Size Of Non-metallic Inclusions Research Articles

Initiation Behavior of Crack Originated from Non-Metallic Inclusion in Rolling Contact Fatigue

Abstract Recently, it has been generally accepted that flaking life is dependent on the size of non-metallic inclusion under proper condition in bearing use. Statistics of extreme values to predict the maximum non-metallic inclusion size and ultrasonic test to assess large non-metallic inclusions in a large volume are widely used as practical methods for the evaluation of bearing steel cleanliness. Murakami’s formula is well known, which describes the relationship between non-metallic inclusion size and fatigue strength. However, the formula cannot be directly applied to rolling contact fatigue because of the difference in applied stress mode. While the equation was invented to predict the fatigue limit in principal stress mode, rolling contact fatigue is mainly caused by shear stress. It is expected that the condition of bearing use will be more critical due to the downsizing tendency in various industrial or automotive machine units. Thus the research on the flaking mechanism becomes more important from the viewpoint of global ecology because that is beneficial to the improvement in bearing life and methods for cleanliness evaluation. Due to the difficulty in experimental observation, however, the details of the flaking mechanism in rolling contact fatigue caused by non-metallic inclusion has not yet been clarified. Focusing on the relationship between non-metallic inclusion and initiation of the crack, the flaking mechanism is proposed in this paper.

Evolution of microstructure and mechanical properties during quenching and tempering of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel

Effects of quenching and tempering treatments on the development of microstructure and mechanical properties of ultrahigh strength 0.3C Si–Mn–Cr–Mo low alloy steel were investigated. Samples were austenitized at 1123–1323 K for 2400 s and oil quenched (OQ) to produce mixed microstructures. Tempering was carried out at 473–773 K for 2–3 h. Phase transformation temperatures were measured using dilatometer. The microstructures were characterized using optical and scanning electron microscope. SEM–EDS analysis was carried out to determine the type and size of non-metallic inclusions. Volume percent of retained austenite was measured by X-ray diffraction technique. Hardness, tensile properties, and impact energies were also determined for all heat treated conditions. Fractography of impact specimens were done using stereomicroscope and SEM. The results showed that newly developed steel exhibited peak hardness, yield strength, and tensile strength of about 600 HV, 1760 MPa, and 1900 MPa, respectively, when OQ from 1203 K and tempered in between 473 and 573 K, combined with adequate ductility and impact toughness. Decrease in hardness and strength was observed with increasing tempering temperature whereas the impact energy was stable up to 623 K, however, impact energy was found to decrease above 632 K due to temper martensite embrittlement.

Relationship among fatigue life, inclusion size and hydrogen concentration for high-strength steel in the VHCF regime

Based on experimental data and from the viewpoint of linear elastic fracture mechanics (LEFM), this paper proposes the relationship among fatigue life, N f, nonmetallic inclusion size, r 0, and hydrogen concentration in steel, C H, for high-strength steels in the very high cycle fatigue (VHCF) regime. Fatigue life, depending on the inclusion size and hydrogen concentration, decreases significantly with inclusion size and/or hydrogen concentration increasing. For the high-strength spring steel 60Si2CrV, if the inclusion size is reduced by one-third or one-half, the fatigue life will be up to about 10 or 100 times, respectively. Finally, the relationships of fatigue strength and fatigue life with inclusion size were compared.

Thermo-Chemistry of Non-Metallic Inclusions in Ductile Iron

Non-metallic inclusions formed during multi-stage ductile iron melt treatment play an important role in solidification, structure, and casting quality. Professor Carl Loper made fundamental studies on the role of inclusions in heterogeneous nucleation of graphite. In this article, a comparison of inclusions in steel and cast iron was done using thermodynamic calculations (FACTSAGE software) and automated SEM/EDS inclusion analysis (ASPEX system). Statistics of non-metallic inclusions (size, shape, composition) were studied in the iron and steel samples collected by quenching after different stages of melt treatment as well as from castings. The suggested methodology of inclusion analysis has the potential to be applied together with adaptive thermal analysis (ATAS) for solving practical problems such as decreasing shrinkage defects in ductile iron.

Effect of loading condition on very high cycle fatigue behavior in a high strength steel

Very high cycle fatigue tests were conducted under rotating bending and axial loading using a high carbon chromium steel, JIS SUJ2, in order to evaluate the effect of loading condition on subsurface fracture with a fish-eye. In very high cycle region where subsurface crack was generated at a non-metallic inclusion, the fatigue lives under axial loading were shorter than those under rotating bending. This was attributed to the maximum size of non-metallic inclusions included in the control volume, because the sizes of non-metallic inclusions that acted as fracture origin were larger under axial loading than under rotating bending. Under both loading conditions, the K max values for crack initiation from non-metallic inclusion decreased with increasing fatigue life and were almost similar irrespective of loading condition, while the threshold K values where the transition from the granular area to the flat area in fish-eye took place depended on the size of the granular area. The transition condition was similar under both loading conditions.

Non-metallic inclusions in the weld metal deposit of shielded electrodes used for welding of low-carbon and low-alloy steel

This paper discusses the relationship between the concentration of oxygen in the weld metal deposit – produced with basic- and rutile-shielded electrodes – and the size of non-metallic inclusions. Furthermore, this paper presents a mathematical formula that allows one to calculate the diameter of non-metallic inclusion in the metal weld deposit depending on the concentration of oxygen in MWD.

Effect of Non-Metallic Inclusion Size and Residual Stresses on Gigacycle Fatigue Properties in High Strength Steel

Fatigue fracture of some high-strength steels occurs at small defect in the subsurface zone of a material at low stress amplitude level and in a high-cycle region of more than 106 cycles (gigacycle fatigue life), whereas surface fatigue crack initiation occurs at high-stress amplitude and low cycles. There is a definite stress range where the crack initiation site changes from a surface to a subsurface defect, giving a step-wise S-N curve or a duplex S-N curve. From the experimental results, fatigue fracture mode was classified into three types, such as, surface inclusion induced fracture mode, subsurface inclusion induced fracture mode without granular bright facet (GBF) area and that with the GBF, depending on stress amplitude level and stress ratio. The GBF area was observed in the vicinity of a non-metallic inclusion at the fracture origin inside the fish-eye in gigacycle fatigue regime. It was made clear from the discussion with fracture mechanics that the transition of fracture mode was affected by compressive residual stresses on the specimen surface. Fracture-mode transition diagram was proposed through the experimental and theoretical investigation. Also, from the evaluation of the fatigue life based on the estimated subsurface crack growth rate from the S-N data, effect of inclusion size on the dispersion of fatigue life was explained, and S-N curve for subsurface inclusion-induced fracture depended on the inclusion size was provided.

Automated SEM-EDX Cleanness Analysis and its Application in Metallurgy

Steel cleanness has become one of the major topics in steel research in respect to both research on production parameters in secondary metallurgy and material properties. In addition to conventional light microscopy and standard ultrasonic testing of slabs and strips other more sophisticated analytical methods have become necessary. One method is automated SEM-EDX cleanness analysis, which also includes information on distribution, size, shape and chemistry of nonmetallic inclusions. Especially the introduction of detailed data from investigations by electron microprobe has been a successful addition for the improvement of the classification system of nonmetallic inclusions consisting of more than one phase. The development of an especially tailored offline evaluation system for the raw data from automated SEM-EDX analyses with 24 inclusion types has been the aim for the product range of Voestalpine Stahl Linz.

New Development in Through Hardened Bearing Steel Grades for Use in Contaminated Lubrication

Abstract Bearing steel quality has been improved over the past decades by reducing the level and the size of nonmetallic inclusions. Therefore, fatigue lifetime of bearings is mainly governed by surface damage, such as debris denting due to foreign particles. Numerous studies have been performed to find out new steel grades or heat treatments to cope with such severe in-service conditions. Most of these studies argued that the level of retained austenite is the main parameter which governs fatigue lifetime. The present study relates to the effect of heat treatment and alloying elements on the fatigue lifetime of bearing steel in contaminated lubrication at room temperature. Several through hardened steel grades have been tested with different methodologies in order to investigate their behavior in case of: (1) Debris denting, (2) Resistance to Tempering, and (3) High loading conditions. An experimental procedure developed by FAG to simulate particle indentation on bearing has been used to study the behavior of each grade. On the other side, torsion tests at room and high temperature have been used to determine the evolution of fatigue limit. At last, flat washer testing at 4200 MPa is used to determine fatigue lifetime in high loading conditions. Results of fatigue tests are presented in this paper and analyzed in terms of tempering resistance and level of retained austenite.

Specific features and mechanisms of fatigue in the ultrahigh-cycle regime

Earlier concepts of ultrahigh-cycle fatigue (UHCF) of both ductile single-phase type I materials that do not contain inclusions and type II materials, e.g., high-strength steels containing non-metallic inclusions as (internal) crack initiation sites, are extended. In particular, the multi-stage fatigue life diagrams with a conventional high-cycle fatigue (HCF) limit and a possible lower fatigue limit in the UHCF range are discussed for both classes of materials in terms of the relevant fatigue cracking mechanisms. In this context, it is emphasized that it is important to consider in both cases details of fatigue crack initiation as a life-determining factor in the HCF and, even more so, in the UHCF regime. Different factors such as the density, size, location and distribution of non-metallic inclusions and details of specimen geometry and surface quality in the case of type II materials are discussed with respect to their influence on the transition stress separating the ranges of surface and internal fatigue cracking and with regard to fatigue lives.

Quantitative Analysis of Inclusions in High-strength Steels by X-ray Computed Tomography Using Ultra-bright Synchrotron Radiation

The observation of internal microstructures in materials is important to elucidate the mechanisms of ultra-long life fatigue of high-strength steels, and to ensure the integrity of structures. By conventional techniques, such as laboratory X-ray and ultrasonic imaging, the size and shape of subsurface non-metallic inclusions and cracks, those are smaller than 100 {mu}m, cannot be measured. Then, in the present study, the ultra-bright synchrotron radiation X-ray was applied to the imaging of subsurface inclusion. To obtain basic data for the measurement, the penetration depth of synchrotron radiation wave in a free-cutting steel was examined. It was found that the depths where the transmitted wave cannot be observed are 100 {mu}m for 15 keV, 200 {mu}m for 20 keV, 600 {mu}m for 25 keV, and 800 {mu}m for 30 keV. For the measurement of size and shape of inclusion, synchrotron radiation computed tomography method (SR-CT) was employed. Metallographic structures can be observed in a free-cutting steel, and the diameter of these structures is about 7 - 10 {mu}m. They are considered to be inclusions, which mainly contain manganese sulfide (MnS) or pearlitic phase. To investigate the possibility of detection of pearlitic phase, SR-CT was applied to observe the metallographic structures in carbon steel (S35C), which contains a lot of pearlitic phase and a little inclusion. In this case, no metallographic structure was observed inside specimen. These results indicate that the microstructures those were observed by SR-CT method was inclusions, not pearlitic phase. (author)

Micro Behavior Study of Non-Metallic Inclusion in High Strength Shaft Steel

The composition, shape, size and distribution of non-metallic inclusions in a kind of high strength shaft steel enriched CoNi have been investigated. In situ tensile tests in special designed SEM have been conducted to trace the whole process of crack initiation and propagation till to fracture at inclusion. The experimental results show that the non-metallic inclusions in the high strength shaft steel are primarily AlN. Their average size is about 3.6µm. Sometimes, a string of inclusions distributes along the rolling direction. Non-metallic inclusion can induce crack to be initiated by inclusion debonding. When the inclusion size is larger than the critical size, the crack can propagate as the main crack that induces the specimen to fracture.

Cleavage Initiation in Ti-V-N and V-N Microalloyed Forging Steels

The effects of matrix microstructure and features of non-metallic inclusion (morphology, type, volume fraction and size) on cleavage initiation in medium-carbon Ti-V-N and V-N microalloyed resulphurized forging steels have been determined by examining fracture surfaces produced in room temperature Charpy impact tests. The steels were generally Al-deoxidised but one V-N steel was Si-deoxidised. It has been found that, in the Ti-treated steel, having a ferrite-pearlite microstructure, brittle fracture initiation occurred at cracked coarse (Ti,V)(C,N) single phase or [(Ti,V)(C,N)/Al2O3/MnS] multi-phase inclusions. In the Ti-free steels, cleavage initiation was dependent on matrix microstructure and non-metallic inclusions. In the low strength Ti-free steels, with a ferrite-pearlite microstructure, the absence of a continuous grain boundary ferrite layer led to initiation from interfacing pearlite colonies. For the bainitic microstructure, cleavage initiated close to the notch, but the microstructural feature responsible could not be identified. For the ferritepearlite microstructure in the Si-deoxidised V-N steel, cleavage initiated at cracked Mn-Al-(Ca) silicate inclusions. The higher matrix strength and more continuous nature of grain boundary allotriomorphic ferrite in the V-N steel deoxidised with Al was associated with cleavage initiation from V-rich (V,Ti)(C,N)-containing inclusions. These were generally of smaller size than those in the Ti-treated steels.

Study of the nature of non-metallic inclusions in samples of aluminum and silicon killed low carbon steels, collected in the refining treatment and continuous casting stages

The amount, distribution, size and chemical composition of non-metallic inclusions have a direct influence on steel properties. By controlling size and chemical composition of these inclusions, it is possible to get a product with good quality. The identification of the nature and the control of inclusion formation are very important for steel cleanness. The behavior of these inclusions is predictable, in some extent, by the determination of the chemical composition of non-metallic phases that form such inclusions. With the objective of studying the chemical composition, the size and the distribution of such inclusions, samples of aluminum and silicon killed low carbon steels were collected in a national steel industry in the secondary refining and continuous casting stages. These samples were analyzed in the scanning electron microscope (SEM) coupled to an energy dispersive analysis system (EDS). From the results, it was possible to evaluate the nature of inclusions and to analyze the effectiveness of the refining process in the reduction of the number and area fraction of the inclusions. It was also possible to verify that the inclusions that remained after treatment, are less damage both to the steel properties as to the continuous casting process (clogging of the submerged valve).

Relationship between through-thickness residual stress of CrN-PVD coatings and fatigue nucleation sites

A CrN monolithic 5 μm thick was deposited on a substrate of H11 tool steel and was characterised by XRD analysis and nanoindentation measurements. Hardness, elastic modulus, chemical composition, texture and lattice constants were determined along with the through-thickness residual stress distribution. A high level of compressive stress was obtained both at the surface and inside the coating; data always higher than 2 GPa were measured. Low values of compressive stresses were measured in the steel side near the coating/substrate interface. Those results, coupled with a good adhesion of the interface, allow, when cycling loads are applied to the coated component, a nucleation of the fatigue crack inside the substrate material. Therefore, the critical microstructural parameter that strongly affects the fatigue resistance of the CrN coated samples investigated becomes the size of nonmetallic inclusions working as discontinuity inside the steel substrate, close to the location of the coating/substrate interface. Indeed, in such a location the compressive residual stresses induced by the deposition technique are low and, therefore, insufficient to oppose the tensile stress induced by the applied loading condition during four-point bending fatigue tests.

Diffusional Dissolution of Alumina in Various Steelmaking Slags

Fast dissolution of nonmetallic inclusions such as alumina particles in molten slags has been required in secondary steelmaking-ladle and tundish-to minimize the amount and the size of nonmetallic inclusions in steel products. In the present work, the dissolution behavior of alumina particles in molten CaO-Al 2 O 3 and CaO-Al 2 O 3 -SiO 2 slags at the temperatures between 1 500 and 1 550°C have been investigated in terms of kinetics and mechanism. Alumina particles were added directly into the slags and the variation of the particle size with time was determined using optical and electron microscopes. The conventional types of slag for ladle and tundish were included in the dissolution study. The effects of Al 2 O 3 , SiO 2 , CaF 2 and MgO contents on the dissolution rate of alumina particles have also been determined. The diffusion boundary layer between alumina particle and bulk slag phase was observed and analyzed using SEM and EDS. Based on the analysis of kinetic data and the boundary layer, the dissolution mechanism has been discussed.

Experimental investigation on statistics of extremes for three-dimensional distribution of non-metallic inclusions

A wide range of studies have shown that the lower bound of fatigue properties of high strength steels is determined by the maximum size of non-metallic inclusions that are present in a component. The maximum size of inclusions in a given component or material volume can be reasonably estimated using the statistics of extremes. However, as long as the estimation is based on microscope inspections of two-dimensional (2D) surfaces, there will be errors and uncertainties in estimating the maximum particle in a three-dimensional (3D) volume. In addition it has been recently found that in some steels the distribution of extreme defects is composed of a mixture of different particle types. The scope of this paper is to clarify the validity of 2D inspections on the basis of 3D distribution of inclusions in a modern super clean steel. The 3D distribution was obtained with a combination of inclusions detected with a repeated slicing procedure and of particles at fatigue fracture origin. The 3D distribution of inclusions is composed of a mixture of two types of particles having similar chemical compositions and different 3D morphological structures: one with a large population and another with few rare particles. The 3D large population can be accurately estimated from maximum inclusions on small polished sections, while in order to estimate the characteristic size of inclusions at fatigue fracture origin by 2D inspections it is necessary to adopt a minimum inspection area Scrit. In the case of the material examined in this study (SCM435 steel) this minimum inspection area is ~ 10 000 mm2.

Experimental and numerical investigations of two material states of the material 15 NiCuMoNb5 (WB 36)

In this contribution, results are presented of mechanical, technological and fracture-mechanical characterization of low alloyed warm strength steel 15 NiCuMoNb5 (WB 36, material No. 1.6368) in two states of the melt E60 at a temperature of 90 °C including the determination of damage mechanical parameters for the Rousselier model. The two states of the material are the initial state (denominated as E60A) and the aged state (denominated as E60B). The aged condition characterizes the material after 57,000 h at a service temperature of 350 °C. In service this material produces significant copper precipitates above 300 °C. These copper precipitates are the reason for strength increase as well as a shift of the transition temperature regime of the “notch” impact energy to higher temperatures. Size, shape, orientation and frequency distribution of non-metallic inclusions are found to be similar for both material states. In comparing the frequency distributions of the copper particles for both material states a significant increase of copper particles with a size between 2 and 7 nm results. The damage mechanical results show that the Rousselier parameters for the initial state can be transferred to the aged state. The strength increase due to aging can be considered through the respective yield curves in the computer simulations. A comparison of multiaxiality relations for different specimen geometries shows that the curves of the multiaxiality coefficient q in the ligament remains principally unchanged by the strength increase at identical specimen geometries. The crack resistance behaviour determined in the experiment at CT specimens correlates with the numerically determined q-values in the ligament for both material states E60A and E60B.

Avaliação do tamanho, da morfologia e da composição química de inclusões não metálicas em aço baixo carbono desoxidado ao alumínio

A presença de inclusões não metálicas endógenas e exógenas prejudica a lingotabilidade e as propriedades mecânicas dos aços. A busca contínua pelo controle da quantidade, do tamanho, da morfologia e da composição química de inclusões não metálicas tem sido tema de estudos nas últimas décadas, em especial durante o processo de refino secundário dos aços. Através do controle, principalmente, do tamanho e da composição química das inclusões, é possível obter um produto final de boa qualidade. A identificação da natureza e o controle da formação de inclusões são de suma importância para a obtenção de aços limpos. Este trabalho tem como objetivo fazer uma investigação de inclusões não metálicas em aço de baixo teor de carbono(SAE 1015), desoxidado ao alumínio, através de um método de dissolução parcial e total da matriz metálica e posterior análise ao Microscópio Eletrônico de Varredura (MEV), acoplado a um sistema de análise por energia dispersiva (EDS), permitindo uma visualização do tamanho, da forma e da composição química das inclusões com maior precisão, sem interferência da matriz ferrítica.

Assessment of a premature failure in a gas turbine part

A premature failure of an inlet guide vane in a gas turbine has been the subject of a rigorous study and the present paper describes it in detail. It was established that failure occurred by fatigue crack growth which was initiated from a surface defect, in the form of a non-metallic inclusion cluster, at the vane root location where working stresses were highest. From a careful fractographic study and fatigue data, it was assessed that the fatigue stress range for this failure was from 260 to 530 MPa. Strain gauge measurements indicated a real working stress range of 210–450 MPa and demonstrated the usefulness of the fractographic predictions. It was recommended that (a) the guide vane root be increased in thickness such that the working stresses be reduced by some 40% and (b) the vane be fabricated from a forged steel rather than a casting. The latter reduced the extent and size of non-metallic inclusions and it was assessed that fatigue crack growth would not be initiated from the worst inclusion cluster observed in the forged steel guide vane component.