Austenite Grain Size Research Articles

Effect of Ti–Mg Treatment on the Impact Toughness of Heat Affected Zone in 0.15%C–1.31%Mn Steel

The influence of Ti–Mg and Ti on the impact toughness of the heat affected zone in steel has been studied comparatively. The changes of microstructure and grain size in the heat affected zone are analyzed. It has been found that the content of acicular ferrite in the steel treated by Ti–Mg is more than that in the steel treated by Ti. The size of the primary austenite grain in Ti–Mg treated steel is smaller than that in the steel treated by Ti. During the welding thermal cycle, the second phase particles in steel can be re‐dissolved and coarsened. After welding, the second phase particles in Ti–Mg treated steel are smaller than those in Ti treated steel, and the number of the second phase particles in Ti–Mg treated steel is more than that in Ti treated steel. The pinning effect of the second phase particles in Ti–Mg treated steel is obvious stronger than that in Ti treated steel. The impact toughness of the heat affected zone has a significant improvement after the addition of Ti–Mg.

Key criterion for achieving giant recovery strains in polycrystalline Fe-Mn-Si based shape memory alloys

In this study, it is proposed that coarsening austenitic grains is a key criterion for achieving giant recovery strains in polycrystalline Fe-Mn-Si based shape memory alloys. In order to verify the hypothesis, the relationship between recovery strains and austenitic grain-sizes in cast and processed Fe-Mn-Si based shape memory alloys was investigated. The recovery strain of cast Fe-19Mn-5.5Si-9Cr-4.5Ni alloy with the coarse austenitic grains of 652µm reached 7.7% while the recovery strain of one with the relatively small austenitic grains of 382µm was only 5.4%. Moreover, a recovery strain of 5.9%, which is the highest previously published value for solution-treated processed Fe-Mn-Si based shape memory alloys, was obtained by coarsening the austenitic grains through only solution treatment at 1483K for 360min in a processed Fe-17Mn-5.5Si-9Cr-5.5Ni-0.12C alloy. However, its recovery strain was still 5.9% after thermo-mechanical treatment consisting of 10% tensile strain at room temperature and annealing at 1073K for 30min. This happens because annealing twins play a negative role, refining the austenitic grains, limiting the recovery strains to below 6%. In summary, coarse austenitic grains enable the achievement large recovery strains by two mechanisms. Firstly, the grains are bigger, and consequently there are fewer grain boundaries, and thus their suppressive effects of grain boundaries on stress-induced ε martensitic transformation is reduced. Secondly, coarse austenitic grains are advantageous to introduce ε martensite with single orientation and reduce the collisions of different martensite colonies, especially when the deformation strain is large. As such, the ceiling of recovery strains is dependent on the austenitic grain-sizes.

Tensile properties and deformation mode of Si-added Fe-18Mn-0.6C steels

The effects of Si concentration and austenite grain size (AGS) on the tensile properties, stacking fault energy (γ), and deformation mode of Fe-18Mn-0.6C (wt.%) steel were investigated to improve the yield strength (YS) of twinning-induced plasticity (TWIP) steel. The 3% Si-added steel revealed the higher YS than previous TWIP steels at the same level of AGS. In particular when the AGS was ∼6.8 μm, its YS reached ∼593 MPa, which is comparable to the YS (613 MPa) of transformation-induced plasticity steel with a tensile strength of 980 MPa. The measured γ of 3% Si-added steel was exponentially decreased with grain coarsening primarily due to the reduction of micro-strain, finally reaching its intrinsic γ (γint) at the AGSs above ∼70 μm. This indicates that the γint measurement by means of X-ray diffractometry must be performed using coarse-grained specimens with the AGSs above ∼70 μm. Critical resolved shear stresses for twinning (τtwin) and ε-martensitic transformation (τε-mart) were evaluated as a function of AGS in (0-3%) Si-added steels. Whereas the τtwin value was slightly decreased with increasing AGS or Si concentration, the τε-mart value was more significantly reduced. This indicates that ε-martensitic transformation precedes mechanical twinning with increasing AGS or Si concentration. As a result, a transition of deformation mode from mechanical twinning to ε-martensitic transformation occurred with grain coarsening in Si-added steels. A critical AGS for the transition of deformation mode was reduced from ∼69 μm to ∼15 μm with increasing Si concentration from 0.5 wt.% to 3.0 wt.%.

Interpretation of dynamic tensile behavior by austenite stability in ferrite-austenite duplex lightweight steels

Phenomena occurring in duplex lightweight steels under dynamic loading are hardly investigated, although its understanding is essentially needed in applications of automotive steels. In this study, quasi-static and dynamic tensile properties of duplex lightweight steels were investigated by focusing on how TRIP and TWIP mechanisms were varied under the quasi-static and dynamic loading conditions. As the annealing temperature increased, the grain size and volume fraction of austenite increased, thereby gradually decreasing austenite stability. The strain-hardening rate curves displayed a multiple-stage strain-hardening behavior, which was closely related with deformation mechanisms. Under the dynamic loading, the temperature rise due to adiabatic heating raised the austenite stability, which resulted in the reduction in the TRIP amount. Though the 950 °C-annealed specimen having the lowest austenite stability showed the very low ductility and strength under the quasi-static loading, it exhibited the tensile elongation up to 54% as well as high strain-hardening rate and tensile strength (1038 MPa) due to appropriate austenite stability under dynamic loading. Since dynamic properties of the present duplex lightweight steels show the excellent strength-ductility combination as well as continuously high strain hardening, they can be sufficiently applied to automotive steel sheets demanded for stronger vehicle bodies and safety enhancement.

Pinning Effect of Cerium Inclusions during Austenite Grains Growth in SS400 Steel at 1300 °C: A Combined Phase Field and Experimental Study

The pinning effect of cerium inclusions in the austenite grain growth of SS400 steel at 1300 °C is investigated by using a semi-empirical-simulation. Firstly, steel samples containing cerium inclusions are prepared; then the properties of inclusions are determined using SEM. In situ observation of austenite grain growth is performed by LSCM, to determine the fitting parameters of the model such as the grain mobility and the pinning parameter. These parameters are directly inserted into our phase field simulation. The time-dependent Ginzburg-Landau (TDGL) equation is implemented in our phase field model, where the effects of inclusion and grain boundary interaction are inserted as a potential term in the local free energy. The results proved that the optimal size of austenite grains can be achieved by changing the volume fraction of inclusions. In fact, by increasing the volume fraction of inclusions from 0 to 0.1, the austenite grain growth can be decreased where the boundary mobility reduces from 2.3×10−12 m4/Js to 1.0×10−12 m4/Js. The results also demonstrated that increasing the temperature can provide more energy for grain to overcome the inclusions’ pinning force. Moreover, it was shown that the classical Zener model, R c = 0.45 r p f i − 1 , describes the pinning effect of cerium inclusions.

Grain size dependence of Young’s modulus and hardness for nanocrystalline NiTi shape memory alloy

In this paper, grain size (GS) dependence of Young’s modulus and hardness for nanocrystalline NiTi shape memory alloy is investigated by experiments. Amorphous NiTi with nanocrystalline debris is fabricated via cold-rolling and polycrystalline NiTi with average GS from 10 nm to 120 nm is obtained by subsequent annealing. Young’s modulus and hardness of nanocrystalline NiTi are quantified by macroscopic isothermal tension and microscopic nanoindentation. It is shown that Young’s modulus of nanocrystalline NiTi first decreases (for GS < 62 nm) and then increases (for GS > 62 nm) with GS in the nano-scale region. The non-monotonic GS dependence of Young’s modulus originates from the combined effects of grain size and volume fractions of austenite, martensite and amorphous phase in the material. It is also shown that with the increase of GS up to 120 nm, hardness of nanocrystalline NiTi monotonically decreases due to the reduced nominal phase transition stress and plastic yielding stress. Such GS dependence of hardness can be utilized for rapid determination of GS in nanocrystalline NiTi via nanoindentation hardness test.

Influence of Ti(C,N) precipitates on austenite growth of micro-alloyed steel during continuous casting

Austenite grain size is an important influence factor for ductility of steel at high temperatures during continuous casting. Thermodynamic and kinetics calculations were performed to analyze the characteristics of Ti(C,N) precipitates formed during the continuous casting of micro-alloyed steel. Based on Andersen-Grong equation, a coupling model of second phase precipitation and austenite grain growth has been established, and the influence of second precipitates on austenite grain growth under different cooling rates is discussed. Calculations show that the final sizes of austenite grains are 2.155, 1.244, 0.965, 0.847 and 0.686 mm, respectively, under the cooling rate of 1, 3, 5, 7, and 10 °C·s-1, when ignoring the pinning effect of precipitation on austenite growth. Whereas, if taking the pinning effect into consideration, the grain growth remains stable from 1,350 °C, the calculated final sizes of austenite grains are 1.46, 1.02, 0.80, 0.67 and 0.57 mm, respectively. The sizes of final Ti(C,N) precipitates are 137, 79, 61, 51 and 43 nm, respectively, with the increase of cooling rate from 1 to 10 °C·s-1. Model validation shows that the austenite size under different cooling rates coincided with the calculation results. Finally, the corresponding measures to strengthen cooling intensity at elevated temperature are proposed to improve the ductility and transverse crack of slab.

Effects of alloying elements and cooling rates on the high-strength pearlite steels

ABSTRACTThe effects of the alloying elements of Cr, Mn and the cooling rates after hot deformation on the microstructures and mechanical properties of pearlite steels were studied. Results show that increasing Cr and decreasing Mn significantly increase the eutectoid transformation temperature of steel. The grain sizes of prior austenite of the steels after hot deformation are ∼12 µm. However, the high-Cr–low-Mn steel exhibits a finer interlamellar spacing and some better mechanical properties than that of the high-Mn–low-Cr steel. A full pearlite microstructure with an interlamellar spacing of 97 nm was obtained on the former steel, which exhibits a hardness of HRC49, a tensile strength of 1700 MPa and an elongation of 19%.

Development of a wear-resistant flux cored wire of Fe-C-Si-Mn-Cr-Ni-Mo-V system for deposit welding of mining equipment parts

The effect of introduction of cobalt in the charge of the flux cored wire of Fe-C-Si-Mn-Cr-Ni-Mo-V system operating under abrasive and abrasive-shock loads is studied. In the laboratory conditions samples of flux cored wires were made, deposition was performed, the effect of cobalt on the hardness and the degree of wear was evaluated, metallographic studies were carried out. The influence of cobalt introduced into the charge of the flux cored wire of Fe-C-Si-Mn-Cr-Ni-Mo-V system on the structure, nature of nonmetallic inclusions, hardness and wear resistance of the weld metal was studied. In the laboratory conditions samples flux cored wire were made using appropriate powdered materials. As a carbon-fluorine-containing material dust from gas cleaning units of aluminum production was used. In the course of the study the chemical composition of the weld metal was determined, metallographic analysis was performed, mechanical properties were determined. As a result of the metallographic analysis the size of the former austenite grain, martensite dispersion in the structure of the weld metal, the level of contamination with its nonmetallic inclusions were established.

Study of the properties of flux cored wire of Fe-C-Si-Mn-Cr-Mo-Ni-V-Co system for the strengthening of nodes and parts of equipment used in the mineral mining

The effect of the introduction of vanadium and cobalt into the charge of the powder surfacing wire of Fe-C-Si-Mn-Cr-Mo-Ni system is studied. In the laboratory conditions, the samples of flux cored wires were produced. The surfacing made by the prepared wire was produced under the flux AN-26C, on the plates of steel St3 in 6 layers with the help of ASAW-1250 welding tractor. Reduction of carbon content in the deposited layer to 0.19-0.2% with simultaneous change in the content of chromium, nickel, molybdenum and other elements present in it contributes to the enlargement of the martensite needles and the increase in the size of the former austenite grain. The obtained dependences of hardness of the deposited layer and its wear resistance on the mass fraction of elements, included in the composition of powder wires of the proposed system, can be used to predict the hardness of the welded layer and its wear resistance under different operating conditions for mining equipment and coal mining equipment.

The Effect of Twice Thermal Cycle on the Microstructure and Mechanical Properties of Coarse Grain Region in X80 Pipeline Steel

Based on the multi-pass welding characteristics of X80 pipeline steel, the influence of twice thermal cycle on the microstructure and mechanical properties of coarse grain region in X80 pipeline steel was investigated. The thermal cycles of weld coarse grain region with different peak temperature for the second thermal cycle were simulated with the Gleeble-3500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding optical micrographs and electron micrographs were systematically investigated to study the effect of the peak temperature on microstructure and impact toughness in the coarse grain region. The results of simulated experiment showed that the microstructure in heat affected zone of coarse grain region is granular bainitic and bainitic ferrite. When the peak temperature of the second thermal cycle is 800°C, the types of microstructure and the grain size of original austenite have no change. However, it forms network microstructure with chain structure in grain boundary and the reduction of toughness may be affected by the M–A constituents. With the peak temperature of 1000°C, the micro structure is composed of granular bainitic and a little bainitic ferrite. In this case, the grain size of austenite can be significantly fined, being helpful to increase the impact toughness.

The microstructure analysis of white etching layer on treads of rails

Abstract Through the utilization of the scanning electron microscope (SEM) and transmission electron microscope (TEM), the microstructure of Martensite White Etching Layers (WELs) on the surface of treads of U71Mn rail(used for a long time and taken from curves of normal railroad tracks) was analyzed and also its formation mechanism was discussed. As the result showed, the WELs observed from the cross-section was crescent-shaped and discontinuously distributed, with the thickness between 30 and 100 μm. Within the microstructure of WELs, the grain size of prior austenite at high temperature was ultra-fine, with the maximum diameter of approximate 1 μm at the outermost layer. As depths increased in the WELs, the grain size of prior austenite inside the WELs gradually became smaller; when the temperature on rail surface reached the temperature of austenitic transformation, austenite grains were less than 200 nm. Under the condition of room temperature, the microstructure inside the WELs was mainly composed of martensite. However, nanocrystalline α-phase, the structure of which was body-centered tetragonal, existed within 300 nm in the outermost layer of the WELs, and its misorientation generally featured random distribution. The organizations in the subsurface of the WELs were plate martensite, and undissolved and ultra-fine cementite, the amount of which rose as depths increase. The formation of the above-mentioned WELs was caused by rapid temperature rises and the subsequent cooling down of rail surface when a train suddenly braked or crept on a track.

Studies on the Mechanism of Work Hardening of Austenitic High Manganese Steel Alloyed with Chromium and Vanadium

This article studies the mechanism of work hardening of austenitic high manganese steel alloyed with chromium and vanadium. The steel was annealed at 650°C before austenitizing at 1100°C, and then was quenched with water. We have observed that after the heat treatment, the size of austenite grain was small (1,950μm2 - level 6). The hardness of the steel was 223HB and the toughness was 115J/cm2. After impact loading, there was no martensite but twinning and sliding in the microstructure of the steel. The nano austenite was found in the microstructure. The steel was also hardened by small austenite grain and the carbide particles were finely dispersed in the microstructure.

Effects of two different types of MX carbonitrides on austenite growth behavior of Nb-V-Ti microalloyed ultra-high strength steel

The size evolution of two different types of MX carbonitrides and its influence on austenite grain growth behavior of Nb-V-Ti microalloyed ultra-high strength steel has been investigated. Based on microstructure examination, the spherical Nb-rich MX is much finer than the cuboidal or rectangle shape Ti-rich MX. The average size of austenite grain and two types of MX carbonitrides sluggishly increased with temperature in the low temperature region. However, a rapid growth has been observed at relative high austenitizing temperatures due to the dissolution and coarsening of precipitates. A grain growth model was developed in view of the pinning effect of different types of carbonitrides. The pinning contribution of two types of MX carbonitrides was calculated using pinning parameter equation, and the pinning effect of Nb-rich MX is much larger than that of Ti-rich MX. This suggests that fine Nb-rich carbonitrides play a more predominant role in retarding austenite grain growth.

Differential heat treatment of one-piece freight-car wheels

Differential quenching of the whole one-piece wheel is shown to be more effective than quenching of only the contact surface at the wheel rim. Hybrid strengthening by differential bulk quenching and surface plasma hardening is very promising in increasing the crack resistance of steel. The martensitic structure of the treated layer in hybrid strengthening is very disperse, thanks to decrease in size of the initial austenite grain. In bulk-quenched components, surface plasma strengthening creates a composite working layer characterized by high wear and crack resistance, whereas the core is relatively soft and plastic.

Effect of Heating and Thermal Deformation on Austenite Grain Size of B1500HS Boron Steel

Abstract To investigate the effect of temperature and soaking time on the austenite grain size of B1500HS boron steel, samples of B1500HS steel were, respectively, heated at 900, 925, 950, 975, and 1000°C for 10, 20, 40, and 60 min, and the grain sizes of austenite for the heated samples were measured. Besides that, samples of B1500HS steel were heated at 1000°C for 10, 20, and 40 min by the thermo-mechanical simulator Gleeble-1500D, and then deformed with the deformation of 10, 20, and 30 % at the strain rate of 0.1 and 0.5 s−1, respectively, the effect of deformation degree and strain rate on the grain size of austenite was also studied. The experimental results showed that the average grain size of austenite rises with the increase of austenitization temperature and soaking time, but the average grain size reduced with the increase of deformation degree and strain rate. Finally, a grain growth model of B1500HS steel was given according to the experimental results.

Microstructural heterogeneity and texture of as-received, vacuum arc-cast, extruded, and re-extruded NiTi shape memory alloy

As-received and vacuum arc-cast near equiatomic Ti-50.6 at.% Ni billets were extruded and re-extruded at 815 °C at Case Western Reserve University (CWRU) with an extrusion ratio of 4 to 1. The effects of arc-casting, extrusion and re-extrusion on the grain size, volume fraction of recrystallized grains, grain boundary misorientation and texture were analyzed via the high-resolution electron backscatter diffraction (EBSD). The results show that extruding and re-extruding the as-received NiTi shape memory alloy produces a 〈111〉texture//ED and {110}〈uvw〉texture//TD. The size of austenite grains and the volume fraction of recrystallized grains significantly decrease but the fraction of LABs slightly increases and the grain size distribution becomes more homogeneous for the re-extruded NiTi in comparison to the first extrusion. For the vacuum arc-cast extruded NiTi shape memory alloy, the microstructure predominantly consists of elongated austenite grains. There is a strong {001}〈001〉texture//TD at the Butt (i.e. as-cast NiTi). Re-extrusion of the vacuum arc-cast NiTi shape memory alloy produces a material that exhibits two strong textures: (110)[11¯0]//CD(ED) and {001}〈100〉//TD in the Extrudate ①. The re-extruded material exhibits few elongated grains at this location. Instead, the second extrusion of the as-cast material produces fine and equiaxed recrystallized grains.

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.