Isothermal Low-Cycle Fatigue Evaluation of External Weld Repair Using Alloy 182 Filler Metal With Backing Plate Design

Coke drums are pressure vessels used in the delayed coking process at oil refineries, which transform heavy residual oil into light-weight hydrocarbon molecules and solid coke through thermal cracking. Due to the severe thermal and mechanical loadings during operation, these vessels experienced low-cycle fatigue failure, which led to shell and skirt damage such as bulging and cracking. External weld repairs using the temper bead technique have been widely applied to repair damaged regions caused by bulging and cracking for preventing the leaks of the residual oil contents. However, a substantial proportion of the external repairs have been reported to experience post-repair cracking issues. In this study, an external weld repair mockup with a backing plate was evaluated through metallurgical characterizations and isothermal low-cycle fatigue tests. The micro-hardness mapping identified the mismatching interfaces from base metal to weld metal (WM) and from root passes to internal clad. Four types of dog-bone samples were extracted from the weld: weld metal, heat affected zone, internal clad and backing plate. These samples were used to evaluate the fatigue resistance of weld metal and transition zone under low-cycle fatigue tests. Failure analysis showed that weld metal samples were susceptible to multiple-crack initiations, while other transition samples failed at mismatching interfaces or stress concentration points at weld toes.

Integrity Analysis of a Vessel Subject to Weld Repair and Post-Weld Heat Treatment: Thermal Stress Distribution Using Numerical Approach

Here, we present a numerical approach to analyze the integrity of a vessel that was subject to a weld repair. A Post-Weld Heat Treatment (PWHT) process was implemented to a vessel undergone weld repair due to leakage. Due to the thick wall of the welded bottom head, this welding process must be followed by the PWHT to relieve the residual stress, as well as to improve the material properties. PWHT process was performed by heating the welded area to reach 675 °C temperature. A numerical approach using finite element analysis (FEA) method was performed to analyze the integrity of the vessel. Based on the analysis, the structure is still stable within the applied load. PWHT process does not lead to buckling on the main structure and the load is still lower than the load required for the occurrence of buckling. A sensitivity analysis was also performed with reduced temperatures to 630 °C or reduction of PWHT area width. These changes were found to have negligible effects in reducing the stress and strains in the vessel. After PWHT is completed, the structure is still considered to be safe to be operated, as indicated by its strain that is still below the allowable strain and only relatively small deflection was occurred.

Repeated Weld Repair and its Influence on Welded Carbon Steel

Weld repair usually comprises of mechanical removal of weld part and redisposition of the filler wire using the same parameters. The defect may be removed by carbon arc gauging and grinding or machining. The strength and the microstructure of the material will changed when the repeated weld repair is applied to the material at the same area. The purpose of this study is to compare and identify the angle of distortion, hardness, and tensile strength and bend strength and to analyze the macro and microstructure between repairing method using carbon arc gauging and mechanical grinding process with the same number of repairing sequence. The result proved that repairing A36 steel increased the strength of the material itself but the ductility was decrease when the number of repair increases. It can be concluded that, the repair using carbon arc gauging can’t be applied to repair weld joint for material because it’s more significant to change the material process compared to mechanical grinding. Overall, the mechanical grinding technique is the most suitable practice which can serve as the suitable method for repairing the weld defect if the repaired focus area received high impact loads.

Enhancement in Welding Performance of 2% Maximum Weld Repair Policy

In the implementation of welding work on a modular oil & gas fabrication project in PT. X Batam to get welding results that are 100% very difficult to achieve and always obtain welding results through visual inspection tests and Non-Destructive Test is rejected or defect that the repair welding must be done to fix it. The causes of welding repair are classified into two, namely repair due to lack of skill welder or due to engineering aspects. With that the management of PT. X Batam issued a policy of 2% maximum welding repair for each structural welding job as a KPI's and part of the company's quality manual to monitor and control of welder’s performance in every project implemented. From the results of the 2% maximum KPI welding repair policy obtained significant enhancement on the performance of welders in every project undertaken and shown from the results of KPI values in 2014 the TEN FPSO E-house project was 1.2%, in 2015 the FPSO Kraken PGM project was 1.5 %, in 2016 the Ghana PGM FPSO project is 0.8%, in 2017 the Adolo Compressor FPSO project is 0.75%, in 2018 the TCO Area E-house project is 0.65% and in 2019 is ongoing the BGC TEG Regeneration unit project is 1.25%.

Mechanical Properties and Microstructure of the Repeated Weld-Repairs of Austenitic Stainless Steel Plates

Joining Most of the repair weld of parts and components cannot be avoided in any manufacturing industry. Weld procedure is commonly used to ensure the welded parts can be useful and safe. Weld repairs have to be carried out with suitable care and avoid premature failures of the weld components. The weld repairs often occur repeatedly on welded parts. Hence the investigation was done to evaluate the effects of repeated weld-repairs of austenitic stainless steels plates on both mechanical and microstructural properties. Weld samples were joined using gas tungsten arc welding (GTAW) with several numbers of weld repairs. The first weld was performed to join metal plates and assigned as 0R. The weld bead was then ground away and followed by the first weld repair using the same GTAW (designated as 1R). This repair process was continual until five times (identified as 5R). All specimen was characterized by the chemical composition test, the microstructure observation, and the mechanical tests. It was found that the HAZ hardness of repeated weld repair decreased when the number of repairs increased. The tensile test results of the repeated weld repair had a few effects on tensile strength. However, the result of the impact test on repeated weld repairs shows a substantial reduction in the toughness properties as the repeated number of weld repairs. The repeated weld repair influenced the mechanical properties of austenitic stainless steel plates and showed a tremendous decrease compared with the type of 304L stainless steel as the repeated numbers of weld repairs.

Weld Repair of Steam Turbine Rotor with 12 Cr Weld to Mitigate Corrosion Issues

Weld repair of steam turbine rotors has become an acceptable practice to extend the life of rotors. Depending on the type of damage, the extent of weld repair can range from weld build-up of integral discs to stubbing a new forging to replace the damaged portion of an old rotor. Steam turbine rotors made of low alloy steels experience corrosion related damages such as stress corrosion cracking, pitting corrosion, corrosion-erosion etc., Traditional weld repairs have been using low alloy steel welds. While the low alloy welds may have mechanical properties comparable to or even slightly exceeding that of the low alloy steel rotor alloys, the corrosion resistance of the low alloy welds are not great. 12 Chromium weld will provide better resistance to corrosion related damages than typical low alloy welds. However using 12 Chromium weld on low alloy steel rotors provides some additional challenges and limitations. These include selection of optimal combination of weld wire and flux for rotor welding, selection of optimal post weld stress relieve temperature and dealing with abrupt change in chemical composition at the weld interface. This paper discusses development of such a weld procedure and provides some examples where the 12 Chromium weld was successfully applied on steam turbine rotors that experienced corrosion related damages.

A Treatise on Weld Evaluation

Welds are inspected by various techniques which include visual examination, surface examination and volumetric examination. While the above techniques would qualify a weld to workmanship criteria, they would not necessarily be indicative of weld properties. Preparation and qualification of welding procedures and testing of production welds are indicative that the weldment would probably provide a safe and satisfactory service life. However, weldments have to operate at their design conditions which may include high temperatures and ASME Codes do not necessarily stipulate tests for verification of high temperature properties. In addition, defective welds are often repaired by removing the originally deposited weld metal and re-welding. The effects of double heat input are not necessarily evaluated. In this paper, an insight is provided into the factors which provide assurance that weldments will perform satisfactorily in service and the combination of non-destructive evaluation methods which would enable effective detection of imperfections. Non-destructive volumetric examination method for welds has traditionally be radiography. With the advent of automated data acquisition methods in Ultrasonics, like Time of Flight Diffraction and Phased Array Ultrasonic Testing, these methods are rapidly replacing radiographic methods for weld inspection. Ultrasonic acceptance criteria in ASME Section VIII Div. 1, ASME Section VIII Div. 2 and ASME Section IX do not include evaluation of porosity as ultrasonic methods do not easily detect porosity. The result of all this is that today we are accepting welders qualified using Ultrasonic examination as per ASME Section IX but on the job there is still the option of inspecting the weld using Radiography in which, excessive porosity can be a cause for weld repair. Considering this and various other criteria, a comprehensive weld evaluation methodology is proposed taking advantage of the strengths of each inspection technique while welding technology used would ensure that welds have required properties at service temperatures. A proposal is also made to improve the detectability of imperfections using modifications of existing Ultrasonic A-scan Techniques.