Hot Water Tank Weld Composition Without Back Gas Sealing
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Solution Overview
Problem
The production cost of hot water tanks is increased due to the necessity of back gas sealing for corrosion resistance in the weld zones, which is a troublesome and gas-intensive process.
Innovation Solution
A hot water tank design with a TIG-welded back bead on the inner surface using steel members with a Cr content of 21-25% by mass, where the Cr concentration in the heat-affected zone is maintained at least 16% by mass in a depth region of 10 nm from the steel basis material/oxide scale interface, eliminating the need for back gas sealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If back gas sealing is performed to prevent corrosion in the weld zone, then corrosion resistance is improved, but production cost and process complexity increase
Solution Approach 1:
The invention changes the chemical composition parameters of the steel material, specifically increasing Cr content to 21-25% by mass and controlling other alloying elements, to achieve sufficient corrosion resistance without requiring back gas sealing. This parameter change in material composition resolves the contradiction by making the back side corrosion-resistant through material properties rather than process measures.
2Reliability
If back gas sealing is performed to prevent corrosion in the weld zone, then corrosion resistance is improved, but production cost increases due to gas consumption
Solution Approach 1:
By changing the material composition parameters to include 21-25% Cr and controlled amounts of other elements, the invention eliminates the need for back gas sealing operations. This resolves the contradiction by achieving corrosion resistance through material composition rather than consuming inert gas during welding.
3Reliability
If back gas sealing is performed to prevent corrosion in the weld zone, then corrosion resistance is improved, but additional flanges and equipment are required
Solution Approach 1:
The invention resolves this contradiction by changing the material composition to achieve inherent corrosion resistance, which eliminates the need for additional flanges and sealing equipment. The simplified structure welds directly without requiring extra components for back gas sealing.
4Quantity of substance
If conventional steel with lower Cr content is used, then production cost is reduced, but corrosion resistance in the weld zone deteriorates
Solution Approach 1:
The invention optimizes the Cr content parameter to 21-25% by mass, which is higher than conventional stainless steel but controlled to balance performance and cost. This parameter optimization achieves sufficient corrosion resistance while managing material costs, resolving the contradiction between cost and reliability.
Solution Approach 2:
The invention uses a composite alloy composition combining Cr (21-25%), Ni (0.1-1%), Mo (0.1-2%), and other elements in specific proportions to achieve enhanced corrosion resistance. This composite material approach resolves the contradiction by creating a cost-effective alloy formulation that outperforms conventional single-element additions.
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 design enhances corrosion resistance in the weld zone without increasing production costs, allowing for a simpler structure and higher cost performance by preventing crevice corrosion and reducing the need for additional flanges and gas usage.
Implementation Method 1
the formation of oxide scale in welding... the Cr concentration in the steel basis material in the area having oxide scale soluble in an aqueous chloride solution
Data Source
Figure 1
Figure 2~3
Figure 4~5
AI summary
A hot water tank having, on the inner surface thereof, a back bead formed by TIG-welding with no back gas sealing of steel members to each other having a composition comprising, % by mass, C: at most 0.025 %, Si: at most 1 %, Mn: at most 1 %, P: at most 0.045 %, S: at most 0.01 %, Ni: from 0.1 to 1 %, Cr: from more than 21 to 25 %, Mo: from 0.1 to 2 %, Al: from 0.02 to 0.3 %, N: at most 0.025 %, and Cu: from 0 to 1 %, and containing at least one of Ti: from 0.05 to 0.4 % and Nb: from 0. 05 to 0. 5 %, with the balance of Fe and inevitable impurities, wherein the Cr concentration in the steel basis material in the area having oxide scale soluble in an aqueous chloride solution, as formed therein in the heat-affected zone on the welded back side thereof, is at least 16 % by mass in the depth region of at least 10 nm from the steel basis material/oxide scale interface.