Female Threaded Steel Pipe Stress Conditioning Against Fatigue Fracture
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Solution Overview
Problem
High pressure gas containers face issues with fatigue fracture originating from the thread portion of threaded steel pipes or tubes, particularly in screw-type lid structures, which are desirable for reducing container size and cost, and this problem is exacerbated by high stress concentrations in the thread bottom.
Innovation Solution
Applying residual compressive stress to the vicinity of the thread bottom of threaded steel pipes or tubes by attaching a lid or jig with a male thread, ensuring the stress conditions do not exceed the material's yield stress and tensile strength, thereby alleviating stress and preventing fatigue fracture.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Volume of moving object
If a screw-type lid structure is used to reduce container size and cost, then container size and cost are reduced, but stress concentration in the thread portion increases leading to fatigue fracture
Solution Approach 1:
The patent applies preliminary action by introducing residual compressive stress to the thread portion before the container is put into service. This is achieved through heat treatment processes that create compressive stresses in advance, which then counteract the tensile stresses that will occur during normal operation, preventing fatigue fracture while maintaining the compact screw-type lid structure
Solution Approach 2:
The patent changes the stress state parameter in the thread portion by transforming the stress distribution through heat treatment. By controlling cooling rates and temperature gradients during heat treatment, the patent creates a favorable stress distribution with compressive stresses at the thread root, fundamentally changing the mechanical behavior of the thread portion under load
2Strength
If heat treatment is applied to improve strength, then strength is improved, but scale and decarburized layer form on the surface causing fatigue resistance degradation
Solution Approach 1:
The patent applies local quality by creating different microstructures and stress states in different regions of the container. The heat treatment process is controlled to produce compressive residual stresses specifically in the thread portion while maintaining appropriate properties in other areas, and the surface is treated to prevent decarburization at critical locations
Solution Approach 2:
The patent converts the potentially harmful effect of heat treatment (which can cause surface degradation) into a beneficial effect by carefully controlling the process to create compressive residual stresses. The heat treatment, which could otherwise cause decarburization and scale, is used strategically to induce favorable stress states that enhance fatigue resistance
3Ease of manufacture
If the cross section of the container is constant to simplify structure, then manufacturing is simplified, but all internal pressure is received by the lid requiring very high pressure resistance
Solution Approach 1:
The patent changes the stress distribution parameters through controlled heat treatment processes. By creating compressive residual stresses in the thread portion and optimizing the microstructure, the patent enables the constant cross-section design to withstand the required pressures without compromising the thread portion, balancing manufacturing simplicity with pressure 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 application of residual compressive stress significantly reduces stress on the thread portion, enhancing fatigue life and allowing the threaded steel pipes or tubes to withstand higher pressures and larger cross sections without fracturing.
Implementation Method 1
a maximum value of residual compressive stress at a position of 0.4 mm in a depth direction from a thread bottom of the female thread portion is 100 MPa or more and less than or equal to tensile strength
Data Source
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AI summary
To alleviate stress exerted on a thread portion of a threaded steel pipe or tube during use to prevent fatigue fracture, there is provided a threaded steel pipe or tube having a female thread portion on an inner peripheral surface of at least one end, wherein a maximum value of residual compressive stress at a position of 0.4 mm in a depth direction from a thread bottom of the female thread portion is 100 MPa or more and less than or equal to tensile strength of a material of the threaded steel pipe or tube.