Duplex Stainless Steel Composition for Deep-Sea Pressure Loads
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Solution Overview
Problem
Existing duplex stainless steels lack sufficient strength and corrosion resistance to withstand increased pressures in deep-sea umbilicals, and are prone to sigma phase formation during welding, leading to potential failure and cracks.
Innovation Solution
A duplex stainless steel with balanced compositions of Cr, W, and N, along with controlled nitride and sigma phase content, ensuring high strength and corrosion resistance by limiting quenched-in nitrides and avoiding sigma phase formation through specific element ranges and heat treatment processes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stress or pressure
If wall thickness is increased to withstand higher pressure, then pressure resistance is improved, but weight increases and strength-to-weight ratio deteriorates
Solution Approach 1:
The patent changes the material parameters by increasing Cr content to 25-35%, W content to 2.0-4.0%, and N content to 0.30-0.55%, while controlling C content to max 0.030%. These compositional parameter changes enable the steel to achieve higher strength and corrosion resistance, allowing thinner walls to withstand the required pressure, thus reducing weight while maintaining pressure resistance.
2Reliability
If Cr content is increased to improve corrosion resistance, then corrosion resistance is improved, but sigma phase formation during welding increases
Solution Approach 1:
The patent changes the parameters by increasing Cr to 25-35% while simultaneously adding W (2.0-4.0%) and N (0.30-0.55%), and controlling C to max 0.030%. This multi-parameter change strategy achieves high corrosion resistance through elevated Cr while the specific combination with W and N, along with low C, suppresses sigma phase formation during welding, resolving the contradiction between corrosion resistance and weldability.
3Strength
If strength is increased to carry own weight at higher wall thickness, then mechanical strength is improved, but ductility and toughness deteriorate
Solution Approach 1:
The patent achieves high strength (Rp0.2 ≥ 650 MPa) through specific compositional parameters: Cr 25-35%, W 2.0-4.0%, N 0.30-0.55%, with C max 0.030%. The nitrogen content specifically contributes to strength while the balanced composition maintains ductility (≥40% elongation) and toughness, avoiding the typical strength-ductility trade-off through optimized multi-element composition.
Data Source
AI summary
The present invention relates to a duplex stainless steel comprising the following elements in weight %: C max 0.030; Si max 0.30; Mn 0.20 to 2.50; P max 0.030; S max 0.030; Cr 28.5 to 30.5; Ni 6.0 to 8.0; Mo 0.70 to 3.00; W 2.00 to 4.40; Cu≤0.50; N 0.30 to 0.55; Balance is Fe and unavoidable impurities; and wherein the ferrite content is of 40-60 vol %; and wherein the duplex stainless steel fulfills the requirements of: a. [Cr]+4.0*[Mo]+2.0*[W]<42.5 wherein the values of [Cr], [Mo] and [W] are in weight %; and b. having less than 10% coverage of quenched-in nitrides in the ferrite grain boundaries when in solution annealed condition. The present invention also relates to objects made of the present duplex stainless steel having a Rp0.2 higher than 650 MPa, ISO 6892-1 2019.