Matrix design of a novel ductile cast iron modified by W and Al: A comparison between thermodynamic modeling and experimental data

In high-temperature applications of ferrous materials, as in the case of exhaust manifolds, high thermal and mechanical stability are required. Stainless steels and Ni-resist alloys having austenitic matrices are good candidates to meet these requirements at elevated temperatures; however, they are expensive materials and present difficulties in casting. Ferritic ductile cast irons, like the commercial SiMo alloy, are comparatively cheaper materials with better castability but they cannot be used above approximately 800 °C. Thus, to meet the requirements with low-cost materials having improved high-temperature properties, new alloys must be developed by ferrite forming elements having the potential to increase equilibrium temperature. In this study, initially, a novel ductile cast iron matrix was designed using 1 W and 0-4 Al wt.-% and their phases stable at room temperature, transformation temperatures, solidification sequences and thermal expansivity values were determined using thermodynamic calculations with Thermo-Calc software. Computational studies revealed that (i) designed alloy matrices had graphite and M6C type carbides embedded in a ferritic matrix at room temperature as expected, (ii) A1 temperature increased as aluminum content increased. The obtained values were all above that of commercial SiMo alloy, (iii) the detrimental effect of increased aluminum addition on graphite content, and thermal expansivity was observed. Secondly, microstructural and thermal characterizations of cast alloys were performed for validation - the obtained data were in good agreement with the thermodynamic calculations

Dispersed-phase transformation toughening in ultrahigh-strength steels

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 1988.Vita.Includes bibliographical references.by Gregory N. Haidemenopoulos.Ph.D

Heat flow and material degradation during laser metal forming

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering and Dept. of Materials Science and Engineering, 1985.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERINGBibliography: leaves 144-146.by Gregory N. Haidemenopoulos.M.S

Microstructure and Salt Fog Corrosion of Wrought Mg-Al-Zn and Mg-RE Alloys

Wrought magnesium alloys have received attention due to their potential application as lightweight materials. However, their use is limited by their poor corrosion resistance. Rare earth additions have the potential to enhance corrosion resistance. The present work included a microstructural investigation and corrosion testing of the alloy WE-43, containing Nd and Y, which was compared against the more conventional compositions of AZ31 and AZ61 alloys. All three alloys exhibited a recrystallized equiaxed structure after hot rolling with the presence of second phases—precipitates. The WE-43 alloy exhibited a better corrosion resistance than AZ31 and AZ61 under salt fog testing, indicated by the lower depth of attack and lower weight loss. The second phases in the microstructure of AZ31 and AZ61 alloys determined their corrosion resistance. The second phases in the AZ31 and AZ61 alloys (based on Al-Mg and Al-Mn phases) were nobler than the Mg matrix and catholically acted, thus sacrificing the Mg matrix. The superior corrosion resistance of WE43 was due to the incorporation of Y in the oxide/hydroxide film. In addition, the second phases in the WE43 consisted of Nd and Y and were less noble than the Mg-matrix. Thus, they acted as anodic sites protecting the Mg-matrix. The above results show the beneficial effect of rare earth additions to wrought Mg alloys towards increased corrosion resistance

Creep rupture in HP-Nb refractory steel tubes due to short-term overheating

The premature creep rupture of cast reformer tubes made of HP-Nb steel due to short-term overheating, was investigated. Investigation was performed on specimens from ruptured, cracked, and uncracked tubes and included metallography, mechanical testing, and SEM/EDX analysis. It was found that accidental short-term overheating, after 5 years of normal operation, caused rapid tertiary creep, manifested by the formation of dense cavities and cracking, which eventually led to the premature creep rupture of several tubes of the reformer. Creep cavities formed preferentially at M7C3 carbides but also at M23C6 and MC carbides while crack propagation took place along the interdendritic carbide network. It appears that the radial direction of the columnar grains from the inside to outside diameter orients the grain boundaries perpendicular to the hoop stress, a situation which degrades creep rupture resistance. An analysis based on the Larson–Miller parameter confirmed the premature creep rupture of the tubes due to overheating

Low Cycle Fatigue Behavior of Plastically Pre-Strained HSLA S355MC and S460MC Steels

Cold roll forming used in the manufacturing of lightweight steel profiles for racking storage systems is associated with localized, non-uniform plastic deformations in the corner sections of the profiles, which act as fatigue damage initiation sites. In order to obtain a clearer insight on the role of existing plastic deformation on material fatigue performance, the effect of plastic pre-straining on the low cycle fatigue behavior of S355MC and S460MC steels was investigated. The steels were plastically deformed at different pre-strain levels under tension, and subsequently subjected to cyclic strain-controlled testing. Plastic pre-straining was found to increase cyclic yield strength, decrease ductility, and induce cyclic softening, which, in S460MC, degrades fatigue resistance compared to the unstrained material. In unstrained conditions, the materials present a cyclic softening to hardening transition with increasing plastic strain amplitude, which in S355MC occurs at lower strain amplitudes and degrades its fatigue resistance with regard to the pre-strained material. Pre-straining also leads to a reduction in transition life from low to high cycle fatigue. SEM fractography, performed following the onset of crack initiation, revealed that plastic pre-straining reduces the fatigue fracture section as well as striation spacing, predominantly in the S355MC steel

Human bone ingrowth into a porous tantalum acetabular cup

Porous Tantalum is increasingly used as a structural scaffold in orthopaedic applications. Information on the mechanisms of human bone ingrowth into trabecular metal implants is rather limited. In this work we have studied, qualitatively, human bone ingrowth into a retrieved porous tantalum monoblock acetabular cup using optical microscopy, scanning electron microscopy and energy dispersive X-ray analysis. According to the results and taking into account the short operational life (4 years) of the implant, bone ingrowth on the acetabular cup took place in the first two-rows of porous tantalum cells to an estimated depth of 1.5 to 2 mm. The bone material, grown inside the first raw of cells, had almost identical composition with the attached bone on the cup surface, as verified by the same Ca:P ratio. Bone ingrowth has been a gradual process starting with Ca deposition on the tantalum struts, followed by bone formation into the tantalum cells, with gradual densification of the bone tissue into hydroxyapatite. A critical step in this process has been the attachment of bone material to the tantalum struts following the topology of the porous tantalum scaffold. These results provide insight to the human bone ingrowth process into porous tantalum implants

Metallographic Index-Based Quantification of the Homogenization State in Extrudable Aluminum Alloys

Extrudability of aluminum alloys of the 6xxx series is highly dependent on the microstructure of the homogenized billets. It is therefore very important to characterize quantitatively the state of homogenization of the as-cast billets. The quantification of the homogenization state was based on the measurement of specific microstructural indices, which describe the size and shape of the intermetallics and indicate the state of homogenization. The indices evaluated were the following: aspect ratio (AR), which is the ratio of the maximum to the minimum diameter of the particles, feret (F), which is the maximum caliper length, and circularity (C), which is a measure of how closely a particle resembles a circle in a 2D metallographic section. The method included extensive metallographic work and the measurement of a large number of particles, including a statistical analysis, in order to investigate the effect of homogenization time. Among the indices examined, the circularity index exhibited the most consistent variation with homogenization time. The lowest value of the circularity index coincided with the metallographic observation for necklace formation. Shorter homogenization times resulted in intermediate homogenization stages involving rounding of edges or particle pinching. The results indicated that the index-based quantification of the homogenization state could provide a credible method for the selection of homogenization process parameters towards enhanced extrudability

Modeling of Microsegregation and Homogenization of 6xxx Al-Alloys Including Precipitation and Strengthening During Homogenization Cooling

Control of the homogenization process is important in obtaining high extrudability and desirable properties in 6xxx aluminum alloys. Three consecutive steps of the process chain were modeled. Microsegregation arising from solidification was described with the Scheil−Gulliver model. Dissolution of Mg2Si, Si (diamond) and β-AlFeSi (β-Al5FeSi) to α-AlFeSi (α-Al12(FeMn)3Si) transformation during homogenization have been described with a CALPHAD-based multicomponent diffusion Dual-Grain Model (DGM), accounting for grain size inhomogeneity. Mg2Si precipitation and associated strengthening during homogenization cooling were modeled with the Kampmann−Wagner Numerical (KWN) precipitation framework. The DGM model indicated that the fractions of β-AlFeSi and α-AlFeSi exhibit an exact spatial and temporal correspondence during transformation. The predictions are in good agreement with experimental data. The KWN model indicated the development of a bimodal particle size distribution during homogenization cooling, arising from corresponding nucleation events. The associated strengthening, arising from solid solution and precipitation strengthening, was in good agreement with experimental results. The proposed modeling approach is a valuable tool for the prediction of microstructure evolution during the homogenization of 6xxx aluminum alloys, including the often-neglected part of homogenization cooling

Investigation of Stress-Oriented Hydrogen-Induced Cracking (SOHIC) in an Amine Absorber Column of an Oil Refinery

Stress-oriented hydrogen-induced cracking (SOHIC) of an amine absorber column made of a Hydrogen Induced Cracking (HIC) resistant steel and operating under wet H2S service was investigated. SOHIC was not related to welds in the column and evolved in two steps: initiation of HIC cracks in the rolling plane and through-thickness linking of the HIC cracks. Both the original HIC cracks as well as the linking cracks propagated with a cleavage mechanism. The key factors identified were periods with high hydrogen charging conditions, manifested by high H2S/amine ratio, and stress triaxiality, imposed by the relatively large thickness of the plate. In addition, the mechanical properties of the steel away from cracked regions were unaffected, indicating the localized nature of SOHIC