DESC:FORM –2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

MARTENSITIC STAINLESS STEEL TURBINE BLADE AND A METHOD OF MANUFACTURING THE SAME

Applicant: BHARAT FORGE LTD.
An Indian Company of
Mundhwa, Pune -411036,
Maharashtra, India.

THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH I TIS TO BE PERFORMED.

FIELD OF THE INVENTION
[001] The present invention relates to a turbine blade and a method of its manufacturing.
[002] Particularly, the present invention relates to a martensitic stainless steel turbine blade and a method of its manufacturing.

BACKGROUND
[003] Stainless steel is typically classified into 5 major types such as Austenitic, Martensitic, Ferritic, Duplex and Precipitation hardened stainless steel. According to the final properties requirement in the components, these grades are selected. Generally, stainless steel grades, because of its high corrosion resistance and good mechanical properties are utilized in various applications of industrial fields.
[004] Martensitic steel grade is basically Fe-Cr alloy with low carbon content. Martensitic steel is mostly used in the power generation sectors and compressor units, because of its high strength at 500-550°C temperature and corrosion resistance. 12% Cr grade with low carbon content (0.1%) is specifically used for manufacturing of turbine blades.
[005] Turbine blade is an important component used in the compressor. The turbine blades are classified into two types; a) Low pressure turbine and b) High pressure turbine - depending upon its application in the industries.
[006] Turbine blade is one of the complex parts due to its intricate shape and varying thickness across profile. Thus, its processing requires special care in heat treatment. Apart from this there is stringent requirement of mechanical and metallurgical properties to withstand at high temperature (around 500°C) as well as subzero temperature.
[007] Particularly, this type of steel is susceptible to embrittlement phenomenon. The embrittlement phenomenon occurred at 350 to 550°C temperature range, the detrimental elements are segregated at the prior austenite grain boundaries. It causes weakening of grain boundaries, which further leads to reduction in impact properties of the component at subzero temperature. It may not affect the room temperature tensile properties, but adversely affect fatigue properties and subzero as well as room temperature impact properties of turbine blade.
[008] Various attempts have been carried out in the past in order to enhance the mechanical properties and achieve good Fracture appearance transition temperature (FATT) in the turbine blade. The techniques disclosed in the prior art particularly involved altering the chemical composition of martensitic stainless steel.
[009] For instance, US7901523 discloses heat treatment process for hardening steel. A spacer is used to provide uniform heat to the surface of the blade. Further, flame hardening or Induction hardening is used to harden the surface of the blade.
[0010] US2014/0007981 discloses precipitation type of the martensitic stainless steel. In said patent a study for the improvement of mechanical properties of the blade viz. strength, toughness and corrosion resistance of the stainless steel is disclosed. Further, the effect of changing the chemical composition of the alloy and heat treatment temperature on the mechanical properties of the steel are disclosed.
[0011] US7931446 discloses increasing surface hardness of the turbine blade by diffusing Boron on the surface of the blade.
[0012] US8632313 discloses turbine rotor blade with enhanced erosion resistance and reduced stress corrosion cracking sensitivity. Induction hardening is used to the portion of leading edge of the blade.
[0013] Though various methods have been practiced to improve the strength of the turbine blade, these methods are inefficient to achieve the desired mechanical properties, FATT and distortion in a specified range.
OBJECT OF THE INVENTION
[0014] Some of the objects of the present disclosure which at least one embodiment herein satisfies are as follows:
[0015] It is an object of the present invention is to provide a martensitic stainless steel turbine blade having improved mechanical properties with minimal distortion.

[0016] It is another object of the present invention is to provide a method for manufacturing a martensitic stainless steel turbine blade having improved mechanical properties with minimal distortion.

[0017] It is still another object of the present invention to provide a method which can eliminate embrittlement phenomenon in tempering stage.

[0018] It is yet another object of the present invention to provide a hardening treatment method to obtain complete transformation of martensite.

[0019] It is a further object of the present invention to establish heat treatment cycle to achieve desired mechanical properties and FATT.
[0020] Other objects and advantages of the present disclosure will be more apparent from the following description which is not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
[0021] Figure 1 illustrates a method of manufacturing of martensitic stainless steel turbine blade in accordance with one embodiment of the present invention.

SUMMARY OF THE INVENTION
[0022] Accordingly, the present invention provides a method of manufacturing a martensitic stainless steel turbine blade, said method comprises forging a martensitic stainless steel billet into a turbine blade, wherein said method characterized in that it comprises a combination of three cooling cycles, said cooling cycles consist of a first cooling cycle, a second cooling cycle and a third cooling cycle, wherein said first cooling cycle comprises furnace cooling which is carried out post forging but before annealing and hardening, wherein said second cooling cycle comprises atmospheric air cooling followed by water spray cooling and is carried out post hardening but before tempering and said third cooling cycle comprises forced air cooling of said turbine blade post tempering.
[0023] The present invention provides a method of manufacturing a martensitic stainless steel turbine blade, said method comprises the following steps:
a. forging a martensitic stainless steel billet into a turbine blade;
b. furnace cooling of said turbine blade in a furnace having a temperature in the range of about 500 to about 600°C;
c. annealing said turbine blade at a temperature ranging from 650 to 9000C;
d. hardening said turbine blade at a temperature ranging from 1000 to 10500C;
e. atmospheric air cooling of said turbine blade at around room temperature followed by water spray cooling;
f. tempering said turbine blade at a temperature ranging from 600 to 7000C; and
g. forced air cooling of said turbine blade after tempering to obtain the final turbine blade.
[0024] In accordance with another aspect of the present invention there is provided a forged and heat treated martensitic stainless steel turbine blade with FATT value in the range of -40 to -50°C. The martensitic stainless steel turbine blade of the present invention is further characterized by ultimate tensile strength (UTS) in the range of 920 to 995 MPa throughout the length of turbine blade; and proof strength (0.2%) in the range of 800 to 875 MPa.

DESCRIPTION OF THE INVENTION
[0025] Martensitic stainless steel is more sensitive to heat treatment operation as compared with other alloy steels. It is one of the reasons, that the rejection rate during heat treatment is comparatively high. Therefore, it is important to design a proper heat treatment cycle so as to achieve optimum combination of required properties at minimum cost of operations.
[0026] Most important property is the Fracture Appearance Transition Temperature (FATT), due to its operative behavior at various climatic conditions. Also this grade is susceptible to embrittlement phenomenon due to presence of trace elements such as P, Sn and the like, therefore special care is required during tempering operation.
[0027] Thermal conductivity of martensitic stainless steel is lower as compare to carbon steels, due to which there is high thermal gradient in the component which further leads to dimensional variation in the component. To overcome this issue, a step of heating and soaking is introduced in such a way that uniformity of temperature is maintained and thermal gradient is minimized.
[0028] In conventional heat treatment process, after hardening process oil quenching is done for complete transformation of Martensite followed by tempering operation which is further followed by atmospheric air cooling. Due to use of oil quenching operation, distortion in the turbine blade goes beyond the specification i.e. more than 3 to 4 mm. To correct this distortion, straightening operation need to be used. After the straightening operation, extra stress reliving cycle is to be performed for reliving residual stresses in the turbine blade which was generated during straightening operation. Achieved mechanical and metallurgical properties are within required specification except FATT which is -18° C. This FATT value is lower than required FATT value which is in the range of -40 to -50° C.
[0029] In present invention, the chemical composition of martensitic stainless steel contains 11-13% Cr; 2-3 % Ni and 2% Mo which are the major alloying element. Turbine blade after forging is transferred to the furnace, which is maintained at 500-600°C followed by furnace cooling to room temperature. This operation is performed for the minimization of thermal gradient in the turbine blade.
[0030] In the present invention, atmospheric air cooling and water spray cooling after hardening treatment is proposed for complete transformation of martensite and simultaneous reduction in distortion as well as thermal gradient. Afterwards, it is followed by tempering operation. After tempering operation, turbine blade is forced air cooled to eliminate embrittlement phenomenon.
[0031] According to the present invention, introducing atmospheric air cooling and water spray cooling after hardening treatment causes uniform heat removal from the surface of the turbine blade wherein heat is transferred from turbine blade thereby reducing the thermal gradient within component. It is found that atmospheric air hardening provides distortion within specification i.e. less than 1.5 mm.
[0032] In accordance with one embodiment of the present invention there is provided a method of manufacturing a martensitic stainless steel turbine blade.
[0033] The method comprises forging a martensitic stainless steel billet into a turbine blade, wherein said method characterized in that it comprises a combination of three cooling cycles, said cooling cycles consist of a first cooling cycle, a second cooling cycle and a third cooling cycle, wherein said first cooling cycle comprises furnace cooling which is carried out post forging but before annealing and hardening, wherein said second cooling cycle comprises atmospheric air cooling and is carried out post hardening but before tempering and said third cooling cycle comprises forced air cooling of said turbine blade post tempering.
[0034] In one embodiment, the method involves the following steps:
In the first step, a martensitic stainless steel billet is forged into a turbine blade. After forging, the turbine blade is subjected to furnace cooling to room temperature. In one embodiment, furnace cooling is employed i.e. furnace cooling of the turbine blade is carried out in a furnace having a temperature in the range of about 500 to about 600°C. In the next step, the turbine blade is annealed at a temperature ranging from 650 to 9000C. The soaking period for annealing is about 150 to about 180 minutes and furnace cooling at around 5000C followed by atmospheric air cool to room temperature. After annealing, the turbine blade is hardened at a temperature ranging from 1000 to 10500C. In the hardening process, stepwise heating of turbine blade is carried out for maintaining temperature uniformity across the variable cross section of turbine blade. In the stepwise heating, initially the turbine blade is heated to 7000C. The soaking time for this step is about 90 to about 120 minutes. Further, the turbine blade is heated from 1000 to 10500C. The soaking time for this step is about 90 to about 120 minutes. The turbine blade is then subjected to atmospheric air cooling to attain room temperature followed by water spray cooling.
In one embodiment, the water spray cooling step is carried out for a time period ranging from 10 to 20 minutes.
The cooled turbine blade is tempered at a temperature ranging from 600-7000C. Finally, the turbine blade is forced air cooled after tempering to obtain the final turbine blade with desired characteristics. In one embodiment the forced air cooling is carried out for a time period ranging from 30 to 60 minutes.
[0035] The present method improves the fracture appearance transition temperature (FATT). Air cooling to room temperature after hardening treatment minimizes distortion. Spray cooling ensures uniform temperature and complete transformation of Martensite in the turbine blade as the Martensite finish temperature of this steel grade is around room temperature. Tempering treatment followed by forced air cooling reduces temper embrittlement phenomenon and subsequently it helps in increasing impact as well as FATT property of turbine blade. FATT value of the turbine blade manufactured in accordance with the present method has been found in the range of -40 to -50°C with good combination of mechanical properties. Distortion observed within the specified range i.e. less than 1.5 mm in turbine blade which obviated the need for straightening and stress relieving operation.

[0036] In accordance with another aspect of the present invention there is provided a martensitic stainless steel turbine blade obtained by the method described herein above.
[0037] The martensitic stainless steel turbine blade of the present invention is a forged and heat treated blade with FATT value in the range of -40 to -50°C.
[0038] The martensitic stainless steel turbine blade of the present invention is further characterized by ultimate tensile strength (UTS) in the range of 920 to 995 MPa throughout the length of turbine blade; and proof strength (0.2%) in the range of 800 to 875 MPa.
[0039] In one embodiment, the turbine blade of the present invention is characterized by FATT value of about -45°C.

Technical Advance and Economic Significance:
[0040] The method of the present invention eliminates straightening and stress relieving operation.
[0041] The method of the present invention eliminates oil quenching.
[0042] The method of the present invention results into economic process.
[0043] The method of the present invention provides minimum distortion and enhanced FATT value.
[0044] The method of the present invention provides efficient manufacturing of complex shaped turbine blade with variable cross section with minimum distortion which is difficult to control otherwise.
[0045] Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
[0046] The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
[0047] Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
[0048] The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
[0049] While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.
,CLAIMS:We claim:
1. A method of manufacturing a martensitic stainless steel turbine blade, said method comprises forging a martensitic stainless steel billet into a turbine blade, wherein said method characterized in that it comprises a combination of three cooling cycles, said cooling cycles consist of a first cooling cycle, a second cooling cycle and a third cooling cycle, wherein said first cooling cycle comprises furnace cooling which is carried out post forging but before annealing and hardening, wherein said second cooling cycle comprises atmospheric air cooling followed by water spray cooling and is carried out post hardening but before tempering, and said third cooling cycle comprises forced air cooling of said turbine blade post tempering.

2. A method of manufacturing a martensitic stainless steel turbine blade, said method comprises the following steps:
- forging a martensitic stainless steel billet into a turbine blade;
- furnace cooling of said turbine blade to room temperature in a furnace having a temperature in the range of 500 to 600°C;
- annealing said turbine blade at a temperature ranging from 650 to 9000C;
- hardening said turbine blade at a temperature ranging from 1000 to 10500C;
- atmospheric air cooling of said turbine blade at around room temperature followed by water spray cooling;
- tempering said turbine blade at a temperature ranging from 600 to 7000C; and
- forced air cooling of said turbine blade after tempering to obtain the final turbine blade.

3. The method as claimed in claim 1 or 2, wherein said process characterized in that said method provide a turbine blade having FATT value of in the range of -40 to -50°C.

4. The method as claimed in claim 1 or 2, wherein the water spray cooling step is carried out for a time period ranging from10 to 20 minutes; and the forced air cooling is carried out for a time period ranging from 30 to 60 minutes.

5. The method as claimed in claim 1 or 2, wherein the hardening step comprises stepwise heating of said turbine blade for maintaining temperature uniformity across the variable cross section of turbine blade; said stepwise heating comprises heating said turbine blade to about 7000C for about 90 to about 120 minutes followed by heating said turbine blade from 1000 to 10500C for about 90 to about 120 minutes.

6. The method as claimed in claim 1 or 2, wherein the said martensitic stainless steel comprises 11-13% Cr; 2-3 % Ni and 2% Mo.
7. The method as claimed in claim 1 or 2, wherein said atmospheric air cooling after hardening treatment provides distortion less than 1.5 mm in said turbine blade.
8. A martensitic stainless steel turbine blade; said blade characterized in that said turbine blade is a forged and heat treated blade with FATT value in the range of -40 to -50°C.
9. The martensitic stainless steel turbine blade as claimed in claim 8 wherein said turbine blade is characterized by ultimate tensile strength (UTS) in the range of 920 to 995 MPa throughout the length of turbine blade; and proof strength (0.2%) in the range of 800 to 875 MPa.

10. The martensitic stainless steel turbine blade as claimed in claim 8 wherein said martensitic stainless steel comprises 11-13% Cr; 2-3 % Ni and 2% Mo.

Dated this April 04, 2017

Prashant Patankar
NOVOIP
Applicant’s Patent Agent (IN/PA-1833)