Olena Volkova

Design of novel materials for additive manufacturing - Isotropic microstructure and high defect tolerance

Electron Beam Melting (EBM) is a powder-bed additive manufacturing technology enabling the production of complex metallic parts with generally good mechanical properties. However, the performance of powder-bed based additively manufactured materials is governed by multiple factors that are difficult to control. Alloys that solidify in cubic crystal structures are usually affected by strong anisotropy due to the formation of columnar grains of preferred orientation. Moreover, processing induced defects and porosity detrimentally influence static and cyclic mechanical properties. The current study presents results on processing of a metastable austenitic CrMnNi steel by EBM. Due to multiple phase transformations induced by intrinsic heat-treatment in the layer-wise EBM process the material develops a fine-grained microstructure almost without a preferred crystallographic grain orientation. The deformation-induced phase transformation yields high damage tolerance and, thus, excellent mechanical properties less sensitive to process-induced inhomogeneities. Various scan strategies were applied to evaluate the width of an appropriate process window in terms of microstructure evolution, porosity and change of chemical composition.

Dilatometry Analysis of Dissolution of Cr-Rich Carbides in Martensitic Stainless Steels

The dissolution of Cr-rich carbides formed in the martensitic constituent of a 13 pct Cr stainless steel was studied by dilatometry and correlative electron channeling contrast examinations. The dissolution of carbides subsequent to the martensite reversion to austenite was associated with a net volume expansion which in turn increased the dilatometry-based apparent coefficient of thermal expansion (CTEa) during continuous heating. The effects of carbides fraction and size on the CTEa variations during carbides dissolution are discussed.

Tensile elongation of lean-alloy austenitic stainless steels: transformation-induced plasticity versus planar glide

ABSTRACT An Fe–13Cr–3.4Mn–0.47C lean-alloy stainless steel was made austenitic by solution annealing at 1250°C. Tensile tests between 20 and 200°C indicated enhancement of ductility at higher temperatures. At 200°C where planar glide, manifested as deformation twinning, was the dominant deformation mechanism, a uniform tensile elongation of 102% was achieved. At 20°C where deformation-induced α′-martensitic transformation replaced deformation twinning as the dominant deformation mechanism, tensile elongation was significantly impaired. The tensile elongation contribution by the planar glide was estimated to be at least four times that of the α′-TRIP (transformation-induced plasticity) mechanism. The results indicate that inexpensive lean-alloy austenitic stainless steels exhibiting pronounced α′-formation at room temperature could become highly formable at higher temperatures.

Estimation of the Temperature-Dependent Nitrogen Solubility in Stainless Fe-Cr-Mn-Ni-Si-C Steel Melts During Processing

The influence of chemical composition, temperature, and pressure on the nitrogen solubility of various high alloy stainless steel grades, namely Fe-14Cr-(0.17-7.77)Mn-6Ni-0.5Si-0.03C [wt pct], Fe-15Cr-3Mn-4Ni-0.5Si-0.1C [wt pct], and Fe-19Cr-3Mn-4Ni-0.5Si-0.15C [wt pct], was studied in the melt. The temperature-dependent N-solubility was determined using an empirical approach proposed by Wada and Pehlke. The thus calculated N-concentrations overestimate the actual N-solubility of all the studied Fe-Cr-Mn-Ni-Si-C steel melts at a given temperature and pressure. Consequently, the calculation model has to be modified by Si and C because both elements are not recognized in the original equation. The addition of the 1st and 2nd order interaction parameters for Si and C to the model by Wada and Pehlke allows a precise estimation of the temperature-dependent nitrogen solubility in the liquid steel bath, and fits very well with the measured nitrogen concentrations during processing of the steels. Moreover, the N-solubility enhancing effect of Cr- and Mn-additions has been demonstrated.

Modelling of temperature and stress profiles in oxygen sensors upon immersion into steel baths

Abstract Using a previously established non-stationary thermal model it is possible to calculate the distribution of temperature and thermal stresses in emf sensors based on stabilised ZrO2 which are frequently applied for the measurement of oxygen activity in steel melts. The model may provide a basis for optimised construction of these sensors. The preheating behaviour is in the first step characterised for the sensor itself and in the second step for the complete one-reading measuring device.

Characterization of Nonmetallic Inclusions in 18CrNiMo7-6

The detrimental effect of nonmetallic inclusions (NMIs) in steels on, e.g., fatigue lifetime is well known. In order to increase the durability and safety of materials and components, inclusion control and a deep understanding of inclusion formation are essential. The present study examines the formation of inclusions as well as their content, type, morphology, and size distribution for different batches of the steel alloy 18CrNiMo7-6 (AISI 4317), which was processed with various refractory crucible materials. To this end, extensive metallographic investigations were carried out including fracture surface analyses, metallographic sections, and the chemical extraction of inclusions. Scanning electron microscopy supported by energy-dispersive X-ray spectroscopy was used alongside electron backscatter diffraction for the qualitative and quantitative analysis of the NMIs. Oxide contents were found to have a significant effect on sulfide precipitation behavior, and had a strong impact on the resulting sizes and numbers of inclusions. The formation and growth of sulfides and oxide-sulfides featuring different morphologies is discussed on the basis of these experimental results.

Density of tin, silver and copper

Purpose: Purpose of this paper is to report on the development of a new density measurement cell. Design/methodology/approach: Measurement cell based on Archimedean principle and consisting of induction furnace and a high/precision balance was applied for measurement of tin, silver and copper density. Findings: It was found that new cell is suitable for high temperature measurement of liquid metals density at temperatures from 700 to 1520°C. Measurement results are in a good agreement with the literature values. Density deviates by 0.5-1% depending on the metal. Research limitations/implications: Accuracy of the density measurement decreases at temperatures below 700°C due to oxidation of the melt surface. More accurate data on thermal expansion coefficient for sinker material is required. Practical implications: Experiments showed applicability of the new measurement cell. Archimedean principle is among the most sensitive density measurement techniques. New cell will be further used for measurement of iron-based alloys. Problems of measurements are discussed. Originality/value: Paper describes application of the known density measurement technique. The paper is of interest for the material scientists working with high-temperature thermophysical properties measurements and users of thermophysical properties data.

Selecting optimal slag conditions in the blast furnace

In selecting the best chemical composition of slag melts, it is expedient to take account of their viscosity and electrical conductivity, which are structure-sensitive properties. The viscosity and electrical conductivity of blast-furnace slag are studied experimentally. To permit correct selection of the slag conditions in the blast furnace, a parameter is proposed for assessing the relation between the structural particles of the melt: the heterogenization temperature, which takes account of the viscosity and electrical conductivity of the slag melts. The discharge temperature of the slag from blast furnace 8 at PJSC ArcelorMittal Kryvyi Rih is measured. Comparison of the actual discharge temperature of the slag and the calculated heterogenization temperature for blast furnace 8 permits identification of the optimal slag basicity (CaO/SiO2).