metals Article An investigation of the Temperature Distribution of a Thin Steel Strip during the Quenching Step of a Hardening Process Pouyan Pirouznia 1,2,3, Nils Å. I. Andersson 1,* , Anders Tilliander 1 and Pär G. Jönsson 1 1 Division of Processes, Department of Material Science and Engineering, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden; [email protected] (P.P.); [email protected] (A.T.); [email protected] (P.G.J.) 2 Department of Material Science and Engineering, Dalarna University, SE-791 88 Falun, Sweden 3 Research & Development Department, voestalpine Precision Strip AB, SE-684 28 Munkfors, Sweden * Correspondence: [email protected]; Tel.: +46-8-7908381 Received: 10 May 2019; Accepted: 8 June 2019; Published: 11 June 2019 Abstract: The dimension quality of the strip within the hardening process is an essential parameter, which great attention needs to be paid. The flatness of the final product is influenced by the temperature distribution of the strip, specifically across the width direction. Therefore, based on physical theories, a numerical model was established. The temperature of the strip for the section before the martensitic transformation was objected in the predicted model by using a steady state approach. In addition an infrared thermal imaging camera was applied in the real process in order to validate the results and to improve the boundary conditions of the numerical model. The results revealed that the temperature of strip decreased up to 250 ◦C within the area between the furnace and the quenching bath. This, in turn, resulted in significant temperature difference across the width of the strip. This difference can be up to 69 ◦C and 41 ◦C according to the numerical results and thermal imaging data, respectively. Overall, this study gave a better insight into the cooling step in the hardening process. In addition, this investigation can be used to improve the hardening process as well as an input for future thermal stress investigations. Keywords: continuous hardening process; martempering; heat transfer; numerical modelling; CFD 1. Introduction Martensitic stainless steel strips, are commonly produced by using a hardening and tempering line. The purpose of the hardening process is to form a desired martensitic structure. The martempering process used as the hardening stage consists of: A controlled atmosphere furnace; • A martempering media where the strip is cooled to the temperature just above the • martensitic transformation; A final quenching where the strip reaches the room temperature. • A molten salt (160 to 400 ◦C) and a hot oil (up to 205 ◦C) are commonly used as the martempering media for a conventional batch hardening of a component [1]. Furthermore, Ebner [2] showed that significant advantages with regards to flatness can be obtained by using a molten lead-bismuth eutectic (LBE) alloy as a martempering media followed by an air jet cooling compared to when using quenching in oil. Later, Lochner [3] compared quenching in a molten metal bath, hydrogen jet and oil and their effects on the flatness of the strip. In addition, Lochner [4] emphasized the usage of the LBE bath as the martempering media by summarizing its important advantages and focusing on improvements in the Metals 2019, 9, 675; doi:10.3390/met9060675 www.mdpi.com/journal/metals Metals 2019, 9, 675x FOR PEER REVIEW 22 of of 15 14 LBE bath as the martempering media by summarizing its important advantages and focusing on dimensional precision and flatness of the strip. Various methods of cooling the LBE bath to obtain a improvements in the dimensional precision and flatness of the strip. Various methods of cooling the natural convection in the media were also compared. LBE bath to obtain a natural convection in the media were also compared. An LBE bath is used as a martempering media in the continuous hardening process which is An LBE bath is used as a martempering media in the continuous hardening process which is mainly used to produce thin strips for: Valve steel, springs and blades for the paper & printing mainly used to produce thin strips for: Valve steel, springs and blades for the paper & printing industry. Figure1 illustrates a schematic view of the specific hardening process, before the martensitic industry. Figure 1 illustrates a schematic view of the specific hardening process, before the transformation step. martensitic transformation step. Figure 1. SchematicSchematic view view of of the the hardening process befo beforere a martensitic phase transformation at voestalpine, Presicion Strips AB, Munkfors, Sweden. The strip is transported within the muffle muffle of the furnace into the LBE LBE bath bath through through a a sealing sealing box, box, a so called gas box. A gas inlet located inside the gas box, which provides the reducing hydrogen atmosphere of the furnace. Flatness and geometrical defects can still be observed in the thin strips despite all advantages of using LBE asas aa martempringmartempring mediamedia in in the the hardening hardening process. process. A A better better comprehension comprehension of of the the process process is requiredis required in in order order to to investigate investigate how how those those defects defects may may form. form. The unevenuneven temperaturetemperature di differencefference were were referred referred by manyby many researchers researchers as one as of one the mainof the reasons main forreasons the cause for the of flatnesscause of defects. flatness Thelning defects. 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