Microstructural evolutions of the railway frog steel solidified under different pressure were studied using OM, FEGSEM, and TEM. The influences of pressure on the solidification, grain sizes, and morphology of carbides of the steel were analyzed. It is found that the melting point of the steel increases with the pressure and the solidified microstructure under high pressure does not vary significantly with the melting temperature. The experimental results show that the solidified microstructure consisting of complete equiaxed dendrites is remarkably refined through the increase of pressure, with the mean dendrite arm spacing of about 24, 18, and 8 μm under 3, 6, and 10 GPa, respectively. It is also revealed by TEM observation that the precipitates change from needle-like and rhombic carbide (M3C) forms during normal (atmospheric) pressure solidification into nodulized hexagonal precipitate M7C3 at 3 GPa, and M23C6 at 6 GPa and 10 GPa, which is associated with the undercooling and distribution of the trace elements. The diameter of the precipitates is between 80 nm and 200 nm.

This paper systematically investigated the effect of cryogenic temperature and soaking time on the 0Cr12Mn5Ni4Mo3Al steel. Microstructure observation and mechanical tests were performed on the specimens by scanning electron microscopy, x-ray diffraction, Vickers hardness tests and tensile tests. Cryogenic treatments were carried out at different temperatures of −73, −120, −160 and −196 °C for a given soaking time of 4 h and at a specific temperature of −73 °C for different soaking time of 8, 12, 21 and 32 h, followed by the subsequent tempering treatment. The results showed that the volume fraction of martensite in this steel has significantly increased and the size of martensite lath has decreased after cryogenic treatment, which leads to the improvement of the mechanical properties of the steel. The cryogenic treatment affected the microstructure by promoting the transformation of retained austenite to martensite and the formation of reversed austenite in the steel. The optimal hardness and strength of this steel were obtained by cryogenic treatment at −73 °C for 8 h. It has been found that the soaking time is a critical parameter for the mechanical properties of 0Cr12Mn5Ni4Mo3Al steel. When the cryogenic temperature is lower than −73 °C, there is no further improvement of the mechanical properties.

•Crack growth rates of multi-cracks deviated from single crack without interaction.•The experimental results cannot be interpreted by the Paris law using ΔK.•A new mechanical driving force parameter ΔKn was proposed.•All da/dN vs ΔKn curves for collinear cracks were overlapped.In this paper, the 2060 Al-Li alloy specimens containing a single crack and multiple cracks were tested under fatigue load. The experimental results showed that the fatigue crack growth rates for multiple cracks deviated from the single crack when there is no interaction between multiple cracks. The results cannot be interpreted by the conventional Paris law using the crack tip stress intensity factor range ΔK calculated with the external applied nominal stress Δσ. At a given ΔK, the fatigue crack growth rates (da/dN) of different cracks for the same homogeneous material should be same based on the Paris law. Therefore, a new mechanical driving force parameter ΔKn that was calculated using the net section stress range Δσn was proposed to describe the single crack and collinear multiple cracks growth behavior under fatigue load. All da/dN vs ΔKn curves for collinear cracks of equal and unequal crack sizes were overlapped, and also overlapped with the single crack curve when no interaction occurred, showing that the ΔKn can be considered as a proper parameter to address the fatigue behavior of materials containing multi-cracks.

•We examined the microstructures of K4648 superalloy specimens after 667, 1121 and 2245 times of service thermal cycles.•The phases in the specimens after 667, 1121 and 2245 times of service thermal cycles were determined by TEM.•Tensile tests were carried out at both room temperature and 800 °C in air.•The fracture surfaces were observed using SEM to investigate the tensile fracture modes of K4648 superalloy.Investigation on the microstructural features of nickel-base K4648 superalloy was conducted on specimens undergone various numbers of service thermal cycles. Our results indicate that during the service thermal cycles, the blocky (TiNb)C phases remain in the K4648 superalloy, whereas the γ′ phases dissolve into the γ matrix. There was no evidence of the existence of η phase in the three different service states of K4648 superalloy specimens. Moreover, the amount of α-Cr precipitates decreases, but their size increases and the shape evolved from small particles into needle-like and then into rod-like long precipitates with increasing number of service thermal cycles, which was analyzed based on the deformation and thermal history. The mechanical property of the specimens decreased with increasing number of service thermal cycles. This is due to the decrease in the amount of (TiNb)C phases and the increase in the size of α-Cr phase, as well as the γ′ phase dissolution. Finally, fracture analysis suggests that the fracture mode of the alloy transforms from ductile mode to mixed mode at room and elevated temperatures.

The 1.1C–1.5Si–1.1Mn–1.4Cr–0.5Mo–0.6Al–0.6Co (in wt%) steel was treated, respectively, by isothermal austempering process and newly developed austempering–partitioning–tempering process (A–P–T). After austempering at 250, 280 and 300 °C for 38, 20 and 10 h, respectively, the sample microstructures were composed of bainitic ferrite plates and film-like retained austenite with thicknesses between 60 and 150 nm. The highest tensile strength of 2003 MPa and hardness value of 53.9 HRC were obtained for the steel after austempering at 250 °C for 38 h, resulting from the combining effect of super-saturated martensite decarburization and stabilization of bainitic formation. After A–P–T treating (heated at 300 °C for 8 h following water cooling, and then heated at 300 °C for 2 h following air cooling), bamboo leaf-like martensite, primary and secondary bainites and retained austenite were observed. The thickness of the secondary bainitic ferrite plates formed during partitioning is much smaller than that of the primary bainite formed during 300 °C austempering. Samples subjected to A–P–T treatment showed improvement in ductility compared to that subjected to austempering.

•Weld joints obtained with different welding parameters were investigated.•The phases of the intermetallic compound layer at the interface were studied.•Effect of heat input in base metal on the intermetallic phases was discussed.•Thermodynamic calculation was carried out for the Fe2Al5 and FeAl3 phases.•The formation process of the intermetallic compound layer was analyzed.The interfacial microstructures of aluminum and galvanized steel dissimilar joint formed by pulsed double electrode gas metal arc (Pulsed DE-GMA) welding–brazing were characterized. Electron probe microanalyzer (EPMA) analysis revealed that the intermetallic compound layer of the welding–brazing joint consisted of Fe2Al5 and FeAl3. Comprehensive analysis of the effect of the heat input parameters showed that, at a constant total welding current (Itotal), the thickness of intermetallic compound at the interface of aluminum and steel decreased with the increase of bypass current. Thermodynamic calculations were carried out to derive the Gibbs free energy diagram for Fe2Al5 and FeAl3. These calculations presented that Fe2Al5 firstly formed, subsequently FeAl3 precipitated during welding process. Further model of the Fe2Al5 and FeAl3 intermetallic compounds formation process was proposed.

Dissimilar linear friction welding of Ti–5Al–2Sn–2Zr–4Mo–4Cr with bimodal and lamellar microstructures was produced. The microstructure evolution of the joint was investigated via OM, SEM, XRD, TEM and microhardness analysis. The temperature field of joint was calculated by a numerical model. The typical microstructures of weld center were recrystallized β grains with some acicular α′′ martensites. In the case of thermo-mechanically affected zone, some partial re-crystallization grains formed in severely deformed microstructures, where a mass of dislocations were observed. However, dislocations were rarely found in the recrystallized β grains of weld center, the temperature field of weld joints calculated was consistent with the microstructural evolution.

Dissimilar welds of Ti17 alloy with lamellar and bimodal structures were produced by friction welding. Microstructural characterizations were performed by means of optical, scanning and transmission electron microscope as well as X-ray diffraction analysis. Mechanical evaluations were conducted by determining microhardness and fracture toughness both in as welded and post weld heat treated (PWHTed) conditions. The results show that in as welded condition, the minimum microhardness and fracture toughness were observed in the weld center. However, in PWHTed condition, the microhardness of weld center was enhanced to a maximum while the toughness was decreased. This is thought to be resulted from fine α and β precipitations upon post weld heat treatment.

Analysis of the dynamic strain aging (DSA) effect was carried out for a reactor pressure vessel (RPV) steel, the A508-class3. The aging process includes the application of various plastic strain values of 1%, 2%, 4%, 6% and 8% with different strain rates at certain tempering temperatures. The microstructure of the specimens was analyzed in detail using optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results indicate that serrated stress–strain curves occurred at the DSA process, and the new hardening phase of M23C6 type carbide is precipitated to pin the mobile dislocations. The hardening effect increases with the increase of the strain value within the uniform plastic strain region, even though the softening effect caused by dynamic recovery is obvious. After analyzing the correlations between aging parameters, hardening, ductile-brittle transition temperature and irradiation effect, it is found that the degeneration of mechanical properties caused by DSA is similar to the irradiation effect.

After direct quenching from 870 °C, the samples were subjected to secondary quenching (L) at different intercritical temperatures within the two-phase region and various tempering temperatures (T). Optical microscopy and scanning electron microscopy were employed to analyse the distributions of phases and grain sizes, and transmission electron microscopy was used to reveal the morphology of martensite and retained austenite. Charpy impact specimens of the steel after different heat treatment were tested. Results show that QLT treatment gives the best impact toughness values compared with the rolling condition and conventional quenching and tempering process, and the optimum QLT heat treatment parameters are determined as Q: 870 °C, L: 770 °C, and T: 580 °C.

In this study, an artificial neural network (ANN) was employed to predict the contact fatigue life of alloy cast steel rolls (ACSRs) as a function of alloy composition, heat treatment parameters, and contact stress by utilizing the back-propagation algorithm. The ANN was trained and tested using experimental data and a very good performance of the neural network was achieved. The well-trained neural network was then adopted to predict the contact fatigue life of chromium alloyed cast steel rolls with different alloy compositions and heat treatment processes. The prediction results showed that the maximum value of contact fatigue life was obtained with quenching at 960 °C, tempering at 520 °C, and under the contact stress of 2355 MPa. The optimal alloy composition was C-0.54, Si-0.66, Mn-0.67, Cr-4.74, Mo-0.46, V-0.13, Ni-0.34, and Fe-balance (wt.%). Some explanations of the predicted results from the metallurgical viewpoints are given. A convenient and powerful method of optimizing alloy composition and heat treatment parameters of ACSRs has been developed.

Compared with high chromium cast iron (HiCr), dry sliding wear behavior of high vanadium high speed steel (HVHSS) containing different amounts of carbon was investigated against a counter-face of 40Cr rings using a block-on-ring setup. Six HVHSS samples with 1.58–2.92 wt% carbon contents were prepared and tested under same conditions to investigate the effect of carbon. The results showed that all HVHSS samples possessed a superior wear resistance over HiCr, and the sample HVHSS4 with 2.58 wt% carbon possessed the highest wear resistance, which was further investigated under the normal load varying from 300 N to 500 N whilst the other wear parameters were kept constant. It was found that the wear loss of both HVHSS4 and HiCr samples increased with the increase of the applied normal loads, but at different increasing rates due to the transition of wear mechanism from micro-cutting to micro-cracking for HiCr.Highlights► We study the dry sliding wear behavior of a high speed steel (HSS) with various C%. ► All HSS exhibit a high wear resistance compared with high chromium cast iron. ► HSS with lath martensite and fine carbides has the highest wear resistance. ► The wear mechanism for HiCr shift from micro-cutting to micro-cracking at 400 N.

Optical microscopy (OM) and transmission electron microscopy (TEM) were used to investigate the effect of tempering temperature on the experimental extra-high carbon steels. It is found that tempering reaction can reduce austenite content and influence the stability of the austenite. As-normalized microstructure is a mixture of twinned martensite and retained austenite. Tempered at 250 °C for 2 h, lath martensite can occasionally be found nearby the diffusionally decomposed austenite area. It also is found that tempering at 650 °C for 2 h, nanoparticles of carbides precipitate in the martensite and decomposed austenite.

TiC particles reinforced Fe matrix composites (TiCp/Fe) with different TiC volume fractions were prepared by in situ reaction synthesis process in conventional melting and casting route. The impact wear resistance and wear mechanism of the TiCp/Fe composites under casting and normalizing conditions were investigated using optical microscopy, SEM and a specially designed impact-abrasive wear device. Results showed that the impact wear of the TiCp/Fe composites occurred through microcutting, microploughing, and partly microcracking. With the increase of TiC volume fraction, the wear resistance of TiCp/Fe composites increased. The normalizing treatment greatly improved the impact wear resistance of the particle-reinforced composite due to formation of the fine secondary TiC particles.

•Q-stress as the function of normalized load was analyzed by FEM.•Effects of Q-constraint on failure assessment diagram (FAD) were discussed.•The predicted results using FAD procedures were compared to experimental values.•The predicted results using the constraint-modified FAD were very satisfactory.The BS 7910 Option 1 and constraint-based failure assessment diagrams (FADs) methodologies were utilized for the integrity assessment of the cracked Al alloy pressure vessel welds (PVWs). To determine the constraint-based FAD curves, finite element analyses were performed to derive the functional relationships between normalized load and Q-constraint for single-edge notched bending (SENB) specimens. The results showed that there was a significant difference between conventional and constraint-based FAD curves for shallow-cracked specimens with low Q-constraint ahead of the crack tip. However, for deeply cracked specimens with high Q-constraint, the effect of constraint-correction on the BS 7910 Option 1 was not pronounced. It was revealed that the prediction based upon constraint-modified FADs was in better agreement with the experimental results of residual strength than the BS 7910 Option 1 procedure which was proved to be conservative for the shallow-cracked vessel specimens.

Ductile-to-brittle transition (DBT) curve fitting approaches are compared over the transition temperature range for reactor pressure vessel steels with different kinds of data, including Charpy-V notch impact energy data and fracture toughness data. Three DBT curve fitting methods have been frequently used in the past, including the Burr S-Weibull and tanh distributions. In general there is greater scatter associated with test data obtained within the transition region. Therefore these methods give results with different accuracies, especially when fitting to small quantities of data. The comparison shows that the Burr distribution and tanh distribution can almost equally fit well distributed and large data sets extending across the test temperature range to include the upper and lower shelves. The S-Weibull distribution fit is poor for the lower shelf of the DBT curve. Overall for both large and small quantities of measured data the Burr distribution provides the best description.Highlights► Burr distribution offers a better fit than that of a S-Weibull and tanh fit. ► Burr and tanh methods show similar fitting ability for a large data set. ► Burr method can fit sparse data well distributed across the test temperature. ► S-Weibull method cannot fit the lower shelf well and show poor fitting quality.

The fracture mechanisms in the ductile-to-brittle transition (DBT) region of the undegraded and degraded A533B steel have been studied. Two mechanisms were observed in the initial stage of the DBT region: transgranular cleavage fracture and intergranular fracture. Factors affecting the proportion of intergranular fracture on the fracture surface have been investigated by four-point-bending (4PB) mechanical tests, SEM observation and statistical fractographic analysis. The effect of intergranular fracture on the critical fracture stress has been discussed and an empirical equation for the mixed critical cleavage/intergranular fracture stress is proposed. Finally, attention is also drawn to the interesting difference in the intergranular fracture percentages shown on the fracture surfaces of fracture toughness test specimens.Highlights► Effect of intergranular on σF can be neglected when it is below a threshold value. ► Linear relationship between σF and intergranular fracture percentage is suggested. ► Effect of intergranular fracture on fracture toughness has been discussed. ► DBTT rise due to embrittlement is explained considering intergranular effect.

In this study, four assessment procedures were used for the integrity analysis of the cracked Al alloy pressure vessel welds (PVWs), and the four methods are (1) to use BS 7910 Option 1 that is a conventional procedure, (2) to involve a modification to the FAD but retain the definition of Kr, (3) to retain the FAD but modifie the definition of Kr (4) to use constraint-based FAD and true Kmat values for SENB specimens with different a/W values. The fracture toughness tests were performed for single-edge notched bending (SENB) specimens with different a/W values to obtain the corresponding Kmat values and to determine the relationship between crack tip constraint (via a/W) and fracture toughness. To determine the constraint-based FAD curves, finite element analyses were performed to derive the functional relationships between normalized load and Q-constraint for SENB specimens. The results showed that the predictions using the second and third procedures was in good agreement with the experimental results of residual strength, and that BS 7910 Option 1 procedure was proved to be conservative for the shallow-cracked vessel specimens, and that the fourth method overestimated the residual strength.