This article deals with the effect of strain-assisted tempering (SAT) on the fatigue properties of 54SiCr6 steel used for spring steel wires in a wide variety of automotive applications, including coil springs. This steel spring wire is extremely strong, having a high elastic limit and yield point, giving the steel excellent energy accumulation and fatigue properties. This combination opens up new possibilities in helical and cylindrical coil spring design, resulting in the reduction of both size and weight. Lightweight coil springs lead to improvements in fuel consumption, stability and vehicle traction. A large plastic deformation and SAT were applied to enhance the yield point of the study material. Improvements in the static and cyclic properties of steel springs were investigated using tensile tests and 3PB fatigue tests at ambient temperature. In addition, an advanced laser shock peening (LSP) process was employed to increase the fatigue resistance of the SAT material. The results presented here show great improvements in the static and fatigue properties over commercial steel treatment. The material quality of the wires was evaluated to be insufficient for further processing with cold coiling.
- Keywords
- 3PB test, LSP, SAT, fatigue properties, spring steels,
- Publication type
- Journal Article MeSH
The paper presents the possibilities of heat treatment of low-density structural steels usable for springs. Heats have been prepared with chemical compositions 0.7 wt% C and 1 wt% C, as well as 7 wt% Al and 5 wt% Al. Samples were prepared from ingots weighing approximately 50 kg. These ingots were homogenised, then forged, and hot rolled. Primary transformation temperatures and specific gravity values were determined for these alloys. For low-density steels, there usually needs to be a solution to achieve the required ductility values. At cooling rates of 50 °C/s and 100 °C/s, the kappa phase is not present. A SEM analysed the fracture surfaces for the presence of transit carbides during tempering. The martensite start temperatures ranged from 55-131 °C, depending on the chemical composition. The densities of the measured alloys were 7.08 g/cm3 and 7.18 g/cm3, respectively. Therefore, heat treatment variation was carried out to achieve a tensile strength of over 2500 MPa, with ductility of almost 4%. Hardnesses above 60 HRC were achieved for 1 wt% C heats using the appropriate heat treatment.
- Keywords
- heat treatment, low density steels, tensile test,
- Publication type
- Journal Article MeSH
This paper introduces a new alloying concept for low-density steels. Based on model calculations, samples-or "heats"-with 0.7 wt% C, 1.45 wt% Si, 2 wt% Cr, 0.5 wt% Ni, and an aluminium content varying from 5 to 7 wt% are prepared. The alloys are designed to obtain steel with reduced density and increased corrosion resistance suitable for products subjected to high dynamic stress during operation. Their density is in the range from 7.2 g cm-3 to 6.96 g cm-3. Basic thermophysical measurements are carried out on all the heats to determine the critical points of each phase transformation in the solid state, supported by metallographic analysis on SEM and LM or the EDS analysis of each phase. It is observed that even at very high austenitisation temperatures of 1100 °C, it is not possible to change the two-phase structure of ferrite and austenite. A substantial part of the austenite is transformed into martensite during cooling at 50 °C s-1. The carbide kappa phase is segregated at lower cooling rates (around 2.5 °C s-1).
- Keywords
- dilatometry, kappa phase, low-density steels,
- Publication type
- Journal Article MeSH
The hot deformation behavior of selected non-alloyed carbon steels was investigated by isothermal continuous uniaxial compression tests. Based on the analysis of experimentally determined flow stress curves, material constants suitable for predicting peak flow stress σp, peak strain εp and critical strain εcrDRX necessary to induce dynamic recrystallization and the corresponding critical flow stresses σcrDRX were determined. The validity of the predicted critical strains εcrDRX was then experimentally verified. Fine dynamically recrystallized grains, which formed at the boundaries of the original austenitic grains, were detected in the microstructure of additionally deformed specimens from low-carbon investigated steels. Furthermore, equations describing with perfect accuracy a simple linear dependence of the critical strain εcrDRX on peak strain εp were derived for all investigated steels. The determined hot deformation activation energy Q decreased with increasing carbon content (also with increasing carbon equivalent value) in all investigated steels. A logarithmic equation described this dependency with reasonable accuracy. Individual flow stress curves of the investigated steels were mathematically described using the Cingara and McQueen model, while the predicted flow stresses showed excellent accuracy, especially in the strains ranging from 0 to εp.
Modern high-strength steels achieve their strength exclusively through the manufacturing process, as the chemical composition of these steels is very similar to the composition of standard-quality steels. Typically, hot-dip galvanizing is used to form a protective zinc layer on the steel parts of structures; nonetheless, the material is exposed to high temperatures during the process. With high-strength steels, this can lead to deterioration of the mechanical properties. This study aims to experimentally examine and evaluate the extent of deterioration of the mechanical properties of high-strength-steel members. The effect was studied on specimens made of three different types of steel with the yield strength ranging from 460 to 1250 MPa. For each type of steel, selected mechanical properties-yield strength, tensile strength, and hardness-were determined on specimens with and without hot-dip galvanization, and the obtained results were mutually compared. Our study shows a significant impact of the hot-dip galvanization process on the mechanical properties of some high-strength steels. With the studied types of steel, the yield strength decreased by up to 18%, the tensile strength by up to 13%, and the hardness by up to 55%.
- Keywords
- hardness, high-strength steels, hot-dip galvanizing, tensile strength, thermal process, yield strength, zinc coating,
- Publication type
- Journal Article MeSH
The phase composition and portion of individual phases in advanced high-strength steels (AHSS) CP1000 and DP1000 was studied by complementary microscopic and diffraction techniques. CP1000 and DP1000 steel grades have a high strength-to-density ratio and they are used in many applications in the automotive industry. The microstructure of the CP1000 "complex phase" steel consists of ferrite, bainite, martensite and a small amount of retained austenite. DP1000 is a dual phase steel, which has a structure of a ferritic matrix with islands of martensite and a minor amount of retained austenite. The influence of selected etchants (Nital, LePera, Beraha I, Nital followed by metabisulfite, Nital followed by LePera, and Nital followed by Beraha I) on the microstructure image is described. X-ray diffraction, neutron diffraction and light optical, scanning and transmission electron microscopy were used in this work for advanced characterization of the microstructure and phase composition. The information provided by each technique is critically compared.
- Keywords
- etching agents, high strength steels, microstructure,
- Publication type
- Journal Article MeSH
Stainless steels are materials that could be used for constructing not only the bearing parts of fuel cells but also the functional ones, particularly the bipolar plates. The advantage of stainless steel is its valuable electrical and thermal conductivity, reasonably low cost, excellent mechanical properties, and good formability. Paradoxically, the self-protection effect resulting from passivation turns into the main disadvantage, which is unacceptable interfacial contact resistance. The aim of this study was to test a number of possible stainless steels in a simulated fuel cell environment, especially those alloyed with boron and manganese, which were found to improve the contact resistance properties of stainless steels. The primary focus of the study is to determine the corrosion resistance of the individual materials tested. Electrochemical tests and contact resistance measurements were performed following the DOE requirements. Manganese-alloyed LDX stainless steel achieved the best results in the electrochemical tests; the worst were achieved by boron-containing steels. Boron-containing stainless steels suffered from localized corrosion resulting from chromium-rich boride formation. All steels tested exceeded the DOE limit in the contact resistance measurement, with 316L reaching the lowest values.
- Keywords
- bipolar plates, corrosion, stainless steels,
- Publication type
- Journal Article MeSH
Conventional heat treatment is not capable of converting a sufficient amount of retained austenite into martensite in high-carbon or high-carbon and high-alloyed iron alloys. Cryogenic treatment induces the following alterations in the microstructures: (i) a considerable reduction in the retained austenite amount, (ii) formation of refined martensite coupled with an increased number of lattice defects, such as dislocations and twins, (iii) changes in the precipitation kinetics of nano-sized transient carbides during tempering, and (iv) an increase in the number of small globular carbides. These microstructural alterations are reflected in mechanical property improvements and better dimensional stability. A common consequence of cryogenic treatment is a significant increase in the wear resistance of steels. The current review deals with all of the mentioned microstructural changes as well as the variations in strength, toughness, wear performance, and corrosion resistance for a variety of iron alloys, such as carburising steels, hot work tool steels, bearing and eutectoid steels, and high-carbon and high-alloyed ledeburitic cold work tool steels.
- Keywords
- carbides, corrosion performance, cryogenic treatment, martensite, mechanical properties, microstructure, retained austenite, steels, wear performance,
- Publication type
- Journal Article MeSH
- Review MeSH
Three types of commercial austenitic stainless steels, 1.4307 (AISI 304 L), 1.4404 (AISI 316 L) 1.4845 (AISI 310 S) with different chemical compositions, are subjected to severe plastic deformation at room temperature by a unique Dual Rolling Equal Channel Extrusion (DRECE) method. Its impact is evaluated from the viewpoint of microstructure analyses, X-ray diffraction, and macroscopic magnetic properties completed by microscopic Mössbauer characteristics. The study also includes the solution annealing at 950 °C for 0.5 h to follow the recovering austenitic structure and paramagnetic state of steels with the aim to offer more information with respect to new technical applications. The results show the importance of the steel's chemical composition and microstructure, mainly grain size, on the stability of the austenitic structure closely associated with the paramagnetic behaviour.
- Keywords
- Magnetic properties, Phase transformation, Stainless steels, microstructure,
- Publication type
- Journal Article MeSH
Understanding the tempering behavior of medium carbon steels is mandatory if their mechanical properties are to be improved. For an optimal technology to be developed for this purpose, a substantial experimental basis is needed to extract quantitative information on the microstructure of the tempered material. This paper reports on the characterization of microstructural changes induced by tempering in medium-carbon steels alloyed with Si, Cr, Cu, and Mn using state-of-the-art experimental techniques. Complementarities among these techniques are highlighted. The evolution of transition carbides, cementite, and copper precipitates is described using data from X-ray diffraction, small and ultra-small angle neutron diffraction, transmission electron microscopy, and dilatometry observation. The effects of silicon, chromium, and copper on the mechanism of carbide and copper precipitation are discussed. The considerable changes found in the size and volume of copper precipitates correlate well with the difference in the yield stress between tempered steels with and without copper.
- Keywords
- SANS, TEM, USANS, XRD, carbides, copper precipitates, dilatometry, medium carbon steels, microstructure, tempering,
- Publication type
- Journal Article MeSH
