F22V steel is widely used in industry for manufacturing large pressure vessels, working at high temperatures. In order to assemble these components, a multi-pass welding process is necessary due to the great thickness of the shells. Because of this reason, the welding process is a critical step; in fact, high residual stresses could be generated influencing the integrity and performances of the product. The ASME-BPVC requires the execution of three different heat treatments, a Dehydrogenation Heat Treatment (DHT), an Intermediate Stress Relieving (ISR) and a Post-Weld Heat Treatment (PWHT), in order to decrease the value of residual stresses deriving from the welding process. In the present work, two different heat treatment sequences, DHT+PWHT or ISR+PWHT, are considered to investigate which configuration could be the more suitable to reduce the costs for the manufacturer, guaranteeing the final component integrity. Furthermore, two different narrow-gap welding techniques are compared: the one bead per layer and the standard one. Specifically, the residual stress distributions after heat treatments for F22V welded steel are analysed by XRD techniques and employing numerical methods. Two coupled thermo-mechanical models, 2D and 3D, are created with the Abaqus/CAE solver implemented with the AWI plug-in. The 2D analysis is used to extract residual stress trends into butt-weld plates in the as-welded condition, with a DHT and with an ISR treatment. The numerical results are in good agreement with those obtained experimentally. Then, the attention is focused on the hoop weld of two shells with an axisymmetric model, to study the residual stress field and to identify the influence of the thermal treatments. The FEAs reveal the presence of a high tensioned area in the material’s FZ and HAZ. We can conclude that the two proposed sequences of heat treatments have the same influence on the final stress field. Indeed, it seems that the PWHT cancels the effects of all the previous heat treatments.
L’acciaio F22V è ampiamente utilizzato nell'industria per la produzione di grandi recipienti in pressione che lavorano ad alte temperature. A causa dell’elevato spessore delle virole è necessario un processo di saldatura multipassata per assemblare questi componenti. Esso è considerato un aspetto fondamentale; infatti tensioni residue molto elevate potrebbero influenzare le prestazioni e l’integrità di questi componenti. L'ASME-BPVC richiede l'esecuzione di tre diversi trattamenti termici: il trattamento di deidrogenizzazione (DHT), il trattamento intermedio di distensione (ISR) e il trattamento post-saldatura (PWHT), al fine di ridurre il valore delle tensioni residue derivanti dal processo di saldatura. Nel presente lavoro, si considerano due diverse sequenze di trattamenti termici, DHT + PWHT o ISR + PWHT, per valutare la configurazione più idonea al fine di diminuire i costi per il produttore, garantendo lo stesso livello di sicurezza del componente. Inoltre si considerano due diverse tecniche di saldatura narrow-gap: quella con una passata per layer e quella standard. In particolare, sia misure XRD che metodi numerici sono utilizzati per analizzare le tensioni residue dopo i trattamenti termici su un acciaio F22V saldato. Due analisi termo-meccaniche, 2D e 3D, sono state eseguite con il software Abaqus/CAE implementato con il plug-in AWI. L'analisi 2D è utilizzata per comprendere gli andamenti delle tensioni residue in piastre saldate di testa in diverse condizioni: post-saldatura, con DHT e con trattamento ISR. I risultati numerici ottenuti sono in accordo con quelli sperimentali. In seguito, ci si è focalizzati sulla saldatura circonferenziale tra due virole con un modello assialsimmetrico, per studiare gli sforzi residui e l'influenza dei trattamenti termici. Le analisi rivelano la presenza di un'area molto sollecitata nella ZF e nella ZTA del materiale. Infine, si può concludere che le due sequenze proposte di trattamento termico hanno la stessa influenza sul valore finale degli sforzi residui. Infatti, il PWHT cancella gli effetti di tutti i precedenti trattamenti termici.
Effect of heat treatments on residual stresses of a F22V welded steel
RIVA, RICCARDO
2017/2018
Abstract
F22V steel is widely used in industry for manufacturing large pressure vessels, working at high temperatures. In order to assemble these components, a multi-pass welding process is necessary due to the great thickness of the shells. Because of this reason, the welding process is a critical step; in fact, high residual stresses could be generated influencing the integrity and performances of the product. The ASME-BPVC requires the execution of three different heat treatments, a Dehydrogenation Heat Treatment (DHT), an Intermediate Stress Relieving (ISR) and a Post-Weld Heat Treatment (PWHT), in order to decrease the value of residual stresses deriving from the welding process. In the present work, two different heat treatment sequences, DHT+PWHT or ISR+PWHT, are considered to investigate which configuration could be the more suitable to reduce the costs for the manufacturer, guaranteeing the final component integrity. Furthermore, two different narrow-gap welding techniques are compared: the one bead per layer and the standard one. Specifically, the residual stress distributions after heat treatments for F22V welded steel are analysed by XRD techniques and employing numerical methods. Two coupled thermo-mechanical models, 2D and 3D, are created with the Abaqus/CAE solver implemented with the AWI plug-in. The 2D analysis is used to extract residual stress trends into butt-weld plates in the as-welded condition, with a DHT and with an ISR treatment. The numerical results are in good agreement with those obtained experimentally. Then, the attention is focused on the hoop weld of two shells with an axisymmetric model, to study the residual stress field and to identify the influence of the thermal treatments. The FEAs reveal the presence of a high tensioned area in the material’s FZ and HAZ. We can conclude that the two proposed sequences of heat treatments have the same influence on the final stress field. Indeed, it seems that the PWHT cancels the effects of all the previous heat treatments.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/145261