Search

The reinforcing bar (rebar) produced by Tempcore process had tempered martensite structure on its surface, owing to quenching and tempering, and ferrite–pearlite structure at the core owing to slow cooling rate. Bainite, a low-temperature transformation phase, was observed in the transition area. Mechanical properties of the rebar are significantly influenced by area ratio and hardness of the tempered martensite. In this study, microstructure and area ratio of the tempered martensite of the rebar produced by the Tempcore process were predicted using finite volume method and a continuous cooling transformation diagram. The area ratio of the tempered martensite was predicted with error range of ± 6%. Furthermore, association between the microstructure and hardness was determined by a non-isothermal tempering experiment. The hardness of the tempered martensite was successfully predicted using the tempering temperature with R-squared 98%. The proposed methodology can be effectively used to control the mechanical properties of the rebars.

Both qualitative and quantitative analyses play a key role in the microstructural characterization of nanobainitic steels focused on their mechanical properties. This research demonstrates various methods of microstructure analysis using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) techniques, taking into account these two approaches. The structural constituents have been qualitatively characterized using TEM and selected area electron diffraction (SAED), together with quantitative analysis based on the misorientation angle (EBSD). Besides, quantitative measurement of austenite with both blocky and film-like morphologies has been carried out. Due to the scale of nanostructured bainite, it is also important to control the thickness of bainitic ferrite and film-like austenite; hence, a method for measuring their thickness is presented. Finally, the possibility of measuring the prior-austenite grain size by the EBSD method is also demonstrated and compared with the conventional grain boundary etching method. The presented methods of qualitative and quantitative analyses form a complementary procedure for the microstructural characterization of nanoscale bainitic steels.

The influence of isothermal holding (IH) near the pearlite start temperature (Ps≈610 °C) and above the martensite start temperature (Ms≈245 °C) were studied in two hot-rolled (FRT≈1000 °C) high carbon steels, without (HC0) and with Nb microalloying (HC1). Optical microscopy (OM), field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) were utilised for observing the microstructural constituents whereas, nanoindentation and microindentation were performed to estimate the average hardness and elastic modulus values of various phase constituents. It has been observed that hot deformation in the austenite region results in the formation of a dual phase-type microstructure consisting of fine pearlite lamella and ferrite grains with some amount of degenerated pearlite. IH at 570 °C leads to a higher volume percentage of ferrite and coarsening of pearlite whereas, IH at 300 °C results in mixed phases comprising bainite, a very low amount of martensite/retained austenite (M/RA), pearlite and some amount of ferrite. Hot rolled microalloyed steel when isothermally held near the Ps results in a significant enhancement of ductility than hot rolled and air-cooled condition while sacrificing tensile strength marginally, which may be attributed to a higher percentage of ferrite and refinement of pearlite due to Nb addition. The estimated yield strength values correlate well with those experimental yield strength values, but a little discrepancy has been noticed for the ultimate tensile strength values. Possible interpretations are suggested in this paper.

Industrially produced high-carbon steel has been heat treated in order to obtain various microstructures of the single phase of martensite, bainite, pearlite, and retained austenite or their combinations. Magnetic properties and the fraction of retained austenite were determined in a magnetometer. Thermomagnetic measurements were also used to record the magnetic response of selected microstructures upon controlled cooling from 300 K to 10 K. Optical metallography, electron scanning microscopy, and electron backscatter diffraction techniques were used to identify selected microstructural features. In general, complex microstructures have higher coercivity and lower remanence. Pearlitic microstructures, obtained through isothermally transformed austenite, showed an increase in coercivity and remanence and a decrease in saturation magnetization with an increase in interlamellar spacing. In martensitic structures, the coercive force decreased by an increase in tempering temperature. The findings that microstructural features have a significant effect on the magnetic properties of the high-carbon steels are important since they reveal that the magnetic measurements have the potential to be used as a reliable non-destructive method to detect and monitor microstructural features in high-carbon steel.

This study analyzes the effects of post-weld heat treatment (PWHT) on the mechanical properties and microstructures of SA-508 Gr.1a welds and proposes a new PWHT exemption criterion based on nonductile fracture evaluation considering welding residual stress. The welding coupons were prepared with submerged-arc welding, gas-tungsten arc welding, and shielded-metal arc welding, using ferritic steel, SA-508 Gr.1a. The microstructure of the heat-affected zone (HAZ) was analyzed using optical microscopy, electron-back-scatter diffraction and Vickers hardness testing. The mechanical properties of the welds were evaluated by uniaxial tensile test, transverse side bend test, Charpy V-notch impact test and side bend test. Bainite and ferrite structures formed mainly in the HAZ, and the grain size became coarser with proximity to the surface and fusion line. The mechanical properties did not depend strongly on PWHT, weldment thickness or welding techniques, and they satisfied the welding procedure qualification test specified in the ASME Boiler & Pressure Vessel code. Welding residual stresses were considered in assessing structural integrity using nonductile fracture evaluation. A margin of safety against nonductile fracture with residual stress was calculated for Korean Standard Nuclear Power Plant steam-generator welds, using its design parameters and operating conditions, and this safety margin is suggested as an acceptance criterion for residual stress for exemption from PWHT. Graphic abstract

Microstructural mechanisms reducing the impact strength values of a coarse grained heat-affected zone are studied for two K60 microalloyed steels. Research is conducted on specimens subjected to simulation of a heat-affected zone. The microstructure and fracture surface are studied using scanning electron microscopy and backscattered electron diffraction. It is shown that titanium nitride inclusions have the greatest influence on impact strength of the heat-affected zone whose cleavage within large bainite packages may cause macro-brittle specimen fracture. The risk of such non-metallic inclusions depends on the their structural features. In particular, presence of aluminum oxide and calcium sulfide blocking contact of the ferritic matrix with nitride leads to an increase in cleavage stress and hinders macroscopic brittle fracture in the early stages of impact bending.

So far, the qualitative and quantitative analysis of bainite has to be carried out by a metallography specialist and often causes ambiguities coming along with poor reproducibility. Possible reasons for a high variance in the description of bainite originate from different expert opinions given the high variety in the appearance of bainite in micrographs. In particular, the applied cooling regime and corresponding temperature gradients in the material dictate the evolving microstructure and its complexity. In recent years, deep learning showed its potential to provide a robust and fast quantification of image data derived from learning large datasets. In order to unfold the potential of deep learning and facilitate its usage for the material science community, a deeper understanding on the role of data pre-processing is necessary to capture the influence of metallography images (and their complexity) on the learning process. In this study, the open-source detection and segmentation library Detectron2 (https://github.com/facebookresearch/detectron2) was used within a framework to quantify a crucial constituent in bainite - the martensite-austenite (M-A) islands - in electron microscopy images. We provide three bainite data sets with image data representing different cooling regimes and therefore different M-A characteristics. From segmentation results, the ratio of constituent to image size manifests as a crucial parameter during pre-processing affecting the accuracy of subsequently trained models.

The features of microstructure and mechanical properties of the aerospace high strength steel were studied after the implementation of various heat treatment modes: conventional oil quenching and tempering, quenching-partitioning, austempering. The dependence of the mechanical properties on the tempering temperature was determined. The basic patterns of the formation of mechanical properties during the implementation of isothermal heat treatment were considered. The optimal heat treatment conditions for the studied steel were established.

This investigation aims to explore the formation mechanism of abnormal martensite in the bainitic rail welding process by combining numerical simulation and physical experiment. For this purpose, the continuous cooling transformation (CCT) diagram of bainitic steel was measured to determine the transformation temperature and cooling rate range of bainite and martensite during phase transformation. Besides, the Eulerian multiphase solidification model was also used to accurately design the experimental plan of the thermosimulated test. The higher cooling rate and microsegregation of the welded joint are important factors affecting the formation of abnormal martensite. The effects of cooling rate and microsegregation on the microstructure during the welding process of the bainitic rail were examined against the martensite fraction and bainite fraction. When the cooling rate is greater than or equal to 1 K/s, abnormal martensite will appear in bainite after the thermosimulated test, regardless of whether there is microsegregation in the sample. The cooling rate, not the microsegregation, is the main factor promoting the formation of abnormal martensite. The reasonable postwelding treatment process has excellent potential for improving the mechanical properties of the welded joint of the bainitic rail.

The microstructure of a low alloy medium carbon bainitic steel, austempered in the temperature range 275 °C to 450 °C has been investigated by detailed electron backscatter diffraction and variant pairing analysis. It is observed that the variant pairing tendency has two distinct changes with varying temperature. At low temperature V1-V6 is the most frequent, whereas V1-V2 is the most frequent at intermediate temperature and at the highest temperature, V1-V4 dominates. This is distinct from the literature on low carbon steel where only two dominant variants pairs, related to the common distinction of bainite into lower and upper bainite, are typically reported. The change of the variant pairing in bainite also has many similarities with the change of variant pairing in martensite when its carbon content changes. Another observation is that the morphological orientation of cementite in the bainite has a strong relation with the variant pairing at lower austempering temperatures.

The effect of quenching after martensitic finish (QAMf) or quenching & partitioning (Q&P) on microstructure evolution, mechanical properties, and strain hardening instability of low and medium carbon hot rolled steels were investigated. Two heats of low and medium carbon steels were cast in an induction open furnace. The chemical composition of low carbon steel is 0.16C–0.27Si–1.47Mn–0.02Al while medium carbon steel is 0.49C–0.30Si–0.91Mn–0.03Al. They were hot-rolled at 1200 °C for 30 min followed by air cooling. The microstructure after hot-rolled gives bands of ferrite and pearlite for 0.16 wt% low carbon steel. On the other hand, 0.49 wt% medium carbon steel produces coarse pearlite islands surrounded by ferrite phase. To enhance mechanical properties, it was necessary to modify the microstructure of low and medium carbon steels using QAMf or Q&P processes. The resultant matrix of microstructure after QAMf and Q&P processes contained ferrite, bainite, lath martensite, and retained austenite for 0.16 wt% low carbon steel, and polygonal ferrite, lath martensite, and retained austenite for 0.49 wt% medium carbon steel, respectively. In low carbon steel, QAMf process increased uniform elongation from 6.6 to 13.5% (105% increase) while ultimate tensile strength (UTS) improved slightly from 645 to 692 MPa (7% increase). However, in medium carbon steel, Q&P reduced uniform elongation from 12.4 to 4.8% (61% decrease) while increased UTS from 769 to 1242 MPa (61.5% increase). It is worthy to mention that QAMf process exhibited strain hardening instability zone (7.8% strain before necking) compared to hot-rolled process (0% strain before necking). On the other hand, Q&P process highly decreased strain hardening instability zone (0.77% strain before necking) compared to hot-rolled process (3.4% strain before necking).

For numerous steel grades, detailed descriptions of different etching techniques and etching times for microstructural analysis are available. However, there are only few reference works for low-alloyed cast iron. Particularly for complex microstructures with combined fractions of bainite, ferrite, pearlite, retained austenite, carbides and martensite, there are only few detailed collections. In addition, the effects of the etchants are rarely investigated for the same image section. Therefore, this study will exclusively compare identical microstructural regions and the effect of different etchants on them. Two specific sample areas were selected in a low-alloyed cast iron and the effect of both surface removal etching and tint etching reagents on them was examined under a reflected light optical microscope and a scanning electron microscope. The results of the study have shown that some etchants for complex microstructures are only suitable in case potentially present phases are already known. However, the combined use of two etching solutions in particular, led to a very detailed and highcontrast image, capable of revealing and resolving microstructures with a variety of phases.

The effect of separation on the fracture surface of ferrite–bainite dual-phase pipeline steels during a drop weight tear test (DWTT) was investigated via microstructural analysis and fracture surface examination. Two specimens (ST1 and ST2) were designed to have a dual-phase microstructure, with different ferrite and bainite fractions, by controlling the reduction ratio and holding time in the ferrite–austenite two-phase region during the thermomechanical process. Notably, ST2, which had a longer holding time for the retained austenite to absorb carbon, exhibited a harder bainite but softer ferrite phase compared to ST1. The greater hardness difference between those phases in ST2 induced the strain incompatibility frequently evident at the phase boundaries, resulting in lower ductility during the tensile test. Owing to the strain incompatibility, ST2 also generated more separation over a wider temperature range during DWTT. At low temperatures, where the brittle fracture was prominent, it was observed that ST2 generated separations on its fracture surface, whereas ST1 did not. These separations formed a local shear lip around themselves, thereby obstructing cleavage fracture propagation from the notch. Hence, it was confirmed that ST2 had a higher DWTT shear area than ST1 over a temperature range near the ductile–brittle transition temperature (DBTT). As a result, the DBTT of ferrite–bainite dual phase steel could be improved by increasing the amount of separation during DWTT.

Fracture resistance of low- and medium-carbon steels depending on type of microstructure (ferritepearlite, bainite, and martensite) and its volume fraction is studied. On the basis of studying the temperature dependence for impact strength of V-notched samples and pre-cracked specimens the contribution of work for crack initiation and propagation to the general specific impact energy of steels with a different structural type is revealed. It is shown, that the main part of failure energy in low-carbon steels with ferrite-pearlite structure is work for crack initiation. For low-carbon martensite steels the work for crack initiation and propagation is comparable. Failure resistance of tempered low- and medium-carbon steels depending on the quantitative relationship of martensite, bainite and ferrite structures is described. Characteristic structural features of lower and upper bainite influencing fracture resistance are detailed. It is shown that failure resistance of such complex structures is governed by properties of the components, and also their amount and location. Polygonal ferrite, whose separation occurs mainly at the boundaries of previous austenite grains, has a particularly unfavorable effect on ductility properties and cold resistance.

As a kind of high strength microalloyed steel, pipeline steel has high strength and toughness. However, the formation of the intercritically reheated coarse-grained heat affected zone (ICCGHAZ) during the welding process is an important limitation affecting its performance. Herein, microstructural transformation in the ICCGHAZ of X100 pipeline steel after a secondary thermal cycle and hydrogen sulfide stress corrosion cracking (SSCC) resistance of the transformed microstructure are investigated. The microstructure of X100 pipeline steel after the secondary thermal cycle is similar to that after the primary thermal cycle, comprising lath bainite and granular bainite. With an increase in t8/5 (time required for the material to cool from 800 °C to 500 °C), the M-A constituents that continuously precipitate along the prior austenite grain boundaries in the second thermal cycle are coarsened and form a necklace-type structure. A variation in t8/5 does not significantly affect the crystallographic characteristics of the ICCGHAZ. The stress corrosion test shows that the resistance to SSCC decreases and cleavage fracture characteristics become more noticeable with an increase in t8/5.

The low-carbon bainitic–martensitic steel added with microalloying elements was designed, and samples with different cooling rates were produced through ingot casting and simulated strip casting. The ingot cast presented a multiphase microstructure constituted of allotriomorphic ferrite, polygonal ferrite, acicular ferrite and pearlite. The air-cooled strip consisted of polygonal ferrite, acicular ferrite and bainite ferrite, and the gas-cooled strip was composed of acicular ferrite, bainite ferrite and martensite. The results indicated that the duplex sulfide-oxide particles were firstly formed during solidification and refined by the high solidification rate during strip casting; the embedded structure of copper sulfide in duplex particles suggested that the copper sulfide precipitated from the liquid oxide. Independent copper sulfides with an average diameter of ~ 80 nm were formed in the ingot cast and a large number of nanoscale carbides or nitrides less than 25 nm was also observed. However, independent copper sulfides and nanoscale precipitates were hardly observed in the gas-cooled strip cast. For the air-cooled strip cast, only a small number of the nanoscale precipitates were observed. Vickers hardness, yield strength (YS) and ultimate tensile strength (UTS) increased with the fraction of bainite ferrite and martensite, in which the gas-cooled strip showed the highest YS and UTS, and the ingot cast presented the highest total elongation (TE). Nevertheless, the gas-cooled strip showed the best toughness due to the highest product of UTS and TE.

ER70-Ti is a high strength gas shielded welding wire steel, which is suitable for ships, bridges and other structures, and can be used for thick plate welding with high current. In the welding wire industry, ER70-Ti is a high-tech deep-processing product with high added value. In this study, the thermal expansion experiment of ER70-Ti wire rod was carried out. The critical temperature of ER70-Ti phase transformation was measured and the continuous cooling transformation curve (CCT curve) of undercooled austenite was drawn. The microstructure and hardness of the samples under different cooling rates were observed. The results show that Ac1 temperature of ER70-Ti sample was 690 °C, Ac3 temperature was 877°C, and Bs temperature was 575°C. When the cooling rate was low (0.1°C/s~2.5°C/s), the phase transformation products of ER70-Ti were equiaxed polygonal ferrite and granular bainite. With the increase of cooling rate, the grain size of ER70-Ti sample was refined and the bainite content increased from 53% to 85%. When the cooling rate was higher than 5°C/s, all the phase transformation products were bainite. The Vickers microhardness also increased with the increase of cooling rate, from 185HV to 325HV.

The bondline of electric-resistance-welded (ERW) linepipe steel, often etched white (i.e., ferrite) in optical microscopy, is generally believed to be carbon depleted. The mechanism for the carbon depletion, however, is not fully understood by researchers. To this end, atom probe tomography (APT) was used to measure elemental segregation of the as-welded and post-weld heat-treated bondline regions of X70 linepipe welds. The thin vertical features at the bondline in the as-welded condition were identified as carbon-rich martensite-austenite (M-A) constituents, and the majority ferrite phase in the bondline was identified as carbon-depleted ferrite. Following the post-weld normalization, all alloying elements, except Nb and Mo, are homogenized across the bondline and heat-affected zone. The carbon depletion in the ERW bondline was accurately measured. A new mechanism for carbon depletion has been proposed using Scheil calculations of elemental partitioning during weld formation. Segregation of elements in the heat-affected zone was shown to follow the negligible partitioning local equilibrium (NPLE) kinetics for bainite transformation.

Hydrogen permeation and distribution at pipeline welds is critical to integrity maintenance of the pipelines, especially for those made of high-strength steels. The situation becomes even more important under stressing conditions. In this work, metallographic characterization and micro-hardness measurements were conducted at an X80 steel weld. Potentiodynamic polarization and electrochemical hydrogen permeation testing were performance at various zones at the weld, along with numerical modeling of hydrogen distribution at the zones. The X80 steel contains a microstructure of bainite bundles and polygonal ferrite. There are more polygonal ferrite, fewer bainite and some segregated cementite at heat-affected zone (HAZ). The weld metal is featured with acicular ferrite and some grain boundary ferrite. HAZ softening occurs at the weld. The hardness of the weld metal, HAZ and base steel is about 290, 248 and 261 HV0.2, respectively. There is the greatest corrosion current density, i.e., corrosion rate, at HAZ under both elastic and plastic stresses. An applied stress further increases the corrosion current density. Under the plastic stress of 1.1σys (σys is yield strength), the corrosion current densities of HAZ, base steel and weld metal are 41.04, 17.03 and 25.49 μA/cm2, respectively. There are always the greatest hydrogen trapping density and the smallest hydrogen diffusivity at HAZ. Hydrogen, once penetrating the welded steel, tends to accumulate at the HAZ, compared with other two zones. When the welded steel is under stresses, especially a plastic stress (i.e., 1.1σys), the hydrogen diffusivity and permeability decrease, while the subsurface hydrogen concentration and hydrogen trapping density increase remarkably. Plastic deformation favors the hydrogen permeation and trapping at weld, especially the HAZ, to elevate the susceptibility to hydrogen damage. The hydrogen distribution at different welding zones can be evaluated and determined by a developed modeling method.

The domestic achievements and research results in the field of the creation of low-carbon, cold-resistant, weldable steels supplied in the form of sheet metal for the shipbuilding industry—in particular, for use in the construction of icebreaker hulls, tankers, gas carriers, ice navigation vessels, and offshore stationary platforms—are reviewed. The requirements of the Russian Maritime Shipping Register for the chemical composition, assortment, and mechanical properties of cold-resistant, structural, low-carbon steels, their certification, and the current state of affairs in the development of cold-resistant steels in Russia and abroad are presented. The features and principles of the alloying and microalloying of low-carbon, cold-resistant, weldable steels of various strengths in the range from 315 to 960 MPa and the mechanisms to harden and enhance resistance to brittle fracture at low temperatures are reviewed. The requirements and technological methods for the formation of a steel structure providing the required cold resistance are formulated, the features of the cold-resistant steel structure are described in detail, and recommendations are formulated for further improvement of the properties and quality of cold-resistant steels.

At present, optimization of mechanical properties in quenching and partitioning (Q&P) steels relies primarily on the tailoring of microstructural characteristics by manipulating the alloying elements and the Q&P process. However, adjusting the final microstructure by taking advantage of the inheritance effect derived from the initial microstructure has received less attention. Based on a typical Fe–C–Mn–Si system, the present investigation explores the effect of microstructural inheritance in an ultrahigh strength hot-rolled Q&P steel. The typical hot rolling process is followed by three cooling rates, including quenching, air cooling, and coil cooling, to obtain different initial microstructures of full martensite, bainite–austenite, and ferrite–pearlite, respectively. The same Q&P treatment is then performed to elucidate the inheritance mechanism, and the final Q&P products consist of different fractions of tempered martensite, retained austenite, bainite and fresh martensite. With decreasing cooling rates after hot rolling, the ultimate tensile strengths of the steels after the Q&P treatment are identical at ~ 1.5 GPa, while the total elongations increase gradually from 10.9, 12.7 to 14.5 pct. The highest elongation of the coil-cooled Q&P steel can be attributed to the effect of microstructural inheritance on the Q&P process and retained austenite, leading to larger austenite grain sizes and more equiaxed morphologies. The combination of austenite chemistry and morphology results in the optimal mechanical stability for retained austenite to provide continuous and durable strain hardening. Compared to cold-rolled products, hot-rolled Q&P steel is very attractive to the industry as it can potentially eliminate the cold rolling step and reduce the production cost. Utilizing the microstructural inheritance effect provides a simple and feasible route to control the microstructures and mechanical properties of ultrahigh strength hot-rolled Q&P steel.

Formation of the microstructure and properties of pipe steel grade K60 (X70) are studied using physical modeling of thermomechanical processing in a DUO 300 laboratory mill and industrial experiment. Steel microstructure is investigated and temperature ranges for formation of various phases and structural components (ferrite, bainite, martensite, M/A-component) with accelerated cooling are determined. It is found that formation of martensitic regions and M/A-component as secondary structural constituents in a ferrite matrix instead of cementite-containing secondary structures facilitates an increase in steel strength and improvement of the σy /σf ratio without a significant reduction in cold resistance. The chemical composition of steel grade K60 (X70) is developed on the basis of laboratory and industrial experiments, as well as a new industrial process for producing rolled product for pipes for the Sila Sibiri gas pipeline sections crossing active tectonic disturbance zones. Sheet and pipes have a low σy /σf ratio, high values of relative and uniform elongation, impact strength and proportion of ductile component in a fracture after DWTT testing at – 20°C.

A concept is developed for assessing the stability of untransformed austenite entering the martensite-austenite component of a bainitic structure, which is determined by the action of hydrostatic stresses caused by difference in the specific volumes of the austenite and ferrite phases. Relations describing the thermal stability of austenite in the M/A phase are derived. The former is affected by the volume fraction of the phase, the fineness, the size distribution, and the difference in the coefficients of thermal expansion of the austenite and of the ferrite component of the bainite.

Neutron radiography using monochromatic neutron radiation makes it possible to visualize the diffraction contrast between different phases in a polycrystalline material due to the difference in the attenuation of the neutron beam intensity in these phases due to coherent neutron scattering. Although this method has already proven itself in studies of the phase distribution in steels, the literature only provides information on phases that differ greatly in structure, such as ferrite and austenite, austenite and martensite, or austenite and bainite. In this work, the possibilities of the method for visualizing the diffraction contrast between the ferrite and martensitic phases of steel with the same chemical composition and similar crystal structures were investigated.

In the present investigation, friction stir welding (FSW) was carried out on C-Mn steel using various welding parameters. The microstructural examination of the weld nugget (WN) region revealed different area fractions of cementite, pearlite, bainite, and martensite within the ferrite matrix. The average hardness in the region of the WN was higher than the base material. The corrosion behavior of the WN and base metal was investigated. The base metal underwent uniform corrosion. Localized pitting corrosion was dominant in the region of the WN. The primary corrosion products were γ-Fe2O3 (maghemite), γ-FeOOH (lepidocrocite), and Fe3O4 (magnetite). The corrosion rate of the WN was significantly higher than that of the base metal. However, the same was considerably lower than the conventionally fusion-welded specimens. Further, the corrosion rate of the WN increased with the increment in the rotational speed of the tool. The change in the corrosion rate at the WN for various specimens was influenced by the matrix grain size, quantity of second phase, area fraction of special boundary, and relative quantity of low- and high-angle grain boundaries.

A description of the experience in the development and implementation of a model for forecasting the microstructure of rolled tubular products steel during thermomechanical processing of a semi-finished product and the relative IT-solution for the plate mill 5000 of PJSC MMK is presented. This solution is based on modeling the kinetics of structure formation in the mill technological line. The article describes briefly the result of analysis of a number of existing kinetics models of phase and structural transformations from the point of view of their application under the conditions of interest to us. An empirical study of the thermal effects of phase transformations and the influence of the heat removal mode on the kinetics of structure formation is described. The process of development of an empirical kinetics model and implementation of IT-solution, that implements this model, is also described.

A study of the low-carbon steel with high hardenability was carried out. The steel contained the following alloying elements, wt. %: C – 0.20; Cr – 2.0; Mn – 2.0; Si – 1.04 Ni – 1.0; Mo – 0.3. The quenching – partitioning treatment of the studied steel was implemented. The microstructure of the steel consisted of the tempered martensite laths, bainite and martensite-austenite regions. The amount of the residual austenite and the carbon concentration in the residual austenite were estimated. The possibility of the quenching – partitioning treatment of the carburized steel was shown.

The microstructure of the advanced low carbon steel with a superior hardenability was studied. The steel contained the following main alloying elements, wt. %: C – 0.20; Cr – 2.0; Mn – 2.0; Si – 1.04 Ni – 1.0; Mo – 0.3. The dilatometer investigation of the steel under consideration revealed the only phase transformation occurring during continuous cooling (0.1...30 °C/s), which started at the martensite start temperature Ms. It was shown that the isothermal treatment of the studied steel led to the bainite formation above and below Ms. The temperature of the bainite morphology shift was determined.

La Effects on bainite transformation behavior including transformation kinetics and the nucleation and growth of bainitic plates of Mn–Cr–Mo bainitic rail steel were investigated using dilatometry, laser scanning confocal microscopy (LSCM) and transmission electron microscope (TEM). Dilatometry analyses show that La postpones the bainite transformation and increases the transformation activation energy by 31 kJ/mol. LSCM results indicate that La stimulates sympathetic nucleation at the beginning of bainite transformation but delays the growth of bainitic plates at the late stage which becomes the dominant factor of transformation, decreasing the transformation rate and prolonging the transformation time simultaneously. Eventually, combined with the refining effects of La and M3C pinning the interfaces between sub-plates, a kind of complicated multi-layer microstructure consisted of less and finer subunits can be achieved at cooling rate of 0.8 °C/s just by air cooling on site.

In this study, the effect of ausforming temperature and strain on the bainite transformation in a model alloy containing 0.39 pct C, 2.0 pct Mn, 1.9 pct Si, and 0.5 pct Cr (mass percent) was investigated. Samples were deformed at 300 °C, followed by isothermal holding to monitor the phase transformation kinetics. Dilatometry results confirm the acceleration of transformation kinetics when the material is deformed to small strains (0.05 to 0.20). Tensile testing results show that, for long isothermal holding times, no improvement in strength or ductility is achieved as a result of ausforming in comparison with the non-deformed state. However, the benefit of ausforming lies in its ability to accelerate the bainite transformation kinetics and to reduce the isothermal holding time needed to avoid premature fracture due to the presence of a high volume-fracture of fresh martensite. Fracture analysis reveals ductile fracture, with cracks along martensitic regions within the microstructure. The distribution of martensite in the final microstructure was associated with bands of Mn segregation. This banding effect was less visible within the ausformed microstructure because bainite was able to nucleate within both the high- and low-Mn regions. A second advantage of ausforming is, therefore, to reduce the microstructure heterogeneity associated with compositional banding within the steel sheets and, by so doing, eliminate crack propagation within the bands. In spite of the fact that ausforming accelerates the bainite transformation kinetics, the required transformation times for the present steel are still too long for use within industrial continuous annealing lines.

Hot stamped boron steel parts with martensite and bainite combined microstructures are widely used in lightweight manufacturing of automotive bodies. The final products usually experience the manufacturing process, such as trimming or punching, which inevitably introduces inner stress and plastic strain. Hot stamped parts with complex microstructures and manufacturing effects exhibit some special characteristics of hydrogen embrittlement (HE) in service. In this paper, hardened boron steels with a total martensite content and different martensite-bainite combinations, called martensite or bainite dominant microstructures (MDM/BDM), are experimentally obtained by a flexible quenching tool, and the HE susceptibility considering the effects of prestrain and inner (tensile/compressive) stress is investigated based on slow strain rate tensile tests by developing hydrogen charging equipment under constant loads. SEM and EBSD analyses on hydrogen-inducted cracks (HICs) and thermal desorption analysis of hydrogen for different microstructures and strain/stress states are carried out to illustrate the HE mechanism. The HE susceptibility of the as-quenched specimen decreases with increasing volume fraction of bainite, with the fractograph exhibiting intergranular fracture in the MDM and the dimples feature in the BDM. Prestrain weakens the HE susceptibility in the MDM but raises it in the BDM due to the enhancement of hydrogen adsorption, promoting local plasticity in the evolution of the HICs. Inner stresses increase the HE susceptibility in the MDM, which lifts the hydrogen segregation at the grain boundaries to intensify the hydrogen-enhanced decohesion effect. Tensile stress intensifies the sensitivity to HE in the BDM. The promotion of hydrogen adsorption facilitates crack initiation. However, compressive stress has opposite effects on the HE susceptibility due to the improvement of the material ductility by reducing the hydrogen content in the specimen.

The microstructural evolution and fire-resistant properties in the weld heat-affected zone (HAZ) of Mo and Mo + Nb-added fire-resistant steels were investigated. For this purpose, three Fe-0.1 wt%C-1.5 wt%Mn-0.1 wt%Si steels containing various Mo and Nb contents were prepared. HAZ samples were experimentally simulated using a Gleeble simulator at a welding heat input of 30 and 300 kJ/cm. The yield strength of the HAZ samples was higher than those of base steels at both room temperature and 600 ℃, whereas a greater decrease in the yield strength at 600 ℃ compared to that at room temperature occurred in the HAZ samples than in the base steels, indicating that the fire-resistance deteriorated in the HAZs as compared to the base steels. This is due to the formation of hard phases such as bainite and martensite in the HAZs, i.e., bainite and martensite phase have very high yield strength with high dislocation density at room temperature, while their strengths decrease rapidly at high temperature due to a great annihilation and recovery of dislocations at high temperature. In addition, the fire-resistance of the HAZ improved as the heat input was increased. The alloying of Mo and Nb improved the fire-resistance of both the base steels and the HAZs. Finally, the changes in the microstructures of the base steels and the HAZs upon alloying and the heat input and corresponding effects on the fire-resistance were carefully explored and discussed through transmission electron microscopy analyses, atom probe tomography analyses, and calculations of continuous cooling transformation diagrams.

Using correlation-regression analysis, the effects of structure and chemical composition on KCV impact strength anisotropy and difference in critical brittleness temperatures for longitudinal and transverse specimens of ferrite-pearlite and high-strength (ferrite + bainite) steels after thermomechanical rolling are studied. The significant effect of pearlite content and sulfide inclusions on impact strength anisotropy and difference in critical brittleness temperatures for longitudinal and transverse specimens of ferrite-pearlite steels is demonstrated. For high strength ferrite-bainite steels prepared using thermomechanical rolling technology the effect of carbon and sulfur on degree of embrittlement for transverse specimens is considered that also depends on the content of titanium carbonitride and Al2O3-based aluminum oxides.

To investigate the mutual effect of hydrogen, microstructures and stress concentration on the fatigue failure, fatigue behaviors of X65 steel welded joints in both air and saturated H2S solution were investigated at high cycle regime. The experimental result demonstrates that due to lower dislocation density observed by electron backscattered diffraction (EBSD), the fine grain heat affected zone (FGHAZ) is prone to induce cyclic strain localization and further lead to fatigue crack propagating along the FGHAZ in air. Furthermore, the quasi-cleavage with brittle-like fatigue striations and secondary crack on the fracture surface in saturated H2S solution is attributed to hydrogen embrittlement. Moreover, compared with base metal (BM) and FGHAZ, the weld metal (WM) and coarse grain heat affected zone (CGHAZ) are composed of bainite and martensite/austenite (M/A) phase, and more sensitive to hydrogen. Therefore, the fatigue crack is prone to grow along the interface between WM and CGHAZ under the normal applied stress.

It is important for the improvement of mechanical properties of quenching and partitioning (QP) steels to control the martensite transformation behavior and the stability of untransformed austenite. In this study, we evaluated the martensite transformation restart temperature (Mr) via a dilatometer during the reheat austempering pattern. This heat pattern corresponding to the typical heat treatment for QP steel involves quenching to TQ, a temperature between the martensite transformation start temperature (Ms) and the finish temperature, and austempering above Ms. In Fe-8Ni-0.2C alloy, Mr equals TQ, regardless of the progress of bainite transformation during austempering at 673 K (400 °C). In Fe-8Ni-1Si-0.2C alloy, Mr decreases parabolically from TQ with the progress of bainite transformation, and this behavior corresponds to the assumed carbon concentration by bainite transformation. These results indicate that the self-stabilization of the entire untransformed austenite during the first quench from Ms to TQ was preserved, regardless the bainite transformation. Moreover, the stabilization by the carbon concentration in untransformed austenite was added to this self-stabilization.

We have determined different phase fractions from microscopy images using semi-automated image analysis fitting technique, and in addition we have classified each phase according to its hardness. The distribution of grayscale pixels of different phases is first characterised separately for each phase, which are sampled from the microscope image. After this the distributions of the separate phases are fitted to give the corresponding distribution of the whole image. The microhardness measurement provides reliability on the classification of the different phases to ferrite, bainite or martensite. In addition to describing the applied techniques in detail, we present the results obtained from the analysis for one steel subjected to isothermal holding experiments at different temperatures.

The authors have developed the in situ neutron diffraction technique focusing on bainite transformation during austempering. Thanks to the features of time-of-flight type neutron diffraction, textures, phase fractions and lattice parameters can be simultaneously measured at high temperature. In this paper, the design of experimental equipment and analytical approach are mainly described.

The growth behavior and crystallographic orientation of bainite in a low carbon steel were investigated by a combination of in situ observation and electron backscattered diffraction. The intersection of different bainitic ferrite (BF) laths was analyzed in detail. The results show that the growth of a given lath stops when it runs into a preexisting lath of bainitic ferrite. Growth resumes, after a short stasis, through the nucleation of a new lath on the other side of the obstructing lath. The newly nucleated lath has been termed BF-clone because it shares the same crystallographic orientation with the prior bainitic lath. The nucleation of the BF-clone is promoted by the accumulated transformation strain in the region where the growing lath intersected the pre-existing lath. During the growth of the bainitic ferrite laths, the transformation strain was accommodated through the formation of twin-related variant pairs, which formed one after another with the identical lengthening rates.

The effect of ten different combinations with various amounts of niobium (0-0.6 wt.%) and chromium (1-4 wt.%) on weldability and mechanical properties of thermomechanically rolled and direct-quenched low-carbon (0.035 wt.%) microalloyed bainitic steel were investigated. Two compositions were alloyed with boron to increase the hardenability, and two with titanium to improve the toughness properties in heat affected zone. The target of the study was to produce steel with 700 MPa yield strength combined with good impact toughness. Coarse grained heat affected zone (CGHAZ) simulations were performed using the Gleeble 3800 thermomechanical simulator to evaluate the weldability of the investigated steels using cooling time from 800 °C to 500 °C (t8/5) of 5 s and 15 s to simulate different heat inputs in actual welding procedure. Microstructures were characterized using light optical microscopy, and hardness profiles of simulated heat affected zones were determined as well as Charpy-V impact toughness at-40 °C and-60 °C. Shorter t8/5 time (5 s) produced generally better impact toughness properties compared to longer t8/5 -time (15 s). Steels with 4 % Cr had the highest impact energies. Generally, more softening occurred with longer t8/5-time (15 s). However, Cr and Nb alloying decreased the amount of softening in the CGHAZ region, especially with longer t8/5 -time. These results indicate that even with higher t8/5 -time, it is possible to achieve strength properties equivalent to the base material in the CGHAZ region by Cr and Nb alloying.

This work presents the results of prior austenite state on the phase transformation behavior in a medium manganese steel alloyed with Al. The austenite was plastically deformed at two different temperatures. The first was at 1050 °C to ensure its recrystallization before cooling. The second treatment included deformation at 900 °C to keep high dislocation density in the austenite. The analysis of recrystallization process or its lack on the phase transformation behavior was analyzed. The study included thermodynamic calculations to analyze proper conditions of selected heat treatments. The dilatometric analysis of the phase transitions dependence on deformation temperatures was carried out. Deformation continuous cooling transformation diagrams were formed on this basis. The metallographic investigations were performed to determine microstructure constituents after cooling. The investigation proved the presence of ferrite untransformed during the austenitization step at 1100 °C. The dominant phase was bainite which was kept present up to 100 °C s−1 cooling rate. The amount of martensite increased with increasing the cooling rate. For the non-recrystallized austenite, more bainite was present in the microstructure for higher cooling rates compared to the recrystallized one. This was the result of higher density of preferable places for bainite nucleation in the non-recrystallized austenite. The Vickers hardness measurements were conducted after the applied heat treatments. The hardness of steel increased together with applying the higher cooling rates, which corresponded to the higher martensite amount. These values were higher for the non-recrystallized austenite because of higher dislocation density.

The effect of austenitizing temperature on microstructure evolution and impact toughness of a newly developed Fe–3.0C–2.8Si–2.0Mn–0.9V–0.2Cr bainite ductile iron was investigated in this research. The ductile iron specimens were heat treated under different continuous cooling process, involving austenitizing between 900 and 980 °C and followed tempering at 200 °C. Optical microscopy, X-ray diffraction, scanning electron microscope and transmission electron microscope tests were conducted to investigate the microstructure evolution. Impact toughness and Rockwell hardness were measured. The results showed that the microstructure of the ductile iron mainly consisted of graphite, acicular bainite and retained austenite after continuous cooling process. The austenitizing temperature could change the volume fraction and size of bainite and retained austenite. There existed a C-area, where retained austenite accumulated near the graphite, except for specimen austenitized at 920 °C. The impact toughness of specimens increased first and then get worse with the increasing of austenitizing temperature. The impact toughness was related with the volume fraction of bainite and the morphology of retained austenite. The fracture mechanism of the bainite ductile iron belonged to cleavage fracture. Chunky graphite acted as the source of microcrack during the impact process. The bulky retained austenite behaved as a prior path for the microcrack propagation, while the bainite and thin filmy retained austenite limited its propagation.

The interaction of different alloying elements has a significant impact on the mechanical and microstructural properties of steel products due to the thermodynamic and kinetic effect. This article presents a statistically developed and validated model for austenite decomposition during cooling, based on a set of experimental continuous cooling transformation diagrams available in literature. In the model, two-way interactions of the alloying elements are included as add-on terms, and the procedure for the analysis ensures there is no overfitting. The model can be used to predict phase transformation temperatures and critical cooling rates for the formation of polygonal ferrite, bainite or martensite for the production of steel.

Austenite → bainite transformation has been documented in situ by confocal scanning laser microscope, and bainite growth kinetics has been quantified in the coarse-grained heat-affected zone of 2.25Cr-1Mo steel. Four typical nucleation sites of bainite laths have been identified. In addition, growth rate of nine distinctive bainite laths varies from 9 to 200 μm/s, depending on the degree of supercooling and nucleation sites of bainite laths.

Medium carbon steels alloyed with two levels each of V and Si were heat treated to form tempered martensite and bainite, followed by nitriding to evaluate their effects on case hardness and residual stress. Microstructures were quantitatively characterized with Vickers microhardness testing, scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), conventional transmission electron microscopy (CTEM), and scanning transmission electron microscopy (STEM). Measurements of subgrain size, cementite size, dislocation density, and volume fractions of nitride precipitates were combined in a strength model to predict the contributions of V and Si to maximum case hardness after nitriding. Higher V contents lead to increases in maximum case hardness by increasing the volume fraction of MX precipitates. Increases in Si content lead to increased maximum case hardness by increasing the volume fraction of amorphous (Si and Mn)-containing nitride precipitates; the presence of Mn in these precipitates has not been previously reported. Higher Si contents also lead to increases in solid solution strengthening because the majority of Si remains in solution in the ferrite matrix. Greater volume fractions of MX and (Si and Mn)-containing nitrides both lead to higher magnitudes of compressive residual stress. The results presented here demonstrate that alloying can improve the properties after nitriding, providing insight into advanced nitriding steel grades and potentially enabling a wider range of steel microstructures to be nitrided without a debit in performance.

This work studies the surface welding parameters for a practical repair for pearlitic rail grades: R260 and R350HT. A filler metal containing low carbon (0.15 %), high silicon (0.5 %) and nickel (2.5 %), self-shielded flux-core welding electrode (FCAW-S) is the candidate in order to ensure the preferable carbide-free bainite. The film-like morphology of the retained austenite is reported to promote the wear resistance and is ensured by silicon and nickel. The effect of preheat temperature and dilution on the microstructure and resulting hardness can be concluded. Too high dilution, as a result of high current and travel speed, and the reheating during the welding of the second layer can result in martensite formation and too high hardness. Proper control of the dilution ensures satisfactorily hardness and avoids martensite formation.

The main factors responsible for the kinetics of isothermal bainitic transformation in alloy steels are determined using computer simulation of the solid-state phase transformation. The effect of the initial configuration of nuclei of the new phase in the volume of the metal on the parameters of the process of bainitic transformation is determined. Comparative analysis of the computed kinetics of phase transformation at different rates of nucleation of the new phase and of the experimentally observed bainite kinetics in steels 25G2S2N2MA, 300M and 50KhMFA is performed.

The EBSD technique is used to study the structural and textural states in an austenitic corrosion-resistant steel of type Kh18N9 after long-term service in a tube at high temperatures. It is shown that the grains of the α-phase nucleate on coherent twin boundaries (Σ3 of the CSL model) between austenite grains, which determines their unique crystallographic orientation.

The microstructure and properties of a Fe-2Cr-Mo-0.12C (wt pct)-tempered steel plate at different normalization temperatures after hot rolling were investigated to enhance its comprehensive mechanical properties. The microstructure of the steel was mainly composed of a ferrite-bainite-tempered martensite mixture after the normalization-tempering process. The tensile properties and the ductile-brittle transition temperature values indicated that the best comprehensive mechanical properties can be obtained by normalizing at 930 °C and tempering at 700 °C. When the normalization temperature was 880 °C to 930 °C, the yield strength increased as the bainite and tempered martensite hard phase volume fractions increased. However, when the normalization temperature surpassed 980 °C, the precipitation strength behavior compensated for the adverse influence of the effective grain size growth, and yield strength was nearly unchanged. Total elongation reached a peak value when normalized at 930 °C, due to the optimal volume fraction ratio of the soft to hard phase. The Charpy impact fracture changed from a ductile to a brittle fracture with an increase in the normalization temperature. When the normalization temperature was 930 °C, the existence of a certain volume fraction of nano-precipitates and soft ferrite in the microstructure contributed to excellent ductile properties.

A low-alloy medium-carbon bainitic steel was isothermally tempered at 300 °C for up to 24 hours which led to a significant hardness decrease. In order to explain the decreasing hardness, extensive microstructural characterization using scanning and transmission electron microscopy, X-ray diffraction, and atom probe tomography was conducted. The experimental work was further supplemented by thermodynamic and kinetic simulations. It is found that the main underlying reason for the hardness reduction during tempering is related to dislocation annihilation, possibly also with corresponding changes in Cottrell atmospheres. On the other hand, cementite precipitate size, effective grain size of the bainite, and retained austenite fraction appear unchanged over the whole tempering cycle.