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Soil-working tools in agriculture are made of boron-containing steels with high wear resistance and hardenability. Nevertheless, these tools are subject to high impacts, abrasive wear, and fatigue and are therefore prone to failure. To combine varying levels of properties within one component in as-quenched condition can be beneficial for such products. To obtain this property variation, a component must undergo a complex and controllable cooling. Therefore, the aim of this work is to obtain a microstructure gradient along two 15 mm-thick steel plates in a newly developed test rig by water jet impingement technique to confirm its controllability and flexibility. Furthermore, a quenching simulation model is created for hardness prediction using phase transformation data from a machine learning tool. Microstructure variation is observed using light optical microscopy and the electron backscatter diffraction technique. Mechanical properties are studied through tensile tests and hardness measurements and are also compared with simulation results. The 0.27 mass% C steel sample is obtained in almost fully martensitic state transitioning to a softer ferritic/bainitic condition, while the 0.38 mass% C steel sample results predominantly into a fully hardened martensitic state and slightly shows ferritic and bainitic features along the sample. The quenching simulation model shows promising hardness prediction for both steels.

Heat-treatment plays a crucial role in manufacturing and production of steel components since its aim is to provide necessary material properties to fulfil the in-service requirements and comply with the safety measures. Cooling of steel components is a necessary step in heat-treatment and associated with different microstructure manipulations that control the mechanical and other properties, depending on the steel type in question. Common issues of this process, for example in large-scale production of thick bars or in small scale-production of automobile or agricultural steel components, include insufficient cooling rate that leads to poor properties, undesirable residual stresses that may lead to cracks and premature failure, as well as insufficient spatial and temporal control over the quenched sample in case when gradients of mechanical properties are required. To avoid the mentioned issues and improve the control during cooling process, a newly developed test rig for Impinging Jet Quenching Technique (IJQT) is used in this thesis to investigate the spatial and temporal behavior of various steel types during different cooling routes. The study is focused on two main cases: continuous cooling of solid cylindrical bars and differential cooling of thick plates. The continuous cooling of bars involves water quench-hardening of low-alloyed carbon steel and different cooling routes of super duplex stainless-steel. The differential cooling of plates involves cooling of different types of carbon steels with various hardenability levels using water and compressed air. The work includes an accurate temperature recording from cooling experiments and metallurgical characterizations for validation of cooling simulation models. The results from continuous cooling case showed that martensite hardening of carbon steel bars with water jets can be controlled using IJQT and correlate well with the simulation model which is validated by hardness measurements, microstructure ob, Värmebehandling spelar en avgörande roll vid tillverkning och produktion av stålkomponenter eftersom dess syfte är att tillhandahålla nödvändiga materialegenskaper för att uppfylla driftkraven och säkerhetsåtgärderna. Kylning av stålkomponenter är ett nödvändigt steg i värmebehandlingen och förknippat med olika mikrostrukturmanipulationer som styr de mekaniska och andra egenskaperna, beroende på vilken ståltyp det gäller. Vanliga problem med denna process, till exempel vid storskalig produktion av tjocka stänger eller vid småskalig produktion av bil- eller jordbruksstålkomponenter, inkluderar otillräcklig kylhastighet som leder till otillräckliga egenskaper, oönskade restspänningar som kan leda till sprickor och för tidigt brott, såväl som otillräcklig rumslig och tidsmässig kontroll över det släckta provet i fall då gradienter av mekaniska egenskaper krävs. För att undvika de nämnda problemen och förbättra kontrollen under kylningsprocessen, används en nyutvecklad testrigg för Impinging Jet Quenching Technique (IJQT) i denna avhandling för att undersöka det rumsliga och tidsmässiga beteendet hos olika ståltyper under olika kylvägar. Studien är fokuserad på två huvudfall: kontinuerlig kylning av cylindriska stänger och differentiell kylning av tjocka plåtar. Den kontinuerliga kylningen av stänger involverar vattenhärdning av låglegerat kolstål och olika kylvägar av superduplext rostfritt stål. Den differentiella kylningen av plåtar innebär kylning av olika typer av kolstål med olika härdbarhetsnivåer med hjälp av vatten och tryckluft. Arbetet inkluderar en noggrann temperaturregistrering från kylexperiment och metallurgiska karakteriseringar för validering av kylsimuleringsmodeller. Resultaten från kontinuerligt kylningsfall visade att martensithärdning av kolstålsstänger med vattenstrålar kan kontrolleras med IJQT och korrelerar väl med simuleringsmodellen som valideras genom hårdhetsmätningar, mikrostrukturobservationer och restspänningsanalys. Kylning av dupl, Funding: Sweden’s Innovation Agency Vinnova, Swedish Knowledge Foundation, Swedish Agency for Economic and Regional Growth, SSAB, Väderstad Components, Outokumpu, and University of Gävle (UoG)

Hardening of carbon steel products by austenitization and immersion in a quenching medium is a widely used heat treatment to obtain a hard and strong martensitic structure. To avoid the undesired consequences, such as residual stresses or insufficient hardening depth, the cooling rates must be accurately measured and controlled. This can be achieved using the impinging water jet quenching technique. The aim of this work is to perform hardening of four low-alloyed 70 mm cylindrical carbon steel bars, using impinging water jet quenching technique with different jet flow rates, and to analyze its effect on thermal evolution and residual stresses. The temperature evolution during quenching experiments is recorded and used as input to a comprehensive quenching model to predict phase transformations, final hardness, and residual stresses of cylindrical bars. All four quenching experiments result in a fully hardened martensitic state. Furthermore, a decrease in jets’ flow rate, within a certain interval, results in different thermal histories and in lower compressive residual stresses on the surface. The results from quenching simulations show promising hardness, microstructure, and residual stress predictions that are validated by hardness measurements, optical microscopy, and residual stress analysis using X-Ray diffraction method.

This report summaries the work within the project ”Effect of spatial-temporal behavior of a newly developed cooling system on carbon and stainless-steel bar properties”. The project was conducted from 2020-01-01 to 2022-12-31 and was co-produced by SSAB, Outokumpu and University of Gävle (UoG). The Knowledge Foundation, SSAB, Outokumpu and UoG financed the project. For the Swedish steel companies SSAB and Outokumpu producing special steels, it is very important to be able to control the cooling process in order to produce steel bars with excellent properties. Both steel companies also want to be able to control the cooling process so that the excellent steel properties become even over the bars’ spatial configuration. The aim of the present project is to reveal the spatial-temporal behavior of a newly developed cooling technology in order to produce steel bars with excellent properties and to control the phase transformation to achieve optimal performance of the steel bars. By using the special test rig at the UoG, detail temperature measurement mapping, invers solution and direct numerical simulation, the present project has identified and quantified several important aspects related to the quenching process, operating conditions, and temperature field development within the investigated products. The result from the proposed cooling process provides an outstanding cooling rate that is very crucial to obtain the required steel phase and thus the correct properties of the bar with different sizes. Results from this study have also shown that the cost per kg product can be reduced by tunning the process parameters such as soaking time and bar temperature before starting the cooling process. In addition, both experimental and numerical results of the material investigation show that the cooling technology has resulted in the desired phase transformation and subsequently the desired steel phases and material properties. The results show that the cooling technology and, Funding: The Knowledge Foundation, SSAB, Outokumpu and University of Gävle, Effect of spatial-temporal behavior of a newly developed cooling system on carbon and stainless steel bar properties

Austenitization followed by quenching is a well-known conventional heat-treating procedure which is widely used on carbon steels with the aim to obtain high strength in as-quenched condition. Such quenching is usually done by immersing a steel product into the cooling medium which provides a uniform cooling of the surface. The cooling rate can be adjusted to a certain degree on a “component” length-scale by using different cooling mediums such as water, oil, polymer solution, etc. However, certain steel products such as beams, pillars in automobile industry or different machinery parts in agriculture require a proper and controllable cooling gradient and thus mechanical property gradient within the product. It is difficult to control the cooling rates locally on the length-scale smaller than the product only by replacing the quenching medium. In addition, quenching by immersing the product into the cooling medium is accompanied by thermal stresses due to the different cooling rates of the surface and the core, and also accompanied by transformation stresses due to the volume change during phase transformations. These stresses may lead to negative effects such as undesired residual stresses or even cracks. Therefore, cooling must be properly optimized and controlled to eliminate these drawbacks. Such a controllable cooling can be performed by several impingements of the water jets onto a hot austenitized surface at certain locations. By controlling the water flow, number of jets, their locations and other parameters, the global and the local cooling rates can be optimized for a specific industrial application later on. This thesis demonstrates the potential and capability of the water Impinging Jet Quenching Technique (IJQT) to provide a flexible and controllable cooling for both differential and for uniform quenching cases. The test rig of IJQT was developed in the University of Gävle and was used to perform quenching experiments in this study: differential cooling, Funding agencies: For financial support Sweden’s Innovation Agency Vinnova, SSAB,Väderstad Components, Swedish Knowledge Foundation and Universityof Gävle are acknowledged.