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

Literature Overview

This paper, authored by Yang Jianguo, Wang Nengdao, Li Minghua, and Li Shaolin from Dongfang Boiler Works, was published in 1991 in the journal Welding (焊接), No. 11, pp. 9-12. The study investigates residual stress distributions in cobalt-based powder plasma arc surfacing deposits applied to valve sealing surfaces, examining the effects of preheat temperature, post-weld cooling methods, and post-weld heat treatment on residual stress levels and crack resistance. This research is particularly relevant to power generation and petrochemical industries where high-pressure valves require durable, wear-resistant sealing surfaces.

Core Technical Findings

The study systematically examined the influence of various process parameters on residual stress in cobalt-based plasma arc surfacing deposits. The key findings are summarized below.

Process Variable	Effect on Residual Stress
Preheat temperature increase	Reduces residual stress magnitude
Post-weld air cooling	Higher residual stress levels
Post-weld slow cooling	Lower residual stress levels
Post-weld stress relief heat treatment	Significantly reduces residual stress
Other plasma arc parameters	Affect deposit stress and crack resistance

The cobalt-based powder used for surfacing typically contains elements such as chromium, tungsten, and molybdenum, providing excellent wear resistance, corrosion resistance, and high-temperature strength for valve sealing applications.

Interpretation of Technical Points

Residual Stress Mechanisms in Plasma Arc Surfacing

Residual stresses in plasma arc surfacing deposits arise from several mechanisms:

Thermal contraction: Differential cooling between the deposit and base material creates thermal stresses.
Phase transformation: If the deposit undergoes phase changes during cooling, volume changes generate transformation stresses.
Plastic deformation: The deposit may yield plastically during cooling, leaving permanent strains that manifest as residual stresses upon unloading.
Metallurgical effects: Differences in thermal expansion coefficients between the deposit and base material create additional stress components.

Cobalt-based alloys typically have lower thermal conductivity than iron-based alloys, which can result in higher thermal gradients and consequently higher residual stresses in the deposit.

Preheat Temperature Effects

Increasing preheat temperature reduces residual stress magnitude through several mechanisms:

Reduced thermal gradient between the deposit and base material during deposition
Lower cooling rates, reducing thermal stresses
Enhanced plastic deformation at higher temperatures, allowing stress relaxation
Reduced risk of cracking during deposition due to lower thermal stresses

For valve sealing surfaces, preheat temperatures of 200-400°C are commonly used to minimize residual stresses while avoiding excessive grain growth in the base material.

Cooling Method Comparison

The comparison between air cooling and slow cooling (such as furnace cooling or insulating blanket cooling) demonstrates the significant impact of cooling rate on residual stress. Air cooling produces higher residual stresses because:

Faster cooling rates increase thermal gradients
Less time for stress relaxation through creep
Greater probability of brittle phase formation

Slow cooling allows for:

More uniform temperature distribution
Extended time for stress relaxation
Reduced thermal shock to the deposit

Stress Relief Heat Treatment

Post-weld stress relief heat treatment is the most effective method for reducing residual stresses in plasma arc surfacing deposits. The typical stress relief temperature for cobalt-based alloys is 600-800°C for 1-2 hours, depending on the specific alloy composition and component geometry. This treatment:

Allows elastic strain energy to be released through plastic deformation
Relieves thermal stresses through creep
Reduces the risk of delayed cracking

However, stress relief heat treatment must be carefully controlled to avoid:

Excessive grain growth in the deposit
Softening of the base material
Distortion of precision valve components

Engineering Practice Integration

Valve Sealing Surface Application

Cobalt-based plasma arc surfacing is widely applied to:

Control valve plug sealing surfaces
Globe valve seat surfaces
Check valve disc and seat
Safety valve seating surfaces
High-pressure gate valve sealing surfaces

These applications demand:

Excellent wear resistance for long service life
Low residual stress to prevent cracking
Dimensional stability for precision sealing
Corrosion resistance for aggressive process media

Process Optimization Strategy

Based on the findings of this study, a comprehensive process optimization strategy for valve sealing surface surfacing can be developed:

Preheating: Apply controlled preheat of 200-400°C to reduce thermal gradients and minimize residual stress initiation.
Interpass temperature control: Maintain interpass temperatures between 150-250°C for multi-pass builds to prevent excessive heat input while allowing stress relaxation.
Cooling management: Use controlled cooling methods (insulating blankets, furnace cooling) to reduce cooling rates and minimize thermal stresses.
Post-weld heat treatment: Perform stress relief annealing at 600-800°C for 1-2 hours to eliminate residual stresses.
Residual stress verification: Conduct XRD or strain gauge measurements to verify residual stress levels after each process step.

Quality Control Considerations

For critical valve applications, the following quality control measures are recommended:

QC Step	Method	Acceptance Criteria
Preheat verification	Infrared thermometer	Temperature within specified range
In-process monitoring	Visual inspection, arc stability	No cracking, uniform deposit
Post-weld residual stress	XRD or hole-drilling method	< 50% of yield strength
Hardness verification	Vickers hardness test	Within specified range
Dimensional accuracy	CMM or optical measurement	Within tolerance

Key Questions and Reflections

An important consideration not fully addressed in this study is the long-term stability of residual stress relief under service conditions. Valves in power generation and petrochemical applications experience thermal cycling, pressure cycling, and mechanical loading that can re-introduce stresses or cause stress relaxation. Understanding the cyclic stability of residual stress relief is critical for predicting long-term component performance.

Additionally, the study does not extensively address the interaction between residual stress and the metallurgical properties of the cobalt-based deposit. For example, how does residual stress affect the wear resistance, corrosion resistance, or fatigue life of the surfacing deposit? These interactions are important for comprehensive component design.

The study also does not discuss the effect of base material condition on residual stress development. Different base materials (carbon steel, stainless steel, low-alloy steel) have different thermal properties and may interact differently with the cobalt-based deposit during surfacing.

Study Insights and Implications

This research provides essential guidance for engineers applying cobalt-based plasma arc surfacing to valve sealing surfaces in critical industrial applications. The systematic examination of preheat temperature, cooling method, and stress relief heat treatment establishes clear process guidelines for minimizing residual stresses. The findings emphasize that residual stress control is not a single-step process but requires integrated management from preheating through post-weld heat treatment. For valve manufacturers in the power generation and petrochemical industries, this study underscores the importance of comprehensive residual stress management in ensuring the reliability and longevity of surfaced valve components. The practical recommendations derived from this research can be directly implemented in production environments to improve quality and reduce failure rates in critical valve applications.