FIELD OF THE INVENTION
The present invention relates to an electrode with coating to enhance
productivity during fabrication of thick walled pressure vessels in a shielded
metal arc welding (SMAW) procedure.
BACKGROUND OF THE INVENTION
In the boiler industry, drums and headers are thick walled pressure vessels
operating at high temperature and pressures. The component wall thickness may
go up from 20-30 mm to even 150-200mm. Attachments like stubs, nozzles and
down-comers are frequently welded in the shop fabrication of such components.
A large number of such welds are carried out in fabrication of boilers.
Since these joints are of medium to high tensile steel, basic coated low hydrogen
electrodes are commonly employed to weld them.
In the prior art, such high quality steels are bought only from approved sources
listed for instance in the Indian Boiler regulations or the plates have to conform
to the stringent American society of mechanical engineer (ASME)'s stringent
Specifications. The SMAW electrodes conforming to ASME SEC iiC E 7018-1 are
used these electrodes are of nominal 1.4 Mn-03 Si composition. A 4.0 mm
electrode (d) conforming to these specifications has a uniform coating thickness
(D) of 6.8 mm (D/d ratio of 1.7 or lower). The electrode coating may contain iron
powder nominally 30 wt% and it is seen that such electrodes generally deposit
around 40-42 g of weld metal per electrode for about 38-40 g of core wire
melted giving a nominal deposition efficiency of in the range 105-110 % when
this is defined as follows:
Nominal deposition efficiency = (weight of weld metal ÷ weight of core wire
melted ) x 100. The deposition efficiency can be further improved by increasing
the coating thickness and increasing the coating of iron powder content.
The weld metal strength is nominally 490 m Pa and the CVN impact strength at
minus 46 deg C is 27 1
The present invention relates to further improvements in deposition efficiency.
International Patent Publication WO 2008068808 entitled 'METHOD OF JOINING
BY WELDING OUTER PANEL/ATTACHMENT OF ROLLING STOCK AND SIDE
STRUCTURE MADE THEREBY', discloses in the joining by welding of outer panel
with attachments, such as skeletal members open frame and material for
reinforcement or interior exterior finishing thereof, the region where the outer
panel and each attachment applied thereto are superimposed one upon the
other is subjected to laser welding along the polishing line aligned in one
direction on the surfaces on the externally exposed side of the outer panel and
attachment. Thus, the joining by welding of the outer panel with the attachment
is accomplished, thereby realizing an outer panel/attachment welding joined
structure that is advantageous in appearance, working efficiency and cost.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention to propose a low hydrogen
electrode with improved deposition efficiency adaptable in SMAW procedures,
which produces the welding joint in shorter time.
Another object of the present invention is to propose a low hydrogen electrode
with improved deposition efficiency adaptable in SMAW procedures, which avoids
any loss of tensile strength or CVN toughness or usability of the welded
components.
A further object of the present invention is to propose a low hydrogen electrode
with improved deposition efficiency adaptable in SMAW procedures which
reduces the possibility of cracking of the welding joint.
SUMMARY OF THE INVENTION
During welding, it becomes imperative to protect the weld metal from
atmospheric contamination. All welding processes rely on either gas shielding or
slag shielding (produced from a welding flux) or a combination thereof to
accomplish this. Thus, the SMAW (shielded metal arc welding) process uses
fluxes as a coating. In SAW (submerged arc welding) the welding arc is buried
under a blanket of flux which on melting protects the weld metal and is finally
discarded as slag. The TIG welding process uses inert gas shielding.
But due to inherent problems for example, the need to clean out solidified slag
(molten and cooled flux) and electrode change time, SMAW process has only low
deposition rate per hour. TIG process also suffers from low productivity. Only
GMAW AND SAW processes are amenable for mechanization and automation. In
terms of deposited metal weight per hour of arc time SMAW is at the lower end
with typical values like 1.5 kg/hr as compared to 2.5-4.5 kg/hr from GMAW and
5-8 kg/hr for SAW.
While the long seam and circumferential seam of drums are welded using SAW
or in some cases GMAW, attachment welds like welding of stubs are still made
using SMAW coated electrodes.
According to the invention, both the coating thickness and the iron powder
content in basic coated low hydrogen electrodes can be further increased than
the current levels of D/d ratio of 1.7 and 30% respectively.
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF
THE INVENTION
All types of coated SMAW electrodes contain slag formers, gas formers, de-
oxidizers, de-nitriders, alloying elements and other materials like mineral silicates
and iron powder. Basic coated low hydrogen electrodes conforming to the
American Welding Society standard AWS5.1 E7018-1 typically have about 30
weight % iron powder in the electrode coating and the balance may comprise of
materials lime limestone and fluorspar and titania and other materials.
The present invention proposes the coating thickness be increased from say 6.8
mm for a 4.0 mm core wire to 8.0 mm and beyond and simultaneously the iron
powder content also be increased from 30% up to a value of 60%. The upper
limit is restricted because of the usability of the electrode for down-hand or flat
welding position without any compromise on mechanical properties like tensile
and yield strength, elongation and subzero notch toughness at -45 *C.
AWS 5.1 standard contains references of two high iron powder containing
electrode classifications:-
1) E7024 electrodes which contain substantial titania in the coating but
these electrodes or their variants exhibit good toughness only up to 20
°C.
2) AWS also refers to E7028 type of electrodes which may have low
hydrogen coating but these electrodes or their variants exhibit good
toughness only up to -20 °C.
Hence, a new basic coated low hydrogen type electrode is proposed herein
for use in attachment welding or other purposes in thick walled pressure
vessels which may have a coating (D) to core wire via (d) ratio D/d more
than 1.7 and up to 60% iron powder in the coating the others in the coating
comprising of slag formers, gas formers, de-oxidisers, de-nitriders. alloying
and microalloying elements subject to the fact in the course of testing as
detailed in AWS 5.1 the following minimum properties are achieved:
The following table illustrates the advantages of the proposed invention.
Plate nominal thickness, 100 mm groove Single V included angle 60 degrees
Nominal volumes 577 cm3 Weld-metal weight 4.53 kg
The projected savings in arc time will be even more for higher diameter
electrodes like 5 mm.
It may be noted that such electrodes are not currently commercially available but
can be developed based on the above points by a person skilled in the art and
the above stated results can even be bettered.
A development batch was extruded with the following coating on a 4 mm mild
steel core wire. The coating thickness is 4 mm (D/d = 2.0 or higher) The dry
powder composition was as follows:

This was mixed with mixture of potassium and sodium silicates as a binder,
made into a cake and extruded into the core wire in an hydraulic extruder.
It was then air-dried for a day and then subsequently over dried up to 450 °C.
When tested as per the provisions of AWS 5.1 E7018 specification, excellent
results were obtained, as tabulated below:
WE CLAIM:
1. A high efficiency electrode with coating thickness/core wire thickness
(D/d) more or equal to 2 containing major quantity of iron powder in
the base coating of low hydrogen electrode, exhibiting UTS of
540 mPa tensile strength after SR of 300 min at 625 deg Celsius, and an
all weld CVU impact value of min 27 J when tested at minus 45 deg
Celsius.
2. A process for producing a shielded metal arc welding ( SMAW) electrode,
comprising:
- providing on a 4 mm mild steel core wire coating thickness of 4 mm
(D/d = 2.0 or higher, the dry powder composition being as under:
- mixing the dry powder with a mixture of potassium and sodium silicates
as a binder, made into a cave and extruded into the core wire in an
hydraulic extruder;
- air drying the coating for a day and then subsequently oven dried up to
450°C.

ABSTRACT

The invention relates to a high efficiency electrode with coating thickness/core
wire thickness (D/d) more or equal to 2 containing major quantity of iron powder
in the base coating of low hydrogen electrode, exhibiting UTS of 518 mPa tensile
strength after SR of 300 min at 625 deg Celcius, and an all weld CVN impact
value of min 27 J when tested at minus 45 deg Celcius.