| Summary: |
The process vessels are very important in power, petroleum and chemical industries, working in environments requiring specific mechanical and corrosion resistance of the vessel walls. In general, these vessels are made in ferritic base alloys cladded by protective layers of austenitic stainless steels. This weld cladding procedure enables production economies by using less expensive and more easily workable materials, coated with expensive metals or alloys, selected to achieve the desired properties in specific
parts of the equipments.
The aim of this project is to study the effect of heat treatment on residual stress fields and corrosion behaviour of weld cladded walls, currently used in reactor pressure vessels. Frequently these vessels present cracking phenomena in the interface metal/
cladding compromising their applications. Cracking seems to be due to the carbon migration to austenitic stainless steel during the welding and during the post heat treatment, and can be exacerbated if hydrogen and tension residual stresses are present in the material.
The present project is particularly focused in determining in what extend the post heat treatment reduces the residual stress level in order to decrease the cracking phenomena in the interface, without significant degradation of the corrosion stainless steel properties. The post weld heat treatment conditions will be analysed in connection with the relaxation of residual stresses and the metallurgical, mechanical and corrosion properties of the cladded walls.
It is expected the results of this research can lead to new PWHT strategies improving safety and reliability of reactor pressure vessels currently used in industrial application. The results of this research will be published in conferences and international journals.
The project starts with the selection of the base materials and filler metals as well the optimization of the welding process parameters, in order to get welds with small dilution. Some of  |
Summary
The process vessels are very important in power, petroleum and chemical industries, working in environments requiring specific mechanical and corrosion resistance of the vessel walls. In general, these vessels are made in ferritic base alloys cladded by protective layers of austenitic stainless steels. This weld cladding procedure enables production economies by using less expensive and more easily workable materials, coated with expensive metals or alloys, selected to achieve the desired properties in specific
parts of the equipments.
The aim of this project is to study the effect of heat treatment on residual stress fields and corrosion behaviour of weld cladded walls, currently used in reactor pressure vessels. Frequently these vessels present cracking phenomena in the interface metal/
cladding compromising their applications. Cracking seems to be due to the carbon migration to austenitic stainless steel during the welding and during the post heat treatment, and can be exacerbated if hydrogen and tension residual stresses are present in the material.
The present project is particularly focused in determining in what extend the post heat treatment reduces the residual stress level in order to decrease the cracking phenomena in the interface, without significant degradation of the corrosion stainless steel properties. The post weld heat treatment conditions will be analysed in connection with the relaxation of residual stresses and the metallurgical, mechanical and corrosion properties of the cladded walls.
It is expected the results of this research can lead to new PWHT strategies improving safety and reliability of reactor pressure vessels currently used in industrial application. The results of this research will be published in conferences and international journals.
The project starts with the selection of the base materials and filler metals as well the optimization of the welding process parameters, in order to get welds with small dilution. Some of the welded coupon plates will be submitted to different heat treatment thermal cycles, mainly changing the holding temperature and time. Metallurgical and mechanical properties of the as welded coupon plates and of the welded plates after suffering the selected heat treatments will be characterized. Residual stress fields will be evaluated by X-ray diffraction, incremental hole-drilling and neutron diffraction. The corrosion resistance of as welded and heat-treated specimens will be evaluated by electrochemical testing and the morphology of the corroded surface analysed by scanning electronic microscopy (SEM) and atomic force microscopy (AFM). Finally metallurgical and mechanical properties of the
welds as well their residual stress fields and corrosion behaviour will be correlated with the welding and heat-treatment procedures. |