Technical Basis for Code Case N-894 Alternative Rules for Repair of Classes 1, 2, and 3 Austenitic Stainless Steel Piping With Thermal Fatigue Cracking

Since 1982 the nuclear industry has employed weld overlay repairs to address intergranular stress corrosion cracking (IGSCC) in boiling water reactors (BWR) and primary water stress corrosion cracking (PWSCC) in pressurized water reactors (PWR). The American Society of Mechanical Engineers (ASME) has created several documents to provide rules and guidelines for weld overlay repair of nuclear components that have experienced stress corrosion cracking (SCC). These documents include ASME Code Case N-504-4 and ASME Section XI, Nonmandatory Appendix Q which specifically address weld overlay repair of stainless steel components. Recently, stainless steel components that have experienced thermal fatigue cracking at the inner diameter surfaces have been repaired with structural weld overlays (SWOL) using the methodology of Code Cases N-504-4 and N-740-2. The SWOL is a good choice for repair of thermal fatigue cracks in piping because it provides structural reinforcement to the affected location and places the inside diameter (ID) surface into compression preventing, or significantly reducing, further flaw growth. However, the rules of Case N-504-4 and N-740 were not specifically written to address thermal fatigue cracking as the primary cause and may not adequately address design, analysis and examination requirements when thermal fatigue is the active mechanism because it is very different in nature than SCC. For example, SCC is driven by a combination of environment, steady state operating stresses, residual stresses from welding and fabrication processes, and operating temperature, whereas thermal fatigue is driven by thermal stress cycles resulting from fluid thermal cycling or stratification. The source of the thermal events that result in cracking may not be as well understood or predictable as SCC degradation. In addition, weld overlays applied to address SCC are constructed of SCC resistant material but are not resistant to thermal fatigue. Therefore, ASME Section XI recognized that alternative rules were needed for repair of piping damaged by thermal fatigue. This paper provides a technical basis for weld overlay repair of components that have experienced thermal fatigue cracking. It addresses design, analysis and examination requirements considering the nature of thermal fatigue in nuclear piping systems. The Code Case was originally drafted based on the industry accepted rules of Case N-504-4 and Appendix Q but includes appropriate modifications needed to address thermal fatigue cracking. These modifications include removing restrictions such as the delta ferrite limit for PWRs that is only applicable to address SCC in BWR environments, and enhancements to the examination requirements to ensure that the repaired location is adequately monitored throughout the remaining service life of the plant. The purpose of this paper is to document the technical basis for Code Case N-894, which is currently still under development by ASME Section XI.

Tempering Efficiency Evaluation for Dissimilar Weld Overlays

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.

Application of Low Heat Input Gas Metal Arc Welding for Corrosion Resistant Weld Overlays

Preventing failure due to corrosion poses a challenge to the oil and gas industry. A cost-effective way to prevent such failures is the application of corrosion-resistant nickel-based weld overlays using arc welding processes. Previous research performed at The Ohio State University indicates low heat input GMAW processes, such as cold metal transfer (CMT), produce weld overlays which corrode up to ten times slower than overlays produced with cold wire GTAW [1, 2], with up to ten times higher deposition rates [3]. However, formation of lack of fusion and lack of penetration defects has been a major concern related to the widespread application of low heat input GMAW processes in the industry. In this study, optimal windows of CMT welding parameters for producing defect-free welds were established using a design of experiment approach. CMT weld overlays were compared with hot wire (HW)-GTAW overlays currently used in industry with respect to bead characteristics, microstructure, and process capability. As compared with the HW-GTAW process, the CMT process produced weld overlays with up to four times lower dilution, seven times smaller interdendritic arm spacing, and four times higher deposition rates. Additionally, average heat affected zone and fusion boundary hardness values in the CMT overlays were below 248 HV0.1 and may not require the post weld heat treatment specified by NACE MR0175.

Example Application of Code Case N-894 Rules for Weld Overlay of Thermal Fatigue Cracking

The ASME Code, Section XI is working on guidance for application of weld overlay repairs to repair thermal fatigue cracking in nuclear piping systems. This new guidance will eventually be published as Code Case N-894 with the original technical basis in PVP2019-93360. Weld overlays have been extensively used in boiling water reactors (BWRs) and pressurized water reactors (PWRs) to mitigate stress corrosion cracking (SCC). The weld overlays applied to date mitigate SCC by putting the flaw into compression and they use materials that are resistant to SCC. Mitigation of thermal fatigue requires the crack to be in compression so that it does not achieve tensile cycling under the thermal fatigue loading condition. Code Case N-894 allows for the use of either stainless steel or nickel alloy filler metals to repair thermal fatigue flaws. This paper will evaluate the use of both filler metals for the weld overlay process to assess the performance difference between the two filler metals such that the welding advantages of stainless steel over nickel alloys can be quantified. Specifically, this paper assesses the residual stress state difference for nominal sized weld overlays on a six-inch pipe. Various cyclic thermal loading conditions are postulated, and the stress intensity factors are determined for both filler metals to assess the difference in mitigation of thermal fatigue flaws.

Properties of weld overlays on regenerated wheel hub of a mining vehicle

The paper presents problems related to the regeneration of wheel hubs for mining machinery vehicles carried out with arc welding using the MAG method. Hubs made of medium-carbon cast steel, type LII500, were weld surfaced with filler metal grade of G4Si. The tests were aimed at determining welding imperfections and metallographic properties of the surfacing welds. The influence of heat treatment on the properties of surfacing was investigated. Hardness distributions in welded hubs after various heat treatment operations were determined. Recommendations for the technology of weld surfacing the wheel hubs were formulated.

Assessment of Weld Overlays in Cladded Piping Systems With Varied Thicknesses

This paper continues the research previously done by authors on computer simulation of the dissimilar welded joints with varying clad thicknesses using numerical methods. For different cladding thicknesses comprising of stainless steel and mild steel, stress curves have been generated. The welding of the two dissimilar materials has been carried out in-house with the aid of a tungsten arc weld with dynamic measurement of the temperature profile in vicinity of the welding track using high temperature thermocouples. Comparison of the experimentally measured stresses from literature versus the simulation results shows close agreement.