Refinery equipment, subjected to high pressures and exposed to hydrogen environment, is commonly made of high strength steels such as 2 1/4 Cr 1 Mo 1/4 V. The adoption of this steel can solve many issues; indeed, it has higher allowable stresses compared to the standard 2 ¼ Cr 1 Mo steel, leading to a significant reduction of pressure vessel wall thickness. On the other hand, one main drawback of this Vanadium modified material is its high hardenability and low critical toughness in the as welded condition. This could lead to potential cracking occurrence, especially in presence of high residual stresses after welding, i.e. before the application of thermal treatments.

Therefore, it is of paramount importance the decision of the specific thermal treatment to perform after welding, such as De-Hydrogenation Heat Treatment (DHT) aimed at reducing the hydrogen content, or the Intermediate Stress Relieving (ISR) specifically aimed at reducing the weld residual stresses. These thermal treatments, which are quite expensive and time demanding for the manufacturer, are still under discussion and not supported by evident scientific findings.

This paper is a contribution to this important subject, focusing the attention on the thermal and stress histories generated during the welding operations and after these heat treatments. The aim is the quantification of residual stresses and the identification of the critical regions where cracks are more prone to occur, based on static failure criteria. With this aim, we considered two weld joint configurations, typical of a pressure equipment: 1) a plate-to-plate butt weld joint, and 2) a nozzle-to-plate weld joint.

The work is divided in three parts. The first part focuses on original and dedicated thermo-mechanical simulations of these two joint configurations. The numerical analyses are implemented with the Finite Elements (FE) software Abaqus, combined with the plugin Abaqus Welding Interface (AWI). The models consider the real geometry of the weld passes and chunks. Material properties are all function of the temperature, based on literature works or standards. Thermal and mechanical boundary conditions are in accordance with the standard procedures adopted by the manufacturer. Numerical analyses are performed by considering three welding conditions: 1) the as welded condition, which is the most critical and experiences higher stresses; 2) after DHT and 3) after ISR, i.e. The corresponding residual stresses, calculated by means of numerical analyses, decrease as a consequence of the thermal treatments.

The second part will describe the experimental tests on two mock-ups, specifically built and including welds generated by the usual procedures of the manufacturer. For both the two analyzed joint configurations, residual stresses are collected by means of an X-ray diffractometer at the most critical points, at the surface and sub-surface.

The third part of the work will discuss the comparison between numerical estimations and experimental measurements of residual stresses. The good accordance between these two approaches will allow a validation of the numerical simulations.

The obtained results will be evidenced and discussed.


Numerical and Experimental Residual Stresses of Different Welded Joint Configurations in Heavy Wall

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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