Seamless Stainless Steel Pipe Microstructure for Sour Service Toughness
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Solution Overview
Problem
Existing stainless steel seamless pipes used in oil country tubular goods lack sufficient high-temperature corrosion resistance, sulfide stress cracking resistance, acid-environment corrosion resistance, and low-temperature toughness, especially in severe environments containing CO2, Cl−, H2S, and high temperatures.
Innovation Solution
A stainless steel seamless pipe with a specific composition and microstructure, including elements like Cr, Mo, Cu, Ni, and Sb, and a manufacturing process involving hot working, quenching, and tempering, to achieve a yield strength of 758 MPa or more, with excellent corrosion resistance and low-temperature toughness.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If 13Cr martensitic stainless steel pipes are used in high-temperature environments, then the strength is maintained, but the corrosion resistance in severe high-temperature corrosive environments containing CO2, Cl−, and H2S is insufficient
Solution Approach 1:
The patent applies parameter changes by optimizing the chemical composition parameters of the stainless steel, specifically setting Cr content at 15.0-19.0%, Mo at 2.0-3.0%, Cu at 0.3-3.5%, Ni at 3.0-5.0%, and controlling C at 0.05% or less, along with specific relationships between these elements (e.g., Cr+1.5Mo+0.5Cu≥18.0, Ni/Cr≤0.33). These compositional parameter changes enable the steel to achieve both high strength and improved corrosion resistance in severe high-temperature environments containing CO2, Cl−, and H2S.
Solution Approach 2:
The patent creates a composite microstructure consisting of multiple phases: 45% or more tempered martensitic phase, 20-40% ferrite phase, and 10-25% retained austenite phase by volume. This composite microstructure combines the high strength of martensite with the corrosion resistance and ductility contributions from ferrite and retained austenite, resolving the contradiction between strength and corrosion resistance in high-temperature environments.
2Strength
If higher strength is achieved through increased alloying, then the yield strength increases, but the corrosion resistance and low-temperature toughness may deteriorate
Solution Approach 1:
The patent achieves high yield strength (862 MPa or more) through precise parameter control of alloying elements and their relationships, while simultaneously maintaining corrosion resistance and low-temperature toughness. The specific compositional parameters and their interrelationships ensure that strength enhancement does not compromise other critical properties.
Solution Approach 2:
The multi-phase composite microstructure (tempered martensite + ferrite + retained austenite) provides a synergistic effect where tempered martensite contributes to high strength, ferrite enhances corrosion resistance and ductility, and retained austenite improves toughness including low-temperature toughness. This composite structure resolves the contradiction between strength and other mechanical properties.
3Strength
If the microstructure is optimized for high strength, then the yield strength increases, but the low-temperature toughness may decrease
Solution Approach 1:
The patent employs a composite microstructure where tempered martensitic phase (≥45% by volume) provides high yield strength (862 MPa or more), while ferrite phase (20-40% by volume) and retained austenite phase (10-25% by volume) contribute to low-temperature toughness. The ferrite phase acts as a ductile component that prevents brittle fracture at low temperatures, while the retained austenite provides strain hardening capability, thus resolving the contradiction between high strength and low-temperature toughness.
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 pipe exhibits a corrosion rate of 0.127 mm/y or less in a 20% NaCl solution at 200°C, sulfide stress cracking resistance under varying stress, and a Charpy impact test absorption energy of 200 J or more at −40°C, while maintaining high strength.
Implementation Method 1
hot working a steel pipe material into a seamless steel pipe
Implementation Method 2
quenching that reheats the seamless steel pipe to a temperature of 850 to 1,150° C., and cools the seamless steel pipe at a cooling rate of air cooling or faster
Implementation Method 3
cools the seamless steel pipe at a cooling rate of air cooling or faster until a pipe surface reaches a cooling stop temperature of 50° C. or less
Implementation Method 4
tempering that heats the quenched seamless steel pipe to a temperature of 500 to 650° C.
Data Source
AI summary
Provided herein is a stainless steel seamless pipe having a composition that contains, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01% or more and 1.0% or less, P: 0.05% or less, S: 0.005% or less, Cr: 15.2% or more and 18.5% or less, Mo: 1.5% or more and 4.3% or less, Cu: 1.1% or more and 3.5% or less, Ni: 3.0% or more and 6.5% or less, Al: 0.10% or less, N: 0.10% or less, O: 0.010% or less, and Sb: 0.001% or more and 1.000% or less, and in which C, Si, Mn, Cr, Ni, Mo, Cu, and N satisfy the predetermined formula, and the balance is Fe and incidental impurities, the stainless steel seamless pipe having a microstructure containing 30% or more martensitic phase, 65% or less ferrite phase, and 40% or less retained austenite phase by volume.