9Cr-1Mo-V steel has excellent creep strength compared to 2 1/4Cr-1Mo steel and 2 1/4Cr-1Mo-V steel, hence the steel has been already put into commercial use for boilers of thermal power plants, etc. However, in case of applying to a reactor in oil refinery plant, it is necessary to ensure not only creep strength but also toughness to prevent brittle fracture as start-up operation of the reactor.

In previous study, the melt-run method, which alternated between welding by GTAW (Gas Tungsten Arc Welding) and arc re-melting without filler to the weld metal, was developed. The weld metal by that melt-run method had better toughness than one by conventional GTAW only.

Meanwhile, in order to ensure mechanical properties of the reactor, several PWHT (PostWeld Heat Treatment) conditions to be simulated shall be required, i.e. Min. PWHT as one cycle for fabrication and Max. PWHT as multiple cycles including repair and spare cycle at site. It was recognized that the toughness of weld metal of 9Cr-1Mo-V steel by GTAW after Max. PWHT was lower than one after Min. PWHT, hence improvement of the toughness after long-time PWHT was a subject to be solved.

Then, mechanical tests and TEM observation were conducted to GTAW test coupon by the melt-run method with various PWHT conditions in order to clarify the factor of toughness decrease after long-time PWHT. As tempering parameter increased, toughness of the weld metal decreased. It was considered that the toughness decrease was attributed to the coarsening of carbide (M23C6) at the grain boundary by TEM observation.

In order to reduce M23C6 precipitation at the grain boundary in weld metal after long-time PWHT, chemical composition of welding consumables was modified. Specifically, carbon content was reduced compared with conventional one and other chemical compositions was also modified to hold mechanical properties. Toughness of weld metal using modified welding consumables did not decrease as the increase of tempering parameter, then the toughness could be stable after both Min. and Max. PWHT condition.

According to these results, it is considered that the coarsening of M23C6 was the factor of toughness decrease of GTAW metal of 9Cr-1Mo-V steel after long-time PWHT. In addition, it was possible to reduce toughness decrease even after long-time PWHT by melt-run method with modified welding consumables.

Based on the above, pressure vessel made of 9Cr-1Mo-V steel, such as a reactor in oil refinery plant required low temperature toughness can be manufactured using these fabrication procedures.


Improvement of Toughness in Weld Metal of 9cr-1mo-v Steel by Gtaw After Long-Time Pwht

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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