Steam Turbine Inner Casing Cladding for Water Droplet Erosion
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Solution Overview
Problem
Low-pressure steam turbines' inner casings made of nodular cast iron suffer from water droplet erosion, particularly at the last stage, where condensed steam impacts and erodes the material, leading to susceptibility to abrasion and erosion due to lack of sufficient work hardening under steam turbine conditions.
Innovation Solution
A cladding of work hardening austenitic steel with specific composition (0.05wt%-0.15wt% C, 18wt%-21 wt% Cr, 8%-10% Ni, 6.5wt%-7.5wt% Mn, and remainder Fe) is applied to the cast iron inner casing components, which work hardens upon water droplet impact, forming a hardened surface resistant to erosion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If nodular cast iron is used for the inner casing, then the base material provides good casting properties and cost-effectiveness, but the material is susceptible to water droplet erosion and abrasion
Solution Approach 1:
The patent applies a composite material solution by cladding the cast iron base with austenitic steel that possesses work-hardening capabilities. This creates a multi-layer structure where the cast iron provides structural integrity and casting advantages, while the austenitic steel cladding provides erosion and abrasion resistance through work hardening during operation.
Solution Approach 2:
The patent changes the material parameters by selecting specific austenitic steel compositions (containing 16-25% Cr, 6-13% Ni, 2-4% Mo) that enable work hardening under impact conditions. The material parameters are optimized to achieve hardness values of 500-600 HB after work hardening, transforming the surface properties to resist erosion while maintaining the base material's casting advantages.
2Reliability
If work hardening austenitic steel cladding is applied, then erosion resistance is improved through work hardening, but the manufacturing complexity increases
Solution Approach 1:
The patent applies preliminary action by pre-cladding the inner casing with austenitic steel before the component enters service. The work hardening process occurs automatically during normal operation when water droplets impact the cladding, eliminating the need for separate post-manufacturing hardening steps and reducing overall manufacturing complexity.
Solution Approach 2:
The cladding material performs self-service by automatically work hardening itself during normal turbine operation. The impact from water droplets and steam flow naturally increases the hardness of the cladding surface in-situ, without requiring external intervention or additional processing steps, thereby simplifying the manufacturing process.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The cladding effectively forms a hardened surface that resists erosion, providing a durable solution to water droplet-induced damage in steam turbine inner casings, enhancing their operational lifespan.
Implementation Method 1
In service the cladding may work harden as a result of water droplet impact to form a hardened surface that is resistant to erosion
Data Source
Figure 1~2
AI summary
The invention relates to an inner casing component (10) configured to form part of a steam flow path (22) of a last stage (20) of an axial flow steam turbine, the steam turbine inner casing component (10) having a base (12) made of nodular cast iron and a cladding (14), on the base (12), in a region exposed to the steam flow path (22), consisting of manganese austenitic steel.