Finite Element Analysis of Surface Residual Stress in Surfacing Layers at Different Depths

Literature Overview

This paper by Zhou Yuming and Shi Haifang, published in "Metal Heat Treatment" in 2014 (Vol. 39, Issue 7, pp. 153-156), presents a combined experimental and numerical investigation of residual stress distribution in surfacing weld layers of varying depths. The study was conducted by researchers from Liaoning Mechanical and Electrical Vocational and Technical College and the School of Materials Science and Engineering at Liaoning Technical University. The work addresses a fundamental challenge in surfacing technology: understanding how weld residual stress evolves with layer depth, which directly impacts the durability, fatigue life, and dimensional stability of surfaced components.

Core Technical Methodology

Finite Element Model Development

The authors used Marc finite element analysis software to build a thermal-mechanical coupled model of the surfacing welding process. The model incorporated several key physical phenomena:

Heat source modeling: A moving heat source representing the welding arc, with parameters calibrated to match the actual welding energy input.
Thermo-mechanical coupling: The thermal field solution was used as input for the mechanical stress analysis, accounting for thermal expansion, plastic deformation, and phase transformation effects.
Material properties: Temperature-dependent elastic modulus, thermal conductivity, specific heat, and yield strength were incorporated to capture the non-linear behavior of the weld metal and base metal.
Layer-by-layer deposition: The model simulated the sequential deposition of weld layers, with each layer subjecting the previously deposited material to thermal and mechanical loading.

The model was constructed under the condition of constant plate thickness, which simplifies the analysis by eliminating variables related to base metal geometry. This controlled approach allows the depth-dependent residual stress trend to be isolated and clearly identified.

Experimental Validation

Residual stress measurements were performed using the blind hole method (also known as the deep hole method), which is a strain gauge-based technique that measures the elastic relaxation of stress when a small hole is drilled into the surface of the component. This method provides accurate point-by-point residual stress measurements but is destructive and time-consuming.

The blind hole method involves the following steps:

Mount a strain gauge rosette on the surface of the surfacing layer.
Drill a small pilot hole to relieve surface stress and measure the initial strain.
Drill a larger blind hole to a specified depth to relieve deeper stress and measure the additional strain.
Calculate the residual stress using the strain relief factors, which are determined from calibration tests on the same material.

Results and Discussion

The study found that welding residual stress increases with increasing weld layer depth. Both the finite element simulation and the blind hole experimental measurements showed good agreement in trend, validating the accuracy of the numerical model. The following table summarizes the key findings:

Depth from Surface	FEA Predicted Stress (MPa)	Blind Hole Measured Stress (MPa)	Agreement
Surface layer	Lowest	Lowest	Good
Intermediate depth	Moderate	Moderate	Good
Deep layer (near base metal)	Highest	Highest	Good

The increasing trend of residual stress with depth can be explained by the following mechanism: the surface layer is deposited last and undergoes the most significant cooling and contraction, but it is also free to relax stress through plastic deformation at the free surface. Deeper layers, deposited earlier, are constrained by the material deposited on top of them and cannot relax as freely. Additionally, the deeper layers experience greater constraint from the base metal, which has a different thermal expansion coefficient and modulus of elasticity than the surfacing alloy.

Process and Standards Analysis

Factors Affecting Residual Stress Distribution

Several process parameters influence the residual stress profile in surfacing layers:

Factor	Effect on Residual Stress	Control Strategy
Welding current	Higher current increases thermal input and residual stress	Use minimum current sufficient for fusion
Travel speed	Faster speed reduces heat input and residual stress	Optimize for adequate fusion with lower stress
Interpass temperature	Higher interpass temperature reduces thermal gradient and stress	Maintain interpass temperature at 150–250 °C
Number of layers	More layers increase cumulative stress	Limit to minimum required layers
Layer thickness	Thinner layers reduce per-pass stress	Use multiple thin layers instead of few thick layers
Welding sequence	Sequential welding from center outward reduces stress concentration	Plan welding sequence to minimize constraint

Residual Stress Management Strategies

To reduce harmful residual stresses in surfacing applications, the following strategies are commonly employed:

Post-weld heat treatment: Stress relief annealing at 550–650 °C for 2–4 hours can reduce residual stress by 50–80%, but may soften the surfacing layer and reduce wear resistance.
Shot peening: Introducing compressive residual stress on the surface layer can counteract tensile residual stress and improve fatigue life.
Vibration stress relief: Applying controlled vibration to the component after welding can relax residual stress without significant thermal softening.
Rolling or burnishing: Mechanically working the surface layer can introduce beneficial compressive stress.
Process optimization: Using low-heat-input welding parameters, proper welding sequences, and controlled interpass temperatures can minimize residual stress during the welding process itself.

Integration with Engineering Practice

Application to Pipeline and Fitting Surfacing

In the context of pipeline and pipe fitting manufacturing, surfacing is commonly used for:

Corrosion-resistant overlay: Applying nickel-based or stainless steel overlay to carbon steel pipelines for chemical resistance.
Wear-resistant overlay: Surfacing pipe elbows, tees, and reducers in slurry service with hardfacing alloys.
Repair surfacing: Restoring worn surfaces on flanges, valve seats, and pipe ends.

The residual stress distribution in these surfacing layers directly affects:

Fatigue life: Tensile residual stress at the surface reduces fatigue life; compressive stress improves it.
Dimensional stability: Residual stress can cause distortion and warping of thin-walled fittings during and after welding.
Corrosion resistance: Tensile residual stress can promote stress corrosion cracking in susceptible alloys, particularly in chloride environments.
Coating adhesion: Residual stress affects the bond strength of subsequent protective coatings.

Quality Control Implications

For surfacing operations in pipeline and fitting manufacturing, the following quality control measures are recommended:

Pre-weld inspection: Verify base metal cleanliness, geometry, and weldability.
Process parameter monitoring: Record and control welding current, voltage, travel speed, and interpass temperature.
Post-weld residual stress measurement: Use ultrasonic or X-ray diffraction methods for non-destructive residual stress assessment.
Stress relief procedures: Apply post-weld heat treatment or mechanical stress relief as specified in the applicable standard (e.g., ASME B31.3, API 5L).
Documentation: Maintain records of welding parameters, residual stress measurements, and stress relief procedures for traceability.

Key Questions and Reflections

The study raises several important questions for further investigation:

How does the residual stress profile change with different surfacing alloy compositions? The study focuses on a specific alloy, but the stress behavior may differ significantly for nickel-based, cobalt-based, or high-chromium alloys.
What is the effect of multi-pass surfacing with different welding sequences on the residual stress distribution?
Can the finite element model be extended to predict residual stress in curved geometries such as pipe elbows and reducers, where geometric constraints significantly affect stress distribution?
How does residual stress interact with the microstructure and mechanical properties of the surfacing layer over long-term service?

The blind hole method, while accurate, is limited by its destructive nature and time-consuming procedure. For industrial quality control, non-destructive methods such as X-ray diffraction (XRD) or ultrasonic methods should be explored as alternatives, with calibration against blind hole measurements to ensure accuracy.

Study Insights and Implications

This study demonstrates the value of combining finite element analysis with experimental validation for understanding residual stress in surfacing welds. The FEA model, when properly calibrated with experimental data, can serve as a powerful tool for predicting residual stress in new surfacing configurations without the need for extensive destructive testing. This is particularly valuable for optimizing surfacing processes on expensive components such as large-diameter pipe fittings or pressure vessels.

The finding that residual stress increases with depth has direct implications for surfacing process design. It suggests that the deepest layers, which are most heavily stressed, should be the focus of stress management strategies. In multi-layer surfacing, the first pass (which becomes the deepest layer) should be deposited with the lowest possible heat input and with careful attention to preheating and welding sequence. Subsequent passes can use slightly higher heat input to ensure good fusion, as the stress in these layers will be lower.

For pipeline and fitting manufacturers, this study underscores the importance of considering residual stress as a critical quality parameter in surfacing operations. The residual stress profile is not merely an academic concern; it directly affects the long-term reliability of surfaced components in service. Incorporating residual stress assessment into the quality control plan for surfacing operations is a recommended best practice, particularly for critical applications such as oil and gas pipelines, power plant components, and chemical processing equipment.