diameter seamless steel pipes of structural-grade steel are widely used in NPP and TPP units for straight and bent sections of pipelines operated at a temperature 450-645°C and pressure 4.0-7,0 MPa. They belong to the increased safety requirement product category that guarantees their uniformity preventing destruction during operation. Such pipes, in the course of fabrication according to the conventional process using cast ingots, go through complex forging operations, which require high labor inputs with low metal utilization factor.
Taking into consideration the current development trends of the global and national power industry related to intensifying operation parameters of power units, there is a need for a significant enhancement of metal physical and mechanical properties of various components of NPP equipment, including pipes for the manufacture of steam pipelines.
To solve this task, in 2019 the specialists of CNIITMASH and JSC Tyazpressmash developed the integrated technology for fabrication of large-diameter pipes (500-1000 mm) of steel 16ГС for NPP equipment combining two hi-tech processes. The first process is casting of a thick-wall hollow ingot in a rotating mold under the intensive impact of centrifugal forces increasing metal density, and the second is plastic deformation of a thick-wall centrifugally cast workpiece by means of radial upsetting with a four-die forging block.
The thick-wall hollow ingot was cast in a centrifugal casting machine RMC 1000 of CNIITMASH and JSC Tyazhpressmash design. This machine makes it possible to fabricate the workpieces of a diameter up to 1000 mm, length up to 3000 mm and wall thickness up to 250 mm, and weight up to 6 tons. The casting process is carried out under the standard operating procedure TTP 01-2019 elaborated and agreed with participation of CNIITMASH. The plastic deformation of the workpiece was performed with a low forging grade (1.5-1.7) enabling to achieve the level of the metal mechanical properties 2-3 times higher than the one specified in the requirements of the relevant technical specifications. The branch pipes manufactured by the specialists of JSC ATM fully comply with the specification requirements in terms of physical and mechanical properties.
The technology of fabrication of centrifugally cast large-diameter tubes of steel 15Х1М1Ф implemented by CNIITMASH and JSC Zavod Bummash in 1980 formed the basis for the new technology. The said pipes are used in over than 40 TPP units of capacity 200-300 MW and in 10 power units of 800 MW capacity. A technically unique centrifugal casting machine of CNIITMASH and JSC Zavod Bummash design was used for fabrication of the workpieces according to this technology, which ensures increased metal density and purity from non-metallic inclusions in the course of fabrication of workpieces of diameter up to 1500 mm, length up to 8500 mm and weight up to 50 t. According to the tests results, the pipes of steel 15Х1М1Ф cast with the said machine in terms of their physical and mechanical properties and operating durability are equal to those of hot-worked blanks, and as for a number of strength and plastic properties, excel them.
“The developed technology of manufacture of large-diameter pipes using centrifugally cast mother pipes instead of solid cast ingots may be successfully applied not only in thermal and nuclear power generation, but also in metallurgy, petrochemistry, heavy engineering and other industries. The process application allows extending the service life of steam pipelines and using them under higher-rated operating parameters,” says the leading expert of CNIITMASH and one of the research authors Genrikh Mirzoyan. “The developed technology, comparing with the conventional process, makes it possible to increase labor productivity 1.5-2 times, cut the consumption of metal by 30-40%, as well as to improve the hygiene and sanitary work conditions for the workers. However, the most important is the enhancement of physical and mechanical properties of centrifugally cast pipe metal 1.5-2 times in comparison with the technical requirements to hot-rolled pipes.”