Increasing the Wear Resistance of Marine Diesel Engines Elements Made of Ductile Iron

This paper examines the influence of austempering and shot peening on the wear resistance of ductile iron. Samples for further testing were made from mechanically processed casts. The hardness and microstructure of ductile iron were examined on the prepared samples. Metallographic analysis of ductile cast iron samples in the cast state determined the pearlitic-ferritic structure of the matrix characteristic of this type of cast iron. Pearlite predominates in the structure, and the rest consists of ferrite and graphite nodules. The samples were then austempered. The isothermal conversion was 240 °C. After austempering, the hardness was measured and the microstructure was characterized, and the changes that occurred in relation to the heat-treated ductile iron were studied. Austempering created a new austempered structure, harder than that in the cast state, which led to improved mechanical properties. A needle-like structure of lower bainite (ADI) with high-carbon austenite, untransformed austenite, martensite and graphite nodules was achieved. The selected parameters of the shot peening process deformed the surface of the ADI and achieved a mostly martensitic structure without the presence of a soft phase, which increased resistance to abrasion. Additional hardening of the substrate by the shot peening process resulted in a composite material with the properties of comparative steel parts, which are lighter, cheaper and easier to manufacture, more resistant to wear and quieter in operation. Such material should have an even wider application in mechanical engineering, and thus in the construction of marine machinery and plants.

Analytical Evaluation of Strengthening, Local Internal Stresses and Microderformations in Welded Joint Metal during Wet Underwater Welding

Analysis of structural factor influence on local internal stresses and zones of deformation localization in upper and lower bainite structures in welded joints of low-alloy steel at wet underwater welding was performed. It is established that when welding joints under the water and applying an external electromagnetic field in the metal of the heat-affected zone (HAZ), a finer-grained substructure is formed with a general decrease in the dislocations density and with their uniform distribution. Estimates of the local internal stresses level considering the dislocation density distribution in the structural zones of their localization show that their maximum level is formed in the metal of the HAZ overheating region at welding without the external electromagnetic field along the upper bainite laths boundaries. The upper bainite structure is characterized by forming localized deformation zones, where the most significant dislocation density gradients are observed. This can lower the crack resistance of welded joints. Low values of local internal stresses are characteristic of welded joints obtained in the modes applying an external electromagnetic field. This is facilitated by the overall decrease in the dislocation density and their uniform distribution in the lower bainite structural components, which provides high crack resistance of welded joints.

Effects of Tempering Temperature on the Microstructure and Mechanical Properties of Low Alloy Ultra-High Strength 45CrNiSiMnMoVA Steel

The effects of different tempering temperatures on the microstructure evolution and mechanical properties of the new low-alloy ultra-high-strength 45CrNiSiMnMoVA steel after quenching were investigated by mechanical property tests, SEM and TEM. The results show that a complex phase organization consisting of martensite/ lower bainite of the tested steel after treated at 920°C×1h+(320~380)°C×4h was obtained, and the partition interface of the lath martensite bundle became blurred from clear with the increase of tempering temperature; In the proposed tempering temperature range, the toughness of the alloy has become better while maintain the strength without decreasing basically, and when the tempering temperature is 350°C, the alloy has the optimal comprehensive mechanical properties of strength, plasticity and toughness together. The analysis concluded that the strong toughening of the tested steel was mainly attributed to the coupling effect of the alloying elements in the steel and the composite toughening of the nano-precipitated phases, among other aspects.

Evaluation of Hardness, Sliding Wear and Strength of a Hypoeutectic White Iron with 25%Cr after Heat Treatments

Hypoeutectic white cast irons with a high chrome content are commonly used in the industrial mining sector where there is a demand for both high resistance to adhesive wear and an acceptable toughness for the absorption of impacts and falls of diverse materials. Through the application of a design of experiment (DoE) technique, factors related to thermal treatment are analyzed with respect to resistance to sliding wear, maximum rupture stress and toughness. The results show that, in order to increase resistance to adhesive wear, it is convenient to use destabilization temperatures of 1050 °C and tempering of two hours at 400 °C. This foments a very hard martensite and a high proportion of highly alloyed retained austenite, which, with low tempering, achieves a precipitation of carbides from this austenite with hardly any loss of hardness of the martensite. In order to increase the energy which this material is capable of absorbing until breakage, furnace cooling set at 150 °C followed by tempering at 550 °C would be favorable. Slower cooling implies a greater quantity of conditioned retained austenite, so that, following this, it may be transformed into lower bainite with a high density of finely dispersed precipitated carbides. Furthermore, this tempering also allows the transformation of martensite into ferrite with finely dispersed carbides.

Phase and structural transformations when forming a welded joint from rail steel. Report 2. Isothermal diagram of decomposition of supercooled austenite of R350LHT rail steel

An isothermal diagram of decomposition of supercooled austenite of R350LHT steel was constructed based on the results of dilatometric, metallographic and hardness analysis of this decomposition during continuous cooling and under isothermal conditions. When comparing the thermokinetic and isothermal diagrams, it was found that the thermokinetic diagram plotted during continuous cooling shifts downward and to the right in comparison with the isothermal diagram. This result is fully consistent with the known regularities. During the research, the critical points of R350LHT steel were determined: Ас1 = 711 °С; Мn = 196 °С. This isothermal diagram was used to determine the temperature of the minimum stability of overcooled austenite, which was 500 °C. Under isothermal conditions, pearlite-type structures appear in the temperature range from 700 to 600 °C. At 550 °C, a mixture of pearlitic and bainitic structures is formed. In the temperature range from 500 to 250 °C, bainitic structures are formed: at 500 – 400 °C – upper bainite; at 350 ° C – a mixture of upper and lower bainite; at 300 – 250 °С – lower bainite. Almost in the entire studied temperature range of overcooled austenite isothermal decomposition, an increase in the hardness of the transformation products is observed with a decrease in the holding temperature from 246 HV (at 700 °C) to 689 HV (at 250 °C). However, at a temperature of 500 °C, a slight drop in hardness occurs, which is apparently caused by the appearance of retained austenite during the development of bainitic transformation.

Copper-Induced Strengthening in 0.2 C Bainite Steel

Bainitic steels were the focus of this study. These steels have the potential to obtain a good combination of strength, ductility, and edge stretchability, which is a very desirable characteristic in the automotive industry. Cu precipitation potential was investigated during prolonged isothermal bainitization treatment. Precipitation strengthening and ductility were measured using a tensile test, and edge stretchability was measured using a hole expansion test. The microstructure was characterized by high-resolution scanning electron microscopy and an electron backscattered diffraction. Lower bainite was obtained by austenitization treatment and subsequent immersion into a salt bath at 400 °C. Cu precipitation occurred after 120 min of holding in the bath and enhanced the yield stress of the Cu-alloyed steel by 120 MPa as compared with a reference steel without Cu. The strengthening did not affect ductility and decreased the edge stretchability by 10%. Steels with different Mn contents were examined. It was found that the enhancement of Mn content from 1 to 2 wt.% did not boost Cu strengthening ability. This result showed that the presence of Mn did not cause an Mn-Cu precipitation strengthening synergy, observed previously during martensite tempering procedure.

Preparation Methods for Lower Bainite after a Continuous Austempering Treatment of Thin Sheets of the Eutectoid Cold Work Steel 80CrV2 (1.2235)

Strip material with a thickness of 0.85 m was austempered in a suitable continuous hardening furnace for the further development of blade body steels used for thin-cutting circular and band saw blades. This work also comprised the search for a rather straightforward metallographic preparation procedure allowing for a reliable identification of the microstructure and microstructural constituents. The prepared microstructure can be revealed using Nital or Beraha and LePera etching solutions. None of the three etchants allows a reliable and reproducible distinction of the microstructural constituents of the extremely fine-grained lower bainite using light-optical techniques. Only the use of the scanning electron microscope at higher magnifications allows a reliable identification of the characteristic bainite fiber bundles. For this examination, etching with Nital is recommended.

Promising Bendability of a New As-Rolled Medium-Carbon, Low-Alloy Steel with Isothermal Upper and Lower Bainitic Microstructure

Bending behavior of a new thermomechanically processed low-alloy steel containing 0.40 wt.% carbon has been investigated. The processing included laboratory hot rolling to 10 mm thick strips followed by direct quenching to different quench-stop temperatures followed by slow furnace cooling to room temperature stimulating hot strip mill processing. The final microstructures were upper and lower bainite with yield strengths of a ~700 and ~1200 MPa, respectively. Local microstructures were characterized using a field-emission scanning electron microscope, microhardness profiles were measured, and bendability was determined using three-point brake press bending. The minimum applicable punch radius for a defect-free bend was 28 mm (2.8 times thickness) for the high-strength lower bainitic microstructure, while it was much smaller, i.e. 20 mm (2.0 times thickness) for the lower strength upper bainitic microstructure. Fractographic examination of the cracked surfaces revealed a more ductile fracture behavior for the upper bainitic microstructure.

Effect of bainitic transformation on the microstructure and wear resistance of pearlitic rail steel

Purpose The purpose of this study is to develop a lower bainite structure consists of a dispersion of fine carbide inside plates of bainitic ferrite from chemical composition unmodified conventional pearlitic steel under bainitic transformation and to investigate its effect on tensile properties and wear resistance. Design/methodology/approach A commercial hypereutectoid pearlitic rail steel was subjected to three different bainitic transformation treatments followed by tempering to develop a desirable microstructure with a DIL805 BÄHR dilatometer. A comprehensive microstructural study was performed by scanning electron microscopy and energy dispersive x-ray spectroscopy. Finally, the mechanical properties and wear resistance were evaluated by tensile, microhardness, and pin-on-disc tests. Findings The results showed that the best combination of mechanical properties and sliding wear resistance was obtained in the sample subjected to bainitic transformation at 300°C for 600 s followed by tempering at 400°C for 300 s. This sample, which contained a bainitic ferrite structure, exhibited approximately 20% higher hardness and approximately 53% less mass loss than the as-received pearlitic sample due to the mechanically induced transformation in the contact surface. Originality/value Although pearlitic steel is widely used in the construction of railways, recent studies have revealed that bainitic transformation at the same rail steels exhibited higher wear resistance and fatigue strengths than conventional pearlitic rail at the same hardness values. Such a bainitic microstructure can improve the mechanical properties and wear resistance, which is a great interest in the railway industry. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-07-2019-0282/

High-Copper Cast Irons for the Products of Tribotechnical Applied

The paper presents the results of the development of a special group of cast composite materials based on nodular cast irons. The alloys were based on alloying cast iron with copper in amounts that exceed its solubility in liquid Fe-C-Si systems (Cu>6 wt.%). A new component is obtained in the structure of castings in the form of a complex mixture based on the Cu-phase. Inclusions are relatively large (up to 200 μm) and globular in shape, with a uniform allocation in the volume of the castings. This favorably distinguishes the obtained alloys from the known cast irons and gives them high antifriction properties. With isothermal hardening, it is possible to obtain the lowest values of wear for structures of lower bainite. Developed cast composite materials are recommended to be used as tribotechnical materials working in conditions of poor lubrication and corrosion.