No abstract

Coke drums are massive pressure vessels used in the oil and gas industry, which are subjected to demanding cyclic thermomechanical loading. Such conditions generate severe plastic deformation that leads to bulging and cracking during service due to low-cycle fatigue. The cracks are often repaired in programed maintenance shutdowns, and the repair procedures can be significantly different depending on the organization and failure characteristics. In this work, two types of weld repairs are evaluated after six months of service: (1) a full-excavation crack weld repair, and (2) a partial-excavation crack weld overlay repair. The repair welds were executed on a 1.25Cr-0.5Mo steel coke drum after 20 years of service. This work evaluates the microstructure of the base material, the fabrication circumferential welds, and the weld repairs. The results show that, after prolonged exposure to elevated temperatures, the originally normalized and tempered heat-treated steel microstructure has undergone severe aging. Thermodynamic equilibrium and para-equilibrium calculations were used to determine the carbide formation sequence, and SEM analysis was used to characterize the carbides present on the as-fabricated and aged microstructures. Analysis of the repaired regions did not reveal new cracks.

Temper bead welding is used routinely for weld repair on low alloy pressure vessel steels in the nuclear power industry. Temper bead procedure qualification is contingent on demonstration that the welding process does not degrade the mechanical properties, including fracture toughness, in the base metal weld heat affected zone (HAZ). Historically for temper bead qualification acceptance, adequate HAZ properties have been verified by tensile, impact, and bend testing, while hardness criteria has not been specified. In fact, temper bead welding has been successfully applied for welding on low alloy steels without any hardness criteria for many years. In 2004, ASME Section IX added hardness testing for temper bead procedure qualification when impact testing is not required. The Eurocode, ISO standards, and numerous other European specifications include maximum hardness criteria for general welding procedure qualification and have invoked these same criteria for temper bead procedures. Test results indicate that imposing maximum hardness criteria can actually lead to acceptance of less than optimum fracture toughness in the temper bead weld HAZ due to rapidly changing microstructures in low alloy steels. Impact properties for such microstructures can vary widely even though similar levels of hardness are exhibited. This paper investigates the legitimacy of using maximum hardness criteria to demonstrate acceptable HAZ fracture toughness in low alloy pressure vessel steels.

Many of the Cr{1-1/4 to 2-1/4}-Mo{1/2 to 1} pressure vessels in the refining and petrochemical industries such as process reactors, distillation columns, separators, pressurized storage vessels, and heat exchangers are typically vertical columns, most often supported by a circular skirt. Typically, design considerations for these vessels and support skirts are for operating under continuous “steady-state” conditions, where temporary stresses due to short-term “transient” events such as start-up and shutdown are often ignored. Consequences of dynamic and cyclic loading play a very significant role in their life and performance. For Coke drums, survey data from API shows that the skirt-to-drum attachment weld and adjoining area appears to be the most problematic, frequently experiencing low-cycle fatigue cracking due to concentrated stresses. A methodology for repairing the skirt attachment weld of Cr-Mo pressure vessels is provided. When designing a repair approach, consideration should include material and aged condition, extent and location of defects, welding process and consumables, and codes, standards, and regulatory guidelines. When repair by weld metal buildup to rebuild a skirt-attachment weld configuration is considered, weld procedure qualification and adequate mock-ups should be performed in order to ensure a sound repair. Further, when invoking a code compliant repair without post-weld heat treatment by controlled deposition welding or temper bead techniques, proper training of welder operators should be conducted to ensure the techniques are implemented properly. A case study is provided for a Coke drum, where the original design and fabrication of the skirt attachment included an initial SAW weld metal buildup on the 2.25Cr (P5A) cone followed by an SMAW/GTAW attachment weld to the 1.25Cr skirt (P4). During a plant shutdown, a surface breaking crack was detected in the skirt to shell attachment weld by Dye Liquid Penetrant Testing (D-LPT) and confirmed with Magnetic Particle Testing (MPT). Subsequent examination by Phased Array Ultrasonic Testing (PAUT) discovered a large number of volumetric indications, oriented towards the knuckle section internally. The repair approach consisted of 1) Completely remove the existing skirt and the attachment weld (knuckle) in segments, 2) Inspect the cone for remaining flaws, 3) Excavate and repair flaws in cone using temper bead technique, 4) Rebuild knuckle area for skirt to cone attachment with an increased radius using temper bead welding techniques, 5) Install new skirt sections using controlled deposition welding technique. Temper Bead and Controlled Deposition repair welding techniques were utilized to avoid conventional post-weld heat treatment requirements, significantly improving the turn-around time in the field.

The objective of this work was to develop a procedure for evaluation and quantification of the tempering efficiency of corrosion resistant weld overlays used in the power generation and oil and gas industries. Three two-layer weld overlays of Alloy 625 on Grade 22 steel plates were produced using GTAW cold wire procedures. Typical welding parameters corresponding to low, medium, and high heat input were utilized. The overlays consisted of nine beads on the first layer and five to seven beads on the second layer. The weld thermal histories experienced in the coarse-grained heat affected zone (CGHAZ) were measured with Type K thermocouples and recorded with a 55 Hz sampling rate. Two rows of seven thermocouples were used in each overlay: one row located in a mid-bead position beneath the center bead of the overlay and the other row located in the nearest bead overlap position. Additionally, one Type C thermocouple was plunged into the weld pool of a second layer weld bead. The acquired thermal histories and the CGHAZ hardness at the thermocouple locations were evaluated to quantify the tempering efficiency in each welding procedure. The weld thermal histories with peak temperatures between 500°C, assumed as the minimum tempering temperature, and the base metal AC1 temperature were considered as tempering thermal cycles. The number of tempering thermal cycles and the sum of tempering cycle’s peak temperatures in each thermocouple location, as well as the corresponding hardness were used to quantify the tempering response efficiency for each of the three welding procedures. The results of this study will be used for validation of a computational model-based approach for prediction of tempering response and optimization of temper bead welding procedures.