Soft Zone Research Articles

Semi in-situ local structure observation during PWHT of Cr–Mo weldments extent of the PWHT on local structure changes in heat affected zone microstructure

Soft zone formation in dissimilar Cr–Mo welding occurs during post weld heat treatment that could compromise the integrity of the weldment. The decomposition of martensite could cause this formation at the weld heat affected zone adjacent to the fusion boundary. There could not yet determine the actual width of this zone. The extent of martensite decomposition in soft zone Cr–Mo steel dissimilar weld was observed in semi in-situ stages using the x-ray absorption spectroscopy (XAS) at Synchrotron facility. 2.25Cr–1Mo was welded using ER90S–B9 using GTAW process. PWHT was performed at 730 °C for 0.5 h and 2.0 h in a chamber with opening for Synchrotron light analysis at various locations away from the fusion boundary. This was complemented by microstructure and grain size analysis. The results showed the decomposition of martensite started at the fusion boundary and could extend beyond 1000 microns covering both CGHAZ and FGHAZ. Since the experiment was performed in normal atmosphere, the oxide film could form during PWHT on the surface of the specimen. More oxide formed was observed more in the location with high carbide contents. There was more oxide formation at the locations further distance from fusion boundary due to less martensite decomposition. Therefore, the extent of the phenomena could be stated by using information from oxide formation and martensite decomposition which could help to determine the risk on both creep life and oxidation.

Experimental investigation on laser cutting of PMMA sheets: Effects of process factors on kerf characteristics

This paper reports an experimental investigation on continuous CO2 laser cutting of polymethylmethacrylate (PMMA) sheet. The influence of four process factors (laser power, cutting speed, assisting gas pressure and sheet thickness) on five process responses (kerf deviation, top heat affected zone, bottom heat affected zone, maximum surface roughness and rough area) has been investigated. The experimental plan was established based on Taguchi L18 mixed design. The kerf geometry and heat affected zones have been measured using polarising light microscopy technique, while the surface roughness was evaluated using 3D laser scanning confocal microscope. Regression models have been derived to correlate different process responses with different process factors. The cut surface could be classified into three zones: rough zone, moderate zone and soft zone. The rough area is increased by increasing gas pressure and laser power and by decreasing the sheet thickness and cutting speed. Increased kerf deviation has been observed at high cutting speed, laser power and gas pressure. High laser power and low cutting speed produced worst surface roughness and wide heat affected zone. Therefore, it is recommended to use low laser power and high cutting speed to minimize the heat affected zone and the surface roughness. However, increasing the cutting speed may result in high kerf deviation.

Microstructure and Mechanical Properties Relationship of Friction Stir- and A-GTA-Welded 9Cr-1Mo to 2.25Cr-1Mo Steel

Dissimilar weldment of 9Cr-1Mo (P9) to 2.25Cr-1Mo (P22) steel processed with fusion welding process must be subjected to post-weld heat treatment (PWHT) prior to operation due to the presence of martensitic microstructure in the weld. However, due to the gradient of carbon activity, PWHT results in the diffusion of carbon from low Cr to high Cr side, which results in a soft zone and hard zone formation in the weld. This would degrade the mechanical properties of the weld joint. In order to tackle this, the present study employed friction stir welding (FSW) process at two different rotational speeds via 200 and 500 rpm. The microstructure and mechanical properties of the FSW specimens were compared with that of activated-gas tungsten arc (A-GTA) weldments. Regardless of the tool speed, the FSW and A-GTA processed specimens indicated martensitic microstructure in the weldment, and hence, PWHT was carried out. The PWHT treated FSW specimen revealed soft and hard zone evolution throughout the weld matrix, whereas in A-GTA welds, no severe diffusional phenomenon was observed across the weld. The A-GTA weld joint exhibited better strength properties than that of the weld joint made by FSW process, and therefore, A-GTA welding can be suitable techniques for welding of P9–P22 steel.

Numerical simulation of fault-slip rockbursts using the distinct element method

Numerical modeling of fault-slip rockbursts remains a great challenge due to the inherent difficulties in the simulation of fault slippage and rock failure under dynamic ground conditions. A sophisticated numerical method based on the distinct element method is proposed to simulate fault-slip rockbursts. The method successfully simulates the generation of a seismic event induced by fault slip. The seismic event generates seismic waves that spread out in a pattern that vibrating in a direction parallel to the fault and moving in the fault normal direction then triggers rockbursts when reaching an opening. Many features associated with seismic waves are produced using the proposed method including the fault growth process, the radiation pattern of the P- and S- waves, and the dissipation of seismic energy. The effect of fault geometry on the characteristic of the seismic event and the triggering of rockbursts is examined. It is found that the magnitude of the seismic event generated by fault slippage is related to the stress field of the fault. Both P and S waves can trigger rockbursts, depending on whether the wave is incident on the tunnel surface at an oblique angle. The seismic waves generated by fault sliding can be successfully blocked by a soft zone. The findings of this study provide useful insight into the source mechanism of rockbursts and the control of the accompanying damage. The proposed method shows promise for simulating earthquakes.

Evolution of the truncated disc and inner hot-flow of GX 339–4

Aims. We study the changes in geometry of the truncated disc and the inner hot-flow of GX 339–4 by analysing the power spectral density (PSD) extracted from six XMM-Newton observations taken at the very end of an outburst. Methods. We developed a theoretical model of the PSD of GX 339–4 in the 0.3–0.7 keV (thermal-reverberation-dominated) and 0.7–1.5 keV (disc-continuum-dominated) energy bands. The model assumes the standard accretion disc to be truncated at a specific radius, inside of which are two distinct hot-flow zones: one spectrally soft and the other spectrally hard. The effects of disc fluctuations and thermal reverberation are taken into account. Results. This model successfully produces the traditional bumpy PSD profiles and provides good fits to the GX 339–4 data. The truncation radius is found to increase from rtrc ∼ 10 to 55rg as the source luminosity decreases, confirming that the truncation radius can be characterized as a function of luminosity. Keeping in mind the large uncertainty in previous measurements of the truncation radius, our values are larger than some obtained from spectroscopic analysis, but smaller than those implied by reverberation lag analysis. Furthermore, the size of two inner hot-flow zones that are spectrally hard and spectrally soft also increases from ∼5 to 27rg and from ∼3 to 26rg, respectively, as the flux decreases. We find that the radial range of the inner hard zone is always larger than the range of the soft hot-flow zone, but by a comparatively small factor of ∼1.1–2.2

Characterization of the soft zone in dissimilar welds joint of 2.25Cr-1Mo and lean duplex LDX2101 steel

• Study on the effect of the post-weld heat treatment on formation of soft proeutectoid ferrite zone near the fusion line. • Effect of the Ni-based filler on microstructure evolution along the weldments of 2.25Cr-1Mo and lean duplex LDX2101 steel. • Role of the filler composition on hardness and Charpy impact toughness variation along the weldments. • Role of the PWHT on mechanical behavior of the soft proeutectoid ferrite zone. The dissimilar welds joint of 2.25Cr-1Mo steel and lean duplex LDX2101 steel was prepared using the Gas Tungsten Arc Welding (GTAW) process and employing the Inconel 617 filler. The microstructural stability of the dissimilar welds joint (DWJ) was studied using the optical microscope and field emission scanning electron microscope (FESEM) for as-welded (AW) and post-weld heat treatment (PWHT) condition. The PWHT was employed at 760 °C (PW 1) and 810 °C (PW 2) for 2 h of duration. The application of the heat-treatment resulted in the formation of the soft zone at the interface of weld metal and HAZ in 2.25 Cr-1Mo side, i.e., low Cr side. The mechanical behavior of the DWJs was also studied for AW and PWHT condition. A measurable change in the width and hardness of the soft and hard zones was measured with an increase in PWHT temperature. For weld metal, the minimum average hardness was measured 237 HV in AW and a maximum of 248 HV for PW 2. In the AW condition, the average CIT of the weld metal was measured 46.5 ± 3.5 J, which meets the minimum recommended value as per EN 1557:1999 standards. The PWHT showed a drastic reduction in CIT and a minimum of 34 ± 3 J was measured for PW 2. The CIT of the 2.25Cr-1Mo HAZ (notch adjacent to fusion boundary includes the soft ferrite region) was measured 45 ± 5 J in AW and get increased drastically after the PWHT and maximum was measured for PW 2 (86 ± 5 J).

Diffusion Bonding of High-Alloyed Tool Steels with Maraging and Precipitation Hardening Steels

Steel composites for application as protective plates were produced via diffusion bonding. Cold work tool steels were combined with a precipitation hardening steel or a maraging steel using a deformation dilatometer and a vacuum furnace at 1150 °C for 8 h in total. Subsequently, a heat treatment was applied to achieve the final mechanical properties. The microstructure of the interface was investigated by optical microscopy, scanning electron microscopy, electron backscatter diffraction, energy dispersive X-ray spectroscopy and hardness measurements. The results are compared with a simulation of the diffusion of elements performed by MatCalc. Both composites achieve high hardness near the surface of the cold work tool steels, which can have a positive effect on the destruction of projectiles on impact. The influence of carbon diffusion can be observed up to a depth of 3 mm from the interface. As a result of carbon diffusion, soft zones are formed on both sides of the interface, which can be attributed to decarburization, phase transformation and carbide formation. The tough back layer is designed to absorb the remaining energy of the projectile. The combination of a hard front layer and a tough rear layer provides an optimal combination of materials against ballistic threats.

Clarification of creep deformation mechanism in heat-affected zone of 9Cr steels with In Situ experiments

This work quantified nonuniform creep deformation across the heterogeneous heat-affected zone (HAZ) of Grade 91 steel with sophisticated experiments, including an electric-thermal finite element model–assisted Gleeble thermomechanical simulation and a high-temperature creep testing with in situ digital image correlation (DIC). High temperature creep properties of HAZ sub-zones were quantitatively measured by the DIC. By utilizing peak temperature, hardness, local creep strain, and underlying microstructures, creep deformation mechanisms in HAZ were further understood. DIC measurements reveal a creep-vulnerable zone (CVZ) exposed to a peak temperature of 932°C (close to AC3) in the intercritical HAZ experienced the fastest creep strength degradation instead of the soft zone with the lowest hardness prior to creep. The significantly reduced precipitation strengthening from misplacement of undissolved and coarsened M23C6 carbides led to a faster recrystallization of tempered martensite in the CVZ. Weak untransformed tempered martensite (ferrite grains) stabilized by local Cr enrichment from dissolved M23C6 also harmed the CVZ's creep resistance.

Soft x-ray microscopy with 7 nm resolution

The availability of intense soft x-ray beams with tunable energy and polarization has pushed the development of highly sensitive, element-specific, and noninvasive microscopy techniques to investigate condensed matter with high spatial and temporal resolution. The short wavelengths of soft x-rays promise to reach spatial resolutions in the deep single-digit nanometer regime, providing unprecedented access to magnetic phenomena at fundamental length scales. Despite considerable efforts in soft x-ray microscopy techniques, a two-dimensional resolution of 10 nm has not yet been surpassed in direct imaging. Here, we report on a significant step beyond this long-standing limit by combining newly developed soft x-ray Fresnel zone plate lenses with advanced precision in scanning control and careful optical design. With this approach, we achieve an image resolution of 7 nm. By combining this highly precise microscopy technique with the x-ray magnetic circular dichroism effect, we reveal dimensionality effects in an ensemble of interacting magnetic nanoparticles. Such effects are topical in current nanomagnetism research and highlight the opportunities of high-resolution soft x-ray microscopy in magnetism research and beyond.

Microstructure and mechanical properties of weld metal in laser and gas metal arc hybrid welding of 440-MPa-grade high-strength steel

Fiber laser and gas metal arc hybrid welding of 440-MPa-grade high-strength marine steel was carried out at different welding speeds. The influence of welding speed on the microstructure and mechanical properties of weld metal was investigated. The weld-metal microstructure mainly consisted of pre-eutectoid ferrite, side-plate ferrite, acicular ferrite and lath bainite at a low welding speed. With the increase in welding speed, acicular ferrite and lath bainite were the dominant weld-metal microstructures. All samples failed at the base metal during tensile tests, which indicates that there is no soft zone in the hybrid welds. The welding speed had a significant effect on the impact toughness of the weld metal. The impact absorbed energy of the weld metal increased from 35 to 105 J with the increase in welding speed from 0.8 to 2.0 m/min. Large amounts of acicular ferrite and lath bainite were formed in the weld metal at a high welding speed, which resulted in an excellent impact toughness.

Numerical and Experimental Investigation on Hot Stamping of TWB B-Pillar

In order to achieve the targeted crashworthiness performance, the application of Tailor-Welded-Blank (TWB) on the crash-relevant components has become a trend in the automotive industry. This paper aims to investigate the mechanical property and structural performance of the hot stamped TWB B-pillars. After hot stamping, the strength grade for the hard zone is 1500 MPa while the soft zone’s strength grade is 590 MPa. The CAE model shows that the intrusion distance is decreased for the TWB design compared to the monolithic material B-pillar. The three point bending performance could be adjusted by changing the range of the soft zone. It is observed that the weld line has a low crack sensitivity during hot stamping. The transition zone between the hard zone and soft zone is less than 1mm. The ATOS scanning results show that no significant effect on the part’s size deviation for the TWB design compared to the monolithic material B-pillar.

Microstructure and fatigue crack growth behavior of Inconel 718 superalloy manufactured by laser directed energy deposition

In this study, Inconel 718 (IN718) specimens, manufactured by laser directed energy deposition (LDED), are subjected to three different heat-treatments to have different microstructural characteristics. Fatigue crack growth (FCG) testing is performed on both as-deposited and heat-treated specimens. The coarse Laves phases with uneven γ′′/γ′ precipitates severely decrease FCG resistance of LDED IN718. The soft precipitate-free zone around short-acicular δ-phase partly reduces FCG resistance of LDED IN718. The FCG threshold (ΔKth) of a judicious heat-treatment specimen increases by ~ 2.6 times vs. as-deposited specimen and reaches 10.8 MPa∙m1/2, and its ΔKth and FCG curve are comparable to those of wrought IN718.

Modelling the creep behavior of an AlSi10Mg alloy produced by additive manufacturing

AlSiMg alloys produced by additive manufacturing possess an extremely fine and complex microstructure that in many ways defies the most widely used phenomenological models, which, in fact, have turned out to be poorly suited for predicting their mechanical properties. The underlying rationale for the peculiar properties of these alloys has been qualitatively established, however the need for a constitutive model with better predictive capability is still strong. To this aim, the ultra-fine microstructure was described by using a model-material (MM) consisting of soft and hard zones deforming under a similar strain rate. A physically-based set of constitutive equations which took into account also the coarsening/ripening phenomena of the Si-particles was used to predict the creep behavior of the MM. In parallel, the creep response of an AlSi10Mg alloy produced by additive manufaturing and tested in the as-deposited condition was investigated at temperatures ranging from 150 to 225 °C. The minimum creep rate curves obtained for the MM by the constitutive model were then compared with the experimental data obtained by testing the real alloy under constant load in different initial states. The excellent correlation between model curves and experimental results was discussed, taking into account the evolution of the microstructure during creep.

The September 19th, 2017 Puebla, Mexico earthquake

This report presents the observations and findings following the 2017 Puebla earthquake that occurred inMexico on September 19th, 2017. The reconnaissance mission was a collaboration between the New ZealandSociety of Earthquake Engineering (NZSEE), the Universidad Autónoma Metropolitana (UAM) Azcapotzalco,the American Concrete Institute (ACI) Disaster Reconnaissance team, and the Colegio de Ingenieros Civilesde Mexico (CICM). During the earthquake, 77 buildings suffered partial or total collapse and more than8,000 buildings experienced damage ranging from slight damage to significant structural damage necessitatingdemolition. As observed in previous earthquakes, the unique soil conditions of Mexico City resulted inextensive damage to the city’s infrastructure, primarily due to local site effects. The earthquake causedrelatively more damage to buildings built on transition and soft soil zones (i.e. between hard and deep softsoils) than those on hard soils.The NZSEE and UAM team focussed on areas with widespread and extensive damage. They also assessedthe performance of repaired and retrofitted buildings after the 1985 Michoacán earthquake. It was found thatthe lessons learnt from the 1985 Michoacán earthquake led to some risk mitigation measures which benefitedseveral buildings in the 2017 earthquake. Retrofitted buildings were found to have performed very well withlittle or no damage when compared to other buildings.

Research on the Re-Deformation Characteristics of Hot Stamping of Boron Steel Parts with Tailored Properties

Traditional hot-stamping products have super-high strength, but their plasticity is usually low and their integrated mechanical properties are not excellent. Functionally graded property structures, a relatively novel configuration with a higher material utilization rate, have increasingly captured the attention of researchers. Hot stamping parts with tailored properties display the characteristics of local high strength and high plasticity, which can make up for the limitations of conventional hot stamping and optimize the crash safety performance of vehicles. This new idea provides a means of personalized control in the hot-stamping process. In this paper, a new strategy of local induction heating and press hardening was used for the hot stamping of boron steel parts with tailored properties, of which the microstructure from the hard zone to the soft zone shows a gradient distribution consisting of a martensite phase, multiphase and initial phase, with the hardness ranging from 550 HV to 180 HV. The re-deformation characteristics of hot stamping parts with tailored properties have been studied through the uniaxial tensile test, in cooperation with digital image correlation (DIC) and electron backscattered diffraction (EBSD) techniques. The experiments show that there are easily observable strain distribution characteristics in the re-deformation of hot stamping parts with tailored properties. In the process of tensile deformation, the initial phase zone takes the role of deformation and energy absorption, with the maximum strain before necking reaching 0.32. The local misorientation of this zone was high, and a large number of low angle grain boundaries were formed, while the proportion of small angle grain boundaries increased from 13.5% to 63.3%, and the average grain size decreased from 8.15 μm to 3.43 μm. Meanwhile, the martensite zone takes on the role of anti-collision protection, with a maximum strain of only 0.006, and its local misorientation is mostly unchanged. The re-deformation experimental results show that the hot stamping of boron steel parts with tailored properties meets the functional requirements of a hard zone for anti-collision and a soft zone for energy absorption, suitable for automobile safety parts.

Performance improvement of tailored die quenching using material combinations with phase change material in hot stamping

The objective of this study is to investigate the performance improvement of tailored die quenching using material combinations with phase change material over the conventional tailored methods in hot stamping. The thermal characteristics of the phase change material, channel, die, block, and blank are analyzed using thermo-fluid transient simulations with the enthalpy method. Four cases of the material combinations with and without the phase change material are adopted as the die and block. A phase change material of lithium nitrate is applied to the soft zone. All cases without the phase change material do not satisfy the soft zone criteria, while all cases with the phase change material satisfy both the soft zone and hard zone criteria, owing to the effects of the heat storage and release in the phase change material. In addition, the martensite finish temperature area ratio increases with increasing the contact pressure, while the martensite start temperature area ratio decreases. The optimum contact pressure is determined to be 15 MPa for H13 and GTCSⓇ with the phase change material to maximize both the strength and ductile area. Overall, the tailored die quenching process using H13 and GTCSⓇ with the phase change material exhibits the best performance for high-quality tailored blanks.

Correlating DYFI Data With Seismic Microzonation in the Region of Montreal

The Western Quebec seismic zone has moderate seismic activity with few historical damaging earthquakes. Nevertheless, recent risk analyses have shown that the combination of a high level of urbanization with soft soil deposits in the metropolitan area of Montreal could lead to significant damage and economic losses. Over the two decades, several projects have been completed to develop a seismic microzonation to identify zones where seismic waves could be amplified. During the same period, Natural Resources Canada developed an internet application to collect reports from the population after an earthquake and to convert them to the Modified Mercalli Intensity scale (MMI). This paper presents a first comparison of the MMI data compiled after eight recent earthquakes felt in Montreal area with the existing zonation in terms of soil classes. It shows that the MMI from individual reports increases when the observer is located in a soft soil zone. Statistics on average MMI over a regular grid confirms this trend. The numerous reports collected through the internet application, and future applications based on data collected from social media, could become a very useful source of information to complement seismic field measurements when developing and validating seismic microzonation maps.

Microstructure evolution and fracture behavior of Mg-9.5Gd-0.9Zn-0.5Zr alloy subjected to different heat treatments

Microstructure evolution and fracture behavior of Mg-9.5Gd-0.9Zn-0.5Zr alloy subjected to different heat treatments were systematically investigated using SEM and TEM as well as tensile testing. It was found that the microstructure of the as-cast alloy consisted of α-Mg matrix, net-like eutectic compounds (α-Mg + Mg3 (Gd, Zn)), cubic GdH2 phases and lamellar 14H LPSO phases. After solution treated at 515 °C for 24 h, two different cooling processes were used to elucidate the effect of cooling rates on the precipitation microstructure. With a hot water quenching, those secondary phases were completely dissolved. Instead, grey-like patches were observed within the α-Mg matrix, which were proposed to be rod-like Zn2Zr3 phases around the α-Zr particle. In contrast, with a furnace cooling, the formation of 14H LPSO and several cubic Mg3 (Gd, Zn) phases was observed. Furthermore, after hot water quenching, the subsequent ageing treatment parameters were also optimized to be 225 °C for 48 h. In the peak-aged condition, a denser and uniform distribution of basal precipitates γ″ and several basal precipitates γ′ together with Zn2Zr3 and ZnZr2 phases were observed. The samples after the solution treatment (for both hot water quenching and furnace cooling) showed a much higher ductility than the as-cast alloy, while the tensile yield strength (TYS) and ultimate tensile strength (UTS) remained unchanged. After the peak-ageing, a significant increase in the TYS and UTS but a great loss in ductility was observed. In the as-cast alloy, the initiation of microcracks occurred from the net-like eutectic compounds, which was believed to be one of the most important reasons for the tensile fracture, and showed a co-existence of intergranular and transgranular fracture behavior. After the solution treatment, with a hot water quenching, the fracture can be mainly related with failures along contraction twins, and then showed a transgranular fracture behavior. While, with a furnace cooling, due to the presence of the kinked 14H LPSO phases, the fracture was caused by the broken of 14H LPSO phases, and then lead to a transgranular fracture. The peak-aged alloy exhibited a brittle intergranular fracture, which can be related with failures along soft precipitation free zones (PFZs).

Tailoring Soft Local Zones in Quenched Blanks of the Steel 22MnB5 by Partial Pre-cooling with Compressed Air

The high-strength boron steel 22MnB5 steel is widely used for automotive lightweight constructions. A novel approach is promising to tailor strength and ductility in the hardened condition using locally pre-tempered sheets for the hot stamping process. It results in the formation of locally soft spots where mechanical joining is subsequently intended. A slow pre-cooling of the later joining zones with cooling rates below a certain critical cooling rate for obtaining a decreased strength in these regions is required. A tubular air cooling system suited for this task is presented and tested in a process where the subsequent quenching of the overall sheet is realized by rapid cooling in a water bath. Varying the air pressure and cooling duration allows controlling the size of the softened local spot in a wide range and still obtaining a bainitic microstructure. Using two-stage cooling with point jet nozzles and a subsequent hot stamping process with water-cooled dies resulted in a main sheet hardness of about 470 HV5 and of 260 HV5 in the pre-cooled spots, respectively.

Experimental and numerical analyses of buried box culverts in trenches using geofoam

In embankment construction, earth pressure on buried culverts is reduced either by placing them in a trench in the ground or by inserting a soft zone in the fill over the culvert to create an ‘induced trench’. Although the performance of both methods has been studied in the literature, there are limited findings for applying the two methods in combination. A study was therefore carried out on the earth pressure exerted on box culverts buried in a trench with a soft zone included above. Laboratory tests and numerical analyses were conducted to study the performance of the proposed combined method. A surcharge load was applied to the backfill, and earth pressure around the buried box culvert was measured. After validation of the test results by a numerical method, parametric studies were performed to investigate the effect of the expanded polystyrene geofoam properties and the culvert dimensions. The results showed that culvert installation using the proposed method significantly decreased the earth pressure applied to the walls of the buried box culvert, especially when compared to the trench-only method. Using a width 1.5 times greater than the culvert width and a low-stiffness geofoam, earth pressure transferred to the buried culvert can be significantly decreased.