Austenite Grain Size Research Articles

Effect of ultrasonic vibration on microstructural evolution and mechanical properties of underwater wet welding joint

The propagation of high-intensity ultrasonic vibration into weld pool can influence on microstructure and mechanical properties of welded joints. This research introduced a hybrid method for improving weld quality by using superimposed ultrasonic vibration on workpiece directly in underwater flux cored arc welding (FCAW). With the purpose of revealing how macrostructure, microstructure and mechanical properties of welded joints are affected by ultrasonic vibration within the weld metal, the present study has been carried out. The results indicated that proeutectoid ferrite, ferrite side plate and acicular ferrite constitute the microstructures of weld metal. Ultrasonic vibration can affect the size and morphology of prior austenite grain during weld pool solidification and eventually accelerate the formation of fine ferrite microstructure of weld metal in high proportion in the subsequently solid-state phase transformation. Excellent balance of high tensile strength and impact toughness is realized as more fine columnar microstructures in weld metal with using ultrasonic vibration. These findings suggest that the addition of ultrasonic vibration is an effective way to improve the tensile strength and impact toughness of weld metal during underwater wet welding.

Microstructure and Mechanical Properties of 9CrNB Steel after Heat Treatment

This paper deals with the analysis of microstructure in the conditions after austenitising at high temperatures (1150 °C, 1180 °C a 1210 °C/45 min) for 9CrNB steel, and with the analysis of microstructure in the conditions after subsequent tempering at temperatures 740 °C, 770 °C, 800 °C/1 hr. After austenitising the microstructure was formed of lath martensite with secondary carbides. The average size of initial austenitic grains ranged from 144 μm (1150 °C) to 163 μm (1210 °C). Increasing the austenitising temperature led to insignificant improvement in strength properties, while the plasticity of the steel decreased. Microstructure in tempered conditions was formed from tempered martensite. This martensite had lath morphology, with inhomogeneus distribution of precipitates. Higher austenitising temperature and increased tempering temperature led to spheroidal and growth of secondary particles. The tempering effect on strength and plastic properties of steel are also described.

Chemical and Microstructural Characterization of Modular Femoral Stems Manufactured in ISO 5832-9 Stainless Steel

The aim of this study is to evaluate and compare the chemical composition and microstructure of two femoral stems manufactured in ISO 5832-9 austenitic stainless steel by different manufacturers. Both the femoral stems under study are used in Brazil. Chemical analyses of the elements were performed by glow discharge optical emission spectrometry. The microstructure of the stems was characterized by optical and scanning electron microscopy and x-ray diffraction. Vickers hardness measurements were also taken. Both stems exhibited small amounts of Z-phase dispersed in the austenitic matrix and had similar grain sizes of austenite. The hardness was 46% higher in one stem, which was attributed mainly to the higher nitrogen content of this stem. The results also showed that small variances, although still within the requirements of the standard, produce differences of the Z-phase, grain size, and hardness of the steels. The control of these characteristics has clinical relevance because it has a direct influence on fatigue and corrosion resistance.

SURFACING OF DETAILS OF MINING EQUIPMENT BY POWDER WIRES OF C – Si – Mn – Mo – V – B AND C – Si – Mn – Cr – Mo – V SYSTEMS

A new composition of the powder wires based on the C – Si – Mn – Mo – V – B and C – Si – Mn – Cr – Mo – V doping systems represented by steels 40GMFR and 40Kh3G2MF was developed. The influence of alloying elements in the composition of C – Si – Mn – Mo – V – B and C – Si – Mn – Cr – Mo – V powder wires on the size of the martensite needles was studied, as well as the primary austenite grain size and the contamination of the weld layer with non-metallic inclusions. The infl uence of the structure on the strength and durability of the deposited metal was defi ned. It is shown that an increase in the content of carbon and alloying elements, in particular chromium, in the composition of the welded steel contributes to the formation of a structure with a medium-carbon fi ne-grained martensite and a small volume fraction of δ-ferrite. It provides reduction in size of the primary austenite grain and decrease in the contamination degree of the deposited layer with non-metallic inclusions. The established improvement in the structure aff ects the hardness increase up to 22 % and the decrease in the abrasion rate of the deposited layer up to 34 %. The chemical composition of samples welded with a fl ux-cored wire of C – Si – Mn – Mo – V – B system is characterized by a lower content of carbon and alloying elements compared to a 40Kh3G2MF steel, which causes the formation of low-carbon martensite in the structure after surfacing. It was established that a signifi cant increase in the content of carbon, manganese, chromium, vanadium and molybdenum in 40GMFR welded steel provides a structure with fine needle and medium needle martensite, reduces the size of the primary austenite grain and the contamination level of the deposited layer with non-metallic inclusions, in particular, non-deformed silicates. Changes in the micro structure are accompanied by an increase in hardness up to 16 % and a decrease in the attrition rate of the deposited layer up to 20 %. As a result of a comparative analysis of the two studied systems of powdered wires, it was found that it is more eff ective to use C – Si – Mn – Cr – Mo – V wire for surfacing mining equipment, since the content and ratio of alloying elements in steel of type 40Kh3G2MF contributes to obtaining a dispersed martensitic structure with an insignifi cant volume fraction of δ-ferrite, which provides high hardness and wear-resistance of the deposited layer.

ANALYSIS OF HEAT TREATMENT EFFECT ON MICROSTRUCTURAL FEATURES EVOLUTION IN A MICRO-ALLOYED MARTENSITIC STEEL

The microstructural evolution of a quenched and tempered medium-C micro-alloyed steel during tempering is analyzed The steel was heat treated in order to develop fully martensitic microstructures after quenching with different prior austenite grain sizes (AGS). Main results are a very poor effect of AGS on packet size is found, as well as, high-angle boundary grains do not significantly grow after tempering; on the contrary, low-angle grain boundaries (cells) move, fully justifying the hardness evolution with tempering temperature.

Effect of Multipass Friction Stir Processing on Mechanical and Corrosion Behavior of 2507 Super Duplex Stainless Steel

The microstructure, mechanical properties, and corrosion behavior of 2507 super duplex stainless steel after multipass friction stir processing (FSP) were examined. A significant refinement in grain size of both ferrite and austenite was observed in stir zone resulting in improved yield and tensile strength. Electrochemical impedance spectroscopy and anodic polarization studies in 3.5 wt.% NaCl solution showed nobler corrosion characteristics with increasing number of FSP passes. This was evident from the decrease in corrosion current density, decrease in passive current density, and increase in polarization resistance. Also, the decrease in density of defects, based on Mott-Schottky analysis, further confirms the improvement in corrosion resistance of 2507 super duplex stainless steel after multipass FSP.

Stainless Steel Microstructural Evolution of Hot-Rolled Clad Plate

The stainless steel microstructure evolution of carbon–stainless-clad steel plate was investigated during vacuum hot-rolling bonding under different deformation conditions. The results show that carbide M2C precipitates in the interior of stainless steel (SS) and carbon content increases with rising reduction ratio (ε). The dislocation density of the SS surface (ρI) is lower than that of the midst (ρM), and ρI decreases with the rising ε. However, ρM increases first and then decreases with rising ε. The dislocation density of bonding interface decreases due to the increasing size of austenite grain. Furthermore, the dislocation density of the midst is high where the high energy of carbide M2C is concentrated for single-pass rolling, and the quantity of M2C increases with reduction ratio. Moreover, carbide almost disappeared while being transformed into austenite, and only minimal granular carbides were formed after two-pass rolling. Bonding strength increases evidently with rising ε and is inversely proportional to ρI.DOI: http://dx.doi.org/10.5755/j01.ms.22.4.12828

Mechanisms of cast structure and stressed state formation in Hadfield steel

The paper describes the investigation of mechanisms of cast structure formation in Hadfield steel depending on the changes in the cooling rate of a casting in the following two temperature ranges: crystallization temperature (1,200-1,390 °С) and the temperature of excessive phase separation (560-790 °С). Changes in the cooling rate of the crystallization temperature range from 1.1 to 25.0 °С•s-1 result in the reduction of the average size of austenite grains from 266 to 131 μm. At the same time, the magnitude of developing shrinkage stresses changes from +195 to 0 MPa. When the cooling rate is higher than 16 °С•s-1, no shrinkage stresses are formed in the casting. Changes in the cooling rate of the casting in the temperature range of the excessive phase separation influence the number of phases, their morphology and chemical composition, the values of phase stresses, and the possibility of martensitic transformation. Changing in the cooling rate from 0.24 to 5.46 °С•s-1 results in the decrease of the amount of the excessive phase from 14.8% to 2.1%, which is composed of eutectic and carbides depending on the cooling rate, their quantitative ratio and morphology change. Such changes in the microstructure are reflected on the changes of value of developing phase stresses. When the cooling rate is 0.24 °С•s-1, it is +100 MPa, while the increase of the cooling rate to 1.4 °С•s-1 results in the decrease of tensile stresses to 0 MPa and their qualitative stresses change to compressive ones. Further increase of the cooling rate results in the increase of the value of compressive stresses. When the cooling rate is 5.5 °С•s-1, their value reaches -92 MPa. Martensite forming in the structure of Hadfield steel is possible if the cooling rate of the casting in the range of excessive phase separation is less than 0.25 °С•s-1.

Effects of welding and post-weld heat treatments on nanoscale precipitation and mechanical properties of an ultra-high strength steel hardened by NiAl and Cu nanoparticles

The effects of welding and post-weld heat treatment (PWHT) on nanoscale co-precipitation, grain structure, and mechanical properties of an ultra-high strength steel were studied through a combination of atom probe tomography (APT) and mechanical tests. Our results indicate that the welding process dissolves all pre-existing nanoparticles and causes grain coarsening in the fusion zone, resulting in a soft and ductile weld without any cracks in the as-welded condition. A 550 °C PWHT induces fine-scale re-precipitation of NiAl and Cu co-precipitates with high number densities and ultra-fine sizes, leading to a large recovery of strength but a loss of ductility with intergranular failure, whereas a 600 °C PWHT gives rise to coarse-scale re-precipitation of nanoparticles together with the formation of a small amount of reverted austenite, resulting in a great recovery in both strength and ductility. Our analysis indicates that the degree of strength recovery is dependent mainly upon the re-precipitation microstructure of nanoparticles, together with grain size and reversion of austenite, while the ductility recovery is sensitive to the grain-boundary structure. APT reveals that the grain-boundary segregation of Mn and P may be the main reason for the 550 °C embrittlement, and the enhanced ductility at 600 °C is ascribed to a possible reduction of the segregation and reversion of austenite.

The effect of initial microstructure on the final properties of press hardened 22MnB5 steels

This paper addresses the relationship between initial microstructure and final properties of press hardened 22MnB5 steels. Four commercial 22MnB5 steels having different initial microstructures were investigated. An experimental press hardening equipment with a flat-die was used to investigate material behavior in the direct press hardening process. Two austenitizing treatments, 450s and 180s at 900°C, were examined. Microstructural characterization with optical and scanning electron microscopes revealed a mixture of martensite and auto-tempered martensite after press hardening. Electron backscatter diffraction data of the transformed martensite was used to reconstruct grain boundary maps of parent austenite. Grain sizes of parent austenite (mean linear intercept) were measured for each material. In addition to microstructural evaluation, quasistatic and high strain rate tensile tests at strain rates of 5×10−4s−1 and 400s−1, respectively, were performed for press hardened samples. The results show that strength and uniform elongation depend on the initial microstructure of the 22MnB5 steel, when parameters typical to the direct press hardening process are used. Parent austenite grain size was shown to influence the morphology of the transformed martensite, which in turn affects the strength and uniform elongation after press hardening. The tensile properties of the press hardened materials are almost strain rate independent in the studied strain rate range. The obtained results can be used to optimize the properties of 22MnB5 steels in the direct press hardening process. In addition, the here revealed connection between the parent austenite grain size and final steel properties should be taken into account in the development of new press hardening steel grades for automotive industry.

Effect of quenching temperature on martensite multi-level microstructures and properties of strength and toughness in 20CrNi2Mo steel

The martensite multi-level microstructures of 20CrNi2Mo steel, which were quenched at the different temperatures of 900–1200°C and tempered at 200°C, were investigated by optical microscope (OM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM), and the relationship between the microstructures and properties of strength and toughness was discussed by the classic formula of Hall–Petch. The results show that the size of prior austenite grain (dr), martensite packet (dp) and block (db) increase with increasing of the quenching temperature, while the martensite lath (dl) size is opposite. On another hand, the confusion degree of the martensite packets changes from disorder to order. The boundaries of prior austenite grain, packet, block and the martensite lath are high angle boundaries (HBs) and low angle boundaries (LBs), respectively, and the ratio of the low angle boundaries increase with the quenching temperature by calculating to the multi-level microstructure size with the mathematical model established by myself. In addition, the relationship between the packet/block and strength follows the classical formula of Hall–Petch, and the size of db is far lower than the size of dp, db is the effective control unit of the strength. Meanwhile, dl is the effective control unit of toughness because it strongly impacts the crack initiation and propagation and follows also the Hall-Petch with toughness in 20CrNi2Mo steel.

Object kinetic Monte Carlo study of the effect of grain boundaries in martensitic Fe–Cr–C alloys

Fe–Cr–C alloys with chromium concentrations in the range from ∼2 to 12 wt.% form ferritic‐martensitic structures by rapid cooling from the austenite state already in the presence of relatively low carbon concentrations. In this process it is possible to obtain different ratios of ferrite and martensite, as well as formation of carbides, by varying the thermal treatment. The presence of ferrite or martensite might have an influence on the nanostructural evolution under irradiation of these alloys. Here, considering a tempered martensite reference alloy with 9% Cr, we make use of an already validated object kinetic Monte Carlo (OKMC) model in order to study the possible effect of the formation of martensite laths on the material nanostructural evolution under neutron irradiation, assuming that the relevant boundaries act as sinks for radiation defects. The results show that the reduction of the grain size (including in this definition the average size of prior austenite grains, packets, blocks, and laths) does not play any relevant role until sizes of the order of ∼0.5 µm are reached: for smaller grains, the number of defects being absorbed by the boundaries becomes dominant. However, this threshold is lower than the experimentally observed martensite lath dimensions, thereby suggesting that what makes the difference in martensitic Fe–Cr–C alloys with respect to ferrite, concerning events and mechanisms taking place during irradiation, are not the lath boundaries as sinks. Differences between the nanostructural evolution under neutron irradiation in ferrite and martensite should therefore be ascribed to other factors.

Austenite grain growth behavior of a GCr15 bearing steel cast billet in the homogenization heat treatment process

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Effect of large-size M23C6-type carbides on the low-temperature toughness of martensitic heat-resistant steels

The relationship between the low-temperature toughness of and the microstructures found in a martensitic heat-resistant steel 10Cr12Ni3Mo2VN was studied using heat treatment and microscopic microstructure analysis. This study discusses the effect of large-size M23C6 type carbides on the low-temperature toughness. Recent results show that the large-size M23C6-type carbides that remain undissolved during austenitizing have a notably adverse effect on the low-temperature toughness after quenching and tempering. The large-size M23C6-type carbides form during normalization and can be eliminated by austenitizing at temperatures of 1050 °C and above. Refinement of prior austenite grains can markedly improve the low-temperature toughness. To meet the requirement of FATT50≤−35 °C in 10Cr12Ni3Mo2VN steel, the average size of prior austenite grains should be less than 65 μm. No large-size M23C6 type carbides should be present after quenching, and the tempering temperature should be 690 °C.

Enhanced intergranular corrosion resistance and tensile strength in 304 stainless steel with dispersed nanocrystallines in microcrystalline austenite

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Effects of rare earth oxide Y2O3 on microstructure and mechanical properties of proeutectoid ferrite/granular bainitic coating

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Effect of Prior Austenite Grain Size Refinement by Thermal Cycling on the Microstructural Features of As-Quenched Lath Martensite

Abstract Current trends in steels are focusing on refined martensitic microstructures to obtain high strength and toughness. An interesting manner to reduce the size of martensitic substructure is by reducing the size of the prior austenite grain (PAG). This work analyzes the effect of PAGS refinement by thermal cycling on different microstructural features of as-quenched lath martensite in a 0.3C-1.6Si-3.5Mn (wt pct) steel. The application of thermal cycling is found to lead to a refinement of the martensitic microstructures and to an increase of the density of high misorientation angle boundaries after quenching; these are commonly discussed to be key structural parameters affecting strength. Moreover, results show that as the PAGS is reduced, the volume fraction of retained austenite increases, carbides are refined and the concentration of carbon in solid solution as well as the dislocation density in martensite increase. All these microstructural modifications are related with the manner in which martensite forms from different prior austenite conditions, influenced by the PAGS.

Effect of Austenite Stability on Microstructural Evolution and Tensile Properties in Intercritically Annealed Medium-Mn Lightweight Steels

The microstructural evolution with varying intercritical-annealing temperatures of medium-Mn (α + γ) duplex lightweight steels and its effects on tensile properties were investigated in relation to the stability of austenite. The size and volume fraction of austenite grains increased as the annealing temperature increased from 1123 K to 1173 K (850 °C to 900 °C), which corresponded with the thermodynamic calculation data. When the annealing temperature increased further to 1223 K (950 °C), the size and volume fraction were reduced by the formation of athermal α′-martensite during the cooling because the thermal stability of austenite deteriorated as a result of the decrease in C and Mn contents. In order to obtain the best combination of strength and ductility by a transformation-induced plasticity (TRIP) mechanism, an appropriate mechanical stability of austenite was needed and could be achieved when fine austenite grains (size: 1.4 μm, volume fraction: 0.26) were homogenously distributed in the ferrite matrix, as in the 1123 K (850 °C)—annealed steel. This best combination was attributed to the requirement of sufficient deformation for TRIP and the formation of many deformation bands at ferrite grains in both austenite and ferrite bands. Since this medium-Mn lightweight steel has excellent tensile properties as well as reduced alloying costs and weight savings, it holds promise for new automotive applications.

Microstructure and Properties of Weld CGHAZ under Different Heat Input for X90 Pipeline Steel

Two kinds of industry trial X90 pipeline steels which have different chemical composition were chosen as test objects, and the grain coarsening, microstructural characteristics and the variation rules of low-temperature impact toughness in weld CGHAZ of this two steel under different welding heat input were studied by physical thermal simulation technology, SEM, optical microscope and Charpy impact tests. The results showed that the microstructure in weld CGHAZ of 1# steel was mainly bainite ferrite (BF) and most of the M/A constituents were blocky or short rod-like; the grains of 2# steel were coarse and there was much granular bainite (GB). Meanwhile M/A constituents became coarser and their morphology changed from block to long bar; alloy content of X90 pipeline steel under different weld heat input had great effect on the grain size of original austenite. When heat input was lower than 20KJ/cm, the impact toughness in CGHAZ of lower alloy content pipeline steel was good; as heat input increased, impact toughness in CGHAZ of 1# steel increased to the values between 260J and 300J when heat input was between 20KJ/cm and 25KJ/cm and the dispersion of impact energy was small. The impact toughness of 2# steel decreased gradually and the impact energy had the obvious dispersion.

Effect of Immersion Routes on the Quenching Distortion of a Long Steel Component Using a Finite Element Model

Distortion while quenching steel is a commonly found problem in industrial practice. This issue arises from the combined effect of thermal contraction and martensite transformation stresses. This problem may be exacerbated in components with long geometries. This work presents the results of a numerical and experimental investigation that assessed the effect of both the austenite grain size (AGS) of three sizes 8, 9 and 10 ASTM, and the immersion route on the distortion of a long component of SAE 5160 steel during oil quenching. The transformation kinetics were calculated using JMatPro and were validated by quench dilatometry. A three-dimensional finite element model was developed in Deform 3D. This model was then validated using thermocouple data. Results showed that the distortion was minimized when the long component was dipped sideways in the quenching medium instead of immersing the component from the distal ends when the AGS was 8 ASTM.