Process equipment which employs a corrosion resistant alloy (CRA) layer cladded to steel is common in refineries, petrochemical plants and other plants processing highly corrosive media. There are two regularly employed methods for welding attachments and internals to clad process vessels. One is to remove the CRA cladding for welding the attachment to the steel base metal assuming dissimilar welds and restoring CRA by weld overlay. The other eliminates the step of removing the cladding, simplifying the attachment process by direct welding of the internals onto the clad layer. With the lack of data to prove the integrity of direct welding attachment onto the clad layer, designers frequently demand the cladding be removed or allow only a conservatively low stress limit for what can be attached directly to the clad surface. It is well understood that eliminating the step of removing clad increases the simplicity, improves the lead-time, and reduces the cost of making these attachments for trays or other internals, but there are concerns about clad disbonding risks. With the aim to provide data around the integrity of direct welding attachments for better risk assessments, a technical study was undertaken. In this study, it will be shown that the bond between clad material and the base steel is robust enough to withstand the heaviest attachments and harshest conditions. The theory behind the technical study will be presented along with the results of this study
This study investigates the influence of thermal cycle as a result of repeated welding heat input on the mechanical and microstructural properties of the SA516 Gr 65 steel plates weldment in as-welded condition. The test specimens used were having an identical joint design, welded with submerged arc welding (SAW) process. Three different heat inputs of 0.96kJ/mm (low heat input), 1.5kJ/mm (medium heat input) and 3.58 kJ/mm (high heat input) were used for welding three individual specimens. All weld longitudinal tensile testing, Charpy V-notch impact test and weld metal & heat affected zone (HAZ) microstructural testing were done. The work shows that low heat input (LHI) produced welds with highest yield, tensile strength and toughness in the weld metal whereas high heat input (HHI) resulted in decrease in yield, tensile strength and toughness in the weld metal. Increased level of acicular ferrite and a fine grain structure in weldment were achieved with LHI, while the HHI produced coarse grain structure in the weldment and in the HAZ.
Residual stresses are self-equilibrating stresses that exist in materials and structures at the absence of instantaneous application of external loadings. In industrial manufacturing and fabrication processes, such stresses can be prominent and may lead to premature failures if uncontrolled. Such failures can be manifested in many forms including stress corrosion cracking, fatigue cracking or brittle facture. This paper is devoted to providing a comprehensive review on residual stress in the manufacturing and fabrication domain with a greater emphasis on welding based residual stresses. Three residual stress evolution mechanisms will be evaluated covering deformation driven stresses during manufacturing, thermally driven during welding and surface modifications such as grinding, carburizing and plating. In welding processes, the residual stresses in the cooling cycle are characterized using Gleeble testing illustrating the stress profiles as a function of temperature. The effect of residual stresses in welded structures will be discussed covering fatigue performance, brittle fracture and effect on Stress Corrosion Cracking resistance. To ensure residual stresses are effectively measured and quantified, a total of nine (9) destructive, semi-destructive and non-destructive residual stress measurement techniques are evaluated. A comparison and evaluation of four (4) common residual stress mitigation techniques are also discussed covering Ultrasonic Impact Treatment, High Frequency Mechanical Impact, shot peening and Post-weld Heat Treatment. The review discussion extends to four (4) factors towards impacting the residual stress magnitude and distribution covering material properties, welding process and clamping and preheating during welding.