Structure Of Lower Bainite Research Articles

Wear behaviour of austempered, ductile iron microalloyed with boron under different contact load by dry sliding wear conditions

In the preformed work were studied the characteristics of Ni, Mo and Cu – free austempered ductile iron microalloyed by Boron under different contact load. For microalloying liquid cast iron with boron, the ferroboron by synthesized SHS - innovation technology was used. The upper and lower bainitic austempered ductile irons were studied with different content of metastable retained austenite. The experimental samples were tested with a fixed linear sliding speed of 2 m / s at a different contact load values, which were changed in the range of 25, 50 and 100 Newtons. It was defined that the value of the contact load has a significant effect on the tribotechnical characteristics of bainitic cast irons in conditions of dry sliding friction. Particularry, they determine the structural changes character and the degree of surface layers degradation of friction. High strength cast irons with the lower bainite structure are more resistant to adhesive wear. According to the obtained data, the presence of up to 80% metastable retained austenite in the metal matrix structure of the of experimental cast irons makes it possible to stabilize the friction coefficient under the investigated contact loads. Thus, to ensure maximum wear resistance, the isothermal quenching of high-strength cast irons must be carried out in the temperature range 220-280 0C with a minimum treatment time.

Povećanje otpornosti na trošenje elemenata brodskih dizelskih motora koji se izrađuju od nodularnog lijeva

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.

Bainitic Cast Irons with Nanosized Particles

In the present study austempered ductile irons (ADI) with lower bainitic structure are investigated. Nanosized particles (50nm) of titanium carbonitride + titanium nitride TiCN+TiN and titanium nitride TiN are added to the casting volume. The samples microstructure is studied by optical metallography and X-Ray analysis. The influence of the nanosized additives on the kinetics of the bainitic transformation and on the morphology of the bainitic structure is investigated. The abrasive wear testing, hardness measurements and impact strength are carried out. It is established that the presence of nanoadditives in the bainitic cast irons leads to the changes in their microstructure which increases their mechanical characteristics and abrasive wear resistance. The studied nanocomposite materials expand the potential for new ADI applications in the industry.

Revisiting Some Prospects for the Further Development of Ideas by Academician V.N. Antsiferov in the Field of Structural Powder Materials

Some developments by Academician V.N. Antsiferov and his scientific school are considered from the viewpoint of their further development when fabricating structural powder materials and products. The prospects of work for the fabrication of concentration-inhomogeneous and trip steels are noted. There is great potential in the possibility of controlling the thickness and volume of the zone of the deformation martensite transformation occurring during the fracture. It is advisable to continue works on the fabrication of fullerene-containing and nitrogen-containing powder compositions, as well as on studying the structural heredity of powder steels. The possibility of synthesizing fullerene-containing phases during the liquid-phase sintering of “iron–cast iron” and “iron–graphite” compositions and their subsequent redistribution in the material bulk during dynamic hot pressing deserves attention. It is reasonable to fabricate nitrogen-containing steels by the mechanical activation of powders with subsequent sintering in dissociated ammonia not only when forming wear-resistant and corrosion-resistant steels, but also heat-resistant materials. Works on studying the decomposition of supercooled austenite in powder steels of various alloying systems having different process prehistories (sintered, hot-deformed, infiltrated, etc.) are promising. Works on studying hot deformed concentration-inhomogeneous materials fabricated, in particular, based on powders of the Distaloy type, are promising. Original procedures developed by the school of Academician V.N. Antsiferov are valuable. This is, in particular, the procedure of determining the concentration variation coefficient, as well as the magnitometric complex and mathematical model providing the possibility to forecast the decomposition of supercooled austenite. Works by V.N. Antsiferov can be in-demand when solving the problem of lean alloyed powder steels with a lower bainite structure, which provide the optimal combination of strength and viscosity.

On some prospects for further development of Academician V.N. Antsiferov ideas in the field of structural powder materials

The paper discusses the further development of some ideas of V.N. Antsiferov and the scholar school created by him in obtaining structural powder materials and products. The prospects for obtaining concentration-inhomogeneous steels and trip-steels are noted. The great potential lies in the control of the thickness and volume of the zone of deformation martensitic transformation occurring during fracture. It is advisable to continue the preparation of fullerene- and nitrogen-containing powder compositions and to study the structural heredity of powder steels. The possibility of the synthesis of fullerene-containing phases during the liquidphase sintering of the iron–cast iron and iron–graphite compositions and their subsequent redistribution in the bulk of the material during dynamic hot pressing is worth noticing. Producing nitrogen-containing steels by mechanical activation of powders followed by sintering in dissociated ammonia is advisable to use for obtaining not only wear-and corrosion-resistant materials, but also heat-resistant ones. The studies on the decomposition of supercooled austenite in powder steels of various doping systems with different technological backgroung (sintered, hot-deformed, infiltrated, etc.) are promising. The potential for development is the research of hot-deformed concentration-inhomogeneous materials, obtained, in particular, on the basis of powders of the Distaloy type. The techniques developed by the Antsiferov’s school are significant. The most important one is the method for determining the concentration variation coefficient, as well as a magnetometric complex and a mathematical model, which makes it possible to predict the decomposition of supercooled austenite. Antsiferov’s works can be used for obtaining lean powder steels with the lower bainite structure, which provides the optimal combination of strength and toughness.

The Role of Bainite in Wear and Friction Behavior of Austempered Ductile Iron.

The austempered ductile iron was austenitized at 900 °C for 1 h and quenched in an isothermal quenching furnace at 380 °C and 280 °C, respectively. This paper aims to investigate the effects of bainite on wear resistance of austempered ductile iron (ADI) at different loads conditions. The micro-structure and phase composition of ADI was characterized and analyzed by metallographic microscope (OM), X-ray diffractometer (XRD) and scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS). The results showed that the volume fraction of retained austenite in ADI is reduced with the increase of austenitizing temperature. Meanwhile, the two kinds of ADI samples showed varied wear resistance when they were worn at different loads conditions. For wearing at a load of 25 N, the wear resistance of ADI mainly depends on matrix micro-hardness. Thus, ADI with lower bainite structure has higher hardness and leads to better wear resistance. When wearing at a load of 100 N, the increase of micro-hardness of upper bainite was significant. As a consequence, upper bainite showed superior friction and wear behavior. It was also found that the form of wear behavior of ADI changed from abrasive wear to fatigue delamination as the wear load increased from 25 N to 100 N according to the observation on worn surface.

Effect of Nanoadditives on the Structure and Properties of Austempered Ductile Irons

In the present study austempered ductile irons (ADI) with an upper and lower bainitic structure with nanosized particles of cubic boron nitride cBN (50nm) are studied. The austempering modes include austenization at 900оС during an hour and isothermal retention at 280оС from 0.5 to 6 hours and at 380oС from 0.5 to 6 hours. The samples microstructure is investigated by means of an optical metallography, scanning electron microscopy and X-ray analysis. Tests for hardness, impact toughness and wear are performed. The influence of nanosized additives on the kinetic of the bainitic transformation and on the morphology of the obtained bainitic structure is studied. It is established that the changes of the bainitic cast irons microstructure under influence of the nanoparticles of cBN lead to the cast irons impact toughness and wear resistance increase. The research is aimed at expanding the scope for new ADI applications in the industry.

Influence of the Modes of Heat Treatment on the Durability of Springs Made of 65G Steel

We study the fatigue life of springs made of manganese steel (0.66% С and 1.02% Mn) after four modes of heat treatment leading to the formation of troostite, martensitic–bainitic, and bainitic–martensitic structures. Springs are studied under the conditions of strain-controlled cyclic loading similar to the operating conditions of products intended for special applications. It is shown that the optimal durability of springs is guaranteed by the mixed structure of lower bainite and martensite obtained after isothermal quenching and special tempering performed by interrupting the process of cooling after quenching in the interval between the points corresponding to the onset and end of the martensitic transformation. The durability of springs is connected with the quantitative parameters of phase composition and dislocation structure and with the micromechanism of fracture of spring steels.

Effect of Isothermal Bainitic Quenching on Rail Steel Impact Strength and Wear Resistance

The effect of heat treatment regimes on hardness, impact strength, and wear resistance of rail steel for high-speed tracks (rail quality category R350HT) is studied. Analysis of steel properties with a different structure is compared: pearlitic, and upper and lower bainite. It is shown that the steel with bainitic structure has the best impact strength, but wear resistance is better for steel with a lower bainite structure.

Increasing the Service Life of Large Rolling-Mill Rolls by Using the Reserve Hardenability of the Steel

Roll steel with a high chromium content (steel 45Kh5MF) is characterized by good hardenability, which provides an opportunity to significantly increase the durability of rolling-mill rolls. Here, the surface layer (working layer) of the rolls is strengthened thanks to the use of a heat treatment that forms a structure of lower bainite while blocking the pearlite transformation of austenite. At the same time, occurrence of the bainite transformation in high-chromium steel requires that the steel be kept in the narrow temperature range associated with this conversion (290–410°C) for a considerable length of time. Large rolls made of steel 45Kh5MF are quenched on a horizontal unit in two stages: intensive water-air cooling for 30–35 min (stage 1); cooling with compressed air for 4–5 h (Stage 2). Such quenching blocks the conversion of austenite to pearlite and ensures the formation of a satisfactorily developed bainitic structure. Efficient regimes for quenching and tempering large rolling-mill rolls were found by using integrated software that makes it possible to track the dynamics of the formation of temporary and residual stresses in rolls.

Structural Features of Structural Steels Microalloyed with Nitrogen and Vanadium

Results are presented for a study of medium-carbon structural steels for the effect of microalloying with nitrogen, vanadium, and other nitride-forming elements on the microstructure and processes of nitride formation after hardening with tempering and rolling. The level of mechanical properties is compared after different treatment. It is shown that a lower bainite structure forms with high dislocation density after rolling and cooling steel to 550°C. Within ferrite plates there is precipitation of finely dispersed densely located vanadium carbonitride. Strength as a result of good dispersion of the fine lower bainite structure, high dislocation density, and dispersion strengthening with precipitation of nanosize carbonitride phase, provides a high level of mechanical properties after rolling and cooling to 550°C, comparable with the level of properties after hardening and tempering.

Effect of chemical composition on microstructure and mechanical properties of laser weld metal of high-tensile-strength steel

Laser beam welding (LBW) and laser-arc hybrid welding are expected to be applied to welding of high-strength steel plates over 780 MPa, because they have advantages such as low distortion and a narrow heat-affected zone. This research examined the relationship between the chemical composition of high-strength steel plates and the microstructure and mechanical properties of the weld metal with the aim of improving the toughness of welded joints of high-strength steels by the LBW process. The results revealed that weld metal toughness is strongly dependent on the carbon (C) content of the steel plates, and excellent toughness can be obtained in weld metals with low C contents having an almost fully lower bainite structure without formation of slender martensite austenite constituent (MA). Based on these results, a guideline for appropriate alloying element design for high-strength steels for LBW was proposed, targeting mechanical properties of tensile strength ≥980 MPa and toughness vE−40 °C ≥ 47 J.

Effect of tempering temperature on failure resistance of wheel steel with a structure of martensite and lower bainite

The effect of wheel steel structural condition on mechanical properties with static and impact loading is studied. It is shown that steel with a lower bainite structure with low-temperature tempering has greater work for failure than steel with a martensitic structure. The superiority of a bainitic over a martensitic structure on toughness in impact tests is established. A difference is revealed in the brittle failure mechanism for martensitic and bainitic high-carbon steel.

High resistance boron treated steels for railway applications

The present study analyses different boron contents (between 10 and 160 ppm) on the structure of a 0·2C–2Mn–1Si (wt-%) steel deformed at a starting temperature of 1050°C in a T. J. Pigott laboratory rolling mill. The steel was made in a laboratory open induction furnace using high purity raw materials and cast into metallic moulds. This experimental steel has proved to have tribological properties, under dry rolling/sliding contact, as good as those for the 0·8% pearlitic steels used in railway applications. Before thermomechanical processing, the steel ingots (70 × 70 × 70 mm) were homogenised at 1100°C for 1·5 h. The thermomechanical treatment was carried out by a reversed multipass process to reach a level of deformation of 60%. Plastic deformation was finished at ∼920°C for all the rolled steels and the plates (70 × 150 × 20 mm) were then water quenched and/or air cooled to room temperature. Results show more bainitic structures as boron content increases in the air cooled steel after hot rolling. For the quenched steels, the structure becomes more martensitic as boron content increases. The best combination of mechanical properties was obtained for the air cooled 76 ppm boron containing steel, which had a lower bainitic structure. This steel had the yield strength of 750 MPa, 15% elongation and the hardness of 40 HRC. Materials characterisation was carried out by optical and transmission electron microscopy (TEM). Results are discussed in terms of the boron segregation towards grain boundaries, the effect of boron on the steel hardenability, as well as on the boron carbonitrides (CNB) precipitation.

On the Microstructure and Fracture Surface of Bainite in JIS SK5 Steel

The microstructure and fracture surface of bainite in JIS SK5 steel have been investigated using transmission electron microscopy (TEM) and electron back-scatter diffraction (EBSD) in this study. Specimens were austenitized at 880 for 30 min and then austempered at 300 and 400 for 1hr, as a result, lower bainite and upper bainite were formed, respectively. The lower bainite phase consists of plate-like bainitic ferrites and embedded cementite platelets, the cementite precipitated unidirectionally at an angle of 55 to 60 deg deviated from the long axes of the ferrites; in addition, the upper bainite phase consists of a parallel array of ferrite laths and discrete cementite layers sandwiched between them. Both the bainite structures have the same ferrite/cementite orientation relationship, which is close to that of the Bagaryatskii relation. The impact fractographs of lower and upper bainite structures exhibit brittle failure with cleavage facets, of which the size is correlated with the width of the bainite sheaves. Moreover, the crystallographic orientation of cleavage facets has been determined directly by EBSD. The results showed that the cleavage facet planes of lower bainite structure are close to the {001} plane of ferrite, and they were close to the {001}, {112} and {123} planes of ferrite for the upper bainite structure.

The Effect of Thermomechanical Processing on the Structure and Mechanical Properties of Ni, Nb and V-Bearing HSLA Steels

The effect of deformation temperature (800-1000°C) - strain accumulation (number of rolling passes: 3÷5) - cooling rate (8,6 – 25°C/s) parameters on the microstructure and mechanical properties of C-Ni-Nb and C-Ni-V steels after High Temperature Thermomechanical Processing (HTMP) are presented. The experiment planning method was used for the simulation and examination of the above mentioned parameters effect on the mechanical properties. The regression equations and diagrams describing the qualitative and quantitative effects of HTMP parameters on the mechanical properties of the steels are derived from the experimental results. The Ni-Nbbearing steel with the lower bainite structure that is a result of the HTMP with the maximum number of passes, the highest deformation temperature and cooling rate shows the highest strength (YS=987 MPa) in combination with a high impact strength (W=154 J). It is shown that the Ni- bbearing steel with the fine lower bainite structure that results from the transformation of the hot deformed fragmented substructure of austenite after water cooling has the best resistance to brittle fracture. The Nb-bearing steel has a higher strength compared to the V-bearing one, although it shows a lower impact strength in the negative testing temperature range.

Contact Durability of Bit Steels in Washing Aqueous Drilling Solutions

It is shown that the contact durability of a new 20KhGNBTChA low-alloy steel in aqueous and mud washing solutions is, respectively, 1.5 and 1.73 times higher than the durability of the 20KhNA commercial bit steel. The roller bearings of drill bits operating in aggressive media should be made of 20KhGNBTChA steel thermally processed to obtain the structure of lower bainite with the lowest susceptibility to hydrogen embrittlement. We propose the most efficient additives of surfactants to drilling solutions guaranteeing high lubricating characteristics of these solutions, inhibiting the anodic and cathodic processes in the course of friction, and increasing the contact durability of bit steels.

Effect of Various Heat Treatments Particularly Austempering on Mechanical and Physical Properties of a High Mn Ductile Iron

The stable and metastable phase diagrams, microsegregation of carbon and alloying additions and the driving force for single and double austempering are reported for a ductile iron (DI) of composition: 3.5% C, 2.64% Si, 0.25% Cu, 0.25% Mo, 0.67% Mn, 0.007% P, 0.013% S, 0.04% Mg The variation with austempering time of the retained austenite volume fraction (VRA), Unreacted Austenite Volume fraction (UAV), austenite carbon content (CJ, UTS, elongation, and unnotched charpy impact energy is reported for single austempering at 400°C, 285°C, and a double austempering treatment (400°C, 120 min., then austempering at 285°C) after austenitizing at 920°C for 120 min. Finally mechanical and physical properties including strength, ductility, toughness, wear resistance, hardness, thermal conductivity, and electrical properties for the following heat treatments are compared: ADI (upper bainitic structure) 870°C, 120 min.; 375°C, 120 min.ADI (lower bainitic structure) 870°C, 120 min., 285°C, 1 dayADI (Double austempered) 870°C, 120 min., 375°C, 120 min., 285°C, 1 dayAir cooled (mainly martensitic/ some widmanstatten ferrite): 870°C, 120 min. furnace cooled (50.7% proeutectoid ferrite/49% pearlite): 870°C, 120 min. Step-cooled (15% proeutectoid ferrite/ 85% pearlite): 870°C, 120 min., 650°C 30 min., air cooled As cast structure (39.5% proeutectoid ferrite/ 60.5% pearlite)The comparison shows that double austempering can be used with the high Mn DI to improve elongation and impact energy obtained by a single austempering. Control over the austenitizing and austempering temperatures and times in the double treatment can be used to change the relative improvements in elongation and impact energy. The results show that furnace cooled irons have the best elongation and physical properties.

Heat treatment of large forgings with heating in the intercritical temperature range

1. In large forgings the investigated Mn-Ni-Mo-steels provide with slow cooling rates of 30–50°C/h a mixed ferritic-pearlitic-bainitic structure with a bainite content of not less than 30%. Such steels must have a low carbon content (about 0.1%) to increase the quantity of ferrite in the ferritic-pearlitic mixture and the necessary quantity of alloy elements to obtain the required stability of the supercooled austenite. 2. Heating to intercritical range temperatures and slow cooling leads to a reduction in the temperature of the bainitic transformation and the formation of a significant quantity of lower bainite causing phase strain hardening of the ferritic matrix. Obtaining of a structure of lower bainite in combination with the strain hardened ferritic matrix and also the presence of a fine grain, provide quite high mechanical properties. 3. After heat treatment from the intercritical temperature range with slow cooling it is necessary to temper at 400–450°C. This provides the greatest stability of the structure, preservation of phase strain hardening, and the maximum strength, plasticity, impact strength, and fracture toughness. 4. To obtain guaranteed mechanical properties of large forgings produced from the heterophase steels (σt ≥ 600 N/mm2, σ0.2 ≥ 350 N/mm2, σ5 ≥ 20%, ψ ≥ 60%,a0.25+20 ≥ 150 J/cm2,a0.2550 ≥ 100 J/cm2) the following heat treatment is recommended: anneal or normalize with heating to 920°C; final heat treatment by heating to 750°C, cooling at a rate of 30–50°C/h, and tempering at 400–500°C.

Cooling large forgings in a water-air mixture

1. A water-air mixture at a water flow rate of 0–100 liters/h possesses insufficient cooling capacity to ensure a martensite and lower-bainite structure across the section of rotor forgings formed from type 35KhN3MFA steel; this does not make it possible to lower significantly the critical brittleness temperature. A mixture with this water flow rate can be used in quenching large forgings after the start of the martensitic transformation has occurred across the entire section, and a more abrupt cooling with a water sprayer should be used in the upper temperature interval. 2. A water-air mixture at water flow rates to 100 liters/h is promising for the quenching of rotors employed in high- and medium-pressure turbines fabricated from steel R2MA. 3. The maximum effect realized from the use of a water-air mixture in the water-flowrate interval under consideration is achieved with the quenching of large forging dies in accordance with the three-stage regime, developed in the present study to prevent the formation of quenching cracks and to ensure the high dynamic strength of the dies. 4. A water-air mixture at water flow rates of up to 100 liters/h makes it possible to attain an allowable level of embrittlement on forgings with a diameter to 2200 mm and completely prevents temper brittleness of forgings to 1100 mm in diameter. 5. Use of a water-air mixture as a cooling medium for quenching and tempering is one of the promising trends in the development of the heat-treatment technology for large forgings.