68
P. K. Nanavati et al.
1 Introduction
Amongst all the joining processes, welding is the most widely used process, and
it can be used for various applications from welding of the door to the welding of
aeroplanes. Gas tungsten arc welding (GTAW) is specially used for high-quality
weld joints used in fabrications of pressure vessels, boilers, reactors and in pipelines
industries but every coin has two sides, so on one side; GTAW offers high-quality
joints, while on the other side, the limited weld penetration depth capability of the
order of 3 mm usually with single pass, which leads to reduced productivity of the
process of the order of 2–3 kg/h deposition rate. To overcome these issues, worldwide
many research studies were undertaken to increase the weld performance employing
some modifications in power source design, filler metal feeding mechanism, etc.
That subsequently gave birth to various successful technological versions known as
variants to address some of the aforesaid limitations. For example, enhanced weld
penetration capability through the addition of filler metal such as auto cold wire
feed drive, hotwire feed GTAW and the most novel process was activated GTAW
(A-TIG) or penetration enhancing TIG which has been claimed to offer 300% more
penetration than conventional GTAW. In the activated flux process, oxide, halides
or fluorides fluxes in powder form are mixed in carrier solvents like acetone and
methanol to form a paste to apply a uniform layer of the flux on the specimen to be
welded [1, 2].
In a study by Vora and Badheka [2], the mechanisms which were responsible for
increasing the penetration of weld bead were examined and amongst them, “Reversed
Marangoni” effect was found to be one of the most prominent mechanisms, which
was enhancing weld penetration by reversing the direction of flow of weld pool from
radially outwards to inward direction with the “reversed fluid flow phenomena” of
increase in surface tension with an increase in temperature. Another most prominent
mechanism was found to be an “arc constriction mechanism” which is accomplished
with either insulating flux and/or negative ions, that helps to achieve high depth-towidth ratio.
P91 steel consists of two phases: ferrite and martensite. It is also known as creep
strength enhanced ferritic steel; the main constituents of alloy are 9% Cr, 1% Mo,
0.25 V and balanced Fe [3].
Modified P91 is widely used in different kinds of application demanding high
temperature and good creep resistance strength such as in steam header, steam pipe,
super-heater, pressure vessels and nuclear power plants [4]. There will be an increase
in demand for this steel since India is planning to build four new nuclear power plants
to produce electrical energy at the most affordable rate.
P91 possesses some excellent properties like high thermal conductivity, low
thermal expansion, good steam corrosion resistance, and excellent creep resistance,
so a combination of these properties makes P91 the best-suited candidate for hightemperature application [5, 6]. Table 1 shows the nominal chemical composition
of P91 material. Chemical composition indicates that is modified with vanadium,
nickel, aluminium, niobium and nitrogen as alloying elements, as a result, weld
P. K. Nanavati et al.
1 Introduction
Amongst all the joining processes, welding is the most widely used process, and
it can be used for various applications from welding of the door to the welding of
aeroplanes. Gas tungsten arc welding (GTAW) is specially used for high-quality
weld joints used in fabrications of pressure vessels, boilers, reactors and in pipelines
industries but every coin has two sides, so on one side; GTAW offers high-quality
joints, while on the other side, the limited weld penetration depth capability of the
order of 3 mm usually with single pass, which leads to reduced productivity of the
process of the order of 2–3 kg/h deposition rate. To overcome these issues, worldwide
many research studies were undertaken to increase the weld performance employing
some modifications in power source design, filler metal feeding mechanism, etc.
That subsequently gave birth to various successful technological versions known as
variants to address some of the aforesaid limitations. For example, enhanced weld
penetration capability through the addition of filler metal such as auto cold wire
feed drive, hotwire feed GTAW and the most novel process was activated GTAW
(A-TIG) or penetration enhancing TIG which has been claimed to offer 300% more
penetration than conventional GTAW. In the activated flux process, oxide, halides
or fluorides fluxes in powder form are mixed in carrier solvents like acetone and
methanol to form a paste to apply a uniform layer of the flux on the specimen to be
welded [1, 2].
In a study by Vora and Badheka [2], the mechanisms which were responsible for
increasing the penetration of weld bead were examined and amongst them, “Reversed
Marangoni” effect was found to be one of the most prominent mechanisms, which
was enhancing weld penetration by reversing the direction of flow of weld pool from
radially outwards to inward direction with the “reversed fluid flow phenomena” of
increase in surface tension with an increase in temperature. Another most prominent
mechanism was found to be an “arc constriction mechanism” which is accomplished
with either insulating flux and/or negative ions, that helps to achieve high depth-towidth ratio.
P91 steel consists of two phases: ferrite and martensite. It is also known as creep
strength enhanced ferritic steel; the main constituents of alloy are 9% Cr, 1% Mo,
0.25 V and balanced Fe [3].
Modified P91 is widely used in different kinds of application demanding high
temperature and good creep resistance strength such as in steam header, steam pipe,
super-heater, pressure vessels and nuclear power plants [4]. There will be an increase
in demand for this steel since India is planning to build four new nuclear power plants
to produce electrical energy at the most affordable rate.
P91 possesses some excellent properties like high thermal conductivity, low
thermal expansion, good steam corrosion resistance, and excellent creep resistance,
so a combination of these properties makes P91 the best-suited candidate for hightemperature application [5, 6]. Table 1 shows the nominal chemical composition
of P91 material. Chemical composition indicates that is modified with vanadium,
nickel, aluminium, niobium and nitrogen as alloying elements, as a result, weld