Austenite Area Research Articles

Phase transformations during partitioning in a Q&P steel with blocky retained austenite

Effect of partitioning on structure and mechanical properties of a Fe-0.44%C-1.8%Si-1.3%Mn-0.82%Cr-0.28%Mo steel processed by quenching-partitioning (Q&P) was considered. This Q&P steel exhibited a superior combination of the yield stress (YS) of ∼1150 MPa, ultimate tensile strength (UTS) of ∼1650 MPa and elongation-to-failure (δ) of ∼21 %. The product of strength and elongation (PSE – UTS × δ) was >30 GPa × % after partitioning, with a duration ranging from 60 to 500 s. During partitioning, three main processes occurred: the carbon partitioning between retained austenite (RA) and martensite/bainitic ferrite (BF), the transformation of blocky RA into bainite, and the precipitation of transition η–Fe2C carbides in the martensitic matrix. The enrichment of RA by carbon changed the mechanism of bainitic transformation and then suppressed the formation of BF and induced the reverse growth of RA upon 500 s partitioning. The rate of carbon partitioning depends on the diffusion path that produces a wide distribution of carbon concentration in areas of RA. The formation of secondary martensite and bainite takes place in areas of untransformed austenite with relatively low carbon content. The effect of RA volume fraction on ductility and the PSE is insignificant. Carbon enrichment of RA ceased its transformation into strain-induced martensite and induced twinning during tension.

Microstructural Investigation and Impact Strength of Sinter-Hardened PM Steels: Influence of Ni Content and Tempering Temperature

This study analyzed the influence of tempering treatment temperature on the microstructural and mechanical behavior of two different powder metallurgy steels containing 0 wt. % Ni and 4 wt. % Ni. The evolution of the microstructure and the macro- and microhardness of the microstructural constituents resulting from tempering treatments conducted on the sinter-hardened materials at temperatures ranging from 160 °C to 300 °C were investigated. The role of the tempering conditions in the impact behavior was assessed using Charpy tests on V-notched and unnotched samples, tempered at 180 °C, 220 °C and 280 °C. The observed macrohardness reduction with increasing tempering temperature was related to martensite transformations. At high tempering temperatures, the remarkable loss in impact energy values was attributed to microfracture modes. The contribution of Ni-rich austenite areas in enhancing impact strength was detected.

Heterogeneous quenching and partitioning from manganese-partitioned pearlite: retained austenite modification and formability improvement

Increasing quenching temperature in conventional quenching and partitioning (Q&P) process usually increases the fraction of retained austenite (RA) being dominant by blocky morphology, which ensures a good global formability but deteriorates the local formability. Instead of homogeneous austenite in conventional Q&P process, we propose a Q&P process purposely based on Mn-heterogeneous austenite inherited from Mn-partitioned pearlite. Therein, Mn-depleted austenite areas originated from ferrite lamellae are readily transformed into lath martensite during quenching; meanwhile, Mn-enriched austenite areas originated from cementite lamellae are readily retained as film RA at room temperature. Consequently, this heterogeneous Q&P process unusually achieves a large RA fraction being dominant by film morphology. The large RA fraction ensures a large total elongation and the dominant film morphology ensures a large post-uniform elongation, indicating good global and local formability, respectively. Additionally, the microstructure is refined through the above divided transformation of heterogeneous austenite, predominantly contributing to the increased yield strength by 200 ∼ 300 MPa. Introducing heterogeneous austenite into Q&P process provides a feasible way to achieve a large RA fraction being dominant by film morphology, leading to the successful production of high-strength Q&P steels simultaneously with good global and local formability.

Excellent tension properties of stainless steel with a 316L/17-4PH/17-4PH laminated structure fabricated through laser additive manufacturing

Laminated metal structures (LMS), which are metals with site-specific properties, exhibit great potential for practical applications. In this study, a stainless steel with a laminated structure of alternating 316L/17-4PH/17-4 PH layer is fabricated using a laser additive manufacturing (AM) technique. Heterogeneous structures in the LMS are observed in two distinct areas, the austenite area (AU) and the martensite area (MA), based on optical microscopy and backscattered electron images. In the electron backscattered diffraction images, the AU area can be divided into austenite and mixture layers, while the MA area represents the martensite layer. Owing to the AM fabrication process, heterogeneity of both the micro-hardness and chemical composition exists among the three layers, which leads to different austenite stabilities in each layer. The tension test results reveal an excellent strength-ductility synergy with stronger ultimate strength and larger elongation than the pure 316L structure produced by AM with the same printing process parameters. During tension testing, the laminated structure causes strain partitioning, which evolves with the applied strain and delays the strain localization process. Owing to the change in austenite stability compared with pure AM 316L, the transformation-induced plasticity effect (TRIP) is triggered successively, and the laminate structure extends the TRIP effect to large plastic strains. As a result, the present study verifies the potential of using AM design for LMS stainless steel and may offer a framework for re-exploring LMS materials and components.

Effect of Carbon Partitioning and Residual Compressive Stresses on the Lattice Strains of Retained Austenite During Quenching and Isothermal Bainitic Holding in a High‐Silicon Medium‐Carbon Steel

The residual compressive stresses and dimensional changes related to the lattice strains of retained austenite (RA) phase in a high‐Si, medium‐carbon steel (Fe‐0.53C‐1.67Si‐0.72Mn‐0.12Cr) are investigated for samples austenitized and quenched for isothermal bainitic transformation (Q&B) in the range 5 s to 1 h at 350 °C. Also, samples are directly quenched in water (DWQ) from the austenitization temperature for comparison with Q&B samples. Field emission scanning electron microscopy (FE‐SEM) combined with electron backscatter diffraction (EBSD) analyses, and X‐ray diffraction are used to investigate the microstructural evolution, phase distribution, and lattice parameters of RA phase. While the Q&B samples showed formation of bainite and high‐carbon fresh martensite in conjunction with stabilization of various fractions of RA, the DWQ samples displayed nearly complete martensitic microstructure. For short holding durations (≪200 s), there was limited formation of bainite and the inadequate carbon partitioning to the adjacent untransformed austenite areas resulted in significant martensite formation and the associated c/a ratio of martensite resulted in high compressive residual stresses within the RA phase. While, at long isothermal holding times (≫ 200 s), there was a significant formation of bainite. The DWQ samples displayed maximum lattice strain in a small fraction of untransformed RA phase.

Scanning Electron Microscopy versus Transmission Electron Microscopy for Material Characterization: A Comparative Study on High-Strength Steels.

The microstructures of quenched and tempered steels have been traditionally explored by transmission electron microscopy (TEM) rather than scanning electron microscopy (SEM) since TEM offers the high resolution necessary to image the structural details that control the mechanical properties. However, scanning electron microscopes, apart from providing larger area coverage, are commonly available and cheaper to purchase and operate compared to TEM and have evolved considerably in terms of resolution. This work presents detailed comparison of the microstructure characterization of quenched and tempered high-strength steels with TEM and SEM electron channeling contrast techniques. For both techniques, similar conclusions were made in terms of large-scale distribution of martensite lath and plates and nanoscale observation of nanotwins and dislocation structures. These observations were completed with electron backscatter diffraction to assess the martensite size distribution and the retained austenite area fraction. Precipitation was characterized using secondary imaging in the SEM, and a deep learning method was used for image segmentation. In this way, carbide size, shape, and distribution were quantitatively measured down to a few nanometers and compared well with the TEM-based measurements. These encouraging results are intended to help the material science community develop characterization techniques at lower cost and higher statistical significance.

Adjusting Mechanical Properties of Forging Dies Produced by Ausforming

Due to high thermo-mechanical loads, tools used in hot forming operations need a high resistance to different damage phenomena, such as deformation, cracking and abrasion. They are exposed to cyclic thermo-mechanical stress conditions, which leads to tool failure and subsequent tool replacement during cost-intensive production interruptions. To increase wear resistance, forging tools can be produced in the metastable austenite area. Forming of steel below the recrystallisation temperature, also known as “ausforming”, offers the possibility to increase strength without affecting ductile properties. This is due to grain refinement during forming. In this study, the thermo-mechanical treatment ausforming will be used to form the final contour of forging dies. For this purpose, an analogy study was performed where a cup-preform is ausformed, which represents the inner contour of a highly mechanically loaded forging die. It is investigated to what extent a fine-grained microstructure generated in the last forming stage can be achieved and how it influences the tool’s performance. The hot-working tool steel X37CrMoV5-1 (AISI H11) was used as workpiece material. To achieve optimal properties, process routes with tempering temperatures from 300 °C to 500 °C and global true plastic strains of φ = 0.25 and φ = 0.45 were examined. The results were evaluated by pulsation tests, metallographic analysis and hardness measurements of the formed parts. Optimal ausforming parameters were derived to produce a high performance forging die.

Novel Cost-Efficient Method of Producing Ausferritic Steels Displaying Excellent Combination of Mechanical Properties

Round bars Ø 53 mm were hot-rolled from a 1.4 tonne ingot forged to 165 × 165 mm. The composition of the steel was 0.45 wt. % C and 3.33 wt. % Si plus alloying elements for hardenability. Microstructure after air cooling from 1010 °C on the cooling bed was predominantly ausferritic. Tensile testing of as-rolled bars resulted in yield strength 846 ± 22 MPa, ultimate tensile strength 1169 ± 99 MPa and A5-elongation of 1.7 ± 0.8 % (without prior necking). When as-rolled steel was baked in air at T = {Ms initial -30 K} for six hours, the yield stress raised to 1121 ± 4 MPa, the ultimate tensile stress raised to 1447 ± 5 MPa and the elongation raised to 22.6 ± 1.6 % (with necking > 18 %). For as-rolled bars during continuous cooling, the exposure time within the temperature range 460 – 320 °C was estimated to be about 10 minutes. The microstructure of as-rolled “semi-finished” bars is stable at room temperature. The first baking was done six months after hot-rolling. Optical and scanning electron microscopy showed that remaining areas of austenite, not transformed during continuous cooling but stable at room temperature, transforms to ausferrite when properly baked.

Solubility of boron and carbon in γ-iron of Fe-B-C alloys

It is known that solubility of elements affects the phase composition of alloys that are formed in the solidification process. To predict the phase composition of alloys, it is necessary to determine the solubility limit in the phases. In the paper the structural properties of austenite of alloys in the system of Fe-B-C are studied and the solubility limit of boron and carbon is determined. The investigation is carried out for the specimens with carbon content of 0.0001–2.3 wt.% and boron content of 0.0001–1.0 wt.%, the rest is iron. To determine the physical properties of alloys, we use the microstructure analysis, X-ray microanalysis, X-ray structure analysis and differential thermal analysis. It is shown experimentally that the maximum shift of the eutectoid point is observed when boron content is up to 0.004 wt.%. When boron content of the alloy increases to 0.01 wt.%, the eutectoid point shifts to the left to 0.21 wt.%-carbon and the austenite area decreases. Further increase in the numerical value of boron content in the alloy is hardly caused the eutectoid point to shift. In this paper, the vertical section of the Fe-B-C system state diagram is obtained from experimental data. For the first time we obtain temperature dependence of the free energy of γ-iron solid solution, using the quasi-chemical method, and determine the solubility limit of carbon and boron. The maximum weight fraction of boron in the austenite can be up to 0.0136 wt.%, and that for carbon – up to 1.12 wt.%.

Pengaruh Hardening dengan Media Quenching Fluida Getah Pohon Pisang terhadap Struktur Mikro dan Komposisi Kimia Baja Karbon Sedang

The purpose of this study was to determine how the chemical composition and microstructure formed in medium carbon steel hardened with banana tree sap fluid compared to oil and water. The process of carbon steel hardening is being heated to the austenite area (850oC ) then held for 10 minutes after which it is cooled quickly (quenching). During the cooling process, it is also measured the cooling rate of the three cooling media. Hardening process material is carried out by microstructure observation using an optical microscope with 400x magnification and also chemical composition testing with SEM-EDX. The results for measuring the cooling rate of banana fluid have a fast cooling rate of 36.9oC/s while water is 16.6oC/s and oil is 7.0oC/s. In observing the microstructure of water it has an unstable micro structure that is still the structure of pearlite, this is because the temperature of the cooling media is too high during quenching. And for oil microstructure and banana fluid get a homogeneous structure, namely martensite and bainite. From the results of testing the chemical composition, it was seen that there was an increase in carbon content in steel after hardening and quenching with the three cooling media. Conclusion The hardening process by quenching oil and banana fluids has a homogeneous micro structure compared to water. Water cooling process has the highest carbon content increase of 1.53 %C from before 0.52 %C.

Improvement of materials for gas welding of cast iron

The aim of the study is to create a bar for gas welding of cast iron parts, with improved mechanical characteristics and lower hardness of the weld metal.A disadvantage of the known welding rods is the high hardness and low mechanical properties of the weld metal. This does not allow the use of the mentioned filler rods for welding critical structures and surfacing of parts to be machined in industrial volumes.As a filler material in gas and electric arc welding of cast iron, rods with a diameter of 8-10 mm from cast iron of grade A or B according to GOST 2671-80 were used. As a flux in gas welding, we used a technical drill according to GOST 8429-77, calcined at a temperature of 300350 ° C for 2 hours.Yttrium and erbium were used as an alloying and rare-earth metals additives in iron bars.The welding-technological properties of the investigated rod compositions were checked during gas (acetylene-oxygen) and electric arc welding of high-strength cast iron of grade VCh 40.Based on the tests of the welding and technological properties of the rods under study, it was determined that rod No. 2 is optimal, which ensures good wettability of the weld pool with molten metal. Slags are fusible and do not impede the conduct of the welding process for cutting large volumes. No weld defects.The structure of the deposited metal on the surface of the weld at a depth of 1.5-2 mm is pearlitic with small areas of austenite of dendritic orientation. Further along the height of the weld to the fusion line, perlite prevails in the structure. The inclusion of ferrite and cementite are single. Spherical graphite of medium size, slightly smaller than in the base metal.The hardness in this zone is 220-229 HB and allows easy machining of welds with conventional metal cutting tools.

INFLUENCE OF CONSTANT MAGNETIC FIELD ON STRUCTURE FORMATION IN STEELS AT HIGH-SPEED LASER PROCESSING

Experimental studies of laser-irradiated layers in a magnetic field (MF) have shown a non-trivial morphology of the surface of handling zone of material in case of reflow. Twisting of a thin layer of liquid metal is observed, irradiated area is getting a crescent appearance, definitely strictly oriented in relation to magnetic flux. This is probably due to the effect of Righi-Leduc, as well as the action of Lorentz forces, which deflect the electrons flow. As a result, there is significant mixing of metal in the irradiation zone, chemical composition equalization, which positively affects the strength properties of the products. One of the important consequences of the MF-effect on the results of laser processing is the phenomenon of magnetostriction. In laser irradiation without MF slide lines were observed on the pre-polished surface patterns resulting from the emerging thermal and structural stresses. By analyzing the topography of irradiated surfaces using modern analysis techniques and computer image processing, it was established that irradiation in MF in conditions of magnetostriction decreases the stress level in irradiated areas and reduces the risk of cracking. The results of temperature measurements at the irradiated spot on cooling stage allow establishing that the cooling rate during laser processing in a MF is considerably higher than without the field. It affects the processes of phase and structural transformations. At laser heating in MF microheterogenic austenite is supercooled with great speed to temperatures of martensite transformation. After that its transformation begins, the sequence of which is determined by the level of local saturation, degree of deformation and is controlled by temperature. The first crystals of martensite are formed in the least saturated areas of austenite, and a very high speed (thousands or tens of thousands of °С/s) of the transformation process beginning γ → α prevents martensite self tempering, which partially can occur when the temperature decreases further due to transformation spread on the remaining volume of austenite, grabbing areas of different saturation. As a result, along with the “fresh-formed” martensite in the areas of laser quenching the martensite is formed, in which segregation of carbon or even ε-carbide may occur and residual austenite with high carbon intensity are formed. Released dispersed carbides contribute to obtaining a sufficiently high hardness values of metals irradiated in a MF.

Effect of the laser heat treatment on the formation of the gradient structures in alloys based on Fe – Cr – Ni system

The possibility of producing gradient materials, i.e. materials with pre-set distribution of areas having fundamentally different physical and mechanical characteristics, with the help of laser heat treatment was investigated. Using as an example austenitic-martensitic alloys of iron-chromium-nickel, subjected to cold plastic deformation led to formation of martensite, we show that using laser at the temperature higher than the temperature of reverse martensite transformation leads to the formation of areas of high-strength austenite having predetermined form inside the martensite matrix. Influence of austenite areas geometry on mechanical properties of gradient material was studied.

Evolution of Microstructure and Mechanical Properties in Steels during Isothermal Holding in the Region of Bainitic Transformation Temperature in Dependence on Silicon Content

Under isothermal treatment conditions, bainite transformation involves decomposition of austenite into a nonequilibrium structure consisting of needles of super-saturated bainitic ferrite and carbide precipitates. Similarly to martensitic transformation, bainitic ferrite forms by shear mechanism. Owing to relatively low temperature, only interstitial elements, predominantly carbon, can migrate by diffusion. Depending on the transformation temperature, carbon migrates from ferrite and forms carbides, either within bainitic ferrite needles and at the interphase interface between bainitic ferrite and austenite, or only within bainitic ferrite needles. In conventional steels, bainite transformation continues until the decomposition of austenite phase is almost complete. If the steel contains enough silicon, carbide precipitation may be suppressed or even prevented altogether. In such case, carbon which diffuses from the needles of bainitic ferrite may enrich adjacent austenite areas. Depending on heat treatment conditions, the carbon-enriched austenite may become sufficiently stable to resist decomposition and remain in the microstructure.

Mutual mechanical effects of ferrite and martensite in a low alloy ferrite-martensite dual phase steel

Abstract In this paper micromechanical behavior of ferrite and martensite microphases was evaluated by means of comprehensive micro-nanohardness measurements supplemented by light and electron microscopies and wavelength-dispersive spectroscopy (WDS). Experimental results indicated that not only ferrite but martensite hardening behavior was significantly influenced by ferrite and martensite volume fractions in dual phase (DP) microstructures. Ferrite hardness was continuously diminished with increasing ferrite volume fraction whereas the martensite hardening was increased with increasing ferrite volume fraction and then it did not varied in line with further progress in ferrite transformation and relevant carbon enrichment of prior austenite. In addition to these unexpected ferrite and martensite hardening variations, the lower hardness of martensite in DP microstructures in comparison to that of martensite in full martensitic microstructure cannot be completely interpreted by carbon alloying effects. These results are rationalized to the introduction of mobile dislocations into prior austenite due to plastically accommodation of martensitic transformation strains by prior austenite areas during quenching and the influence that formation of ferrite would have on this hardening mechanism.

Role of Microstructure Heterogeneity on Fatigue Crack Propagation of Low-Alloyed PM Steels in the As-Sintered Condition

Due to their lower production costs, powder metallurgy (PM) steels are increasingly being considered for replacing wrought counterparts. Nevertheless, the presence of a non-negligible volume fraction of porosity in typical PM steels makes their use difficult, especially in applications where cyclic loading is involved. On the other hand, PM offers the possibility of obtaining steel microstructures that cannot be found in wrought. Indeed, by adequately using alloying strategies based on admixing, pre-alloying, diffusion bonding or combinations of those, it is possible to tailor the final microstructure to obtain a distribution of phases that could possibly increase the fatigue resistance of PM steel components. Therefore, a detailed study of the effect of different microstructural phases on fatigue crack propagation in PM steels was performed using admixed nickel PM steels (FN0208) as well as pre-alloyed PM steels (FL5208). Specimens were pressed and sintered to a density of 7.3 g/cm3 in order to specifically investigate the effect of matrix microstructure on fatigue properties. Fatigue crack growth rates were measured at four different R-ratios, 0.1, 0.3, 0.5 and 0.7 for both PM steels. The negative effect of increasing the R-ratio on fatigue properties was observed for both alloys. The crack propagation path was characterized using quantitative image analysis of fracture surfaces. Measurements of roughness profile and volume fractions of each phase along the crack path were made to determine the preferred crack path. Weak Ni-rich ferritic rings in the FN0208 series (heterogeneous microstructure) caused a larger crack deflection compared to the more homogeneous microstructure of the FL5208 series. It was determined that, contrary to results reported in literature, crack propagation does not pass through retained austenite areas even though fatigue cracks propagated predominantly along prior particle boundaries, i.e., intergranular fracture.

Role of Bulky Retained Austenite in Austempered Ductile Iron

In this investigation, the characteristics of bulky retained austenite in an austempered ductile iron are evaluated in two tempered conditions. which were intially tempered at 200oC for 1h before cooling to room temperature, and then tempered at 350oC for 1h. The result shows that the hardness within retained austenite areas is distributed unevenly with a range from 423 HV to 897 HV, which is attributed to the transformation from austenite to martensite during austempering. Also, the mechanism regarding the quenched marteniste formation is discussed. The poor fatigue resistance of ADI is hypothesized to be due to the amount of austenite transformed to martensite.

Effect of alloying element partitioning on ferrite hardening in a low alloy ferrite-martensite dual phase steel

In this paper, the effect of carbon and other alloying elements partitioning on ferrite hardening behavior were studied in details using a low alloy AISI4340 ferrite-martensite dual phase (DP) steel. To do so, various re-austenitised samples at 860°C for 60min were isothermally heated at 650°C from 3 to 60min and then water–quenched to obtain the final ferrite-martensite DP microstructures containing different ferrite and martensite volume fractions. Light and electron microscopic observations were supplemented with electron dispersive spectroscopy (EDS) and nanoindentation tests to explore the localized compositional and hardening variations within ferrite grains in DP samples. The experimental results showed that the ferrite hardness was varied with progress of austenite to ferrite phase transformation in DP samples. In the case of a particular ferrite grain in a particular DP sample, despite a homogeneous distribution of carbon concentration, the ferrite hardness was significantly increased by increasing distance from the central location toward the interfacial α/γ areas. Beside a considerable influence of martensitic phase transformation on adjacent ferrite hardness, these results were rationalized in part to the significant level of Cr and Mo pile-up at α/γ interfaces leading to higher solid solution hardening effect of these regions. The reduction of potential energy developed by attractive interaction between C-Cr and C-Mo couples toward the carbon enriched prior austenite areas were the dominating driving force for pile-up segregation.

Investigation of the effect of cyclic laser heating for creating dispersed structures in the austenitic-martensitic alloys based on Fe-Cr-Ni system

The effect of cyclic laser heating on the formation of the austenite structure in the austenitic-martensitic alloys based on Fe-Cr-Ni system is investigated. It is shown that under the influence of ultra-fast laser heating on the martensite, which was formed during plastic deformation, the reverse martensitic transformation occurs, and austenite with high strength characteristics is formed. Repeated and multiple laser heating effectively grinds areas of austenite to a size close to the large nanoparticles. There is an additional increase in the strength characteristics of austenite as a result of this fragmentation.

The Influence of Quench Tempering and Carburazing Treatment towards Mechanical Properties and Microstructure of Medium Carbon Steel for Automotive Application

Gear is one of the machine components that is widely used in industrial and automotive fields. In machinery process, gear has a very important function to forward speed, power, or torque from one engine component to other components as a mechanical drive. Today a lot of development to obtain a good quality of gear, due to many gears were damage, worn out, and broken because they were not strong enough to resist friction and pressure. In addition, broken gears due to pressure and friction make them did not last long. To increase the hardness value of gear, then it needs though material that can be used when the gear reach optimum rotation. The material that is widely used for gear application is medium carbon steel. The medium carbon steel is a metal material that has carbon composition ranging from 0.30 to 0.59%. This medium carbon steel has hardness value of 174.501 HVN without treatment. The process of quench tempering and carburizing are conducted to increase hardness and toughness value of the material. The hardness value of gear is 140 HVN. The result of the research showed the hardness value at various temperature 780°C, 810°C, and 840°C. The optimum hardness values ​is 165.355 HVN at the temperature of 840°C. Medium carbon steel is expected to be an alternative to produce steel material with certain mechanical properties. This research also conducted heat treatment in austenite area and then detained with holding time of 20, 40, and 60 minutes. Furthermore, quench tempering was conducted and followed by carburizing to obtain a ferrite phase and coarse pearlite and to increase hardness value after quech tempering. It is expected that after quech tempering and carburizing process, steel with better mechanical properties can be produced. This research obtained the increase of hardness value and the number of pearlite and ferrite.