Stress Analysis of Dissimilar Steel Welded Oblique Tees Using Finite Element Method

Literature Overview

This 2016 paper by Nie Lei and Wang Lei, published in Fertilizer Design, presents a finite element analysis of dissimilar steel welded oblique tees using ANSYS Workbench. The primary objective is to determine stress concentration factors (SCFs) that are more accurate than those specified in ASME B31.3-2014, thereby improving the accuracy of piping stress calculations performed using the CARSAR II software. The study also compares the SCFs for dissimilar steel welds with those for homogeneous steel welds, finding that the dissimilar steel condition has a greater influence on stress concentration factors. This work is directly relevant to piping engineers designing process piping systems where dissimilar material connections are common, such as in fertilizer plants, petrochemical facilities, and power generation stations.

Technical Background

Oblique tees are pipe fittings where the branch pipe connects to the main pipe at an angle other than 90 degrees. They are commonly used in process piping to redirect flow or connect to equipment at specific angles. When the branch and main pipes are made of different materials—such as carbon steel main pipe with stainless steel branch, or carbon steel with alloy steel—the weld joint presents unique challenges:

• Thermal expansion mismatch: Different coefficients of thermal expansion cause differential movement during thermal cycling, generating additional stresses at the weld.
• Stiffness mismatch: Different elastic moduli and yield strengths create stress concentrations at the material interface.
• Weld metallurgy: Dissimilar metal welds may have different mechanical properties from both parent materials, further affecting stress distribution.

These factors combine to produce stress concentration factors that differ from those of homogeneous welds and from the generic values specified in ASME B31.3.

Methodology and Results

The finite element analysis was performed using ANSYS Workbench with the following approach:

The study compared SCFs for two configurations:

• Dissimilar steel weld: Branch and main pipe made of different materials
• Homogeneous steel weld: Branch and main pipe made of the same material

The key finding is that dissimilar steel welding produces higher SCFs than homogeneous welding under the same loading conditions. This is attributed to the combined effects of stiffness mismatch and thermal expansion mismatch at the weld interface. The magnitude of the difference depends on the specific material combination and the loading conditions, but the trend is consistent across the cases examined.

The study also demonstrates that the ASME B31.3 SCF values, which are based on homogeneous material assumptions, are conservative for dissimilar steel configurations. While conservatism is generally acceptable in design, it can lead to unnecessarily thick pipe walls, heavier supports, and higher costs. Using geometry-specific SCFs from finite element analysis allows for more rational and economical design.

Engineering Practice Implications

For piping engineers, this research has several practical implications:

• Design optimization: Using more accurate SCFs can reduce conservatism in pipe wall thickness selection, leading to cost savings without compromising safety.
• Weld qualification: The higher SCFs in dissimilar steel welds highlight the importance of proper weld qualification and inspection, as these joints are more susceptible to stress-related failures.
• Software improvement: The study provides data to improve the SCF databases in piping stress analysis software such as CARSAR II, CAESAR II, and AutoPIPE, leading to more accurate system-level stress calculations.
• Material selection: The findings support the practice of minimizing dissimilar steel welds where possible, or selecting material combinations with similar thermal expansion coefficients and mechanical properties to reduce stress concentrations.

In the context of fertilizer plant design, where the authors are based, dissimilar steel welds are common due to the need for corrosion resistance in certain sections while maintaining economic carbon steel for others. The ability to accurately predict stress concentrations at these joints is essential for ensuring long-term integrity under cyclic thermal and pressure loading.

Key Reflections

This paper makes a valuable contribution to piping stress analysis by addressing a specific and common engineering problem: the stress concentration behavior of dissimilar steel welded oblique tees. The finding that dissimilar steel welds have higher SCFs than homogeneous welds is intuitive but had not been quantified with sufficient rigor for practical engineering use. The development of more accurate SCF values through finite element analysis represents a methodological advancement that can be applied to other piping component types and material combinations.

The study also highlights an important aspect of modern piping engineering: the integration of detailed component-level finite element analysis with system-level piping stress analysis. The SCFs derived from detailed component analysis can be incorporated into system analysis software to improve overall accuracy, creating a more rational design process. This approach—using detailed analysis to calibrate and improve simplified methods—is a powerful engineering methodology that should be more widely adopted.

The paper is a useful reference for piping engineers working on projects involving dissimilar steel connections, and its methodology provides a template for similar analyses of other complex piping geometries where standard SCF values may be inadequate.