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 observations and residual stress analysis. Cooling of the duplex stainless-steel bars and their metallurgical characterizations as well as impact toughness results show the technique’s high flexibility and control over temperature evolutions during both water and compressed air cooling. The study of continuous cooling using IJQT in this thesis clearly demonstrates its high potential to be used for other sizes and geometries.  

The results from differential cooling case show that IJQT is flexible enough to provide a wide range of simultaneous cooling rates along the steel plates of different carbon content using both water jets and air jets resulting in various microstructure and hardness gradients. For 0.38-mass% C steel the gradient includes fully martensitic condition with high hardness level transitioning to a slightly softer bainitic region. For the 0.27-mass% C steel the gradient includes almost fully hardened state smoothly transitioning to a soft pearlitic region covering a wide range of hardness levels within a component. The results from physical experiments on differential quenching facilitated further modelling approach for exploring carbon steels in terms of their applicability for microstructure- and hardness gradient formation using different cooling strategies.  

The research in this thesis provides a deeper understanding of how microstructures and properties of steel components can be manipulated using IJQT to achieve specific requirements depending on the steel type in question. In long term, it is believed that the approach used in this thesis will contribute to the development and establishment of an advanced digital tool for optimal selection of alloys and corresponding cooling strategies thus reducing the experimental time and unnecessary emissions. 

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 duplexstängerna av rostfritt stål och deras metallurgiska egenskaper samt resultat av slagseghetstester visar teknikens höga flexibilitet och kontroll över temperaturutvecklingen under både vatten- och tryckluftkylning. Studiet av kontinuerlig kylning med IJQT i denna avhandling visar tydligt dess höga potential att användas för andra storlekar och geometrier.  

Resultaten från differentiell kylning visar att IJQT är tillräckligt flexibel för att tillhandahålla ett brett utbud av samtidiga kylningshastigheter längs stålplåtarna med olika kolhalter genom att använda både vattenstrålar och luftstrålar, vilket resulterar i olika mikrostruktur- och hårdhetsgradienter. För 0.38 mass% C stål inkluderar gradienten helt martensitiskt tillstånd med hög hårdhetsnivå som övergår till ett något mjukare bainitisktt område. För 0.27 mass% C stål inkluderar gradienten nästan helt härdat tillstånd som smidigt övergår till ett mjukt perlitiskt område som täcker ett brett spektrum av hårdhetsnivåer inom en komponent. Resultaten från fysikaliska experiment på differentiell härdning underlättade ytterligare modelleringsmetoder för att utforska kolstål i termer av deras tillämpbarhet för mikrostruktur- och hårdhetsgradientsbildning med hjälp av olika kylningsstrategier.  

Forskningen i denna avhandling ger en djupare förståelse för hur mikrostrukturer och egenskaper hos stålkomponenter kan manipuleras med IJQT för att uppnå specifika krav beroende på vilken ståltyp det gäller. På lång sikt är förhoppningen att det tillvägagångssättet som används i denna avhandling kommer att bidra till utvecklingen och etableringen av ett avancerat digitalt verktyg för optimalt val av legeringar och motsvarande kylningsstrategier, vilket minskar experimenttiden och onödiga utsläpp. 

The demand for steel components with tailored properties is constantly growing. To obtain a specific variation of microstructures and mechanical properties along the component it must undergo a controllable cooling. One way to control the cooling rates along the component is by using different simultaneous water jet impingements on a hot austenitized surface. This can be done by a newly developed test rig for water Impinging Jet Quenching Technique (IJQT). This work discusses the effect of IJQT on mechanical properties and fracture behavior of 15 mm steel plates containing 0.27 and 0.38 mass-% carbon. The samples were cooled in a specifically designed setup of the technique to obtain simultaneous water and air cooling resulting in diverse microstructures. The mechanical property gradients of both steels were analyzed through hardness measurements and tensile tests. The fracture surfaces and the near fracture regions were observed using scanning electron microscope and light optical microscope respectively. The results from tensile tests showed that the larger part of the sample with higher carbon content was fully hardened, however smoothly transitioning to a more ductile region. The sample with lower carbon content combined various degrees of hardening and transitioned from higher to lower ultimate tensile strength values. Fracture behavior of higher carbon steel was predominantly brittle transitioning to a ductile, while the lower carbon steel had a small region showing brittle fracture transitioning to a larger region of predominant ductile fracture behavior.

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 of thick sheets and uniform quenching of bars to different depths. Differential cooling was performed on square-shaped carbon steel sheets with thickness of 15 mm, and the uniform quenching with different flow rates was performed on carbon steel cylindrical bars with 100 mm in diameter. Along with the physical experiments, Comsol Multiphysics 5.6 software was used to solve a 1D heat transfer problem to estimate the cooling rate profile along the radius of the bar. The experiments were verified by observations and characterization of the microstructure using light optical microscopy (LOM), and by examining the mechanical properties through tensile tests and hardness measurements. The results of the quenching experiments and verifications showed a high potential and flexibility of the IJQT in differential cooling case as well as in the uniform quenching case.