Corrosion Resistant Steel Microstructure for Oil and Gas
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Solution Overview
Problem
Current steel grades used in oil and gas wells lack sufficient corrosion resistance and fracture toughness while being difficult to produce in hot conditions, especially for applications requiring high yield strength and resistance to sulphide stress cracking and high-temperature corrosion.
Innovation Solution
A stainless steel with a chemical composition of 0.005-0.03% C, 14-17% Cr, 2.3-3.5% Mo, 3.2-4.5% Ni, Si ≤ 0.6%, 0.4-1.3% Mn, 0.35-0.6% V, 0.02-0.05% N, and balanced by Fe and impurities, processed through hot forming, austenitization, quenching, and tempering to achieve a microstructure of 30-50% ferrite, 5-15% austenite, and 35-65% martensite, with minimal intermetallics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If standard martensitic stainless steel (13% Cr) is used, then manufacturing cost is low and production is easy, but corrosion resistance and fracture toughness are insufficient
Solution Approach 1:
The patent changes the chemical composition parameters by increasing Cr content to 14-17% (from standard 13%), adding specific amounts of Mo (2.3-3.5%), Ni (3.2-4.5%), and other alloying elements. These parameter changes transform the steel from standard martensitic to a modified composition that achieves both high corrosion resistance and hot workability, resolving the contradiction between improved reliability and manufacturing ease.
Solution Approach 2:
The patent creates a composite microstructure containing multiple phases (ferrite 30-50%, austenite 5-15%, martensite 35-65%) within the steel material. This composite microstructure combines the corrosion resistance of ferrite, the ductility of austenite, and the strength of martensite, achieving superior overall performance while maintaining manufacturability through controlled phase distribution.
2Reliability
If duplex material is used, then corrosion resistance is improved, but mechanical properties do not match requirements for production tubing
Solution Approach 1:
The patent creates a tri-phase composite microstructure (ferrite + austenite + martensite) that improves upon duplex material by adding the martensitic phase for enhanced strength and mechanical properties. The controlled distribution of all three phases provides both the corrosion resistance of ferrite/austenite and the mechanical strength required for production tubing applications.
Solution Approach 2:
The patent applies local quality by distributing different phases in specific proportions and locations within the microstructure. The ferrite phase (30-50%) provides corrosion resistance in regions exposed to corrosive environments, while martensite (35-65%) provides strength in load-bearing regions, achieving localized optimization of both corrosion resistance and mechanical properties.
3Reliability
If low-C, high-Cr alloy steel is used, then corrosion resistance in severe environments is improved, but hot condition production becomes difficult
Solution Approach 1:
The patent modifies the composition parameters by optimizing Cr content to 14-17% (not excessively high), controlling C content to 0.005-0.03%, and adding specific amounts of Ni (3.2-4.5%) and Mo (2.3-3.5%). These parameter changes reduce hot cracking susceptibility while maintaining corrosion resistance, enabling successful hot condition production.
Solution Approach 2:
The patent uses Ni and Mo as intermediary elements that mediate between the conflicting requirements of high corrosion resistance and hot workability. These elements improve hot ductility and reduce hot cracking tendency, allowing the steel to be produced in hot conditions while maintaining the high Cr content needed for corrosion resistance.
4Strength
If yield strength is increased to at least 758 MPa, then mechanical strength is improved, but ductility and fracture toughness may deteriorate
Solution Approach 1:
The patent creates a composite microstructure where martensite (35-65%) provides the high yield strength (≥758 MPa), while ferrite (30-50%) and retained austenite (5-15%) provide ductility and fracture toughness. The interaction between these phases creates a synergistic effect where the hard martensite provides strength and the softer ferrite/austenite phases provide toughness, preventing brittleness despite high strength.
Solution Approach 2:
The patent applies local quality by creating regions with different phase compositions and properties. The martensitic regions provide high strength locally, while ferritic and austenitic regions provide ductility and toughness locally. This spatial distribution of properties allows the material to achieve both high yield strength and adequate fracture toughness simultaneously.
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 steel achieves a yield strength of at least 758 MPa, improved corrosion resistance, and fracture toughness of at least 68 J at -10°C, with a corrosion rate below 0.13 mm/year and excellent sulphide stress corrosion cracking resistance, making it suitable for production tubing and liners.
Implementation Method 1
hot formed at a temperature comprised between 1150° C. and 1260° C. through commonly known hot forming processes
Implementation Method 2
the tube is heated up to a temperature AT comprised between 920° C. and 1050° C. and kept at the temperature AT during a time comprised between 5 and 30 minutes
Implementation Method 3
followed by cooling to the ambient temperature to obtain a quenched tube
Implementation Method 4
the quenched tube is heated up to a temperature TT comprised between 500° C. and 700° C. and kept at the temperature TT during a time Tt comprised between 5 and 60 minutes followed by cooling to the ambient temperature
Data Source
AI summary
A corrosion resistant steel having a yield strength of at least 758 MPa is described. The corrosion resistant steel comprises in weight %: 0.005≤C<0.03, 14≤Cr≤17, 2.3≤Mo≤3.5, 3.2≤Ni≤4.5, Si≤0.6, 0.5≤Cu≤1.5, 0.4≤Mn≤1.3, 0.35≤V≤0.6, 3.2×C≤Nb≤0.1, W≤1.5, 0.5≤Co≤1.5, 0.02≤N≤0.05, Ti≤0.05, P≤0.03, S≤0.005, Al≤0.05, with the balance of the chemical composition of said corrosion resistant steel being constituted by Fe and inevitable impurities. A manufacturing method of such steel to obtain a quenched and tempered semi finished product is also described.