Steel Pipe Microstructure for Sour Cracking Resistance
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Solution Overview
Problem
Existing steel pipes and plates fail to provide sufficient sulfide stress cracking (SSC) and hydrogen induced cracking (HIC) resistance in harsh high-pressure hydrogen sulfide environments, limiting their application in oil and gas pipelines, and existing solutions do not adequately control microstructure and hardness to meet the increased resistance requirements.
Innovation Solution
A steel pipe and plate with a specific chemical composition and controlled microstructure, including a chemical composition that meets ESSP 1.5 to 3.0 and Ceq 0.20 to 0.50, with a surface layer microstructure of at least 80% granular bainite, acicular ferrite, or tempered bainite, and tempered martensite, and a maximum hardness of 250 Hv or lower, ensuring high yield strength and resistance to cracking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high-strengthening is applied to increase yield strength for deeper drilling, then the strength increases, but sulfide stress cracking and hydrogen induced cracking occur in harsh acidic environments
Solution Approach 1:
The patent applies parameter changes by precisely controlling chemical composition parameters (C: 0.05-0.15%, Si: 0.01-0.50%, Mn: 1.50-3.00%, P: 0.010-0.030%, S: 0.005-0.020%, Al: 0.010-0.060%, Ti: 0.001-0.050%, V: 0.010-0.100%, Nb: 0.001-0.050%, B: 0.0005-0.0050%) and microstructural parameters (grain size, phase distribution) to achieve both high strength and sour resistance. This systematic parameter optimization resolves the contradiction between strength enhancement and cracking resistance.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases (ferrite, bainite, martensite) with specific area fractions (ferrite: 20-60%, bainite: 30-50%, martensite: 5-20%). This multi-phase composite structure provides both the strength needed for deep drilling and the ductility required to prevent sulfide stress cracking and hydrogen induced cracking in corrosive environments.
2Reliability
If seamless steel pipe is used to improve sour resistance, then SSC and HIC resistance improve, but application range is limited and cannot be used for large-diameter welded steel pipe
Solution Approach 1:
The patent applies local quality by specifying different microstructural requirements for different regions of the steel plate. The surface layer (0-1mm depth) requires a specific microstructure with ferrite 20-60%, bainite 30-50%, and martensite 5-20% to ensure sour resistance, while the internal structure can have different characteristics. This regional differentiation allows the material to simultaneously achieve high sour resistance and suitability for welded construction.
Solution Approach 2:
The patent uses parameter changes to optimize the steel composition for weldability while maintaining sour resistance. By controlling C equivalent (0.25-0.45%) and specific alloying elements, the steel achieves both the mechanical properties needed for seamless pipe performance and the weldability required for large-diameter welded pipe applications, thereby expanding the application range.
3Ease of manufacture
If common steel or low alloy steel is used to reduce manufacturing costs, then manufacturing cost decreases, but insufficient sour resistance in high-pressure hydrogen sulfide environment
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition to achieve cost-effective sour resistance. The specified ranges (C: 0.05-0.15%, Mn: 1.50-3.00%, Si: 0.01-0.50%, and controlled impurities) balance material cost with performance requirements, providing sufficient sour resistance for high-pressure hydrogen sulfide environments without requiring expensive high-alloy compositions.
Solution Approach 2:
The patent applies local quality by concentrating alloying elements and microstructural control where most needed - in the surface layer (0-1mm depth) where sour resistance is critical. This localized optimization allows cost-effective material design by not over-alloying the entire cross-section, thereby reducing manufacturing costs while maintaining adequate sour resistance for pipeline applications.
Data Source
Figure 1~2A
Figure 2B~2C
Figure 3A~3B
AI summary
A steel pipe includes: a base metal that includes a cylindrical steel plate; and a weld that is provided in a seam portion of the steel plate and extends in a longitudinal direction of the steel plate, in which the steel plate has a predetermined chemical composition and satisfies ESSP: 1.5 to 3.0 and Ceq: 0.20 to 0.50, a total area fraction of one or more kinds selected from the group consisting of granular bainite, acicular ferrite, tempered bainite, and tempered martensite in a microstructure of a surface layer in a range up to a depth of 1.0 mm from a surface of the base metal is more than 80%, Hvmax that is the maximum hardness in the surface layer of the base metal is 250 Hv or lower, and the yield ratio is 85% or higher.