The relationship between the welding conditions, thermal cycles, microstructure and toughness of weld metal in C»~Mn steels

Submerged arc weld deposits were produced using a 40 mm thick low Sulphur, low Phosphorous, Carbon Manganese microalloyed steel to B»S. 45d0 : 50D. The welding consumables used were a 4 mm diameter C. 1'2/fc Mn Wire (SD5) in conjunction with the OP 41 TT ffully basicf flux. Two series of three welds were made at three different calculated heat inputs of 5.8 EJ/nnn, 3.9 KJ/mm- and 2.9 KJ/mm. For the first series . the welding current was kept constant at 650 amp and the welding speed was varied from 200 mn/min to 400 mm/min. For the second series the welding speed was kept constant at 300 mm/min, but the welding current varied from 850 amp to 480 amp. For both the sub-surface and root regions of each weld the relationship between weld metal post solidification cooling cycle, transformation temperature, weld metal microstructure and toughness was examined and it was shown primarily that there is not a simple relationship between heat input as conventionally measured and the weld metal cooling cycle. The weld metal cooling cycle was found to be dependent upon various factors such as :

1. The actual heat input, measured in terms of weld metal bead volume.
2. Weld bead shape measured in terms of width to depth ratio,
3. Flux consumption measured in weight of the slag removed per unit volume of weld bead.
4. The relationship between the size of the weld bead and the geometry of the immediately surrounding plate.
5. The post solidification thermal effects imposed by the subsequent weld runs. From the thermal analysis measurements made whilst welding was in progress, two transformation reactions were identified. À high temperature transformation occuring at approximately 85Q°C identified by subsequent metallographic examinations as the pro-eutectoid ferrite transformation, and a low temperature transformation occuring at approximately 650 C identified as the acicular ferrite phase trans-. formation. The thermal analysis results also showed that .the weld metal cooling rate had an effect on the weld metal transformation temperatures. For each transformation an increase in the weld cooling rate lead to a depression of the transformation Temperature. The present results indicate that the most desirable welding condition from a toughness point of view, should give a weld metal cooling cycle which was "slow" for the 1400°C - 900°C temperature range, but "fast" below the temperature of 900°C. This would lead to a microstructure formed of large columnar grains, but with a high acicular ferrite volume fraction. 1 All welds showed a through thickness toughness variation. These differences in the through thickness properties were mainly attributed to the large differences in the thermal history between the sub-surface and the root beads which in turn lead to different microstructures, the sub-surface beads were formed by a larger columnar grain and a higher volume fraction of acicular ferrite than the root beads. The root beads Charpy V specimens also contained some refined equiaxed ferrite grains while the sub-surface Charpy V specimens contained solely as deposited weld metal. These differences in the microstructure features between the sub-surface and the root beads in turn appear to be, for the present welds, the main cause for the differences in the through thickness properties. The overall conclusion from the present work is therefore that the weld metal deposits made at the same calculated heat input do not necessarily show the same toughness properties. This results from the fact that the cooling cycle, transformation temperature and amount of weld metal reheated by the subsequent runs are determined by the precise welding conditions.