The last seven hot rolling passes of the ferritic stainless steels (FSS) AISI 430 and AISI 433 (the latter an Al-added variant of 430) were simulated on Gleeble-1500D® and Gleeble-3800TM® thermo-mechanical simulators to investigate the effect of temperature, strain rate and inter-pass time on the development of texture in these steel grades and its subsequent influence on ridging. The compression tests were carried out over a wide range of strain rates (0.1 s-1 to 5 s-1, 25 s-1 and 50 s-1) and temperatures (1100 to 820 oC) with different inter-pass times (2 s, 10 s, 20 s and 30 s). The transition temperature from dynamic recrystallization (which may introduce a texture change) to dynamic recovery (in which no texture changes are expected) was determined by examining the relationship between the mean flow stress and the deformation temperature in multi-pass tests. Both macrotexture (XRD) and microtexture (EBSD) analyses were employed to characterise and study the texture present in these steels. It was found that the texture in the central layer of the compressed sample is a strong recrystallization-type. The through-thickness textural and microstructural banding was found to be responsible for ridging in these grades of stainless steels. Dynamic recrystallization which promotes the formation of the desired ã-fibre texture leading to high ductility, formability and eventually reduction or elimination of ridging, was found to occur in both AISI 430 and AISI 433 at high temperatures, low strain rates and longer inter-pass times with multi-pass testing. Generally AISI 433 has a stronger gamma texture developed than the AISI 430 when hot rolled under similar conditions, which leads to improved ductility and less ridging in AISI 433 than AISI 430. / Dissertation (MSc)--University of Pretoria, 2012. / Materials Science and Metallurgical Engineering / unrestricted

Additive manufacturing (AM) has the possibility of producing complex-shaped components which can not be produced by conventional manufacturing methods. This gives the opportunity for designers to freely think outside the design spectra which is otherwise limited by conventional manufacturing methods. AM of metal has rapidly been developed for the last three decades, and they now are reaching industrial acceptance levels, metal feedstock for use in AM is also rapidly growing. AM of metals is especially of interest for the tooling industry. The design freedom which AM offers the tooling manufacturer can design complex cooling channels within moulds, which could reduce cycle time and enhance the quality of components produced with the moulds. Maraging steels have been proven to both be able to be processed with AM but also have comparable performance to traditionally carbon-based used tool steels. Laser Powder Bed Fusion (LPBF) is one of the most promising AM systems today, by using powder as a feedstock it can produce high-resolution parts without needing to process them after they have been produced. However, there is a need to better understand processing within the LPBF system. This master thesis is aimed to process a newly developed maraging steel from Uddeholm, and conduct process parameter experiment and study their correlation to be able to produce samples with as few defects possible. It is crucial to conform to a good methodology for how to find those correlations and see how they influence the printed material. LPBF process has a multi-complex variable system, and by narrowing down the complexity by focus on the most influencing parameters as has been proven by many researchers. Even with a reduced focus, it is still a multi-variable problem. In this study a methodology of finding process parameters relations, a Design Of Experiment software was used, namely, MODDE. By screening of process parameter ranges, within the software, a statistical evaluation of operational process window can be found with fewer conducted experiment. Development of process parameter can traditionally be time-consuming and result in an unnecessary large number of experiments to find the operational window. The experiment showed that laser power and point distance had the most influencing effect on relative density, followed by exposure time and hatch distance. The experiment was firstly conducted with a layer thickness of 50 µm, the achieved relative density resulted in over 99.8 percent. However, a large lack of fusion defects was observed inside the specimens. Even though a high relative density was measured, a pore analysis has to be conducted to fully understand the size and shape of defects since they can have a severe impact on mechanical properties. It was believed that the layer thickness was too high and that the defects could be reduced by printing a set with same process parameters but with a lower layer thickness of 40 µm instead. The reduction of layer thickness did result in a significant decrease of the defects observed. However, future work after this thesis must be continued to further optimize and to increase the solidity of printed material to reach closer to its conventional produced relatives / Additiv tillverkning har möjligheten att producera komplext konstruerade komponenter som inte kan produceras med konventionella tillverkningsmetoder. Detta ger konstruktörer möjligheten att fritt tänka utanför designspektra som annars begränsas av konventionella tillverkningsmetoder. Additiv tillverkning av metall har snabbt utvecklats under de senaste tre decennierna och har nu nått industriella acceptansnivåer. Metallråvara för användning i additiv tillverkning växer snabbt. Additiv tillverkning av metaller är särskilt intressant för verktygsindustrin, designfriheten som additiv tillverkning kan erbjuda verktygstillverkaren för att kunna utforma komplexa kylkanaler inuti formar. Det kan således reducera cykeltiden och förbättra kvaliteten på komponenter som produceras med formarna. Maråldringsststål har visat sig att både kunna processas i additiv tillverkning och har jämförbara egenskaper med traditionellt kolbaserade verktygsstål. Pulverbäddsystemet är ett av de mest lovande systemen idag, genom att använda pulver som råmaterial kan systemet producera komponenter med hög noggranhet utan att behöva bearbeta dem efter att processen är klar. Det finns emellertid ett behov av att bättre förstå själva processen inom pulverbädds teknologin. Den här masteruppsatsen syftar till att additivt tillverka ett nyutvecklat maråldringsstål från Uddeholm. Samt att genomföra processparameterexperiment och studera deras korrelation för att kunna producera prover med så få defekter som möjligt. Det är avgörande att hitta en metod för hur man hittar korrelationerna och se hur de påverkar det tillverkade materialet. Pulverbäddsystemet har ett multikomplext variabelsystem. För att minska komplexiteten kan fokus läggas på de mest inflytelserika processparametrarna, vilket har bevisats av många forskare. Även med ett reducerat fokus är det fortfarande ett flervariabelsproblem. I denna studie användes en metod för att hitta relationer mellan processparametrar och en Design Of Experiment-programvara, nämligen MODDE. Genom screening av processparametrar, inom programvaran, kan en statistisk utvärdering av operativt processfönster hittas med färre genomförda experiment. Utvecklingen av processparametrar kan traditionellt vara tidskrävande och resultera i ett onödigt stort antal experiment för att hitta det operativa fönstret av processparametrar. Experimentet visade att lasereffekt och punktavstånd påverkande den relativa densiteten mest, följt av exponeringstiden och spåravståndet. Experimentet genomfördes först med en lagertjocklek av 50 mikrometer, lagertjockleken resulterade i en relativ densitet på över 99,8 procent. Emellertid observerades stora fusionsdefekter inuti proverna. Även om en hög relativ densitet mättes, måste en poranalys genomföras för att fullt ut förstå storleken och formen på defekter eftersom de kan ha en avgörande inverkan på mekaniska egenskaperna. Det misstänktes att lagertjockleken var för hög och att defekterna kunde minskas genom att tillverka en ytterligare uppsättning av samma processparametrar men med en lägre lagertjocklek på 40 mikrometer istället. Minskningen av lagertjockleken resulterade i en signifikant minskning av de observerade defekterna. Framgent efter den här avhandlingen måste dock arbetet fortsätta att ytterligare optimera och öka soliditeten i det additivt tillverkade materialet. Det för att uppnå bättre prover och komma ännu närmre det konventionellt tillverkade materialets egenskaper.

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

The “475°C embrittlement” occurring in ferritic and duplex stainless steel is considered to be detrimental and it limits the application of ferritic and duplex stainless steel at elevated temperatures, i.e., above about 300°C . In this study, the effect from alloying elements Ni, Cu and Mn on 475°C embrittlement was examined based on microhardness measurement and Charpy V-notch tests as well as atom probe tomography (APT). It was found that, after aging for 10h, 3% Ni accelerates the ferrite decomposition dramatically, 5% Mn has minor effect and no effect of 1.5% Cu was seen. The hardness increase tested at 450°C and 500°C was consistent with the observations from APT. The embrittlement based on room temperature Charpy tests was observed mainly during the first 10h. The embrittlement in Fe-20Cr-3Ni alloy was attributed to ferrite decomposition, while the other three alloys may be influenced by other phenomenon as well. A clustering effect of Cu has been observed in Fe-20Cr-1.5Cu and it was supposed to contribute to the mechanical changes.

If these macro inclusions are not detected before the material is put in used, its mechanical properties are greatly affected and this can lead to outrageous consequences in the engineering application.

A drill tool body operates under tough cutting conditions where it is subjected to severe rubbing at the drill flute margin by the sliding chips. The tool wear caused by the contact stresses of the high-speed sliding chips is one of the common damage mechanisms observed in drill tool bodies. The present investigation is aimed at evaluating the wear properties, mechanical properties and tribological characteristics of the selective laser melting (SLM) produced drill bodies of Co-alloyed tool steels. Two different maraging steel powders, Modified 18Ni300 and Osprey MAR were the materials of interest in this work. A ranking was obtained after a detailed examination to select the most suitable cutting tool body material for drilling application. The microstructures of SLM-produced materials after aging were carefully characterized and analyzed. The result shows that both the materials are composed of fine dendritic cellular structures and even exhibit melt pool boundaries. The microhardness values taken on the as-polished samples indicate that Modified 18Ni300 is marginally harder than Osprey MAR. The material pick-up tendency, friction characteristics and wear properties of the two material samples at room temperature were also evaluated using a scratch tester and a pin-on-disc tribometer. The wear volume for the scratch test and the pin-on-disc test was quantified by a 3D optical profilometer. The overall coefficient of friction (COF) and the wear volume was considerably higher for Osprey MAR than Modified 18Ni300 in scratch testing. The increase in COF for Osprey MAR can be attributed to the build-up edge adhered to the moving stylus. Wear characterization of the scratched surface shows secondary plowing which validates the adhesion tendency of Osprey MAR. The results from the pin-on-disc test conform to the scratch results displaying marginally lower COF and wear volume for the Modified 18Ni300 disc specimen. The higher volume loss of pin manufactured from workpiece material SS2541 used against Modified 18Ni300 disc substantiates the better wear characteristics of this material. Characterization of wear on the surface and cross-section of disc samples suggests that the total wear is the result of adhesion by delamination and build-up edge, abrasion, and cellular fracture at the interacting interface. Machining application tests were also conducted to study the chip wear characteristics and facilitate the ranking of the materials concerning wear resistance. Chip breaker wear test was performed with chip breakers of two different surface topography, milled and ground, to evaluate the effect of surface texture and roughness on the wear behavior. The result shows adhesion on the chip breaker surface for both the materials of interest. The wear is higher in Osprey MAR than Modified 18Ni300 in both milled and ground conditions. A series of actual drill tests from SLM-produced drill bodies indicates wear at the drill flute margin. The drill body material at the chip flute margin is abraded by the sliding chips for Osprey MAR exhibiting greater wear than Modified 18Ni300 in which case the flute margin is intact. Adhesion is also seen at the flute surface for both materials. EDS analysis undertaken at the site of wear confirms the adhered material is of the workpiece SS2541. The results from all the aforementioned tests suggest that Modified 18Ni300 has better wear resistance than Osprey MAR. It also indicates that the cellular microstructure of SLM-produced maraging steels is not suitable for sliding wear resistance.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18266号 / 工博第3858号 / 新制||工||1592(附属図書館) / 31124 / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 福永 俊晴, 教授 白井 泰治, 准教授 徐 蛟 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21889号 / エネ博第390号 / 新制||エネ||75(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー変換科学専攻 / (主査)教授 木村 晃彦, 教授 星出 敏彦, 教授 今谷 勝次 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

In recent years, the automotive industry has turned to press hardened steels (PHS) to improve passenger safety while enabling vehicle weight reduction. To form the complex shapes required for this purpose, they are often direct hot press formed. It is possible to provide corrosion resistance to these parts by galvanizing the PHS sheets prior to direct hot press forming (DHPF). However, the austenitization of the galvanized steel causes the Zn-based coating to transform into two intermetallic phases. These are iron-rich α-Fe(Zn) and zinc-rich Г-Fe3Zn10. The Г-Fe3Zn10 is liquid during traditional DHPF, and the applied stress can result in liquid metal embrittlement (LME). Recently, two new grades of PHS have been developed, which allow for DHPF at 600-700°C, below the Fe-Zn peritectic temperature at 782°C, thus avoiding LME. These prototype PHS grades are designated 2%Mn (0.2C-2Mn-0.25Si-0.005B (wt%)) and 2.5%Mn (0.2C-2.5Mn-0.25Si-0.005B (wt%)). The objective of this work is to determine the effect of DHPF on the ability of a Zn-based coating to provide robust cathodic protection to the two prototype PHS. Galvanized panels of both the 2%Mn and 2.5%Mn steel were DHPF with a U-shape die at 700°C. The surface and cross-section of the coating were examined to determine the effects of DHPF on the coating surface. Die friction during DHPF resulted in die wiping on the wall of the part, leading to removal of surface Г-Fe3Zn10. In cross-section, coating cracks were present at the wall and corner of the U-shape part due to the deformation during DHPF. Potentiodynamic polarization scans were used to determine the corrosion potential of the coating, and this was used to calculate the driving force for cathodic protection using the difference in corrosion potential between the coating phases and the substrate. It was found that only Γ-Fe3Zn10 provided robust cathodic protection to both steel substrates, and the driving force for cathodic protection was lower for the coated DHPF 2.5%Mn steel. Galvanostatic scans were used to evaluate dissolution kinetics of coating phases. Robust cathodic protection was provided by the galvanized coating for austenitization times of 30 - 120 s for the 2%Mn substrate and 30 - 60 s for the 2.5%Mn substrate. The duration that robust cathodic protection was provided was shortest at the wall of the U-shape part. This result was attributed to die wiping caused by DHPF, where the surface is smoothed by die friction. When there is less Г-Fe3Zn10 in the coating, such as at longer austenization times, surface Г-Fe3Zn10 was removed and an increased amount of α-Fe(Zn) is exposed, which does not provide robust cathodic protection. In addition, coating cracks form along α-Fe(Zn) grain boundaries after austenitization for 180 s on all examined regions of the U-shape part, allowing a greater surface area of the coating exposed to electrolyte, further increasing dissolution of the coating. / Thesis / Master of Applied Science (MASc)

The process optimization of continuous annealing furnace, RHF 125, for recrystallization annealing of two steel grades, Sandvik 13C26 and Sandvik 20C has been carried out. To recreate the continuous annealing process carried out in the roller hearth furnace in the industry, samples with different cold reduction rates were chosen from ongoing production lines. An experimental heat treatment model was chosen by the ‘Design of Experiments’ approach from MODDE (from U-Metrics). The annealing temperature was chosen below the austenitization temperature for both steel grades and soaking time of 30 seconds to 240 seconds were chosen. Microscopic estimation of fraction recrystallized was performed with the help of Electron Back Scattered Diffraction, accompanied by mechanical testing methods to measure the hardness and yield strength of the steel strips. The experimental output was used to create a model to correlate between the different cold reduction rates and annealing parameters to achieve a higher degree of recrystallization along with desirable mechanical properties. Also, a modified Johnson-Mehl-Avrami-Kolomogrov model, based on hardness values, to determine the transformation kinetics by tracking the progress of recrystallization was developed. The model was verified with EBSD measurements for Sandvik 13C26 strip steels. For 20C, inhomogeneous recrystallization was observed, thus limiting the model’s adaptability to steels which exhibit homogeneous recrystallization behavior and negligible change in precipitation and/or coarsening of secondary phases. / Processoptimering av en kontinuerlig glödgningsugn, RHF 125, för rekristallisationsglödgning av två Sandvik-stål, Sandvik 13C26 och Sandvik 20C, har genomförts. För att återskapa den kontinuerliga glödgningsprocessen som utförs den verkilga processen i valdes prover och olika kallreduktionshastigheter från pågående produktionslinjer. En experimentell värmebehandlingsmodell valdes med metoden 'Design of Experiments' med MODDE (från U-Metrics). Glödgningstemperaturen valdes till en temperatur under austeniseringstemperaturen för båda stålen och hålltider varierade från 30 s till 240 s. Mikroskopisk uppskattning av fraktionen rekristalliserat material utfördes med hjälp av Electron Back Scatter Diffraktion (EBSD), åtföljd av mekaniska testmetoder för att mäta hårdheten och sträckgränsen för stålproverna. De experimentella resultaten användes för att skapa en modell för att korrelera mellan de olika reduktionshastigheterna och glödgningsparametrarna för att uppnå högre grad av rekristallisation tillsammans med önskvärda mekaniska egenskaper. Dessutom utvecklades en modifierad Johnson-Mehl-Avrami-Kolomogrov-modell, baserad på hårdhetsvärden, för att bestämma transformationskinetiken genom att spåra evolutionen för rekristallisation. Modellen verifierades genom jämförelse med EBSD-mätningarna för Sandvik 13C26 bandstål. För 20 °C observerades inhomogen rekristallisation, vilket begränsade modellens användbarhet till stål som uppvisade homogent rekristallisationsbeteende och försumbar förändring i utskiljning och/eller förgrovning av sekundära faser.