Ferritic Steel Weld Heat Treatment Against Type IV Damage
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Solution Overview
Problem
Ferritic heat-resistant steel used in high-temperature applications suffers from Type IV damage in the weld heat-affected zone, leading to creep deformation and fracture, which is difficult to prevent due to the coarsening of carbides and loss of precipitation strengthening properties during thermal cycles.
Innovation Solution
A method involving a pre-weld heat treatment that completely dissolves carbides in the weld heat-affected zone by heating to temperatures above the Ac3 point for 2 minutes or longer, followed by a post-weld heat treatment to control the precipitation of M23C6-type carbides on large-angle grain boundaries, optimizing the chemical composition to prevent Type IV damage without adding boron in large concentrations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pre-weld heat treatment is applied to dissolve carbides in the weld heat-affected zone, then Type IV damage is prevented, but additional processing time and complexity are required
Solution Approach 1:
The patent applies pre-weld heat treatment to completely dissolve carbides in the weld heat-affected zone before welding occurs. This preliminary action prevents carbide coarsening during the welding thermal cycle, thereby preventing Type IV damage and maintaining creep rupture strength comparable to the base material.
Solution Approach 2:
The patent specifies precise heat treatment parameters: heating to temperatures above the Ac3 point (specifically 1000-1200°C) and holding for 2 minutes or longer. These parameter changes ensure complete carbide dissolution while controlling the transformation to austenite, which prevents Type IV damage when followed by controlled cooling.
2Reliability
If heating temperature is increased above Ac3 point for carbide dissolution, then Type IV damage is prevented, but energy consumption increases
Solution Approach 1:
The patent optimizes the heat treatment temperature range to 1000-1200°C, which is above the Ac3 point for complete carbide dissolution but not excessively high. The holding time is minimized to 2 minutes or longer, balancing thorough carbide dissolution with energy efficiency. This parameter optimization prevents Type IV damage while controlling energy consumption.
3Strength
If post-weld heat treatment is applied to control carbide precipitation, then creep strength is maintained, but processing time and cost increase
Solution Approach 1:
The patent specifies post-weld heat treatment parameters of 720-780°C for 30 minutes or longer. This temperature range controls the precipitation of M23C6-type carbides on large-angle grain boundaries, maintaining creep rupture strength. The time parameter is optimized to achieve sufficient precipitation without excessive processing time.
Solution Approach 2:
The patent focuses heat treatment on the weld heat-affected zone where carbide precipitation is most critical for preventing Type IV damage. By controlling carbide distribution locally in this region rather than treating the entire structure, the patent maintains creep strength where needed while reducing overall processing time and cost.
4Strength
If B is added in large concentrations to prevent Type IV damage, then creep strength improves, but material cost increases
Solution Approach 1:
The patent removes boron from the chemical composition entirely (B: 0.0005-0.005 mass%, effectively eliminating the need for large concentrations). Instead of relying on boron addition to prevent Type IV damage, the patent uses controlled heat treatment processes to achieve complete carbide dissolution and controlled re-precipitation, thereby maintaining creep rupture strength without expensive boron additives.
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 method effectively prevents Type IV damage by maintaining the creep rupture strength of the weld joint comparable to the base material, ensuring long-term durability and preventing weld cracking, while maintaining on-site operability and economic viability.
Implementation Method 1
a pre-weld heat treatment that completely dissolves carbides in the weld heat-affected zone by heating to temperatures above the Ac3 point for 2 minutes or longer
Implementation Method 2
completely dissolves carbides in the weld heat-affected zone by heating to temperatures above the Ac3 point
Implementation Method 3
a post-weld heat treatment to control the precipitation of M23C6-type carbides on large-angle grain boundaries
Implementation Method 4
a welded structure of a ferritic heat-resistant steel having portions joined together by welding
Data Source
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AI summary
A method of manufacturing a welded structure of a ferritic heat-resistant steel is provided that prevents Type IV damage and that has good on-site operability without adding a high B concentration. The method includes: the step of preparing a base material including 8.0 to 12.0 % Cr, less than 0.005 % B and other elements; the step of forming an edge on the base material; a pre-weld heat treatment step in which a region located between a surface of the edge and a position distant from the surface of the edge by a pre-weld heat treatment depth of 30 to 100 mm is heated to a temperature of 1050 to 1200 °C and is held at this temperature for 2 to 30 minutes; a welding step in which the edge is welded to form the weld metal; and a post-weld heat treatment step in which a region located between the surface of the edge and a position distant from the surface of the edge by a distance not smaller than the pre-weld heat treatment depth and not greater than 100 mm is heated to a temperature of 720 to 780°C and is held at this temperature for a time period not shorter than 30 minutes and satisfying the following formula, (1): Logt+12⋅T+273<13810