[en] The martensitic transformation during gas tungsten arc (GTA) welding of steel 42CrMo4 has been studied using the acoustic emission (AE) monitoring technique. Welds were produced under static conditions (spot welding) and under stationary conditions (travelling arc welding). After spot welding, the root mean square (RMS) value of the continuous acoustic emission was measured, revealing a peak that reflects the evolution of martensite formation during cooling of the spot weld. The RMS value was also measured during travelling arc welding at different heat inputs and corrected for the noise of the welding process to obtain the RMS value due to martensite formation. After welding, optical metallography was carried out to quantify the amount of martensite formed during cooling of the weld. An analysis of the results shows that the squared RMS value is proportional to the volume rate of martensite formation during welding, which is consistent with theory and in good agreement with the results obtained in the case of spot welding. The obtained results suggest that AE can be applied as a real time monitoring technique for the detection of martensite formation during steel welding. (author)

[en] Acoustic emission (AE) signals generated during bainite and martensite formation in steel C45 have been measured, and the AE energy has been correlated with the strain energy accompanying both displacive transformations. The gas tungsten arc welding process was used to vary the volume transformation rates of bainite and martensite formation. The root mean square (rms) voltage U_rms of the continuous AE signals was measured during travelling arc welding and after spot welding. Depending on the cooling rate and the mean austenite grain size, martensite or bainite is formed in the weld. After spot welding with moderate arc currents, only martensite was formed during cooling, which was reflected by a peak in the U_rms data: the martensite peak. An analysis of the results shows that the AE energy produced during the transformation (∫ U-bar2_m dt) is proportional to the volume V_m of martensite in the spot weld, with proportionality factor k_m. During travelling arc welding, bainite and martensite formation occur simultaneously and both displacive transformations contribute to the measured AE power at each moment. The AE power due to bainite formation (U-bar2_b) was calculated using the obtained proportionality factor k_m and was found to be proportional to the volume rate of bainite formation dV_b/dt with proportionality factor k_b. (author)

[en] Highlights: • This is the first detailed report on the fabrication of ODS Eurofer via mechanical alloying and spark plasma sintering. • The optimal fabrication conditions are determined. • Bimodal microstructure, carbides and Y₂O₃ contribute to the mechanical properties of the bulk steel. • Spark plasma sintering is proved to be a promising consolidation technique to fabricate high performance ODS Eurofer. -- Abstract: Oxide dispersion strengthened (ODS) Eurofer steel was prepared via mechanical alloying (MA) and spark plasma sintering (SPS). Different combinations of MA and SPS parameters were adopted in order to optimise the fabrication process. The experimental results show that the sample milled for 30 h, sintered at 1373 K at a pressure of 60 MPa has the highest density and microhardness among all the results obtained. As-produced ODS Eurofer shows a bimodal microstructure with homogeneously dispersed nanoscale Y₂O₃, that is beneficial for the mechanical properties. The yield and tensile strengths are higher while the elongation is lower in the top and bottom surfaces compared to the middle area of the sample. This is due to the presence of a larger number of M₂₃C₆ carbides, resulting from carbon diffusion from the mould material. As-produced samples were also subjected to a heat treatment. A good balance is achieved between the strength and ductility of the heat-treated material. Yielding properties comparable to hot isostatic pressing or hot extrusion as reported in the literature, the processing route presented by this study shows potential to produce high performance ODS Eurofer.

[en] Highlights: • Weld solidification cracking is studied in two advanced high strength TRIP (high phosphorus) and DP (low phosphorus) steel sheets. • Metallurgical factors for cracking includes morphology of the solidifying grains, interface growth rate and segregation of phosphorus. • Thermal diffusivity is an important consideration that affects the weld pool shape and also the cracking behaviour. • A sufficiently accurate thermal model can predict the weld pool shape and allows to achieve favourable conditions to avoid cracking. -- Abstract: Solidification cracking susceptibility during laser welding was studied experimentally and numerically in advanced high strength steel sheets, namely transformation-induced plasticity (TRIP) and dual phase (DP) steel. Using the same heat input, laser bead-on-plate welding was carried out on single sided clamped specimens at various starting distances from the free edge. It was observed that TRIP steel with high phosphorus is susceptible to cracking while in DP steel with low phosphorus, solidification cracking was not observed. The metallurgical factors affecting the solidification cracking were studied and it was found that solidification morphology, phosphorus segregation at the prior austenite grain boundaries, inclusions, interface growth rate and interdendritic liquid feeding have a prominent effect on the strength of the mushy zone. These results are discussed pertaining to the cracking mechanism. For the same welding parameters, a difference in the weld pool shape was observed in both the steels, which is attributed to the high temperature thermophysical properties. Weld pool shape affects the strain distribution in the mushy region and thus the cracking behaviour. The cracking phenomenon is further described using hot ductility curves.

[en] High strength steels combine good formability with excellent mechanical properties and have developed continuously in recent years. Joining these materials is however increasingly difficult as fusion joining processes destroy the carefully constructed microstructure. To counteract this problem, joining processes which require less heat input have been investigated. Laser brazing is a relatively new technique and a potential candidate which has found application in the automotive industry. In this paper the fatigue lifetime properties of laser-brazed Dual Phase (DP600) and TRansformation Induced Plasticity (TRIP700) steel joints made with a copper-aluminium consumable are reported. Joints created with DP600 steel showed fracture through the steel due to a brass present in the stress concentration region at the edge of the reinforcement. TRIP700 steels show similar results if the applied maximum stress is in excess of 280 MPa. However, at maximum stresses of 230 MPa, failure occurred across the interface between the braze metal and the steel. A basic fatigue crack path model is presented for the two competing failure mechanisms.

[en] The softening effect in metals due to ultrasonic vibration is used in many industrial applications. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic treatment either imposes additional stress waves to supplement the quasi-static applied load or causes heating of the metal. In both cases the intrinsic deformation resistance and/or mechanisms of the metal are assumed to be unaltered by the ultrasound. In this study, the effect of an in situ ultrasonic treatment on the microstructure of low-carbon steel (Fe–0.051C–0.002Si–0.224Mn–0.045Al (wt.%)) under tensile deformation is reported. Detailed microstructural analyses reveal that the ultrasonic treatment intrinsically alters the deformation characteristics of the metal. The deformation microstructure underneath the area of treatment in the deformed samples was investigated by a combination of optical microscopy, scanning electron microscopy, crystal orientation mapping by electron backscattered diffraction and X-ray diffraction. The results show that the dislocation density and the fraction of low-angle grain boundaries decrease significantly, accompanied by preferential grain rotation. The softening effect of the ultrasound is found to drive recovery associated with a significant reduction in subgrain formation during deformation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, the reduction in subgrain formation is shown to occur due to the combined application of the quasi-static loading and the ultrasound, but is not a simple addition of the two factors acting separately. The effect of the ultrasound can be attributed to its ability to enhance dislocation dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances, thereby increasing the probability of annihilation

[en] Highlights: • Nanoclusters in ODS Eurofer steel were characterised by APT. • The effect of V and Ta on the formation of nanoclusters was investigated. • The small particles have a variable stoichiometry while the large particles are likely to have Y₂O₃ stoichiometry. • The nanoclusters exhibit a core/shell structure, possibly due to a competition of elements binding with O. Oxide dispersion strengthened (ODS) steels are leading candidates for structural materials in nuclear fission and fusion power plants. Understanding the nature of nano-oxide particles in ODS steels is vital for a better control of the microstructure and mechanical properties to further their applications. In this study, electron microscopy and atom probe tomography (APT) have been used to investigate the nanocluster features in ODS Eurofer steel. With the addition of V and Ta in ODS Eurofer, the nanoclusters exhibit a higher number density with a decreased average diameter, indicating that V and Ta are beneficial for the formation of small clusters. Irrespective of the composition of the base material, the smaller particles have a variable stoichiometry while the larger particles are likely to have Y₂O₃ stoichiometry. The nanoclusters were found to have a core/shell structure, where Y, O and Ta are enriched in the core and Cr and V are predominant in the shell. The formation of the complex structure is possibly the result of a competing effect between Ta, Y, V and Cr binding with O. It is deduced that Ta tends to combine with O in the core (Y₂O₃) of the clusters due to a higher affinity, and pushes V and Cr to the surrounding shell during the formation of nanoclusters.

[en] Sufficient liquid feeding under constrained solidification conditions like, those experienced in welding and casting, is vital to avoid solidification cracking. We present the results of unique in-situ experimental observations of liquid feeding in a solidifying steel melt pool. Liquid feeding was observed in the inter-cellular regions during the terminal stage of solidification. An average liquid flow speed of 450–500 μm s ⁻¹ was found. A pressure difference of the order of 10⁴ Pa is calculated to cause the liquid flow. The rate of solidification shrinkage and the rate of deformation were found to be less than the rate of liquid feeding.

[en] The temperature-dependent plane-specific diffraction elastic constants (DEC) of ferrite in a high-strength quenched and tempered structural steel, S690QL1 (Fe–0.16C–0.2Si–0.87Mn–0.33Cr–0.21Mo (wt.%)), have been determined with a high degree of precision. For this purpose, in situ tensile tests have been carried out at different temperatures in a high-energy synchrotron X-ray diffractometer. The data reported are of considerable significance, since they allow an accurate determination of the residual stress states from the measured local d-spacings between lattice planes