Improvement of the technology of welding high-temperature diaphragms in steam turbine flow section

Abstract

Special features and causes for operational damage in welded components of high-temperature diaphragms are considered. Comparative testing was performed of welding-technological properties of some commercially available coated electrode grades for welding heat-resistant steels of Cr-Mo and Cr-Mo-V compositions. Implant procedure and welding of Tekken technological samples were used to assess the delayed fracture susceptibility of 15Kh12VNMF + 15Kh1M1FL steel welded joints at application of selected electrodes. It is established that to ensure a high cold cracking resistance of these steel welded joints, it is necessary to apply preheating at 200-250 °C. Proceeding from the data of measurement of metal hardness in welds and HAZ of welded joints and weld metal impact toughness, an optimum mode of heat treatment of 15Kh12VNMF + 15Kh1M1FL steel welded joints was determined. It is shown that a softening zone of lower hardness forms in the HAZ metal structure of 15Kh12VNMF steel, in which the samples fail at testing for static tension at working temperature of 570 °C.

Improvement of the Mechanical Characteristics, Hydrogen Crack Resistance and Durability of Turbine Rotor Steels Welded Joints

“…The combined rotor is made of alloy steels that meet the special operating conditions of high-pressure (HP), medium-pressure (IP) and low-pressure (LP) cylinders. For the stages of the rotor, which operates in low temperatures at JSC "Turboatom", the steels 25H2NMFA and 20H3NMFA were proposed [10][11][12][13][14][15].…”

“…The welding of the 25H2NMFA and 20H3NMFA steels (Table 1) is an extremely difficult task [10][11][12][13][14], which is associated with the formation of defective structures in the alloying area, as well as in the area of incomplete recrystallization of the heat-affected zone (HAZ) of the 20H3NMFA steel welded joint. An urgent requirement is also to ensure the necessary resistance against the formation of cold cracks.…”

“…Impact toughness of samples with a V-notch along the HAZ (fusion and overheating areas) (Figure 4). The metallographic analysis of the structure of combined welded joints showed that the metal of HAZ sections has a mainly sorbitol and bainitic-ferritic structure on the 25H2NMFA side, as well as a bainitic martensite structure on the 20H3NMFA side [10][11][12][13]20].…”

“…The hydrogen content in the metal of the ingots should not exceed 2.23 ppm [10][11][12][13][14][15], and the decrease in its value in steel and weldments has led to the improvement of the mechanical characteristics, hydrogen resistance and durability of steam turbine rotor steels welded joints. The initial steel structure is tempered bainite (Figure 4).…”

“…(b1, b2)-schematic of a built-up stream turbine rotor with shrunk-on discs; (c)-schematic of a welded stream turbine rotor. The welds connect the discs (ABB Power Generation, Baden, Switzerland) [2,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23]. Rotor body with detailed drawing of gaps for wedges and winding (d) [5].…”