Super Duplex Pipe Coil Processing for Deep-Sea Pitting Resistance
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Solution Overview
Problem
Existing duplex stainless steel pipes used in deep-sea umbilical cables face challenges with insufficient mechanical properties, pitting resistance, and fatigue performance, particularly under harsh deep-sea conditions, necessitating improved materials and manufacturing methods.
Innovation Solution
A method involving precise control of alloying elements, high-compression forging, and advanced metallurgical processes to produce a welded and extended super duplex stainless steel seamless pipe coil, including steps like triple stripping pretreatment, controlled casting, high-deformation forging, and solution heat treatment, followed by self-fusion welding and polishing to enhance mechanical properties and pitting resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If S31803 duplex stainless steel is used, then the cost is lower, but the pitting resistance and mechanical properties are insufficient
Solution Approach 1:
The patent changes the chemical composition parameters by increasing Cr from 22% to 25-28%, Mo from 3% to 4-6%, and N from 0.15% to 0.25-0.35%, transforming the material from S31803 to S32750 super duplex stainless steel, thereby significantly improving pitting resistance equivalent from 34.3 to above 40 while maintaining manufacturing feasibility
Solution Approach 2:
The patent creates a composite microstructure consisting of austenite and ferrite phases with specific volume ratios (40-60% austenite, 40-60% ferrite), achieving superior mechanical properties and corrosion resistance that neither phase could provide alone
2Strength
If S32750 super duplex stainless steel is used, then the mechanical properties and pitting resistance are improved, but the manufacturing complexity increases
Solution Approach 1:
The patent performs preliminary phase balance control during the casting and hot working stages, establishing the correct austenite-ferrite ratio early in the process before final forming operations, which simplifies subsequent manufacturing by preventing phase-related defects
Solution Approach 2:
The patent maintains continuous temperature control and phase balance management throughout the entire manufacturing process from casting through hot rolling to final forming, ensuring the austenite-ferrite structure remains stable and preventing detrimental phase transformations that would complicate manufacturing
3Reliability
If high alloying elements content is increased, then the pitting resistance equivalent is improved, but the cost increases
Solution Approach 1:
The patent optimizes the alloying element parameters by precisely controlling Cr at 25-28%, Mo at 4-6%, and N at 0.25-0.35%, achieving a pitting resistance equivalent of above 40 with balanced cost-performance ratio, avoiding excessive alloying that would unnecessarily increase cost
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 results in a pipe coil with enhanced tensile strength, yield strength, and pitting resistance, meeting the demanding requirements of deep-sea umbilical cables, ensuring high reliability and durability under extreme conditions.
Implementation Method 1
adding aluminum powder to the slag surface to perform deoxidation so that O element exists in the liquid steel in the form of Al2O3 inclusion
Implementation Method 2
the hard non-deforming Al2O3 inclusion is transformed into a plastic calcium aluminate inclusion with a low melting point
Implementation Method 3
performing calcium treatment for optimization so that Al in the Al2O3 inclusion is replaced by Ca and enters the liquid steel without generating AlN
Implementation Method 4
in argon-oxygen refining in an AOD furnace, performing an oxidation reaction to raise the furnace temperature
Implementation Method 5
adding ferrosilicon during a reduction period to remove CaO+MgO+Al2O3+SiO2 slag from the steel, performing a decarbonization reaction to remove carbon to 0.03% or below
Implementation Method 6
heating the ingot first, and then performing longitudinal compression on a high-speed forging press followed by transverse compression, so that the ingot is longitudinally compressed with a compression deformation ratio of at least 1
Implementation Method 7
performing cold-deformation plastic processing and solution heat treatment on the cold rolled pipe, wherein the temperature of solution heat treatment is controlled to be 1080±10° C., the two-phase ratio is accurately controlled to be 1:1, wherein the ferrite content is controlled to be 49-52%, and the detrimental σ phase in the steel is eliminated
Implementation Method 8
performing a self-fusion welding and cosmetic circumferential girth welding process, wherein the outside of the pipe is protected by a high-purity argon-nitrogen mixture (Ar: 98% and N2: 2%), and the inside of the pipe is protected by pure N2
Data Source
AI summary
The invention discloses a preparation method for a welded and extended super duplex stainless steel seamless pipe coil for a deep-sea umbilical cable. An AOD furnace, Al deoxidation, calcium treatment and external refining are adopted to ensure that the oxygen content in steel is at most 25 ppm. The key technology of integrated cold deformation control for hot piercing high-temperature intermediate solution heat treatment, deformation quantity and distribution is adopted. As stated above, the preparation method for a welded and extended super duplex stainless steel seamless pipe coil for a deep-sea umbilical cable according to the invention can meet the standards for deep-sea oil and gas production and use and ensure that the coil has high mechanical properties, good pitting resistance, and is suitable for a harsh seawater application medium environment in deep-sea water.