热加工对硫化物及氧化物夹杂的影响

随着制造业的发展,人们对钢材的质量提出了更高的要求,而夹杂物是影响钢材质量十分重要的因素之一.特别是对高强钢而言,夹杂物对其韧性的影响更加敏感.因此,研究冶炼过程中对夹杂物的控制及后续热轧和热处理工艺对钢中夹杂物的影响,是钢铁材料制造和加工过程中的一个重要课题.夹杂物对钢材的强度、塑性、韧性、抗疲劳性能、耐腐蚀性能都会产生重要影响.要减少夹杂物对钢材性能的影响,需从优化冶炼工艺开始.随着气泡去除夹杂物技术、真空碳脱氧技术及中间包电磁搅拌技术在冶炼过程中的应用,钢材中的夹杂物含量明显降低.然而,在目前工业化的生产环境中,仍然无法实现将钢中的非金属夹杂物完全消除.目前的研究已经表明,热轧和热处理过程对夹杂物也有重要影响,这对进一步控制夹杂物的尺寸、分布、形态,达到减轻夹杂物对钢材性能影响的目的具有重要的意义.因此,目前除了发展新的冶炼工艺以进一步降低钢中的夹杂物含量外,研究和阐明热加工工艺对夹杂物的影响也是研究重点.文中归纳了冶炼过程中先进的夹杂物控制技术及其原理,评述了热变形和热处理过程对夹杂物的数量、尺寸、分布、形态特征及类型产生的影响,对研究钢材在制造、加工过程中夹杂物的变化规律及控制具有参考价值.     

大锻件内非金属夹杂物的形成及防止措施

介绍了大锻件内非金属夹杂物的形成过程,非金属夹杂物的机械性能和特性对钢质零件内在性能的影响及防止非金属夹杂物产生的措施.     

钢中非金属夹杂物对疲劳性能的影响

本文研究了38CrMoAlA和30CrMnSiA两种钢中所含夹杂物对疲劳性能的影响。研究结果表明:在夹杂物含量基本相同的条件下,夹杂物的大小、形状及分布特徵对疲劳寿命具有明显的影响。夹杂物的颗粒大小直接支配着裂纹的成核与扩展,当夹杂物的平均直径大于40μm时,由夹杂物引起疲劳失效的比率可达95％以上。因此,在控制夹杂物含量的基础上,通过减小夹杂物颗粒尺寸和改变不利的几何形状及分布,可减缓夹杂物对疲劳性能的有害影响。     

钢中非金属夹杂物及其金相检验

对钢中非金属夹杂物按来源、化学成分和形态进行了分类,并依据金属学原理讨论了非金属夹杂物的危害以及对钢的各种性能的影响,详细介绍了各类非金属夹杂物的形态特征和分布特点以及在光学显微镜明场、暗场和偏光下的颜色,最后简单介绍了非金属夹杂物检验标准GB/T10561-2005的主要内容和技术要点。     

非金属夹杂物对钢性能的影响

本文分析了钢中非金属夹杂物的形成以及非金属夹杂物的分布状态与材料性能的关系,钢中非金属夹杂物对加工工艺的影响和性能改善的方法,以及在选材时对非金属夹杂物的综合因素分析。     

浅析硬线钢中非金属夹杂物

硬线盘钢制品对钢的纯净度,夹杂物的尺寸、分布以及形态都有严格的要求,非金属夹杂物又是影响硬线钢质量的主要因素之一。本文通过介绍硬线钢中非金属夹杂物的来源和分类,分析了不同类型夹杂物的形成机理及形貌,指出了非金属夹杂物对硬线钢的影响和危害。     

轴承钢大颗粒夹杂物的高频超声波C型扫描检测

大颗粒夹杂物对轴承钢的疲劳寿命有重大的影响.针对轴承钢非金属夹杂物的评价方法进行了比较和分析.利用高频超声波C型扫描法对轴承钢钢材试样进行检测,并利用金相法对超声波C型扫描法检测的结果进行验证.结果表明:高频超声波C型扫描法用于检测轴承钢中的大颗粒夹杂物是非常有效的.     

非金属夹杂物对高铁轴承疲劳寿命的影响

在高铁轴承钢的冶炼和生产制造过程中,总是不可避免地带有非金属夹杂物,这些非金属夹杂物的存在会破坏金属基体的连续性,严重影响高铁轴承的疲劳寿命。本文通过求出在径向力和轴向力作用下高铁轴承的接触载荷分布,考虑受力最大的滚子与轴承套圈接触时,轴承外圈滚道所包含的夹杂物,采用疲劳寿命基础理论,利用局部应力-应变的方法,分析了夹杂物的位置、大小和数量对轴承疲劳寿命的影响,建立了高铁轴承钢中非金属夹杂物对轴承接触疲劳寿命影响的定量关系。     

X80管线钢中非金属夹杂物的检验及其对钢性能的影响

根据GB／T10561--2005并结合检验实例,介绍了X80管线钢中各种非金属夹杂物的形态及其特征;并分析了夹杂物对钢性能尤其是焊接性能的影响。结果表明：X80管线钢中含有GB／T10561--2005中规定的A,B,C,D及DS各类夹杂物;钢中的夹杂物除了对力学性能和耐腐蚀性能有影响外,对X80管线的钢焊接性能影响也较大。     

FGH96合金反复镦拔与挤压缺陷对比分析

对FGH96合金反复镦拔变形与挤压变形两种制坯工艺的夹杂缺陷水平进行了比较,并分析了不同变形状态下非金属夹杂物的形变特性。结果表明:反复镦拔变形后,锻件内检测到的非金属夹杂物数量有不同程度的增多,超标缺陷成倍增长;而FGH96合金挤压+等温锻造变形后,锻件内检测到的非金属夹杂物数量明显减少,纯净度水平有大幅提高。从微观夹杂物形貌的变形特性来看,等温锻造变形时在垂直于变形方向合金中夹杂物缺陷的尺寸增大;反复镦拔变形时,合金中原有非金属夹杂物缺陷聚集使可检测到的缺陷面积增大;而在挤压变形过程中,夹杂物缺陷在挤出方向被拉长成不连续的线状,每个方向上夹杂物的面积均减小;FGH96合金挤压+等温锻造变形后夹杂缺陷的大小主要由挤压变形后夹杂缺陷破碎情况决定;因此大挤压比变形可有效破碎合金中非金属夹杂物,改善锻件质量。     

A 3D numerical and experimental investigation of microstructural alterations around non-metallic inclusions in bearing steel

Non-metallic inclusions such as sulfides and oxides are byproducts of steel manufacturing process. When a component is subjected to repetitive loading, fatigue cracks can emanate from these inclusions due to stress concentrations that happen because of mismatch in elastic–plastic properties of inclusions and matrix. In certain applications such as gears and bearings, crack initiation from inclusions is accompanied with microstructural alteration. This paper employs a numerical as well an experimental approach to investigate these microstructural changes which are so-called “butterfly wings”. A 3D finite element model was developed to obtain the stress distribution in a domain subjected to Hertzian loading with an embedded non-metallic inclusion. A formerly introduced 2D model based on continuum damage mechanics (CDM) was developed to simulate the butterfly wing formation in 3D. Wingspan-to-inclusion ratios were observed at different cross sections following an analytical serial sectioning procedure. A closed form solution was suggested for the wingspan-to-observed-inclusion-diameter ratio and the results were corroborated with the data available in the open literature. On the experimental front, nonmetallic inclusions inside a sample made of bearing steel was detected using ultrasonic inspection method. Rolling contact fatigue (RCF) tests were run on the specimen and post-failure serial sectioning was conducted to understand the 3D shape of butterflies formed around an inclusion detected by ultrasound. Comparison of experimental and numerical serial sectioning of the wings showed a close correlation in the butterfly wings geometry.     

Characterization of inclusions in clean steels: a review including the statistics of extremes methods

The application of new secondary refining techniques and non-metallic inclusion reduction techniques in steel production processes has greatly reduced the size and amount of non-metallic inclusions remaining in molten steels and steel products. This makes the inspection of inclusions difficult. Here the main methods used for the characterization of inclusions in clean steels are reviewed. The influences of inclusions on the properties of steels are discussed. Statistical methods for the prediction of the maximum inclusion size in a large volume of steel are introduced. Methods based on the statistics of extremes are described in detail. The methodology for the practical application of these methods is described and the factors affecting the precision of the estimation are discussed.     

Variables affecting the fatigue resistance of PVD-coated components

The effect of intrinsic properties of CrN coatings on fatigue behaviour was studied in this paper. The coating layer microhardness and the residual stresses characterising the surface film were measured and the obtained results were introduced in a numerical modelling predicting fatigue life procedure of coated components. The effect of a CrN monolayer film deposited on bulk samples, produced in 2205 duplex stainless steel, H11 tool steel or 6082 aluminium alloy was investigated. The fatigue limit of coated and uncoated samples was experimentally determined while the development of FEM models, confirmed by means of experimental tests, represents a powerful tool to predict fatigue life of coated components. The effects on the fatigue strength of coating and bulk material defects like droplets and non-metallic inclusions were considered along with the residual stress gradient characterising the coating and evaluated by means of X-ray measurements. The influence of the substrate material plastic deformation on the integrity of the coating was evaluated too.     

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.     

Anisotropic distribution of non-metallic inclusions in a forged steel roll and its influence on fatigue limit

The objective of this work is to determine the fatigue limit of a forged heavy-weight steel roll, and to compare the estimated fatigue limit and its standard deviation with the theoretically predicted lower fatigue limit. The examination of the extreme value distribution of the inclusions located at the site of fatigue crack nucleation on the fracture surface of the fatigue test bars is compared with the extreme value distribution of the inclusions found on polished specimen. The extreme value distribution of the non-metallic inclusions is used to predict the theoretical lower fatigue limit of the steel. The experimental part of the paper consists of the fatigue testing of 61 test bars that were taken from a process industry forged steel roll. The estimation and comparison of the fatigue limit and its deviation was done using the least squares and binomial analysis methods. The results from this analysis are compared with the theoretically predicted upper and lower fatigue limit. The fatigue tests show that the fatigue test bars orientated tangentially to the axis of the forged steel roll have significantly lower fatigue lives when compared to the fatigue lives of the test specimens orientated along the axial direction of the forged steel roll.     

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.     

Study of rolling contact fatigue of bearing steels in relation to various oxide inclusions

Flaking failure of rolling contact fatigue is known to originate from non-metallic inclusions, where fatigue cracks occur at subsurface region of materials. It is fundamental to understand the main factors governing the rolling contact fatigue life. Steel samples with different size and chemical composition of oxide inclusions were evaluated on rolling contact fatigue life. As a result,     

Corrosion fatigue crack initiation and growth in 18 Ni maraging steel

Nucleation of fatigue cracks in air and 3.5 wt% NaCl solution has been studied in an 18 wt% Ni maraging steel. Specimens tested on reverse bending fatigue machine showed a marked decrease in fatigue strength of the steel in NaCl solution reducing the 10⁷ cycles endurance limit from 410 MPa in air to 120 MPa. Microscopic studies revealed crack initiation to be predominantly associated with non-metallic silicate inclusions in both cases. In air, initiation is caused by decohesion of the inclusion/matrix interface, while in NaCl solution complete detachment of inclusions from the matrix results due to the dissolution of the interface. 70% more inclusions are quantitatively shown to be associated with cracks in NaCl solution than in air at the same stress levels. Experimental and theoreticalS-N curves and inclusion cracking sensitivity data are consistent with the mechanism suggested. The final fracture occurs by the main crack consuming the inclusions ahead of it by the “unzipping” of the shear band produced between the crack tip and the inclusion ahead.     

光学显微镜检测非金属夹杂物长度的不确定度评定

根据JJF 1059-1999和GB/T 10561-2005,采用光学显微镜及金相图像分析软件对某轧制比大于3的LZ50钢轴坯的非金属夹杂物长度进行了观察和测量,按A法检验,放大100倍观察,对检测非金属夹杂物长度的测量不确定度来源进行了分析,并对每个不确定度分量进行了评定,最终给出了扩展不确定度及测量结果的不确度报告。结果表明:LZ50钢轴坯中的非金属夹杂物主要为C类细系硅酸盐夹杂物;测量结果的不确定度报告为,最严重视场C类非金属夹杂物总长度的测量值为(632±6)μm,k=2。     

Fatigue Fracture of a High-Resistance Structural Steel Component Destined to Sustain Severe Loads Under Service Conditions

A structural steel component that failed under fatigue was examined with the aim to identify the root causes of this failure. Fractographic examination revealed the presence of multiple beach marks; the position and arrangement of those signified the occurrence of fatigue fracture under the presence of combined loading conditions, involving torsion and bending stress components. Crack initiation was observed also at the corners of the steel plate where non-metallic inclusions were located. Stereo-microscopical examination of the fracture surface likely revealed the presence of casting inclusions, probably fluxes or slag particles, near the surface and in the interior of the component. These inclusions could be considered inherent—metallurgical stress raisers, behaving as locations of prominent crack nucleation under cyclic fatigue loading, stimulating subsequent crack propagation and final ultimate rupture.