AREVA NP Uddcomb AB wants to replace the material used for a specific valve seat used in boiling water reactors, BWR. Their solution is a weld overlay of different stainless steels composed of two buffer layers of the steel 309 MoL followed by two layers of the filler material SKWAM welded on type 316 stainless steel or carbon steel. The report focuses on the long term structural effects in the weld overlay due to the operating temperature in BWRs, in this case 270 °C. To investigate the thermodynamic stability in the weld overlay the computer software Thermo-Calc was used and a metallographic examination was carried out. The results from these procedures were compared and possible long term effects were discussed. Most likely spinodal decomposition is the most severe structural change that may appear in the material. At equilibrium conditions at the operating temperature ferrite is decomposed into Fe-rich and Cr-rich ferrite but since the kinetics is not included in the calculations it is not possible to determine the rate of decomposition.

The background to this bachelor thesis is an on-going project within the VINN Excellence Center Hero-m. The task in this thesis is to perform a literature survey about the martensite transformation and investigate how the resulting fraction depends on cooling below the Ms-temperature. Instead of calculating the undercooling for each of the known fractions of martensite the driving force will be evaluated. Several efforts have been made through the years to describe the relationships between fraction transformed austenite and temperature. The approaches to the first models were empirical and derived from collections of data regarding the amount of retained austenite at different quenching temperatures. Lately, studies have been made to derive a thermodynamical relationship using how the Gibbs energy is affected by increments in volume transformed austenite. Two equations are derived by calculating the resulting driving force at different known quenching temperatures and the respective percentage transformed martensite found in previous works. The data for the steels used show a characteristic slope when linearised. A trend for the steels which have a high characteristic slope is that they also have a high Ms temperature, and the steels which have a low characteristic slope tend to have a low Ms. Previous relationships which describe the martensitic transformation have considered the importance of the Ms temperature only in it being a starting temperature for the transformation. To further incorporate the Ms temperature in the equations presented, further research of the martensitic transformation is required. The approach in this thesis of using thermodynamically calculated data is a base for further investigation of the range of the martensite transformation.

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

The influence of basicity, heat treatment as well as different oxygen partial pressures on the phase relationships in the CaO-MgO-SiO2-Cr2O3 slags was studied with a view to control the precipitation of Cr-spinel in the slag phase. The equilibrium phases in CaO-MgO-SiO2-Cr2O3 slag system in the range on 1673-1873 K have been investigated under low oxygen partial pressure as well as in as air atmosphere. In low oxygen partial pressure experiments, a suitable mixture of CO and CO2 was used to control the oxygen partial pressure. The oxygen partial pressure was kept at 10-4 Pa. The Cr2O3 and MgO contents in the slag were fixed to be 6 and 8wt% respectively. The basicity (CaO/ SiO2) of the slag was varied in the range 1.0-2.0. Gas/slag equilibrium technique was adopted to synthesize the slag at a suitable temperature above the liquidus point. One heat treatment procedure is that the samples were heated to and soaked at 1873 K for 24h in order to achieve the equilibrium state and subsequently quenched in water. The other is that the samples were heated to and soaked at 1873 K for 24h, then slow cooled to 1673 K and soaked at this temperature for additional 24h in order to achieve the equilibrium state at lower temperature before quenching in water. The chromium distribution and phase compositions in the quenched slag were studied using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction techniques (XRD). FACTsage software was used for the phase equilibrium calculations. The experimental results obtained from the present work are compared with the calculation results from FACTsage software as well as with results from samples directly quenched after soaking at 1873K. It is found that the spinel formation at 1873 K in air atmosphere is favored in the slag basicity range of 1.0 to 1.6. The size of spinel crystals increased drastically after slow cooling followed by annealing compared to samples being quenched after soaking at 1873 K. The amount of foreign elements dissolved in the spinel phase, and matrix phases decreased after slow cooling followed by annealing at lower temperature, resulting in purer phases with less defects. It was found that the amount of foreign elements in the spinel phase, and other phases decreased after soaking at very low PO2. The size of the spinel crystals was found to be larger in samples with low basicity. Spinel phase precipitation has improved in the samples with higher basicities compared to the results obtained in air. / QC 20111208

The production of metal powders by water atomisation is a well established process, which can be used to produce a wide range of particle sizes. A careful control of the particle size distribution is necessary, to atomise powders with a high quality and at a low production cost. Therefore, it is necessary to have a substantial knowledge of the relation between operational parameters and the particle size, to be able to produce water atomised metal powders with consistent and high yields. The main purpose with this thesis was to increase the knowledge about factors which affect the mass median particle size (d50) for water atomised metal powders. The specific objectives with the study were to develop a theoretical d50 model and to investigate the relation between the particle size and the physical properties of the liquid metal. Pilot scale experiments for liquid iron showed that alloy additions of carbon and sulphur decreased the d50 value, at a maintained liquid steel temperature before atomisation. Moreover, it was indicated that the reduced particle size at increased %C and %S contents may be related to a decreased viscosity and surface tension of the liquid metal, respectively. An alternative explanation could be that raised superheats at increased carbon contents increased the total available time for atomisation, which may have contributed to a reduction of the d50 value. The theoretical d50 model developed in this work showed a very good correlation to the current experimental data. The model considers the influence of surface tension, viscosity, melt stream diameter, water pressure, water jet angle and water to metal ratio. This model was further used to analyse how the d50 value was influenced by the viscosity and the surface tension. A reduced viscosity from 4∙9 to 2∙1 mPa s decreased the d50 value with 33%. In addition, the particle size was estimated to decrease with 21% by decreasing the surface tension from 1840 to 900 mN m-1. / Q 20120529

In this thesis work, some basic concepts about desulphurizationof hot metal have been done based on a literature study. Two experimentaltrials have also been carried out to study the slags: one consider as areference and in the other trial, the amount of calcium carbide was reduced by150 kg. The average carbide efficiency has been improved from 21.3% in trial 1 to 26.0% in trail 2. Metaldroplets containing iron oxides are found in three of eight heats and most ofmetal droplets are surrounded by Ti-compounds. The slags mainly consisted of (Ca,O, Si) and (Ca, S), with some low content of other elements.

If these macro inclusions are not detected before the material is put in used, its mechanical properties are greatly affected and this can lead to outrageous consequences in the engineering application.

Stainless steel family grades are very famous for their combined corrosion resistance and high mechanical properties. These properties can be improved further by decreasing the content of impurities like carbon and oxygen. The main purpose of this research work is to study the possibility of stainless steel powder decarburization by vacuum annealing. The influence of different process parameters like treatment time, temperature, fraction size and depth of the powder layer on the decarburization process was analyzed. The investigation results showed that it is possible to achieve extra low values of carbon and oxygen in steel powder by processing it with optimum process parameters.

This report is about continuous temperature measurements of pig-iron at tapping from blast furnace 2 in Oxelösund, SSAB. Nowadays the temperature is only checked once at every tapping. The purpose of this project is to see if the process stability increases by continuously knowing the temperature and to compare the costs of this new technique with the technique used today. Possible savings due to less consumption of coke/coal if the silicon amount and the temperature are closer to their aim values will be regarded and if as little steam consumption as possible are used. The process stability can be divided into different sub goals. The ordinary measuring techniques were investigated as a part of the main goal; such as the ordinary measured temperature, the pig-iron and slag samples. The new continuous temperature measuring technique was compared with the ordinary temperature measurement and investigations of the life length were done. How representative the pig-iron and slag samples are, when taking them at the time they are today, are also looked into. The continuous measured temperature showed around 0,37 % higher temperature than the ordinary measured temperature. The pig-iron and slag samples should be taken as they are today, for mainly safety aspects. By using continuous temperature measurement, some of the sub goals can be achieved for a more stable process. The economy on the other hand has shown that large savings can be done by using this continuous temperature method due to a more stable process. This is mainly because of a decrease in steam usage in the experimental period. By regarding only the material of the methods the continuous temperature equipment is a bit more expensive, but the savings are much larger so the continuous temperature method is beneficial. With time this method could probably improve the process stability even more since the operators will deal with the information and the probe better.

The mechanical properties of components made by PM steels are normally inferior to those made by alternative processes. One of the main reasons is that a large amount of pores are present in sintered components. The other main reason is that the alloying elements, particularly Ni, are not uniformly distributed after conventional sintering procedures.  This work is aimed at a better understanding of the influence of alloying additions on mechanical properties and homogeneity of the microstructure. The experimental work has been carried out in two trials. Trial 1 was performed to investigate mechanical properties of Distaloy powders (commercial grades) and second trial to examine influence of alloying additions on homogeneity of microstructure.  For trial 1, as-sintered and heat treated specimens were produced by mixing commercial powders with two different carbon levels. Whereas, alloying elements were admixed to base iron powder for producing  sintered specimens for trial 2. Mechanical properties including dimensional changes, micro-hardness, tensile strength and impact resistance were measured. Distribution of alloying elements was studied using LOM and SEM-EDS analysis. The results obtained show that additions of alloying elements enhance the mechanical properties. Moreover, interaction of C with Cu and Ni as well as interaction between Cu and Ni have a deceive role in determining final properties of the components. The metallographic investigation indicated that major reasons of heterogeneous microstructure are slow diffusion of Ni in Fe matrix and interaction of other alloying elements with Ni.  The results of trial 2 showed that addition of Mo and Cu to Ni-containing PM steels improves the distribution of Ni in Fe matrix. Mo results in improved uniformity of microstructure by lowering the chemical potential of carbon. In Ni and Cu containing alloys, the interaction between Ni and Cu is responsible for enhanced distribution of Ni. However, the improved Ni distribution is achieved at the expense of non-uniform distribution of Cu. In Ni-containing PM steels, improved microstructure homogenization can be attained by increasing Ni-Cu interaction, lowering the surface energy of Ni-Cu liquid and decreasing the chemical potential of carbon.