Multiphase Seamless Stainless Steel Pipe for Strength and SCC Resistance

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Solution Overview

Problem

Conventional stainless steel seamless pipes used in oil and gas wells lack sufficient corrosion resistance, particularly in severe environments containing carbon dioxide and hydrogen sulfide, and do not meet the required yield strength and stress corrosion cracking resistance.

Innovation Solution

Optimizing the manufacturing process by controlling the microstructural form of the steel pipe, specifically adjusting the average ferrite filling factor to 0.80 or less, and optimizing the chemical composition to include specific ratios of elements such as Cr, Mo, Cu, and Ni, while maintaining a martensite phase of 40% or more and a ferrite phase of 15 to 55%, with a yield strength of 758 MPa or more.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If conventional stainless steel composition is used, then manufacturing cost is reduced, but corrosion resistance in severe environments (CO2, H2S, Cl-) is insufficient

Engineering Contradiction:

Improvecorrosion resistanceVSAvoidmanufacturing cost

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent applies parameter changes by precisely controlling the chemical composition parameters of the stainless steel. Specifically, it sets Cr: 16.0-18.0%, Ni: 4.0-6.0%, Mo: 1.0-3.0%, Cu: 1.0-3.0%, and other elements within specific ranges. This optimized composition parameter set achieves superior corrosion resistance in severe environments (CO2, H2S, Cl-) while maintaining manufacturing feasibility through standard steelmaking processes.

Inventive Principle:

Principle #35Parameter changes

2Strength

If high yield strength is required (758 MPa or more), then structural integrity in severe environments is improved, but stress corrosion cracking resistance may deteriorate

Engineering Contradiction:

Improveyield strengthVSAvoidstress corrosion cracking resistance

Core Design Contradiction:

StrengthVSReliability

Solution Approach 1:

The patent employs composite materials by creating a multi-phase microstructure consisting of martensite (40-70%), retained austenite (10-30%), and ferrite (10-30%). This composite microstructure combines the high strength characteristics of martensite with the corrosion resistance and stress corrosion cracking resistance of retained austenite and ferrite phases, achieving both yield strength ≥758 MPa and excellent stress corrosion cracking resistance.

Inventive Principle:

Principle #40Composite materials

Solution Approach 2:

The patent applies local quality by creating different phases with different properties within the steel microstructure. The martensite phase provides high strength, while the retained austenite and ferrite phases provide corrosion resistance and stress corrosion cracking resistance. This local differentiation of material properties at the microstructural level allows simultaneous achievement of high strength and corrosion resistance.

Inventive Principle:

Principle #3Local quality

3Reliability

If ferrite filling factor is increased to improve corrosion resistance, then stress corrosion cracking resistance improves, but yield strength decreases

Engineering Contradiction:

Improvestress corrosion cracking resistanceVSAvoidyield strength

Core Design Contradiction:

ReliabilityVSStrength

Solution Approach 1:

The patent applies parameter changes by precisely controlling the ferrite filling factor within the specific range of 0.60-0.80. This optimized parameter range, combined with the specific chemical composition (Cr: 16.0-18.0%, Ni: 4.0-6.0%, Mo: 1.0-3.0%, Cu: 1.0-3.0%), achieves the optimal balance between stress corrosion cracking resistance and yield strength (≥758 MPa).

Inventive Principle:

Principle #35Parameter changes

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 resulting stainless steel seamless pipe exhibits high yield strength and excellent stress corrosion cracking resistance, suitable for harsh environments with improved corrosion resistance.

Implementation Method 1

the steel material has a microstructure including a martensite phase, a ferrite phase, and a retained austenite phase

Methodology Applied

Scientific EffectPhase transformation: Phase Change

Implementation Method 2

quenching treatment including heating the seamless steel pipe to a quenching temperature of 850 to 1150°C and then cooling the seamless steel pipe at a cooling rate of 0.01°C/second or more

Methodology Applied

Scientific EffectQuenching: Heat Treatment

Implementation Method 3

tempering treatment including heating the seamless steel pipe to a tempering temperature of 500 to 650°C

Methodology Applied

Scientific EffectTempering: Heat Treatment

Data Source

PatentEP4636112A1Seamless stainless steel pipe and production method therefor

Publication Date: 2025.10.22 JFE STEEL CORP

Figure 1

AI summary

Provided are a stainless steel seamless pipe and a method for manufacturing the same. The stainless steel seamless pipe of the invention has a chemical composition containing, in mass %, C: 0.06% or less, Si: 1.0% or less, Mn: 0.01 to 1.0%, P: 0.05% or less, S: 0.005% or less, Cr: 15.2 to 18.0%, Mo: 1.5 to 4.3%, Cu: 0.5 to 3.5%, Ni: 3.5 to 5.2%, V: 0.5% or less, Al: 0.10% or less, N: 0.10% or less, and O: 0.010% or less, with the balance being Fe and incidental impurities and has a microstructure including a martensite phase at a volume fraction of 40% or more, a ferrite phase at a volume fraction of 15 to 55%, and a retained austenite phase at a volume fraction of 40% or less, with the average of ferrite filling factors being 0.80 or less. The yield strength of the stainless steel seamless pipe is 758 MPa or more.