High-Strength Line Pipe Weld Joint Strength Control
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Solution Overview
Problem
Conventional methods for forming circumferential welded joints in high strength line pipes, particularly in seismic and frozen ground areas, face challenges in preventing ductile fracture and achieving sufficient joint strength when using submerged arc welding, as they often result in excessive overmatching of weld metal strength, which is difficult to achieve without causing weld cracks.
Innovation Solution
A circumferential welded joint is formed using steel pipes with a yield strength of at least 555 N/mm2, employing a submerged arc welding method with a one pass per layer laminating technique and specific weld conditions to ensure a joint strength ratio and critical equivalent plastic strain that satisfy certain equations, allowing for a joint strength equivalent to the base material tensile strength without excessive overmatching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If conventional overmatching design (110% or greater) is applied to prevent brittle fracture, then joint strength is improved, but weld cracks are more likely to occur and the design becomes excessively conservative
Solution Approach 1:
The invention changes the design parameter from conventional overmatching (110% or greater) to a new criterion based on Equation (1) involving joint strength ratio σmatch and critical equivalent plastic strain εp-cri. This parameter change allows achieving sufficient joint strength while avoiding excessive overmatching that causes weld cracks
Solution Approach 2:
Instead of designing from the viewpoint of preventing brittle fracture through high overmatching, the invention inverts the approach by designing from the viewpoint of preventing ductile fracture through controlling the relationship between joint strength ratio and critical equivalent plastic strain, thereby avoiding excessive conservative design
2Productivity
If submerged arc welding is used to increase welding efficiency, then productivity is improved, but achieving joint strength overmatching of 110% or greater without causing weld cracks becomes difficult
Solution Approach 1:
The invention changes the joint strength criterion from fixed overmatching percentage to a dynamic criterion based on Equation (1) that relates joint strength ratio σmatch to critical equivalent plastic strain εp-cri. This allows submerged arc welding to achieve sufficient joint strength without causing weld cracks
Solution Approach 2:
The invention accepts that weld metal strength may not always exceed base material strength (allowing undermatching in some cases), as long as the overall joint strength satisfies Equation (1). This relaxes the requirement for high-strength weld metal, making submerged arc welding more feasible
3Stress or pressure
If high strength steel pipes (yield strength ≥ 555 N/mm2) are used to improve transport efficiency, then operating pressure is increased, but ductile fracture risk increases under large plastic deformation
Solution Approach 1:
The invention performs preliminary evaluation of critical equivalent plastic strain εp-cri for ductile crack generation in the base material heat affected zone before finalizing the welding design. This allows predicting and preventing ductile fracture under large plastic deformation conditions
Solution Approach 2:
The invention uses the relationship expressed in Equation (1) as a feedback criterion to evaluate whether the welded joint design is sufficient. The joint strength ratio σmatch is compared against the critical equivalent plastic strain εp-cri to ensure adequate ductile fracture resistance
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method ensures a joint strength equivalent to the base material tensile strength, even with defects, and provides a reasonable and required strength for the welded joint, suppressing ductile crack growth and maintaining joint integrity during large plastic deformations.
Implementation Method 1
welding the butted portions in a circumferential direction... employing a submerged arc welding method
Implementation Method 2
a welding method, such as gas shield metal arc welding (GMAW)... submerged arc welding, which enables welding of higher efficiency
Data Source
AI summary
A circumferential welded joint of a line pipe is formed by butting against each other end portions of steel pipes having a yield strength according to 5L Specification of API Standards not smaller than 555 N/mm2 and welding the butted portions in a circumferential direction. A joint strength ratio σmatch=(TS-w/TS-b)·(YS-w/YS-b) represented by a product of a ratio between a tensile strength TS-w of a weld metal and a tensile strength TS-b of a base material and a ratio between a yield strength YS-w of the weld metal and a yield strength YS-b of the base material, and a critical equivalent plastic strain εp-cri [%] for ductile crack generation in a base material heat affected zone satisfy Equation (1), and the yield strengths YS-w, YS-b of the weld metal and base material satisfy Equation (2).σmatch>4.85εp-cri−0.31 (1)YS-w/YS-b≧1.0 (2)