Stainless Steel Pipe Surface Martensite for Corrosion and Workability
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Solution Overview
Problem
Conventional high-strength seamless stainless steel pipes for oil country tubular goods face challenges in achieving excellent hot workability, sulfide stress cracking resistance, and corrosion resistance while minimizing the use of expensive alloy chemical elements, which often leads to deterioration in pipe-making capability.
Innovation Solution
The development of a stainless steel pipe with a surface layer microstructure featuring a white phase formed by controlling the concentration of Cr and Ni, which improves corrosion resistance, hot workability, and sulfide stress cracking resistance, achieved by controlling the oxygen concentration and heating conditions during the manufacturing process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If large amount of expensive alloy chemical elements are added to achieve satisfactory corrosion resistance, then corrosion resistance is improved, but hot workability deteriorates
Solution Approach 1:
The invention creates a dual microstructure with different properties in different regions: the surface layer contains a white phase (martensite) that provides excellent corrosion resistance, while the base material maintains an austenite-ferrite dual phase structure that ensures good hot workability. This local differentiation allows each region to optimize its function without compromising the other.
Solution Approach 2:
The white phase is formed on the surface layer before the pipe making process through controlled oxidation during heating. This preliminary formation of the protective surface layer ensures that subsequent manufacturing processes benefit from both the corrosion resistance of the white phase and the good hot workability of the underlying austenite-ferrite structure.
2Reliability
If large amount of alloy chemical elements are added to achieve satisfactory corrosion resistance, then corrosion resistance is improved, but pipe making capability deteriorates
Solution Approach 1:
The invention creates a dual microstructure with different properties in different regions: the surface layer contains a white phase (martensite) that provides excellent corrosion resistance, while the base material maintains an austenite-ferrite dual phase structure that ensures good hot workability. This local differentiation allows each region to optimize its function without compromising the other.
Solution Approach 2:
The invention changes the microstructural parameters of the steel by controlling the heating conditions to form a white phase on the surface. By adjusting the heating temperature and atmosphere composition, the surface layer transforms to contain martensite phase while the base material maintains austenite-ferrite dual phase, achieving both corrosion resistance and manufacturability.
3Strength
If conventional heat treatment is performed to achieve high strength, then strength is improved, but corrosion resistance may be compromised
Solution Approach 1:
The white phase is formed on the surface layer before the pipe making process through controlled oxidation during heating. This preliminary formation of the protective surface layer ensures that subsequent manufacturing processes benefit from both the corrosion resistance of the white phase and the good hot workability of the underlying austenite-ferrite structure.
Solution Approach 2:
The invention creates a composite microstructure where the surface layer contains a white phase (martensite) and the base material contains an austenite-ferrite dual phase structure. This composite structure combines the corrosion resistance of martensite with the ductility and hot workability of austenite-ferrite, achieving both high strength and excellent corrosion resistance.
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 steel pipe exhibits high strength, excellent corrosion resistance in harsh environments with CO2 and Cl-, and improved hot workability, while reducing material costs and enhancing productivity.
Implementation Method 1
a microstructure including a tempered martensite phase, a martensite phase, and a retained austenite phase
Implementation Method 2
performing a two-step heat treatment including a heat treatment at a temperature equal to or lower than a temperature at which an austenite phase fraction is 80% and a heat treatment at a temperature equal to or lower than a temperature at which an austenite phase fraction is 60%
Implementation Method 3
performing a quenching treatment including performing heating to a temperature equal to or higher than the Ac3 transformation temperature and then performing cooling to room temperature at a cooling rate equal to or larger than that of air cooling
Implementation Method 4
performing a tempering treatment at a temperature equal to or lower than the Ac1 transformation temperature
Data Source
Figure 1
AI summary
Provided are a high-strength seamless stainless steel pipe for oil country tubular goods excellent in terms of hot workability, sulfide stress cracking resistance, and corrosion resistance and a method for manufacturing the steel pipe. The steel pipe has a chemical composition containing Cr and Ni so that the relationship Cr/Ni ≤ 5.3 is satisfied and a microstructure including mainly a tempered martensite phase in which a surface layer microstructure includes a phase which looks white when subjected to etching with a Vilella etching solution, which has a thickness in the wall thickness direction from the outer surface of the pipe of 10 µm or more and 100 µm or less, and which disperses in the outer surface of the pipe in an amount of 50% or more in terms of area fraction. The chemical composition may be a chemical composition containing, by mass%, C: 0.005% or more and 0.05% or less, Si: 0.05% or more and 1.50% or less, Mn: 0.2% or more and 1.8% or less, P: 0.02% or less, S: 0.005% or less, Cr: 11% or more and 18% or less, Ni: 0.10% or more and 8.0% or less, Mo: 0.6% or more and 3.5% or less, and the balance being Fe and inevitable impurities.