Influence of soaking time in deep cryogenic treatment on the microstructure and mechanical properties of low-alloy medium-carbon HY-TUF steel

The influence of soaking time in deep cryogenic treatment on the tensile and impact properties of low-alloy medium-carbon HY-TUF steel was investigated in this study. Microstructural studies based on phase distribution mapping by electron backscatter diffraction show that the deep cryogenic process causes a decrease in the content of retained austenite and an increase in the volume fraction of η-carbide with increasing soaking time up to 48 h. The decrease in the content of retained austenite from ~1.23vol% to 0.48vol% suggests an isothermal martensitic transformation at 77 K. The η-type precipitates formed in deep cryogenic-treated martensite over 48 h have the Hirotsu and Nagakura orientation relation with the martensitic matrix. Furthermore, a high coherency between η-carbide and the martensitic matrix is observed by high-resolution transmission electron microscopy. The variations in macrohardness, yield strength, ultimate tensile strength, and ductility with soaking time in the deep cryogenic process show a peak/plateau trend.

INVESTIGATION OF FRACTURE DESIGN FOR MEDIUM CARBON STEEL UNDER EXTRA-LOW CYCLIC FATIGUE IN AXIAL LOADING

The extra-low cyclic fracture problem of medium carbon steel under axial fatigue loading was investigated. Several problems, such as the relations of the cycle times to the depth and tip radius of the notch, loading frequency, loading range and the parameters of fracture design for medium carbon steel on condition of extra-low axial fatigue loading were discussed based on the experiments. Experimental results indicated that the tension-pressure fatigue loading mode was suitable for extra-low cyclic fatigue fracture design of medium carbon steel and it resulted in low energy consumption, fracture surface with high quality, low cycle times, and high efficiency. The appropriate parameters were as follows： loading frequency 3-5 Hz, notch tip radius r = （0.2-0.3） mm, opening angle α = 60°, and notch depth t = （0.14-0.17）D.

Fretting Wear Behavior of Medium Carbon Steel Modified by Low Temperature Gas Multi-component Thermo-chemical Treatment

The introduction of surface engineering is expected to be an effective strategy against fretting damage. A large number of studies show that the low gas multi-component （such as carbon, nitrogen, sulphur and oxygen, etc） thermo-chemical treatment（LTGMTT） can overcome the brittleness of nitriding process, and upgrade the surface hardness and improve the wear resistance and fatigue properties of the work-pieces significantly. However, there are few reports on the anti-fretting properties of the LTGMTT modified layer up to now, which limits the applications of fretting. So this paper discusses the fretting wear behavior of modified layer on the surface of LZ50 （0.48%C） steel prepared by low temperature gas multi-component thermo-chemical treatment （LTGMTT） technology. The fretting wear tests of the modified layer flat specimens and its substrate （LZ50 steel） against 52100 steel balls with diameter of 40 mm are carried out under normal load of 150 N and displacement amplitudes varied from 2 μm to 40 μm. Characterization of the modified layer and dynamic analyses in combination with microscopic examinations were performed through the means of scanning electron microscope（SEM）, optical microscope（OM）, X-ray diffraction（XRD） and surface profilometer. The experimental results showed that the modified layer with a total thickness of 60 μm was consisted of three parts, i.e., loose layer, compound layer and diffusion layer. Compared with the substrate, the range of the mixed fretting regime（MFR） of the LTGMTT modified layer diminished, and the slip regime（SR） of the modified layer shifted to the direction of smaller displacement amplitude. The coefficient of friction（COF） of the modified layer was lower than that of the substrate in the initial stage. For the modified layer, the damage in partial slip regime（PSR） was very slight. The fretting wear mechanism of the modified layer both in MFR and SR was abrasive wear and delamination. The modified layer presented better wear resistance than the substrate in PSR and MFR; however, in SR, the wear resistance of the modified layer decreased with the increase of the displacement amplitudes. The experimental results can provide some experimental bases for promoting industrial application of LTGMTT modified layer in anti-fretting wear.

Effects of cooling rate and Al on MnS formation in medium-carbon non-quenched and tempered steels

The effect of Al on the morphology of MnS in medium-carbon non-quenched and tempered steel was investigated at three different cooling rates of 0.24, 0.43, and 200°C·s^-1. The formation mechanisms of three types of MnS were elucidated based on phase diagram information combined with crystal growth models. The morphology of MnS is governed by the precipitation mode and the growth conditions. A monotectic reaction and subsequent fast solidification lead to globular Type I MnS. Type II MnS inclusions with different morphological characteristics form as a result of a eutectic reaction followed by the growth in the Fe matrix. Type III MnS presents a divorced eutectic morphology. At the cooling rate of 0.24°C·s^-1, the precipitation of dispersed Type III MnS is significantly enhanced by the addition of 0.044wt% acid-soluble Al（Als）, while Type II MnS clusters prefer to form in steels with either 0.034wt% or 0.052wt% Als. At the relatively higher cooling rates of 200°C·s^-1 and 0.43°C·s^-1, the formation of Type I and Type II MnS inclusions is promoted, and the influence of Al is negligible. The results of this work are expected to be employed in practice to improve the mechanical properties of non-quenched and tempered steels.

Ratcheting led surface failure of medium carbon bainitic steel under mild operation conditions

The behavior of rolling contact fatigue (RCF) of medium carbon bainitic back-up roll steel was investigated under its actual work conditions. A kind of asperity-scale surface originated cracks, which is lying parallel or at an acute angle to the surfaces, initiated after unidirectional plastic flow of the material in thin surface layer had occurred. Theoretical analysis indicates that they nucleate due to plastic ratcheting induced by asperity contact stresses, and consequently are named as ratcheting cracks. After nucleating and initially propagating, they arrest at some depth and resume propagating till about 70%-80% of the RCF failure life by initially turning parallel to contact surfaces. Their behavior of initiation and propagation is confined to a thin layer prior to the formation of surface distress. According to the critical principle of the preventive grinding strategy, removing the asperity influenced surface layer at about 70%-80% of the RCF failure life can effectively prevent the ratcheting cracks from developing into surface distress, which can lead to the formation of macro-RCF failure soon.

Corrosion Behaviour of Heat Treated Rolled Medium Carbon Steel in Marine Environment

Investigation were carried out to study the corrosion behaviour of heat treated rolled medium carbon steel and as-rolled medium carbon steel in sodium chloride medium. The as-rolled medium carbon steel was heated to a temperature of 830℃ to completely austenize it and water quenched;it was reheated to the ferrite-austenite dual phase region at a temperature of 745℃ below the effective Ac3 point. The steel was then rapidly quenched in water and tempered at a temperature of 480℃. The corrosion behaviour of the steel in marine medium (NaCl) was studied by weight loss measurement. The weight loss is between 0.02g-0.11g for the as-rolled steel and 0.01g – 0.013g for the heat treated steel. The results obtained showed that the as-rolled medium carbon steel is more susceptible to corrosion than the heat treated rolled medium carbon steel.

Effect of Boron on Delayed Fracture Resistance of Medium-Carbon High Strength Spring Steel

The delayed fracture behavior of medium carbon high strength spring steel containing different amounts of boron （0. 000 5%, 0. 001 6 %） was studied using sustained load delayed fracture test. The results show that delayed fracture resistance of boron containing steels is higher than that of conventional steel 60Si2MnA at the same strength level and it increases with the increase of boron eontent from 0. 000 5% to 0. 001 6%. The delayed fracture mode is mainly intergranular in the boron containing steels tempered at 350℃, which indicates that the addition of boron does not change the fracture character. However, the increase of boron content enlarges the size of the crack initia tion area. Further study of phase analysis indicates that most boron is in solid solution, and only a very small quantity of boron is in the M3 （C, B） phase.

Effect of RE-Modifier on Microstructure and Mechanical Property of High-Carbon Medium-Manganese Steel

The effect of RE-modifier on the microstructure and mechanical properties of high carbon-medium manganese steel has been investigated in present work.The results showed that the RE-modifier can refine the crystalline grain of high-carbon medium-manganese steel.The shape and distribution of carbides are improved and the columnar grains and phosphide in grain boundary are eliminated.Consequently,the impact toughness of the steel is increased by more than one time,compared with no addition of RE-modifier.

Cold extrusion deformation behavior of medium carbon steel after quenching and tempering

By taking 40Cr as a specific object, cold extrusion deformation behavior of medium carbon steel after quenching and tempering was studied by experimental works. The influence of deformation extent (10%-50%), cone angle of die (90 °-120 °), hardness after quenching and tempering (HRC21-29) and lubricated condition on the forming load was analyzed. The results show that there is no central bursting and micro crack in the inner of the extruded specimen, and the forming quality is good. The double-peak phenomenon takes place at the front-end of the specimen; the double-peak index increases with deformation extent, and larger deformation can avoid the double-peak phenomenon. The deformation extent is the most important influencing factor, and the lubricated condition almost has no influence, which means that the phosphate coating plus soap process is still a proper lubrication method for cold extrusion of medium carbon steel after quenching and tempering. By investigating the microscopic structure before and after deformation, the initial equiaxed grain is elongated in the extrusion direction, and this feature is more significant at the front-end of specimen.

PHASE TRANSFORMATION UNIT OF BAINITIC FERRITE AND ITS SURFACE RELIEF IN LOW AND MEDIUM CARBON ALLOY STEELS

The lath-or plate-shaped bainitic ferrite of low and medium carbon alloy steels consists of packets of ferrite sublaths which are composed of many finer and regular ferrite blocks.They are uniform shear growth units of bainitic phase transformation.No carbide is precipitated from them.The bainitic O-carbides are precipitated from γ-α interface or carbon-rich austenite.The mode of arrangement of the units in ferrite sublath packet is in uni-or bi-di- rection.Single surface relief is produced by the accumulation of uniform shear strains with all the ferrite units arranged unidirectionally in a sublath packet,while tent-shaped surface relief is formed by the integration of the uniform shear strains of two groups with ferrite units piling up in two directions and growing face to face;whereas if they grow back to back,the integra- tion will be responsible for invert-tent-shaped surface relief.The interface trace between two groups of ferrite units in a sublath packet is shown as“midrib”.

Impact property analysis of weld metal and heat-affected zone for low-alloy carbon steel multi-pass welded joint

Welded joint impact performances of low-alloy carbon steel plates welded by full-automatic gas metal arc welding （GMAW） were evaluated. To clarity the effect of impact temperature on impact properties of weld metal （WM） and heat- affected zone （ HAZ）, Charpy V impact tests at different temperatures and fracture surface analysis were carried out. The Charpy V impact energy decreases with the decreasing test temperature both for the WM and HAZ, while the proportion of crystal zone on WM and HAZ impact fracture surface increases with the decreasing test temperature. Research results indicate that the welding defects （void and slag） make the impact energy of WM more scattered and lower than that of HAZ.

Effect of Heat Treatment Processes on the Mechanical Properties of Medium Carbon Steel

The importance of various form of heat treatment operations on medium carbon steel in order to forester the problem that may arise in making a wrong choice of these steel materials or faulty heat treatment operations which may give rise to serious disruption in terms of human safety, higher cost and untimely failure of the machine components is of great concern. The mechanical properties such as ductility, toughness, strength, hardness and tensile strength can easily be modified by heat treating the medium carbon steel to suit a particular design purpose. Tensile specimens were produced from medium carbon steel and were subjected to various forms of heat treatment processes like annealing, normalizing, hardening and tempering. The stiffness, ductility, ultimate tensile strength, yield strength and hardness of the heat treated samples were observed from their stress-strain curve. The value of the yield strength (σy) was observed to be higher for the tempered specimen possibly as a result of the grain re-arrangement, followed by the hardened, normalized and annealed specimens. The value of the ultimate tensile strength (σu) were also observed to be in the order;hardened> tempered>normalized>annealed.

Evaluation of Mechanical Properties of Medium Carbon Steel Quenched in Water and Oil

Samples of medium carbon steel were examined after heating between 900℃ - 980℃ and soaked for 45 minutes in a muffle furnace before quenching in palm oil and water separately. The mechanical behavior of the samples was investigated using universal tensile testing machine for tensile test and Vickers pyramid method for hardness testing. The microstructure of the quenched samples was studied using optical microscope. The tensile strength and hardness values of the quenched samples were relatively higher than those of the ascast samples, suggesting improved mechanical properties. However, samples quenched in palm oil displayed better properties compared with that of water-quenched samples. This behavior was traced to the fact that the carbon particles in palm oil quenched samples were more uniform and evenly distributed, indicating the formation of more pearlite structure, than those quenched in water and the as-received samples.

Texture of Rotary-Friction-Welded from Dissimilar Medium Carbon Steels

The purpose of the present study was to investigate the texture in dissimilar medium carbon steels welded by rotary friction technique. The Electron Backscatter Diffraction (EBSD) technique was the main technique used to investigate the effect of welding on grain size and grain crystallographic orientation in the welded joint. Moreover, the effect of isothermal heat treatment at 600°C on welded joint has been studied knowing that this annealing allows to decrease the residual stresses. EBSD results revealed different subzones in welded joint. The texture in the weld is essentially composed of three components: Goss {110} <001>, Rotated Cube {100} <110>, and Rotated Goss {110} <110> orientation. The heat treatments applied on welded material had a slight effect on texture and grain size.

Development of Microalloyed Medium-Carbon Steel with Improved Toughness

The application of the hot-rolled micronlloyed medium-carbon steel is limited due to its lower toughness compared with quenched and tempered steels.A high strength microalloyed medium-carbon steel has been developed to meet the increasing demand for eliminating heat treatment.The effects of chemical compositions,hot-deforming temperatures and cooling rates after hot-deforming on mechanical properties of the steel were investigated.Attention was given to the steel toughness above the required value for safety components in application.The basic microstructural variables controlling the properties were discussed.It is demonstrated that toughness and machinability can be improved through the addition of 0.5%Si,0.5%S and 0.01%Ti.Toughness can be further improved by controlling hot-deforming temperature and subsequent cooling rate.

The Study on the Corrosion Behaviour of Welded and Unwelded Medium Carbon Steel in Sodium Chloride(Nacl)Solutions

The research work was based on the study of the corrosion behaviour of the welded and un-welded medium carbon steel in sodium chloride solutions.The Sodium chloride solutions used are 1ml,2ml,3ml and 4ml for both welded and un-welded medium carbon steel in NaCl.The experiments were conducted in two ways,the weight loss analyses of measurements and using the electrochemical analyzer workstation to determine the potential dynamic of the samples.The samples for the weight loss measurements were prepared from rolled products obtained at the foundry shop.Two medium carbon steel materials were sourced with different chemical compositions as sample A and B.The materials were prepared to accommodate the experiments for the determination of welded and un-welded medium carbon steel.A total of sixty-eight(68)samples were produced,prepared and used for the weight loss measurements/analyses the experiments.Thirty-four of the samples each were prepared for both the welded and un-welded experiments.All the samples were produced and prepared through the use of various machining processes with the use of a lathe machine for planning,milling.Thirty-four(34)of the sample preparation were further welded in readiness of the experiments.Sixty-eight breakers were sourced for and used.Ten(10)other samples were used for the determination with the use of the electrochemical analyzer.The chemical compositions of the medium carbon steel were determined with the use of SPECTRO Analytical Instruments.A metallurgical inverted optical microscope was used to determine the microstructures of the materials.The Scanning Electron Microscopy with EDS was used to determine the morphologies of the materials.The thirty-four of the samples were welded this process was performed to determine the effects of welding on the material surrounding the weldments.These materials were made into sizes with the use of power hacksaw(i.e.2cm by 2cm).Other materials were prepared to 1cm x 1cm thickness from the same materials.The Tafel plot experiments and that of the open Circuit Potential Time(OCPT)were carried out with the use of Electrochemical Analyzer/Workstation.The Medium carbon steel materials were exposed for fifty-four(54)days,with an interval of 3days.The corrosion rates analyses were determined and the graphs of the corrosion rates(mm/yr.)and other parameters were used plotted against No of days exposed.

Hot deformation behavior of medium carbon V-N microalloyed steel

Processing maps for a medium carbon V-N microalloyed steel(designated as VN steel) and a medium carbon V-N bared steel(designated as Non-VN steel) were developed to study the hot deformation behavior and the influence of vanadium and nitrogen, in the temperature range of 750-1 100 ℃ and strain rate range of 0.005-30 s-1. Experimental results show that the processing map for the VN steel exhibits two dynamic recrystallization and three instability domains, while that for the Non-VN steel has one dynamic recrystallization and three instability domains. The instability domains of VN steel are larger than those of the Non-VN steel, and the VN steel is easier to be unstable when being hot deformed at high temperature and high stain rate. The addition and precipitation of vanadium and nitrogen can hinder the dynamic recrystallization. Compared with the Non-VN steel, the VN steel has higher dynamic recrystallization critical strain and the corresponding stress.

Ductile Fracture Characterization for Medium Carbon Steel Using Continuum Damage Mechanics

This paper presents the ductility characterization for a medium carbon steel, for two microstructural conditions, that has been evaluated using the continuum damage mechanics theory, as proposed by Kachanov and developed by Lemaitre. Tensile tests were carried out using loading-unloading cycles in order to capture the gradual deterioration of the elastic modulus, which may be linked to the ductile damage increase with increasing plastic strain. The mechanical parameters for the isotropic damage evolution equation were obtained and then used as inputs for a plasticity-damage coupled nu- merical algorithm, validated through numerical simulations of the experimental tensile tests. A comparison between the SAE 1050 steels studied and two carbon steel alloys (obtained from the literature), provided some basic understanding of the influence of the carbon level on the evolution of the damage parameters. An empiric relationship for this set of parameters, which can provide useful data for preliminary studies envisaging prediction of ductile failure in carbon steels, is also presented.

Quasi-Static and Dynanmic Deformation Behaviors of Medium-Carbon Steels in a Wide Temperature Range

This paper presents a study of the quasistatic and dynamic deformation behaviors of conventional and microalloyed medium-carbon steels in a wide temperature range. As strain rate increased, the flow stress increased at room temperature, but occasionally did not at elevated temperatures. The flow stress of the microalloyed steel containing precipitates was less sensitive to strain rate at room temperature than that of the conventional steel due to a relatively larger activation length. Microstructural observation of the steels deformed after compression test indicated that inhomogeneous deformation became more serious with increasing strain rate and temperature without fracturing in the highly localized region.

Hardening Characteristics of Plain Carbon Steel and Ductile Cast Iron Using Neem Oil as Quenchant

The hardening characteristics of medium carbon steel and ductile cast iron using neem oil as quenching medium has been investigated. The samples were quenched to room temperature in Neem oil. To compare the effectiveness of the neem oil samples were also quenched in water and SAE engine oil the commercial quenchants. The microstructures and mechanical properties of the quenched samples were used to determine the quench severity of the neem oil. The result shows that hardness value of the medium carbon steel increased from 18.30HVN in the as-cast condition to 21.60, 20.30and 20.70HVN while that of ductile cast iron samples increased from 18.90HVN in the as-cast condition to 22.65, 20.30 and 21.30HVN for water, neem oil and SAE40 engine oil respectively. The as-received steel sample gave the highest impact strength value and water quenched sample gave the least impact strength. The impact strength of the medium carbon steel samples is 50.84, 41.35, 30.50 and 45.15 Joule and that of ductile iron is 2.71, 1.02, 0.68 and 1.70 Joule for as-cast condition, neem oil, water and SAE 40 engine oil quenched respectively. The microstructure of the samples quenched in the Neem oil revealed the formation of martensite. Hence, neem oil can be used where cooling severity less than that of water but greater than SAE 40 engine oil is required for hardening of plain carbon steels and ductile cast iron.