The determination of steel cleanliness in the as-cast steel ingot 26NiCrMoV145

The content and the type of non-metallic inclusions in the as-cast ingot for open-die forgings made from 26NiCrMoV145 steel were determined. The standard methods for the determination of non-metallic inclusions are suitable only for the wrought steels, so a modified standard method for the determination of the non-metallic inclusions in the as-cast steel was performed. Analyzed samples were taken from three different positions on the ingot. The determination of the type of the non-metallic inclusions was made by EDS analysis. The content of the non-metallic inclusions was determined using an optical microscope.     

Effect of desulphurization and calcium treatments on the inclusion morphology of 0.4C-Cr-Mo-Ni steel

Several 0.4C-Cr-Mo-Ni steels with different sulphur and calcium content levels have been studied to determine the effect of desulphurization and calcium treatments on the morphology of non-metallic inclusions in low-alloy steels. The volume fraction and size parameters of the inclusions were determined using optical microscopy. The inclusions were identified by electron probe microanalysis. The volume fraction and mean aspect ratio of a stringy type of inclusion, which consisted predominantly of MnS, was reduced significantly with a decrease in sulphur content through desulphurization. A cluster type of composite inclusion was also decreased with decreasing sulphur content. Calcium treatment of steel with a commercial sulphur level was not very effective for modification of the inclusions, producing two types of cluster composite inclusion. However, the calcium treatment of desulphurized steel modified dramatically the stringy type to a particle type (mean diameter: 1.3 m) which consisted predominantly of CaS-CaO, while small amounts of Ti and Al were also detected.     

Investigation on defect distribution and its statistical evaluation for case hardened material states

The fracture behaviour of steel components with very hard material states under cyclic loading is mostly defined by non-metallic inclusions present in the material. The inclusion distribution is a result of specific conditions in steel making, casting and rolling processes. Standard methods (e.g. DIN EN 10247:2007) are commonly used for estimation of the distribution of non-metallic inclusions. These methods are based on random metallographic examination of longitudinal microsections and evaluation with standard pictures. For numerical evaluation of the endurance limit of very hard material states the knowledge of defect distribution of the material is necessary. However, the standard methods are not suitable to find the actual inclusion distribution.In a large investigation program on the fatigue strength of very hard material states, thin wall tubes made of a case hardening steel were tested. All fracture surfaces were investigated, size and coordinates of the fracture initiating defects were documented and analysed statistically. The defects appear as inclusion stringers stretched in rolling direction mainly containing Al-oxide. Several distribution functions for statistical analysis were investigated.Furthermore, transverse microsections with a total area of about 2000 mm² were investigated and the data of the maximum width of inclusion stringers were also analysed statistically. Comparison of the extreme value distribution of inclusions made by metallography and fractography, respectively, show that the metallographic method considerable underestimates the actual size of inclusions. Therefore micrographic methods are not suitable to generate input data for inclusion distribution based on fatigue design.     

Improved fracture toughness of ultrahigh strength steel through control of non-metallic inclusions

In recent years, ultrahigh-strength steels, which can be employed successfully at yield strengths of 1400 MPa or higher, have been used increasingly for critical structural applications in aircraft and aerospace vehicles. Most recently, there has been increased demand, however, for ultrahigh-strength steel with superior plane-strain fracture toughness, K _IC, and for the steels suitable for large-sized structural applications; isotropy regarding the property has especially been required. One potential solution to this problem is to control nonmetallic inclusions of the steels. This review concentrates on recent topics concerning improved K _IC of ultrahigh-strength steels, i.e. low-alloy and highly alloyed secondary hardening steels, through control of non-metallic inclusions. The major factors controlling the property are discussed for each of the techniques.     

非金属夹杂物特性对钢铁材料疲劳性能影响的研究进展EI北大核心CSCD

本文综述了非金属夹杂物对钢铁材料疲劳性能的影响及研究现状,从夹杂物的角度出发,首先介绍非金属夹杂物特征提取的最新研究进展,分别从实验测量方法和数学公式科学统计方法两方面进行论述;其次根据夹杂物对于疲劳损伤的主要原理,介绍5种应用较为广泛的定量化分析夹杂物特征参数与钢材疲劳性能的数学模型;然后以夹杂物的形貌特征、力学性能以及与基体之间的相互作用为出发点,探究非金属夹杂物的特性对重载零件钢材疲劳性能的影响。最后指出从多角度解析非金属夹杂物对钢材疲劳性能的主要作用机理,构建非金属夹杂物对钢材疲劳寿命预测模型是未来该领域的研究重点。     

Creep ductility considerations for high energy components manufactured from creep strength enhanced steels

Abstract

It is well established that the tendency for low ductility ‘creep brittle’ fracture behaviour in tempered martensitic steels is linked to the formation and growth of micro voids or ‘cavities’. Details of the contributions of all factors affecting damage development are still under investigation. However, it is known that for tempered martensitic steels voids often initiate over most of the creep life. Nucleation has been recorded on both prior austenite grain boundaries and at other micro structural features such as lath boundaries. The number of voids formed, and the fracture behaviour observed, depend on the type of creep strength enhanced ferritic (CSEF) steel and specific details of fabrication and heat treatment. In Grade 91 steel, void nucleation is sensitive to metallurgical factors such as composition and steel making practices. Key indicators of susceptibility to creep cavitation also include the levels of trace elements present and the presence of hard non-metallic inclusions. In Grade 92 steel, creep void formation has been linked to boron nitrides and other inclusions. These inclusions are present when there has been insufficient control of composition and heat treatment. Metallurgical factors linked to whether a particle will nucleate a void include the nature of the inclusion/matrix interface, the shape and size and the location of the inclusions within the microstructure. This paper describes the results of critical uniaxial and multiaxial testing for CSEF steels and compares data from nominally the same steels which have different metallurgical susceptibilities to void formation.     

Effects of rare earth elements on inclusions and impact toughness of high-carbon chromium bearing steel

High-carbon chromium bearing steels with different rare earth (RE) contents were prepared to investigate the effects of RE on inclusions and impact toughness by different techniques. The results showed that RE addition could modify irregular Al₂O₃ and MnS into regular RE inclusions. With the increase of RE content, the reaction sequence of RE and potential inclusion forming elements should be O, S, As, P and C successively. RE inclusions containing C might precipitate in molten steel and solid state, but the precipitation temperature was significantly higher than that of carbides in high-carbon chromium bearing steel. For experimental bearing steels, the volume fraction of inclusions increased steadily with the increase of RE content, but smaller and more dispersed inclusions could be obtained by 0.018% RE content compared with bearing steel without RE, whereas the continuous increase of RE content led to an increasing trend for inclusion size and a gradual deterioration for inclusion distribution. RE addition could improve the transverse impact toughness and isotropy of bearing steel, and for modified high-carbon chromium bearing steel by RE alloying, the increase of RE content continuously increased both transverse and longitudinal impact toughness until excessive RE addition.     

Observed behavior of various oxide inclusions in front of a solidifying low-carbon steel shell

The engulfment and pushing (extrusion) of inclusions during solidification play an important role in the formation of a steel structure and, as a result, for the mechanical properties of the final steel product. The aim of this study is to gain knowledge about the behavior of non-metallic inclusions at the interface between a growing solid front and a liquid phase. The focus is on the effect of the titanium and titanium oxide content on the inclusions and the different phenomena, which occurs at the solid/liquid interface. This was studied in samples of low-carbon steels de-oxidized by different combinations of Al, Ca, and Ti. For this purpose, each metal sample of 0.19 g was melted at a temperature close to 1550 °C in an argon atmosphere and solidified under different solidification rates. A direct observation of inclusion behavior during solidification was made using a confocal scanning laser microscope equipped with an infrared gold image furnace. The alloying elements in the sample varied between: C 0.002–0.044; Si 0.02–1.33; Mn 0.12–1.33; P 0.003–0.016; S 0.003–0.01; Al 0.002–0,033; Ni 0–0.28; Cr 0–0.25; Ti 0.008–0.065; Ca 0.0007–0.002; O 0.002–0.0114 and N 0.0028–0.0056 (mass%). Several types of inclusions with different morphologies were found within the sample. The morphology of the observed inclusions on the molten steel surface varied from round alumina and calcium-oxide-rich inclusion to needle-shaped titanium oxide-rich inclusions. The observed motions of the inclusions at the vicinity of the front of the solidifying steels were classified. At a low solidifying velocity and a small inclusion size, inclusions flowed away from the solidifying front. Furthermore, they also or got pushed a distance and thereafter flowed away from the interface. At a medium velocity and a slightly bigger size, inclusions tend to get pushed in front of the solidifying front. Another observation was that at a high velocity and a large particle size, inclusions tend to get engulfed or pushed and then engulfed by the progressing front. The relationship among the morphology, chemical composition of inclusions and the solidifying velocity is discussed in this article.     

渗碳Cr-Ni高强硬度合金钢超高周疲劳寿命预测

为了给渗碳合金钢提供一种有效可行的超高周疲劳寿命预测方法,在应力比为0和0.3两种情况下,对渗碳Cr-Ni高强硬度合金钢展开疲劳试验研究.通过对试样断口的微观组织观测,发现渗碳层与基体材料中均有非金属夹杂的存在;通过对裂纹萌生位置和疲劳断口形貌的观察,将疲劳失效分为带有细晶粒区(Fine Granular Area,F...     

Development of TiOx-bearing steels with superior heat-affected zone toughness

TiO_x steels with superior toughness of the heat-affected zone (HAZ) and applicable to a wide range of welding heat inputs have been developed using advance metallurgical techniques. A promising practice in industrial production is adopting a Ti-killing process in which ferro-titanium alloys are utilized as killing agents in an Al-free molten steel. This process creates Ti-oxides with various crystalline structures (referred to as TiO_x) in a steel matrix. These TiO_xinclusions improve the toughness of coarse-grained HAZ by promoting the formation of intragranular acicular ferrite (IAF), which can section an austenite grain into several colonies and refine the effective grain size to fracture resistance. The volume fraction of IAF is closely related to the number of TiO_x inclusions, inclusion diameter, austenite grain size and the hardenability of steels. The favourable conditions for the formation of IAF and the concept of process control are discussed.     

Effect of Zr–Ti deoxidisation on the hydrogen-induced cracking of X65 pipeline steels

The effect of Zr–Ti combined deoxidisation, compared with the traditional Al deoxidisation, on inclusion and microstructure in X65 pipeline steels was investigated by means of analytical characterisation techniques such as non-aqueous solution electrolysis method, optical and electron microscopy in conjunction with energy dispersive X-ray spectroscopy and hydrogen-induced cracking test. Large inclusions as well as elongated MnS particles were observed in the conventional Al-deoxidised steel. However, the type and size of inclusions were efficiently controlled in the Zr–Ti-deoxidised steel, in which MnS particles were spheroidised and evenly dispersed in the matrix. Hydrogen-induced cracking test results showed that centre segregation was the main factor accountable for hydrogen-induced cracking in X65 pipeline steels. The Zr–Ti-deoxidised X65 pipeline steel revealed better HIC-resistance performance.     

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

Abstract

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 S _crit. In the case of the material examined in this study (SCM435 steel) this minimum inspection area is ~ 10 000 mm².     

A new approach to model the fatigue anisotropy due to non-metallic inclusions in forged steels

The objective of this work is to propose an anisotropic fatigue criterion for the sizing of industrial forged components. The results from different experimental campaigns using three different rolled steels are first presented. The effect of inclusions and the microstructure on the fatigue behaviour are investigated. For the two ferrite–pearlitic steels tested, the presence of a microstructure consisting of elongated grains has no observable effects on the fatigue behaviour. For two of the three steels studied the presence of non-metallic inclusions, elongated in the rolling direction, form the origin of the anisotropic fatigue behaviour.The proposed probabilistic model is based on the competition between two possible fatigue crack initiation mechanisms. The anisotropic character of the fatigue resistance of forged components is taken into account by the definition of the geometry and the orientation of the non-metallic inclusion. This criterion results in the establishment of a probabilistic Kitagawa type diagram.     

电渣重熔马氏体时效钢中的非金属夹杂物研究

本文应用电子扫描法(SEM+EDS)和金相法,对真空感应熔炼(VIM)及随后进行电渣重熔(ESR)的改进型18NiNb马氏体时效钢中的非金属夹杂物进行了定性和评级分析。结果表明,电渣重熔对去除钢中的硫化物和尺寸较大的单颗粒夹杂效果明显,电渣重熔后钢中的夹杂物主要为细小的氧化物夹杂,数量明显减少、且呈弥散分布,进一步提高了18NiNb合金钢的钢锭质量。     

夹杂物特征参数对40CrNi2Mo钢塑性的影响

用定量金相、扫描电镜和能谱分析等方法获取了用2种工艺冶炼的40CrNi2Mo钢中夹杂物尺寸特征参数,研究了这些参数对钢塑性的影响。结果表明:由于夹杂物的体积分数和尺寸比较小,夹杂物间距的增大,在塑性断裂时由夹杂物形成的微孔洞很难以内颈缩形式聚合,微孔洞间的局部剪切也不容易扩展。由此判断,真空自耗重熔钢的断面收缩率比电渣重熔钢的高。钢中的夹杂物对其均匀延伸率、形变硬化指数和拉伸强度没有影响。     

Influence of non-metallic inclusions on fatigue life in the very high cycle fatigue regime

The present paper deals with the influence of non-metallic inclusions on fatigue life in the high cycle fatigue and the very high cycle fatigue regime. For that purpose, several castings of steel 42CrMo4 (AISI 4140, DIN EN 1.7225) were produced by using recently developed novel metal-melt filters. The specimens were tested in hot-isostatically pressed and heat treated condition. After fatigue failure every fracture surface was intensively investigated by scanning electron microscopy in order to define the type, the size, the chemical composition, the morphology and the location of the crack initiating discontinuity. Subsequently, Murakami’s √area model was used for the evaluation of the influence of non-metallic inclusions on the fatigue life. In the present investigation four common types of chemical compositions of crack initiating discontinuities were identified. Furthermore, four different internal failure types and their influence on the fatigue life in cast steel were investigated and described. Thus, the present contribution proposes a basic correlation determined from fatigue lives in case of various internal crack initiation types. The key parameters for fatigue life prediction in case of internal fatigue failure in the very high cycle fatigue regime are (i) the size of the crack initiating discontinuity, (ii) the inclusion depth and (iii) the crack initiating failure type.     

Quantitative evaluation of effects of shape and size of artificially introduced alumina particles on the fatigue strength of 1.5Ni---Cr---Mo (En24) steel

In order to investigate the effects of non-metallic inclusion on the fatigue strength of high-strength steels, in 1963 W.E. Duckworth and E. Ineson conducted fatigue tests using specimens that contained artificially added spherical and angular alumina particles of various controlled sizes. Although the fatigue tests were carried out under the same nominal stresses in rotating-bending and tension-compression tests the fatigue lives of specimens showed a large scatter. They reported in some detail typical complicated aspects of the effects of non-metallic inclusions on the fatigue strength.

In the present study the authors have reanalysed these complicated fatigue data using the prediction equation that was proposed by Murakami et al for the quantitative evaluation of the effects of small defects on fatigue strength. The geometrical parameter that controls the scatter of the fatigue strength is the square root of the projection area √ area and not the shape of the inclusions, whether they are spherical or angular. It is shown from the data from the failed specimens that the fatigue strength of materials containing inclusions larger than a critical size can be predicted by the Vickers hardness (Hv) of the matrix and √ area of the inclusion regardless of the shape.     

圆珠笔头用新型易切削不锈钢的组织与力学性能

对圆珠笔头用新型高铬硫系铁素体易切削不锈钢冷拉线材的微观组织与力学性能进行了研究.结果表明,钢中易切削相为MnS、Pb及其与Te形成的复合夹杂物,均呈细小球状或椭球形沿拉拔方向断续、串链状分布.MnS夹杂物大小横向约1μm,纵向约10μm.钢中Cr、Mo的碳化物对改善MnS夹杂物形态有显著作用.该钢成品线材基体组织为细小铁素体晶粒,晶粒度约为10级,力学性能与进口同类钢线材相近,作为替代进口材料,其用于制造圆珠笔头,具有良好的切削性能,能满足生产加工和使用要求,填补了国内空白.     

SEM based automatic image analysis of non-metallic inclusions in steel weld metals

Abstract

The application of SEM based automatic image analysis to the determination of the volume fraction, size distribution, shape, and chemical composition of non-metallic inclusions in steel weld metals is described. Both carbon extraction replicas and mechanically polished specimens are used to obtain reliable data which, in turn, can serve as a basis for future theoretical treatments of weld metal microstructure development.

MST/750     

High-cycle Fatigue Fracture Behavior of Ultrahigh Strength Steels

The fatigue fracture behavior of four ultrahigh strength steels with different melting processes and therefore different inclusion sizes were studied by using a rotating bar two-point bending fatigue machine in the high-cycle regime up to 107 cycles of loading. The fracture surfaces were observed by field emission scanning electron microscopy （FESEM）. It was found that the size of inclusion has significant effect on the fatigue behavior. For AtSI 4340 steel in which the inclusion size is smaller than 5.5 μm, all the fatigue cracks except one did not initiated from inclusion but from specimen surface and conventional S-N curve exists. For 65Si2MnWE and Aermet 100 steels in which the average inclusion sizes are 12.2 and 14.9 μm, respectively, fatigue cracks initiated from inclusions at lower stress amplitudes and stepwise S-N curves were observed. The S-N curve displays a continuous decline and fatigue failures originated from large oxide inclusion for 60Si2CrVA steel in which the average inclusion size is 44.4 pro. In the case of internal inclusion-induced fractures at cycles beyond about 1×10^6 for 65Si2MnWE and 60Si2CrVA steels, inclusion was always found inside the fish-eye and a granular bright facet （GBF） was observed in the vicinity around the inclusion. The GBF sizes increase with increasing the number of cycles to failure Nf in the long-life regime. The values of stress intensity factor range at crack initiation site for the GBF are almost constant with Nf, and are almost equal to that for the surface inclusion and the internal inclusion at cycles lower than about 1×10^6. Neither fish-eye nor GBF was observed for Aermet 100 steel in the present study.