Diffusion kinetics of borided AISI 52100 and AISI 440C steels

In this study, the case properties and diffusion kinetics of AISI 440C and AISI 52100 steels borided in Ekabor-II powder were investigated by conducting a series of experiments at temperatures of 1123, 1173 and 1223 K for 2, 4 and 8 h.The boride layer was characterized by optical microscopy, X-ray diffraction technique and micro-Vickers hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed the existence of FeB, Fe₂B and CrB compounds.The thickness of boride layer increases by increasing boriding time and temperature for all steels. The hardness of the boride compounds formed on the surface of steels AISI 52100 and AISI 440C ranged from 1530 to 2170 HV_0.05 and 1620 to 1989 HV_0.05, respectively whereas Vickers hardness values of untreated steels AISI 440C and AISI 52100 were 400 HV_0.05 and 311 HV_0.05, respectively. The activation energies (Q) of borided steels were 340.426 kJ/mol for AISI 440C and 269.638 kJ/mol for AISI 52100. The growth kinetics of the boride layers forming on the AISI 440C and AISI 52100 steels and thickness of boride layers were also investigated.     

Structural behaviour of aeronautical tungsten carbide/carbon-coated titanium ball screws under space thermal-vacuum conditions

In view of the pressing need for improvements in space mechanisms and tribology research work, a thermal high‐vacuum mechanism test facility has been constructed at Rome ‘La Sapienza’ University. The purpose of this facility is to provide fundamental data on the performance of mechanical components and materials in high vacuum in order to test the validity of space mechanism designs. The facility also makes it possible to examine the behaviour of both mechanisms and materials under combined environmental factors, such as ultraviolet radiation, atomic oxygen and temperature. This paper reports on the behaviour of titanium recirculating ball screws provided by Umbra Cuscinetti SpA (Foligno, Italy) under thermal‐vacuum conditions. It is to be noted that these ball screws—well known in the aeronautical field—may be particularly attractive as a new generation of space mechanisms. A solid multilayer film of tungsten carbide/carbon (WC/C) was deposited on screw surfaces as a lubricant and was then compared with the case of no lubrication, which presently appears promising for many mechanisms. In order to compare experimental results, traditional stainless steel mechanisms were also tested and analysed. The requisites imposed by the space environment, as well as by contact mechanical stresses, have given useful information on a preliminary geometrical configuration and choice of materials, both of which are presented in this paper. Thermal‐vacuum effects on overall ball screw efficiency and on ball screw coating friction and wear are also reported and commented on herein as well as numerical contact simulations of crack mechanisms.     

Failure analysis of stress corrosion cracking of 316L structured packing in a distillation tower

The present study was done to find the probable causes of AISI316L structured packings (SPs) failure in a dewatering phenol tower. In this tower, the SPs would provide a high contact surface between streams to maximize and accelerate the distillation process. Increasing vibration of discharging pump showed that there were detached pieces of SPs in pump's strainer. During plant overhaul, corrosion inspection was done and some of the broken packings were gathered for further analysis. The samples were evaluated by visual observation, optical and scanning electron microscope. Transgranular cracks in packing sheets and screws, used for restraining layers of packing together, were seen. Energy-dispersive X-ray spectroscopy (EDS) of corrosion products on packing and the fracture surface of the screw was conducted. The results showed the occurrence of chloride stress corrosion cracking. Even though the chlorine contents of tower streams were low, it seemed that process fluids have been mal-distributed leading to the local concentration of the chlorine on SPs surface. In addition to chlorine, high working temperature and applied stress provided a favorable condition for stress corrosion cracking.     

Cracking of a Tool Steel Guide Roller in a Bar Mill

Failure analysis of a guide roller used in a bar mill of an integrated steel plant has been presented. The guide rollers are positioned at the entry of rolling stands and act as guide in the multi-grooved pass for aligning the hot bars and hold the twisted oval bar while it enters a round pass. The hot bar is at a temperature of ??1100°C, and the rollers are water cooled. The component is subjected to cyclic thermal stress depending on the mill operating conditions. The rollers are cracking longitudinally leading to failure. The investigation consists of visual inspection, chemical analysis, fractography, characterization of microstructures using optical and scanning electron microscopes (SEM), energy dispersive spectroscopy (EDS) analysis, and measurement of micro-hardness. The chemical analysis indicates the material as AISI D2 grade of cold work tool steel. Visual observation of the failed component shows multiple longitudinal cracks on the roller surface associated with a dark circular band of oxidation. The fracture surface shows a dark oxidized area propagating from the roller surface followed by flat bright appearance indicating final brittle fracture. Fractography of the dark surface from where the crack initiates shows fatigue striations. Microstructural examination under optical and SEM shows a network of coarse carbide particles at the grain boundary as well as uniformly distributed fine carbide precipitates within the martensite matrix. Multiple cracks are observed to initiate and propagate from the surface through the clusters of grain boundary carbides. EDS analysis and elemental mapping suggest the carbides to be chromium carbides. SEM shows micro-cracking of carbide particles associated with the crack. Clustered distribution of carbides deteriorates toughness and initiates cracking at the roller surface subjected to thermal cycling because of their differential thermal expansion coefficient leading to the failure of the component.     

A general elastic–plastic approach to impact analisys for stress state limit evaluation in ball screw bearings return system

A methodology for stress state limit recovering in ball screws return system is presented in this paper. A lot of different researches and standards have been already performed for rolling bearings load static and dynamic rating (ISO 76, ISO 281), but nothing has been said for ball screws except in some drafting standards (Draft standard DIN 69051-4). Also the overall stress limits introduced for rolling bearings in ISO standards are not valid for ball screws bearing because of their different contact geometry. Moreover, the plastic contact due to impact between the spheres and the ball return system, very important for ball screw bearing life determination, has been investigated in very recent works only for nominal impacts by Hung et al. [Impact failure analysis of re-circulating mechanism in ball screw. Eng Failure Anal 2004;11:561–73].     

Failure of alloy steel socket-head cap screws used in offshore oil production

Low alloy quenched and tempered steels are widely used as screws and studs. In this work screws of AISI 4140 steel were used to bolt the Christma's tree to his cap in offshore oil and gas production. In the moment of the cap re-motion to perform a routine maintenance five 1″ screws failed. The screws worked under cathodic protection, which means that they must have undergone hydrogen pick-up. The investigation showed that the screws had been correctly heat treated by quenching and high temperature tempering, but the sulfur content and the level of inclusions were too high. As a consequence, the screws were highly susceptible to hydrogen induced cracking (HIC). On the other hand, the analysis of the fracture surface revealed that the main cracking mechanism was fatigue. The main recommendation resulted from this work is that the specification for new screws for this kind of service must be more restrictive to sulfur and inclusions contents.     

自攻螺钉双面剪切连接抗剪性能研究

该文通过ABAQUS有限元软件对自攻螺钉双面剪切连接抗剪性能进行了研究,通过已有试验结果验证了有限元模型的准确性,基于此模型讨论了钢材强度等级、钢板厚度、螺钉直径以及不同中间钢板厚度对螺钉双面剪切连接的破坏模式和抗剪承载力的影响。结果表明:螺钉双面剪切连接构件的破坏模式可分成3类,分别为:承压破坏、承压-剪切破坏和剪切破坏。螺钉双面剪切连接抗剪承载力随着钢材强度等级和钢板厚度的增加而提高并趋于稳定;随着螺钉直径的增加,螺钉双面剪切连接抗剪承载力呈线性增加;在一定范围增加中间钢板厚度也可显著提高螺钉双面剪切连接抗剪承载力。将数值模拟结果与中美欧规范计算值进行比较分析得出,当钢板发生承压破坏时,中国、欧洲和AISC规范公式计算值偏于保守,AISI规范计算值较接近有限元值;当螺钉发生剪切破坏时,AISI和中欧规范过于保守。AISC规范公式计算值与有限元值吻合较好,因此,当螺钉发生剪切破坏时,使用AISC规范公式计算具有一定参考价值。     

Failure analysis of H13 working die used in plastic injection moulding

The present study is focused on the failure of a die used in plastic injection moulding. The die was made from AISI H13 steel and was intended for the production of plastic cups used for the outer closure of cylindrical aluminium cans in coffee packaging. The appearance of the die provides a clear picture of degradations. Extended corrosion damage on various areas of the metallic part and a wide crack can be observed by the naked eye. Hardness measurements and chemical analysis eliminated the probability of faulty material selection or improper heat treatment. Visual inspection, macro-examination and microscopic observations of representative failed parts revealed that the failure was caused by corrosion that led to the total cracking of the die. The design deficiency and improper cooling conditions generated a complex fatigue-corrosion cracking mechanism that lead to the damage of the die after half of it’s predicted service life.     

Evolution of microstructure in laser clad Fe–Cr–Mn–C alloy

Abstract

The laser surface cladding technique was used to form in situ Fe–Cr–Mn–C alloys on AISI 1016 steel substrate. In this process, mixed powders containing Cr, Mn, and C in the weight ratio 10: 1 : 1 were delivered using a screw feed, gravity flow, carrier gas aided system into the melt pool generated by a 10 kW CO₂ laser. This technique produced an ultrafine microstructure in the clad alloy layer. The microstructure of the laser surface clad region was investigated by optical, scanning and transmission electron microscopy, and X-ray microanalysis techniques. Microstructural study showed a high degree of grain refinement and an increase in solid solubility of alloying elements which, in turn, produced a fine distribution of complex types of carbide precipitates in the ferrite matrix because of the high cooling rate. An alloy of this composition does not show any martensitic transformation or retained austenite phase.

MST/356     

滚珠丝杠副刚度的影响因素及其试验研究

为了提高滚珠丝杠副静态和动态分析的精度,利用滚珠丝杠副中丝杠滚道的力和力矩平衡方程,并考虑变化的接触角和接触变形的影响,建立了联合载荷作用下精确的滚珠丝杠副刚度的数学模型,利用模型分析了滚珠丝杠副刚度的影响因素和规律,结果发现：滚珠丝杠副的刚度随轴向和径向载荷的增大,随之变大,而随倾覆力矩的增加,随之减小;增大导程和减小滚道曲率比可提高滚珠丝杠副的刚度;轴向刚度随轴向载荷、导程和滚道曲率比的改变所受影响最大,角刚度主要受径向载荷和倾覆力矩的影响,径向刚度受载荷和结构参数的影响都较小。通过与滚珠丝杠副轴向静刚度测试平台上测量的结果比较表明,建立的模型是合理和可靠的。     

螺母安装方式对滚珠丝杠载荷特性的影响规律研究

针对一端固定、一端支承的滚珠丝杠上工作螺母的两种不同安装方式,分析、比较了丝杠受力状态的差异;通过建立计及接触角变化的滚珠丝杠载荷分布模型,定量研究了不同螺母安装方式对钢球法向载荷分布和实际接触角的影响规律。数值分析结果表明在螺母和工作台的连接法兰远离固定端的安装方式下,钢球载荷分布和接触角分布均更为均衡。另外螺母安装方式对滚珠丝杠轴向刚度的影响并不显著。这一结论为滚珠丝杠的安装和维护提供了理论依据,对保证滚珠丝杠使用性能和提高其工作寿命具有工程指导意义。     

Degradation of mechanical and corrosion resistance properties of AISI 317L steel exposed at 550 °C

The use of austenitic stainless steel type AISI 317L has increased in the last years, in substitution to AISI 316L and other austenitic grades. The higher Mo content (3.0 wt.%. at least) gives higher corrosion resistance to AISI 317L. However, some concern arises when this material is selected to high temperature process services in refineries. Microstructural changes such as chromium carbide precipitation and sigma phase formation may occur in prolonged exposure above 450 °C. In this work, the microstructure evolution of AISI 317L steel during aging at 550 °C was analyzed. Thermodynamic calculations with Thermocalc® and detailed microstructural analysis were performed in steel plate base metal and in weld metal produced by GTAW process. The aging for 200, 300 and 400 h promoted gradual embrittlement and deterioration of corrosion resistance of both weld and base metal. The results show that the selection of AISI 317L steel to services where temperatures can reach 550 °C is not recommended.     

Investigation of crack initiation and propagation behavior of AISI 310 stainless steel up to very high cycle fatigue

Fatigue behavior up to very high cycles for AISI 310 stainless steel has been investigated. The fatigue crack initiated from the surface of the material. It was found that up to 10⁶ cycles, cracks initiated from the carbide precipitates at grain boundaries. However, above 10⁶ cycles, the cracks initiated from persistent slip bands found at the surface of the specimen. At lower stress levels, slip bands were developed without initiating the cracks. The horizontal asymptote S–N curve from 10⁶ to 10⁹ cycles was attributed to the development of slip bands all over the surface of the specimen, before crack initiation.     

Metallographic analysis of embedded crack in electron beam welded austenitic stainless steel chemical storage tank

An AISI 304L austenitic stainless steel tank used for chemical storage showed cracks during the post weld qualification programme. A crack of 75 mm length embedded within the weld pool was subjected to detailed metallographic analysis. The results revealed that the cracking was due to a shift in the solidification mode from primary ferrite to primary austenite. The residual stress introduced during rolling and forming of material as well as additional contractional strain during welding under fixtured condition, are additional factors which caused cracking.     

Plasma paste boronizing of AISI 8620, 52100 and 440C steels

In the present study, AISI 8620, 52100 and 440C steels were plasma paste boronized (PPB) by using 100% borax paste. PPB process was carried out in a dc plasma system at temperature of 700 and 800 °C for 3 and 5 h in a gas mixture of 70%H₂–30%Ar under a constant pressure of 4 mbar. The properties of boride layer were evaluated by optical microscopy, X-ray diffraction and Vickers micro-hardness tester. X-ray diffraction analysis of boride layers on the surface of the steels revealed FeB and Fe₂B phases for 52100 and 8620 steels and FeB, Fe₂B, CrB and Cr₂B borides for 440C steel. PPB process showed that since the plasma activated the chemical reaction more, a thicker boride layer was formed than conventional boronizing methods at similar temperatures. It was possible to establish boride layer with the same thickness at lower temperatures in plasma environment by using borax paste.     

Hydrogen induced cracking of a tappet adjusting screw

The electroplated tappet adjusting screws used in diesel engines failed during initial bend testing. The analysis of the failure showed that the fracture was nucleated from the subsurface of the screw. The fracture surface was intergranular at the ID and OD region and microvoid coalesence in the center. The improper baking after electroplating of the screw led to H₂-induced blistering/cracking. The high strength of the threaded region of the adjusting screw increased the failure propensity.     

Effect of high temperature annealing on texture and microstructure on an AISI-444 ferritic stainless steel

AISI 444 is a Mo-alloyed ferritic stainless steel which presents good naphthenic corrosion resistance, making it attractive for applications in petroleum refining plants; however, good formability is also important. To achieve good formability with this alloy the annealing process is crucial. The annealing temperature in ferritic stainless steel is usually around 850 °C, which falls in the range of sigma phase precipitation. A means to avoid this precipitation is to anneal at temperatures around 1000 °C followed by rapid cooling. Annealing at high temperatures can cause grain growth and carbide or nitride precipitation which can result in a reduction of room temperature toughness. In this paper, the rolling and recrystallization textures of AISI 444 steel were studied in samples cold rolled with different thickness reductions (30%, 60%, 80% and 90%) followed by annealing at 955, 980 and 1010 °C. Aspects of grain size and carbide precipitation after annealing were characterized using EBSD and AFM. The material drawability was analyzed through strain rate or Lankford (r) coefficients calculated from texture results.     

Failure analysis and weld ability improvement of 35%Cr–45%Ni heat resistant alloy

The failure of Ethan cracking tubes in radiant chambers after four years service has been analyzed. The microstructure evolution during solution annealing and its affect on the mechanical properties and repair welding of 35Cr–45Ni heat resistant alloy was studied by means of optical and electron microscopy and by mechanical testing in samples welded both by GTAW and SMAW process. HAZ cracking after welding of cracked or creeped tubes due to formation of continuous brittle carbide network was overcome by localized solution heat treatment. Analysis revealed that dissolution of continuous brittle secondary carbide precipitates promoted an increase in ductility of about 20% which is enough to accommodate the thermal stresses during thermal fluctuations at service.     

Atom probe tomographic observation of hydrogen trapping at carbides/ferrite interfaces for a high strength steel

A three-dimensional atom probe (3DAP) technique has been used to characterize the hydrogen distribution on carbides for a high strength AISI 4140 steel. Direct evidence of H atoms trapped at the carbide/ferrite interfaces has been revealed by 3DAP mapping. Hydrogen is mainly trapped on carbide/ferrite interfaces along the grain boundaries. Slow strain rate tensile (SSRT) testing shows that the AISI 4140 steel is highly sensitive to hydrogen embrittlement. The corresponding fractographic morphologies of hydrogen charged specimen exhibit brittle fracture feature. Combined with these results, it is proposed that the hydrogen trapping sites present in the grain boundaries are responsible for the hydrogen-induced intergranular fracture of AISI 4140. The direct observation of hydrogen distribution contributes to a better understanding of the mechanism of hydrogen embrittlement.     

Effect of laser surface hardening on the hydrogen embrittlement of AISI 420: Martensitic stainless steel

The susceptibility of stress corrosion cracking (SCC) of AISI 420 which was surface transformed hardened by a pulsed Nd:YAG laser, was investigated in 5% sodium chloride + 0.5% acetic acid solution by the U-Bend method, in the range of pH value from 3.5 to 6, in the absence and presence of 1 ppm thiosulphate ion, at 25 and 60 °C. The results showed that the laser-treated areas are more susceptible to SCC than the base metal. Hydrogen embrittlement (HE) is the main cause of crack propagating, mostly effective on the grain boundaries and the interface between carbide particles and second phases; tempered martensite or ferrite.