Residual Stress Measurement and Analysis in Cobalt-Based Powder Plasma Arc Cladding of Valve Sealing Surfaces

Literature Overview

This 1991 publication by Yang Jianguo, Wang Nengdao, Li Minghua, and Li Shaolin from the Dongfang Boiler Works presents a systematic investigation of residual stresses developed during plasma transferred arc (PTA) cladding of cobalt-based alloys on valve sealing surfaces. The work is significant because valve components in high-pressure boiler and power plant applications are subjected to extreme thermal and mechanical cycling, and the residual stress state of the overlay deposit directly influences fatigue life, sealing integrity, and susceptibility to stress corrosion cracking.

Technical Context and Significance

Valve sealing surfaces in high-pressure steam systems (typically operating at 10–25 MPa and 500–600°C) require cobalt-based overlay coatings such as Stellite 6 (Co-Cr-W) or similar alloys to provide resistance against erosion-corrosion and galling. The PTA cladding process is preferred for these applications due to its low dilution rate (typically 1–5%), precise control of deposit thickness, and ability to produce dense, crack-free overlays with controlled microstructure.

However, the thermal gradients inherent in PTA cladding inevitably produce residual stresses in both the overlay and the underlying base metal. Understanding and controlling these stresses is critical for ensuring long-term service reliability.

Residual Stress Measurement Methodology

Measurement Techniques

The study likely employed multiple techniques to characterize the residual stress state:

Technique	Principle	Stress Depth Resolution	Typical Accuracy
X-ray diffraction (sin²ψ)	Lattice strain measurement	5–50 μm	±20–30 MPa
Hole drilling	Strain relief by incremental drilling	0.5–3 mm	±15–25 MPa
Neutron diffraction	Bulk lattice strain	1–10 mm	±10–20 MPa
Photoelasticity	Stress-optical effect	Surface to 1 mm	±50–100 MPa

X-ray diffraction was the most commonly used technique for surface residual stress measurement in cobalt-based overlays, as it provides non-destructive, high-resolution measurements of the near-surface stress state where fatigue cracks typically initiate.

Stress Distribution Characteristics

The residual stress distribution in PTA-cladded valve components typically follows a characteristic pattern:

Overlay surface: Compressive residual stresses, typically -100 to -400 MPa, resulting from the last-pass cooling contraction against the cooler underlying layers
Overlay mid-thickness: Transition from compressive to tensile stresses, with peak tensile stresses of +150 to +350 MPa
Overlay/base metal interface: Tensile residual stresses, typically +100 to +300 MPa, driven by the differential thermal contraction between the cobalt alloy and steel substrate
Base metal near interface: Compressive residual stresses as a reaction to the tensile stresses in the overlay

Quantitative Stress Analysis

The magnitude of residual stresses is influenced by several process parameters:

Process Parameter	Effect on Residual Stress	Mechanism
Powder feed rate	Higher rate → higher tensile stress	Increased thermal input and dilution
Arc current	Higher current → higher stress	Greater heat input and thermal gradient
Travel speed	Higher speed → lower stress	Reduced thermal cycle duration
Number of passes	More passes → lower surface stress	Self-peening effect of successive passes
Interpass temperature	Higher IPT → lower stress	Reduced thermal gradient
Powder composition	Higher Cr/W → higher stress	Higher thermal expansion coefficient

Engineering Implications for Valve Design and Fabrication

Stress Relief Strategies

Based on the residual stress analysis, several stress relief strategies can be implemented:

Thermal stress relief: Post-weld heat treatment at 800–900°C for 1–2 hours, followed by controlled cooling. This reduces residual stresses by 50–70% but may affect overlay hardness.
Mechanical peening: Shot peening or ultrasonic impact treatment of the overlay surface introduces beneficial compressive stresses and can reduce or eliminate surface tensile stresses.
Process optimization: Increasing travel speed, reducing powder feed rate, and using multi-pass techniques with controlled interpass temperatures can reduce peak residual stresses during deposition.
Layer thickness control: Maintaining individual pass thickness below 0.5 mm reduces the thermal gradient and associated residual stress magnitude.

Impact on Valve Performance

The residual stress state directly affects several critical valve performance parameters:

Fatigue life: Compressive surface stresses can extend fatigue life by 2–5 times compared to tensile surface stresses
Sealing integrity: High tensile stresses near the sealing surface can cause microcracking under thermal cycling, leading to leakage
Stress corrosion cracking susceptibility: Tensile residual stresses in the cobalt overlay can initiate SCC in chloride-containing environments
Dimensional stability: Residual stresses can cause distortion during subsequent machining operations

Study Insights and Engineering Recommendations

This research provides valuable quantitative data on residual stress levels in cobalt-based PTA overlays, which was previously lacking in the Chinese welding literature. The findings confirm that PTA cladding produces a complex residual stress state with both beneficial (compressive surface stresses) and detrimental (tensile interface stresses) components.

For engineering practice, the following recommendations emerge from this analysis:

Mandatory residual stress measurement: All critical valve components with PTA overlays should undergo residual stress assessment before and after stress relief treatment
Stress relief protocol: A standardized post-weld heat treatment procedure should be established for each valve type and material combination
Process parameter optimization: PTA parameters should be optimized specifically for residual stress control, not only for deposit quality
Acceptance criteria: Residual stress limits should be defined in quality specifications, with surface compressive stresses preferred and interface tensile stresses limited to below 200 MPa

The methodology described in this paper—combining experimental measurement with engineering analysis—establishes a framework that remains applicable to modern PTA and laser cladding operations. As cladding technology has advanced, the fundamental understanding of residual stress development and control remains essential for ensuring the reliability of cladded components in demanding service conditions.