Severe Plastic Deformation of Ferritic and Austenitic Steels

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Chapter IV the designations denote the different processing temperatures of each sample. The hardness of the initial as-received material is about 160HV1. Fig. 4.2: Vickers hardness distributions given as a function of the distance for grade A220 (Fe-Cr-Ni) and grade P555 (Fe-Cr-Mn-N) at different HPT deformation temperatures. See text for further details. The characteristic feature of HPT deformed discs is the development of an inhomogeneous radial dependent microstructure since the deformation is based on the torsional straining of the small sample. Fig. 4.2 demonstrates the radial distributions of microhardness measurements in several HPT samples with various processing temperatures. The error ranges were calculated from 6 measurements at each distance with the 95% confidence interval. It is apparent that the highest hardness values are found at the edges and the lowest at the centre of the disks. However, the induced microstructural refinement depends strongly on the local strain at any point within the disk. Independent on the investigated austenitic steel grade, the radial hardness dependency is intensified when the deformation temperature is increased (Fig. 4.2b and 4.2d). By comparing the two austenitic steels it is obvious that in grade P555 (Fe-Cr-Mn-N) higher values are found than in grade A220 (Fe-Cr-Ni). The high solubility of nitrogen in this austenite with a coarse grained microstructure results in higher toughness and improved strength. Thus, Fe- 30
Chapter IV Cr-Mn-N alloys are promising alternatives to Fe-Cr-Ni-based austenitic stainless steels. Fig. 4.3: Vickers hardness in the saturation as a function of the processing temperature for a) grade A220 (Fe-Cr-Ni) and b) grade P555 (Fe-Cr-Mn-N). In Fig. 4.3 the hardness is summarized as a function of the applied processing temperature. The hardness values correspond to the saturated edge near region at a radius of 3 mm. The most obvious characteristic is the distinctive rise in hardness with increasing deformation temperature, especially in Fig 4.3b. By increasing the processing temperature the hardness values rise continuously up to a temperature of about 600°C independent on the grade. Beyond this critical temperature a rapid decline in strength is observed. Within a temperature range of -196°C and 450°C almost the same strength was observed for grade A220 (Fe-Cr-Ni). By contrast, a steep increase in hardness was detected for grade P555 (Fe-Cr-Mn-N) which is based on the alloying concept as well as on the different deformation mechanisms. By contrast, one would expect higher values of the microhardness at cold-HPT and lower values at warm-HPT deformation, respectively. It seems that both investigated austenitic steels have their best performance in strength at a temperature of about 600°C. Such high temperatures lead to the evolution of a soft centre with a pronounced gradient in the Vickers hardness (see Fig. 4.2b and 4.2d). The clarification of the present abnormal hardness evolution with increasing deformation temperature is the main focus of the microstructural investigations and is discussed below. It is important to note that the present measurements provide information on the distribution of the microhardness of the deformed disks rather than direct information on the homogeneities of the internal microstructure. The significant influence of the deformation temperature on the in-situ torque in respect to the applied strain is depicted in Fig. 4.4 below. The curves were carefully collected by a strain-gauge unit during HPT at elevated processing temperatures. Fig 4.4a represents the torque curves of grade A220 in a temperature range between -196°C and 540°C. 31
Page 1: Severe Plastic Deformation of Ferri
Page 4 and 5: Affidavit IV I declare in lieu of o
Page 6 and 7: Danksagung Mein Dank gilt natürlic
Page 8 and 9: Abstract thermal treatments for sho
Page 10 and 11: Kurzfassung Das großtechnisch am w
Page 12 and 13: Contents IV.4 Evolution of the Micr
Page 14 and 15: Chapter I SPD provide several benef
Page 16 and 17: Chapter I I.3 Principle of High Pre
Page 18 and 19: Chapter I Fig. 1.3: The big HPT too
Page 20 and 21: Chapter I behaviour. Slip dissoluti
Page 22 and 23: Chapter II Fig. 2.1: Steady-state m
Page 24 and 25: Chapter II Fig. 2.4: In-situ torque
Page 26 and 27: Chapter II 14
Page 28 and 29: Chapter III 16 Fe C Cr Cu Ni Co Si
Page 30 and 31: Chapter III III.2 Magnetic Properti
Page 32 and 33: Chapter III cooling-down process to
Page 34 and 35: Chapter III Ring Method The SQUID a
Page 36 and 37: Chapter III independent on the appl
Page 38 and 39: Chapter III 26
Page 40 and 41: Chapter IV The effect of Mn on aust
Page 44 and 45: Chapter IV Fig. 4.4: In-situ torque
Page 46 and 47: Chapter IV Fig. 4.5: a) Microstruct
Page 48 and 49: Chapter IV TEM studies. Transmissio
Page 50 and 51: Chapter IV conditions such as the a
Page 52 and 53: Chapter IV the average grain size i
Page 54 and 55: Chapter IV 42 For each processing t
Page 56 and 57: Chapter V substructure-free, fully
Page 58 and 59: Chapter V (I) High local stresses o
Page 60 and 61: Chapter V Fig. 5.4: Stress-strain c
Page 62 and 63: Chapter V Fig. 5.6: SSRT fracture s
Page 64 and 65: Chapter V (III) A high hydrogen abs
Page 66 and 67: Conclusions discontinuous dynamic r
Page 68 and 69: Conclusions From the technical poin
Page 70 and 71: Appendix 58
Page 72 and 73: Appendix VII.2 HR-TEM microstructur
Page 74 and 75: Appendix VII.4 SSRT fracture surfac
Page 76 and 77: Appendix Fig A.9: Higher magnificat
Page 78 and 79: Appendix VII.5 Microstructures foll
Page 80 and 81: List of Appended Papers Paper E S.
Page 82 and 83: Paper A 1. Introduction Over the la
Page 84 and 85: Paper A transmission electron micro
Page 86 and 87: Paper A Fig. 4: BSE images of HPT d
Page 88 and 89: Paper A Fig. 7: SEM images of HPT d
Page 90 and 91: Paper A temperatures and it seems t
Page 92 and 93:
Abstract Paper B iIIiB Tailoring th
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Fig. 1: Dimensions and orientation
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Paper B evaluated to quantify the s
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Paper B grained metallic materials
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Paper C iIIiC Using Ring Samples to
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Paper C All sets of samples were de
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Paper C Therefore the rings for the
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Paper C (iv) The stress distributio
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Paper D 1. Introduction Metals and
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Paper D Fig. 2: Sequence of microgr
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Paper D approximately 0.4 µm; a se
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Paper D behaviour was studied in-si
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Paper E 1. Introduction Recrystalli
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Paper E consist of a substructure w
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Paper E the centre to the edge afte
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Paper E suspended. This indicates t
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Paper F 1. Introduction Over the pa
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Paper F F - 4 Fig. 1: Schematic ill
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Paper F Fig. 2 demonstrates the rad
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Paper F strains the initial coarse
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Paper F Fig. 6: a) TEM micrograph o
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Paper F Fig. 7: Sequence of TEM mic
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Paper F Fig. 7g represents the micr
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Paper F 4. Discussion The present s
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Paper F Ad iii) It is well known th
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Paper F 5. Conclusions A deformatio
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Severe Plastic Deformation of Ferritic and Austenitic Steels

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