<p>This thesis focuses on the numerical and theoretical studies of gas quenching in industrial furnaces. Gas quenching is the rapid cooling of metal pieces, aiming at forcing a phase transformation of the metal structure to improve its mechanical properties. The numerical methodology has been evaluated with respect to the desired accuracy and different aspects of the flow with importance for achieving an optimized process have been investigated. Initially, attention was paid to the flow and heat transfer fields both in an empty furnace and in a furnace loaded with different charges with the objective to study the influence of the charge configuration on the flow and heat transfer uniformity. This study led to the identification of several possible improvements, which are currently being implemented by the industrial partners of this project. As earlier studies had shown the importance of flow uniformity on the quality of the heat treatment, the subsequent work focused substantially on the flow uniformity upstream of the quenching zone resulting in design recommendations for the particular type of furnace under consideration. The dependence of the performance of the coolant medium on its composition was investigated theoretically and an analysis of most important parameters was carried out. Improved knowledge of the effect of gas mixture composition on heat transfer was added to the body of knowledge already available.</p>

This thesis focuses on the numerical and theoretical studies of gas quenching in industrial furnaces. Gas quenching is the rapid cooling of metal pieces, aiming at forcing a phase transformation of the metal structure to improve its mechanical properties. The numerical methodology has been evaluated with respect to the desired accuracy and different aspects of the flow with importance for achieving an optimized process have been investigated. Initially, attention was paid to the flow and heat transfer fields both in an empty furnace and in a furnace loaded with different charges with the objective to study the influence of the charge configuration on the flow and heat transfer uniformity. This study led to the identification of several possible improvements, which are currently being implemented by the industrial partners of this project. As earlier studies had shown the importance of flow uniformity on the quality of the heat treatment, the subsequent work focused substantially on the flow uniformity upstream of the quenching zone resulting in design recommendations for the particular type of furnace under consideration. The dependence of the performance of the coolant medium on its composition was investigated theoretically and an analysis of most important parameters was carried out. Improved knowledge of the effect of gas mixture composition on heat transfer was added to the body of knowledge already available. / QC 20101019

In this master's thesis a design of door for helium quenching furnace is described. Based on background research, several options for the design have been compared. All calculations were carried out based on structural needs resulting from high pressure gas quenching technology. By the ending of this thesis a mounting procedure and rough cost price estimation is described. Part of master's thesis is also a virtual 3D prototype and a drawing documentation.

L’utilisation de fluides non vaporisables tels que les gaz pour tremper différentes pièces d’acier rend possible, avec les outils numériques actuels, la simulation de l’ensemble des phénomènes physiques mis en jeu dans ce traitement thermique et ainsi de prédire les comportements thermique, métallurgique et mécanique des pièces. Plusieurs auteurs s’intéressent à la construction de ce type de modèles dont les enjeux sont de promouvoir l’utilisation de la trempe gazeuse haute pression, en cherchant à remplacer les trempes à l’huile plus polluantes et/ou à maîtriser les déformations pendant ou après la trempe. Lors de notre travail, un modèle décrivant ces phénomènes physiques, de l’écoulement gazeux aux déformations, et leurs couplages a été construit par étapes en couplant les deux codes de calcul commerciaux Fluent et Sysweld. Les résultats de simulations ont ensuite été confrontés à des mesures de vitesses d’écoulements de gaz (par PIV), de températures et de déformations in situ (détection de contours). Malgré quelques écarts dont les origines ont été analysées, la comparaison a révélé une très bonne concordance des résultats de calculs et expérimentaux, validant ainsi la démarche de calcul. Le modèle pourrait maintenant être testé pour des refroidissements plus rapides que ceux considérés mais son utilisation pour des applications industrielles (pièces de géométrie plus complexe, refroidissement de charges entières, …) peut d’ores et déjà être envisagée avec confiance / The use of non vaporizable fluids such as gas to quench steel bodies makes the simulation of all the coupled physical phenomena involved in this heat treatment possible thanks to current numerical simulation tools. It gives access to the prediction of thermal, metallurgical and mechanical behaviour of steel pieces during quenching operation. Several authors have tried and build such kind of models in order to promote gas quenching use either by trying to replace less environmental friendly oil treatments or by trying to control distortions during or after quenching. In our work a model describing the physical coupled phenomena, from gas flow to distortion has been built step by step by coupling both commercial numerical codes Fluent and Sysweld. Simulation results have then been compared to gas flow velocities measurements (by PIV), temperature measurements and in situ distortions measurements (contour’s detection method). This confrontation revealed a very good agreement between calculated and experimental data despite some slight differences, which have been analysed in detail. This validated the calculation method used in the model. One of the perspectives of this work could be the validation of the model for higher quenching rate. However, some industrial applications (tridimensional more complex bodies quenching simulation, quenching of full loads, …) can be run from now on with confidence

Heat treatment of different steel products have existed for thousands of years. It has always been an important tool to get the microstructure and resulting properties such as hardness and case hardness and it is even more important today than ever before. This project concentrated on the quenching process and means to decrease the distortion caused by this process. The effect of different oils, temperatures, agitation and if gas quenching could give better results were investigated. The results showed that Miller´s 75 quench oil was better than Park´s 420 at slow agitation and that the viscosity of the oils influenced how much changes in agitation speed and oil temperature affected the distortion. It also shows that gas quenching is an alternative to oil quenching if the microstructure can be improved. Otherwise using Miller´s 75 with low agitation in the Surface combustion furnace will give best results.

The increasing demands for lightweight designs with high strength call for improved manufacturing processes regarding heat treatment of steel. The manufacturing process has considerable potential to improve the mechanical performance and to obtain more reliable results with less variation. The goal of this thesis is to establish new knowledge regarding improved manufacturing processes in industrial heat treatment applications. Three research questions with associated hypotheses are formulated. Process experiments, evaluation of the mechanical performance, and modelling of the fatigue behaviour assist in answering the questions. The gas quenching procedure following low-pressure carburising differs from the conventional procedure of gas carburising and oil quenching. It is shown that the introduction of a holding time during the low-temperature part of the quench has a positive effect on mechanical properties, with some 20 percent increase in fatigue strength. This is attributed to increased compressive surface residual stress and stabilisation of austenite. Tempering is a common manufacturing process step following hardening in order to increase the toughness of the steel. However, the research shows that the higher hardness from eliminating tempering from the manufacturing process is beneficial for contact fatigue resistance. The untempered steel showed not only less contact fatigue damage but also a different contact fatigue mechanism. Straightening of elongated components is made after heat treatment in order to compensate for distortions. The research shows that straightening of induction hardened shafts may lead to lowering of the fatigue strength of up to 20 percent. A fracture mechanics based model is developed to estimate the effects of straightening on fatigue strength. / Ökande krav på höghållfasta lättviktskonstruktioner kräver förbättrade tillverkningsprocesser för värmebehandling av stål. Det finns stor potential att förbättra mekanisk prestanda och att erhålla mer tillförlitliga resultat med mindre variation genom att förbättra tillverkningsprocessen. Målet med denna avhandling är att etablera ny kunskap kring tillverkningsprocesser inom industriella värmebehandlingsapplikationer. Tre forskningsfrågor med tillhörande hypoteser formuleras. Processexperiment, utvärdering av mekanisk hållfasthet och modellering av utmattningsbeteende bygger upp besvarandet av frågorna. Gaskylning som följer lågtrycksuppkolning skiljer sig från det konventionella förfarandet med gasuppkolning och släckning i olja. Resultaten visar att en hålltid i den nedre delen av kylningsförloppet har positiv inverkan på utmattningshållfastheten. Orsaken till förbättringen hänförs till ökade tryckrestspänningar samt stabilisering av austenit. Anlöpning är en vanlig tillverkningsprocess som efterföljer härdning för att öka stålets seghet. Forskningen visar däremot att den högre hårdheten för oanlöpt stål är fördelaktig för motstånd mot kontaktutmattning. Oanlöpt stål visade mindre mängd kontaktutmattningsskador och även en annan skademekanism. Riktning av långa komponenter görs efter värmebehandling för att kompensera för de formförändringar som uppstår. Forskningen visar att riktning av induktionshärdade axlar kan leda till sänkning av utmattningshållfastheten med upp till 20 procent. En brottmekanisk modell som uppskattar effekten av riktning på utmattningshållfasthet presenteras. / <p>QC 20150410</p>

Research on bainite and martensite structures has indicated that lower bainite needles have a refining effect on the lath martensitic structure. Lower bainte needles partitions prior austenite grains and will consequently have a refining effect on the subsequent formed lath martensite. Smaller austenite grains will result in smaller lath martensitic packets and blocks and will result in enhanced mechanical properties.   In order to create a variation of lower bainte structure in a matrix of martensite, two different heat treating methods were tested. The work was focused towards the formation of lower bainite during isothermal heat treating in molten salt, above and below the MS-temperature. Both un-tempered and tempered samples were analyzed .Two different materials were tested, both were AISI 4140 but with a slightly difference in hardenability. The material provided by Ovako Steel is 326C and 326F the later had a higher hardenability. In order to better distinguish the two structures from each other when studied under a microscope, a variation of etching methods were tested.  It was possible to create a variation of lower bainite structures in a matrix of martensite.  326F shows less amount of lower bainite and provides a higher average surface hardness before tempering.

During the last 10 years, low pressure carburizing and high pressure gas quenching has become more popular since it is a “new” process and researchers reports that the distortion characteristics gained from these processes is improved in relation to conventional processes. The aim of this work was to investigate the distortions gained from three different heat treatment processes on main shaft gears and crown wheels. Experiments with atmospheric carburization with oil quenching and low pressure carburization with nitrogen gas quenching or oil quenching were made and distortion characteristics gained from these processes were then measured and compared. It was found that components treaded in the relatively new LPC-HPGQ process are similar and often even better than the conventional method. The larger main shaft gear improves the most with newer method but instead gets a lower core hardness and systematic diametrical shrinkage. Some measurements on the crown wheel were also improved but others were affected negatively.