FIELD OF THE INVENTION:
The invention relates to a realistic determination of lower critical transformation
temperature in grade 91 weldments based on the experimental physical simulation
conducted in thermo mechanical simulator, in particular to estimate the maximum
allowable post weld heat treatment temperature without causing severe changes in the
final microstructure of tempered martensite.
BACKGROUND OF THE INVENTION AND PRIOR ART:
Low alloy steel of Grade 91 has lower thermal expansion coefficient, higher thermal
conductivity and superior resistance to thermal fatigue and are widely used as super
heater material in thermal power plants. Moreover, this steel has relatively good
corrosion and oxidation resistance and can serve at high temperature upto about
600°C. The steel obtains its good creep resistance and impact toughness from the
tempered martensitic microstructure and its fine dispersed of second phase particles
such as M23C6 carbide and the MX carbo-nitrides. When such steels are welded,
undesirable microstructures may be obtained in the weld metal and the Heat Affected
Zone(HAZ) in as welded condition and as a result of this, the mechanical properties may
be inferior. In order to get acceptable mechanical properties, all these weldments are
subjected to Post Weld Heat Treatment (PWHT) irrespective of the diameter and the

thickness at a temperature range of 730 to 780°C for stress relieving purpose and to
obtain optimal mechanical properties. Post Weld Heat Treatment (PWHT) is an
annealing process in which hardened or normalized steels are held at elevated
temperatures below where the austenite phase would normally be stable. The PWHT
process causes a reduction in hardness and increase in toughness due to a fine
precipitation of secondary phases. If the PWHT temperature is lower, the required
precipitation cannot occur and the prescribed impact toughness cannot be obtained.
Conversely, when the PWHT temperature is high, the creep-rupture performance
becomes degraded due to coarsening of the precipitates. PWHT within the prescribed
range of temperature ensures this fine tempered martensite matrix with uniformly
distributed carbides and carbonitride precipitates.
PWHT is generally carried out at the temperature of about 30-50°C lower from lower
critical temperature (called as Ac1). Ac1 is the temperature at which the austenitic phase
begins to form during the heating process. This Ac1temperature strongly depends on
the chemical composition of the alloy, its processing history and the heat treatment
parameters such as heating and cooling rate. There can be deleterious consequences if
PWHT temperature exceeds the Ac1 temperature of the steel. The microstructure will
transform to austenite above the Ac1 temperature, resulting in the formation of ferrite
or fresh martensite upon cooling. Also, heating above the Ac1 temperature will cause
the secondary particles to coarsen and lose their strengthening effect. All ferrite

stabilizing elements tend to raise the Ac1 temperature and all austenite stabilizers tend
to decrease the Ac1 temperature.
In this invention an experimental method for the determination of lower critical
transformation temperature in grade 91 weldments is disclosed.
PRIOR ART SEARCH
US patent 6676777 B2 refers the method of post weld heat treatment process for
carbon steel and low alloy steel comprising the steps of holding a welded joint steel
within austenite single phase temperature range for a given time and subsequently
cooling the joint by air-cooling or by slow cooling at a cooling rate lower than that of
the air-cooling.
US patent 7837810 B2describes method of post weld heat treatment for chemically
stabilized austenitic stainless steel. In particular the patent elaborates required a
specific ramp up heating rate, soaking time and cooling using ramp down rate to stress
relief the weld without any formation of deleterious precipitates.
US patent 4475963 A relates to a method for post weld heat treatment of a welded
portion of thick base metal, and more particularly to a method for appropriately judging
the point in time for terminating the PWHT in dissipating residual diffusible hydrogen in
a welded metal by after heating.

CN patent 102618713 relates to heat treatment method for welding SA335-P91/P92
steel and in particular refers a specific heating rate upto 500°C and then slight lower
heating rate above 500°C for the required PWHT temperature. Also it provides
information on heating width in relation to thickness.
OBJECTS OF THE INVENTION:
The object of the invention is to determine the lower critical transformation temperature
in grade 91 weldments through simulation process.
Further object of the invention is to develop a method for more accuracy of result than
conventional system.
SUMMARY OF THE INVENTION:
This invention relates to a realistic determination of lower critical transformation
temperature in grade 91 weldments based on the experimental physical simulation
conducted in thermo mechanical simulator. Determination of lower critical
transformation temperature helps to optimize the appropriate range of post weld heat
treatment (PWHT) temperature. It has been reported that higher (Mn + Ni) content
present in weld tends to decrease the Ac1 temperature. The ASTM A335 specification
limits the nickel content in P91 steel to maximum of 0.4% and Manganese to 0.3 –

0.6%. For filler metal, ASME section IIC SFA 5.5M specification limits Nickel to 0.8%
and Manganese to 1.2%. ASME Boiler and Pressure Vessel Code VIII specifies the
maximum allowable PWHT temperatures in P91 steel welds as 800°C if (Mn + Ni) is
below 1% and 790°C if (Mn + Ni) is between 1-1.5%. Although Nickel and Manganese
have strong influence on Ac1 temperature, other elements such as silicon, nitrogen,
chromium, molybdenum, carbon, vanadium and copper also have significant effect on
Ac1 temperature. Since the chemical composition of the P91 base material and the weld
metal are different, their response to PWHT process will also be different. For this
reason it is necessary to investigate Ac1 temperature of the weld metal and base metal
separately. Here the specimen extracted from weldment is taken and subjected to heat
treatment simulation in a thermo mechanical simulator with simultaneous recording of
dilation using dilatometer. The specimen was subjected to austenitising treatment
followed by stress relieving treatment at appropriate temperature close to
Ac1temperature at a heating and cooling rate similar to actual product. Plot of
dilatometer readings versus the temperature was generated and transformation
temperature of martensite, ferrite and austenite formation are noted. Based on the
phase transformation analysis derived from dilatometer plots appropriate PWHT
temperature is envisaged. The same microstructure is further confirmed again by
analysis using the optical metallography.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1 – Specimen prepared from weldment for conducting experiment
Figure 2 – Heating and cooling schedule of PWHT process
Figure 3 – Dilatometry plot of weld metal at particular PWHT temperature
Figure 4 - Dilatometry plot of weld metal at particular PWHT temperature
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT:
According to the invention, a method of determination of lower critical transformation
temperature in grade 91 weldments is disclosed. Cylindrical specimen with 10 mm
diameter (Figure 1) were extracted from welded pipe in weld transverse direction
having weld region at the middle portion. Similar specimens were also taken from P91
base metal for the comparative study. Experiments on phase changes during
austenitising and tempering were conducted through Gleeble 3500 thermo mechanical
simulator. Type K thermocouple was welded to the middle portion of the specimen as
shown. High resolution dilatometer with range of 5 mm placed on the cross-section of
cylindrical specimen was used to plot the dilatometric curves during austenitising and
PWHT treatment. All the critical phase transformation temperatures were evaluated
from plots of dilatometer using method of tangent. In a practical PWHT process,
maximum heating rate per unit inch thickness can reach upto 220°C/hour (i.e.
0.06°C/s), whereas the maximum cooling rate can be upto 280°C/hour (i.e. 0.07°C/s).
To simulate this real situation, the weld specimens were heated to the austenitising

temperature of 1050ºC at heating rate mentioned above and soaked for 30 minutes
(Figure 2). Then the specimen were subjected to PWHT process at the same
heating/cooling rate to specific temperature and soaked for 30 minutes. Plot of
dilatometer readings versus the temperature was generated (Figure 3 and 4) and
analysis of phase transformation is conducted.

WE CLAIM
1. A method for determination of lower critical transformation temperature in grade 91
weldments, characterized by dilatometry based heat treatment simulation, predicting
maximum allowable PWHT temperature for steel alloy material.
2. The method as claimed in claim 1, wherein austenitizing and post weld heat
treatment (PWHT) is conducted for a grade 91 cylindrical specimen material selected
from welded region, on a thermo mechanical simulator similar to actual fabrication.
3. The method as claimed in claim 2, wherein the specimen is heated at a temperature
of 1050 degree centigrade (Ac1) at a maximum heating rate upto 0.06 degree
centigrade/sec. per unit inch thickness.
4. The method as claimed in claim 3, wherein, the specimen is soaked for 30 minutes
at a temperature of 1050 degree Celsius (Ac1).
5. The method as claimed in claim 4, wherein, the maximum cooling rate being upto
0.07 degree Celsius/sec. per unit inch thickness.

6. The method as claimed in claim 1, wherein high resolution dilatometer with 5 mm
range is placed on the cylindrical cross section of the specimen to plot the
dilatometric curves during post weld heat treatment.
7. The method as claimed in claim 1, wherein Type K thermo couple is welded on the
middle portion of the specimen for recording of temperature.
8. The method as claimed in claims 6 & 7, wherein plot of dilation versus temperature
is generated for the weld material and compared with the base material in the
micrograph detecting phase changes with determination of lower transformation
temperature.
9. The method is repeated for specimens and base material to compare the results as
a part of simulation technique and appropriateness.