The BWR RPV Internals Management Program of the German NPP Gundremmingen, Units B and C: Results and Conclusions

2002 ◽  
Author(s):  
Martin Widera
Keyword(s):  
Heat Treatment ◽  
Crack Initiation ◽  
Residual Stresses ◽  
Surface Condition ◽  
Management Program ◽  
Post Weld Heat Treatment ◽  
Interim Result ◽  
The Core ◽  
Weld Heat ◽  
Core Shroud

Due to the core shroud cracks reported from numerous BWRs including the German KWU type BWR Wuergassen, a RPV internals management program for the Gundremmingen NPP (KRB-II) has been initiated in 1994. Major steps and the main results of this program are presented. As an interim result, surface condition of the weld regions and controlled post weld heat treatment (PWHT) in order to reduce the residual stresses seem to play an important role for resistance to crack initiation and growth. To support these assumptions, results of related R&D projects of the German utilities (VGB) are presented.

Establishing Recommended Guidance for Local Post Weld Heat Treatment Configurations Based on Thermal-Mechanical Finite Element Analysis

2016 ◽  
Author(s):  
Phillip E. Prueter ◽  
Brian Macejko
Keyword(s):  
Heat Treatment ◽  
Finite Element Analysis ◽  
Finite Element ◽  
Crack Initiation ◽  
Residual Stresses ◽  
Pressure Vessel ◽  
Post Weld Heat Treatment ◽  
Element Analysis ◽  
Non Linear ◽  
Weld Heat

Post weld heat treatment (PWHT) is an effective way to minimize weld residual stresses in pressure vessels and piping equipment. PWHT is required for carbon steels above a Code-defined thickness threshold and other low-alloy steels to mitigate the propensity for crack initiation and ultimately, brittle fracture. Additionally, PWHT is often employed to mitigate stress corrosion cracking due to environmental conditions. Performing local PWHT following component repairs or alterations is often more practical and cost effective than heat treating an entire vessel or a large portion of the pressure boundary. In particular, spot or bulls eye configurations are often employed in industry to perform PWHT following local weld repairs to regions of the pressure boundary. Both the ASME Boiler and Pressure Vessel (B&PV) Code and the National Board Inspection Code (NBIC) permit the use of local PWHT around nozzles or other pressure boundary repairs or alterations. Additionally, Welding Research Council (WRC) Bulletin 452 [1] offers detailed guidance relating to local PWHT and compares some of the Code-based methodologies for implementing local PWHT on pressure retaining equipment. Specifically, local PWHT methodologies provided in design Codes: ASME Section VIII Division 1 [2] and Division 2 [3], ASME Section III Subsection NB [4], British Standard 5500 [5], Australian Standard 1210 [6], and repair Codes: American Petroleum Institute (API) 510 [7] and NBIC [8] are discussed and compared in this study. While spot PWHT may be appropriate in certain cases, if the soak, heating, and gradient control bands are not properly sized and positioned, it can lead to permanent vessel distortion or detrimental residual stresses that can increase the likelihood of in-service crack initiation and possible catastrophic failure due to unstable flaw propagation. It is essential to properly engineer local or spot PWHT configurations to ensure that distortion, cracking of adjacent welds, and severe residual stresses are avoided. In some cases, this may require advanced thermal-mechanical finite element analysis (FEA) to simulate the local PWHT process and to predict the ensuing residual stress state of the repaired area. This paper investigates several case studies of local PWHT configurations where advanced, three-dimensional FEA is used to simulate the thermal-mechanical response of the repaired region on a pressure vessel and to optimize the most ideal PWHT arrangement. Local plasticity and distortion are quantified using advanced non-linear elastic-plastic analysis. Commentary on the ASME and NBIC Code-specified local PWHT requirements is rendered based on the detailed non-linear FEA results, and recommended good practice for typical local PWHT configurations is provided. Advanced computational simulation techniques such as the ones employed in this investigation offer a means for analysts to ensure that local PWHT configurations implemented following equipment repairs will not lead to costly additional damage, such as distortion or cracking that can ultimately prolong equipment downtime.

scholarly journals Residual Stress State of X65 Pipeline Girth Welds Before and After Local and Furnace Post Weld Heat Treatment

2017 ◽  
Vol 139 (4) ◽  
Author(s):  
Yao Ren ◽  
Anna Paradowska ◽  
Bin Wang ◽  
Elvin Eren ◽  
Yin Jin Janin
Keyword(s):  
Heat Treatment ◽  
Residual Stress ◽  
Residual Stresses ◽  
Outer Surface ◽  
Post Weld Heat Treatment ◽  
Pipe Length ◽  
Stress Profiles ◽  
Girth Welds ◽  
Welded Pipe ◽  
Weld Heat

This research investigated the effects of global (in other words, furnace-based) and local post weld heat treatment (PWHT) on residual stress (RS) relaxation in API 5L X65 pipe girth welds. All pipe spools were fabricated using identical pipeline production procedures for manufacturing multipass narrow gap welds. Nondestructive neutron diffraction (ND) strain scanning was carried out on girth welded pipe spools and strain-free comb samples for the determination of the lattice spacing. All residual stress measurements were carried out at the KOWARI strain scanning instrument at the Australian Nuclear Science and Technology Organization (ANSTO). Residual stresses were measured on two pipe spools in as-welded condition and two pipe spools after local and furnace PWHT. Measurements were conducted through the thickness in the weld material and adjacent parent metal starting from the weld toes. Besides, three line-scans along pipe length were made 3 mm below outer surface, at pipe wall midthickness, and 3 mm above the inner surface. PWHT was carried out for stress relief; one pipe was conventionally heat treated entirely in an enclosed furnace, and the other was locally heated by a flexible ceramic heating pad. Residual stresses measured after PWHT were at exactly the same locations as those in as-welded condition. Residual stress states of the pipe spools in as-welded condition and after PWHT were compared, and the results were presented in full stress maps. Additionally, through-thickness residual stress profiles and the results of one line scan (3 mm below outer surface) were compared with the respective residual stress profiles advised in British Standard BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures” and the UK nuclear industry's R6 procedure. The residual stress profiles in as-welded condition were similar. With the given parameters, local PWHT has effectively reduced residual stresses in the pipe spool to such a level that it prompted the thought that local PWHT can be considered a substitute for global PWHT.

Engineering Critical Assessment of Post Weld Heat Treatment for Full-Encirclement Tees Greater Than 1.25 Inches

2019 ◽  
Author(s):  
Kolton Landreth ◽  
Qi Li ◽  
Raghav Marwaha
Keyword(s):  
Heat Treatment ◽  
Residual Stresses ◽  
Welding Process ◽  
Post Weld Heat Treatment ◽  
Critical Assessment ◽  
Material Strength ◽  
Mechanical Reinforcement ◽  
Element Analysis ◽  
Pipeline Design ◽  
Weld Heat

Abstract Full-encirclement split tee fittings for hot tapping and plugging (HT&P) wrap completely around the pipeline and are welded in place. The welded joint provides mechanical reinforcement of the pipe and branch. When full-encirclement hot tap tees are welded to pipelines 24 inches in diameter or larger, the header must often be at least 1.25 inches thick to pass the required calculations for reinforcement. This means the joint will require post weld heat treatment (PWHT) according to ASME B31.8 and CSA Z662. However, PWHT can be extremely dangerous and impractical, potentially elevating temperature to the point where material strength of the pressurized pipeline is compromised. An engineering critical assessment per ASME FFS-1/API 579 indicated PWHT may not be required for a full-encirclement hot tap tee over 1.25 inches thick. Specifically, research showed that the residual stresses developed during the welding process may not limit the design of a full-encirclement tee or lead to shorter pipeline design life. This paper illustrates how a “more rigorous analysis” per paragraph 802.2.2[b] of ASME B31.8 and paragraph 4.3.12.2 of CSA Z662 may help operators avoid the PWHT requirement. It discusses the finite element analysis (FEA) simulations researchers used to induce residual stresses in a carbon steel fitting. The residual stresses induced in the fitting were used as initial condition for plastic collapse and fatigue evaluations.

Effect of Post Weld Heat Treatment on Weld Hardness Quality of P91 Welded Pipes

2015 ◽  
Vol 799-800 ◽  
pp. 377-381
Author(s):  
Mohd Amin Abd Majid ◽  
Muhammad Sarwar
Keyword(s):  
Mechanical Properties ◽  
Heat Treatment ◽  
Residual Stresses ◽  
Weld Metal ◽  
Welded Joint ◽  
Post Weld Heat Treatment ◽  
Bead Geometry ◽  
Construction Sites ◽  
Weld Heat

Quality of a weld produced is generally evaluated by different parameters such as weld size, bead geometry, deposition rate, hardness and strength. A common problem that has been faced at the construction sites is to obtain a good welded joint having the required strength with minimal detrimental residual stresses to avoid any premature cracking due to high variation of hardness. In order to address this issue and to attain good weld quality, this study has been made to comprehend the effect of the post weld heat treatment on P91 material welds produced by TIG welding. Findings from the studies indicate that the PWHT has significant influence on the weld hardness of Alloy Steel-A335 P91 pipes. It is eminent that during cooling, after welding of P91, quenched martensite was formed in the HAZ that results in an increased hardness to an undesirable level of more than 250HB. PWHT at temperature of 760°C for 2 hours has good influence on mechanical properties as the hardness decreases and turns out to be uniformly distributed. If the PWHT is correctly carried out, the hardness of parent metals, heat affected zones and weld metal can be brought into the required limits to avoid any premature cracking due to high variation of hardness.

scholarly journals Residual Stress State of X65 Pipeline Girth Welds Before and After Local and Furnace Post Weld Heat Treatment

2016 ◽  
Author(s):  
Yao Ren ◽  
Anna Paradowska ◽  
Bin Wang ◽  
Elvin Eren
Keyword(s):  
Heat Treatment ◽  
Residual Stress ◽  
Residual Stresses ◽  
Outer Surface ◽  
Post Weld Heat Treatment ◽  
Stress Profiles ◽  
Stress States ◽  
Girth Welds ◽  
Welded Pipe ◽  
Weld Heat

This research investigated the effects of global (in other words, furnace-based) and local post weld heat treatment (PWHT) on residual stress (RS) relaxation in API 5L X65 pipe girth welds. Two pipe spools were fabricated using identical pipeline production procedures for manufacturing multi-pass narrow gap welds. Non-destructive neutron diffraction strain scanning was carried out on girth welded pipe spools and stress-free comb samples and also thin slices for the determination of lattice spacing. All residual stress measurements were carried out at the KOWARI strain scanning instrument at the Australian Nuclear Science and Technology Organization (ANSTO). Residual stresses of two pipe spools (in the as-welded condition) were measured through the thickness in the weld material and adjacent parent metal starting from the weld toe. Three line-scans were completed 3mm below outer surface, at mid thickness and 3mm above the inner surface. PWHT was adopted for stress relaxation; one pipe was conventionally heat treated entirely in an enclosed furnace and the other was locally heated by a flexible ceramic heating pad. Residual stresses were measured after PWHT at exactly the same locations as those used for the as-welded condition. Residual stress states of the two pipe spools in as-welded condition and after PWHT were compared and the results were presented in full stress maps. Additionally, through thickness residual stress profiles and the results of one line scan (3mm below outer surface) were compared with the respective residual stress profiles advised in British Standard BS 7910 “Guide to methods for assessing the acceptability of flaws in metallic structures” and the UK nuclear industry’s R6 procedure. The residual stress states of the two pipe spools measured in the as-welded condition were similar. With the given parameters, local PWHT has effectively reduced residual stresses in the pipe spool to such a level that it prompted the thought that local PWHT can be considered a substitute for global PWHT.

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