Study Note on Microstructure and Cavitation Resistance of Arc Weld Overlay Repair on Martensitic Stainless Steel Turbine Blades

Literature Overview

This paper investigates the microstructure evolution and cavitation erosion resistance of martensitic stainless steel turbine blades after arc weld overlay repair. Turbine blades in hydropower and steam power applications are subjected to severe cavitation erosion, which can significantly reduce their service life. When blade damage is localized, arc weld overlay repair is an economical alternative to blade replacement. However, the weld overlay process introduces a heat-affected zone (HAZ) with altered microstructure, which may have different cavitation resistance characteristics from the original base material. This study provides a systematic analysis of the repair process, microstructural changes, and cavitation performance evaluation.

Core Technical Content and Key Points

Base Material and Repair Configuration

The turbine blades are made of martensitic stainless steel, typically grades such as 410 (12Cr13), 420 (20Cr13), or 440C (39Cr13), which offer a combination of strength, wear resistance, and moderate corrosion resistance. The base material is in the martensitic state after heat treatment, with a hardness typically in the range of 35–45 HRC. The repair process uses arc welding, with the specific method being either submerged arc welding (SAW) or gas metal arc welding (GMAW), depending on the blade geometry and accessibility.

Weld Overlay Process Parameters

The repair welding process requires careful parameter selection to minimize the thermal impact on the surrounding base material while ensuring adequate fusion and repair quality. The following table summarizes the key parameters:

Parameter	SAW Repair	GMAW Repair	Notes
Current	200–350 A	120–200 A	Lower current for thin blade sections
Voltage	25–32 V	20–28 V	Adjusted for arc stability
Travel speed	80–150 mm/min	100–200 mm/min	Higher speed for GMAW
Filler wire	0Cr13 or 1Cr13	0Cr13 or 1Cr13	Match base material chemistry
Preheat	100–150 °C	100–150 °C	To reduce residual stress
Interpass temp	≤ 200 °C	≤ 200 °C	Critical for martensitic control

Microstructural Analysis

The microstructural evolution across the repair zone is a critical aspect of this study. The following zones are identified from the base material to the weld centerline:

Base material zone: Retains the original martensitic microstructure with lenticular or acicular morphology, possibly with some tempering effects if the local temperature exceeded 400 °C.
Heat-affected zone (HAZ): This zone experiences a range of thermal cycles, from temperatures below the Ac1 transformation temperature to temperatures well above the Ac3 transformation temperature. The HAZ near the fusion line may show a mixture of martensite and retained austenite, while the outer HAZ may exhibit tempered martensite with some grain coarsening.
Weld metal: The weld metal solidifies as martensite due to the high carbon and chromium content, with some retained austenite depending on the cooling rate and composition.

The paper reports that the HAZ width is typically 1–3 mm for SAW repair and 0.5–1.5 mm for GMAW repair, with the narrower HAZ of GMAW being advantageous for preserving the original microstructure of the surrounding blade material.

Cavitation Erosion Performance Evaluation

Cavitation erosion testing was conducted using a standard ultrasonic vibration apparatus or a rotating ring-on-disk apparatus. The key findings are summarized in the following table:

Test Condition	Base Material (Loss rate)	Weld Metal (Loss rate)	HAZ (Loss rate)
Ultrasonic vibration, 30 min	0.08 mg/h	0.12 mg/h	0.15 mg/h
Ultrasonic vibration, 60 min	0.15 mg/h	0.22 mg/h	0.28 mg/h
Rotating ring-on-disk, 2 h	0.05 mg/h	0.07 mg/h	0.09 mg/h

The results indicate that the HAZ exhibits the poorest cavitation resistance among the three zones, likely due to the presence of tempered martensite with lower hardness and the potential for microcracking at the fusion line. The weld metal, while having slightly lower cavitation resistance than the base material, is generally acceptable for repair applications.

Engineering Practice Integration

Repair Process Optimization

The study provides several recommendations for optimizing the arc weld overlay repair of martensitic stainless steel turbine blades:

Minimize heat input: Use the lowest current and highest travel speed that still achieves adequate fusion. This reduces the HAZ width and minimizes microstructural changes in the surrounding base material.
Control interpass temperature: Maintaining the interpass temperature below 200 °C prevents excessive grain growth and reduces the risk of cracking in the martensitic weld metal.
Post-weld heat treatment: A tempering treatment at 400–500 °C for 1–2 hours can relieve residual stresses and improve the toughness of the weld metal and HAZ without significantly reducing hardness.
Surface finishing: Grinding and polishing the repair area to match the original blade surface profile is essential for restoring aerodynamic performance and reducing stress concentrations.

Defect Prevention and Quality Control

The following quality control measures are recommended for arc weld overlay repair of turbine blades:

Inspection Method	Purpose	Acceptance Criteria
Visual inspection	Surface defects, undercut, spatter	No visible defects after cleaning
Magnetic particle testing (MT)	Surface and near-surface cracks	No linear indications > 1 mm
Ultrasonic testing (UT)	Volumetric defects, bond integrity	No indications > 3 mm
Hardness testing	Microstructural verification	Weld metal hardness within 5 HRC of base material
Cavitation testing	Performance verification	Loss rate within 20% of base material

Practical Lessons

The most significant finding from this study is that the HAZ, rather than the weld metal itself, represents the weakest link in terms of cavitation resistance. This has important implications for repair strategy: the goal should be to minimize the HAZ width rather than simply matching the weld metal composition to the base material. GMAW repair is preferred over SAW repair for thin blade sections because of the narrower HAZ and lower heat input. Additionally, the study highlights the importance of post-weld surface finishing in restoring cavitation resistance, as surface roughness significantly affects cavitation damage initiation.

Key Questions and Reflections

Several questions remain open for further investigation. First, the long-term cavitation performance of the repaired blade under actual service conditions, which involve complex flow patterns and variable cavitation intensity, is difficult to predict from laboratory testing alone. Second, the effect of repeated repair cycles on the microstructure and cavitation resistance of the blade is not addressed. In practice, blades may undergo multiple repair cycles over their service life, and the cumulative thermal history could lead to progressive microstructural degradation. Third, the study does not explore alternative repair methods such as laser cladding or cold spray, which may offer lower heat input and potentially better cavitation resistance.

Study Insights and Implications

This paper provides a solid foundation for understanding the relationship between arc weld overlay repair and cavitation performance of martensitic stainless steel turbine blades. The key insight is that the HAZ is the critical zone for cavitation resistance, and process optimization should focus on minimizing HAZ width and thermal effects. Engineers should adopt a systematic approach to repair qualification, including microstructural analysis, hardness mapping, and cavitation testing, to ensure that repaired blades meet performance requirements. The study also underscores the importance of post-weld surface finishing in restoring blade performance, which is often overlooked in repair practices. Future work should explore advanced repair technologies with lower thermal impact and investigate the effects of multiple repair cycles on blade integrity.