Cryogenic Impact Toughness Research Articles

Enhanced ultra-cryogenic impact toughness in 9 wt% Ni steel through lamellar microstructure refinement

The research aims to effectively enhance the low-temperature toughness of 9 wt% Ni steel, which is used in storage containers for alternative energy sources such as liquefied natural gas. To achieve this, a lamellarizing heat treatment was applied to 9 wt% Ni steel, initially manufactured using the conventional quenching-tempering (QT) process, thereby introducing a quenching-lamellarizing-tempering (QLT) process. The Charpy impact toughness of the 9 wt% Ni steel was then evaluated under two extreme cryogenic conditions: 196 °C and −253 °C. The 9 wt% Ni steels subjected to the QLT treatment demonstrated exceptional Charpy impact toughness values of 238 J at −196 °C and 217 J at −253 °C. This study presents an examination of the microstructural and micro-hardness evolution throughout the various stages of the QLT heat treatment process. A comparative investigation was undertaken by contrasting these results with samples treated using quenching (Q), quenching-tempering (QT), and quenching-lamellarizing (QL) treatments. The QLT treatment induces a substantial volume fraction of retained austenite, combined with a strategically distributed micro-hardness profile. In this profile, the soft phase interspersed among hard phases can alter the crack propagation path and absorb energy, thereby enhancing toughness. This synergy contributes to the exceptional Charpy impact toughness observed at −253 °C. These results suggest that the introduction of the QLT heat treatment is an effective method for enhancing the cryogenic impact toughness of 9 wt% Ni steel at temperatures below −196 °C, compared to the conventional QT process.

Effect of Welding Thermal Cycling on Microstructures and Cryogenic Impact Toughness of Medium-Mn Low-Temperature Steel

Effect of Welding Thermal Cycling on Microstructures and Cryogenic Impact Toughness of Medium-Mn Low-Temperature Steel

On the role of chemically heterogeneous austenite in cryogenic toughness of maraging steels manufactured via laser powder bed fusion

Steels manufactured via Laser powder bed fusion (LPBF) usually exhibit a good synergy of strength and ductility due to their ultrafine microstructure. Yet, their toughness, in particular cryogenic toughness is intrinsically inferior as the formation of micro-voids and oxide inclusions can hardly be fully prevented during LPBF. In this study, a toughening strategy based on chemically heterogenous metastable austenite was proposed to improve the impact toughness of LPBF manufactured high strength steels. As demonstrated in a maraging stainless steel, cryogenic (-196 °C) impact toughness can be enhanced by three times without a sacrifice of strength via tailoring chemically heterogenous austenite in the strong martensitic matrix. Both experiments and molecular dynamic simulations demonstrate that upon impact deformation chemically heterogenous austenite could transform into martensite in a stepwise manner, which could not only absorb massive energy via deformation induced martensite transformation but also make a contribution to local stress mitigation, crack passivation and deflection. The chemically heterogenous austenite strategy has the potential to be utilized for improving the toughness of other high-strength steels.

Effect of grain size and segregation on the cryogenic toughness mechanism in heat-affected zone of high manganese steel

High manganese steels have been nominated as desirable materials used for various cryogenic applications due to their excellent cryogenic toughness caused by twinning. High manganese steel welded joints were obtained through gas metal arc welding, and the microstructural differences at three subzones of heat-affected zone (HAZ) were utilized. The aim was to investigate the effect of grain size and Mn/C/Cu segregation on the cryogenic toughness mechanism of high manganese steel welded joints. The results indicate that the average impact absorbed energies at −196 °C for 1.0, 3.0, and 5.0 mm from the fusion line (denoted by FL + 1, FL + 3, and FL + 5) is 118.6 J, 102.4 J, and 117.2 J, respectively. Compared with base metal, the embrittlement occurred in HAZ. Large grains improve toughness, while small grain boundaries, segregation, and precipitation in HAZ deteriorate toughness. Larger grain size facilitates the primary twins with thinner inter-twin spacing and twin thickness, and the high density of these primary twins inhibits the activation of secondary twins. This, in turn, enhances cryogenic impact toughness. Although the stacking fault energy (SFE) throughout the welding joint lays within the range for the twinning-induced plasticity effect, the enrichment of Mn/C/Cu slightly increases the local SFE and critical resolved shear stress for twining within the segregation zone. This phenomenon makes the activation of primary twins more challenging. Furthermore, the C segregation at grain boundaries leads to the precipitation of fine Cr23C6 carbides. These carbides serve as crack initiation points during impact tests, contributing to degradation in cryogenic impact toughness.

Cooperative roles of twinning, transformation and strain partitioning on the excellent cryogenic strength-toughness synergy of dual-heterogeneous metastable high entropy alloys

In a metastable Fe50Mn30Co10Cr10 high entropy alloy (HEA), a dual-heterogeneous structure, encompassing both grain size and dislocation heterogeneity, has been constructed to obtain excellent cryogenic mechanical performance by varying the annealing temperature (630 and 740 °C) following severe deformation. The HS-630 sample, consisting of recovered coarse-grains (CG), recrystallized fine-grains (FGs) and un-recrystallized ultrafine-grains (UFGs), possesses higher yield strength (YS) of ∼965 MPa, ultimate tensile strength (UTS) of ∼1680 MPa and uniform elongation (UEL) of ∼45 % when compared to the fully-recrystallized HS-740 sample at liquid nitrogen temperature (LNT). The difference in YS is primarily caused by the grain boundary and dislocation strengthening of UFGs, whereas the higher ductility of the HS-630 sample is associated with the hierarchical activation of transformation- and twinning-induced plasticity (TRIP and TWIP) effects, and compatible strain partitioning between heterogeneous domains. The dual-heterogeneity results in different critical stresses of ε-phase nucleation in heterogeneous domains, and promotes sustainable TRIP effect in CGs and the emergence of TWIP effect in UFGs with strain progressing. Moreover, the strain of the ductile transformed ε-phase is mainly accommodated by non-basal slip, which contributes to the strain hardening in the later deformation stage. Furthermore, the higher cryogenic impact toughness of the HS-630 sample can be attributed to the synergistic effects of the fracture mode, the transformation characteristics, and the heterogeneous strain partitioning.

Influence of Nickel on Microstructure and Mechanical Properties in Medium-Carbon Spring Steel.

The effects of adding nickel on the phase transition temperature, microstructure, and mechanical properties of medium-carbon spring steel have been investigated. The results show that adding nickel reduces the martensite start (Ms) temperature, improves hardenability, and refines the sub-microstructure of the martensite, thereby improving yield stress. The yield strength of martensitic steel increases by approximately 100 MPa due to a synergistic combination of grain refinement strengthening and dislocation strengthening, with an increase in the nickel content from 0 wt.% to 1 wt.%. The cryogenic impact toughness of martensitic steel also improved with a higher nickel content due to packet and block refinement and an increase in the proportion of high-angle grain boundaries (HAGBs).

Exceptional cryogenic-to-ambient impact toughness of a low carbon micro-alloyed steel with a multi-heterogeneous structure

A low-carbon micro-alloyed (LCMA) steel with a body-centered cubic (bcc) crystal structure suitable for extremely low temperatures was developed by overcoming the intrinsic ductile-to-brittle transition in bcc alloys at cryogenic temperatures. The excellent cryogenic-to-ambient impact toughness in the LCMA rolled plate results from its heterogeneous microstructure, which gradually changes from bamboo-like ultrafine grains (∼ 1.1 μm) on the surface to relatively equiaxed coarse grains in the core (∼ 3.4 μm), accompanied by a distinct texture gradient variation. The heterostructured LCMA steel displays a cryogenic impact toughness of ∼200 J/cm2 at 77 K, which is 24 times higher than the coarse-grained LCMA steel. Such high impact toughness of heterostructured LCMA arises from the coordinated deformation mechanisms over different length-scales coupled with delamination toughening. At 77 K, the heterostructured steel plate deforms by forming cellular sub-structures at the core to the surface, which refines the microstructure and promotes hetero-deformation induced (HDI) hardening to improve intrinsic toughening. Moreover, the subsequent delamination process induces extrinsic toughening by shielding and blunting the cracks, with the local plane-stress conditions induced by delamination promoting ductile fracture of the coarse grains in the core regions. This low alloy steel with its heterogeneous microstructure exhibits extraordinary impact toughness at cryogenic temperatures highlights the possibility of materials design strategies for sustainable development.

Effect of Nb content variations on the microstructure and mechanical properties of NiCrMo welded joints

The effects of varying the Nb content (1.04 wt%-1.55 wt%) in NiCrMo weld metals on the microstructure, tensile strength and cryogenic impact toughness were investigated. The microstructure of the three different weld metals was observed to be composed of austenite matrix and precipitates by characterization techniques such as SEM, EDS, EBSD, and TEM. The precipitates within the grains were NbC and Ni3Nb phases, and the precipitates on the grain boundaries were NbC phases. With the increase of Nb content, the average grain size of austenite decreased from 205.3 μm to 161.4 μm, and the area fraction of precipitates increased from 1.41% to 3.97%. The cryogenic impact value of the welded joints decreased from 97 J to 76.3 J, while the tensile strength increased from 682.5 MPa to 710 MPa, indicating that the increase in the area fraction of the Nb-rich phase had a deleterious effect on the cryogenic impact toughness and was conducive to the elevation of the tensile strength. When the Nb content in the weld metal was 1.25 wt%, the comprehensive mechanical properties of NiCrMo welded joints were the best, with a tensile strength of 695.5 MPa and a cryogenic impact value of 86.7 J.

Effects of cooling process on the microstructure and mechanical properties of a novel high-V high-Mn TWIP steel for cryogenic storage tank applications

In the present work, a novel high-V high-Mn twinning induced plasticity (TWIP) steel for cryogenic storage tank applications was developed. Furthermore, innovative two-stage cooling processes encompassing ultra-fast cooling and the subsequent furnace cooling were proposed. Effects of cooling process on the microstructure and mechanical properties of the experimental steel were investigated. It was observed that with the change in cooling process (the initial temperature of furnace cooling rised gradually), the dwell time of hot-rolled plates above 600 °C was extended. The extension in dwell time facilitated the precipitation of secondary phase particles, leading to the change in average diameter and volume fraction of secondary phase particles. The yield strength increased with the increasing initial temperature of furnace cooling due to the combined effect of small-sized secondary phase particles inside the grains and large-sized ones located at the grain boundaries. The occurrence of intergranular cracking, primarily caused by the presence of large-sized secondary phase particles at the grain boundaries, was the main factor responsible for the significant reduction in total elongation. The small-sized secondary phase particles inside the grains inhibited deformation twinning, while the large-sized ones at the grain boundaries weaken the binding force between grain boundaries, which ultimately resulted in a decrease in cryogenic impact toughness. The implementation of two-stage cooling processes effectively mitigated the trade-off between yield strength and cryogenic impact toughness, offering a novel approach and valuable insights for optimizing the strength and toughness characteristics of cryogenic high-Mn steels.

Microstructure evolution and enhanced cryogenic toughness of laser welded dissimilar SA645/AISI304L joints via intercritical tempering

In this study, we explored the impact of intercritical tempering on the microstructural evolution and mechanical properties of dissimilar welds formed through laser welding between 5%Ni steel (SA645) and austenitic stainless steel (AISI304L). Upon tempering at 500 °C, reversed austenite manifested within the weld metal. The volume proportion of reversed austenite in weld metal at room temperature increased first, exhibiting a maximum (∼10%) at ∼600 °C, and then decreased with increasing tempering temperature, which was associated with the diminished stability of austenite. Due to the change in tempering temperatures, two distinct types of reversed austenite were present, with one being located at the high-angle grain boundaries and the other within the martensite blocks. As the tempering temperature increased, the weld metal hardness decreased, reaching a minimum of ∼306 HV at 600 °C, and then increased. The dislocation density within the martensite matrix and the quantity of reversed austenite are the crucial factors to determine the hardness of weld metal. The cryogenic impact toughness of weld metal increased at first and then decreased as the tempering temperature increased. The maximum of ∼40 J for the cryogenic impact toughness was obtained when tempering at 600 °C. The low-temperature impact toughness of the weld metal was determined by the synergy of the TRIP effect of reversed austenite, the compatible deformation capability of dual-phase structure, and the crack propagation behavior.

Effect of Mo/Nb ratio variations on the microstructure and cryogenic impact toughness of NiCrMo deposited metals

Effects of varying the Mo/Nb (in weight percent) ratio in ENiCrMo-6-deposited metals on the microstructure and cryogenic impact toughness of the metals were investigated. When the Mo/Nb ratio decreased from 6.09 to 3.28, the Nb-rich phase on the grain boundaries was transformed from un-continuous precipitation to continuous precipitation, the area fraction and average size of the precipitates within the grains were significantly increased, and a eutectic-type Laves phase appeared. Although the austenitic matrix of the deposited metals indicated a ductile fracture mode, the precipitates significantly deteriorated the cryogenic impact toughness. As the average size and area fraction of the precipitates increased, the cryogenic impact value decreased significantly.

An Improved Method for Deriving the Heat Source Model for FCAW of 9% Nickel Steel for Cryogenic Tanks.

The International Maritime Organization (IMO) is tightening regulations on air pollutants. Consequently, more LNG-powered ships are being used to adhere to the sulfur oxide regulations. Among the tank materials for storing LNG, 9% nickel steel is widely used for cryogenic tanks and containers due to its high cryogenic impact toughness and high yield strength. Hence, numerous studies have sought to predict 9% nickel steel welding distortion. Previously, a methodology to derive the optimal parameters constituting the Goldak welding heat source for arc welding was developed. This was achieved by integrating heat transfer finite element analysis and optimization algorithms. However, this process is time-consuming, and the resulting shape of the weld differs by ~15% from its actual size. Therefore, this study proposes a simplified model to reduce the analysis time required for the arc welding process. Moreover, a new objective function and temperature constraints are presented to derive a more sophisticated heat source model for arc welding. As a result, the analysis time was reduced by ~70% compared to that previously reported, and the error rates of the weld geometry and HAZ size were within 10% and 15% of the actual weld, respectively. The findings of this study provide a strategy to rapidly predict welding distortion in the field, which can inform the revision of welding guidelines and overall welded structure designs.

ENHANCING THE CRYOGENIC IMPACT TOUGHNESS OF FERRITIC STEEL BY MULTI-TYPE AND FINE-MICROSTRUCTURE DESIGN

Based on pure melting, multi-type and fine-microstructure design, a ferritic steel with an outstanding combination of cryogenic impact toughness and strength was fabricated. The microstructure was characterized by means of optical microscopy, electron back-scattered diffraction and transmission electron microscopy. The steels with different final cooling temperatures of 663 °C and 560 °C show ferrite and perlite microstructure, and ferrite grain size can be refined from 3.15 µm to 2.67 µm by lowering the final cooling temperature from 663 °C to 560 °C. Moreover, a higher volume fraction of acicular ferrite and polygonal ferrite can be obtained for a lower final cooling temperature of 560 °C. Hence, the cryogenic impact toughness can be enhanced, and the brittle failure can be suppressed, even at –110 °C, showing a higher impact toughness compared with other ferritic steels.

Effect of tempering process on the cryogenic impact toughness of 13Cr4NiMo martensitic stainless steel

The effects of tempering process on the cryogenic impact toughness of 13Cr4NiMo martensitic stainless steel (13-4MSS) were investigated by experiments and crystal plasticity finite element modeling (CPFEM). The tempering structure mainly includes tempered martensite lath, reversed austenite and M23C6 carbide. The tempered martensite lath is refined by the tiny new martensite transformed by reversed austenite, and the refinement degree increases with the tempering temperature. The nucleation of reversed austenite depends on the new martensite and carbide generated during tempering, and more reversed austenite is obtained by double tempering. The cryogenic impact toughness of the material changes nonlinearly with the increase of tempering temperature. It indicates that fine martensitic lath and inverse austenite are beneficial to improve the cryogenic impact toughness of the material, while carbide particles play the opposite role. The results of CPFEM show that reversed austenite coordinates the impact force of the surrounding martensite through large plastic deformation, and eases the stress concentration. Reversed austenite even occurs DIMT behavior under large plastic deformation to further reduce the stress concentration and prevent the initiation and propagation of cracks. As a hard particle, carbide causes the stress concentration, leading to cracks initiation and propagation along the martensite boundary.

Achieving superior cryogenic impact toughness and sufficient tensile properties in a novel high-Mn austenitic steel weld metal via cerium addition

The microstructure and mechanical properties of the novel cryogenic high-Mn austenitic steel weld metals with different cerium (Ce) contents were investigated via electron back-scattered diffraction, scanning and spherical aberration-corrected transmission electron microscopy, and cryogenic impact and tensile test. The results indicated that the crystallographic grain size was significantly refined, and the relative frequency of high-angle grain boundaries increased with the increase of Ce content. Ce element addition could modify the main inclusions from irregular Al2O3 into regular Ce-containing inclusions. Meanwhile, the inclusions gradually became fine and dispersed, and the volume fraction of inclusions decreased with the increase of Ce content from 0 to 870 ppm. The increase of Ce content continuously improved cryogenic impact toughness from 82 J to 116 J, and yield strength, tensile strength and total elongation all increased to different degrees. We found that in addition to the beneficial changes in inclusion modification, grain refinement and increase in the proportion of high-angle grain boundaries, the solid solution of Ce also played a crucial role in the mechanical properties.

A novel Fe–Cr–Ni–Co–Mo maraging stainless steel with enhanced strength and cryogenic toughness: Role of austenite with core-shell structures

Metastable austenite plays an important role in improving the cryogenic impact toughness of maraging steels by strain-induced martensitic transformations. However, a significant volume fraction of austenite decreases the yield strength of the material. In this study, to overcome the strength-toughness issue in maraging steels, austenite with a compositional core-shell structure and specific volume fraction are designed in Fe–Cr–Ni–Co–Mo maraging stainless steel. The core-shell compositional structure comprises retained austenite with bulk content in the core region and high Ni content reverted austenite layers in the shell region. During tensile testing, a step transformation of austenite with a continuous lattice is observed; herein, the shell regions are preserved. In contrast, the core regions transform to martensite. Close to the impact fracture, the thin austenite shell layers are retained from the martensitic transformation, which further contribute to the impact toughness. Under the combined influence of austenite and nanoprecipitates (Laves phase), the investigated alloy reaches a yield strength >1200 MPa at room temperature with good cryogenic impact toughness (77 K, >90 J), thereby outperforming conventional maraging steels and several high-entropy alloys. The current study demonstrates that the chemical heterogeneity within metastable austenite may create unique mechanical properties in structural materials.

A Study on the Enhanced Process of Elaborate Heat Source Model Parameters for Flux Core Arc Welding of 9% Nickel Steel for Cryogenic Storage Tank

Various regulations are being devised and implemented to prevent the environmental pollution that is threatening mankind. The International Maritime Organization has strengthened regulations on sulfur, a notorious pollutant, to prevent sea pollution. In addition, the production of LNG fueled ships is increasing. Among various metals, 9% nickel steel is widely used in the shipbuilding industry because it is advantageous in terms of material strength and cryogenic impact toughness. Various studies are being carried out to predict and prevent its distortion, caused by welding, in the design. To predict welding distortion during flux core arc welding, this study found a way to refine the parameters constituting the Goldak welding heat source. The optimal heat source parameters were derived by using BOP experiments, cross-sectional analysis, finite element analysis and global optimization algorithm. When re-analyzed and verified based on the values, an error of up to 6.3% was found between simulation results and experimental values. The process was improved by clarifying the objective function and reducing the range of candidate welding efficiencies during global optimization and the process efficiency was also improved by reducing analysis time with a simplified model. Therefore, it is thought that this study can contribute to the productivity improvement of LNG storage containers, helping engineers apply it immediately in the industrial field.

Exploring the improvement of strength and cryogenic impact toughness in hot-rolled high Mn austenitic steel for cryogenic application

In general, there is a steep decrease in cryogenic (−196 °C) impact absorbed energy of high Mn austenitic steels after aging at 600–800 °C, which is also the temperature range of aging precipitation. Thus, the precipitation hardening is hardly used in hot-rolled high Mn austenitic steel for cryogenic application and its effect on cryogenic impact toughness is also not understood up to now. In the present work, A high Mn austenitic steel alloyed with Al and V was designed, and the effect of aging temperature on the evolution of microstructure and mechanical properties was investigated by means of electron backscattered diffraction, transmission electron microscopy and X-ray diffraction. The grain size is nearly not affected by aging temperature, exhibiting similar grain size of 6.7–8.1 μm, but the volume fraction and size of precipitated particles greatly depend on aging temperature. The average particle size slightly increases from 5.6 to 6.7 nm with increasing aging temperature from 550 to 650 °C, whereas the precipitated particles are greatly coarsened at 700 °C and the average particle size reaches 16.3 nm. These fine particles can provide a sufficient precipitation hardening of about 200 MPa, leading to a high yield strength of 587 MPa. Meanwhile, this strong precipitation hardening only slight lowers cryogenic impact absorbed energy from 132 to 97 J, also implying that the above-mentioned aging brittle effect should be suppressed or very weak.

Twinning-induced high impact toughness of titanium alloy at cryogenic temperature

The impact toughness of titanium alloys is one of the key factors determining their safe service in cryogenic temperature environments. In this work, the temperature-related impact properties of grade 2 pure titanium and Ti-2.5Al–3Zr–1Mo alloys are investigated. It is found that the impact properties and deformation mechanisms of the above two materials are different with decreasing temperature (20 °C, 0 °C, −50 °C, −100 °C and −196 °C). Speaking of pure titanium: the impact toughness doesn't change obviously, and crack initiation energy (Wi) gradually increases while crack propagation energy (Wp) decreases (Wp of −196 °C is 40% lower than that of 20 °C while Wi is 90% higher). The slip-dominated deformation mechanism at 20 °C gradually transforms into twinning-dominated deformation mechanism under cryogenic temperature. High-density twins formed at −100 °C and −196 °C can effectively relieve stress concentration and lead to better cryogenic impact toughness. By contrast, Wi and Wp of Ti-2.5Al–3Zr–1Mo alloy decrease with decreasing temperature. The impact toughness of Ti-2.5Al–3Zr–1Mo alloy is higher than that of pure titanium when the temperature is above −100 °C. At 20 °C, the synergistic effect of the tortuous crack path and deformation twins of Ti-2.5Al–3Zr–1Mo alloy promotes its impact toughness higher than that of pure titanium. However, the twinning density of Ti-2.5Al–3Zr–1Mo at −196 °C is much lower than that of pure titanium, which cannot release stress effectively and cause a brittle fracture. The research provides theoretical and experimental basis for the application of titanium alloys in cryogenic environment, and provides guidance for the development of titanium alloys with better cryogenic impact toughness.

Mg effect on the cryogenic temperature toughness of Al-Mg alloys

In this study, the Mg and temperature effects on cryogenic impact toughness of Al-Mg alloys are investigated. Cryogenic Charpy impact tests are conducted for several Al-Mg alloys: AA5083 (=reference), Al-6 Mg, Al-8 Mg, and Al-8.5 Mg. The temperature range is – 196 ˚C to 100 ˚C. In all Al-Mg alloys, the impact toughness is improved at higher temperatures. The Al-6 Mg alloy exhibits the largest impact toughness, whereas the lowest impact toughness is observed in AA5083 over the temperature range. Beyond the Mg content of 6 wt%, the impact toughness of Al-Mg alloys decreases with increasing Mg. The planar anisotropy (Δr) is low in Al-Mg alloys of higher impact toughness. The largest amounts of coarse inclusions (>10 µm) are present in the AA5083, providing favorable cracking sites and thereby its poor impact toughness. The grain size and intergranular Mg segregation do not appear to influence the toughness of Al-Mg alloys. Weaker texture in the most ductile Al-6 Mg appears beneficial to gain more homogeneous deformation and lower Δr. Brass {110} 〈112〉, S {123} 〈634〉, and Copper {112} 〈111〉 textures evolve at the expense of a Goss {110} 〈001〉 weakening by increasing the Mg level. This texture evolution illustrates the toughness degradation of Al-Mg alloys of higher Mg levels.