Role Of Non-metallic Inclusions Research Articles

An investigation into the role of non-metallic inclusions in cleavage fracture of medium carbon pearlitic steels for high-speed railway wheel

Non-metallic inclusions are often considered to be detrimental to the toughness of steels. Here, we found that, in a cleavage-dominated fracture mode, the toughness, particularly the room temperature fracture toughness of pearlitic wheel steel can be improved by refining oxide inclusions or by increasing sulfur content. The roles of these inclusions in crack initiation and propagation were carefully investigated. Intriguingly, no spherical oxides or elongated sulfides were found to be the direct sources of cracks in the cleavage fracture within both Charpy V-notch and compact tension specimens. Cleavage cracks were found to be originated only from the steel matrix. Based on the theory of cleavage fracture and direct microscopic observations, the mechanisms were proposed for understanding the inclusion-toughness relations: (i) the oxide inclusions could indirectly induce crack initiation by enhancing local stress concentration, therefore promoting cleavage fracture and being detrimental to the toughness; (ii) in addition to enveloping the oxides to reduce the local stress concentration, the sulfide inclusions could also break the continuous propagation of cleavage cracks in the longitudinal direction by sulfide cracking or debonding, which released the stress concentration at the crack tip and enhanced the plastic work of fracture, thereby being beneficial to the toughness improvement.

Application of microcrystalline complex modifiers for steel ladle treatment. Part 1. The role of nonmetallic inclusions in steel quality forming

Contamination of steel by nonmetallic inclusions (NI) has a negative effect on mechanical characteristics of metal used under no favorable conditions. Conditions of NI forming in the process of steel smelting, ladle treatment and casting considered. It was shown that it is impossible to get rid of many NI. However, the task of forming less “harmful” NI having minimal effect on the decrease of finished products indices is quite practicable. To refine steel of NI it is reasonable to accomplish operations in a melt to modify NI morphology from dangerous acute-angled aluminous to globular oxide-sulphide. This task can be solved by introduction into metal complex modifiers comprising calcium, barium, strontium and rare earth metals. Addition of complex modifiers is a good alternative to complicative and long-time operations to decrease NI general content to lower levels, for example, by long-time metal ladle treatment. Application of the method enables in some situation to avoid expensive operations related to deep metal desulphuri zation and its dehydronization. Clean steel production becomes considerably easier at application of multicomponent alloys, obtained by a technology of accelerated crystallization. Application of such compositions results in forming globular oxide and oxide-sulphide compounds, as well as eutectics with low-melting point, which are comparatively quickly removed out of liquid metal. At that due to decreasing of liquation processes forming in the liquid metal, higher quality of large ingots and work-pieces, obtained from 420 t mass ingots can be reached.

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.

OXIDATION OF NON-METAL INCLUSIONS AND STEEL MATRIX WITH LASER ACTION

Purpose. It is necessary to study the processes of oxidation of steels during their interaction with the atmosphere at the time of laser treatment. The aim of the work was to study the processes of steel oxidation under laser action. Methods. The materials for investigation were commercial steels containing different non-metallic inclusions. The specimens of different steels with preliminary polished surface were exposed to laser beaming on the installations GOS-30M and GUANTUM-16. Methods of investigation – petrography and optical microscopy (Neophot-21) – were used. Results. It was found that at the time of laser exposure due to the interaction of the steel surface with the air atmosphere, steel oxidizes. At the same time, the role of nonmetallic inclusions in which phase transformations occur is of great importance. The oxidation products have different morphology and chemical composition, depending on the chemical composition of steel, as well as on the wettability in the steel-liquid oxide system. The structural features of oxide films are shown. The interaction of steel with the air atmosphere during laser processing is analyzed. Scientific novelty. The features of the oxidation of the surface of steels in the process of interaction with the air atmosphere under conditions of laser exposure are established. The features of phase transformations in nonmetallic inclusions and local zones of a steel matrix near inclusions are shown. The effect of inclusions on oxidative processes is shown. The features of the morphology and chemical composition of oxide films in different steels are shown. Practical significance. The use of the results obtained will allow us to develop laser processing technologies for steels with regulated surface quality parameters, which will prevent the formation of various kinds of defects. A method for removing oxide films from the surface of products is proposed.

Role of Nonmetallic Inclusions in Premature Stress-Corrosion Fractures of Drill Pipes

We analyze the causes of the in-service loss of integrity of drill pipes made of G105 steel connected with their premature stress-corrosion fractures. In this case, the mechanical characteristics of steel are satisfactory, except a somewhat lowered elongation. At the same time, both in the structure and on the fracture surfaces of impact test specimens, we reveal a large number of corrosion-active nonmetallic inclusions of the calcium-aluminate type, which may increase the rate of local corrosion by an order of magnitude, promote the formation of deep pits on the inner surface of the pipe, and play the role of sites of initiation of corrosion-fatigue cracks. This is why we assume that the stage of crack initiation, which mainly determines the total durability of drill pipes after premature failures is controlled by the presence of corrosion-active nonmetallic inclusions and their density in the structure of steel.

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.

A general model for hydrogen trapping at the inclusion-matrix interface and its relation to crack initiation

The role of non-metallic inclusions in hydrogen-induced failure of structural materials has been a controversial topic for many years. In this paper, hydrogen trapping and its relation to the crack initiation at the inclusion-matrix interfaces are studied by considering the interfacial structure and the interaction between the dissolved hydrogen atoms and the elastic strains produced by lattice matching and misfit dislocations. A model is proposed to analyse the change of interfacial structure with inclusion size and its relation to hydrogen trapping. Hydrogen accumulation at the interfaces is quantitatively analysed. The obtained results are in good agreement with the experimental observations. The multiple factors, such as interfacial structure, chemical composition, elastic properties of matrix and inclusions, crystallographic relationship between inclusions and matrix, inclusion morphology and size, simultaneously control hydrogen trapping. In addition, the mechanism of hydrogen-induced crack initiation at the interface is investigated. A criterion is proposed to determine critical conditions for crack initiation. For the first time, the inherent relationship between hydrogen trapping and hydrogen-induced cracking at the interface is clarified. This work paves a way for an in-depth understanding of the effects of inclusions on hydrogen-induced degradation of mechanical properties.

Recent Aspects on the Effect of Inclusion Characteristics on the Intragranular Ferrite Formation in Low Alloy Steels: A Review

AbstractIntragranular ferrite (IGF), which nucleates from specific inclusion surfaces in low alloy steels, is the desired microstructure to improve mechanical properties of steel such as the toughness. This microstructure is especially important in the coarse grain heat affected zone (CGHAZ) of weldments. The latest review paper focusing on the role of non-metallic inclusions in the IGF formation in steels has been reported by Sarma et al. in 2009 (ISIJ int., 49(2009), 1063–1074). In recent years, large amount of papers have been presented to investigate different issues of this topic. This paper mainly highlights the frontiers of experimental and theoretical investigations on the effects of inclusion characteristics, such as the composition, size distribution and number density, on the IGF formation in low carbon low-alloyed steels, undertaken by the group of Applied Process Metallurgy, KTH Royal Institute of Technology. Related results reported in previous studies are also introduced. Also, plausible future work regarding various items of IGF formation is mentioned in each section. This work aims to give a better control of improving the steel quality during casting and in the heat affected zone (HAZ) of weldment, according to the concept of oxide metallurgy.

Crack initiation sensitivity of wrought direct aged alloy 718 in the very high cycle fatigue regime: the role of non-metallic inclusions

Fatigue crack initiation in the direct aged version of the nickel-based superalloy Inconel 718 has been investigated at room temperature in the low stress/very high cycle regime via ultrasonic fatigue testing. Three different microstructures have been examined at the same strain amplitude in order to understand the influence of non-metallic inclusions (NMIs), i.e. carbides, carbonitrides and nitrides, and Σ3 twin boundary density on lifetime and failure mode. A slight refinement in grain structure and a higher Σ3 twin boundary density is associated with substantial reductions in lifetime. Decreasing Σ3 twin boundary density for fine grain microstructures results in a change in crack initiation mechanism from strain localization within grains at the high end of the grain size distribution to cracking of NMIs. To study the early stages of crack initiation and growth, specimens with pre-cracked NMIs were also tested in order to examine the role of the surrounding grain structure. Pre-cracked NMIs mainly result in macroscopic failure initiation at NMIs independent of the wrought microstructure. However, pre-loading specimens within the plastic domain highlighted the competition in crack initiation mode between cracked-NMIs and favorably oriented twin boundaries. Crack arrest from most of the pre-cracked NMIs demonstrates that surrounding grain structure (grain orientation, local plasticity and roughness in the vicinity of crack tip due to pre-straining) play a key role in the fatigue life of components stressed in the nominal elastic regime.

Role of Nonmetallic Inclusions in Hydrogen Embrittlement of High-Strength Carbon Steels with Different Microalloying

High-strength carbon steels of 1200 MPa strength level with different microalloying were tensile tested at constant extension rate and constant load under continuous electrochemical hydrogen charging. The results show that hydrogen markedly reduces elongation and time to fracture of all the studied steels. Fractography of the steels shows that nonmetallic inclusions (NMIs) play the major role in crack initiation in hydrogen-charged specimens. The role of NMIs in the hydrogen-induced fracture of steels is discussed.

Very high cycle fatigue behavior of a low strength welded joint at moderate temperature

Very high cycle fatigue (VHCF) behavior of a low strength weldment was investigated by fully reversed axial tests in air at room temperature and 370°C. The role of non-metallic inclusions in the VHCF was addressed in terms of experimental results and finite element simulations. The higher potential for interior crack nucleation at higher temperature was ascribed to matrix softening, surface oxidation and surface compressive residual stress. A new model for interpretation of the role of inclusion in the transition of crack initiation modes was developed.

A design approach for components in ultralong fatigue life with step loading

An overview of critical variables that affect fatigue failure with respect to steel components in ultralong life regimes is presented. The key role of hydrogen trapped by non-metallic inclusions in the ultralong life fatigue failure process is documented. The role of non-metallic inclusions on ultralong fatigue life is shown in the master curve of ODA (Optically Dark Area surrounding a non-metallic inclusion at fracture origin) growth. The master curve of ODA growth shows the correlation of the size of the ODA with the size of the non-metallic inclusion as it corresponds to fatigue life. The ability to predict the presence of non-metallic inclusions in steels with extreme value methods is incorporated with the master curve of ODA growth to determine the maximum threshold stress for ultralong fatigue life using the area parameter model. Most machine components experience variable loads in service. A design approach is introduced for calculating the effects of different loading levels for ultralong fatigue life.

On the Role of Non-metallic Inclusions in the Nucleation of Acicular Ferrite in Steels

The effects of non-metallic inclusions in nucleating acicular ferrite in steels during cooling from a weld or cooling from an austenitic temperature are reviewed. The influence of the acicular ferrite (AF) structure on mechanical properties of steels such as strength and toughness is briefly mentioned. The different factors affecting the formation of acicular ferrite, such as the soluble content of alloying elements in steel, cooling rate from austenitizing temperature, austenite grain size and inclusion characteristics in steel, are discussed. The mechanisms of acicular ferrite formation on non-metallic inclusions, such as reduction of interfacial energy, mismatch strain between the inclusion and ferrite/austenite, thermal strains at the inclusions and changes in matrix composition near the inclusions are also discussed. Finally, the effects of inclusion characteristics, such as size, number and composition are described and their effectiveness in nucleating acicular ferrite is discussed.

THE EFFECT OF NON-METALLIC INCLUSIONS ON THE CRAK PROPAGATION IMPACT ENERGY OF TOUGHENED 35B2+Cr STEEL

35B2+Cr steel was designed as a material for screws. The investigations were carried out on toughened samples, taken from three ingots of 35B2+Cr steel (delivered from three various steel suppliers), and of different content of non-metallic inclusions. In each case, the fraction of non-metallic inclusions was in agreement with the corresponding standard. This study presents the results of the research focused on the influence of non-metallic inclusions on the impact energy of crack development in 35B2+Cr steel. The role of non-metallic inclusions was considered both in relation to their total fraction and in relation to the fraction of oxides, sulfides, nitrides and exogenous inclusions separately. Assuming linear dependence between notch-root radius and the impact energy of crack nucleation, basing on the impact toughness tests, the impact energy of the crack development was evaluated. It was proved, that in spite of the level of fraction of non-metallic inclusions compatible with the corresponding standards, they directly or indirectly influence the impact energy of 35B2+Cr steel. This influence depends on the character of the inclusions.

High-Machineability Steels

Collaborations between metallurgists and industrial engineers have led to the development and successful introduction of high-machineability steels that make it possible to significantly improve machining indices at machine factories. The highest level of machineability is seen in free-cutting steels alloyed with sulfur, lead, and bismuth. This article examines aspects of their composition and structure and explains the role of nonmetallic inclusions, the importance of their morphology, and the need to have them distributed uniformly in the metal. A metallurgical technology can be used to improve the machineability of ordinary structural steels through inoculation with calcium or microalloying with sulfur. In the latter case, while sulfur content is kept to the level stipulated by standards for structural steels (S ≤ 0.04%), the machineability of these steels is improved as a result of a change in the morphology of the sulfides. A diagram is presented to show the areas in which different variants of high-machineability steels are used most effectively.

The Role of Nonmetallic Inclusions in Accelerating the Local Corrosion of Metal Products Made of Plain-Carbon and Low-Alloy Steels

Using the principles of physicochemical modeling to describe the deoxidation and desulfurization of steel in the ladle and interactions between the metallic and slag phases with allowance for features of the equipment in use at specific plants, the paper shows that it is possible to quickly optimize the parameters of out-of-furnace treatments of the steel without losing productivity. The optimum parameters make it possible to fully realize all of the main advantages of such treatments: further refining of the steel, modification of the nonmetallic inclusions, guaranteeing that the steel will be clean in terms of the presence of corrosion-active nonmetallic inclusions, and, thus, ensuring that the steel will be resistant to local corrosion.

Ferrite nucleation potency of non-metallic inclusions in medium carbon steels

In recent years, there has been renewed interest in the role of non-metallic inclusions in controlling the microstructure of steels. The potency of various inclusions and precipitates such as SiO 2, MnO·SiO 2, MnS, Al 2O 3, Ti 2O 3 and VN for the nucleation of intragranular ferrite has been examined in the present study. Among them, single SiO 2, MnO·SiO 2, Al 2O 3, TiN and MnS particles seem to be inert to the nucleation of intragranular ferrite under the present experimental condition. Ti 2O 3 particles in a Mn-containing steel are very effective for the nucleation of intragranular ferrite, being (Ti,Mn) 2O 3 particles by absorbing Mn atoms from a steel matrix. On the other hand, Ti 2O 3 particles in a Mn-free steel are not effective. MnS and Al 2O 3 particles in high nitrogen steels containing vanadium also appear to be potent for the nucleation of intragranular ferrite. The decrease in transformation temperature causes a change in the morphology of intragranular ferrite from idiomorphic ferrite to acicular ferrite.

Role of non-metallic inclusions in formation of acicular ferrite in low alloy weld metals

A critical investigation into the role of non-metallic inclusions in the nucleation of acicular ferrite in low alloy C–Mn–Ni weld metals was carried out using rapid in situ quenching, transmission electron microscopy, energy dispersive X-ray spectroscopy analysis, and dilatometric techniques. The relationship between the transformed acicular ferrite, the inclusions, and the chemical nature of the inclusions which had actually nucleated ferrite laths was examined in detail. By employing weld metals with either intermediate (200–350 ppm) or low (15–30 ppm) oxygen levels, the inclusion chemistry and nucleation behaviour of acicular ferrite in welds with different oxygen contents were compared. In addition, the influence of the [Al]/[O] ratio in lower oxygen content weld metals was also examined. It was found that the first acicular ferrite laths nucleated from the inclusions at both oxygen levels, and the chemical composition of the active inclusions varied considerably. The inclusions nucleated acicular ferrite by acting simply as inert substrates with the ability to reduce the free energy barrier to nucleation. The number and size distribution of the inclusions appeared to be the dominant factors; the thermal stress-strain fields and lattice disregistry were less important. The most favourable type of inclusions were within the size range 0·3–0·9 μm for the higher oxygen content weld metals and 0·35–0·75 μm for the lower oxygen level weld metals. A high [Al]/[O] ratio depresses the formation of acicular ferrite by increasing the soluble matrix aluminium content, which encourages extensive ferrite sideplate formation; the concurrent variation in inclusion chemical composition did not appear to influence the nucleation of acicular ferrite.MST/3209

Role of non-metallic inclusions in formation of acicular ferrite in low alloy weld metals

AbstractA critical investigation into the role of non-metallic inclusions in the nucleation of acicular ferrite in low alloy C–Mn–Ni weld metals was carried out using rapid in situ quenching, transmission electron microscopy, energy dispersive X-ray spectroscopy analysis, and dilatometric techniques. The relationship between the transformed acicular ferrite, the inclusions, and the chemical nature of the inclusions which had actually nucleated ferrite laths was examined in detail. By employing weld metals with either intermediate (200–350 ppm) or low (15–30 ppm) oxygen levels, the inclusion chemistry and nucleation behaviour of acicular ferrite in welds with different oxygen contents were compared. In addition, the influence of the [Al]/[O] ratio in lower oxygen content weld metals was also examined. It was found that the first acicular ferrite laths nucleated from the inclusions at both oxygen levels, and the chemical composition of the active inclusions varied considerably. The inclusions nucleated acic...

Role of non-metallic inclusions in heat-treated steels for sour environment

Sulphide stress corrosion cracking (SSCC) studies on a set of three steels with different inclusion morphologies were conducted. Effect of heat treatment of the steels with and without rare-earth metal (RE) treatment upon SSCC resistance was studied. Threshold stress, corrosion rate and hydrogen content in the steels were determined. Quenched and tempered structure was found to improve the SSCC resistance only when the steel is RE treated. Corrosion rate and hydrogen content in the steels were found to decrease as the inclusion morphology is changed from elongated stringers to globular shape, which improves the SSCC resistance