9Cr-1Mo-V steel with excellent high temperature strength is one of candidate materials for advanced pressure vessels in oil-refining plants, whose process temperature is expected to be around 500°C. Although 9Cr-1Mo-V steel has been applied as boiler tube material in power generation for a few decades, it was reported that embrittlement occurred after long-term aging around 600°C which is accelerated condition for pressure vessel operation. Since pressure vessels are more sensitive in stress-concentration around crack tip than boiler tube because of its large wall thickness, fracture toughness is an important property of concern when 9Cr-1Mo-V steel is applied to pressure vessels. In this research, 9Cr-1Mo-V steel with tempered-martensitic microstructure was aged up to max. 10000 hr at 500, 550 and 600°C, and fracture toughness was evaluated after the aging by Charpy impact test.The influence of heat treatment conditions such as austenitizing, tempering and PWHT were also investigated, because the heat treatment conditions used in pressure vessels are different from those of boiler tube. In case of samples treated under the conditions for pressure vessels, Charpy impact values at 0°C were sufficient around 200J before aging, and decreased after aging depending on its conditions, and longer time and higher temperature led to more severe degradation. When the aging time at 550°C and 600°C was converted to the equivalent aging time at 500°C by Larson-Miller-equation, the impact value was estimated to keep over 50J after several decades at the operating temperature for pressure vessels. In contrast to the conditions for pressure vessels, the heat treatment conditions used in boiler tube made initial impact value decreased significantly, because tempering and PWHT were shorter than those of pressure vessels. Therefore, the samples heat treated under boiler tube conditions showed lower impact values around 50J in the earlier stage of aging. Considering all obtained results, it was suggested that the serious degradation of fracture toughness in 9Cr-1Mo-V after long term aging would not occur in actual service time for pressure vessels.
Due to the increasing demands for light oil in newly industrializing countries and depletion of conventional oil resources, upgrading of heavy oil and coal-to-liquid processes have been a focus in recent years. The efficiency of these processes depends on temperature and pressure conditions, where a higher temperature, around 500ºC, is likely to be used. However, 2 1 / 4 Cr-1Mo-V steels which have been widely used for heavy-wall pressure vessels for many years cannot be applied to a high temperature process around 500ºC since the design temperature of this material is limited to 482ºC by ASME Code Section VIII, Division 2 [1].On the other hand, 9Cr-1Mo-V steels (Grade 91), which has an excellent performance at high temperature in mechanical properties and hydrogen resistance, has been used for tubing and piping materials in power industries and it can be a candidate material for the high temperature processes. However it has not been used for pressure vessels in refining industries.In order to manufacture heavy-wall pressure vessels using 9Cr-1Mo-V steels, essential techniques including manufacture of large forged shell rings, thick wall welding and overlay welding have been developed.
9Cr-1Mo-V steel, which was developed for application as a steam generator for fast breed reactors in the 1970s, has a higher strength at high temperatures and has been used for equipment and piping systems in the fossil power industries. ASME, Section VIII, Division 2 [1] gives 9Cr-1Mo-V steel a maximum design temperature of 649°C and an operating temperature of 500°C. And it has higher allowable stresses at 450°C or over, compared to 2¼Cr-1Mo-V steel. Therefore, if this material can be used, more economical pressure vessels operating at 454–500°C can be designed and manufactured. In our previous study for base metal, a large forged shell ring of 9Cr-1Mo-V steel was manufactured and for base metal welding, cracking susceptibilities and weldability were investigated. For overlay, welding consumables with high resistance to sigma phase embrittlement were developed [2]. In this study, highly efficient welding consumables for tandem SAW designed for circumferential welding of heavy wall shells were developed and welding using the full-scaled shell ring was demonstrated, and then the mechanical properties of the weld metal were evaluated. Results indicated that, regardless of the weld thickness, a minimum of 8 hours postweld heat treatment (PWHT) at 745°C was required to meet hardness and toughness requirements for conventional reactors. The strength of the materials can comply with the Code requirements after 3 cycles of PWHT considering the PWHT in fabrication and after weld repair. Furthermore, the following new Code Cases and Code revision were proposed and approved in order that pressure vessels can be designed in accordance with ASME, Section VIII, Division 2. • New Code Case for Fatigue Evaluation • New Code Case to apply SA-336-F91 in ASME Section VIII, Division 2 • Revision of Table 5A to add SA-336-F91 As a result, it has become possible to design and manufacture refining reactors to operate at 454–500°C.
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