FIELD OF THE INVENTION
This invention relates to a method for improving toughness and cold cracking
resistance at heat affected zone (HAZ) on weld products by correlating the
microstructure and mechanical properties of the materials.
BACKGROUND OF THE INVENTION
During welding operation, designed microstructure of the material changes in the
narrow heat affected zone (HAZ), because the transformation temperature exceeds in
this zone. The coarse grained heat affected zone (HAZ) where the grain growth is more
pronounced at peak temperature above 1200°C has the greatest potential for
weldability problems. Heat affected zone (HAZ) is the region in base metal which is
affected by the welding heat. The HAZ locates from the fusion zone to few millimetres
into the base metal. If the high hardness martensite forms in the HAZ, it can
potentially make the HAZ susceptible to hydrogen induced cracking. The formation of
martensite depends on cooling rate, composition, and thickness of the carbon steel
material to be welded. Cooling time is measured in terms of t8/5 the time taken for
cooling from the temperature of 800°C to 500°C.When the hardness of simulated
region measures less than 350Hv, then there is limited chance of cold cracking
tendency. The presence of martensite is believed to increase the hardness to very
higher value and increases the susceptibility of cold cracking. Similarly the phase
modification in heat affected zone affects the toughness property and which is the

ability to withstand the brittle fracture. In this invention, cooling time ts/5 is correlated
to hardness property and toughness property in the simulated heat affected zone at
particular peak temperature.
US patent 0082039 Al describes a method for optimizing weld performance. The
patent correlates the chemical composition of base metal material, composition of weld
metal material and the welding process conditions and how that interaction affects the
performance of the weld produced. Using the thermo mechanical simulation,
mechanical properties such as hardness and toughness property are determined for a
selected base metal, selected weld metal and selected process condition. This patent is
based on construction of continuous cooling transformation (CCT) diagram for base
metal and welds metal and then correlates the phases with hardness. And our invention
relates to improve the hydrogen cold cracking resistance and toughness of heat
affected zone in boiler material.
US patent No.5080732 describes a method of determining the relative toughness of
heat affected zone on steel. This invention is based on determining the relative
toughness of offshore platform grade steel. Crack tip opening displacement (CTOD) test
is one of the mostly used methods to measure HAZ toughness of steel. In order to
avoid this time consuming procedure, the invention measures the relative toughness
after two thermal cycle simulation using the thermal cycle simulator. The intention of
our invention is to establish the safe welding window through which the cold cracking
resistance and toughness are improved.

SUMMARY OF THE INVENTION
Accordingly, there is provided a method for improving toughness and cold cracking
resistance at heat affected zone (HAZ) on weld products by correlating the
microstructure and mechanical properties of the materials. It is known that during
welding, the critical parameters such as preheat temperature, heat input, thickness of
the material, chemical composition of steel, affect the cooling rate. On the other hand,
toughness and hardness at HAZ region depends on microstructural phase formation and
this microstructural phase formation is controlled by controlling the cooling rate.
Accordingly, an experiment has been conducted on a HAZ in a thermal cycle simulator,
for a range of cooling time T8/5 wherein T8/5 is the time taken for cooling from the
temperature of 800°C to 500°C and evaluated using the known Rykalin 2D method for
a set of welding parameters including the preheat temperature, welding heat input and
physical constant of material such a density, specific heat and thermal conductivity.
Hardness and toughness properties are measured for the range of cooling time T8/5 and
correlated to the phenomenon of microstructural phase formation. The presence of
higher amount of martensite is believed to raise the cold cracking tendency and
deteriorate the toughness. When the hardness of an experimented region measures
less than 350Hv, then there would be limited chance of cold cracking tendency. And
when the toughness measures above 27 Joules at room temperature, then the material
is devoid of brittle fracture. The welding process conditions i.e. preheat temperature
and heat input are suitably raised to avoid to the high hardness above 350Hv and low
toughness below 27J. Using the correlation among welding process conditions including
microstructural phases, hardness and toughness, a safe welding window is generated.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l illustrates a single thermal cycle applied in coarse grain heat affected zone
simulation for different cooling time t8/5 of 5, 10,15,20,50, and 100 seconds
Fig.2 illustrates tested Vicker's hardness values in simulated CGHAZ at different cooling
time t8/5
Fig.3 illustrates a Charpy V-notch toughness (unit: Joules) in simulated CGHAZ as a
function of different cooling time t8/5
Fig.4 illustrates microstructural phase formation and a,b,c,d,e and f corresponds to the
cooling t8/5 of 5, 10, 15, 20, 50 and 100 seconds. (M- Martensite, AC- Aligned carbide
FN- Non-aligned carbide, GbF- Grain boundary ferrite, P- Pearlite, FC - ferrite carbide
aggregate)
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Normally in the prior art, the carbon steel seamless pipes of grade C as per standard
SA106 (ASME section II Part-A) are used as header pipes in thermal power stations.
The material has minimum yield strength of 40000psi (275Mpa) and minimum tensile
strength of 70000 psi (485Mpa). The typical composition of material is C: 0.18,
Si:0.226, Mn:0.948, P:0.022, S:0.023, Cr:0.03, Mo:0.03, Ni: 0.02, V:0.02, Al:0.023 and
Cu:0.05.

Generally, the main process conditions in welding are heat input and preheating
temperature. The weld heat input is the measure of the amount of energy deposited in
a weld per unit of weld length. Heat input of the welding process is measured by
calculating HI = n x {(I X V)/ S}, where S is travel speed of welding, I is welding
current in Amps, V is the voltage during the welding (in volts) and rj is the efficiency of
the heat transferred. The preheat temperature is the temperature to which the base
metal is heated before welding to avoid hydrogen or cold cracking. Cooling rate
depends on chemical composition of base material, thickness of material to be welded
and other process condition such as preheating temperature and heat input. The
cooling time ts/5 is computed using Rykalin 2D model from the equation given below

Where Q is heat input J/mm, p is density in gm/cc, C is specific heat capacity J/g-C, K is
thermal conductivity J/cm-s-C, d is plate thickness, de is equivalent plate thickness, T0
is preheat temperature, °C, Ti is temperature taken as 800°C and T2istemperature
taken as 500°C. The density of material is taken as 7.85g/cc and thermal conductivity is
taken as 0.502 J/cm-s-C and specific heat taken as 0.46 J/g-C. Heat input are varied as
1.4, 2.0, 2.45, 2.82, 4.4 and 6.32kJ/mm and cooling time t8/5 is computed from the
equation mention and corresponds to t8/5 of 5,10,15, 20, 50 and 100 seconds
respectively.
According to the invention, the steel sample is clamped between two water-cooled
copper grips and heated by resistance heating method in Gleeble thermal cycle
simulator. A single weld thermal cycle is simulated by rapidly heating to 1350°C
corresponding to CGHAZ. Generally, second and further thermal cycles do not decrease

the HAZ toughness of the steel and are not taken into consideration in this invention.
The distance between the copper grips is varied to obtain the required cooling rate. As
the spacing is too wide, the sample does not cool fast enough. A spacing between the
Copper grips is kept for fast cooling rate (say t8/5 of 5 and 10s) and the gap is
increased by 50% of the original, for slower cooling rates (t8/5 of 15,20,50 and 100s).
R-type (Platinum-Platinum 13%Rh) thermocouple is welded to the middle of the sample
using a thermocouple welder R-type thermocouple is chosen as simulation temperature
is 1350°C. The temperature of each sample is measured at least 100 times per second
(sampling rate of 100Hz).
Hardness and toughness properties are measured from simulated specimens of
different cooling time. Hardness test indicates the materials ability to withstand
indentation or plastic deformation and can be determined by calculating the Hv =
(1.854xP)/nD2, where P is the load and D is average diagonal of the indentation. The
corresponding unit of Vickers hardness is Kgf /mm2.The toughness test measures the
material ability to withstand fracture in rapid loading condition and measured by
Charpy-V notch impact test and the unit of measurement is joules.
There is a phase transformation from the original ferrite and pearlitic structure to other
forms like Martensite, Aligned carbide, Non-aligned carbide, Grain boundary ferrite,
Pearlite, or ferrite carbide aggregate depends on the cooling time t8/5. The phases
directly link with high hardness and poor toughness property in the simulated region. In
the real welding operation, the preheating/ heat input is suitably modified to obtain the
improvement in toughness and cold cracking resistance.

4. The method as claimed in claim 1, wherein the microstructural phase such as
martensite-M, ferrite with aligned carbide-AC, Non-aligned ferrite-NF,
Pearlite-P, ferrite carbide aggregate-FC and grain boundary ferrite-GbF of the
heat affected zone under varied cooling rage, is correlated to toughness
property of the welded sample.
5. The method as claimed in claim 1, wherein the preheat temperature of heat
input is raised to avoid hydrogen induced cracking on the sample.

ABSTRACT

The present invention relates to a method for improving toughness and cold cracking
resistance at heat affected zone (HAZ) on weld products by correlating the
microstructure and mechanical properties of the materials, comprising the steps of
simulating thermal cycles by heating and cooling the sample adapting a thermocouple
welded at the centre of the sample; adjusting a distance between two copper wedge
shaped grips to obtain a desired cooling time for the heated sample; and machining
the simulated sample for testing impact toughness at room temperature.