Abstract: Austempered ductile iron has been considered as a substitution to replace conventional steel castings, steel forgings and even aluminum in many applications. Its excellent mechanical properties are mainly attributed to its unique ausferritic structure containing acicular ferrite and carbon saturated austenite. Some ADI components such as camshaft and engine valve commonly experience surface contact and relative motion under external load and pressure. Outstanding wear resistance of ADI is required to ensure the satisfactory working performance and service life of these key components. In order to meet the advanced requirements of mechanical properties and wear behavior of ADI, a dual-step austempering process and additional surface hardening treatments are suggested to be utilized. In this review, conventional single-step austempering process and corresponding ausferritic structure are introduced. Second, the positive effects of a novel dual-step austempering process on microstructure and mechanical properties of ADI are provided. Next, wear behavior of single-step and dual-step ADIs and wear mechanisms are discussed. Then, studies regarding the applications of surface hardening treatments on wear resistance of ADI are reviewed. Finally, a summary and expectations are discussed. The information presented in this paper would be beneficial to future studies and applications for any grade of ADI.

Abstract: The present research studied the combined effects of quench-tempering and laser surface hardening treatments on wear behavior of gray cast iron, and compared results with conventional austempered gray cast iron. Four tempering temperatures of 316 °C (600 °F), 399 °C (750 °F), 482 °C (900 °F) or 552 °C (1025 °F) with a constant holding time of 60 min and four austempering temperatures of 232 °C (450 °F), 288 °C (550 °F), 343 °C (650 °F) or 399 °C (750 °F) with a constant holding time of 120 min were utilized in the heat treatment design. The wear tests were carried out on a universal mechanical tribometer with a reciprocating ball-on-plate sliding configuration. Also, the microstructure, micro-hardness profiles and worn tracks were examined. Through this work, it was found that three zones existed under the laser hardened surface. Zone 1 was the laser hardened zone containing ledeburite with hardness of approximately 68HRC. Zone 2 was the heat affected zone containing the martensite with hardness of approximately 66HRC. Zone 3 was the substrate with hardness ranging from 42.1 to 24.8HRC. In the sliding wear tests, the quench-tempering treatment only resulted in higher wear resistance of gray cast iron when compared with untreated specimens, but lower wear resistance than that of austempered gray cast iron under similar macro-hardness. The wear performance of the quench-tempered gray cast iron was enhanced after receiving the laser surface hardening treatment. Finally, the laser hardened and quench-tempered gray cast iron with tempering temperature of 552 °C showed similar mass loss due to wear as austempered gray cast iron with an austempering temperature of 232 °C. By observing the worn surfaces, the laser hardened regions could effectively inhibit the formation and propagation of cracks developed within the substrate regions. In addition, the substrate with low hardness in laser hardened and quench-tempered gray cast iron may provide enhanced ductility and toughness for gray cast iron engineering components. The results obtained in this research have significant value in selecting the optimum heat treatment process for laser hardened gray cast iron components.

Abstract: The present study mainly investigated the phase transformation and wear performance of laser hardened austempered gray cast iron specimens. Ball-on-Plate reciprocating sliding wear tests with PAO4 base oil were carried out on untreated, quench-tempered, austempered, laser hardened austempered gray cast iron specimens. Micro hardness profiles and worn surfaces were analyzed for specific wear mechanisms. It was found that four different zones were formed beneath the laser hardened surface including laser hardened zone (Ledeburite, ≈67HRC), upper heat affected zone (Martensite, ≈69HRC), lower heat affected zone (Tempered Ausferrite, ≈36HRC) and substrate (Ausferrite, ≈39HRC). In the sliding wear tests, the laser hardened austempered gray cast iron specimens showed higher wear resistance than others, which could be associated with the benefits of ausferritic structure with high fracture toughness and laser hardened regions with high hardness. By observing the worn surface, grooves and micro-sized material removal occurred within the substrate. The surface of the substrate ploughed by the asperities of the ball and hard wear debris stopped on the edge of the laser hardened regions since ledeburite and martensite had high hardness and abrasive wear resistance. However, some micro-cracks and spalls were developed within the laser hardened zone. The results obtained from this study could be used as reference in future research and applications of laser hardened austempered gray cast iron.

Abstract: The grinding process is a finishing operation that guarantees the surface of the workpieces with geometric and dimensional precision. This machining requires significant efforts to remove material, which can cause excessive wheel wear and even distortions on the workpiece surface. Also, the productivity and efficiency of this process are directly affected by the properties of the workpiece, wheel friability, and feed rate. In this context, it is essential to find the best machining conditions. Thus, this article makes a comparative analysis between the cylindrical grinding of SAE D-7003 ductile iron (DI) and SAE D-7003 austempered ductile iron (ADI), with two wheels of cubic boron nitride (CBN), one with low and the other with high friability, using feed rate of 0.50, 1.00, and 1.50 mm/min. The lubri-refrigeration of the process was done by the conventional method. The output parameters evaluated were surface roughness (Ra), roundness error, diametral wheel wear, tangential cutting force, microhardness, and micrograph. The tool with less friable grains reduces the surface roughness by 18%, 21%, and 29% compared with the tool with more friable grains for grinding of the DI with the feed rate of 0.50, 1.00, and 1.50 mm/min, respectively. In the ADI, the surface roughness is reduced by 27%, 26%, and 29% with a feed rate of 0.50, 1.00, and 1.50 mm/min, respectively. The results show that the low friability tool performs better in most parameters.

Abstract: High Silicon Austempered steels (AHSS) are materials of great interest due to their excellent combination of high strength, ductility, toughness, and limited costs. These steel grades are characterized by a microstructure consisting of ferrite and bainite, accompanied by a high quantity retained austenite (RA). The aim of this study is to analyze the effect of an innovative heat treatment, consisting of intercritical annealing at 780 °C and austempering at 400 °C for 30 min, on the microstructure and mechanical properties of a novel high silicon steel (0.43C-3.26Si-2.72Mn wt.%). The microstructure was characterized by optical and electron microscopy and XRD analysis. Hardness and tensile tests were performed. A multiphase ferritic-martensitic microstructure was obtained. A hardness of 426 HV and a tensile strength of 1650 MPa were measured, with an elongation of 4.5%. The results were compared with those ones obtained with annealing and Q&T treatments.

Abstract: In the present work, mechanical and microstructural properties of Austempered Ductile Iron (ADI) alloyed with Cu-Ni-Mn-Mo submitted to different hot air austempering cycles in a two-step mechanism were evaluated. In the first step, cooling and austempering were performed in an experimental hot air blower device. In the second step, the austempering was performed in a muffle furnace. Bars with O16.5 mm and 90 mm of length were austenitized in an electric resistive furnace for 90 min at 860°C. Austempering first step was performed for 10 min at 290°C, 320°C and 350°C corresponding to cycles I, II and III, respectively. Austempering second step was performed for 60 min at 380°C for all cycles. The measurements of Brinell hardness, elongation, yield strength, and ultimate tensile strength for cycle I were 288 HB, 5.0%, 840 MPa, and 988 MPa. For cycle II, the measurements were: 264 HB, 5.9%, 717 MPa, and 876 MPa. And for cycle III: 257 HB, 6.6%, 711 MPa, and 888 MPa. Microstructural analysis indicated the presence of pearlite-free ausferrite.

Abstract: The martensitic transformation in a Fe–C–Mn–Si bainitic steel was examined by in situ high-temperature laser scanning confocal microscopy (LSCM) and dilatometry. The phenomenon of continuous martensitic transformation during austempering was firstly dynamically observed by LSCM. Differing from the commonly accepted viewpoint on martensite formation in bainitic steels, the martensitic transformation in the conventional medium-carbon bainitic steel was not instantaneous and proceeded gradually when the sample was austempered below martensite starting temperature (MS). It can be attributed to the generation of internal stresses, thermal activation, stimulating nucleation, and the segregation of Mn. In addition, apart from the continuous martensitic transformation, the bainitic transformation was also directly observed by LSCM during austempering below MS. Moreover, it was clear from the results of dilatation during austempering that the inflection point in the dilatation curve against time was not the demarcation point between martensitic and bainitic transformation, and in situ observations confirmed that martensite was still formed after the inflection point. Therefore, the obtained results could be an excellent reference to further understand the mechanism of bainitic and martensitic transformations in Fe–C–Mn–Si bainitic steel during austempering below MS. Graphical abstract presents a series of frames that reveals the rapid growth of some black units during austempering at 240 °C, and these rapidly growing units (marked by arrows) were not present in the previous frame of the video. These black growing units (martensite laths) shown in Graphical abstract appeared over a time interval of ~ 420 s from 837.39 to 1257.37 s during austempering. This phenomenon differs from the previously held viewpoint where martensitic transformation is expected to be finished in a very short period of time after a sample is cooled to a certain temperature below MS because of the diffusionless nature of martensite transformation in conventional medium-carbon bainitic steels. The growth of martensite laths during the austempering process at 240 °C.

Abstract: In the work here presented, the high performance of ceramic insert tools in milling of ADI 1000 iron casting is analyzed. Austempered ductile irons (ADI) are ductile iron castings with strength and mechanical properties enhanced after specific heat treatment, achieving 1000 MPa or even more. Sintered carbide tools are state of the art in many industrial applications, including iron casting machining, but ceramic inserts are a feasible and promising option since cutting speed can be improved by 5 or even 10 times. A complete testing campaign was performed, starting with coated sintered carbides and aiming at the use of whisker reinforced Al2O3 ceramics and Si3N4 tools. The two most important conclusions are as follows: firstly, that the milling type so-called up-milling (or conventional) is more recommended than down-milling, also known as climb milling, and secondly, that dry machining enhances ceramics performance in comparison with using emulsion coolants (oil in water). Finally, results regarding economic aspects were analyzed based on the tools cost-performance ratio.

Abstract: The microstructure, hardness, impact toughness and ring block wear properties of a novel bainitic steel were investigated by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), laser confocal microscopy (LCM), X-ray diffraction (XRD), Rockwell hardness tester, impact tester and MM-200 wear tester (Jinan Sida Testing Technology Co., LTD, China). The above steel was austenitized at 900 °C, and was isothermally quenched at 250, 280, 310, 340, 370, 400 and 430 °C respectively. The results showed that the microstructure of the bainitic steel was mainly composed of bainitic ferrite (BF), retained austenite (RA), and martensite/austenite (M/A). A part of the RA underwent a phase transition and transformed into martensite during the wear process, resulting in the increase of the hardness. The wear morphology of bainitic steel was dominated by straight furrows. The wear mechanisms were mainly abrasive wear and oxidative wear. With the increase of the austempering temperature, the microstructure coarsened, the amount of the blocky RA content increased, the hardness of the high silicon bainitic steel increased slightly first and then decreased, the impact toughness improved significantly first and then reduced, and the wear resistance increased first and then decreased. Under the same test conditions, the high silicon bainitic steel after austempering at 340 °C had better combination of hardness and toughness, stable and low friction coefficient, and excellent wear resistance.

Abstract: The interplay between ausforming strain level applied prior to austempering step and bainite transformation kinetic as well as plastic deformation mechanism of nanostructured bainitic steel was essentially investigated. The results indicated that the bainite transformation proceeded rapidly with increasing ausforming strain owing to the added heterogeneous nucleation sites and overall storage energy. Nevertheless, when the ausforming strain further increased to 40%, the acceleration effect became less pronounced attributing to the counteraction mechanism by inhibition effect of retarded bainite growth. Increasing ausforming strain could effectively refine bainite sheaves and minimize the dimension of blocky retained austenite, yet strengthen undercooled austenite, leading to a decreased volume fraction of bainitic ferrite at the termination of transformation. Ausforming strain had a positive effect on mechanical properties, particularly, the sample ausformed at 300 °C for 40% strain achieved the largest tensile strength of 1900 MPa and the best total elongation of 24%. This enhanced strength–ductility synergy mainly ascribed to the benefit of the refined bainitic ferrite lath and the increased dislocation density together with transformation induced plasticity effect. Unfortunately, the apparent variant selection brought about by ausforming could offset the increased impact toughness resulting from microstructure refinement when ausforming strain was higher than 10%, which leaded to an intensive deterioration of impact property.

Abstract: The transformation kinetics of a low carbon bainitic steel was investigated using in situ confocal microscopy and dilatometry for isothermal treatments above and below the martensite start (MS) temperature. Both martensite and bainite are formed during isothermal holding below the MS, but bainite was the dominant transformation product. The prior martensite formed during cooling to the holding temperature strongly influenced the bainite precipitation kinetics. A critical volume fraction of prior martensite, f c P M , was identified. In order to accelerate the bainite transformation kinetics by austempering below the MS, the prior martensite fraction should be less than f c P M . The best combinations of strength and ductility were obtained in steels by austempering below the MS temperature with an optimum martensite fraction (∼5%). Finally, the isothermal bainite transformation kinetics is analyzed by Johnson-Mehl-Avrami-Kolgomorov (JMAK) kinetics model. The analysis suggests that the nucleation of bainite approaches site saturation when the isothermal holding temperature is below the MS.

Abstract: Carburizing implies the existence of a carbon gradient from the surface to the core of the steel, which in turn will affect both the critical temperature for austenite formation and the kinetics of the bainitic transformation during the austempering treatment. Therefore, for future development of carbo-austempered steels with nanobainitic microstructures in the case, it is key to understand the effect of such carbon gradient has on the final microstructure and the mechanical properties reached by the heat treatments used. This work was divided into two parts, firstly two alloys with similar carbon content to those at the surface and center of the carburized steel were used to establish the optimal heat treatment parameters and to study bainite transformation kinetics by high resolution dilatometry. In a second step, a carburized alloy is produced and subjected to the designed heat treatments, in order to evaluate the microstructure and mechanical properties developed. Results thus obtained are compared with those obtained in the same carburized alloy after following the most common quench and temper treatment.

Abstract: A novel hot-galvanized quenching and partitioning (Q&P) steel (0.225C-0.85Si-2.02Mn-0.91Al, in wt.%) with excellent strength-ductility balance was fabricated by intercritical annealing followed by high-temperature short-time overaging treatment. The resulting microstructure evolution and its relationship with mechanical properties were investigated using dilatometry, SEM, EBSD, TEM, XRD, nanoindentation and tensile testing, and compared with samples subjected to conventional quenching and austempering (QAT) process. The combined effects of structural subdivision by primary martensite and carbon partitioning simultaneously from martensite and bainite lead to the retention of larger amounts of retained austenite (RA) constituents with different morphologies, sizes and stabilities at room temperature. On the contrary, the QAT samples have coarser martensite-austenite (M-A) islands and smaller fraction of RA due to relatively insufficient partition and bainitic transformation kinetics. As compared to QAT treatment, the better mechanical properties with a yield strength (YS) of 736 MPa, tensile strength up to 1020 MPa, total elongation (TEL) of around 24%, and product of strength and elongation (PSE) above 24 GPa% can be achieved after the Q&P process, and particularly, more ductile tempered martensite and refined M-A particles result in a simultaneous increase of YS and TEL. The instantaneous work-hardening exponent vs. true strain curve of Q&P samples displays a small peak followed by a long plateau region before break, which can be attributed to a less active and sustained TRIP effect.

Abstract: This research studies the kinetic transformation and austempering temperature effect on the morphologies of bainite for AISI 6150 steel. A minimum of 10 different holding times for each austempering temperature were utilized. All the AISI 6150 steel samples with original spheroidal pearlite were austenized at 855 °C for 20 min, followed by a lower temperature salt bath for austempering at various holding times. The hardness for each austempered temperature and holding time was measured, and the microstructures were observed by optical microscopy. The kinetic energy for needle like lower bainite and granular like upper bainite was analyzed by using kinetic transformation equations. The aim of this research was to study the transformation of bainite, in particular lower bainite, to obtain improved mechanical properties and higher ductility of AISI 6150 steel, and then apply AISI 6150 steel to more applications.

Abstract: This study describes the laser surface hardening process effect on microstructures produced and their wear behavior. Ductile iron samples were austempered at three different austempering temperatures: 232 °C/288 °C/398 °C respectively. Then, the laser surface hardening process was applied on each sample. Different laser gaps were designed: 1.5mm/3mm/4 mm. The microstructures were observed using optical microscopy and tribo-tests were run using a UMT-3 tribo tester. A Rockwell hardness tester was used to measure the hardness after heat-treatment. Optical microscopy and SEM were used to observe the different microstructures and their distribution on the worn surface. The results showed that the laser processing generated ledeburite, martensite, or tempered bainite microstructures. Vickers hardness tests were carried out on these microstructures. The original needle-like microstructures were observed on the samples with 4 mm laser gap but not with 1.5 mm or 3 mm laser gap. This is because part of the 4 mm laser gap is beyond the laser heat effected zone. Generally, severe ploughing wear and smearing wear were observed on the tempered bainite zone but not the bainite zone. Debris was generated during the tribo-tests and led to three body abrasive wear which not only resulted in higher wear loss, but also polished the worn surface.

Abstract: The microstructure and mechanical properties of M50 steel subjected to combining cold rolling (CR) with austempering are investigated. The microstructure is characterized using X-ray diffraction and scanning and transmission electron microscopy. The mechanical properties are measured using the uniaxial tensile and Charpy impact tests. It is observed that an excellent combination of ultimate tensile strength (2536 MPa) and impact toughness (128 J) was achieved by combining austempering with CR; of which the ultimate tensile strength increased by 10% compared with traditional martensite quenching-tempering (Q-T) specimen and the impact absorbed energy exhibited 2.3 times of that in Q-T specimen. The observation of the microstructure indicates that CR obviously refines the thickness of bainite sheaves, which is favorable for the formation of ultrafine equiaxed ferrite during tempering.

Abstract: Both ausforming and austempering below the martensite start temperature (MS) have been shown to accelerate the bainitic transformation. This work investigates the combined effects of deformation and austempering below the MS on the isothermal transformation kinetics, microstructure, and hardness in a medium-carbon bainitic steel. The results show that, deformation below the MS can delay the onset of the baintic transformation compared to the case in which no deformation is applied. The larger the amount of martensite presents prior to the deformation step, the greater the delay in the onset of bainitic transformation. This behavior is attributed to the development of strong misorientation gradients within austenite during the co-deformation of the austenite-martensite microstructure. These large misorientation gradients make the pre-existing martensite a less effective nucleation site for bainite.

Abstract: This research presents the results of tribo-fatigue behavior of austempered ductile cast iron MoNiCa and gives a comparison with standard grades of steel and cast iron. Due to the possibility to combine the castability of cast iron and toughness of steel in one material, new structural material MoNiCa attracted attention of industry and science because of economic benefits and high performance at the different application areas. After successful former experiments the main directions of further development of research for solving relevant practical wear and fatigue problems in rail-wheel system were framed. The complex experimental studies have demonstrated that MoNiCa is consistent with heat treated steels including the rail steels: required tensile strength of rail steel ranges from 1180 MPa to 1280 MPa when rolling surface hardness have to be from 38 HRC to 44 HRC whereas new structural material showed higher tensile strength up to 1400 MPa and slightly higher hardness up to 50 HRC. Herewith the workability of frictional couple cast iron MoNiCa/steel 20MnCr5G exceeds work performance of steel/steel system by 14 %.

Abstract: It is an important challenge to reduce the carbon content in nanostructured bainitic steels for commercialization purposes while still being able to gain the desired microstructural characteristics in nanoscale and not to deteriorate the strength–ductility combinations That is the point at which an appropriate heat treatment procedure design would be an important parameter This article aims to investigate how to obtain nanostructured bainite in steel with 026 wt% carbon content by applying multi-step austempering procedures One-, two- and three-step austempering processes have been implemented, and proper heat treatment temperatures and approaches were selected based on dilatometry tests Results indicated that it has become possible to achieve bainitic ferrites and austenite films with overall thicknesses of 164, 145 and 132 nm and 134, 105 and 90 nm at the end of one-, two- and three-step austempering heat treatments, respectively Meanwhile, microstructural characteristics resulted in enhanced mechanical properties with ultimate tensile strength (UTS) of 1435, 1455 and 1428 MPa in combination with elongation levels of 154, 136 and 114% after implementing those heat treatments Finally, it has been shown that applying the multi-step austempering heat treatments resulted in enhanced yield strength and impact toughness values due to the microstructural characteristics and proper heat treatment procedure design

Abstract: The quest for vehicle safety improvement with weight reduction and consequently lower fuel consumption led the automotive industry to begin research into the third generation of advanced high-strength steels. These steels present complex microstructures, composed of martensite, bainite and stable retained austenite. Two of the main treatments for obtaining these microstructures are the low-temperature austempering and quenching and partitioning (Q&P). The objective of this work is to evaluate the microhardness and adhesive wear performance of C-Si-Mn and C-Si-Mn-Nb steels submitted to the treatments mentioned above. Both austempering and the partitioning steps were conducted at Ms+50 °C. Treatment times were 1 and 3 h for austempering and 10, 30 and 60 min for Q&P. Results show that niobium addition promotes improvement in wear performance despite inferior levels of austenite stabilization and changes in the bainite morphology.

Abstract: Medium-carbon dual-phase bainite/ferrite steels with different bainite and carbide volume fractions were prepared via intercritical annealing and austempering. Bainite dominated the microstructure ...

Abstract: The tensile test is an important test to determine several mechanical properties in which a sample is subjected to a constant strain rate until failure. Properties that are directly measured in a tensile test are ultimate tensile strength, breaking strength, maximum elongation, and reduction in area. The present research compared the tensile properties between austempered SAE52100 steel and quench-tempered SAE52100 steel under equivalent hardness. The experimental results showed that the ultimate tensile strength and yield strength of quench-tempered specimens was higher than austempered specimens. However, the total elongation and area reduction of quench-tempered specimens was lower than austempered specimens. Transgranular quasi-cleavage fracture was found on the austempered SAE52100 steel specimens. While using the austempering temperature of 288 °C, slight plastic deformation with existence of cleavage facets and ductile risers around the edges was obtained. However, while using the austempering temperature of 427 °C, the presence of tear marks on the cleavage facets surrounded by dimples indicated significant plastic deformation or ductile rupture. The fracture analysis could be correlated with the elongation obtained in tensile tests.

Abstract: Austempered Ductile Iron (ADI) is widely implemented to achieve lightweight and optimized components in power transmissions (e.g., internal gear in planetary stages), by substituting steel castings or a combination of Ductile Iron and steel-assembled structures. In this paper, the contact fatigue of two ADI grades (ADI J/S900-8; ADI J/S1200-3) is investigated by disk-on-disk tests. The disk-on-disk contact is an analogy model of the tooth contact and represents the tribological conditions at a specific point on the path of contact. The experimental results cover the endurance limit as well as the short time fatigue behavior for both ADI variants. In addition, the pitting characteristics and microstructural changes are analyzed in order to get a further understanding of the fatigue behavior of ADI.

Abstract: A promising martensitic steel with good hardenability is studied. In the cooling rate range 0.1–30°C/sec and the only transformation recorded by a dilatometer starts at an Ms temperature of 355 ± 10°C. Microstructure and mechanical properties of the steel studied are analyzed after various heat treatment regimes: cooling from the austenitizing temperature at various rates (from 0.02 to 20°C/sec), austempering at 280–380°C, and oil quenching and tempering at 200–600°C. It is shown that a 1000-fold change in the cooling rate has a weak effect on steel strength. Bainitic transformation in the steel in question occurs in the martensitic transformation temperature range, which leads to a reduction in impact strength compared with a martensitic microstructure both after slow cooling and isothermal hardening.