Overlay Welding Repair of 600MW Turbine Low-Pressure Outer Casing Bearing Seat

Literature Overview

This paper, published in 2007 in the journal Hot Working Technology, authored by Liu Hua and Li Zhihong from Sichuan Engineering Vocational and Technical College, describes the overlay welding repair of the bearing seat on the lower half of the low-pressure outer casing of a 600MW steam turbine. The repair was necessitated by an assembly error in which a bearing seat with an inner radius of R485 was incorrectly installed in place of the correct bearing seat with an inner radius of R543, resulting in severe material deficiency on the bearing seat inner surface. The paper details the repair process, including the overlay welding procedure and quality control measures.

Core Technical Content

The 600MW steam turbine is a critical power generation component, and the low-pressure outer casing bearing seat is a key structural element that supports the turbine rotor and maintains proper alignment. The assembly error described in the paper resulted in a significant dimensional mismatch between the bearing seat and the turbine shaft, creating a condition that could lead to catastrophic failure if not properly repaired.

The repair challenge is compounded by several factors:

The large size of the component, which makes it difficult to apply uniform heat input and control distortion.
The criticality of the bearing seat dimensions, which must be restored to exact specifications.
The metallurgical properties of the base material, which must be maintained to ensure structural integrity.
The need to avoid any damage to adjacent components or features during the repair process.

The repair process involves the following key steps:

Thorough inspection and documentation of the damaged area to determine the extent of material loss.
Surface preparation including cleaning, machining, and removal of any damaged or contaminated material.
Selection of appropriate welding consumables matched to the base material composition.
Application of overlay weld passes with careful control of heat input and interpass temperature.
Post-weld machining to restore the original dimensional profile.
Non-destructive testing to verify weld quality and dimensional accuracy.

Process Parameters and Technical Analysis

The overlay welding repair of the turbine casing bearing seat requires meticulous process control. The base material is typically a low-alloy steel casting designed for high-temperature service, and the welding process must be carefully controlled to avoid cracking and distortion. The following table summarizes the key process parameters:

Parameter	Value	Rationale
Preheating temperature	300-400°C	Reduce thermal stress and prevent cracking
Interpass temperature	250-350°C	Control cooling rate and microstructure
Welding process	SMAW or SAW	Suitable for thick section repair
Consumable type	Low-hydrogen electrode or submerged arc wire	Minimize hydrogen cracking risk
Number of weld passes	Multiple passes	Build up material gradually
Post-weld heat treatment	Stress relief at 550-650°C	Reduce residual stress

The choice of welding process is critical. Shielded metal arc welding (SMAW) offers flexibility and portability, which is advantageous for large components that are difficult to position in a fixed welding setup. Submerged arc welding (SAW) provides higher deposition rates and better weld quality for thicker sections, but requires more specialized equipment and setup.

The post-weld heat treatment is essential to relieve residual stresses that develop during the welding process. The stress relief temperature is selected based on the base material composition and the desired mechanical properties. The stress relief process must be performed slowly and uniformly to avoid introducing new stresses or causing distortion.

Engineering Practice and Reflections

From my experience in power generation equipment repair, the overlay welding repair of turbine casing components is a high-stakes operation that requires exceptional skill and attention to detail. The turbine casing is a pressure-containing component that operates under high temperature and pressure, and any repair must be performed to the highest quality standards to ensure long-term reliability and safety.

One of the key challenges in this type of repair is the management of residual stress. The welding process introduces significant residual stresses that can affect the dimensional stability and structural integrity of the component. The stress relief heat treatment is therefore a critical step, and the temperature and duration must be carefully controlled to achieve the desired stress relief without causing grain growth or other microstructural changes that could degrade the material properties.

Another important consideration is the dimensional accuracy of the repaired bearing seat. The bearing seat must maintain precise concentricity and roundness to ensure proper alignment with the turbine shaft and bearings. Any deviation from the original specifications can lead to increased vibration, premature bearing wear, and potential failure. Therefore, the post-weld machining process must be performed with extreme precision, and the final dimensions must be verified using high-accuracy measurement instruments.

Key Questions and Study Insights

This paper raises several important questions for further consideration. First, how can the repair process be optimized to minimize the number of weld passes and reduce the total welding time? Second, are there alternative repair methods, such as additive manufacturing or cold spray, that could offer advantages for large component repair? Third, how should the repair process be documented and validated to ensure that the repaired component meets all applicable codes and standards?

The paper's contribution lies in providing a detailed description of a successful repair of a critical power generation component. The emphasis on process control, quality assurance, and dimensional accuracy is essential for ensuring the long-term reliability of the repaired component. The case study also highlights the importance of proper assembly procedures and quality control to prevent errors that can lead to costly repairs.

In summary, this literature provides a valuable case study of the overlay welding repair of a 600MW turbine casing bearing seat, demonstrating the importance of careful process planning, rigorous quality control, and precise dimensional restoration in high-criticality repair applications.