Excellent Low Temperature Toughness Research Articles

Crystallographic Study of Transformation Products of Heat-Affected Zone and Correlation with Properties of FH690 Heavy-Gauge Marine Steel by Multi-Pass Submerged Arc Welding

In this work, the microstructure–property relationship of the heat-affected zone (HAZ) of a FH690 ultra-heavy marine steel plate was investigated based on insight of microstructure and crystallographic features. After multi-pass welding with a heat input of ~30 kJ/cm, an ~8 mm wide HAZ was obtained with a coarse grain HAZ (CGHAZ) of ~3.8 mm, fine grain HAZ (FGHAZ) of ~3.4 mm, and intercritical HAZ (ICHAZ) of ~1 mm. High impact toughness values of ~120 and 140 J at -60 °C were obtained for coarse grain HAZ and fine grain HAZ, respectively. The microstructure of the CGHAZ and FGHAZ was fine lath bainite. Although the average prior austenite grain size for the CGHAZ was ~75 μm, which was five times that of the FGHAZ (15 μm), a high density of high-angle grain boundaries (HAGBs) with misorientation higher than 45° was obtained in the CGHAZ. This is the underlying reason for the excellent low-temperature toughness of the HAZ. Thermo-dynamic calculations indicated that the high density of HAGBs in the CGHAZ was attributed to the decreased bainitic transformation temperature due to the reduced phase transformation driving force via the high nickel addition, leading to weak variant selection. In addition, the high nickel addition offered high hardenability for high hardness in the FGHAZ. The outcome of this study could provide an experimental and fundamental basis for designing high-strength ultra-heavy steel plates with excellent weldability.

Improving the Mechanical Properties of Hot Rolled Low-Carbon Copper-Containing Steel by Adjusting Quenching Roll Speed.

This paper presents a comprehensive study of the impact of quenching roll speed on enhancing the low-temperature toughness of a low-carbon copper-containing steel. The microstructure characteristics, such as the prior austenite grains, and the distribution and volume fraction of precipitates, are observed using optical microscopy, scanning electron microscopy, transmission electron microscopy, and small-angle scattering X-ray. The results show that a decrease in the quenching roller speed (2 m/min) contributes to the achievement of more excellent low-temperature toughness (the average value is 232 J), although the prior austenite grains exhibit a relatively larger size in this case. The tempering treatment results in the precipitation of a large amount of 9R-type Cu-rich particles, regardless of the quenching roller speed. Reducing the quenching roller speed contributes to the increase in the volume fraction of Cu-rich particles, which is considered to be the main factor contributing to the achievement of excellent low-temperature toughness.

Excellent low-temperature toughness of 1 GPa grade microlaminated 5Mn steel containing retained austenite

The development of high-strength steels with excellent low-temperature toughness continues to present significant challenges. Delamination toughening and retained austenite (RA) toughening are effective methods for improving the low-temperature toughness of steels. In the present study, medium manganese steel with a microlaminated microstructure containing blocky RA is prepared by intercritical rolling (IR). A comparative steel with a microstructure containing annealed martensite and film-like and blocky RA is generated by hot rolling and intercritical annealing (HRIA). The different microstructure morphologies lead to different fracture characteristics. The IR steel plate exhibited a delaminated fracture that changes the crack propagation path, inducing large-scale and severe plastic deformation in the area below the delaminated crack in the entire temperature range (RT ∼ −196 °C). The Charpy impact V-notch absorbed energy (CVN) is as high as 450 ± 7 J at −60 °C. Moreover, the fracture behavior of the HRIA steel was similar to that of steels with equiaxed microstructures; ductile fracture takes place (above −40 °C) first, followed by brittle fracture, and the CVN is only 150 ± 83 J at −60 °C. The RA contents of the IR and HRIA steels at RT are 25.8% and 22.3%, respectively. Upon impact at −60 °C, the RA content becomes 0 and 4% in the IR and HRIA steels, respectively. Finally, a toughening mechanism is established for IR steel from the perspectives of crack propagation and microstructure evolution.

The Influence of Centerline Segregation on Impact Toughness in Welding Heat-Affected Zone of X70 Pipeline Steel

The influence of centerline segregation on the low-temperature impact toughness of the heat-affected zone (HAZ) of welded joints was studied by welding experiments on X70 steel plates rolled from continuous casting slabs with segregation grades of class 2 and class 3. The experimental results show that the impact toughness at HAZ from class 2 slab steel plate is more stable and has excellent low-temperature toughness than that of class 3 slab steel plate. The impact toughness of the HAZ of the class 3 slab steel plate is low to 100 J at −40 °C and has a severe fluctuation range (~150 J), and the delamination phenomenon is also observed in the fracture cross-section. The reason for this phenomenon is due to the enrichment of C and Mn elements in the centerline segregation zone. The formation of abnormal microstructure (martensite/bainite) in the segregation zone leads to stress concentration, which easily weakens the low-temperature toughness of the joint.

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.

The co-precipitation evolution of NiAl and Cu nanoparticles and its influence on strengthening and toughening mechanisms in low-carbon ultra-high strength martensite seamless tube steel

The designed low-carbon ultra-high strength martensite seamless tube steel was manufactured by hot rolling and quenching-tempering processes. The multiple strengthening mechanisms are evaluated depending on the microstructure and co-precipitation evolution mechanism of Cu and NiAl, and the toughening mechanisms associated with multiscale microstructures are systematically discussed. The results show that the microstructure of the experimental steel in the quenched state consists of 87.8% lath martensite (LM) and 12.2% granular bainite (GB), while the microstructure in the QT state includes tempered martensite (TM), GB and a small amount of reversed austenite. The TEM morphology of QT steel shows three types of nanoparticles co-precipitated by Cu-rich, NiAl and Cu-NiAl, and the nanoparticles coarsen significantly and the number density decreases dramatically as the aging temperature increases from 500 °C to 650 °C. The co-precipitation evolution mechanism of nanoparticles elucidates that high density of small-sized BCCCu and B2-NiAl particles is optimal for strengthening increment. The experimental steel has an maximum yield strength of 1332.5 MPa aged at 500 °C, which is attributed to high precipitation strengthening of 651.2 MPa (general superposition of shear strengthening and Orowan strengthening) and dislocation strengthening of 454.8 MPa. The experimental steel has obvious low-temperature toughening, and the impact energy at -40 °C increases from 5 J to 237 J as the aging temperature increases from 500 °C to 650 °C. The excellent low-temperature toughness is attributed to the reduction of dislocation density, the weakening of the shear mechanism and the transformation of a small amount of reversed austenite to increase the crack nucleation energy, and the increase of the number fraction of HAGB and the significant plastic deformation increase the crack propagation energy.

Effect of Al Addition on Corrosion Behavior of High Mn Steels in a Cl–-Containing Environment

The high Mn steels are expected to become a novel steel for liquefied natural gas (LNG) tank building because of their low cost, high strength, and excellent low-temperature impact toughness. Up until now, it is still limited for studies on corrosion behavior of high Mn steel in a Cl–-containing environment. We found that strong Mn enrichment layers always exist in the outer rust layer, whereas strong Al enrichment layers always exist in the inner rust layer. However, the Al and Cl simultaneously enrich in the same area. Although the corrosion resistance can be further improved by increasing Al content from 5.0 mass% to 8.0 mass%, the improvement degree becomes weak and the pitting corrosion becomes serious due to the formation of δ-ferrite. There are two aspects to explain why Al improves corrosion resistance. (1) More Al addition can enhance the resistance of passive oxide. (2) The α-FeOOH content can be increased and the compactness of the rust layer can be also enhanced by increasing Al content.

Macroporous epoxy resin mixture – Structural design and performance study

At the problems of insufficient bonding force and low overall strength of asphalt as the binder in the porous mixture, this paper developed a new epoxy resin binder with high bonding strength, good durability, and easy operation and excellent low-temperature toughness, prepares macroporous epoxy resin mixture, put forward the corresponding mix design method, and studied its performance. The mass ratio of the component of the new epoxy resin binder is modified epoxy resin:diluent:curing agent:accelerator = 120:65:45:3. The mixture ratio design method of macroporous (18 %∼26 %) epoxy resin mixture can directly calculate the target mixture ratio according to the porosity requirement, and the prepared mixture not only shows good mechanical strength and deformation coordination ability.. When the void ratio is greater than 22 %, the permeability of the mixture is significantly improved, and the anti-scattering performance still meets the specification requirements. The dynamic stability times of macroporous epoxy resin mixture at 60 °C are 18000 ∼ 157500 times/mm; the ratio of 15 °C splitting strength to-10 °C splitting strength is between 80 % and 100 %, shows good toughness at low temperature and is insensitive to temperature; the immersion Marshall stability is between 85.97 % and 92.51 %, and the road performance is better than that of porous asphalt mixture. This mix proportion method effectively avoids the randomness of determining the gradation composition of the macroporous mixture and the content of binder and dramatically reduces the test time and cost in the process of mix proportion design.

Microstructure characteristics and mechanical properties of deposited metals with different types of bainite

In this work, three types of bainite microstructure have been gained in the deposited metals by adjusting the Ni content (1 wt %, 2.5 wt %, and 4 wt %). The microstructure feature of different types of bainite has been characterized by optical microscope, scanning electron microscope, transmission electron microscope, and electron backscattering diffraction. Microstructure changed from granular bainite with different morphology to lath bainite with the increase of Ni content. The number of elongated Martensite-Austenite constituents in granular bainite increased with the increasing Ni content from 1 wt % to 2.5 wt % due to the increase in hardenability. The complex orientation in the deposited metal with 4 wt % Ni content attributed to the numerous autocatalytic nucleation occurrence during the bainite transformation. After crystallographic characterization, the bainite variant selection tendency decreased with the bainite type change from granular bainite to lath bainite with autocatalytic nucleation, and the density of high-angle grain boundaries increases obviously. The deposited metal with numerous autocatalytic nucleation has an outstanding match of high strength and excellent low-temperature toughness due to the effective grain-refinement strengthening. The large-sized elongated Martensite-Austenite constituents in the deposited metals with 1 wt % and 2.5 wt % Ni contents deteriorate low-temperature toughness in two aspects, promoting crack nucleation and reducing the crack propagation resistance. Interlaced bainite is the most optimized among different bainite types. This work guides the microstructure design in high-strength deposited metals.

Superior Toughened Biodegradable Poly(L-lactic acid)-based Blends with Enhanced Melt Strength and Excellent Low-temperature Toughness via In situ Reaction Compatibilization

Superior Toughened Biodegradable Poly(L-lactic acid)-based Blends with Enhanced Melt Strength and Excellent Low-temperature Toughness via In situ Reaction Compatibilization

Ultrahigh Low-Temperature Toughness of Polypropylene Composites with Low Ductile-Brittle Transition Temperature by Introducing Traces of Needlelike Wollastonite

In order to survive harsh settings, such as outer space, deep sea and high plains, needlelike wollastonite fibers (WF) were incorporated into polypropylene/ethylene octene block copolymer/β-nucleating agent (PP/OBC/β-NA) blends with the hope of tailoring the low-temperature toughness. The results revealed that WF showed very satisfactory toughening effects in the PP/OBC/β-NA/WF composites at low temperature (–20 °C). The impact strength of the WF-0.05 was as high as 32.1 kJ/m2 at −20 °C, which was 286.6% higher than that of the WF-0 sample, and even 832.6% higher than that of the pure PP sample. SEM characterization showed that by simultaneously adding WF and β-NAs, much coarse and tremendous plastic deformation appeared in the WF-0.05 after impacting. The low-temperature toughening mechanism is systematically discussed. The increase of β-crystallinity, caused by the synergy of β-nucleating agent and WF fillers, was mainly responsible for the enhancement of the impact strength of the composites with WF. On the other hand, when the transmitting of the stress, the overlapping of the stress and the hindering effect of WF on cracks occurred simultaneously; it resulted in the excellent low temperature toughness of the PP composites. Such outstanding-performance material with the advantages of facile processing and low cost is unprecedented. It shows a promising future application in many fields, especially aerospace, navigation, and the automotive industry.

Ultra-high impact PPR composites at low-temperature through enhanced preferential loading of nanoparticles at polymeric interface induced by properly vulcanized rubber dispersed phase

A variety of low-temperature toughened, rigid-tough balanced polypropylene random copolymer/ethylene propylene rubber/silica (PPR/EPM/SiO2) nanocomposites were prepared by properly crosslinked rubber phase. The impact test results showed that the slightly vulcanized PPR/EPM/SiO2 composites presented lower ductile-to-brittle transition temperature (Tbd) and more excellent low-temperature toughness than unvulcanized or highly vulcanized composites, and there was hardly rigidity loss induced by slight vulcanization. SiO2 distribution, the size of EPM dispersed phase and EPM relaxation were investigated by TEM, SEM, rheology and DMA, respectively. It was found that the remarkable decrease of Tbd for slightly vulcanized PPR/EPM/SiO2 composites was due to the synergistic effect of SiO2 nanoparticles and vulcanization of rubber phase. Furthermore, the origin of excellent low-temperature toughness for vulcanized PPR/EPM/SiO2 composites was ascribed to the enhanced interfacial distribution of SiO2 nanoparticles induced by moderate vulcanization of rubber phase, which increased damping loss of rubber phase, increased interfacial area and decreased the matrix ligament thickness.

Correlation between Microstructure and Mechanical Properties of Welded Joint of X70 Submarine Pipeline Steel with Heavy Wall Thickness

This paper aims to study the relationship between the microstructure and the mechanical properties of X70 submarine pipeline steel with 40.5 mm thickness. The microstructure was examined by using optical microscopy, scanning electron microscopy and an electron backscattered diffractometer, while the mechanical properties were examined by using a hardness test, a tensile test, a Charpy impact test and a drop weight tear test (DWTT), respectively. The results show that the base metal (BM) of the pipe has a low yield ratio of 0.83 and an excellent elongation of more than 45%. The DWTT shear area of the steel plate reaches 87%, showing excellent low-temperature toughness. The Charpy impact energy increases when the distance from the fusion line increases, and it reaches a maximum at the BM near the heat-affected zone (HAZ) due to the small martensite-austenite (MA) constituents and fine grains. The concentrated distribution of blocky/slender MA constituents along the prior austenite grain boundaries of the intercritically reheated coarse-grained HAZ and the large MA constituents are the main reasons for the deteriorating impact toughness. Delamination cracks in the DWTT fracture surface only occurred in the midthickness of a sample with a small opening width that spread about 2.1 mm perpendicular to the DWTT fracture surface and were finally arrested at the acicular ferrite clusters containing a high density of high-angle boundaries.

Poly (lactic acid) blends with excellent low temperature toughness: A comparative study on poly (lactic acid) blends with different toughening agents

Poly (lactic acid) (PLA) blends with different toughening agents were prepared by melt compounding, and the effects of toughening agents on the toughness of PLA, especially the low-temperature toughness, were investigated. All blends were immiscible systems, but the rheological Cole-Cole diagram showed that the blends had certain compatibility, and the interfacial bonding of PLA/Ethylene/butyl methacrylate/Glycidyl Methacrylate Terpolymer (GEBMA) blend was the best. With addition of the toughening agents, all blends showed improvement of the tensile and impact toughness both at room temperature and low temperature. GEBMA was the best toughening agent, the elongation at break and impact strength at room temperature and low temperature were greatly improved. The elongation at break, tensile strength and impact strength of PLA blend with 20 wt% GEBMA at −20 °C was 55.8 MPa, 195.9% and 18.8 kJ/m2, respectively, which showed the reinforcement and super ductility at low temperature. However, the toughening effect of Poly (propylene carbonate) polyurethane (PPCU) at low temperature was poor. The Tg and interfacial bonding were the main factors affecting the toughness of the blends, especially at low temperature. The lower the Tg and the better the interfacial bonding, the better the toughness of the blends.

Synergy of strengthening and toughening of a Cu-rich precipitate-strengthened steel

A high strength steel with a combination of ∼930 MPa yield strength and excellent low temperature toughness with an upper shelf energy of above 200 J and ductile brittle transition temperature (DBTT) of lower than −90 °C is developed. The strengthening and toughening mechanisms are investigated systematically based on the detailed characterization on microstructures including the matrix and precipitates. The results indicate that the steel is composed of a fine lath martensite with rod-like Cu precipitates. The high strength is achieved by a combination of solid-solution strengthening, dislocation strengthening, grain boundary strengthening and precipitation strengthening of Cu-precipitates. The instrumented Charpy impact results further indicate that the crack propagation is the main factor affecting DBTT while the dislocation density has an obvious effect on both crack initiation and propagation. The fine lath structure of the low carbon martensite enhances the crack resistance and delays the rapid unstable crack propagation at low temperatures. Both the strengthening and toughening are thoroughly discussed in details.

Microstructure features and formation mechanism in a newly developed electroslag welding

Electroslag welding (ESW) is known to show higher heat input than electrogas welding (EGW), resulting in poor low-temperature toughness. However, a newly developed ESW (dev. ESW) method using low-resistivity slag bath exhibited excellent low-temperature toughness as a result of lower effective heat input than conventional EGW, as demonstrated by the faster cooling rates measured in weld metals and estimated using finite element method analyses. This led to much shallower molten pool in the dev. ESW, resulting in much finer columnar grains and thinner centerline axial grains. High cooling speed in the dev. ESW method appeared to contribute to increased acicular ferrite proportion. The uniform microstructure with large acicular ferrite proportion and small number of inclusions in the weld metal permitted the dev. ESW weld metal to possess little variation in Charpy impact energy across the center of weld metal.

Ultrafine microstructure design of high strength pipeline steel for low temperature service: The significant impact on toughness

An X80 (80 ksi, 555 MPa) pipeline steel was innovatively processed in the laboratory using the intercritical rolling process and excellent low temperature toughness was obtained. The Charpy v-notch impact energy was greater than 400 J at −80 °C and the shear fracture area in drop-weight tear test approached ~100% at −60 °C. In order to unravel the low temperature toughening mechanism, the crystallography of the microstructure was studied in detail by electron back scattered diffraction. The results indicated that the microstructure mainly consisted fine polygonal ferrite and granular bainite. The ferrite grains were refined to less than 3 μm and the ferrite grain boundaries predominantly had high misorientation angle. The granular bainite was also refined by some sub-boundaries, but these sub-boundaries were largely low angle grain boundaries. Therefore, the ultrafine ferrite contributed to the effective refinement of microstructure. Increasing the proportion of fine polygonal ferrite is the key to improve the low temperature toughness of pipeline steel.

Preparation and properties of poly(L-lactic acid) blends with excellent low-temperature toughness by blending acrylic ester based impact resistance agent

Poly(L-lactic acid) (PLLA) blends with excellent low-temperature toughness and strength were prepared by melt compounding with acrylic ester based impact resistance agent (AEIR). The morphology, thermal properties, mechanical properties and biodegradability of the blends were investigated. Morphology observations revealed the blend was immiscible but had good compatibility with the dispersed phase size of about 200–300 nm. With the addition of AEIR, dramatic improvement in toughness of PLLA was achieved in a wide temperature range, especially at low temperatures the tensile strength was effectively remained. For the blend with 20 wt% AEIR, the tensile strength, elongation at break and impact strength were 51.6 MPa, 72% and 77.1 KJ/m2 at −20 °C, respectively, much greater than that reported. The calculated Tg of AEIR was lower than the test temperatures, and the brittle-tough transition occurred. The PLLA matrix demonstrated obvious shear yielding which induced energy dissipation and therefore lead to excellent toughness of the blends. Moreover, the biodegradation of PLLA was enhanced after blends preparation.

Determining role of heterogeneous microstructure in lowering yield ratio and enhancing impact toughness in high-strength low-alloy steel

Here we present a novel approach of intercritical heat treatment for microstructure tailoring, in which intercritical annealing is introduced between conventional quenching and tempering. This induced a heterogeneous microstructure consisting of soft intercritical ferrite and hard tempered martensite, resulting in a low yield ratio (YR) and high impact toughness in a high-strength low-alloy steel. The initial yielding and subsequent work hardening behavior of the steel during tensile deformation were modified by the presence of soft intercritical ferrite after intercritical annealing, in comparison to the steel with full martensitic microstructure. The increase in YR was related to the reduction in hardness difference between the soft and hard phases due to the precipitation of nano-carbides and the recovery of dislocations during tempering. The excellent low-temperature toughness was ascribed not only to the decrease in probability of microcrack initiation for the reduction of hardness difference between two phases, but also to the increase in resistance of microcrack propagation caused by the high density of high angle grain boundaries.

Unusual relationship between impact toughness and grain size in a high-manganese steel

The high-manganese steels are important structural materials, owing to their excellent toughness at low temperatures. However, the microstructural causes for their unusual properties have not adequately been understood thus far. Here, we report a reversal relationship between impact toughness and grain size in a high-manganese steel and its unrevealed microscopic mechanisms, which result in an excellent low-temperature toughness of the steel. Our investigations show that with increasing grain size the impact toughness of the steel can be improved drastically, especially at low-temperatures. Advanced electron microscopy characterization reveals that the enhanced impact toughness of the coarse-grained steel is attributed to the twinning induced plasticity and transformation induced plasticity effects, which produce large quantities of deformation twins, εhcp-martensite and α′bcc-martensite. Inversely, in the fine-grained steels, the formation of deformation twins and martensite is significantly inhibited, leading to the decrease of impact toughness. Microstructural characterizations also indicate that εhcp-martensite becomes more stable than α′bcc-martensite with decreasing temperature, resulting in characteristic microstructures in the coarse-grained samples after impact deformation at liquid nitrogen temperature. In the coarse-grained samples under impact deformation at -80 °C, εhcp-martensite transformation, α′bcc-martensite transformation and deformation twinning all occur simultaneously, which greatly improves the toughness of the steel.