[ Power Generation ] gradients , they are commonly made from high-strength , good ductility materials that are heat and corrosion resistant . Martensitic stainless AISI 422 is an ideal choice . AISI 422 is a hardenable martensitic stainless steel designed for use at temperatures of up to 645 ° C . It has good resistance to scaling and oxidation . AISI 403 is also being used for the same purpose . The use of medium carbon Cr-Mo steel for steam turbine blades has been observed in a few exceptional instances although it does not meet purpose . Other types of martensitic stainless steel such as 403 and 410 posses high corrosion resistance yet are only considered for the manufacture of steam turbine buckets , valves and bucket covers , low pressure blades , pump parts , compressor blades , nuclear reactor control , gas turbine compressor , bolts and other components , etc . In today ’ s marketplace , the selection of materials for the various components for centrifugal compressors and steam turbines is very competitive and an important factor in the overall cost and delivery of the product . Material selection for major components - shafts , impellers , blading , bolting , seals , etc - needs to take corrosion , erosion and fouling into account to prolong turbine lifetimes . Note that components such as rotors and blades have long lead times .
[ Power Generation ] gradients , they are commonly made from high-strength , good ductility materials that are heat and corrosion resistant . Martensitic stainless AISI 422 is an ideal choice . AISI 422 is a hardenable martensitic stainless steel designed for use at temperatures of up to 645 ° C . It has good resistance to scaling and oxidation . AISI 403 is also being used for the same purpose . The use of medium carbon Cr-Mo steel for steam turbine blades has been observed in a few exceptional instances although it does not meet purpose . Other types of martensitic stainless steel such as 403 and 410 posses high corrosion resistance yet are only considered for the manufacture of steam turbine buckets , valves and bucket covers , low pressure blades , pump parts , compressor blades , nuclear reactor control , gas turbine compressor , bolts and other components , etc . In today ’ s marketplace , the selection of materials for the various components for centrifugal compressors and steam turbines is very competitive and an important factor in the overall cost and delivery of the product . Material selection for major components - shafts , impellers , blading , bolting , seals , etc - needs to take corrosion , erosion and fouling into account to prolong turbine lifetimes . Note that components such as rotors and blades have long lead times .
AISI 422 Chemical composition (%) Chromium 11-13 Nickel 0.5-1.0 Molybdenum 0.75-1.25 Manganese Silicon
1.0 max
0.75 max Nitrogen 0.04-0.15 Tungsten 0.75-1.25 Niobium and / or tantalum 0.02-0.3 Vanadium 0.15-0.30 Carbon 0.20-0.25 Sulphur and Phosphorous 0.025 Physical & mechanical properties Density
7.75 gm / cc
Tensile strength |
965 MPa |
|
Yield Strength at 0.02
Offset
|
793 MPa |
Elongation at 4D |
13 % |
Reduction in area 25 % Hardness Rockwell B
95 max
Turbine blades - forging AISI 422 grade is melted in an electric arc furnace and refined using argon oxygen decarburization ( AOD ). Ingots or concast blooms are then forged into blades as stock at a temperature of 1040-1175 ° C . Before heating the stock to forging temperature , pre-heating may be given at 650-760 ° C . After forging , parts are slow-cooled to room temperature followed by annealing and tempering . If hardening is required , a temperature of 1040 ° C is recommended followed by oil quenching . Parts with complex shapes can be mar-quenched ( quenching at a temperature right above the martensite starting temperature ) at 340 ° C . Tempering should be started after the part has fully cooled to room temperature . Steam turbine blades made of AISI 422 display good corrosion , scaling and oxidation resistance with excellent creep-rupture properties at temperatures up to 649 ° C , as well as highest electrical conductivity and high strength at high temperature .
Final shaping Turbine blade machining is a precise and intricate manufacturing process that plays a crucial role in the production of high-performance turbines used in various industries , including aerospace , power generation , and automotive . Machining of blades requires advanced techniques , state-of-the-art equipment , and skilled craftsmanship to ensure the highest quality and efficiency . As per the design requirements , shaping and finishing is performed to optimizing intricate geometry so as to enable the blade to withstand extreme temperatures , pressures , and rotational speeds . The demand for precise blade machining is gradually increasing in meeting the world ’ s energy needs and driving innovation forward with respect to cost , performance and environment issues . CNC milling along with CAD / CAM technologies are being used in almost all the industries in turbine blade manufacturing sector to improve productivity whilst achieving highly precise machining .
Turbine blades - investment casting Investment casting enables a smooth surface to be obtained , eliminating the need for secondary surface finishing and machining operations for certain
Blades in a steam turbine are subjected to high centrifugal forces and temperature gradients .
applications . Investment casting is a well-known method for stainless steel parts production . It can create complex , near-net shape products with excellent dimensional tolerances , even without secondary machining . However , machining will be required if closer tolerances and / or surface finishes are specified . Occasionally , customers try to compare total cost and performance between investment casting and forging before making procurement decisions for turbine blades . Both processes are effective metal-forming techniques , with advantages and disadvantages at opposite ends of the spectrum . The factor that usually pushes most customers towards investment casting is the limitation on product complexity that forging imposes . While some simple parts with limited surface finish and tolerance can be very economically forged in large quantities , nearly any complex geometry or special tolerances make investment casting the better choice . Finally , also note that 3D printing is receiving a lot of attention . There are now several developments using 3D printing of these blades , with companies such as GE , Siemens etc involved in extensive trials .
Conclusion The blades of a steam turbine play a critical role in converting the energy of high-pressure steam into rotational mechanical energy . Forged finished martensitic stainless AISI 422 blades are advantageous for applications requiring high strength , corrosion and heat resistant , fatigue resistance , and efficient material usage . However , due to tooling and die design limitations , it may not be suitable for highly intricate shapes where cast turbine blades satisfy cost and supply lead times .
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