Special Topic : Heat Exchangers
Productivity Boosting Tubes for High Temperature Applications
Reduction of overall maintenance costs for critical equipment has been the need of the hour with many high temperature applications . The service conditions seen in many high-temperature applications are extremely aggressive . The life for a high-temperature component is often relatively short compared to applications at ‘ normal ’ temperatures . Replacement of high-temperature components is therefore often necessary during expensive planned maintenance breaks . Unplanned maintenance breaks that are required when a component failure occurs are increasingly costly to implement . Specially designed high temperature grades can increase the service life of many components and improve the overall productivity of the equipment .
Venkat Ramesh , Global Product Manager ( Heat Exchanger tubing ) and Shabareesh Nair , Director ( Technical Marketing Asia ) Tubacex Group , Spain
Obstacles in High Temperature Applications
Some of the common obstacles in high temperature applications include cracking of tubes in the application , the wall thickness of the tubes tends to reduce in size , sometimes the surfaces develop scaling and lastly these components can also demonstrate erosion and corrosion . These obstacles typically occur when the production produces solids from the process and erosion occurs due to solid particles from the process . Hot corrosion often causes oxidization , carburization , sulfidation , and nitriding . Low creep resistance of the tubes is also an obstacle users experience .
Hot corrosion occurs when service conditions become more aggressive and effect the tubes at temperatures above 400 º C . Apart from this , the creep strength of the material becomes a design factor and may not yield the needed strength . Normal temperatures and structural stability of the material becomes very important .
For example , Tubacex high temperature grade ( Tubacex TX 253 MA ® ) is suitable for many high-temperature conditions . These tubes can provide substantial savings by increasing the service life of many components as compared to other standard stainless-steel grades .
Tubacex TX 253 MA ® is an austenitic 20Cr 10Ni + Si type of alloy with very well-defined contents of rare earth metals ( REM ) and nitrogen .
The REM in combination with the silicon contribute to the alloy high oxidation resistance and the nitrogen to its high creep strength and structural stability up to high temperatures . Maximum service temperature reaches 1150 ° C .
The material has silicon in its chemical composition which helps in formation of Silicon oxide layer ( SiO 2
) in addition to the already existing protective oxide layer ( Cr 2
O 3
). This prevents the diffusion of Oxygen ; Carbon is present in the service condition and it prevents risk of oxidation and carburization .
Silicon ( Si ) is used for additional protection and acts as a silicon oxide layer , REM ( Ce ) provides a thinner , tougher , and more adherent oxide layer , and lastly Nitrogen ( N ) raises the strength and delays the precipitation of the sigma phase . The presence of micro-alloying elements like Cerium help the oxide layers remain integral to the tube in service conditions , creating optimal protection for the tube .
Creep Strength
The high carbon and nitrogen contents present are are mainly responsible for the superior high creep strength in this
grade compared to other high temperature grades ( Figure 1 ). The high carbon and nitrogen contents result in the solution hardening effect and the precipitation hardening effect is generated by the precipitation of fine chromium carbides and nitrides .
Microstructure Stability
This type of material presents a fully austenitic matrix . Due to its lower chromium content and its higher nitrogen content , the alloy is much less sensitive to the sigma phase formation , as opposed to other high temperature grades , such as AISI 309S , 310 and 310S . Sigma phase severely impairs toughness , ductility , and corrosion resistance .
High carbon and nitrogen can lead to the formation of chromium carbides and nitrides , which can reduce toughness and corrosion properties but with an effect less pronounced than sigma phase .
Corrosion Resistance Oxidation
This material also has an extraordinary resistance to oxidation . The well-defined
6 Stainless Steel World Americas - August 2023 | www . ssw-americas . com