Weldability Evaluation of a Cu-Bearing High-Strength Blast-Resistant Steel

BlastAlloy160 (BA-160) steel, with a nominal composition of Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct), is strengthened by Cu-rich precipitates and M₂C carbides. This alloy was subjected to several weldability tests to assess its susceptibility to certain weld cracking mechanisms. Hot ductility testing revealed a liquation cracking temperature range (LCTR) of 148 K (–125 °C), which suggested moderate susceptibility to heat-affected zone (HAZ) liquation cracking. The enrichment of Ni and Cu was measured along the prior austenite grain boundaries in the simulated partially melted zone (PMZ) and was consistent with similar enrichment at interdendritic boundaries of the simulated fusion zone (FZ). Good wetting and penetration of liquid films along the austenite grain boundaries of the PMZ was also observed. Associated with that finding were thermodynamic calculations indicating a completely austenitic (face-centered cubic) microstructure at elevated temperatures. In testing to determine reheat cracking susceptibility, ductility values of 41 to 78 pct RA were established for the 723 K to 973 K (450 °C to 700 °C) temperature range. The good ductility values precluded susceptibility to reheat cracking according to the test criterion. Dilatometric measurements and thermodynamic calculations revealed the formation of austenite in the reheat cracking temperature range, which was attributed to the high Ni content of the BA-160 alloy.     

New Composition Based Technique for Solidification Cracking Resistance Evaluation

Predicting the occurrence of solidification cracking during the solidification of metallic alloys by numerical simulation is a crucial move for avoiding such defects. Several models are widely available, however, the application of such are impacted due to the specific and not accessible parameters required. A simple, composition-based approach to rank solidification cracking susceptibility is presented. The procedure links computational thermodynamic and computational fluid dynamics (CFD) to provide an evaluation tool for solidification cracking. The method is related to the liquid filling phenomena in dendritic arms during solidification, which plays a critical role in solidification cracking phenomena. The dendritic profiles were constructed using the fraction of solid calculated by commercial thermodynamic software packages. The calculated results were compared with experimental solidification cracking data and showed satisfactory accuracy. The method capability to rank the solidification cracking propensity of similar alloys based on composition provides an important new operative tool to aid alloy development in welding and additive manufacturing related areas.

     

Towards the Optimization of Post-Laser Powder Bed Fusion Stress-Relieve Treatments of Stainless Steel 316L

The use of post-processing heat treatments is often considered a necessary approach to relax high-magnitude residual stresses (RS) formed during the layerwise additive manufacturing laser powder bed fusion (LPBF). In this work, three heat treatment strategies using temperatures of 450 °C, 800 °C, and 900 °C are applied to austenitic stainless steel 316L samples manufactured by LPBF. These temperatures encompass the suggested lower and upper bounds of heat treatment temperatures of conventionally processed 316L. The relaxation of the RS is characterized by neutron diffraction (ND), and the associated changes of the microstructure are analyzed using electron backscattered diffraction (EBSD) and scanning electron microscopy (SEM). The lower bound heat treatment variant of 450 °C for 4 hours exhibited high tensile and compressive RS. When applying subsequent heat treatments, we show that stress gradients are still observed after applying 800 °C for 1 hour but almost completely vanish when applying 900 °C for 1 hour. The observed near complete relaxation of the RS appears to be closely related to the evolution of the characteristic subgrain solidification cellular microstructure.

     

Effects of Ti-Ce refiners on solidification structure and hot ductility of Fe-36Ni invar alloy

Effects of Ti-Ce refiners on the solidification structure and the hot ductility of Fe-36Ni invar alloy were investigated, the corresponding mechanisms were also discussed. The results showed that the s...     

Cold Cracking Development in AA7050 Direct Chill–Cast Billets under Various Casting Conditions

Cold cracking is a potentially catastrophic phenomenon in direct chill (DC) casting of 7xxx series aluminum alloys that leads to safety hazards and loss of production. The relatively low thermal conductivity and wide solidification temperature range in these alloys results in accumulation of residual thermal stress under nonuniform cooling conditions of the billets. In addition, such alloys show a severe loss in ductility below a critical temperature of 573 K (300 °C). This brittleness along with high stress concentration at the tips of voids and microcracks can lead to catastrophic failure. Casting process parameters affect the magnitude and distribution of stresses in the billet and increase the susceptibility of the material to cold cracking. In order to investigate the effect of casting process parameters such as casting speed, billet size, and water flow rate, thermomechanical simulations were applied using ALSIM5 casting simulation software. Among the studied casting process parameters, the increased billet size and high casting speed resulted in the most dramatic increase in residual stress level. Critical crack sizes that led to catastrophic failure were also calculated and are reported against process parameters.     

Validation of Predicted Residual Stresses within Direct Chill Cast Magnesium Alloy Slab

A significant level of cold cracking has been observed within direct chill (DC) cast, high-strength magnesium alloy Elektron WE43. These cracks have been attributed to the formation of significant residual stresses during casting. A finite-element modeling (FEM) code, which is called ALSIM, has been used to predict the residual stress within the DC-cast slab. Verification of the predicted residual stress field within an 870 × 315-mm sized slab has been carried out using neutron diffraction measurements. Given that measurements in such large-scale components using diffraction measurements are particularly challenging and expensive, the efficient use of neutron diffraction measurements is emphasized. This has included the use of sectioning, allowing the residual stress within the slab to be mapped in detail.     

Fracture Behavior of W-Ni-Fe Heavy Alloys

Heavy alloys were liquid phase sintered from compacts of mixed W, Ni, and Fe powders. The alloy compositions ranged from 93 to 97 wt pct W, with the Ni:Fe ratio maintained at 7:3. Sintering was performed under hydrogen in the 1465 to 1485 °C temperature range, giving full density within the first 15 minutes. The room temperature strength and ductility showed major degradation with sintering times in excess of two hours. Tensile tests and bend tests have been performed to isolate the fracture mode and the property determining features. Initial cracking occurs at the tungsten-tungsten grain boundaries and in the tungsten grains. These latter cracks propagate through the structure to give eventual failure. The ductility to failure is shown to be governed by the strength of the tungsten-matrix interface. The maximum elongation depends on the contiguity, which in turn is set by the alloy composition. Formerly a Postdoctoral Fellow at RPI under a fellowship from the Korea Science and Engineering Foundation     

An Investigation of the Massive Transformation from Ferrite to Austenite in Laser-Welded Mo-Bearing Stainless Steels

A series of 31 Mo-bearing stainless steel compositions with Mo contents ranging from 0 to 10 wt pct and exhibiting primary δ-ferrite solidification were analyzed over a range of laser welding conditions to evaluate the effect of composition and cooling rate on the solid-state transformation to γ-austenite. Alloys exhibiting this microstructural development sequence are of particular interest to the welding community because of their reduced susceptibility to solidification cracking and the potential reduction of microsegregation (which can affect corrosion resistance), all while harnessing the high toughness of γ-austenite. Alloys were created using the arc button melting process, and laser welds were prepared on each alloy at constant power and travel speeds ranging from 4.2 to 42 mm/s. The cooling rates of these processes were estimated to range from 10 K (°C)/s for arc buttons to 10⁵ K (°C)/s for the fastest laser welds. No shift in solidification mode from primary δ-ferrite to primary γ-austenite was observed in the range of compositions or welding conditions studied. Metastable microstructural features were observed in many laser weld fusion zones, as well as a massive transformation from δ-ferrite to γ-austenite. Evidence of epitaxial massive growth without nucleation was also found when intercellular γ-austenite was already present from a solidification reaction. The resulting single-phase γ-austenite in both cases exhibited a homogenous distribution of Mo, Cr, Ni, and Fe at nominal levels.     

Flow Behavior and Hot Workability of Nb-15Si-22Ti-5Cr-3Al-2.5Hf Alloy

Constitutive models for flow behaviors of an arc-melted Nb-15Si-22Ti-5Cr-3Al-2.5Hf alloy at temperatures of 1350 °C to 1500 °C and strain rates of 0.001 to 0.1 s⁻¹ have been successfully established during work hardening and dynamic softening stages, respectively, and relatively small average absolute relative errors of the predicted flow stresses are reached (7.7 pct for the work hardening stage and 5.7 pct for the dynamic softening stage). The hot processing map has also been established successfully for this Nb-Si-based ultrahigh temperature alloy. The favorable conditions for hot working of this alloy have been determined as 1350 °C to 1380 °C/0.001 to 0.003 s⁻¹ and 1380 °C to 1440 °C/0.001 to 0.01 s⁻¹. The deformed microstructures under different conditions have been explored and the mechanisms for flow instability of this alloy have been revealed. Flow instability at relatively low temperatures and high strain rates (1350 °C and 1410 °C, 0.1 s⁻¹) is mainly derived from the cracking of Nb₅Si₃ and the debonding of Nbss/Nb₅Si₃ interfaces, while flow instability at higher temperatures (1500 °C) should primarily result from the development of cracks and holes within the Nbss phase because of excessive strain accumulation.

     

The Investigation of Strain-Induced Martensite Reverse Transformation in AISI 304 Austenitic Stainless Steel

This paper presents a comprehensive study on the strain-induced martensitic transformation and reversion transformation of the strain-induced martensite in AISI 304 stainless steel using a number of complementary techniques such as dilatometry, calorimetry, magnetometry, and in-situ X-ray diffraction, coupled with high-resolution microstructural transmission Kikuchi diffraction analysis. Tensile deformation was applied at temperatures between room temperature and 213 K (−60 °C) in order to obtain a different volume fraction of strain-induced martensite (up to ~70 pct). The volume fraction of the strain-induced martensite, measured by the magnetometric method, was correlated with the total elongation, hardness, and linear thermal expansion coefficient. The thermal expansion coefficient, as well as the hardness of the strain-induced martensitic phase was evaluated. The in-situ thermal treatment experiments showed unusual changes in the kinetics of the reverse transformation (α′ → γ). The X-ray diffraction analysis revealed that the reverse transformation may be stress assisted—strains inherited from the martensitic transformation may increase its kinetics at the lower annealing temperature range. More importantly, the transmission Kikuchi diffraction measurements showed that the reverse transformation of the strain-induced martensite proceeds through a displacive, diffusionless mechanism, maintaining the Kurdjumov–Sachs crystallographic relationship between the martensite and the reverted austenite. This finding is in contradiction to the results reported by other researchers for a similar alloy composition.

     

Creep and fatigue crack propagation in a directionally-solidified carbide eutectic alloy

Creep and fatigue crack growth rates and threshold stress intensity amplitudes have been measured for a directionally solidified carbide-eutectic alloy, C73. Fatigue testing temperatures have been ambient, 750 and 950°C for cracking perpendicular and parallel to the solidification direction. In the former cracking direction comparative propagation rates may be understood in terms of the properties of the matrix, which shows a phase transformation from hexagonal to cubic above ~900°C. A situation where crack growth rates decrease with increasing apparent stress intensity amplitudes (ΔK) has been found to exist for propagation parallel to the solidification direction at low ΔK values and high temperatures only. This phenomenon can be related to the occurrence of crack branching and multiple cracking of the carbide fibers. Considerations of plastic zone sizes and critical defect sizes for crack propagation are consistent with the conditions necessary for such crack deceleration to occur. Transformation of the M₇C₃ fibers, present in the as-cast condition, to M₂₃C₆ at cell boundaries of the solidification structure occurs at a temperature of 950°C. Although M₂₃C₆ carbides are easily cracked and therefore probably reduce propagation rates by causing secondary cracking, their presence is known to be detrimental to creep properties. High cycle fatigue threshold stress intensity amplitudes for C73 in either loading direction at room temperature, 750 and 950°C are ~20 pct lower than for the cast nickel-base alloy, EST 738LC,i.e. critical defect sizes are ~10 pct smaller in C73. Despite the known sensitivity of cracking rates and threshold values to factors such as minor fluctuations in loading amplitude it is believed that these differences are significant.     

CSP热轧板卷边部裂纹成因及控制

为了抑制CSP热轧板卷边部裂纹,对CSP热轧板卷边部裂纹的成因进行了研究.CSP热轧板卷边部裂纹缺陷主要有3类:边部横裂纹、边部纵裂纹、边部烂边或掉块等.板卷产生边部裂纹的主要原因是:连铸坯表面边部横裂纹(包括深的振痕)和边部的细小纵裂纹,在加热和轧制过程中不断扩展;钢液在凝固以及铸坯在冷却、均热、轧制、层流冷却和卷取等过程中的热应力、机械应力以及相变应力等作用力超过钢的塑性变形抗力.抑制CSP热轧板卷产生边部裂纹的主要措施是:控制好合适的钢水成分;制定有效的工艺参数,如结晶器热流密度、结晶器振动参数、二冷冷却强度等.工业试验结果表明,CSP热轧板卷边部裂纹率由7.93%降低到1.81%.     

Stress Corrosion Behavior of Low-temperature Liquid-Nitrided 316 Austenitic Stainless Steel in a Sour Environment

Low-temperature nitridation is a widely used surface heat treatment. Low-temperature liquid nitridation was applied to 316 austenitic stainless steel and an S-phase (expanded austenite) layer was achieved on the alloy surface. The effect of the S-phase layer on corrosion resistance and stress corrosion cracking was investigated in a sour environment. When a bending stress of 164 MPa (80 pct yield stress, YS) was applied, no macroscopic corrosion cracking and pits were observed on the nitrided samples and the S-phase layer stayed intact. Although no macroscopic corrosion cracking was observed on the non-nitrided samples under 205 MPa (100 pct YS), some pits were formed on the alloy surface. This could be attributed to the high stresses and hardness, and the excellent corrosion resistance of the S-phase layer introduced by low-temperature nitridation. Supersaturated nitrogen atoms in the S-phase layer can effectively prevent the decrease in pH of the corrosive medium and accelerate the alloy repassivation kinetics. However, when the bending stress was increased to 205 and 246 MPa (100 pct YS, 120 pct YS), macroscopic cracks were observed in the presence of both tensile stress and a corrosive medium.

     

Mapping of Diffusion and Nanohardness Properties of Fcc Co-Al-V Alloys Using Ternary Diffusion Couples

Ternary diffusion behavior in Co-Al-V ternary alloys was investigated at 1373 K and 1473 K (1100 °C and 1200 °C) by the solid-state diffusion-couple technique. The extraction and interpolation of diffusion data allows the diffusion properties of Fcc Co-Al-V alloys to be mapped in the composition arrays of Al and V. A full picture of the diffusion properties was then constructed by interpolating all accessible interdiffusivities and impurity diffusivities of Co-Al binary and Co-Al-V ternary with a Redlich–Kister polynomial, in a graphic manner depicting a rapid increase of Al diffusion with increasing Al and a weak decrease with the V addition alone. Further incorporation of a nanoindentation technique enables the nanohardness property of the Co-Al-V fcc alloys to be screened in the Al and V arrays. The hardenability in the Co-Al-V alloy system has been evidenced; specifically, the alloy arrays containing higher contents of V, being solution-and-quenching processed, exhibit more effective strengthening than those with the addition of Al. The discovery of Co-Al-V alloys with comparable nanohardness but differing alloy compositions could facilitate the strengthening design of next generation Co-based alloys.

     

Effect of Cyclic Oxidation Exposure on Tensile Properties of a Pt-Aluminide Bond-Coated Ni-Base Superalloy

The tensile behavior of a directionally solidified (DS) Ni-base superalloy, namely, CM-247LC, was evaluated in the presence of a Pt-aluminide bond coat. The effect of the thermal cycling exposure of the coated alloy at 1373 K (1100 °C) on its tensile properties was examined. The tensile properties were evaluated at a temperature of 1143 K (870 °C). The presence of the bond coating caused an approximately 8 pct drop in the strength of the alloy in the as-coated condition. However, the coating did not appreciably affect the tensile ductility of the substrate alloy. The bond coat prevented oxidation-related surface damage to the superalloy during thermal cycling exposure in air at 1373 K (1100 °C). Such cyclic oxidation exposure (up to 750 hours) did not cause any further reduction in yield strength (YS) of the coated alloy. There was a marginal decrease in the ultimate tensile strength (UTS) with increased exposure duration. Because of the oxidation protection provided by the bond coat, the drastic loss in ductility of the alloy, which would have happened in the absence of the coating, was prevented.     

Matrix cracking of cross-ply ceramic composites

A micromechanics study is presented of the matrix cracking behavior of laminated, fiber-reinforced ceramic cross-ply composites when subject to tensile stressing parallel to fibers in the 0° plies. Cracks extending across the 90° plies are assumed to exist, having developed at relatively low tensile stresses by the tunnel cracking mechanism. The problem addressed in this study is the subsequent extension of these initial cracks into and across the 0° plies. Of special interest is the relation between the stress level at which matrix cracks are able to extend all the way through the 0° plies and the well known matrix cracking stress for steady-state crack extension through a uni-directional fiber-reinforced composite. Depending on the initial crack distribution in the 90° plies, this stress level can be as large as the uni-directional matrix cracking stress or it can be as low as about one half that value. The cracking process involves a competition between crack bridging by the fibers in the 0° plies and interaction among multiple cracks. Crack bridging is modeled by a line-spring formulation where the nonlinear springs characterize the sliding resistance between fibers and matrix. Crack interaction is modeled by two representative doubly periodic crack patterns, one with collinear arrays and the other with staggered arrays. Material heterogeneity and anisotropy are addressed, and it is shown that a homogeneous, isotropic average approximation can be employed. In addition to conditions for matrix cracking, the study provides results which enable the tensile stress-strain behavior of the cross-ply to be predicted, and it provides estimates of the maximum stress concentration in the bridging fibers. Residual stress effects are included.     

稀土对Fe-43Ni膨胀合金热塑性的影响

Fe-43Ni膨胀合金因其凝固组织粗大导致合金的热塑性较差,在热加工过程中开裂严重。针对该问题,通过Gleeble高温拉伸试验、SEM、EDS等手段研究了稀土Ce及其含量对Fe-43Ni膨胀合金组织和热塑性的影响。结果表明,常规Fe-43Ni合金在温度小于1 000℃时热塑性较差。添加质量分数为0.025%的Ce,合金中形成了大量的Ce₂O₃和Ce₂O₂S稀土夹杂物,合金的热轧组织显著细化。850～1 100℃温度区间的热塑性明显提高,热塑性良好的温度区间向低温区扩大了50℃。但是添加质量分数为0.063%的Ce时,合金铸锭在热轧过程即发生开裂,热塑性严重恶化。稀土添加适量可提高合金的热塑性,反之过量会恶化热塑性。试验结果为大生产中热轧稀土微合金化Fe-43Ni膨胀合金提供一定的指导。     

Test temperature and strain rate effects on the properties of a tungsten heavy alloy

Liquid phase sintered tungsten heavy alloy specimens with a 90W-7Ni-3Fe composition were tested for temperature and strain rate effects on mechanical behavior. Both fracture stress and strain were measured for samples tested at 20, 300, or 600 °C, with crosshead speeds ranging from 0.004 to 400 mmJs in an argon atmosphere. Fracture surface examinations showed a dramatic increase in tungsten cleavage as the ductility increased. The effect of an increasing strain rate is a slight strength increase with a concomitant ductility decrease. Alternatively, higher test temperatures degrade strength with a nonsystematic effect on ductility; maximum ductility occurs at 300 °C and a slow strain rate. Surface oxidation at 600 °C greatly degrades ductility. The results are mathematically modeled using classic strain rate dependent equations.     

The Role of Eta Phase Formation on the Creep Strength and Ductility of INCONEL Alloy 740 at 1023 K (750 °C)

INCONEL alloy 740 is an age-hardenable nickel-based superalloy proposed for advanced ultrasupercritical steam boiler applications operating at high stress and long times above 973 K (700 °C), where creep will be the dominate deformation mode. During high-temperature exposure, the alloy can form eta phase platelets that many have suggested may be detrimental to creep strength and ductility. In this study, creep-rupture tests were conducted on smooth and notched bars of INCONEL alloy 740 at 1023 K (750 °C) for times up to 20,000 hours. Examination of the creep-rupture life, creep ductility, failure modes, and microstructure by quantitative electron microscopy shows that a small amount of eta phase does not diminish the creep performance. Applied stress appears to have a minor effect on the precipitation of the eta phase but not its growth rate. Based on the observation that the microstructure after 20,000 hours of creep exposure has reached equilibrium in comparison to thermodynamic calculations, it is concluded that 20,000 hour creep tests are adequate for prediction of long-term creep performance.