High-strength Fasteners Research Articles

METALS HEAT TREATMENT MODEL

This paper considers the problem of developing a model of thermal metal processing by multiprocessor computing systems. The obtained metal is used for high-strength fasteners manufactured by cold forging method without final heat treatment. The model is based on the heat treatment method of a billet from low- and medium-carbon steels intended for cold heading. The model aims at improving technological properties of a billet by ensuring high dispersion and uniformity of a billet structure across the entire plane of its cross-section.Implementation of the proposed model ensures the technical result of high dispersion and uniformity of the structure of the billet. The technological process of steel heat treatment is characterized by high performance, low power consumption, and improved performance characteristics. The apparatus for implementation of the spheroidization annealing regime determines the uniform distribution of cementite globules in the ferrite matrix, which means that it provides the necessary mechanical properties of the metal for its further cold deformation. The multiprocessor computing system software allows controlling the temperature conditions, both on the entire plane of the billet section, and across its length. Such temperature conditions are controlled in the center of the plane of the billet cross-section.Experimental studies of the heat treatment of metal products were conducted. In order to test the functions of the proposed model, several experiments were performed when a 20 mm diameter wire from 20G2G steel was subjected to heat treatment. Experimental studies have shown that metal has the necessary elasticity properties, saving the required hardness.

Materials for Manufacturing Automotive Industry High-Strength Fasteners

The main requirements and criteria are provided in the article for selecting steels for cold bulk stamping of fasteners in the automobile industry: stamping capacity, component dimensions and configuration, the possibility of achieving the required component mechanical properties by heat treatment and strain hardening, and also the cost of starting workpiece and product manufacturing.

Experimental Investigation on the Effect of Shot Peening and Deep Rolling on the Fatigue Response of High Strength Fasteners

Shot-peening and deep rolling are mechanical surface treatments that are commonly applied to enhance the fatigue performances of components, owing to their capacity to generate compressive residual stresses and induce work hardening. However, literature is still poor of published data concerning the application of these treatments to high strength steels fasteners, although these represent a class of components among the most widespread. In the present work, the impact of deep rolling and shot-peening performed in the underhead radius of two set of fasteners made of 36NiCrMo and 42CrMoV for fatigue life enhancement has been investigated. The experimental tests consisted of six combinations of shot-peening and deep rolling, including the non-treated state. Two test campaigns have been sequentially carried out with different process parameters and treatment sequences. The results always showed a beneficial impact of the deep rolling on fatigue, especially for the 42CrMoV steel. Conversely, the effect of the shot-peening strongly depended on the selected set of parameters, alternatively leading to an improvement or a worsening of the fatigue life in relation to the level of induced surface roughness.

Effects of Surface Treatments on the Fatigue Response of High Strength Fasteners

The aim of the present work was the assessment of the impact of deep-rolling and shot-peening performed in the underhead and in the unthreaded shank of two high strength screws (36 NiCrMo and 42 CrMoV) for fatigue life enhancement. The experimental campaign consisted of six combinations, including the non-treated state. The aforementioned treatments were evaluated alone or with shot-peening performed after deep-rolling in the underhead fillet of the screws. Deep rolling was carried out at the optimal rolling force, whereas two shot diameters were considered for shot-peening (Z100 and UFS70). The results have been evaluated in terms of fatigue limits and factor effects have been assessed by marginal mean plots.

Energy-Saving Technology for the Manufacture of High-Strength Threaded Fasteners

An energy-saving technology is proposed for the manufacture of high-strength fasteners (strength class no less than 8.8) on the basis of the principles adopted in thermal–mechanical–thermal technology (TMT technology). This technology is tested in industrial conditions. The technology relies on cumulative increase in the strength at all stages of production: wire rod → blank → fastener. The proposed technology decreases the energy consumption in the manufacture of high-strength fasteners and so increases their competitiveness in the global marketplace.

Dependence of mechanical properties on the microstructure characteristics of a near β titanium alloy Ti-7333

In this work, the microstructure and the corresponding tensile properties of the rolled Ti-7Mo-3Nb-3Cr-3Al (Ti-7333) alloy before and after the thermal treatments were investigated. The results show that a strong α-fiber texture is developed in the rolled Ti-7333 alloy. The deformed matrix and the texture significantly induce the variant selection of α phase. The high strength of the rolled Ti-7333 alloy is attributed to the <110> texture parallel to the tensile direction and the dispersed α phase within the matrix. After the solution treatment followed by the aging treatment, the texture decreases and the microstructure consists of the equiaxed β grains, the spheroidal αp phase and various needle-like α variants. Eventually, the alloy could achieve an optimal combination with the strength of about 1450 MPa, the ductility of about 10.5% and a considerable shear strength of about 775 MPa. This balance can be ascribed to the performance of the spheroidal αp phase and various needle-like αs variants. The results indicate that the Ti-7333 alloy could be a promising candidate material for the high-strength fastener.

Experimental study of the pull-out resistance of alternative high-strength fasteners for wood-based materials

ABSTRACTMore curtain-wall facings are fastened non-visibly to create a uniform building surface. As a consequence of bigger panels and the invisible attachment, the load effects on the fasteners and the fixing areas of the panels are increased. These include the tare weight and the tensile load due to the wind load. Therefore, this study assessed the tensile load of novel fasteners in two different wood materials. A new façade panel material is high-pressure laminate. It consists of cellulose layers soaked in epoxy resin. The second tested material was a modified pine wood, which is produced by impregnation and subsequent heat-curing. It is comparable to highly durable tropical wood. The survey focused on three alternative fixing materials such as screw inserts, drive-in nuts and undercut anchors. The pull-out resistance and failure pattern of the specimens with an alternative fastener are compared to those with wood screws. Furthermore, it was tested whether different temperatures between −20°C and +80°C have an impact on it. The results demonstrate that the alternative fasteners are much more effective than normal wood screws. The pull-out resistance out of the same thin plate material was up to four times higher.

EVALUATION OF QUALITY OF ROLLED STEEL FOR COLD VOLUME FORGING

Metal products obtained by cold heading from rolled metal are used in all branches of mechanical engineering. Its quality is estimated by the required chemical composition and plasticity, the absence of a dispersion of mechanical characteristics along the entire length, the absence of internal and surface defects. The competitive advantage of metalware is the result of optimization during all technological pro­cesses: from rolled metal smelting to heading of finished metalware. At the same time, to reduce costs and to achieve the required quality of metalware, an important condition is to ensure safety and to reduce the energy intensity and labor input of its manufacturing process. An important controlling role in this technological chain is the preparation of the material for its cold volume forging. High-strength fasteners, obtained in conditions of cold heading, are most often obtained from chromium steels. Recently, alternative boron-containing steels have been actively introduced. However, because of the possible formation of boron oxides and nitrides, which lead to a decrease in hardenability, they exhibit instability of thermal hardening during thermohardening of metal products. In addition, the rolled metal of chromium steels, as a rule, is cheaper by 12 – 16 %. And as the fact that foreign deliveries of such steels are associated with additional costs, the fasteners produced from boron-containing steel are obtained with even higher increase in value, which again shows the favor of chromium steels. The standard mechanical characteristics were obtained as well as the destruction cri­teria of 40Kh rolled steel, subjected to patenting in a nylon bath with different temperatures and subsequent drawing with different degrees of deformation during cobbing. The optimum mode of preparation of structure parameters and mechanical characteristics of rolled products before the operation of metalware cold volume forging was identified: patenting (temperature of salt bath of 400 °С) and drawing (deforma­tion degree in the range of 5 – 10 %). It was established that treatment under this mode ensures the obtaining of the required quality of rolled products and is more preferable than the one that operates in produc­tion.

Effect of Environment Variables and Material Properties on the Limiting Current Density for Oxygen Reduction

For aircraft alloys that have received surface treatments, primers, and topcoats to control self-corrosion due to precipitate phases, high-strength fasteners made of stainless steel or titanium alloys are the most obvious point of attack due to galvanic mismatches between the materials. However, the electrolytes that form from atmospheric processes can differ widely in composition and thickness. Dissolved oxygen concentrations vary due to temperature and salinity changes in the electrolyte. Recent attempts to model atmospheric corrosion have suggested that, at high salt loadings on the surface, electrolytes can form that have water layer thicknesses that appear as bulk electrolytes in terms of oxygen transport1. Experiments using rotating disk electrode (RDE) measurements and polarization curve data,2 have suggested an upper bound of approximately 800 mm on the diffusion layer thickness for oxygen that can be expected to form in a thin film of water. At distances beyond this thickness, convective flow is expected to dominate oxygen transport. In this report, we compare the results of RDE experiments that simulate thin film behavior on different substrate materials in different electrolytes with polarization data obtained under actual electrolyte thin films equilibrated at different relative humidity values, and bulk polarization measurements, as shown in Figure 1, in order to determine the effects of ohmic drop on the measurements and isolate relevant kinetic parameters. Isolating these effects is important in order to accurately model the corrosion currents arising from galvanic couples in which the oxides at which the reduction reactions are occurring can be changing in response to the changes in the electrolyte as the environment changes. Figure 1. Bulk cathodic polarization curves in 0.6M NaCl at 25◦C for Pt, UNS S31600, and UNS R56400. This work was sponsored by the Office of Naval Research, ONR, under grant/contract number N0001418WX00826; the views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy or the U.S. government. References 1 Van den Steen, N., Simillion, H., Dolgikh, O., Terryn, H. & Deconinck, J. An integrated modeling approach for atmospheric corrosion in presence of a varying electrolyte film. Electrochimica Acta 187, 714-723, doi:10.1016/j.electacta.2015.11.010 (2016). 2 Liu, C., Srinivasan, J. & Kelly, R. G. Electrolyte Film Thickness Effects on the Cathodic Current Availability in a Galvanic Couple. J Electrochem Soc 164, C845-C855, doi:10.1149/2.1641713jes (2017). Figure 1

Quality of Rolled Steel for Cold Bulk Stamping

Metal products obtained from rolled blanks by cold upsetting are used in every branch of manufacturing. The quality of such components is assessed in terms of the required chemical composition and the plasticity, the consistency of the mechanical characteristics over the whole length, and the lack of internal and surface defects. If such metal components are to be competitive, all the steps in the production chain must be optimized: from smelting of the steel to cold upsetting of the product. In attempting to minimize costs and ensure the required quality, it is important to ensure safety and to decrease energy and labor costs in manufacturing. In this chain, the preparation of the material for cold bulk stamping is a key step. The high-strength fasteners obtained by cold upsetting are most often made of chrome steel. Recently, alternative boron-bearing steels have been actively introduced. However, their thermal hardening in quenching is unstable because boron oxides and nitrides may be formed, with decrease in their hardenability. In addition, chrome steels are 12–16% cheaper, as a rule. Also, since foreign supplies of boron steels are associated with additional costs, fasteners made of boron steels are even more expensive. That is a further impetus to using chrome steels. In the present work, we obtain standard mechanical characteristics and failure criteria for 40Kh steel components patented in a saltpeter bath at different temperatures, with subsequent drawing. The optimal preparation of the rolled steel in terms of its structural and mechanical characteristics after cold bulk stamping is determined: patenting (in a saltpeter bath at 400°C) and drawing (with 5–10% reduction). Such treatment yields products of the required quality and is preferable to the existing technology.

Comparative study of the testing approaches for the susceptibility of high-strength fasteners to environmental hydrogen embrittlement (EHE)

Abstract Low-alloyed, high-strength steels with yield strength above 1200 MPa are frequently used in the automotive industry, in order to reduce the weight of the components. However, hydrogen embrittlement susceptibility is often a concern for this group of materials. Hydrogen can be produced on the metal surface, as a result of electrochemical corrosion reaction during service. The aim of this study is to evaluate the impact of the various loading procedures according to the existing technical specifications for environmental hydrogen embrittlement (EHE) testing. The hydrogen embrittlement susceptibility of high strength fasteners (strength class 14.8) was tested according to the well-established testing specifications: DIN 50969-2, DIN EN ISO 7539, ASTM F1624-12 in 5 wt.-% sodium chloride solution acidified to pH 3 with hydrochloric acid. Open circuit potential measurements were carried out during the testing procedure to gain understanding of the damage process in each of the testing approaches. Under the chosen testing conditions, rising step load (RSL) testing approach, specified in ASTM F1624-12, appeared as the most sensitive method for the evaluation of the material's hydrogen embrittlement susceptibility.

Effect of Nonmetallic Inclusions and Impurities on the Properties and Quality Characteristics of Round Rolled Product Made of Special Alloyed Steels

It is found from results of studying rolled steel from special alloyed steels of industrial grades 30G1R, 40Kh and laboratory grades 12KhN, 38KhGNM, 41Kh1 that a change in the ratio of Mn:S concentration in the range of 21–620, P content from 0.004 to 0.022 wt.%, non-metallic inclusions up to point 2.5 according to GOST 1778, N up to 0.010%, H up to 0.0005%, with total pressure created by gas-forming elements up to 1.05 atm, are not critical for occurrence of defects and rolling capacity for cold upsetting. Determination of the characteristics of NI, and CANI content in steel 40Kh round rolled product shows that metal corrosion resistance increases by more than a factor of three with a reduction in CANI2 content and inclusions of the system CaO–Al2O3–MgO containing up to 5 wt.% SiO2, and the insignificant effect of other types of inclusions. A study of the metal of high-strength fasteners manufactured by JH (Taiwan), OF, AL (USA) shows that it has a significantly lower content of other types (corundum, calcium, aluminate, sulphide) of inclusions, and conversely better corrosion resistance.

Galvanic Corrosion of AA7075-T6 Caused by Doped Titanium Oxides in a Controlled Atmospheric Environment

Reduction reactions, such as oxygen reduction, hydrogen evolution, nitrate reduction, and sulfate reduction, are of interest in atmospheric corrosion because they are more positive than many structural airframe alloy oxidation reactions and can therefore drive material breakdown. The rate controlling reduction reaction is the one capable of supporting the largest corrosion rate at a given potential. In the case of heterogeneous corrosion, in which the anodic and cathodic sites are spatially distinct and stable, as is found in a galvanic couple; the ability of the noble metal to support a reduction reaction strongly influences the corrosion of the active metal. For aircraft alloys having received conversion coatings and topcoats to control self-corrosion due to precipitate phases, high-strength fasteners made of stainless steel or titanium alloys are the most obvious point of attack for galvanic corrosion. In atmospheric electrolytes, droplet chloride concentrations can exceed 4M, and the oxygen reduction reaction (ORR) is frequently the dominant reduction reaction driving corrosion. Novel binary titanium alloys that had been developed using density functional theory and Monte Carlo simulations that predicted oxygen reduction reaction (ORR) rates on the metal oxides, were coupled to structural aluminum alloy (AA7075-T6) panels and exposed to a simulated tropical rainforest environment to compare predicted galvanic corrosion rates with actual galvanic corrosion in an environment in which Cl- ions were applied in a controlled manner.

Galvanic Corrosion of AA7075-T6 Caused By Doped Titanium Oxides in a Controlled Atmospheric Environment

Reduction reactions, such as oxygen reduction, hydrogen evolution, nitrate reduction, and sulfate reduction, are of interest in atmospheric corrosion because they are more positive than many structural airframe alloy oxidation reactions and can therefore drive material breakdown. The rate controlling reduction reaction is the one capable of supporting the largest corrosion rate at a given potential. In the case of heterogeneous corrosion, in which the anodic and cathodic sites are spatially distinct and stable, as is found in a galvanic couple; the ability of the noble metal to support a reduction reaction strongly influences the corrosion of the active metal. For aircraft alloys having received conversion coatings and topcoats to control self-corrosion due to precipitate phases, high-strength fasteners made of stainless steel or titanium alloys are the most obvious point of attack for galvanic corrosion. In atmospheric electrolytes, droplet chloride concentrations can exceed 4M, and the oxygen reduction reaction (ORR) is frequently the dominant reduction reaction driving corrosion. Novel binary titanium alloys that had been developed using density functional theory and Monte Carlo simulations that predicted oxygen reduction reaction (ORR) rates on the metal oxides, were coupled to structural aluminum alloy (AA7075-T6) panels and exposed to a simulated tropical rainforest environment for 30 days in order to compare predicted galvanic corrosion rates with actual galvanic corrosion in an environment in which Cl-ions were applied in a controlled manner. Acknowledgment This work was sponsored by the Office of Naval Research, ONR, under grant/contract number N0001415WX00948; the views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Office of Naval Research, the U.S. Navy or the U.S. government. Figure 1. Galvanic corrosion test panels exposed to a simulated tropical rainforest environment with controlled chloride dosing. Figure 1

Alloy and composition dependence of hydrogen embrittlement susceptibility in high-strength steel fasteners.

High-strength steel fasteners characterized by tensile strengths above 1100 MPa are often used in critical applications where a failure can have catastrophic consequences. Preventing hydrogen embrittlement (HE) failure is a fundamental concern implicating the entire fastener supply chain. Research is typically conducted under idealized conditions that cannot be translated into know-how prescribed in fastener industry standards and practices. Additionally, inconsistencies and even contradictions in fastener industry standards have led to much confusion and many preventable or misdiagnosed fastener failures. HE susceptibility is a function of the material condition, which is comprehensively described by the metallurgical and mechanical properties. Material strength has a first-order effect on HE susceptibility, which increases significantly above 1200 MPa and is characterized by a ductile--brittle transition. For a given concentration of hydrogen and at equal strength, the critical strength above which the ductile-brittle transition begins can vary due to second-order effects of chemistry, tempering temperature and sub-microstructure. Additionally, non-homogeneity of the metallurgical structure resulting from poorly controlled heat treatment, impurities and non-metallic inclusions can increase HE susceptibility of steel in ways that are measurable but unpredictable. Below 1200 MPa, non-conforming quality is often the root cause of real-life failures.This article is part of the themed issue 'The challenges of hydrogen and metals'.

A Comparison between Ultra-high-strength and Conventional High-strength Fastener Steels: Mechanical Properties at Elevated Temperature and Microstructural Mechanisms

A comparison is made between the mechanical properties of the ultra-high-strength steel KNDS4 of fastener grade 14.9 and of conventional, high-strength steels 34Cr4 of fastener grade 12.9 and 33B2 of grade 10.9. The results show that the ratio of the yield strength at elevated temperatures to the yield strength at room temperature is higher for the ultra-high-strength steel than for both conventional highstrength steels, especially at 500°C. Moreover, the results show a trend in which the nano-indentation creep rate is lower as the strength of the steels is higher. The improved mechanical properties of the KNDS4 steel compared to the conventional high-strength steels are related to the smaller size of the alloy carbides in the KNDS4 steel. Furthermore, the effect of an alternative (industrial) heat-treatment on the evolution of the microstructure and hardness of the KNDS4 steel was investigated. Changing the industrial heat treatment can increase the hardness of KNDS4 by about 8%, since more alloy carbides can nucleate and grow. However, the standard industrial heat treatment results in a refinement of the martensite microstructure (grain size), which might be more beneficial for the toughness of the steel. Independent of the heat treatment, the mechanical performance of KNDS4 fasteners at elevated temperature and the low nano-indentation creep rates are two strong indicators that fasteners made from KNDS4 steel might be used at higher service temperatures than traditional high strength fasteners.

Microstructure Control and Mechanical Property Correlation in Titanium Alloy Ti-6Al-4V Fasteners for Space Applications

High strength fasteners of Titanium alloy Ti-6Al-4V are widely used in both launch vehicle and satellite structures. Ti-6Al-4V fasteners having three different types of microstructures were analysed for mechanical properties to understand the role of two phase α+β or transformed β phase in the head region. Based on the microstructure-mechanical property correlations, it was concluded that the presence of transformed beta phase will not affect the strength properties, but will produce scatter in percentage elongation, percentage reduction in area and failure torque values. Therefore, it is recommended that the microstructure of aerospace titanium alloy Ti-6Al-4V fasteners should contain homogeneous two phase α+β throughout.

Mechanisms of hydrogen‐induced cracking in high‐strength screws

Low alloyed high strength steels with yield strength above 1200 MPa are increasingly implemented for the reduction of the component weight in the automotive industry. This material type is known for its susceptibility to hydrogen assisted cracking. Both, manufacturing process induced and hydrogen induced embrittlement during component operation have to be considered. The aim of this study was to evaluate the impact of different zinc‐containing topcoats on hydrogen re‐embrittlement. The susceptibility of high strength fasteners (strength class 12.9) with various zinc‐based topcoats (electroplated zinc, electroplated zinc‐nickel, galvanized zinc and zinc‐flake) were tested under various corrosive environmental conditions. It is shown that the zinc‐containing topcoats impose a different susceptibility on hydrogen re‐embrittlement. Under the tested environmental conditions, fasteners with electroplated zinc revealed a decrease in the fracture load compared to the heat treated black specimen. In contrast to that, an electroplated zinc‐nickel topcoat demonstrated a failure at the loads slightly exceeding the fracture load of the uncoated specimen.

Comparison of Mechanical and Microstructural Characteristics in Maraging 300 Steel Welded by three Different Processes: LASER, PLASMA and TIG

The maraging steel are considered ultra high strength due to its yield strength greater than 1400MPa and are part of a set of advanced materials of interest for technological development, mainly for aeronautics and aerospace industry. For this purpose should submit good toughness, fatigue resistance and acceptable weldability. These steels are used in the aerospace industry as high-strength fasteners, engine casings and missiles, landing gear structures, among others.There are few studies of the process of laser welding of this material, making it important to study the feasibility of welding these steels. A comparison between the traditional welding processes (TIG-Tungsten Inert Gas and PAW-Weldding Plasma Arc) and the laser welding process (LBW - Laser Beam Welding) was performed.To evaluate the mechanical properties were used tensile and hardness tests by microindentation, showing that the maraging steel can be welded with little loss in mechanical properties, with advantages for the laser welding process.Microstructural characterization was performed by optic and scanning electron microscopy, showing that the fused and heat affected area in the process LBW is about 10 times lower that the area affected by heat by TIG and PAW process.

Failure of Large High-strength Fasteners by Hydrogen Embrittlement

Abstract Five out of several tens of large high-strength fasteners, used for attaching compressor-end supports of a large gas turbine engine in a test rig to its foundation, failed by deferred fracture after tensioning. The hexagonal bolt heads fractured off at the fillet of the shaft end. This happened in a delayed manner, in some cases six hours or so after tensioning. Because of this peculiar delayed fracture, hydrogen embrittlement (HE) was suspected as the metallurgical cause of failure. Delayed cracking is typical of and consistent with HE. On the fracture surfaces of the liberated hexagonal heads, some microfractographic fracture features, consistent with hydrogen embrittlement, were found.