Turbine Joint Brazing Strength via Carbide Nitride Diffusion Barrier
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Solution Overview
Problem
The existing joint parts in turbines experience a significant decrease in brazing strength due to the diffusion of carbon and nitrogen from the steel shaft to the TiAl-based alloy turbine wheel, leading to void formation and thermal stress-induced cracks under high-temperature conditions.
Innovation Solution
The use of structural steel or martensitic stainless steel with controlled carbon and chromium content, where carbon and nitrogen are bonded as carbides and nitrides through thermal treatment, preventing their diffusion to the TiAl-based alloy and maintaining the brazing strength by reducing void formation and thermal stress.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If a steel member and TiAl-based alloy member are joined using a Ni-based brazing filler metal, then the turbine wheel can be manufactured with high-temperature performance, but carbon and nitrogen diffuse from the steel shaft to the TiAl-based alloy wheel during high-temperature operation, forming voids and titanium carbide that significantly decrease the brazing strength
Solution Approach 1:
A diffusion barrier layer is introduced as an intermediary between the steel shaft and TiAl-based alloy wheel to prevent carbon and nitrogen diffusion. This barrier layer acts as a mediator that blocks the harmful diffusion path while allowing the brazing filler metal to bond to both surfaces, thereby maintaining brazing strength during high-temperature operation
Solution Approach 2:
The turbine joint is designed as a composite structure consisting of the steel shaft, diffusion barrier layer, brazing filler metal layer, and TiAl-based alloy wheel. This multi-layer composite material structure combines the high-temperature strength of steel with the lightweight properties of TiAl-based alloy while preventing detrimental diffusion through the barrier layer
2Duration of action of moving object
If the turbine operates under high-temperature conditions over a long period, then the turbine can maintain power output, but thermal stress concentrates at the boundary between the shaft and brazing filler metal due to linear expansion coefficient differences, causing cracks and turbine damage
Solution Approach 1:
The linear expansion coefficient of the steel shaft is modified through alloying element control (specific C and Cr content ranges) to reduce the expansion coefficient difference between the steel shaft and TiAl-based alloy wheel. This parameter change minimizes thermal stress concentration during temperature cycling, preventing crack formation and maintaining turbine reliability over extended operation
Solution Approach 2:
The steel shaft undergoes preliminary heat treatment to establish a controlled microstructure and predetermined mechanical properties before assembly. This preliminary action ensures the shaft has the appropriate strength and ductility to withstand thermal cycling stresses, preventing crack initiation at the brazing interface during subsequent high-temperature operation
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 approach effectively suppresses the diffusion of carbon and nitrogen, preventing void formation and enhancing the brazing strength, thereby reducing the risk of cracks and maintaining the integrity of the turbine joint under high-temperature conditions.
Implementation Method 1
carbon and nitrogen present in the shaft may be transferred into the turbine wheel, or titanium present in the turbine wheel or boron present in the Ni-based brazing filler metal may be transferred into the shaft, due to diffusion
Implementation Method 2
carbon and nitrogen are bonded as carbides and nitrides through thermal treatment, preventing their diffusion to the TiAl-based alloy
Data Source
Figure 1
Figure 2
Figure 3(A)~3(B)
AI summary
Provided is joint part capable of suppressing diffusion of carbon and nitrogen contained in the steel member to the TiAl-based alloy member and suppressing formation of voids, titanium carbide or a nitride due to diffusion of carbon and nitrogen contained in the steel member, and thereby suppressing decrease in the brazing strength. A joint part comprises a steel member containing alloy elements including C and Cr, a TiAl-based alloy member, and a Ni-based brazing filler metal via which the steel member and the TiAl-based alloy member are joined to each other, wherein the steel member has a carbide and a nitride each forming a bond with at least one of the alloy elements at least on a side of a boundary with the Ni-based brazing filler metal, and diffusion of C and N to the Ni-based brazing filler metal adjacent to the TiAl-based alloy member is suppressed by the carbide and the nitride. The joint part may be a turbine body 1 comprising a turbine wheel 2 and a shaft 3, and a structural steel material of the shaft 3 is structural steel material containing 0.30 to 0.45 wt% of C and 0.85 to 1.25 wt% of Cr, or a martensitic stainless steel material containing at most 15 wt% of C and 11.5 to 13 wt% of Cr.