Steam Turbine Casing Manufacturing Process and Machining Technology

The steam turbine casing, often referred to as the cylinder or chamber, is the core component of a steam turbine, and its role is far more complex and demanding than that of a hydro turbine casing. It must withstand not only enormous internal pressure but also the core challenge of extreme high temperatures.

The primary considerations for selecting materials for steam turbine casings are high-temperature strength, creep resistance, oxidation resistance, and thermal fatigue resistance. Cast iron and ordinary carbon steel are completely inadequate, and specialized high-temperature alloy cast steel must be used.

The manufacturing of steam turbine casings is a prime example of high-end heavy equipment manufacturing, and the process is extremely complex:

Casting: Precision sand casting is performed in large foundries to ensure the casting is dense and free of internal defects. This is the most critical and challenging step.

Heat Treatment: Including annealing, normalizing + tempering, etc., the purpose is to eliminate casting stress, refine grain size, obtain the required microstructure (usually tempered sorbite), and mechanical properties.

Rough Machining: Removing most of the machining allowance.

Non-Destructive Testing (NDT):

• Ultrasonic Testing (UT): Inspecting for internal casting defects (shrinkage cavities, slag inclusions, cracks).

• Magnetic Particle Testing (MT) or Penetrant Testing (PT): Inspecting for surface and near-surface defects.

• Radiographic Testing (RT): Inspecting for internal defects in critical areas.

Thermal Stabilization Treatment (Stress Relief Annealing): Performed again after rough machining to eliminate machining stress and stabilize dimensions.

Finish Machining:

• Large CNC Boring and Milling Machines / Vertical Lathes: Machining critical areas such as the split plane, bearing seat holes, and steam seal grooves. Ensuring the flatness and sealing performance of the split plane and the concentricity of each hole is a core requirement.

• Drilling: Drilling various connecting bolt holes and instrument holes.

Hydrostatic Testing: Overpressure testing the finished cylinder to verify its pressure-bearing strength and sealing performance.