Steam turbines used to generate electricity are at the heart of a modern society. This article highlights the key role played by AISI 422 martensitic steel in turbine production.
By Srikumar Chakraborty, ex ASP/SAIL, Freelance Consultant
Introduction
Steam turbines are rotary generator heat engines that produce electricity. They rely on a heat source such as gas, coal, nuclear or solar energy which is used to convert water into steam at extremely high temperatures; the steam then flows past the turbine’s blades, causing it to spin. The potential energy of the steam is thus turned into kinetic energy in the rotating turbine. This rotary motion is suited for driving electrical generators. Typically, turbines are connected to generators with an axle.
The blades of a steam turbine play a critical role in converting the energy of the high-pressure steam into rotational mechanical energy. There are two main types of blades in a steam turbine: stationary blades, also known as nozzles or diaphragms, and rotating blades, often referred to as buckets or rotor blades. About 90% of all electricity generation in the world is by use of steam turbines. The modern steam turbine was invented in 1884 by Sir Charles Parsons, whose first model was connected to a dynamo that generated 7.5 kW (10 hp) of electricity. The turbine is a common feature of all modern and also future thermal power plants.
Blade design
Steam turbine blades are fundamental components in power generation systems. They are responsible for extracting the maximum energy from the steam, driving the turbine to produce electricity. Blades are subjected to high centrifugal forces and temperature gradients. Material selection is therefore critical to ensure the blades can withstand the harsh operating conditions.
They are commonly made from high-strength, heat-resistant materials such as stainless steel or nickel-based alloys. The efficiency and reliability of a turbine depend on the proper design of the blades which is multi-disciplinary task and involves thermodynamic, aerodynamic, mechanical and material science disciplines with experts from all related fields contributing as a team towards component performance. Efficiency of the turbine depends on parameters such as the inlet and outlet angle of the blade, blade materials, and blade profile design.
The key aspects are blade design, blade material and manufacturing technologies. Manufacturing can involve precision forging and CNC machining to produce turbine blades with tight tolerances and an excellent surface finish. Rigorous quality control measures are required throughout the manufacturing process to ensure that every steam turbine blade meets or exceeds industry standards.
Material selection
The material chosen for turbine blade manufacture should not only meet the desired properties for the application, but should also be assessed in line with the expected service conditions. Other factors include sustainability, environmental impact and cost. Since turbine blades are subjected to high centrifugal forces and temperature 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 possess 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.
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 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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