Low Temperature Toughness Research Articles

Investigation of Crystallization, Morphology, and Mechanical Properties of Polypropylene/Polypropylene-Polyethylene Block Copolymer Blends.

Polyethylene (PE)-based elastomers are the ideal choice for enhancing the compatibility of polypropylene/polyethylene (PP/PE) blends and improving the mechanical properties of PP-based materials. However, the issue of blend systems lies in the interplay between the crystallization processes. Therefore, we investigated the crystallization behavior during the cooling process of a new generation of PP/PE block copolymers (PP-b-PE) and random polypropylene (PPR, a copolymer of propylene and a small amount of ethylene or an alpha-olefin) blends using in-situ X-ray diffraction/scattering and differential scanning calorimetry (DSC) techniques. We also conducted mechanical performance tests on PPR/PP-b-PE blends at room temperature and low temperature (-5 °C). The results indicate that during the cooling process, the PP phase of PP-b-PE will follow the PPR to crystallize in advance and form a eutectic mixture, thereby enhancing the compatibility of PP/PE. Moreover, the PPR/PP-b-PE blend will form stable β-(300) crystals with excellent mechanical properties. Due to the improved compatibility of PP/PE with PP-b-PE, PE crystals are dispersed within PP crystals, providing bonding that improves the toughness of PPR under the low stiffness failure conditions of PPR/PP-b-PE blends, thereby enhancing their impact performance at low and room temperatures. This research has great significance for both recycling waste plastics and enhancing the low-temperature toughness of PPR.

Unveiling the intrinsic rhenium effect in Tungsten

Body-centered cubic refractory metals, as a class of critical structural materials, are often restricted by poor low-temperature toughness, which is a common, undesirable phenomenon that continues to be the focus of intense research. Historically, rhenium (Re) has been considered the most promising alloying element in improving ductility and reducing the ductile-to-brittle transition (DBT) temperature of group-VI metals, known as the “Re effect”. However, whether this is indeed possible remains uncertain so far. Here, in the modelled W-xRe (x = 0, 3, 5, 10 wt.%) alloys, we reveal that only in a limited temperature range of 50 °C∼200 °C does the Re softening occur, imparting a positive but small effect on fracture toughness. At higher temperatures (>300 °C), the dynamic interactions between Re solutes and dislocations produce profuse jogs and sessile loops that strongly hinder dislocation motion and cause hardening. The resultant difficulty in plastic flow significantly degrades the high-temperature (>300 °C) toughness of W-Re alloys, leading to an undesirable increase of the DBT temperature, which shifts higher with increasing Re content. Our discovery of the solution softening-to-hardening transition, including its dislocation mechanism, clarifies the intrinsic effect of Re on the toughness of recrystallized W alloy, providing essential knowledge for the compositional design of refractory alloys in general.

Influence of the Holding Thickness and Process in the Microstructure and Mechanical Properties of a Microalloyed 380 MPa Structural Steel

Flat low carbon microalloyed structural steels processed by controlled rolling have excellent mechanical properties, particularly regarding toughness, since austenite is rolled with full recrystallization between rolling passes (roughing stage at high temperatures), and, subsequently, with no recrystallization at all between passes (finishing stage at low temperatures), maximizing nucleation of ferrite grains during austenite transformation after hot rolling. However, rolling must be prompted in an intermediate temperature range where recrystallization of austenite between rolling passes occurs partially, as this can lead to great heterogeneity in the grain size distribution of the product, a potential condition to impairs its low temperature toughness. On the other hand, this holding period that is necessary to avoid rolling within this temperature range can last several minutes, reducing the productivity of the rolling line and, in this way, potentially harming the financial performance of the plant. This work was developed to analyze several proposed rolling conditions for the finishing stage during controlled rolling of 16 mm thick strips of a low C Nb-Ti microalloyed steel strip, 380 MPa structural grade, processed in a Steckel mill, to identify the best condition that provides maximum productivity without affecting the required microstructural, toughness and mechanical characteristics of the final product.

Enhancement of low-temperature toughness of Fe–Mn–C–Al alloy by rare earth Ce-modified inclusions

Enhancement of low-temperature toughness of Fe–Mn–C–Al alloy by rare earth Ce-modified inclusions

Design of smart transforming metal ceramic multilayers

Novel metal ceramic multilayer composites with the ability to phase transform into a single phase ultra-high temperature ceramic are described. These materials offer enhanced low-temperature fracture toughness and reliability without sacrificing high-temperature mechanical properties by eliminating the metal reinforcement layers upon reaching temperatures sufficient for the activation of carbon diffusion. At low temperatures, these materials are practically indefinitely stable because of the high activation energy for carbon diffusion in the carbide layers that control the phase transformation kinetics. Here, knowledge of the overall thermodynamics, i.e., carbon content, is combined with prediction of phase transformation kinetics and toughening in the limit of small-scale yielding to characterize the design space of these novel composites. It is proposed that the ability to control thermodynamic and mechanical properties of the post-transformation ceramic via selection of metal-to-ceramic layer ratios makes these novel composites an excellent alternative to current methods for reinforcement of ultra-high temperature ceramics.

Crystallographic texture and the mechanical properties of API 5L X70 pipeline steel designated for an arctic environment

The aim of this research is to examine the effect of roughing and finishing reductions on crystallographic texture. Three rolling conditions were applied with the same total reduction but different combinations of the roughing and finishing reductions, identified by their finishing reductions as low (X7FL), medium (X7FM), and high (X7FH). Texture measurements showed significant inhomogeneity at the centreline region that had higher intensities of Epsilon, Alpha, and Gamma fibre textures compared to the quarter-thickness and surface layers. Drop Weight Tear Test (DWTT) indicated that X7FL and X7FM specimens, with a relatively higher fraction of texture components of {332}<113>, Gamma fibre, and Transformed Copper (TC), exhibited superior low-temperature impact toughness compared to the X7FH steel. The electrochemical hydrogen charging experiments showed that the internal hydrogen cracks appeared only in X7FL and X7FM steels due to the large presence of detrimental {001} grain orientation on surface layer, perpendicular to ND.

Maintaining modulus of polypropylene composites with ultra-high low temperature toughness by synergistic branched polyethylene and nucleating agents

In this work, the synergistic effect of highly branched polyethylene (HBPEs, s = 1, 2) elastomers featuring dendritic structure and α-type nucleating agents (NA1, NA2) on the toughness and stiffness of block polypropylene (PP–B) was investigated. Blends of PP-B/HBPE/nucleating agent ternary systems were prepared, yielding materials with exceptionally high low-temperature toughness and minimal stiffness loss. The results indicate that HBPE enhances the low-temperature impact strength of the system but leads to a reduction in modulus. After the addition of the nucleating agent, both the low-temperature toughness and stiffness of the blend are improved simultaneously. The PP-B/HBPE2/NA2 (90/10/0.2) blend achieves the optimal balance between low-temperature toughness and stiffness, with its low-temperature impact strength increasing by a factor of 7 compared to PP-B, reaching 56.9 kJ/m2, and a bending modulus of 1110 MPa, nearly identical to PP-B. Dynamic mechanical properties and phase morphology results indicate that HBPE reduces the glass transition temperature of the rubber phase in the blended system and exhibits good compatibility with PP-B. Analysis of the crystallization behavior reveals that the significant increase in the crystallization temperature of PP-B and the substantial reduction in the size of spherical particles observed in PP-B/HBPE/NA blends are facilitated by NA1 and NA2. Additionally, NA1 and NA2 promote the transformation of the PP-B matrix into α-type crystals and enhance the crystallinity of PP-B.

Lowering the ductile-to-brittle transition temperature to -36 °C via fine-grained structures in chromium

The inherent ductile-to-brittle transition (DBT) behavior of body-centered cubic (BCC) metals severely limits their applications, and leads to low-temperature brittleness. To enhance the low temperature toughness of BCC metals, we take chromium as an example and design a fine-grained structure with average grain size of 5 μm, which shows a substantial decrease of DBT temperature to -36 °C. Compared with other samples with larger average grain sizes (45–135 μm), the fine-grained chromium still exhibits excellent toughness at -20 °C, and has the highest fracture energy and the yielding load. Our results reveal that numerous grain boundaries act as dislocation sources to emit easy glide edge dislocations at low temperature and also effectively slow down crack propagation, all together toughen ambient brittle chromium.

Effect of microstructural characteristics on the impact fracture behavior of cryogenic 9Ni steel

The percentage of shear fracture largely determines the service performance of 9Ni steel used in low-temperature pressure vessels. Through elemental analysis, microstructural characterization, and mechanical property tests, this study investigates why the percentage of shear fractures is low in 9Ni steel and clarifies the mechanism by which the microstructural characteristics influence the low-temperature impact behavior of 9Ni steel. It was found that cleavage fracture zones, formed when segregation bands appear in the microstructure, decrease the percentage of shear fractures at the impact fracture surface. Specifically, as the segregation area increases from 0.9% to 7.1%, the shear-fracture percentage in 9Ni steel sharply decreases from 100% to 65%, accompanied by a deterioration in low-temperature toughness. The segregation zone is enriched in austenite-forming elements (Ni, C, Mn), leading to a tempered martensite microstructure with a lath shape. The small number of high-angle grain boundaries and low interface bonding strength cannot effectively prevent crack initiation and propagation, resulting in brittle cleavage fracture. In contrast, the non-segregated zone is tempered sorbite with a uniform structure, several high-angle grain boundaries, and a high interface bonding strength. These features hinder crack initiation and propagation. Furthermore, the shear-fracture zone generated in the non-segregated zone exhibits ductile fracture characteristics.

Obvious different formability and mechanical properties of warm caliber rolling Q345 steel with normalized and tempered states

The formability and strengthening-toughening behaviors of the warm rolled steels are highly dependent on the raw heat treatment state. Herein, the normalized and tempered Q345 steels were both subjected to caliber rolling at 500 °C with different rolling reduction ratios (18%, 44%, and 68%) to obtain elongation grains and carbide particles with different distribution and size. With the increase of caliber rolling reduction ratio, the strength and fracture elongation of the rolled normalized Q345 steels are gradually decreased, whereas the rolled tempered Q345 steels exhibit obvious improvement in these properties. The warm rolled normalized steel contains severe uneven grain distribution and coarse carbides, which leads to high stress concentration and low mechanical properties, as well as poor formability. On the other hand, warm rolled tempered steel contains nanoscale carbides and homogeneous elongation grains that help to release high stress and strain concentration, resulting in improved formability and mechanical properties to a certain extent. Moreover, the warm rolled tempered steel (Tempformed/TF) exhibited a significant improvement in the impact absorb energy, about 3.1 and 47 times higher than those of the quenched and tempered (QT) state at 20 °C and −80 °C, respectively. This remarkable improvement in low-temperature impact toughness can be attributed to the delamination behavior and subsequent induced super-high plastic deformation energy. These findings have important implications for the design and development of high-performance materials with superior strength-toughness combinations.

Effect of accelerated cooling after cross-helical rolling on formation of structure and low-temperature fracture toughness of low-carbon steel

The effect of accelerated cooling after cross-helical rolling of X70 low-carbon steel on the formation of structures and mechanical properties under static tension and impact bending was investigated. The use of interrupted accelerated cooling of steel after cross-helical rolling with exposure at 530 °C (mode I) and continuous accelerated cooling (mode II) leads to the formation of different types and ratios of structures in steel. After rolling according to mode I, the structure is characterized by the presence of ferrite, troostite, granular bainite, and fine Fe3C carbides. After rolling according to mode II, the structure is characterized by the formation of lath bainite and large sections of the martensitic-austenitic (MA) component up to 1 – 2 µm in size. It is shown that a decrease in the fineness of ferrite grains in steel after cross-helical rolling in modes I and II from 12 to 4.6 – 4.3 μm, the formation of a bainitic phase, and hardening of the matrix with carbides led to an increase in the yield strength of steel up to 440 and 490 MPa and tensile strength up to 760 and 880 MPa. Carrying out helical rolling according to mode I makes it possible to significantly increase the low-temperature fracture toughness of steel (KCV–70 °С = 160 J/cm2) compared to the hot-rolled state (KCV–70 °С = 11 J/cm2) and reduce the cold brittleness of steel to the temperatures below –50 °C. The use of continuous accelerated cooling (mode II) does not allow increasing the cold resistance of steel due to the formation of the lath bainite structure and large areas of the MA component.

Preparation and characterization of polyurethane-modified asphalt containing dynamic covalent bonds

Self-healing behavior of binder has important potential for promoting low-carbon service and extending service life of asphalt pavement. A novel self-healing polyurethane modified asphalt with dynamic covalent bonds was prepared in this work. The chemical structure, thermal-oxygen stability, rheological properties, fatigue properties and self-healing capability of polyurethane modified asphalt with dynamic covalent bonds were systemically investigated through Fourier infrared spectrum test, thermal gravimetric test, dynamic shear rheology test, linear amplitude sweep(LAS) test and multiple fatigue-healing-fatigue test. The result indicates disulfide dynamic covalent bond and oxime urethane dynamic covalent bond were successfully introduced into polyurethane cross-linking structure in asphalt, and the thermal-oxygen stability of asphalt has a significant increase after modification under the environmental conditions of construction and service. Rheological properties analysis suggests the introduction of dynamic covalent bonds is beneficial to improve the low-temperature toughness of polyurethane modified asphalt, and enhances its low-temperature cracking resistance. LAS analysis indicates the fatigue life of polyurethane modified asphalt can also be lengthened by over 70% at different strain levels(1.5%, 2.5% and 5%) with the introduction of dynamic covalent bonds, which is attributed to the excellent conformation adjustment ability and stress dissipation ability of dynamic cross-linking structure in asphalt. The multiple fatigue-healing-fatigue test shows the self-healing capability of polyurethane modified asphalt has an improvement with the introduction of dynamic covalent bonds, especially at the later stage with serious fatigue damage, and oxime urethane dynamic covalent bond shows the better contribution effect as compared with disulfide bond. This is due to the reversible fracture-rearrangement characteristics of dynamic covalent bonds. This work provides a new idea for improving the self-healing capacity and rheological properties of binder, reducing the frequency of pavement maintenance and extending the pavement service life.

Study on Characteristics of Through-Thickness Crack When Cryogenic Materials are Applied to Independent B Type Tank

Due to climate change, interest in eco-friendly energy is continuously increasing around the world. LNG is an intermediate energy source and its use increases continuously because of its established infrastructure. LNG temperature is -165°C and requires low temperature toughness of tank material. High manganese austenitic steel has similar mechanical properties to cryogenic material for LNG listed in IGC and IGF code. High manganese austenitic steel easily satisfies the low temperature toughness required at cryogenic temperature and shows superior performance in yield strength, tensile strength and elongation, compared to existing materials in IGC and IGF code.&lt;/br&gt;In this study, the thickness ratio is reviewed while loading the same LNG fuel. Based on the thickness ratio, the thickness for 9%Ni steel, STS304 and Al5083 is calculated for two standard thickness of 10 mm and 20 mm for the high manganese austenitic steel. 9%Ni steel, STS304, Al5083 and high manganese austenitic steel are compared and reviewed from the viewpoint of the required time for initial surface cracks to grow into through-thickness cracks and the size of through-thickness cracks under the same amount of LNG fuel. The effect of different thickness, welded joint and stress sequence on through-thickness cracks are investigated.

Insight into the Role of Mo Content on the Microstructure and Impact Toughness of X80 Thick-Walled Low-Temperature Pipeline Steel

In this manuscript, the effects of Mo content on the microstructure and impact toughness of X80 thick-walled low-temperature pipeline steel were studied. Two test steels with different Mo content (0.25% and 0.40%) were prepared by the thermo-mechanical control process. The impact properties were measured at −45 °C, and the microstructure evolution was observed via an optical microscope (OM), a scanning electron microscope (SEM), electron back-scattered diffraction (EBSD), and a transmission electron microscope (TEM). Each steel showed the formation of a mixed microstructure consisting of polygonal ferrite (PF), granular bainite (GB), and lath bainite (LB). Increasing Mo content resulted in the rise of LB at the expense of PF and GB. At the same time, the morphology of martensite/austenite (M/A) constituents changed from blocky to slender. The dislocation density in the ferrite matrix around the M/A constituents enhanced with an increase in Mo content. This also led to an increase in the microstrains around the M/A constituents. Also, the number fraction of the high angle grain boundary (HAGB) (MTA &gt; 15°) decreased with the addition of more Mo content. Furthermore, with an increase in Mo content from 0.25% to 0.40%, the low-temperature impact toughness decreased from 206 to 57 J. Both an increase in the slender M/A constituents and a decrease in the HAGB number fraction deteriorated the low-temperature impact toughness of the X80 thick-walled low-temperature pipeline steel.

Improvement in low-temperature toughness of Fe-6.5Mn-0.08C Medium-Mn steel by multi-step heat treatment

In this study, a novel multi-step heat treatment method, including intercritical annealing and tempering, is proposed to improve the low-temperature toughness of Fe-6.5Mn-0.08C medium-manganese (Mn) steel. The effects of the subsequent tempering treatment on the microstructural evolution and low-temperature toughness of medium-Mn steel were investigated and compared with those of a single intercritical annealing treatment. Medium-Mn steel subjected to intercritical annealing exhibited a duplex microstructure composed of martensite and nanolaminate-retained austenite. The subsequent tempering treatment after intercritical annealing generated a more homogenous distribution of retained austenite, together with an increase in the volume fraction of the retained austenite. The uniformly distributed retained austenite prevented Mn segregation at prior austenite grain boundaries, and the high fraction of retained austenite effectively inhibited crack propagation, leading to an improvement in the low-temperature toughness. In addition, the subsequent tempered medium-Mn steel contained a considerable amount of retained austenite, which could transform into martensite during the impact test, resulting in an impact fracture morphology that changed from cleavage facets to dimples at low temperatures.

Synergistic effect of compound rubber and SiO2 nanoparticles on low-temperature toughening and balanced stiffness-toughness of random copolymer polypropylene nanocomposites

Random copolymer polypropylene (PPR)/rubber/silica nanocomposites with excellent low-temperature toughness and balanced rigidity-toughness were prepared by using compound rubber composed of ethylene propylene rubber (EPR) and isoprene rubber (IR). Effects of compound rubber on reinforcement and toughening of PPR were systematically investigated through mechanical properties, dynamic mechanical behavior, morphologies and rheological behavior. Mechanical results showed compound rubber had higher efficiency in both reinforcement and toughening than single rubber. According to morphological analysis and rheological results, it was found that the EPR/IR compound rubber had a better distribution in PPR matrix due to the close viscosity to PPR matrix and maintained the characteristics of silica distribution at the interface between rubber and matrix. Dynamic mechanical results revealed PPR/compound rubber blends showed larger area of loss factor. Consequently, the better toughening effect and achieving good rigidity-toughness balance of compound rubber in PPR nanocomposites was ascribed to the synergistic effect of compound rubber and SiO2 nanoparticles, which results in smaller particle spacing and larger loss factor area of rubber phase in nanocomposites without changing preferential loading of nanoparticles at polymeric interface.

Low-temperature impact toughness of laser–arc hybrid welded low-carbon bainitic steel

Low-temperature impact toughness of laser–arc hybrid welded low-carbon bainitic steel

Effects of Ti/N Ratio on Coarse-Grain Heat-Affected Zone Microstructure Evolution and Low-Temperature Impact Toughness of High Heat Input Welding Steel

Coarse-grain heat-affected zone (CGHAZ) properties of steel deteriorate when it is welded using high heat input, which always restricts the promotion and use of high heat input welding steel. TiN particles significantly inhibit the growth of austenite and improve the microstructure and properties of high heat input welding steel. Effects of different Ti/N ratios on the CGHAZ microstructure and properties of high heat input welding steel were studied using welding thermal simulations and in situ observations. Results showed that a higher Ti/N ratio led to the abnormal growth of austenite grains and promoted the nucleation and growth of lath ferrite, which made the microstructure of the CGHAZ heterogeneous. In contrast, austenite grains were more uniform at a lower Ti/N ratio. Thus, the microstructure was refined, the brittle structure was reduced, and the properties of the CGHAZ were improved. In addition, when Ti/N = 5.85, the impact absorption energy of the CGHAZ obviously fluctuated. However, when Ti/N = 2.82, the impact absorption energy of the CGHAZ was higher and more stable. These results provided a new idea for the development of high heat input welding steel based on TiN theory.

Effect of Heat Treatment on Microstructure and Properties of Quenched and Tempered Steel with High Vanadium Contents

After heat treatment, the microstructure of quenched and tempered (Q&T) steel with high vanadium (V) content was characterized using metallographic and scanning electron microscopes (SEM). The proto-austenite grains of the tested steel were compared at different quenching temperatures, and the precipitation behavior of precipitates and matrix microstructure characteristics at various tempering temperatures were studied. Tensile properties and low-temperature impact toughness of the tested steel with different heat treatments were measured using an electronic universal testing machine and an impact testing machine. The findings indicate that as the quenching temperature increases, the size of the proto-austenite grains gradually increases. When the quenching temperature surpasses 920 °C, the size of the proto-austenite grains obviously increases. With a constant tempering temperature, an increase in quenching temperature results in a decrease in the size of precipitates. With a constant quenching temperature, an increase in tempering temperature results in an increase in the size of precipitates. Therefore, the tested steel quenched at 920 °C for 1 hour and then tempered at 630 °C for 1.5 hours would obtain excellent comprehensive mechanical properties, with a tensile strength of 1233 MPa and a low-temperature impact value of 64 J.

A phenomenological understanding of the novel design of hierarchical structure for 1 GPa ultrahigh strength and high toughness combination low alloy steel

In the present study we propose a novel design of hierarchical structure with the aim to obtain high strength of 1 GPa and low temperature toughness in a heavy low alloy steel plate by intercritical quenching and tempering (IQ&T). The hierarchical microstructure consisted of bimodal prior austenite grains with complex microstructure obtained via intercritical quenching from 810 °C and tempering at 500 °C. The large size grains with average diameter of ∼22 μm consisted of tempered martensite and fine intercritical ferrite. While the surrounding fine grains with average diameter of ∼12 μm consisted of tempered lower bainite and tempered lath martensite. Ultra-high yield strength of 998 MPa and tensile strength of 1059 MPa were obtained for the novel hierarchical microstructure steel, which was similar to the conventional quenched and tempered (Q&T) steel. Whereas, the low temperature impact energies at both −40 and −80 °C were ∼100% higher for the hierarchical microstructure steel than the conventional Q&T steel. The significant improvement of low temperature toughness is attributed to the synergistical effect of hierarchical microstructure. The fine intercritical ferrite in large grains increased the plastic deformation ability during impact and was beneficial for formation of dimples, and the surrounding fine grains offered high density of large mis-orientation grain boundaries to deflect crack propagation, leading to high impact energy at low temperature.