Failure Analysis of Stellite Alloy Overlay on Turbine Valve Bodies

Literature Overview

Published in Thermal Processing Technology in 2025, this study by researchers from the Henan Boiler and Pressure Vessel Inspection Technology Research Institute and Henan University of Technology investigates the failure mechanism of Stellite alloy overlay layers on steam turbine valve bodies. Funded by the Henan Province Key Science and Technology Project (212102210350) and the Henan University of Technology High-Level Talent Research Startup Fund (2019BS052), this work addresses a critical safety and reliability concern in power generation equipment. Steam turbine valves experience severe erosion-corrosion conditions where high-velocity steam at temperatures of 540–620°C contains trace moisture, creating an aggressive environment that demands exceptional overlay performance.

Failure Phenomena and Macroscopic Observations

Service Conditions

The turbine valve bodies examined in this study operated under the following conditions: steam temperature of 565–590°C, steam pressure of 16.7 MPa, valve opening/closing cycles of approximately 8,000–12,000 times over the service period, and service duration of 18,000–24,000 hours before failure was detected. The overlay was applied as a 3.0–4.0 mm thick Stellite 6 alloy layer using oxy-fuel welding followed by machining to final dimensions.

Failure Modes Identified

The failure manifested as a combination of three distinct damage mechanisms:

Failure Mode	Location	Proportion of Total Damage	Severity
Erosion-corrosion pitting	Valve seat sealing surface	45%	High - loss of sealing integrity
Intergranular cracking	Overlay/base metal interface	30%	Critical - delamination risk
Surface spalling	Valve stem guide surface	25%	Moderate - accelerated wear

Microstructural Analysis and Failure Mechanism

Overlay Microstructure After Service

Metallographic examination revealed that the as-welded Stellite 6 overlay, characterized by a matrix of austenite with M6C and M23C6 carbide networks, underwent significant microstructural degradation during service. The carbide network transformed from fine, dispersed M6C particles to coarse M23C6 carbides with dimensions exceeding 5 μm, indicating extensive carbide phase transformation at elevated temperature. This transformation depleted chromium from the matrix, reducing local Cr content below 20% at carbide/matrix interfaces, which is below the critical threshold for passivity in oxidizing environments.

Interfacial Degradation

The most critical finding concerns the overlay/base metal interface. The base metal was a low-alloy steel (12Cr1MoV equivalent) with a dilution zone extending 0.3–0.5 mm into the substrate. This dilution zone, containing 4–8% Cr, developed intergranular cracks along prior austenite grain boundaries during thermal cycling. The cracks propagated under the combined action of thermal stress cycling (±50°C per operating cycle), mechanical stress from valve actuation, and stress corrosion cracking in the presence of trace moisture in the steam.

Erosion-Corrosion Synergy

The erosion-corrosion damage on valve seat surfaces resulted from the synergistic interaction between high-velocity steam impact (velocity exceeding 300 m/s during valve operation) and electrochemical corrosion. The mechanism follows a cyclic pattern:

Steam impact removes the protective chromium oxide film from the overlay surface.
Freshly exposed Stellite surface undergoes rapid oxidation in the presence of trace moisture.
The newly formed oxide layer is mechanically removed by subsequent steam impact.
Progressive material loss creates pits that concentrate stress and accelerate further degradation.

Standards and Specification Compliance

Applicable Standards Review

Standard	Requirement	Compliance Assessment
ASME IX QW-441	Oxy-fuel welding qualification for overlay	Qualified procedure used, but operator skill verification lacking
ASME VIII Div.1 UG-91	Overlay thickness requirements	Meets minimum 3.0 mm specification
API 934	Clad plate/weld overlay fabrication	Dilution ratio not controlled per API 934 Section 5
NACE MR0175	Sulfide-resistant overlay requirements	Not applicable to this service
NB/T 47014	Chinese welder qualification	Qualified, but requalification interval may have been exceeded

Root Cause Analysis Using FMEA Approach

Failure Mode and Effects Analysis

Failure Mode	Potential Cause	Severity (1-10)	Occurrence (1-10)	Detection (1-10)	RPN
Interfacial cracking	High dilution ratio (>30%)	10	7	4	280
Erosion-corrosion pitting	Carbide coarsening, Cr depletion	9	8	5	360
Surface spalling	Excessive residual stress	7	6	6	252
Overlay delamination	Poor base metal preparation	10	3	3	90
Hot cracking	Excessive sulfur in base metal	8	4	5	160

Recommendations and Engineering Countermeasures

Material Selection Improvements

For improved service life under similar conditions, the study recommends considering the following alternatives:

Stellite 21 (high Cr-Ni content, 25% Cr, 20% Ni) for superior corrosion resistance at the cost of slightly lower erosion resistance.
Alloy 625 overlay (Inconel 625) for maximum corrosion resistance in high-temperature steam environments, with hardness of 35–40 HRC.
Multi-layer approach: Stellite 6 as wear layer over Alloy 625 as corrosion-resistant barrier layer.

Process Optimization

Improvement Area	Current Practice	Recommended Practice	Expected Benefit
Base metal preparation	Grit blast to Sa 2.0	Grit blast to Sa 2.5 with acid pickling	Reduce interfacial contamination
Preheat temperature	200°C	300–350°C	Reduce residual stress, minimize dilution
Welding sequence	Single direction	Step-back from center	Reduce distortion and stress concentration
Post-weld treatment	None	Solution treatment at 1100°C/2h + water quench	Homogenize carbide distribution
Dilution control	Not monitored	UT thickness measurement per pass	Ensure dilution below 20%

Study Insights and Reflections

This failure analysis provides valuable lessons for the industry regarding the limitations of Stellite alloy overlays in prolonged high-temperature steam service. The fundamental issue is not the inherent quality of Stellite 6 as an overlay material, but rather the inadequacy of the fabrication process to control dilution and the absence of post-weld heat treatment to optimize carbide morphology. The study underscores the importance of integrating metallurgical understanding into quality control procedures, particularly the need for dilution monitoring and interfacial integrity verification. For future turbine valve overlay applications, a holistic approach combining optimized material selection, rigorous process control, and appropriate post-weld treatment is essential to achieve the required service life. The integration of NACE and ASME standards with Chinese NB standards provides a comprehensive quality framework that should be adopted in all critical overlay applications.