Naval applications require the use of steels with high strength and high toughness over a wide range of service and welding conditions. A 10 wt% Ni steel welding consumable is currently under development with strength and toughness values exceeding the naval steels currently in use. This 10 wt% Ni steel welding consumable could provide several societal benefits, including: 1) a matching filler metal system for the 10 wt% Ni steel base metal currently under development, 2) a more cost effective option for 9 wt% Ni steel used extensively in the LNG sector, which is currently welded with Ni-based filler metals, and 3) a high strength/high toughness consumable that could be used with other naval steels for which the properties are currently undermatched. A robust consumable will be able to produce exceptional welds using a variety of fusion welding processes, therefore the objective of this work was to establish a scientific basis for the effects of microstructural constituents on the mechanical properties in welds produced using the gas tungsten arc (GTAW) and gas metal arc welding processes (GMAW).
Fusion welds were made using the GTAW and GMAW welding processes. For both processes, a single vee-groove butt joint without root face design was used, and the thickness of plate was 19mm. Two welds were made using the GMAW process – one where the shielding gas used was 98% Ar / 2% O2 and the other 100% Ar. For the GTAW process, 100% Ar was used for shielding. For all three welds, the preheat/interpass temperature was maintained between 135 and 148°C. Reheat experiments were performed on the welds to enable identical and uniform microstructures using a Gleeble 3500 thermal-mechanical simulator. Two different reheat cycles were used to create a coarse grain and fine grain microstructure. The reheat thermal cycles were above the Ac3 temperature to allow for reaustenization, thereby eliminating any previous microstructure. The mechanical properties of the welds and reheats were evaluated using Charpy impact energy and microhardness testing. Charpy V-notch testing was performed at -51°C in accordance with ASTM E23 using the standard 10mm x 10mm x 55mm geometry. Microstructure characterization was performed using an FEI Scios DualBeam FIB/SEM for scanning electron microscopy (SEM) analysis. Quantitative analysis of the oxide inclusion size distributions was performed in the as-polished condition on all welds and reheats. Electron backscattered diffraction was used to understand the scale of the lath martensitic microstructure. Finally, fractographic analysis was performed on the surfaces of the fractured Charpy samples using the scanning electron microscope.
Results and Discussion:
Mechanical property measurements for the GTAW and GMAW demonstrated that the GTAW has superior toughness (217 ± 3J at -51°C) to the GMAW using 98% Ar / 2% O2 shielding gas (81 ± 19J). While 81J is still quite an impressive impact energy compared to commercially available high strength welding consumables, it was of interest to understand the fundamental metallurgical influences affecting this large difference in toughness. Detailed microstructural characterization revealed several key differences between the two welds which could be contributing to the dissimilarity of toughness: (1) a finer effective grain size of the martensitic microstructure in the GTAW, (2) a larger presence of a coarse martensite constituent, suggested to be detrimental to toughness, throughout the reheated regions of the GMAW, and (3) fewer and smaller oxide inclusions in the GTAW. Reheat experiments were performed to isolate the effects of the oxide inclusions by producing identical martensitic microstructures of the GTAW and GMAW. The results of those experiments demonstrated that the non-metallic oxide inclusions are the controlling microstructural factor in the inferior toughness of the GMAW, as even with a very fine effective grain size in the GMAW, which was much finer than in the as-welded condition, the toughness was still only 97 ± 5J at -51°C. However, in order to maintain high toughness in the GTAW a fine grain size is necessary the reheat experiments demonstrated that a coarse effective grain size produced a toughness that was 38 ± 8 J. Therefore, in the as-welded GTAW, it is both the lack of oxide inclusions and the fine effective grain size that promotes high toughness.
A novel 10wt% Ni steel welding consumable has been fabricated that produces excellent toughness using both the GTAW and GMAW processes. The results of this research demonstrate the influence that the non-metallic oxide inclusions and effective grain size of the lath martensitic microstructure have on the toughness in this alloy system. These results are significant as now further modifications to the welding processes can be driven based on a sound scientific basis, which previously was incomplete.