FIELD OF INVENTION
The present invention relates to a method for optimizing hot workability of Nickel base alloy with respect to hot working temperature and strain rate by conducting hot compression test.
BACKGROUND OF THE INVENTION
Hot working of materials is an art known to mankind for several centuries and has been continuously evolving to newer dimensions in the recent past. There are several manufacturing methods covered under the class of metal working or metal forming methods. These include forging, rolling, extrusion, wire drawing, deep drawing etc. These metal forming methods can be used for producing both near net shape as well as final shape of components. The typical shapes produced using such forming methods include extrusion of tubes, forming of plates to pipes and plate and sheet production from rolling of billets. These forming methods can be done as either cold forming or hot forming. If the temperature of forming is kept below the recrystallization temperature of the material, then it is called as cold forming and if the same is kept above recrystallization temperature, then it is called as hot forming. Since at elevated

temperatures, the material strength levels significantly drop, hot forming methods are preferred for a wide range of materials and products. In order to go about with a hot forming method, the flow stress required for producing a required level of straining in them materials at a defined strain rate and at the required processing temperature are to be determined beforehand. This can be decided only after conducting exhaustive experimentation with the actual components.
Pipes bends involved in thermal power plant are required to be manufactured by hot forming method like incremental hot bending technique to restore the creep properties of materials. In hot working process, the material are heated to temperature of above 0.5Tm in Kelvin scale (melting point of material). The deformed grains are replaced by new set of un-deformed grains at particular temperature called as dynamic recrystallization (DRX) temperature. The recrystallization temperature is mainly depend on amount of deformation/ strain and rate of deformation i.e. strain rate. Some Patent application have been filed relating to the mentioned subject matter and are illustrated as below for better understanding of the present invention.

PRIOR ART SEARCH
US4614550 A discloses a thermo-mechanical treatment of superalloys enabling simultaneously the production of a structure which is fine and homogeneous, with work hardened grains, a reduction in the stresses resulting from cooling and the absence of parasitic phase, characterized by an isothermal aging of predetermined duration after deformation in the final shaping sequence. Whereas the present invention is focused on the finding out the optimum hot workability parameters for a required degree of strain rate at a required temperature so as to achieve completely recrystallized grains in the structure for a nickel base alloy material.
US 5879818 discloses the method of producing a nickel alloy which possesses excellent workability and corrosion resistance characteristics. This invention is based on optimizing the chemical element present in alloy to achieve these properties. In our invention the optimum hot working parameters leading to dynamic recrystallisation is revealed.
CN 103424315 A discloses an experimental facility for the measurement of the hot forming limit of a laser tailor-welded blank. The experimental facility is characterized by comprising an upper mould base and a lower mould base; a spherical punch placed in the heating and temperature-controlling bell jar and a

setup for measuring high temperature strains. Whereas our invention is different in the sense that it discloses the method of determining the optimized hot workability parameters for the nickel base alloy material under a given combination of temperature and strain rate that would enable the production of dynamically recrystallized structure in the material.
US 6193823 B1 discloses a method of fabricating an article from an ingot of a nickel-iron-base alloy having a controlled chemistry in order to produce a microstructure with an array of intragranular delta-phase precipitates within the ingot and deforming the ingot having the array of delta-phase precipitates therein at a temperature below a delta-phase solvus temperature of the alloy. But our invention relates to a method of producing a dynamically recrystallized grains in nickel base alloy material by way of optimizing the flow stresses at a given temperature and strain rate. In the present invention, a method of determining the optimum hot workability parameters on a nickel base alloy for a required level of strain and at selecting the temperature and strain rate has been disclosed.

OBJECTS OF THE INVENTION
The object of the present invention is to focus on finding out the optimum hot workability parameters for a required degree of strain rate at a required temperature.
Another object of the present invention is to disclose optimum hot working parameters leading to dynamic recrystallisation.
Another object of the present invention is to produce dynamically recrystallized grains in nickel base alloy material by way of optimizing the flow stresses at a given temperature and strain rate.
SUMMARY OF THE INVENTION
This invention relates to a a method for optimising the hot workability with respect to a given combination of temperature, and strain rate and total strain in nickel base alloy using thermo mechanical simulator. Wherein the phenomena of dynamic recrystallisation (DRX) was observed through metallographic analysis after the completion of hot compression test, comprising. Simulating to thermal cycles by heating and cooling the sample adapting R-type thermocouple welded

at the centre of the sample. Isothermal condition applied in 10mm diameter cylindrical specimen using tungsten carbide type anvil. A method for optimising the hot workability parameter with respect to hot working temperature, strain rate and total strain in Nickel base alloy is developed for this nickel base alloy. The ideal combination of hot working temperature, strain rate for the particular strain magnitude which will result in dynamic recrystallisation phenomena for nickel base alloy has been disclosed.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be explained with reference to the accompanying
drawings where
Fig 1. illustrates the schematic flow chart narrating the inventive steps
Table 1. Provides various test conditions involved in the invention
Fig 2. Illustrates true stress-true strain curves of hot compression test at
different temperature of 1000, 1050, 1100, 1150 and 1200°C. Encircled portion indicates occurrence of dynamic recrystallisation.
Fig 3. Illustrates optical micrograph of hot compressed specimen at
different temperature of 1000, 1050, 1100, 1150 and 1200°C at the

strain rate of 0.1s-1. The microstructure of 1150 and 1200°C clearly depicts the recrystallized new grains.
Fig 4. illustrates optical micrograph of hot compressed specimen at
different temperature of 1000, 1050, 1100, 1150 and 1200°C at the strain rate of 1 s-1. The microstructure of 1150 and 1200°C clearly depicts the recrystallized new grains.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION
This invention relates to the field of hot workability in nickel base alloy. In particular, this invention relates to hot working nickel-base alloys having 23.7% Cr, 9.5%Mo, 11.1% Co, 1.37%Al, 0.38%Ti, 0.068%C, 0.028%V, 0.125%Nb, 0.088%W, 1.735Fe and rest Nickel. A method for optimising the hot workability parameter with respect to hot working temperature, strain rate and total strain in Nickel base alloy is developed for this nickel base alloy. The optimisation was based on hot compression test using Gleeble thermo mechanical simulator. The cylindrical specimen of size 10mm diameter and 12mm length is prepared from Nickel base alloy tube material. Hot compression test is carried out at temperature of 1000, 1050, 1100, 1150 and 1200°C using Gleeble thermo

mechanical simulator. At each test temperature, sample is compressed to true axial strain of -1.09 (i.e. 12mm length sample is compressed to 4mm) with strain rate of 0.01, 0.1,1 and 10s-1. The axial strain is calculated as In (h/h0), where h0 and h are initial and final height of cylinder. It can be observed that flow lines (i.e. carbide particles) are visible in the specimens hot worked at 1000 and 1050°C. The dynamic recrystallization (DRX) begins where new grain starts to nucleate is confirmed in the micrograph.
The schematic flow chart given in fig.1 depicts different steps to be followed and
various test conditions involved in the invention is given in table 1. In the true
stress –true strain plot generated at different temperatures, the portion of curve
corresponding minor serration indicates the occurrence of dynamic
recrystallisation. This effect is more pronounced at temperature of 1150°C and above at the strain rate of 0.01- 1 per sec as shown in encircled regions in fig 2. The corresponding microstructure indicating the phenomena of dynamic recrystallisation (DRX) are shown in fig.3 and fig.4.

WE CLAIM
1. A method for optimising the hot workability with respect to a given
combination of temperature, and strain rate and total strain in nickel base
alloy using thermo mechanical simulator wherein
The phenomena of dynamic recrystallisation (DRX) was observed through metallographic analysis after the completion of hot compression test, comprising the steps of:-a) Simulating to thermal cycles by heating and cooling the sample adapting
R-type thermocouple welded at the centre of the sample; b) Isothermal condition applied in 10mm diameter cylindrical specimen using
tungsten carbide type anvil.
2. The method as claimed in claim 1, wherein the nickel base alloy comprises 23.7% Cr, 9.5%Mo, 11.1% Co, 1.37%Al, 0.38%Ti, 0.068%C, 0.028%V, 0.125%Nb, 0.088%W, 1.735Fe.
3. The method as claimed in claim 1, wherein optimum workability parameters is ascertained for a required degree of strain rate at a required temperature.

4. The method as claimed in claim 1, wherein the method is performed based on metallographic analysis for occurrence of dynamic recrystallisation (DRX).