Constantinos Goulas

University of Twente | UT · Department of Design, Production and Management

In this paper, the effect of chemical inhomogeneity on the isothermal bainite formation is investigated in medium-carbon low-silicon spring steel by dilatometry and microscopy. The analysis of the microstructure at different times during transformation shows that chemical segregation of substitutional alloying elements resulting from casting strong...

Intravascular surgical instruments require precise navigation within narrow vessels, necessitating maximum flexibility, minimal diameter, and high degrees of freedom. Existing tools often lack control during insertion due to undesirable bending, limiting vessel accessibility and risking tissue damage. Next-generation instruments aim to develop hemo...

The concept of Industry 4.0, as a means to move forward in the industrial ecosystem, has reached an important turning point. Where do we stand now in terms of industrial innovation and transition? This opinion paper provides an overview of the situation in the Netherlands, a reflection on what has been achieved by the Industry 4.0 paradigm, and the...

Purpose The purpose of this paper is to assess the feasibility of analytical models, specifically the radial basis function method, Akbari–Ganji method and Gaussian method, in conjunction with the finite element method. The aim is to examine the impact of processing parameters on temperature history. Design/methodology/approach Through analytical...

The Ti-XSn (X = 5, 10, 15) (wt.%) alloys are fabricated by mixing TiH2/Sn powders, then compacting and extruding. The microstructure, mechanical properties, and biocorrosion resistivity of the Ti-XSn rods are studied and compared with the CP Ti fabricated with the same condition by the powder compact extrusion of TiH2. The Ti-XSn alloys exhibit sig...

Intravascular surgical instruments necessitate precise navigation within narrow vessels, requiring maximum flexibility, minimal diameter, and high degrees of freedom. However, existing tools often lack adequate control during insertion due to undesirable bending, which not only limits vessel accessibility but also poses a risk of tissue damage. The...

TX-80 low-transformation-temperature (LTT) welding wire was used to replace the traditional ER 307Si welding wire to realize the connection of 22SiMn2TiB armor steel in manual overlay welding. The previously existing issues, such as welding cracks, large welding deformation, and severe welding residual stress, were solved to ensure good strength an...

This paper presents the simultaneous influence of two different activators (NH4Cl and NaF) on the kinetic evolution during siliconising of steel. The AISI D2 tool steel surface was coated by pack siliconising through the growing of silicon on the surface. Pack siliconising was accomplished by employing a mixture of powders of Si 12 wt% + NH4Cl 0.5...

Low-temperature transformation (LTT) welding wire was initially developed to mitigate residual stress in the weld. It could also be used for internal stress optimization in Wire and Arc Additive Manufacturing (WAAM) process. In this study, a 26 layers LTT wall sample fabricated by using the WAAM technique was investigated. The microstructure of the...

In the present study, the deposition of silicon nitride (Si 3 N 4 ) coating on Inconel 738 through plasma spray has been investigated. For producing the Si 3 N 4 coating, different times (6, 8 and 10 s), different distances of the plasma spray torch from the substrate (30, 35 and 40 mm) and different currents for plasma spraying (3000, 3250 and 350...

The present study explores the microstructure and the depth-dependence of hardness after a single pass laser heat-treatment. The initial microstructure was selected as ferrite and pearlite in different fractions, with the aim of studying low, medium, and high carbon steels. A modification of an existing model is proposed, whereby the cementite diss...

Additive manufacturing of NiTi shape memory alloys has attracted attention in recent years, due to design flexibility and feasibility to achieve four-dimensional (4D) function response. To obtain customized 4D functional responses in NiTi structures, tailorable phase transformation temperatures and stress windows as well as one-way or two-way shape...

One approach to the wireless actuation and gravity compensation of untethered helical magnetic devices (UHMD) is through swimming with a non-zero angle of attack (AoA). This configuration allows us to counteract gravity, so that for a given desired path, we can move the UHMD controllably without drifting downward under its own weight. This study se...

Invar alloys exhibit low thermal expansion and are useful in applications requiring dimensional stability when subject to temperature changes. Conventional production of Invar faces certain challenges that can be offset by exploiting additive manufacturing processes. This study employed pulsed gas tungsten arc welding (GTAW) to deposit Invar 36 all...

Isothermal heat treatments below the martensite start temperature are carried out in a 0.25C‐1.4Mn‐1.4Si‐0.32Mo steel (in wt%). The microstructural observation reveals a combination of tempered lath and plate martensite, accompanied by bainite and retained austenite. The temperature variation of the austenitic carbon content under different constra...

Through additive manufacturing (AM), it is now possible to produce functionally gradient materials (FGM) by depositing different metal alloys at a specific location to locally improve mechanical properties and enhance product performance. Despite recent developments, however, there are still some important trade-offs to consider and inherent challe...

Wire arc additive manufacturing (WAAM) is a class of technologies suitable for producing large parts due to its high material deposition and building rates. Among the many possible materials processed by WAAM, austenitic stainless steels, e.g. 316 L, are commonly employed. The structure of WAAM 316 L thin parts has been studied extensively before....

The final carbon content of austenite in equilibrium with tempered martensite can be estimated by the so-called constrained carbon equilibrium in the presence of carbide (CCE theta) model. However, the linear predictions under CCE theta deviate from both the initial and the experimentally measured carbon content. A modified approach to the CCE thet...

Welcome scholars to submit their recent research results to this Special Issue. The suitable article themes include, but are not limited to, the following: - Additive manufacturing for large-scale structures; - Innovative, advanced manufacturing; - Theories, techniques, and methods of robotic fabrication; - Metal additive manufacturing; - 3D-printe...

Stellite 6 is a cobalt-based superalloy which has a good wear and corrosion resistance and retains these properties at high temperatures. In this study, wire and arc additive manufacturing (WAAM based on the GMAW) was employed to deposit Stellite 6 wire on low alloy high strength steel (S355) and stainless steel (AISI 420) plates. One of the main i...

The majority of glass used in load-bearing structures is as planar elements. Some projects exist that use linear glass elements. This paper discusses in broad terms the design, engineering, and fabrication of a unique vector active glass structure consisting of glass bundles and partly printed steel connections. A structure was conceived that utili...

High-strength low-alloy steels (HSLA) are gaining popularity in structural applications in which weight reduction is of interest, such as heavy duty machinery, bridges, and offshore structures. Since the fatigue behavior of welds appears to be almost independent of the base material and displays little improvement when more resistant steel grades a...

Wire and arc additive manufacturing (WAAM) is a 3D metal printing technique based on the arc welding process. WAAM is considered to be suitable to produce large-scale metallic components by combining high deposition rate and low cost. WAAM uses conventional welding consumable wires as feedstock. In some applications of steel components, one-off spa...

A novel high-strength steel design is proposed, with a fine bainitic microstructure free from inter-lath carbides, for railway crossings applications. The design is based on the phase transformation theory and avoids microstructural constituents like martensite, cementite and large blocky retained austenite islands in the microstructure which are c...

High strength steels (HSS) offer a unique opportunity to reduce the weight of civil and off-shore structures and heavy-duty machinery. Such equipment is subjected to continuous cyclic loading and therefore fatigue failure often happens where concentrations of stresses are present, the latter most of the time being caused by connection details. Prev...

Atomic-scale investigation was performed on 51CrV4 steel, isothermally held at different temperatures within the bainitic temperature range. Transmission electron microscopy (TEM) analysis revealed three different morphologies: lower, upper, and inverse bainite. Atom Probe Tomography (APT) analysis of lower bainite revealed cementite particles, whi...

The aim of the present work is to study the microstructure evolution of a cold‐rolled low carbon steel alloyed with Cr, Mn, Mo, and Nb during continuous heating. The formation of austenite and its further transformation is examined by means of peak annealing experiments at three different heating rates, followed by quenching. The effect of carbide‐...

The Ph.D. thesis investigates the substitution of quenching and tempering treatments by isothermal bainitic treatments in automotive spring production. An isothermal bainitic treatment has benefits mainly in terms of energy savings, but it can also prevent quench cracking, distortion and residual stresses, commonly found in quenched and tempered co...

A comparative study on the microstructural changes after conventional (20°Cs⁻¹) and ultrafast (300°Cs⁻¹) heating is performed on a medium carbon steel in the soft annealed condition. Continuous-heating dilatometry experiments are carried out. The phase transformations and the volume phase fraction of austenite are determined among other microstruct...

This work focuses on the effect of a pre-heating stage on the microstructure evolution during continuous heating of 50% and 75% cold-rolled low carbon steel under conventional and ultrafast heating rates. Peak annealing experiments under two heating rates (10 °C/s and 400 °C/s) are applied to the samples after a pre-heating stage of 10 s. The selec...

A structural steel component that failed under fatigue was examined with the aim to identify the root causes of this failure. Fractographic examination revealed the presence of multiple beach marks; the position and arrangement of those signified the occurrence of fatigue fracture under the presence of combined loading conditions, involving torsion...

Purpose – Identification of the critical process conditions that enhance Cu diffusion in ferrite grain boundaries and promote precipitation of Cu-rich particles in the proximity of steel semi-finished products surface is crucial for every steel maker as it leads to the creation of hot shortness cracks in final products deteriorating surface conditi...

Automobile scrap re-melting results in noticeable Copper (Cu) amounts in EAF (Electric Arc Furnace)-produced steel. Presence of Cu-particles in grain boundaries near the surface of intermediate or final products deteriorates surface quality through relevant defects. Surface defects, usually hot tears in the surface of "as-cast" material, lead to fl...