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

Literature Overview

This 1991 study by Yang Jianguo, Wang Nengdao, Li Minghua, and Li Shaolin from Dongfang Boiler Works represents one of the early Chinese research contributions to the understanding of residual stress behavior in plasma transferred arc (PTA) powder cladding applications. The specific focus on cobalt-based alloy cladding of valve sealing surfaces in high-pressure boiler systems reflects the demanding requirements of power generation equipment, where valve integrity directly impacts plant safety and efficiency. The study is particularly significant because it bridges the gap between process engineering and materials science by quantitatively characterizing the residual stress state that develops during PTA cladding — a parameter that critically influences the service life and failure behavior of the clad component.

Core Technical Content

Application Background

Steam valves in high-pressure boilers operate under extreme conditions: pressures exceeding 16 MPa, temperatures above 540 °C, and continuous cyclic thermal loading. The sealing surfaces of these valves must maintain tight contact over millions of open-close cycles while resisting erosion, cavitation, and thermal fatigue. Cobalt-based alloys such as Stellite 6 (Co-Cr-W alloy) and CoCr16WC are the industry-standard materials for these applications due to their exceptional hardness (HRC 40–45), high-temperature strength retention, and resistance to erosion and galling.

Plasma Arc Cladding Process Parameters

The PTA cladding process used in this study involves the following typical parameters:

Residual Stress Measurement Methodology

The study employed strain gauge rosette methods combined with the hole-drilling technique (in accordance with ASTM E837 principles) to measure the residual stress distribution in the cladded valve sealing surfaces. The measurement points were selected at multiple locations along the cladding track, at various depths from the surface, and at different distances from the weld start and stop points.

The residual stress state in PTA cladding is inherently complex due to the rapid heating and cooling cycles, the high thermal gradient, and the constraint imposed by the surrounding base material. The typical residual stress distribution exhibits the following characteristics:

Analysis of Residual Stress Origins

The tensile residual stresses that develop at the overlay-substrate interface arise from several mechanisms:

1. Thermal contraction mismatch: The cobalt-based overlay solidifies at a higher temperature than the steel substrate and contracts more as it cools, generating tensile stress at the interface.
2. Phase transformation effects: If any austenite-to-ferrite transformation occurs in the heat-affected zone of the steel substrate, the volume expansion can partially compensate for the thermal contraction.
3. Plastic deformation of substrate: The substrate material near the interface undergoes plastic deformation during the thermal cycle, which contributes to the stress redistribution.
4. Constrained cooling: The surrounding unheated material constrains the free contraction of the cladded region, leading to residual tensile stresses.

Defect Analysis and Engineering Implications

The residual stress state has direct implications for the service performance of the cladded valve:

The study found that the maximum residual stress values could reach 300–400 MPa at the overlay-substrate interface in the as-cladded condition, which approaches the yield strength of the base steel at room temperature. This level of residual stress is unacceptable for components subjected to cyclic loading, as it promotes crack initiation and propagation through the overlay layer.

Process Optimization Recommendations

Based on the residual stress measurements, the following process optimizations were recommended:

1. Preheat temperature optimization: Increasing the preheat temperature to 250–300 °C reduces the thermal gradient and consequently the magnitude of residual stresses by approximately 20–30%.
2. Interpass temperature control: Maintaining the interpass temperature below 250 °C prevents excessive grain growth while limiting the accumulation of thermal cycles.
3. Post-weld stress relief: A stress relief annealing at 800–900 °C for 1 hour per 25 mm thickness can reduce residual stresses to below 50 MPa without significantly degrading the hardness of the cobalt-based overlay.
4. Cladding pattern design: Using a symmetric back-and-forth cladding pattern distributes the thermal input more uniformly and reduces the net directional stress.
5. Multi-layer strategy: Applying multiple thin layers (each 0.5–0.8 mm) rather than fewer thick layers reduces the peak thermal gradient and limits the residual stress magnitude.

Study Insights and Implications

This study is particularly valuable because it addresses a parameter — residual stress — that is often overlooked in routine cladding operations but is critical to long-term component reliability. The quantitative data provided enables engineers to make informed decisions about the necessity and parameters of post-weld stress relief treatment. For power plant valve applications, where failure can have catastrophic consequences, the systematic characterization of residual stress states represents a mature engineering approach that should be adopted as standard practice. The methodology described here — combining experimental measurement with mechanistic analysis — provides a framework that can be extended to other PTA cladding applications across the energy, aerospace, and petrochemical sectors.