Double Pipe Welded Structure Using Austenitic Ferritic Steel
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Solution Overview
Problem
High-temperature equipment, such as furnace wall tubes, face challenges with weld crack resistance due to high temperature and pressure conditions, particularly in the use of 9%Cr and 2.25%Cr heat resistant ferritic steel and heat resistant austenitic steel, where preheating and postheating are costly and not always effective in preventing low-temperature and hot cracking issues during fillet welding.
Innovation Solution
A double tube structure is proposed, where the inside tube is made of heat resistant ferritic steel and the outside tube is made of heat resistant austenitic steel, with specific wall thickness conditions to prevent weld penetration and hydrogen diffusion, thereby reducing the occurrence of low-temperature and liquation cracks during submerged arc welding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If preheating and postheating are performed during fillet welding of heat resistant ferritic steel, then weld crack resistance is improved, but production cost and manufacturing complexity increase
Solution Approach 1:
The invention changes the material parameter by using heat resistant austenitic steel instead of heat resistant ferritic steel for the tube requiring fillet welding. This material substitution fundamentally alters the welding characteristics, allowing fillet welding to be performed without preheating or postheating while maintaining excellent weld crack resistance, thus resolving the contradiction between reliability and ease of manufacture
Solution Approach 2:
The invention employs a composite structure with an inside tube made of heat resistant ferritic steel and an outside tube made of heat resistant austenitic steel. The outside tube specifically addresses the welding requirement by providing a material that is inherently resistant to both low-temperature and hot cracking during fillet welding, eliminating the need for costly preheating and postheating processes
2Reliability
If preheating and postheating are performed during fillet welding of heat resistant austenitic steel, then hot crack resistance is improved, but production cost and manufacturing complexity increase
Solution Approach 1:
The invention changes the material parameter by selecting heat resistant austenitic steel with specific composition ranges (Cr: 15-35%, Ni: 5-50%) for the outside tube. This material parameter optimization provides inherent hot crack resistance during fillet welding, eliminating the need for preheating and postheating processes and reducing manufacturing costs
3Reliability
If the outside tube wall thickness is increased to prevent weld penetration, then weld crack resistance is improved, but hydrogen diffusion and low-temperature cracking increase
Solution Approach 1:
The invention optimizes the wall thickness parameter of the outside tube to a specific range (0.5-3.0mm) that balances two opposing requirements: it is thick enough to prevent weld penetration and protect the inside tube, yet thin enough to allow hydrogen to diffuse outward rather than accumulate and cause low-temperature cracking in the inside tube
Solution Approach 2:
The outside tube acts as an intermediary layer that mediates between the welding process and the inside tube. It provides protection against weld penetration while its controlled thickness allows it to serve as a hydrogen diffusion pathway, preventing hydrogen accumulation that would lead to low-temperature cracking in the inside tube
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 double tube structure significantly enhances weld crack resistance, allowing for fillet welding without preheating or postheating, thereby reducing production costs and ensuring high-temperature equipment can be manufactured with improved dimensional accuracy and reduced cracking issues.
Implementation Method 1
the outside tube is made of heat resistant austenitic steel, with specific wall thickness conditions to prevent weld penetration and hydrogen diffusion
Implementation Method 2
reducing the occurrence of low-temperature and hot cracking issues during fillet welding
Data Source
Figure 1~2
AI summary
A double tube for welded structure comprises an inside tube of a heat resistant ferritic steel and an outside tube of a heat resistant austenitic steel. The inside tube has a chemical composition which comprises specific amounts of C, Si, Mn P, S, Ni + Cu, Cr, Mo + W, V, Nb, Ti, B, Al, N and O with the balance of Fe and impurities. The outside tube has a chemical composition which comprises specific amounts of C, Si, Mn, P, S, Ni, Cr, Al, N and O with the balance of Fe and impurities. A fillet weld zone, which satisfies Formula of [wall thickness of outside tube ≥ fusion depth in fillet weld + 0.3 mm], is formed on the outside surface of the double tube, and the wall thicknesses (mm) of the outside tube and inside tube satisfy Formula of [wall thickness of outside tube / (wall thickness of outside tube + wall thickness of inside tube) ≤ 0.4]. The double tube is excellent in weld crack resistance. If this double tube is used, a welded structure forming various types of high-temperature equipment can be manufactured by fillet-welding plates, metal fittings, and the like to the outside surface of the tube by means of ordinary submerged arc welding without performing treatment such as preheating, postheating, and cleaning of steel. The inside tube may contain one or more kinds of elements selected from Ca, Mg and REM, and the outside tube may contain one or more kinds of elements selected from Mo, W, Cu, Co, Nb, Ti, V, B, Ca, Mg and REM.