Influence of welding parameters on microstructure of welded joints SMAW/GTAW steel X10 CrMoVNb 9-1 (P91)

This paper presents weldability results of P91 creep resistant steels performed through two different welding technologies: The " fully mechanized (GTAW)", and the "Shielding Metal Arc Welding (SMAW)" It was evaluated the influence of welding parameters on the microstructure of the joint and on the consequent mechanical characteristics (Strength, toughness and ductility) of the fused zone (ZF). With reference to the P91 creep resistant steel (Tab.1), the RCC-MRx code for the design and production of components for nuclear power plants, have data sheet about the chemical and mechanical features of "Base Material" (BM) but does not have data sheet about filler materials, as qualified for welding nuclear components class 1. Scope of this work is to evaluate possible candidate filler material (Tab.2) and to suggest welding technologies and related operative welding conditions to ensure that N1 class could be produced with the same requirements as the Base Material, in compliance to the design code. The main objective of this study is to evaluate the candidate filler materials and suggest welding conditions optimized for PWTH, Heat Input (HI), and Deposition Mode/Ratio(DM/DR), to obtain an elongation of more than 20 % and a toughness of more than 60 J to 0°C (as evaluated by Charpy KV test)in the fused zone (ZF). For both processes, according to the literature data, it was decided to analyze the influence of four parameters: Speed of deposition (deposition ratio DR ); mode of deposition (DM); Heat input (HI); duration of PWHT . To varying DR/ DM were chosen two different bevel angles (60 ° and 75 °), two modes of deposition (" String Beads"," Weave Beads "), obtained through different welding speed Ws. Two different holding times (2h , 4h ) for PWHT were investigate to evaluate the influence of Time on the feature of FZ. The test was carried out in compliance within the RCC- Mr and the correlate ISO Standard: ( UNI EN ISO 15614-1 ; Transversal tensile test: ISO 4136-2011 & ISO 6892-1 ; Longitudinal tensile test: IS05178 - 2011 and ISO 6892-2 , ISO 9015). The Data about the performed test are reported in Tab. 6. In general for both the welding technology evaluate, the Fused Zone as welded have a fully martensitic microstructu- res, this is demonstrated by the high value of hardness. After the PWHT the hardness in FZ decreases from more of 400 to 230-250 HV5, typical values of tempering martensite. The comparison between the hardness profiles measured at different conditions and PWHT show that the duration of the holding time influences the hardness (Fig.1). If the hoi-ding time increases the hardness in FZ decreases. About SMAW, the toughness improves with increasing the holding time of the PWHT. However only for the condition of welding "weave beads" the value is in compliance with the RCC- MR (Tab. 6). In general, a little reduction of tensile strength was observed after the PWHT, and the differences increase with the increase in holding time. At the same way, the test on GTAW coupons, confirming that longer PWHT holding time induces a reduction in the hardness of fused zone. The micro hardness test highlighted the well know problem of softening in the interface between of HAZ and BM, where the material was subjected to an unavoidable over-tempering (Fig.3). The V notch Charpy tests on GTAW weld shown very high values (higher than SMAW ) in all the case in compliance , with the requirement of RCC-MR. The longitudinal tensile test at room temperature for GTAW ZF specimens (Tab.10), shows very high values of Rm than SMAW. However, this high values are not in compliance with the requirements of the RCC-MR (Max 760 MPa). At the same time the values of elongation, both for GTAW and SMAW processes, is more than 17%, which is promising for a filler material but less than the 20%, threshold required for the BM and the target for the welds. The influence of holding time of PWHT on the mechanical features is the same for the two processes analyzed. The strength is reduced and the elongation increased with augment of holding time of the PWHT. ' The longitudinal tensile test at 550°C were performed only for the GTAW coupons and shown values of Rp 0,2 more than required (260 MPa) and an elongation more than 20 (Tab.11). The SMAW and GTAW shown that the "weave beads" Deposition Mode allowed obtaining the best results in terms of toughness and ductility. The holding time of PWHT influenced the toughness, hardness and strength of the welds. The holding time of 4 h appear to allow the best results. The elongation in all tests for processes at room temperature were less than 20 %. The fully mechanized GTAW showed values of strength very high in association with very good toughness. In all the processes two main problem remain: The softening zone between HAZ and BM (Type IV Cracking zone); The elongation less than 20% , which is requested for the Base Material. Values of elongation of 17-18% are commonly achievable from a wide range of processes and welding parameters. To increase this value, the augment of the PWHT holding time seems to improve the elongation but a long time of PWHT ; means a higher over-tempering of the Type IV Cracking zone. So, in conclusion, the filler material used in this work is a good candidate to weld Nuclear component made of P91,; however the target in terms of ductility and maximum strength, required as BM of RCC-Mr, needs to be reviewed.