Thin-Walled Pipe Welding with Peak Temperature Control
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Solution Overview
Problem
Air-hardening steel alloys used in steam generator boiler walls, such as T23 and T24, tend to harden during welding, leading to crack formation and leakage due to geometric irregularities, high pressure, and high temperatures, especially in thin-walled pipes, which existing methods like post-heat treatment or welding above the martensite starting temperature are impractical or ineffective.
Innovation Solution
A multi-layer weld seam method using TIG welding with a root layer, filling layer, and cover layers, where the cover layers are welded with optimized parameters to maintain the heat-affected zone within a temperature range of 600°C to 1000°C, reducing hardness and preventing martensite formation, and using a pendulum welding technique to moderate heat input.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If air-hardening steel alloys (T23, T24) are used for boiler walls to meet heat dissipation requirements, then heat dissipation performance is improved, but hardening during welding occurs leading to crack formation and reduced reliability
Solution Approach 1:
The invention changes the thermal parameter by controlling the temperature in the heat-affected zone during welding. By maintaining the temperature between Ac1 and Ac3 transformation points through optimized welding parameters (current, speed, layers), the material undergoes phase transformation that prevents hardening and martensite formation, thus resolving the contradiction between heat dissipation performance and crack resistance
Solution Approach 2:
The invention utilizes phase transitions of steel during welding by controlling the temperature to stay within the Ac1-Ac3 range where austenite formation occurs. This phase transition prevents the formation of hard and brittle martensite structure, thereby preventing cracks while maintaining the air-hardening alloy properties for heat dissipation
2Reliability
If post-heat treatment with tempering temperature is applied to eliminate hardening, then crack resistance is improved, but device complexity and process time increase significantly
Solution Approach 1:
The invention extracts or removes the need for separate post-heat treatment processes by incorporating the hardening prevention directly into the welding process itself. Through controlled temperature management during welding, the harmful hardening is prevented from occurring in the first place, eliminating the requirement for additional tempering operations and reducing overall process complexity
Solution Approach 2:
The invention performs preliminary action by preventing hardening during the welding process itself rather than addressing it afterward. By controlling thermal parameters and maintaining temperature in the Ac1-Ac3 range during welding, the material structure is pre-conditioned to avoid hardening, thus eliminating the need for subsequent corrective heat treatment
3Reliability
If welding is performed above the martensite starting temperature to prevent hardening, then crack resistance is improved, but material properties deteriorate due to overheating structure
Solution Approach 1:
The invention optimizes the temperature parameter by establishing a precise control range between Ac1 and Ac3 transformation points. This optimized parameter range prevents both hardening (by avoiding martensite formation) and overheating damage (by not exceeding Ac3), thus simultaneously improving crack resistance while preserving material strength properties
Solution Approach 2:
The invention applies dynamic control of welding parameters (current, speed, number of layers) to maintain the temperature within the optimal Ac1-Ac3 range throughout the welding process. This dynamic adjustment ensures the temperature remains high enough to prevent hardening but low enough to avoid overheating, resolving the contradiction between crack resistance and material strength
4Productivity
If thin-walled pipes (≤10 mm) are welded with conventional parameters, then welding speed is maintained, but hardening occurs in the heat-affected zone leading to cracks
Solution Approach 1:
The invention changes the welding parameters (current, speed, number of layers) to control the thermal input and maintain temperature in the Ac1-Ac3 range during welding of thin-walled pipes. This parameter optimization prevents hardening in the heat-affected zone while maintaining efficient welding speeds, thus resolving the contradiction between productivity and crack resistance
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
This method effectively reduces hardness in the heat-affected zone, minimizing the risk of crack formation and stress corrosion, ensuring a reliable and durable connection for air-hardening steel alloys in steam generator boiler walls.
Implementation Method 1
The welding process used is so-called tungsten inert gas welding (TIG welding)
Implementation Method 2
in the area of the root layer 20 on the inside 14 in the area of a heat-affected zone (Z) there is a temperature in an optimization temperature range of 600° C. to 1000° C.
Data Source
Figure 1~2
Figure 3~4
Figure 5~6
AI summary
The method comprises providing a joint, extending between an inner side (14) to an outer side (15), between parts (11, 12) to be connected, welding a root layer (20) in an area of the inner side, welding a filling layer (21) adjacent to the root layer, and welding a cover layer adjacent to the filling layer in an area of the outer side with given welding parameters so that a temperature is adjusted in an optimization temperature range in the area of the root layer at the inner side. The method comprises providing a joint, extending between an inner side (14) to an outer side (15), between parts (11, 12) to be connected, welding a root layer (20) in an area of the inner side, welding a filling layer (21) adjacent to the root layer, and welding a cover layer adjacent to the filling layer in an area of the outer side with given welding parameters so that a temperature is adjusted in an optimization temperature range in the area of the root layer at the inner side. One of the two parts or a welding material used in the manufacture of welded joint consists of an air-hardening steel alloy. A weld bead of the cover layer is designed as a pendulum bead. The optimization temperature range lies between a material-specific A(C1)-temperature and A(C3)-temperature of the parts to be connected. A welding current used to weld the cover layer is 85-95 A. The connected parts have a wall thickness of 5.6-6.3 mm. Each of the layers comprises weld bead paths.