Mika Kemppainen

On the mechanisms of environment sensitive cyclic crack growth of nuclear reactor pressure vessel steels

Abstract An analysis of the cyclic crack growth rate data generated so far for pressure vessel materials in simulated light water reactor environments suggests a strong dependence on frequency, load ratio, waveform, temperature and material composition. To account for these observations a mechanistic crack growth model has been advanced based on hydrogen-induced cracking, where anodic dissolution creates the conditions for hydrogen absorption at the crack tip. Hydrogen-induced cracking starts from the manganese sulfide inclusions, which act as strong hydrogen traps. The hydrogen-induced crack growth around the manganese sulfide inclusions generally spans several prior austenite grains. At high hydrogen input rates brittle crack growth also occurs, this being unrelated to inclusions. When the crack growth exposes manganese sulfide inclusions, these dissolve and the crack tip environment becomes aggressive and conducive to hydrogen absorption. This hydrogen-induced cracking model explains why inclusions form a preferred crack path, and accounts for the effect of sulfur on crack growth rate both in PWR and BWR conditions. Based on the model, the observed crack growth rate dependence on different testing variables can also be explained.

Effects of MnS inclusion dissolution on environmentally assisted cracking in low-alloy and carbon steels

Abstract In fatigue both monotonic and cyclic plastic zones are formed ahead of the crack tip, inside which the strain history can be studied on the basis of stable hysteresis loops and structure anticipated in the crack-tip process zone and in the area of the maximum tensile stress (MTS) ahead of the crack tip. With these materials, slow strain rate tests (SSRT) were performed both in bulk pressurized water reactor (PWR) environments and in the simulated crack tip environments (MnS-contaminated PWR water). Environment-sensitive cracking occurred in the simulated crack tip environment at low corrosion potentials, −550 to −700 mV (vs a standard hydrogen reference electrode [SHE]), and in pure PWR water after anodic polarization to 0.0 mVSHE.

Advanced flaw production method for in-service inspection qualification mock-ups

One of the key issues in in-service inspection qualification is the representativeness of the defects used in qualification specimens. The best representativeness is achieved with realistic defects. However, present specimen production techniques have some significant weaknesses, such as unrealistic defects or additional alterations induced in the surrounding material. Specimens manufactured, for example, by weld implantation or with weld solidification defects always result in one or more extra weld interfaces. These interfaces can be detected by NDT. To overcome problems with the current specimens, a new defect manufacturing technique was developed. The new technique produces natural, representative defects without introducing additional weld metal or other unwanted alterations to the specimen. The new method enables artificial production of single, separate fatigue cracks by thermal loading. The method is based on a natural thermal fatigue damage mechanism and enables production of real cracks directly into the samples. Cracks are produced without welding or machining and without any preliminary surface treatment or artificial initiator such as a notch or a precrack. Single crack or a network of cracks can be induced into the base material, welded areas, HAZ, weld claddings, threaded areas, T-joints, etc. The location, orientation and size of produced cracks can be accurately controlled. Produced cracks can be used to simulate different types of service-induced cracks such as thermal fatigue, mechanical fatigue and stress corrosion cracks. It is shown that artificially produced thermal fatigue cracks correspond well with the real, service-induced cracks and overcome the problems of traditional qualification specimen manufacturing techniques.

Dynamic strain aging of austenitic high nitrogen Cr-Ni and Cr-Mn steels

Dynamic strain aging (DSA) of solution annealed Cr-Ni and Cr-Mn type austenitic high nitrogen steels (HNS) was investigated by means of tensile tests. Nitrogen content of the steels varied from 0.26 to 1.00 wt. %. Tensile tests were performed at strain rates from 0.00005 to 0.03 1/s between room temperature and 800 °C. Austenitic HNSs exhibit DSA at elevated temperatures, which is caused by interactions between diffusing solute atoms and moving dislocations. Low activation energies for plastic deformation in the studied steels indicate that the diffusion mechanism is pipe diffusion below 600 °C, and also bulk diffusion at 800 °C. An activation energy for nitrogen diffusion of about 1.95 eV was determined by internal friction measurements.

Effect of Thermal Stresses Along Crack Surface on Ultrasonic Response

Artificial flaws can be manufactured by controlled thermal fatigue loading. The produced cracks can be introduced to a wide variety of materials. This technology gives also a unique opportunity to monitor the ultrasonic response of a crack during thermal loading. This paper reports studies on the effects of different thermal load cycles on the ultrasonic response. The loads are analyzed with FEM. Two cracked samples were loaded with different thermal load cycles.

ADVANCED FLAW MANUFACTURING AND CRACK GROWTH CONTROL

Advanced artificial flaw manufacturing method has become available. The method produces true fatigue cracks, which are representative of most service‐induced cracks. These cracks can be used to simulate behaviour of realistic cracks under service conditions. This paper introduces studies of the effects of different thermal loading cycles to crack opening and residual stress state as seen at the surface of the sample and in the ultrasonic signal. In‐situ measurements were performed under dynamic thermal fatigue loading of a 20 mm long artificial crack.

New Flaws for Qualification of Cast Stainless Steel Inspection

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.Before being applied on larger scale, these new inspection methods must be proven to be effective at identifying discontinuities in CASS material. This presents a problem of its own. Several traditional flaw manufacturing methods cannot be applied to CASS due to the disruption of the parent material. Excavation and welding changes the cast material microstructure and thus significantly affects the inspection results. At the same time, due to the significant wall thickness and inspection limitations, the required qualification flaws can be quite large. Until recently, modern flaw manufacturing techniques, that do not require welding, have not been applied to flaws of this size.In this paper, recent developments will be presented on the manufacturing of thermal fatigue cracks in centrifically CASS material. The presented developments make it possible to use real cracks for demonstrating the effectiveness of CASS inspection techniques.The results also contain first published UT data on this kind of thermal fatigue cracks in CASS and reveal new insight on the inspectability of this difficult material.Copyright

Effect of Stress on Ultrasonic Response in Detection and Sizing of Cracks

Abstract In different NDT techniques huge developments have been achieved during the last few years with regard to crack detection and sizing. In a large range of materials, cracks are one of the most dangerous defect types. A crack is a planar reflector, which is sometimes extremely difficult to detect and to be sized. A crack can be totally open, partly closed or even totally closed because of compressive stresses. The effects of stresses cause problems, for instance, in in-service inspections of nuclear power plants in detection and sizing of closed cracks. This phenomenon causes similar effects in all kinds of plants and components. In this study some experimental inspections have been carried out as well as some FEM calculations of stress field around the crack and compared to corresponding measurements in literature. Materials used for this study are austenitic and ferritic piping steel. The dynamic load applied to the cracks was in form of different thermal cycles. The maximum temperature variations were from 20°C to 600°C depending on each dynamic loading cycle. Different types of ultrasonic methods were used in the measurements. The effect of closure on the response of normal ultrasonic practical probes was recorded. Dynamic loading during ultrasonic measurement gives clear evidence on the effect of the crack closure as well as on the amplitude variation limits in ultrasonic testing.