Abstract:
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.