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From Bloomery Furnace to Blast Furnace Archeometallurgical Analysis of Medieval Iron Objects from Sigtuna and Lapphyttan, Sverige

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From Bloomery Furnace to Blast Furnace Archeometallurgical Analysis of Medieval Iron Objects from Sigtuna and Lapphyttan, Sverige EXAMENSARBETE INOM TEKNIK, GRUNDNIVÅ, 15 HP STOCKHOLM, SVERIGE 2019 From Bloomery Furnace to Blast Furnace Archeometallurgical Analysis of Medieval Iron Objects From Sigtuna and Lapphyttan, Sverige ANDREAS HELÉN ANDREAS PETTERSSON KTH SKOLAN FÖR INDUSTRIELL TEKNIK OCH MANAGEMENT Abstract During the Early Middle Ages, the iron production in Sweden depended on the bloomery furnace, which up to that point was well established as the only way to produce iron. Around the Late Middle Ages, the blast furnace was introduced in Sweden. This made it possible to melt the iron, allowing it to obtain a higher carbon composition and thereby form new iron-carbon phases. This study examines the microstructure and hardness of several tools and objects originating from archaeological excavations of Medieval Sigtuna and Lapphyttan. The aim is to examine the differences in quality and material properties of iron produced by respectively blast furnaces and bloomery furnaces. Both methods required post-processing of the produced iron, i.e. decarburization for blast furnaces and carburization for bloomeries. These processes were also studied, to better understand why and how the material properties and qualities of the items may differ. The results show that some of the studied items must have been produced from blast furnace iron, due to their material composition and structure. These items showed overall better material quality and contained less slag. This was concluded because of the increased carbon concentration that allowed harder and more durable structures such as pearlite to form. The study also involved an investigation of medieval scissors, also known as shears, made from carburized bloomery furnace iron. Here, one specific aim was to find out if the different sections of the shears had different properties, and if so, if these properties correlated with the functions of the different parts of the shears. Our microstructure analysis showed that the blade indeed was the hardest part due to intentional carburization and forming of martensite. The blade is connected to a softer ferritic handle, which in turn is connected to a ductile bow, also ferritic but with a larger grain size. Keywords: bloomery furnace, blast furnace, iron production, Middle Ages Sammanfattning Den svenska järnproduktionen var under medeltiden beroende av blästerugnen som då var väl etablerad i hela landet. Under denna period introducerades även masugnen i Sverige, vilket gjorde det möjligt att smälta järn. Den nya tekniken gjorde det möjligt att uppnå en ökad kolhalt och därmed bilda nya järn/kol-faser. Den här studien undersöker mikrostrukturer och sammansättningar i medeltida järnföremål från arkeologiska utgrävningar i Sigtuna och Lapphyttan. Syftet är att undersöka vilka egenskaper och materialkvaliteter som gick att uppnå i järn som tillverkats med masugn respektive blästerugn. Båda tillverkningsmetoderna kräver efterbearbetningar som förbättrar järnets egenskaper. Även dessa efterbearbetningsmetoder studeras, för att kunna dra slutsatser angående hur det slutligt producerade järnets egenskaper och materialkvalitet skiljer sig mellan de två produktionsmetoderna. Analyserna visade att järn som tillverkats i masugn innehöll mindre slagg och generellt var av bättre kvalitet. Detta järn innehöll även mer kol vilket tillät perlit att bildas. Därmed blev järnet betydlig hårdare. Slutsatsen är att dessa järnföremål hade övergripande bättre mekaniska egenskaper och materialkvalitet än de järnföremål som tillverkats med järn från en blästerugn. I studien undersöks även medeltida fårsaxar, tillverkade av järn från en blästerugn som sedan genomgått uppkolning. Syftet är att utifrån mikrostrukturen i materialet ta reda på om de olika sektionerna i en fårsax har olika egenskaper, och om dessa egenskaper i så fall är anpassade till den aktuella sektionens användning.Analysen visade att bladet var hårdast på grund av avsiktlig uppkolning och martensitbildning. Därefter följde ett mjukare handtag med en ferritisk struktur. Den böjda delen av saxen var den mest duktila och fjädrande, eftersom den uppvisade en rent ferritisk struktur med större kornstorlekar än i handtaget. Nyckelord: Blästerugn, masugn, järnproduktion, medeltiden Table of Contents 1. Introduction ........................................................................................................................................ 1 1.1 Purpose ......................................................................................................................................... 1 1.2 Limitations ..................................................................................................................................... 1 2. Background ......................................................................................................................................... 2 2.1 The importance of carbon in steel ................................................................................................ 2 2.2 Bloomery Furnace ......................................................................................................................... 3 2.3 Blast furnace ................................................................................................................................. 4 2.4 Osmund process ............................................................................................................................ 5 2.5 Carburization ................................................................................................................................. 6 2.6 Finery forge ................................................................................................................................... 6 3. Methods and material ........................................................................................................................ 7 3.1 Knives ............................................................................................................................................ 7 3.2 Shears ............................................................................................................................................ 7 3.3 Ingots............................................................................................................................................. 8 3.5 Sample preparation .................................................................................................................... 10 3.6 Microstructure and analysis ........................................................................................................ 10 3.7 Vickers test .................................................................................................................................. 10 3.8 SEM ............................................................................................................................................. 11 4. Results ............................................................................................................................................... 12 4.1 Light microscope images ............................................................................................................. 12 4.2 Vickers hardness test .................................................................................................................. 21 4.3 Sem analysis ................................................................................................................................ 21 5. Discussion .......................................................................................................................................... 25 5.1 Ingots........................................................................................................................................... 25 5.2 Knives .......................................................................................................................................... 26 5.3 Shears .......................................................................................................................................... 26 5.4 Vickers hardness test .................................................................................................................. 27 5.5 Ethical aspects ............................................................................................................................. 28 5.6 Sources of error .......................................................................................................................... 28 6. Conclusion ......................................................................................................................................... 29 7. Further investigation ......................................................................................................................... 30 8. Acknowledgement ............................................................................................................................ 31 9. References ........................................................................................................................................ 32 1. Introduction During the middle ages Sweden experienced many technological advances in the field of metallurgy, one of which was the ability to melt iron and achieve a much higher carbon content. This was made possible thanks to the blast furnace which could reach a much higher temperature than the bloomery furnace that had been used before. In conjunction with this came the Osmund
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