External Attachment Weld PWHT Thresholds for SCC Avoidance
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Solution Overview
Problem
Current methods for determining whether post weld heat treatment (PWHT) is required for external attachment welds in the petrochemical industry are impractical due to the difficulty in measuring residual stresses and the time-consuming nature of time-to-failure testing, which is not typically adopted in the Oil and Gas industries.
Innovation Solution
A method involving the preparation of test samples with varying wall thicknesses welded at different heat inputs, measuring residual stresses, and plotting trendlines to determine a residual stress threshold, allowing for the determination of whether PWHT is required based on the intersection points of these trendlines and the subject pipe's wall thickness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If time-to-failure testing is performed to determine threshold residual stress for PWHT requirements, then the accuracy of determining whether PWHT is needed is improved, but the time and complexity of the process increases significantly
Solution Approach 1:
The patent performs preliminary time-to-failure testing to establish threshold residual stress values for different wall thicknesses before actual piping construction. These pre-determined thresholds are then used as reference values in the visual tool, eliminating the need to perform time-consuming failure tests during construction while maintaining accurate determination capability.
Solution Approach 2:
The patent creates a visual tool that copies and represents the complex relationship between wall thickness, residual stress, and PWHT requirements in a simplified graphical format. Instead of performing complex testing during construction, engineers can visually compare measured residual stress against the pre-established threshold curves in the tool, achieving accurate determination without repeating the lengthy testing process.
2Reliability
If residual stress measurement is performed on every weld to determine PWHT requirements, then the reliability of avoiding stress corrosion cracking is improved, but the complexity and difficulty of the process increases
Solution Approach 1:
The patent introduces a visual tool as an intermediary between residual stress measurement and PWHT decision-making. The tool incorporates pre-established threshold curves that account for wall thickness and material properties, serving as a mediator that translates complex engineering criteria into simple visual comparisons. This maintains reliable SCC prevention while eliminating complex determination procedures.
3Reliability
If post weld heat treatment is applied to all external attachment welds, then the protection against stress corrosion cracking is improved, but the cost and time of construction increases
Solution Approach 1:
The patent enables local decision-making for PWHT application based on specific weld characteristics (wall thickness, measured residual stress, material type) rather than applying a blanket requirement to all external attachment welds. The visual tool allows engineers to identify which specific welds require PWHT and which do not, optimizing protection against SCC while avoiding unnecessary treatments that would reduce construction productivity and increase costs.
Data Source
AI summary
A method is provided for determining whether post-weld heat treatments should be required for external attachment welds in applications where stress corrosion cracking is a possibility. According to the method, residual stress values are measured in test samples welded at high input heat levels and test samples welded at low input heat levels for a variety of different wall thickness. A threshold residual stress level is determined according to a method of selecting a common pipe used in industry and measuring the residual stress level in the longitudinal seam of the pipe. The measured residual stress values for the high and low heat input welds are compared against the threshold to determine whether for a particular wall thickness, the high and low input data exceeds the threshold, indicating whether post-weld heat treatments are required.