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

Literature Overview

Liu Hua and Li Zhihong's paper, published in Hot Working Technology (2007, Vol. 36, No. 19, pp. 87–89), documents the overlay welding repair of the lower half bearing seat of the low-pressure outer casing of a D600BN5 steam turbine. The repair was necessitated by a manufacturing error: during assembly, a bearing seat with an inner radius of R485 was incorrectly installed instead of the correct bearing seat with an inner radius of R543, resulting in a severe material deficiency (excess machining allowance) on the inner bore surface. This case study is particularly instructive because it involves the repair of a large, thick-walled, safety-critical power generation component where dimensional accuracy and metallurgical integrity are non-negotiable.

Core Technical Content

The D600BN5 is a 600 MW subcritical once-through boiler steam turbine, and the low-pressure outer casing is one of the largest pressure-retaining components in the turbine. The bearing seat is a precision-machined bore that supports the turbine rotor bearing, and its inner diameter must conform to tight dimensional tolerances to ensure proper bearing fit and rotor alignment. The material deficiency caused by the wrong bearing seat installation created a gap of approximately 58 mm in radius (from R485 to R543), representing a substantial volume of material that needed to be rebuilt through overlay welding.

The base material of the low-pressure outer casing is typically a low-carbon steel or low-alloy steel (such as 20G or 12Cr1MoV), depending on the specific design and operating conditions. The repair required the addition of several millimeters of weld metal to restore the original bore diameter, with the final surface to be machined to the precise R543 dimension.

Repair Strategy and Process Parameters

The repair strategy involved a multi-stage approach:

Assessment and preparation: The extent of the material deficiency was quantified by dimensional measurement. The surface was cleaned and prepped for welding.
Welding consumable selection: A low-hydrogen, low-carbon steel electrode or wire was selected to match the base material composition and ensure weldability. The consumable was chosen to produce a weld metal with adequate ductility and low cracking susceptibility.
Preheating: Given the thick section and the large volume of weld metal required, preheating was essential. The preheat temperature was set to ensure a slow cooling rate and minimize residual stress.
Multi-pass overlay welding: The repair was executed in multiple passes, with each pass carefully controlled to manage heat input and dilution.
Post-weld machining: After the overlay welding was complete, the bore surface was machined to the final R543 dimension.
Inspection: Comprehensive NDT (non-destructive testing) was performed on the repair area.

Parameter	Value	Rationale
Base material	Low-carbon/low-alloy steel (e.g., 20G or 12Cr1MoV)	Typical for LP casing components
Welding consumable	Low-hydrogen electrode (e.g., E5015 or equivalent)	Matches base composition, low cracking tendency
Preheat temperature	200–300°C	Reduces cooling rate, minimizes HAZ hardness
Interpass temperature	≤ 250°C	Controls thermal cycle and grain growth
Welding current	200–280 A (SMAW)	Adequate deposition rate with controlled heat input
Number of passes	Multiple (depending on build-up height)	Ensures uniform microstructure and stress distribution
Post-weld stress relief	550–620°C for 2–4 hours (if permitted)	Reduces residual stress in thick section

Metallurgical Considerations and Defect Prevention

The primary metallurgical concern in this repair is the formation of a hard, brittle HAZ in the base material adjacent to the weld. Low-alloy steels containing chromium and molybdenum (such as 12Cr1MoV) are particularly susceptible to HAZ hardening and cracking due to the formation of hard martensitic structures during rapid cooling. The preheat temperature and interpass temperature are therefore critical parameters that must be carefully controlled.

The large volume of weld metal required for this repair also introduces the challenge of cumulative heat input. Each successive pass adds heat to the previously deposited weld metal, potentially causing grain coarsening and reduced mechanical properties in the earlier passes. To mitigate this, the welding sequence should be planned to distribute the heat evenly and to avoid excessive reheating of any single area.

Common defects to watch for in this type of repair include:

Hydrogen-induced cracking: Can occur if the consumable is not properly stored or if the preheat is insufficient. Low-hydrogen electrodes must be baked at 300–350°C for 2 hours before use.
Hot cracking: Can occur in the weld metal if the sulfur and phosphorus content is too high or if the travel speed is too slow.
Lack of fusion: Can occur at the root of the first pass if the surface is not properly prepared or if the welding parameters are inadequate.
Residual stress: The large volume of weld metal can produce significant residual stresses that may cause distortion or delayed cracking.

Engineering Practice and Safety Considerations

This repair was performed on a safety-critical component of a power generation facility. The bearing seat of a 600 MW turbine casing must withstand extreme mechanical loads, thermal cycling, and pressure differentials. Any repair must be executed to a standard that ensures the component will perform reliably throughout its remaining service life. This requires:

A thorough engineering assessment of the repair feasibility and risk.
Approval from the relevant regulatory authority and the turbine manufacturer.
Use of qualified welding procedures and certified welders.
Comprehensive NDT inspection including RT (radiographic testing) and MT (magnetic particle testing) on the repair area.
Mechanical property testing on witness coupons welded with the same parameters.
Documentation of all parameters, inspection results, and approvals.

Study Insights and Implications

This case study highlights the importance of proper component identification and installation procedures in power plant assembly. The root cause of the repair was a simple but costly error: the wrong bearing seat was installed. This underscores the need for robust quality control systems, including barcode tracking, visual verification, and multi-level inspection before assembly proceeds.

From a welding perspective, the repair demonstrates that overlay welding can be used to restore significant material loss on thick-walled components, provided that the process is carefully planned and executed. The key success factors are: proper consumable selection to match the base material, controlled preheat and interpass temperatures to minimize HAZ hardening, and comprehensive post-weld inspection to verify the integrity of the repair.

For engineers involved in power plant maintenance and repair, this case study serves as a reminder that even seemingly straightforward repairs on safety-critical components require rigorous engineering analysis, documented procedures, and thorough inspection. The consequences of inadequate repair can be catastrophic, both in terms of equipment failure and potential safety incidents.