Duplex Stainless Steel Nitride Precipitation Control
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Solution Overview
Problem
Duplex stainless steel faces challenges in maintaining low temperature toughness due to the precipitation of harmful Al nitrides and Cr nitrides, which decreases corrosion resistance and toughness, especially in high-latitude marine environments where temperatures are below freezing.
Innovation Solution
The composition of duplex stainless steel is optimized within specific ranges for elements like Ni, Cr, Mo, N, and Mn to limit the precipitation of Al nitrides and Cr nitrides, with formulas governing their relationships to ensure high corrosion resistance and toughness, including controlling particle sizes and distributions of these nitrides.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Al and N content are increased to improve corrosion resistance, then pitting resistance is improved, but AlN nitride precipitation is promoted which decreases low temperature toughness
Solution Approach 1:
The invention changes the chemical composition parameters by strictly limiting Al content to 0.005-0.03% and N content to 0.20-0.40%, while optimizing Cr (23-26%), Mo (2-4.0%), and Ni (6-7.5%) contents. This parameter optimization prevents AlN precipitation while maintaining corrosion resistance through controlled alloying element relationships.
2Reliability
If Cr content is increased to improve corrosion resistance, then pitting resistance is improved, but Cr nitride precipitation is promoted which decreases low temperature toughness
Solution Approach 1:
The invention optimizes Cr content within 23-26% and establishes quantitative relationships between Cr, Mo, and N through the formula ([%Cr]+6.5534×[%Mo])2×[%N]≤−215.6×[%Ni]+1708.3×[%Mn]+2150. This controlled parameter relationship prevents Cr nitride precipitation while maintaining necessary corrosion resistance.
3Strength
If Mn content is increased to increase N solubility and prevent nitride precipitation, then low temperature toughness is improved, but σ phase precipitation is promoted which decreases corrosion resistance
Solution Approach 1:
The invention limits Mn content to 0.05-0.30% and establishes a balanced relationship with Ni content through the formula [%Al]×[%N]≤(−22.78×[%Mo]−5×[%Cr]−3.611×[%Ni]+323)×10−4. This controlled parameter relationship prevents both nitride precipitation and excessive σ phase formation, achieving simultaneous improvement in toughness and corrosion resistance.
4Reliability
If Ni content is increased to improve corrosion resistance and austenite stability, then pitting resistance is improved, but ferrite phase stability decreases which promotes nitride precipitation
Solution Approach 1:
The invention optimizes Ni content within 6-7.5% and establishes quantitative relationships with Cr and Mo through the formula [%Al]×[%N]≤(−22.78×[%Mo]−5×[%Cr]−3.611×[%Ni]+323)×10−4. This balanced parameter relationship maintains ferrite phase stability while ensuring adequate corrosion resistance through controlled austenite formation.
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 optimized composition achieves superior low temperature toughness and corrosion resistance, as evidenced by impact values exceeding 87.5 J/cm2 at -46°C, making it suitable for severe corrosive environments such as marine structures and oil well components.
Implementation Method 1
hard and brittle nitrides mainly containing Cr, Al and N are easily deposited. In a case in which the nitrides represented by these AlN or Cr2N are deposited, toughness of the material is decreased
Data Source
AI summary
A duplex stainless steel has reduced precipitation risks of Al nitride and Cr nitride which are undesirable precipitates, and has superior low temperature toughness. The duplex stainless steel has in mass %, indicated as “%”, C: 0.001 to 0.030%, Si: 0.05 to 0.5%, S: not more than 0.002%, Ni: 6 to 7.5%, Cr: 23 to 26%, Mo: 2 to 4.0%, N: 0.20 to 0.40%, Al: 0.005 to 0.03%, Mn: 0.05 to 0.3%, B: 0.0001 to 0.0050% and Fe, and the remainder being inevitable impurities. The duplex stainless steel has an impact value of not less than 87.5 J/cm2 at −46±2° C. as defined in Japanese Industrial Standards Z2242.