Distribution Of Non-metallic Inclusions Research Articles

New insight into the role of inclusions in hydrogen-induced degradation of fracture toughness: three-dimensional imaging and modeling

ABSTRACT An in-depth understanding of the role of non-metallic inclusions (NMIs) in the hydrogen-induced failure of structural materials is important in the manufacture and application of steels. In this paper we investigated the type, the amount and the distribution of NMIs in pipeline steels and their relation to the susceptibility to hydrogen-induced cracking (HIC). Scanning Electron Microscope (SEM) observation confirmed the interlinking of the microcracks formed at the inclusion-matrix interfaces. By combining with the nondestructive three-dimensional (3D) mapping of crack propagation paths, based on the synchrotron tomography technique, we obtained direct evidence that the cracking initiation at the NMIs and then interlinking on the (nearly) same plane are the main mechanism of HIC failure in X70 steels. We built an elastic-energy-based model, which can be applied to quantitatively predict the HIC-induced dependence of the fracture toughness on NMIs, based on statistical information of the NMIs. The theoretical results show a good agreement with the experimental observation. We proposed a criterion to determine the susceptibility to HIC by the observation of NMIs. We also developed a thermodynamics-based model to analyze the hydrogen trapping at the inclusion-matrix interface and its dependence on the strength of the steel matrix. For the first time, the direct relationship between NMIs and HIC-induced degradation of fracture toughness is obtained.

Хімічна і фізична неоднорідності та гази у великому сталевому зливку

For creation of the high-tech equipment that is used in energy, heavy engineering, chemistry and transport, the unique large-sized steel products are required. In the manufacture of such products, large forging ingots in the mass to 600 tons are used. However, an increase in the mass of the ingots leads to the formation of chemical and physical heterogeneity, enlargement and unfavorable distribution of non-metallic inclusions, of the development of segregation defects in them, which reduce the strength and exploitation characteristics of the metal. In this connection, the quality forgings and finished parts are not always meet the producing demands and the loss of metal, in the form of technological waste and rejects are reaching significant values. It is known that eccentric zonal segregation, especially it’s the most dangerous variety - cords, significantly reduce the quality and properties of products from large steel ingots. In connection with the continuous expansion of the production of large ingots, the problem of creating optimal technologies for their formation, which reduce or exclude the possibility of the formation of chemical heterogeneity and cords in steel during crystallization, it is currently important and relevant. In this paper it are presented the results of studies of the structure, gas distribution, physical and chemical heterogeneities in the cross section and height of an ingot in the mass of 140 tons, which was casted in vacuum from steel 25KHN3MFA. It is shown that depending on the temperature and time conditions of ingot hardening, among which the crystallization interval (due to the chemical composition of steels), cooling intensity in different volumes in height and cross section of ingot, temperature gradient before the crystallization front, solubility of alloying elements and gas content in the melt, etc. Based on this, when developing technology for large ingots to ensure their quality, optimal structure and properties should take into account not only their dimensions, but also the combination of these thermokinetic parameters on the crystallization process, dendritic structure formation, manifestations of liquation in different ingot volumes. Keywords: ingot, segregation strip and inclusions, dendrites, structure, oxygen, oxides, sulfides.

Характер розподілу неметалевих включень в структурних складових металу зварних швів при дугових методах зварювання

Характер розподілу неметалевих включень в структурних складових металу зварних швів при дугових методах зварювання

The nature of nonmetallic inclusion distribution in the weld metal structural components at arc welding methods

The nature of nonmetallic inclusion distribution in the weld metal structural components at arc welding methods

ИССЛЕДОВАНИЕ ПРИЧИН СНИЖЕНИЯ УДАРНОЙ ВЯЗКОСТИ СОЕДИНИТЕЛЬНЫХ ДЕТАЛЕЙ ТРУБОПРОВОДОВ ИЗ СТАЛИ 10Г2ФБЮ

Testing of mechanical properties of products is one of the most important production stages because they, most objectively, allow determining whether products manufactured by a certain technological process meet the necessary requirements. Impact strength is the most important parameter in assessing the mechanical characteristics of tubular steel products, impact values at low temperatures are especially important. Brittle fracture is very dangerous, since it proceeds at a high speed and with a load much less than with viscous fracture. This characteristic depends on many factors, including: the number, size and nature of the distribution of non-metallic inclusions, grain size, the presence of reinforcing particles, the uniformity of their distribution, and much more. Impact strength is strictly controlled during testing of pipeline components operating at elevated pressures and freezing temperatures; transition to brittle fracture can provoke disruption of the entire pipeline. The task of determining the causes of a local decrease in impact strength remains relevant due to the many individual factors acting on it, or even a complex of factors. Due to the fact that the probability of a defect contributing to brittle fracture, because of the relatively small crosssectional area of impact samples, is not high, it is necessary to test several samples from the same area of the product to average the results. The aim of this work was to determine the causes of reduced viscosity when testing pipeline components made of 10G2FBY steel by comparing fractures and sample structures with reduced and required impact strengths.

Technology for Complex Deoxidation of Steel with Silicon Carbide and Calcium Carbide in Order to Obtain Steel with Low Nonmetallic Inclusion and Oxygen Contents

Advantages and disadvantages of deoxidizing steel with silicon carbide and calcium carbide are considered. The advantage is demonstrated of joint diffusion deoxidization technology for steel with silicon carbide and calcium carbide. Use of classical technology of metal deoxidation with aluminum leads in a number of cases to the appearance of individual alumina inclusions of the reject type with general high purity of steel with respect to non-metallic inclusions. Inclusions are exogenous in nature, and their origin is presumably entry of alumina growths from the CBCM submerged nozzle. The technology of diffusion deoxidation of metal during ladle treatment with combined use of silicon carbide and calcium carbide makes it possible to exclude entry into steel of large alumina inclusions. Types, composition, sizes and distribution of nonmetallic inclusions in cast metal produced by this technology are investigated. Use of diffusion deoxidation with silicon carbide and calcium carbide makes it possible to form fine globular non-metallic inclusions, predominantly small calcium aluminosilicate, and manganese and calcium sulfides.

INFLUENCE OF METAL OXIDATION AND FEATURES OF VACUUMING ON THE FORMATION AND LOCATION OF NONMETALLIC INCLUSIONS IN LARGE STEEL INGOTS 38ХН3MФA

The article deals with the influence of steel oxidation and features of vacuuming on the processes of formation and distribution of non-metallic inclusions in large ingots and forgings intended for power engineering. It is shown that the distribution of sulfides and oxysulfides across the ingot cross-section is inversely related to each other, due to changes in oxygen and sulfur concentrations during the crystallization of the melt. Under the influence of intensification of degassing process at vacuum casting of ingots at the expense of purging of a jet of metal argon, the formed nonmetallic inclusions had the minimum sizes, more favorable distribution that caused increase in plastic characteristics of the received forgings on the average for 15-20 %

Analyzing the Features of Non-Metallic Inclusion Distribution in Ø410 mm Continuously Cast Billets of Low Carbon Steel Grades

The paper reports findings on the morphology of non-metallic inclusions in low carbon pre-peritectic and peritectic steel grades used for the fabrication of seamless pipes. It is demonstrated that the distribution of non-metallic inclusions over the cross section area of continuously cast billets is of a step-like nature conditioned by the features of billet solidification. In all the steels analyzed the non-metallic inclusions are presented by oxides, sulfides and complex oxi-sulfides not larger than 2 μm.

Bayesian analysis of critical fatigue failure sources

A novel approach for inferring the underlying non-metallic inclusion distribution from fatigue test fractography is presented. It is shown that the non-metallic inclusion size distribution obtained from fatigue testing differs from the extreme value distributions, which do not take fatigue into account. Fatigue, as a process, acts as a filter for the observed inclusions, and by taking advantage of this allows us to extract more refined information from the fractography using statistical inference. The emphasis in this paper is on analysis of axial fatigue testing of smooth specimens. The concepts presented here apply to all fatigue testing where the data from fracture surfaces is collected.

Numerical study on the removal and distribution of non-metallic inclusions in electroslag remelting process

A transient model coupling the magnetohydrodynamic flow and heat transfer is developed to predict the removal and distribution of non-metallic inclusions in electroslag remelting process. The interaction between the melt and non-metallic inclusions was described by two-way coupled Euler-Lagrange method. Both original inclusions from electrode and newly precipitated ones with solidification were considered. The effect of current on the inclusion behaviour was also studied. The model was validated by experiments. The overall removal ratio of original inclusions 2–20 µm in diameter increases from 80.47% to 94.75%. The inclusions with 30 µm even is completely removed. A proper increase of current is conductive to the removal efficiency. In the mushy zone the drag force and buoyancy force play a major role in the entrapment and distribution of newly precipitated inclusions. The inclusion amount increases along radius, but slightly decreases when ranging from 0.9 to 1.0 radius in the solidified ingot. The buoyancy force is great enough for almost all of the 20 µm inclusions to escape from the mushy zone. The absorption ratio of inclusions into slag decreases from 50.5% to 28.7% when the current increases from 3100 A to 3700 A due to the increase of solidification rate.

Effect of continuous casting speed on mold surface flow and the related near-surface distribution of non-metallic inclusions

For the control of surface defects in interstitial-free (IF) steel, quantitative metallographic analyses of near-surface inclusions and surface liquid flow detection via the nail-board tipping method were conducted. The results show that, at casting speeds of 0.8 and 1.0 m/min, a thin liquid mold flux layer forms and non-uniform floating of argon bubbles occurs, inducing the entrainment and subsequent entrapment of the liquid flux; fine inclusion particles of Al2O3 can also aggregate at the solidification front. At higher casting speeds of 1.4 and 1.6 m/min, the liquid mold flux can be entrained and carried deeper into the liquid steel pool because of strong level fluctuations of the liquid steel and the flux. The optimal casting speed is approximately 1.2 m/min, with the most favorable surface flow status and, correspondingly, the lowest number of inclusions near the slab surface.

Comparative Analysis of Mechanical Properties and Cold Resistance of Metal of Steel Castings from the Special Alloyed Steel

In this paper, the experimental observation of influence of casting conditions on steel alloy quality is presented. We studied the casted steel samples obtained with the following methods: sand casting mold (as a reference sample), thin-wall mold with height-adjusted ceramic layer and forced water-air mixture cooling, same as above but with micro-cooling channels used during liquid steel casting. We showed that the steel density as well as typical distribution of non-metallic inclusions and mechanical properties are dependent on casting technology been used. We present the figures of impact toughness, crack growth energy and filamentation ratio with respect to the test temperature. We observed that the impact toughness of the samples is higher than for the reference one within the entire test temperature range. In particular, the impact toughness exhibits a smooth dependence versus temperature as opposed to its abrupt decrease for a reference sample. We showed, that the samples obtained according to the new technology are less fragile and have better cold resistance, similar results were obtained after crack filamentation ratio analysis.

Distribution of nonmetallic inclusions in molten steel under hot-top pulsed magneto-oscillation treatment

To control the distribution of nonmetallic inclusions in molten steel is of highly commercial importance. A newly developed method, hot-top pulsed magneto-oscillation (HPMO), was employed to control nonmetallic inclusions in a medium-carbon steel ingot. The experimental results present that the position of nonmetallic inclusions of spinel (Al–Mg–O) and Mn–S inclusions is significantly influenced by HPMO. The number of nonmetallic inclusions gradually increased from the lateral wall to the center of ingot along the radial direction under the impact of HPMO treatment, whereas this distribution trend cannot be observed in the reference sample without HPMO treatment. In addition, the number of inclusions along vertical direction is proportional to the height of ingot, especially for the inclusions with the size of larger than 10 μm. It indicates that the application of HPMO can push away inclusions from lateral to center, and then the expelled inclusions aggregate and rise up to the top of ingot.

Influence of non-metallic inclusions and products of corrosion on the wear of railway wheels tread

Formulation of the problem . It is of interest to investigate the effect of nonmetallic inclusions and corrosion products of wheel steel on the formation of wear particles on the tread of railway wheels. It is also necessary to identify the specifics of the interaction of wheel steel with the surrounding atmosphere during the operation of railway wheels. The purpose of the article is to study the mechanisms of formation of wear particles on the surface of rolling of railway wheels during operation. Methodology . The effect of inclusions and corrosion products on the mechanism of wear of railway wheels made of R7 steel, which worked for about 5 years under the passenger train, was studied. The wheels were removed from service due to the extreme wear of the rims. We studied the macrostructure and microstructure of wheel steel along the width of the rims. The research was carried out by several methods: metallographic (Neophot−21), electron microscopy (JSM−35). Findings . The features of the formation of wear particles near nonmetallic inclusions and corrosion products related to structural changes near the rolling surface of railway wheels as well as to the interaction of steel with the surrounding atmosphere during operation were investigated. The influence of various factors on the role of nonmetallic inclusions and corrosion products in the formation of wear particles is shown, which makes it possible to predict the nature of wear of the rolling surface of railway wheels under various operating conditions. Originality . The main sources and the mutual influence of various factors on the formation of wear particles near nonmetallic inclusions and corrosion products have been established, which makes it possible to predict the nature of wear of the rolling surface of railway wheels under various operating conditions. Practical value . The use of the obtained results will allow to develop methods and regimes of smelting of wheel steel, allowing to influence the size, composition, structure and distribution of non-metallic inclusions, as well as to develop anticorrosion measures in order to improve the reliability and durability of railway wheels.

Numerical Investigation and Experimental Validation of Motion and Distribution of Nonmetallic Inclusions in Argon Protection Electroslag Remelting Process

A transient coupled mathematical model has been proposed to predict the motion and distribution of nonmetallic inclusions in the electroslag remelting process. The electromagnetic field, temperature distribution, flow pattern, solidification, and inclusion behavior were solved simultaneously. The Euler-Lagrange approach was used to describe the interaction between continuous phase and inclusions. The discrete phase model (DPM) was adopted to track the particles acted by a number of forces. An experiment under argon protection was implemented, and it agrees well with the simulation. The effect of inclusion diameter was also discussed. The experiment result shows that the typical inclusions in final ingot are Al2O3 core surrounded by an outer sulfide layer and pure MnS inclusions. The total area of inclusions increases from center toward ingot surface, while the average equivalent diameter decreases. The simulated result indicates a denser distribution of inclusions locations at the region ranging from 0.7 to 0.9 radii. Few inclusions are entrapped near the mold due to the strong descending flow. Moreover, the inclusion amount increases with height. The total amounts of inclusions with diameter of 2 μm, 5 μm, and 10 μm in ingot are 2199, 1871, and 1704, respectively. The imposed buoyancy and floatation contribute to this phenomenon.

Microstructural analysis and fatigue fracture behavior of rail steel

ABSTRACTThe purpose of this paper is an experimental study on fatigue fracture performance of rail steel which is used in Iran railway. For this purpose, the fatigue, fracture toughness and fatigue crack growth tests were carried out and have been checked by fractography studies on fractured specimens. Since fatigue failure occurs in micro-dimensions, it can be used for fracture surface analysis and study of the distribution of non-metallic inclusions. The experimental results provide a good basis for assessing the numerical results and have been used in the assessment of structural continuity of rail steel in railway networks.

Effect of Structure on Ductile–Brittle Transition in Steels with BCC-Lattice

The effect of structural characteristics (ferrite, ferrite + pearlite, sorbite) including morphology, size and distribution of nonmetallic inclusions of different morphology, sizes and distribution, on ductile–brittle transition in structural steels is considered. It is shown that an elementary ductile microcrack is initiated by particles of critical size. For steels with ferrite-pearlite and sorbite microstructure, the particle size (carbide, nitride and nonmetallic inclusions) is almost the same. The ductile–brittle transition is caused by competition of the size of the brittle and ductile (pits) microcracks. As a result, the critical brittleness temperature T c in simplified form looks like $$ {T}_c={T}_c^0+B{\Lambda}_d^{1/2}/{\Lambda}_b, $$ where elementary microcracks Λ b and Λ d correspond to the size of a brittle crack of transcrystalline cleavage (Λ b ) and the size of ductile crack (Λ d ), Т c 0 reflects the contribution of different strengthening mechanisms to metal yield strength: dislocation, solid solution, grain size, precipitation hardening, and others, and B is a coefficient taking into account the stress-strain state in the fracture zone. As part of this model of failure, it becomes clear why for hardened steel the grain size does not change up to the recrystallization temperature, and yield strength and the value of T c have a complex dependence on the tempering temperature. Since the steel has particles of different sizes, they initiate the occurrence of pits of different sizes. The larger the particle, the sooner a micropore originates around it and pit growth is faster. A wide range of pit sizes arises that causes a wide range of T c values. A close connection is demonstrated for the width of the ΔT c ductile–brittle transition temperature range with a change in the distance between particles, including between pearlite colonies. This relationship can be positive and negative, which is confirmed by experiment.

Nonmetallic Inclusion Distribution within Ingots for Power Generation Engineering Forgings

Results are provided for a study of the distribution and location of nonmetallic inclusions in large forgings of steel 38KhN3MFA weighing 24.2 and 23.52 tons with a normal configuration and with a changed shape of the bottom part (“convex” bottom) cast in a vacuum. It is shown that the change in ingot bottom section geometry leads to an increase in temperature gradient and as a consequence to an increase solid phase advance intensity. In turn, this leads to more uniform distribution of oxysulfide inclusions. The sulfide inclusion content in the ingots compared increases from the periphery to the ingot axis. Dependences are obtained for metal ductility properties, i.e., relative elongation, relative reduction of area, and impact strength on sulfide inclusions distribution over ingot levels, and their unfavorable effect on these properties is established. It is shown that a reason for the reduction in ductility properties is sulfide phase location in the form of films of dendrite boundaries that occurs under conditions of a shortage of oxygen in the melt. This leads to a reduction in oxysulfide content and formation of a sulfide component in an unfavorable form distributed over cast grain boundaries. The ingots are destined for domestic power generation engineering forgings.

Effect of observation position of SUJ2 bar specimens on inclusions distribution

The size and distribution of non-metallic inclusions are strongly related to rolling contact fatigue (RCF) life. SUJ2 steel is manufactured as a round bar, and is cooled from the surface during casting. This indicates that the inclusions may be distributed non-uniformly in the center direction. The objective of this study is a clarification of the difference of the inclusion distribution in the SUJ2 round bar. It was found that the distribution of inclusions in SUJ2 was distributed uniformly in a transverse direction.

Relation Between the Structural Components and Nonmetallic Inclusions and the Operating Properties of Bearing Steel

The endurance of bearing steel ShKh15SG under cyclic bending tests is studied. The relation between the cyclic bending frequency and the ultimate bending strength is determined. It is shown that the endurance calculated by the prototype method does not match the results of full-scale tests of bearings. A new method for determining the endurance of bearings from steel ShKh15SG allowing for the distribution of nonmetallic inclusions in the carbide bands and outside them is suggested.