Overlay Welding Alloy for Reaction Tube Projections Against Coking
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Reaction tubes used in olefin production face challenges with oxidation, carburization, and nitridization due to projections on their inner surfaces, leading to decreased heat transfer efficiency and increased pressure loss, as well as the risk of coke attachment and frequent decoking tasks.
Innovation Solution
An alloy for overlay welding containing a high amount of Al, along with Nb, is used to form NbC, which enhances creep strength and reduces carbon concentration, allowing for the formation of an alumina barrier layer on projections, thereby preventing coke attachment and maintaining heat transfer efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If projections are formed on the inner surface of the tube main body to increase heat transfer efficiency and decrease pressure loss, then heat transfer efficiency is improved and pressure loss is reduced, but the projections are more exposed to gas and thus more influenced by oxidation, carburization, and nitridization
Solution Approach 1:
The invention applies a specific alloy composition (containing 2.0-5.0% Al, 0.05-2.0% Nb, and controlled C content) specifically to the projections through overlay welding, giving them different chemical properties than the base material. This local quality change enables the projections to form protective alumina barrier layers that resist oxidation, carburization, and nitridization while maintaining their heat transfer function.
Solution Approach 2:
The invention creates a composite structure where the projection alloy (with specific Al-Nb-C composition) is overlay-welded onto the tube main body. This composite material approach combines the heat transfer efficiency of projected surfaces with the corrosion resistance of alumina-forming alloy composition, resolving the contradiction between heat transfer performance and resistance to harmful chemical factors.
2Reliability
If a large amount of Al is added to the alloy for overlay welding to form alumina barrier layer, then oxidation resistance is improved, but weldability and elongation deteriorate
Solution Approach 1:
The invention optimizes the Al content to a specific range (2.0-5.0%) rather than using excessive amounts, and combines it with Nb addition (0.05-2.0%). This parameter optimization allows sufficient alumina barrier layer formation for oxidation resistance while maintaining weldability and elongation properties within acceptable ranges.
Solution Approach 2:
The invention creates a composite alloy system combining Al and Nb elements with controlled C content. The Nb forms NbC that binds carbon, preventing carbon-related welding defects, while Al forms the protective alumina barrier. This composite material approach achieves oxidation resistance without sacrificing weldability or elongation.
3Strength
If a large amount of C is added to the tube main body to increase creep strength, then creep strength is improved, but weldability decreases and elongation decreases
Solution Approach 1:
The invention maintains lower C content (0.2-0.6%) in the overlay welding alloy compared to conventional high-C alloys, while achieving sufficient creep strength through Nb addition. The Nb forms NbC precipitates that provide strengthening without the excessive carbon content that would harm weldability. This local quality adjustment in the projection alloy resolves the contradiction between creep strength and weldability.
Solution Approach 2:
The invention changes the chemical composition parameters of the overlay welding alloy, specifically limiting C to 0.2-0.6% and adding Nb (0.05-2.0%). This parameter change allows the formation of NbC that provides creep strength enhancement through precipitation hardening, while avoiding the weldability deterioration associated with high carbon content.
4Strength
If Nb is added to the alloy for overlay welding to form NbC and increase creep strength, then creep strength is improved and carbon concentration is reduced, but the alloy composition becomes more complex
Solution Approach 1:
The invention adds Nb to the overlay welding alloy at controlled levels (0.05-2.0%), which forms NbC precipitates that strengthen the material through precipitation hardening. This single element addition achieves creep strength improvement and carbon binding without requiring multiple alloying elements or complex compositional adjustments, thus minimizing alloy complexity while achieving the desired mechanical properties.
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 alumina barrier layer effectively prevents oxidation, carburization, and nitridization, reducing pressure loss and heat transfer efficiency issues, while also decreasing the frequency of decoking tasks and increasing the yield of olefins.
Implementation Method 1
Nb binds to C during welding so as to form NbC
Implementation Method 2
the alumina barrier layer containing an Al oxide... makes it possible to realize excellent oxidation resistance
Implementation Method 3
excellent oxidation resistance, carburization resistance, nitridization resistance
Implementation Method 4
excellent oxidation resistance, carburization resistance, nitridization resistance
Implementation Method 5
Nb included in the projections formed through welding forms NbC (niobium carbide) during welding and can increase the creep strength through crystal boundary strengthening
Data Source
Figure 1(a)~1(c)
Figure 2
Figure 3
AI summary
The present invention provides an alloy for overlay welding with which an alumina barrier layer containing an Al oxide can be formed on a projection that is overlay welded on an inner surface of a reaction tube, powder for welding, and a reaction tube having a projection that is overlay welded on the inner surface as a stirring member. An alloy for overlay welding according to the present invention is an alloy for overlay welding that is to be used in overlay welding, and the alloy contains C in an amount of 0.2 mass% to 0.6 mass%, Si in an amount of more than 0 mass% to 1.0 mass%, Mn in an amount of more than 0 mass% to 0.6 mass% or less, Cr in an amount of 25 mass% to 35 mass%, Ni in an amount of 35 mass% to 50 mass%, Nb in an amount of 0.5 mass% to 2.0 mass%, Al in an amount of 3.0 mass% to 6.0 mass%, Y in an amount of 0.005 mass% to 0.05 mass%, and Fe and inevitable impurities as a remaining portion.