Thermal fatigue of austenitic and duplex stainless steels

Thermal fatigue behavior of AISI 304L, AISI 316, AISI 321, and AISI 347 austenitic stainless steels as well as 3RE60 and ACX-100 duplex stainless steels was studied. Test samples were subjected to cyclic thermal transients in the temperature range 20 - 600°C. The resulting thermal strains were analyzed with measurements and numerical calculations. The evolution of thermal fatigue damage was monitored with periodic residual stress measurements and replica-assisted microscopy. The elastic strains of the ferrite phase in duplex stainless steels were studied using Barkhausen noise. Finally, destructive analyses including fractographic scanning electron microscopy (SEM) studies and transmission electron microscopy (TEM) analyses were performed. The surface residual stresses changed markedly during the first load cycles. In the austenitic stainless steels yielding during the rapid cooling resulted in compressive residual stresses from -200 MPa (20 - 300°C temperature cycle) to -600 MPa (20 - 600°C temperature cycle). After 10 cycles the residual stresses stabilized and then started to relax due to crack formation. Cracks were seen to initiate from persistent slip bands (PSBs) and in 3RE60 from MnS inclusions. In duplex stainless steels the phase boundaries retarded crack growth markedly. In the austenitic stainless steels, the fracture surfaces of thermal fatigue cracks showed extensive striation formation, i.e. they were similar to mechanical fatigue. The dislocation density was lower than expected based on mechanical fatigue data. Dislocation tangles and occasional cell tendency was observed. In duplex stainless steels the plastic deformation concentrated to the austenite phase. The obtained thermal fatigue data were compared with mechanical fatigue data from literature and with the ASME design curve. The ASME design curve was found to give safe design life, although the remaining safety factor on strain is decreased to 1.5. The total strain (elastic+plastic) caused by thermal loading was solved with linear-elastic FE-analysis. Thermal fatigue crack growth was predicted successively using total strain solution of an uncracked component and a strainbased growth model: da / dN = C7 Δε tot m7  a, where C7=1.6 and m7=1.3 for the studied austenitic stainless steels. The model is applicable to small fatigue cracks (0.05 - 4 mm) growing in varying temperature and strain fields and is temperature-independent in the studied range.reviewe

A round-robin project in Japan for the evaluation of nondestructive responses of natural flaws

This paper introduces the current status of a round-robin project aiming at gathering non-destructive data of natural flaws. The project, which was launched in 2009, prepared specimens containing artificial stress corrosion cracks and thermal fatigue cracks, and served the specimens to a round-robin test to gather non-destructive data. A total of 12 universities and research institutes have participated to the round-robin test. Some of the specimens are already destroyed to confirm the true profiles of the cracks, whereas others remain undestroyed. All the data are presented at a dedicated webpage, together with the results of the destructive tests, so that they are freely available for anybody. Keywords: thermal fatigue crack, stress corrosion cracking, electromagnetic nondestructive testing, ultrasonic testing, numerical modelin

NEW FLAWS FOR QUALIFICATION OF CAST STAINLESS STEEL INSPECTION

ABSTRACT For decades, cast austenitic stainless steels (CASS) have presented a challenge for inspection. However, recent advanced inspection technologies have shown promise in inspecting CASS materials with wall thicknesses that were once considered impossible

A52M/SA52 Dissimilar Metal RPV Repair Weld : Experimental Evaluation and Post-Weld Characterizations

Aging management of the existing fleet of nuclear power plants is becoming an increasingly important topic, especially as many units are approaching their design lifetimes or are entering long-term operation. As these plants continue to age, there is an increased probability for the need of repairs due to extended exposure to a harsh environment. It is paramount that qualified and validated solutions are readily available. A repair method for a postulated through cladding crack into the low alloy steel of a nuclear power plant’s reactor pressure vessel has been investigated in this study. This paper is part of larger study that evaluates the current possibilities of such repair welds. The present paper documents the weld-trials and method selection. A parallel paper describes numerical simulations and optimization of weld parameters. The presented weld-trial represents a case where a postulated crack has been excavated and repaired using a nickel base Alloy 52M filler metal by gas metal arc welding-cold metal transfer with a robotic arm. A SA235 structural steel has been used as a base material in this weld-trial. No pre-heating or post-weld heat treatment will be applied, as it would be nearly impossible to apply these treatments in a reactor pressure vessel repair situation. While Alloy 52M presents good material properties, in terms of resistance to environmentally assisted degradation mechanisms, such as primary water stress corrosion cracking, it is notoriously difficult to weld. Some difficulties and challenges during welding include a sluggish weld puddle, formation of titanium and/or aluminium oxides and its susceptibility to lack of fusion defects and weld metal cracking, such as ductility dip cracking and solidification cracking. Moreover, gas metal arc welding-cold metal transfer is not traditionally used in the nuclear industry. Nonetheless, it presents some interesting advantages, specifically concerning heat input requirements and automation possibilities, as compared to traditional welding methods. The mechanical properties, in terms of indentation hardness, and microstructure of a weld-trial sample have been evaluated in this study. The fusion boundary and heat affected zone were the main areas of focus when evaluating the mechanical and microstructural properties. Detailed microstructural characterization using electron backscatter diffraction and nanoindentation were performed across the weld interface. Based on these results, the gas metal arc welding cold metal transfer is seen as a potential high-quality weld method for reactor pressure vessel repair cases.acceptedVersionPeer reviewe

The effect of subsurface crack opening on the stress intensity factor under cyclic thermal loads

This study considers surface crack behaviour under periodic thermal loads using weight functions and finite element calculations. A surface and subsurface crack opening mode during the load cycle was observed. The subsurface mode where the crack mouth is closed while the crack tip remains open can be more critical for deeper cracks. Crack face contact is required to obtain the subsurface mode both with finite elements and weight functions. A combination of surface and subsurface weight functions can capture the crack behaviour throughout the load cycle. This behaviour is demonstrated using numerous simulations with periodic thermal loads with different extent and frequency.Peer reviewe