Duplex Stainless Steel Pipe Thread Geometry Against Bauschinger Fatigue
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Solution Overview
Problem
Duplex stainless steel pipes used in severe corrosive environments face issues with reduced axial compressive yield strength due to the Bauschinger effect from conventional cold rolling, leading to decreased fatigue life and corrosion resistance in threaded portions.
Innovation Solution
A stainless steel pipe composition with controlled nitrogen content (0.005% to 0.150%) and a specific microstructure (20-80% ferrite phase with austenite phase) is subjected to controlled heat treatments and cold working processes to maintain high axial tensile yield strength and reduce the difference between axial tensile and compressive yield strength, along with a curvature radius of 0.2 mm or more at thread corners to enhance fatigue characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If cold rolling is applied to increase axial tensile yield strength, then tensile strength improves, but fatigue characteristics of threaded portions deteriorate
Solution Approach 1:
The patent employs warm rolling at elevated temperatures (150°C to 600°C) to achieve dislocation strengthening. This temperature parameter change allows the material to accumulate dislocations that strengthen the matrix without creating the severe residual stress patterns and microstructural damage that occur with conventional cold rolling, thereby improving fatigue resistance in threaded portions while still achieving high tensile strength
Solution Approach 2:
The patent converts the potential harm of cold working (which causes the Bauschinger effect and reduces fatigue life) into a benefit by performing the rolling at warm temperatures. The warm temperature allows the dislocation strengthening mechanism to work effectively while avoiding the harmful residual stresses and microstructural degradation that would otherwise occur, thus turning a potentially harmful process into a beneficial one
2Reliability
If nitrogen content is increased to improve corrosion resistance, then corrosion resistance improves, but excessive nitrogen causes brittleness and processing difficulties
Solution Approach 1:
The patent optimizes the nitrogen content parameter to a specific range (0.010% to 0.060%) that provides sufficient corrosion resistance through the formation of a protective passive film containing nitrogen, while avoiding the excessive nitrogen levels that would cause severe brittleness and processing difficulties. This precise parameter control allows the material to achieve the desired corrosion resistance without the harmful effects of over-alloying
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 solution provides a stainless steel pipe with improved corrosion resistance and fatigue characteristics in threaded portions, enabling easier joint construction in oil and gas wells while maintaining high axial compressive yield strength.
Implementation Method 1
reduced axial compressive yield strength due to the Bauschinger effect from conventional cold rolling
Data Source
AI summary
A stainless steel pipe of a predetermined composition is provided that has an axial tensile yield strength of 689 MPa or more, an axial compressive yield strength/axial tensile yield strength ratio of 0.85 to 1.15, and a microstructure that is 20 to 80% ferrite phase by volume with the remainder containing an austenite phase, the stainless steel pipe having pipe end portions at least one of which has a fastening portion for an external thread or an internal thread, and having a curvature radius of 0.2 mm or more for a corner R formed by a bottom surface of a thread root and a pressure-side flank surface of the thread, measured in an axial plane section of the fastening portion.