Duplex Stainless Steel Composition for Urea Stripper Corrosion
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Solution Overview
Problem
Existing duplex stainless steels used in urea production plants face high passive corrosion rates at elevated temperatures and microstructural instability, particularly in stripper tubes, necessitating improved corrosion resistance and structural stability.
Innovation Solution
A duplex stainless steel composition with specific weight percentages of C, Si, Mn, Cr, Ni, Mo, W, N, S, and P, along with a Mo+W content between 3.0 and 4.0 wt%, optimized for reduced passive corrosion and structural stability, particularly in high-temperature carbamate environments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If existing duplex stainless steel compositions are used in urea production plants, then the equipment can operate in corrosive environments, but the passive corrosion rate increases at elevated temperatures
Solution Approach 1:
The patent applies parameter changes by precisely controlling the chemical composition parameters of the duplex stainless steel. Specifically, it limits C to ≤0.030%, Si to ≤0.8%, Mn to ≤2.0%, Cr to 29.0-31.0%, Ni to 5.0-9.0%, Mo to <4.0%, W to <4.0%, N to 0.25-0.45%, Cu to ≤2.0%, S to ≤0.02%, and P to ≤0.03%, with Mo+W content between 3.0-4.0%. This compositional parameter optimization reduces passive corrosion rates at elevated temperatures while maintaining corrosion resistance in urea production environments.
Solution Approach 2:
The patent utilizes composite materials by creating a duplex stainless steel with a controlled two-phase microstructure (ferritic and austenitic phases). The specific composition ratio and phase distribution are optimized to achieve both corrosion resistance and low passive corrosion rates. The ferritic phase provides corrosion resistance while the austenitic phase contributes to ductility and toughness, creating a synergistic composite material system.
2Temperature
If existing duplex stainless steel is used in high-temperature carbamate environments, then equipment can function, but microstructural instability occurs
Solution Approach 1:
The patent applies parameter changes by optimizing alloying element concentrations to stabilize the duplex microstructure at high temperatures. The controlled levels of Cr (29.0-31.0%), Ni (5.0-9.0%), and N (0.25-0.45%) specifically prevent phase transformations and microstructural degradation in high-temperature carbamate environments, ensuring long-term structural stability.
Solution Approach 2:
This principle does not apply to this patent as the invention focuses on creating durable, long-lasting stainless steel material rather than temporary or disposable components.
Data Source
AI summary
A corrosion resistant duplex stainless steel (ferritic austenitic alloy) is suitable for use in a plant for the production of urea. Uses of the corrosion resistant duplex stainless steel include in objects made of said duplex stainless steel, in methods for the production of urea, in a plant for the production of urea comprising one or more parts made from said duplex stainless steel, and in methods of modifying an existing plant for the production of urea. In one aspect, the composition consists of in weight % (wt %): C 0.010 to 0.015; S 0.08 to 0.23; Mn 1.0 to 1.05; Cr 29.07 to 29.92; Ni 5.76 to 7.17; Mo 3.0 to 4.0; W max 2.55; N 0.3 to 0.35; Cu max 0.01; S max 0.008; P max 0.008; at least one of Ti, Nb, Hf, Ca, Ba, V, and B max 0.5 total; balance Fe and unavoidable occurring impurities.