1 Overview
Turbine 12Cr rotor is widely used in ultra-supercritical units at home and abroad for its excellent toughness, high temperature durability, low creep deformation after long-term operation, and loss of life. However, the 12Cr rotor body and bearing alloy are resistant to work. The characteristics of poor wear and easy adhesion are also outstanding. Therefore, solving the problem of rotor wear failure by surfacing the wear-resistant layer has become an urgent task for turbine welding workers. This paper briefly introduces the method of 12Cr rotor surfacing wear layer in combination with the current domestic development status.
2. Difficulty in rotor surfacing
The ultra-supercritical 12Cr rotor is a martensitic stainless steel forging. The weldability is poor, the crack tendency is serious, and the rotor service conditions are quite complicated. The difficulties in surfacing are mainly concentrated on:
(1) Surfacing material Surfacing material not only withstands the thermal stress generated by the rapid change of the steam turbine unit during the start-stop process and load, but also withstands the centrifugal force of high-speed rotation. Therefore, the surfacing material is required to have high resistance under high temperature and high pressure. Creep rupture strength.
(2) Multi-layer multi-pass welding of thermal cycle rotor surfacing, surfacing alloys need to undergo complex thermal cycling. If the temperature control or welding parameters of each link are improperly adjusted, it will cause uneven organization, coarse grain, and segregation of components. Severe and low hardness.
(3) Properties of fusion zone Since the weld overlay is a fusion weld of a dissimilar material, there is a fusion zone in which the composition and the structure are different from the heat-affected zone of the deposited metal and the substrate, and thus may be a brittle layer having poor performance. Under high temperature conditions, the weld overlay layer is cracked or peeled off due to the difference in thermal expansion coefficient.
3. Surfacing method
At present, there are three main methods used in the domestic 12Cr rotor wear layer surfacing:
(1) The quality of submerged arc surfacing weld is stable, easy to realize mechanization and automation, continuous welding and high welding efficiency. It is the most important surfacing method for rough rotor. Pre-heating before welding; the welding process strictly controls the interlayer temperature to avoid the low hardness of the surfacing layer; the dehydrogenation and heat treatment should be carried out in time after welding; the slag should be paid attention to during the welding process to avoid excessive pores. The first three layers during surfacing are to reduce the dilution rate, and the small specification parameters should be selected as much as possible to ensure good fusion.
(2) Tungsten argon arc welding has good visibility, stable arc, less spatter, and excellent quality of surfacing layer. It is often used for rework of weld seams with partial undercuts, unfused, and vent holes during submerged arc welding. Through parameter optimization, a weld overlay layer with low dilution rate, high quality and stability can be obtained, but the surfacing efficiency is low. In certain cases, the method can also be used for defect surfacing at local locations in the finished state.
(3) Micro-arc plasma welding uses ion arc as a high-temperature heat source, using powder or filament as a filling metal, which is concentrated in heat and easy to be automated. Because of its small welding deformation and low dilution rate, it is mainly used for rework soldering of rotor wear layer with relatively large amount of surfacing during finishing. After welding, it can ensure that the rotor deformation is controlled within 0.05mm. Preheating may not be performed before welding, but local heat treatment is required after welding. This method requires relatively high precision for the welding equipment.
4. Conclusion
With the continuous maturity of 12Cr rotor wear-resistant layer surfacing technology, some domestic large-scale steam turbine manufacturers have gradually mastered relevant technologies and have achieved successful operation results. However, with the rapid development of thermal turbines in the international market to high parameters and large capacity, unit steam Temperature, pressure and other parameters are also constantly improving, and welding workers will also face new challenges. Only by continuously exploring, independent innovation, and in-depth study of the welding mechanism can we be in line with international standards and continuously promote the domestic power equipment manufacturing enterprises to move toward the world first-class.
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