The Research On Joint Welds Of NiCrMoV Dissimilar Steel Properties And Microstructure
The rotor is one of the core components of large steam turbine equipment. At present, there are mainly three manufacturing methods: set, whole section and welding. Compared with the previous two methods, the welded rotor has flexible design, compact structure, light weight, good rigidity and operating stress. The advantages of low, segmented forgings with small size, easy quality assurance and short production cycle have been selected by many steam turbine manufacturers in the world. For high, medium and low pressure integrated rotors, different grades of materials can be used for different parts to maximize the characteristics of the materials, achieve the ideal combination, save valuable elements, and further reduce production costs.
Dissimilar steel welding technology has gradually been applied in the production of single-cylinder steam turbines, gas turbines/combined cycle steam turbines, and high/medium pressure rotors of super-supercritical steam turbines. NiCrMoV steel is a type of low-alloy high-strength steel commonly used in large steam turbines. It has high strength and good low-temperature impact performance. According to the different proportions of several basic elements such as Ni, Cr, Mo, V, it can be composed of different strength levels, The structure and properties of steel.
This kind of material has a high sensitivity to temper embrittlement, and it is easy to produce brittleness when used for a long time at 350～500℃. There is no research report on whether brittleness occurs in NiCrMoV dissimilar steel welded rotors after aging. It is pointed out in the literature that the ageing brittleness of NiCrMoV steel in the range of 250～400℃ and the flaky carbides precipitated along the grain boundary and sub-grain boundary during aging, the decomposition of retained austenite and the segregation of solute atoms at the original austenite grain boundary related.
In this paper, the welded joint of the welded rotor simulation part composed of dissimilar NiCrMoV steel is selected to age for 3500h under the simulated service environment, and then samples are taken from each area on the simulation part for impact test, and the chemical composition and metallography of the impacted sample are carried out. Microscopy, scanning electron microscopy, electron backscattering and other experimental analysis to study the internal relationship between the toughness and microstructure of the welded joint after aging. The research results provide a basis for the analysis of material damage and embrittlement of steam turbine welded rotors in service, and propose control strategies.
The test materials are 22Cr2NiMoWV and 30Cr2Ni4MoV steel forgings. Both of these materials are ultra-pure steel, which reduces the content of impurities such as S, P, As, Sn, Mn, etc. to a minimum. The mechanical properties are shown in Table 1.
| ReL/MPa | Rm/MPa | A(%) | Z(%) | AKV/J | |
| 22Cr2NiMoWV | 650~720 | ≤860 | ≥15 | ≥40 | ≥80 |
| 30Cr2Ni4MoV | 760~860 | 860~1040 | ≥15 | ≥40 | ≥140 |
The dissimilar steel welding simulation parts of these two kinds of steel are formed by narrow gap groove, argon arc welding backing and deep narrow gap submerged arc welding, multi-layer and multi-pass welding, and the welding material is a special welding wire containing 2% Ni. After welding, the simulated parts are subjected to local stress-relieving heat treatment and 350℃×1000h aging treatment. Then, mechanically cut the simulated parts in the aged state and the non-aging state to prepare samples for impact test, metallographic analysis, scanning electron microscopy analysis, and EBSD analysis.
The retained austenite in the weld metal of the dissimilar NiCrMoV steel undergoes thermal instability decomposition during the aging process, and carbides are precipitated between the martensite bars along the lath. The observation and analysis of the microstructure by metallography and scanning electron microscope show that the dissimilar joints of NiCrMoV steel are aging treated, and carbides are precipitated at the grain boundary and sub-grain boundary, which weakens the grain boundary.
After the dissimilar NiCrMoV steel weld metal is aged, the decomposition of retained austenite, the segregation of N grain boundaries and the precipitation of carbides along the grain boundaries together contribute to the decrease in its toughness.
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