Improvement In Impact Toughness Research Articles

Investigation of morphologies and tensile impact toughness of immiscible polyphenylene sulfide/polyether sulfone films and carbon fiber composites by quantitative optical methods

Targeting the transfer and improvement of impact toughness in composites deriving from polymer mixtures, we investigated the effectiveness of immiscible polyphenylene sulfide (PPS)/polyether sulfone (PES) blends reinforced by endless carbon fibers. Tensile impact testing of films and composites was performed. Impact strengths of films showed high dependency of the PPS crystallinity and crystal orientation influenced by the present amount of PES. Intermolecular interactions between the immiscible phases and the presence of amorphous areas were assumed to be the driving factor for the load transfer in these films. Optical evaluation of local and global strain at break of films revealed a significant increase in plasticization. Tensile impact tested ±45° carbon composites exhibited high improvements of toughness and deformation ability provided by the PES spheres. The quantification of morphologies evidenced a strong correlation of PES profile's mean diameters and next neighbor distances with impact toughness and deformation ability, qualifying these novel materials as potential alternatives for conventional high‐end composites. POLYM. COMPOS., 40:3725–3736, 2019. © 2019 Society of Plastics Engineers

Research on the impact response and model of hybrid basalt-macro synthetic polypropylene fiber reinforced concrete

This paper experimentally studied the impact behavior of hybrid basalt-macro synthetic polypropylene fiber reinforced concrete under impact loads (101–102 /s) with a split Hopkinson pressure bar (SHPB) device. The fiber content of basalt fiber (BF) was 0.05%, 0.075% and 0.1% and that of macro synthetic polypropylene fiber (SF) was 0.15%, 0.25%, 0.35% and 0.5%. Both static and impact tests were conducted to investigate the effect of strain rate and fiber hybrid ratio on the dynamic performance of the hybrid FRC, i.e. impact strength, dynamic increase factor (DIF), impact strain and toughness. The test results indicate that the hybrid FRC is strain-rate sensitive and a proper fiber hybrid ratio can improve the impact performance of concrete. DIF and lgε̇ are linearly related, whereas the impact strain, toughness and strain rate present quadratic polynomial relationship. Both BF and SF can enhance the impact strength of concrete. However, BF has a better enhancement effect than SF in terms of improving impact strength but not as good as SF in improving impact toughness. Also, appropriate BF and SF hybrid ratio can enhance the impact strength and toughness of concrete but excess fiber content has a weakening effect. In this study, the optimal fiber hybrid ratio was 0.075%–0.35% (BF-SF) which had the best impact resistance. Finally, a damage dynamic constitutive model suitable for the hybrid FRC was proposed to fit the test curves from SHPB test based on Zhu-Wang-Tang (ZWT) constitutive model.

Effects of Initial Austenite Grain Size on Microstructure and Mechanical Properties of 5% Nickel Cryogenic Steel

In the present study, the relationship between initial austenite grain size and mechanical properties of a 5% nickel cryogenic steel was investigated. The microstructures of initial austenite grains and tempered martensites were analyzed by an optical microscope. In addition, a transmission electron microscope was employed to examine the morphologies of reversed austenite and refined tempered martensite laths. Further, tensile and cryogenic impact tests were conducted to evaluate the mechanical properties of the experimental steel. It was found that the impact toughness of the sample steel increased as the size of initial austenite grain decreased and the thicknesses of martensite laths decreased with the decrease in initial austenite grain size. The average grain size of initial austenite in the specimen austenitized at 830 °C for 60 min followed by tempering at 600 °C for 60 min was found to be 19.2 μm, and the maximum value of impact toughness reached 292.5 J. Therefore, the improvement in impact toughness can be attributed to the existence of refined tempered martensite laths and more amount of reversed austenites in the specimen.

Effects of TiN and B on Grain Refinement of HAZ Microstructure and Improvement of Mechanical Properties of High-strength Structural Steel Under High Heat Input Welding

In the current steel structures of high-rise buildings, high heat input welding techniques are used to improve productivity in the construction industry. Under the high heat input welding, however, the microstructures of the weld metal (WM) and heat-affected zone (HAZ) coarsen, resulting in the deterioration of impact toughness. This study focuses mainly on the effects of fine TiN precipitates dispersed in steel plates and B addition in welding materials on grain refinement of the HAZ microstructure under submerged arc welding (SAW) with a high heat input of 200 kJ/cm. The study reveals that, different from that in conventional steel, the γ grain coarsening is notably retarded in the coarse grain HAZ (CGHAZ) of a newly developed steel with TiN precipitates below 70 nm in size even under the high heat input welding, and the refinement of HAZ microstructure is confirmed to have improved impact toughness. Furthermore, energy dispersive spectroscopy (EDS) and secondary-ion mass spectrometry (SIMS) analyses demonstrate that B is was identified at the interface of TiN in CGHAZ. It is likely that B atoms in the WM are diffused to CGHAZ and are segregated at the outer part of undissolved TiN, which contributes partly to a further grain refinement, and consequently, improved mechanical properties are achieved.

Effect of Nb on improving the impact toughness of Mo-containing low-alloyed steels

The microalloying of low-alloyed steels with Nb can improve the strength-to-toughness balance. Such an effect of Nb is usually ascribed to the refinement of the grain structure occurring in the austenite regime during hot forming. In the present work, we report that Nb enhances the impact toughness of a low-alloyed Cr–Mo steel by a mechanism which has not been appreciated so far. The lower impact toughness in the Nb-free Cr–Mo steel is due to segregation of Mo to boundaries, which facilitates the formation of fine Mo-rich ξ-phase carbides lining up along the boundaries. This further promotes the nucleation and propagation of microcracks. The addition of Nb leads to the formation of Mo-enriched NbC particles. The interfaces between these particles and the matrix supply new preferential sites for precipitation of Mo-rich ξ-phase carbides upon subsequent tempering. In this way, Nb additions result in a decrease of Mo segregation to boundaries, significantly reducing the precipitation of ξ-phase carbides on grain boundaries, thus leading to improved impact toughness. In addition to the classical microstructural explanation (grain size effect), this chemical role of Nb sheds new light on the design strategies of advanced low-alloyed steels with optimized strength-to-toughness ratios.

Cold-bending of linepipe steel plate to pipe, detrimental or beneficial?

Systematic investigation was carried out to understand the effect of plate-to-pipe forming (U-bending) strain on the microstructure and mechanical properties of X80 linepipe steel and to establish a structure-property correlation. Finite element simulation predicted the von Mises stress state and through-thickness residual stress distribution. Transmission electron microscopy and electron back scattered diffraction studies detected the local strain gradients, which were verified by nanoindentation testing along with the estimation of through-thickness residual stress. Both yield strength and tensile strength increased after cold-bending operation due to dislocation generation. Also, post-uniform elongation increased, especially at inner diameter (ID) portion due to an overall more dislocation interaction. In the presence of compressive residual stress, ID sample revealed improved impact toughness than the as-received (AR) one at all tested temperatures in the range of −80 °C to room temperature. Low temperature (-80 °C) impact toughness increased by ~16% even at outer diameter (OD) of the pipe, despite the presence of a tensile residual stress state. This improvement in toughness is contributed by many factors, such as the increase in critical stress for yielding, the decrease in crystallographic unit size, less availability of potential sites for cleavage crack initiation, texture strengthening, and crack-divider type fissure formation.

Greatly improved toughness of isotactic polypropylene blends with traces of carbon nanotubes

In this work, the β‐nucleated isotactic polypropylene (iPP)/ ethylene‐octene copolymer (POE) blends demonstrated greatly enhanced impact toughness by adding traces of carbon nanotubes (CNTs) (only 0.05 wt%). When the POE content was 30 wt%, the impact strength of β‐nucleated iPP/POE blends with CNTs was as high as 51.7 kJ/m2, about 5.6 kJ/m2 higher than β‐nucleated iPP/POE blends, 15.2 kJ/m2 higher than CNTs‐filled iPP/POE blends, and almost 19 times of pure iPP sample. This significantly improved impact toughness was considered to be attributed to the shear yielding and multiple‐crazing, originating from the presence of abundant β‐crystals in the iPP matrix, the enhanced mobility of the molecular chains in the confined amorphous region of iPP lamellae and the homogenous distribution of POE dispersed phase with a small size, indicating the synergistic effect of CNTs, β‐nucleating agent and POE on the toughness of iPP. POLYM. ENG. SCI., 59:757–764, 2019. © 2018 Society of Plastics Engineers

Effects of cooling path and resulting microstructure on the impact toughness of a hot stamping martensitic stainless steel

The present study examined the effect of microstructural characteristics on the toughness properties of a hot stamping martensitic stainless steel. Moderately slow cooling during the martensitic transformation leads to the auto-tempering of the martensite laths and the stabilization of thin austenite films. The amounts of retained austenite and cementite precipitates were quantified for various cooling conditions. Charpy impact toughness tests were performed over a large range of temperatures to characterize the ductile-to-brittle transition. Decreasing the cooling rate from 300 °C/s down to 3 °C/s increased the retained austenite fraction from 0.6% up to 2.6% and decreased the ductile-to-brittle transition temperature by 140 °C. The critical cleavage fracture stress was determined to be around 2400 MPa whatever the cooling rate, by applying the local approach to fracture. However, it has been demonstrated that a higher retained austenite fraction modifies incipient plasticity and decreases the yield stress by 60 MPa. As a result, retained austenite delays cleavage fracture by increasing the strain necessary to reach the critical cleavage fracture stress required to trigger cleavage initiation in the ductile-to-brittle transition domain. In this way, retained austenite plays a determining role to decrease the ductile-to-brittle transition temperature. It is thus beneficial to design cooling rates in order to increase the retained austenite fraction and to improve impact toughness at low temperatures.

Role of Reversed Austenite Behavior in Determining Microstructure and Toughness of Advanced Medium Mn Steel by Welding Thermal Cycle.

Reversed austenite transformation behavior plays a significant role in determining the microstructure and mechanical properties of heat affected zones of steels, involving the nucleation and growth of reversed austenite. Confocal Laser Scanning Microscope (CLSM) was used in the present work to in situ observe the reversed austenite transformation by simulating welding thermal cycles for advance 5Mn steels. No thermal inertia was found on cooling process after temperature reached the peak temperature of 1320 °C. Therefore, too large grain was not generated in coarse-grained heat-affected zone (CGHAZ). The pre-existing film retained austenite in base metal and acted as additional favorable nucleation sites for reversed austenite during the thermal cycle. A much great nucleation number led to the finer grain in the fine-grained heat-affected zone (FGHAZ). The continuous cooling transformation for CGHAZ and FGHAZ revealed that the martensite was the main transformed product. Martensite transformation temperature (Tm) was higher in FGHAZ than in CGHAZ. Martensite transformation rate was higher in FGHAZ than in CGHAZ, which is due to the different grain size and assumed atom (Mn and C) segregation. Consequently, the softer martensite was measured in CGHAZ than in FGHAZ. Although 10~11% austenite retained in FGHAZ, the possible Transformation Induced Plasticity (TRIP) effect at −60 °C test temperature may lower the impact toughness to some degree. Therefore, the mean absorbed energy of 31, 39 and 42 J in CGHAZ and 56, 45 and 36 J in FGHAZ were exhibited at the same welding heat input. The more stable retained austenite was speculated to improve impact toughness in heat-affected zone (HAZ). For these 5Mn steels, reversed austenite plays a significant role in affecting impact toughness of heat-affected zones more than grain size.

Crucial microstructural feature to determine the impact toughness of intercritically annealed medium-Mn steel with triplex-phase microstructure

We investigated the correlation between the impact toughness and microstructures of annealed Fe-8Mn-0.2C-3Al-1.3Si (wt.%) steel to identify the key microstructural feature determining the impact toughness of medium-Mn steel. The microstructural constituents were varied by changing the hot-rolling temperature in the range of 1000–1200 °C before intercritical annealing. The annealed steels exhibited a triplex-phase microstructure consisting of δ ferrite with coarse grains and an elongated structure along the rolling and transverse directions and nanolaminate α martensite plus γR retained austenite with ultrafine size. While the volume fraction of γR remained almost constant regardless of the hot-rolling temperature, the volume fraction of δ increased and that of α decreased with increase in the hot-rolling temperature. The average grain size for all phases increased with the hot-rolling temperature. The stability of γR decreased with the increase of the hot-rolling temperature owing to grain coarsening and a reduction in the Mn and C concentrations. A lower hot-rolling temperature resulted in improved impact toughness. We observed that deep parallel cracks formed and propagated along the δ interface decorated with Mn, ultimately causing a fracture. This result indicates that δ ferrite was the crucial factor determining the toughness among the existing phases, and the steels with a higher fraction of δ exhibited a lower impact toughness. The decrease of the retained austenite stability and the increase of the size of prior γ grains with increasing hot-rolling temperature were identified as other microstructural factors determining the impact toughness.

The toughening mechanisms of microstructural variation and Ni addition in direct-cooled microalloyed ferrite-pearlite steels

The toughening mechanisms of microstructural variation and Ni addition were investigated in two direct-cooled microalloyed ferrite-pearlite steels by using the U-notched Charpy tests. Results show that the impact toughness of the low Ni steel decreases dramatically from 110 J to 20 J when the prior austenite grain size increases from 10 µm to 37 µm, and then stays unchanged as the prior austenite grain size continues increasing from 37 µm to 90 µm. When the prior austenite grains are finer than 37 µm, the cleavage fracture is much postponed and the area of ductile fracture is much enlarged by the significantly improved ability to perform plastic deformation. When the prior austenite grains are coarse, the energy absorbed by cleavage, which is the main mode of fracture, conforms to the Griffith type equation and is much less compared to that consumed by ductile fracture, leading to the low impact toughness. The cumulative effect of ferrite content and pearlite interlamellar spacing on impact toughness is not obvious, because their influences are contradictory as the cooling rate is raised. There is a steady enhancement of about 20 J in impact toughness of the high Ni steel compared to the low Ni steel, when the austenite grain size exceeds about 37 µm. The area of ductile fracture is enlarged by the increased dislocation mobility in high Ni steel, resulting in the improvement of impact toughness. It is difficult to refine the prior austenite grain size to less than 37 µm of microalloyed ferrite-pearlite steels through optimizing forging and cooling process. However, small amount of Ni addition is beneficial to the impact toughness and does not increase the cost significantly.

INFLUENCE OF HEAT TREATMENT ON THE ABSORBED ENERGY OF CARBON STEEL ALLOYS USING OIL QUENCHING AND WATER QUENCHING

Carbon steel alloys are modified by heat treatment processes so that structural components are able to withstand and sustain external forces and loads which specified operating conditions and have desired useful life. In this work, oil quenching and water quenching were tested and compared to low carbon steel, medium carbon steel and high carbon steel alloys. During quenching process, the specimens were heated at 900C° and soaked for 20 minutes in the furnace, three specimens were then quenched in oil and other three in water with the objective to improve impact toughness, the results obtained with instrumented Charpy impact tests showed that in low carbon steel the oil quenching treatment was able to increase impact toughness 159% and 107% by water quenching, for medium carbon steel, impact toughness by oil quenching increased 70% and water quenching increase 59%.The different behavior of high carbon steel appears when specimens heat treated, 85% impact toughness decrease in oil quenching and 50% decrease in water quenched.

A novel large cross-section quenching and tempering mold steel matching excellent strength–hardness–toughness properties

According to the synergistic effect of multiple elements, a novel large cross-section pre–hardened mold steel is designed with higher strength and hardness as well as toughness (HSTPM steel) compared with industrial high–end 718H steel during the normal tempering temperature range (530–650 °C). The achievements on the strength and hardness of HSTPM steel are attributed to the effects of 0.5 wt% molybdenum (Mo) and 0.1 wt% vanadium (V) additions on the matrix structure, including misorientation gradient, dislocation density, and nano–sized precipitates. The main strengthening precipitate (V, Mo)C was finely characterized by high angle annular dark field–scanning transmission electron microscope (HADDF–STEM) analysis. In addition, it was found that rare earth (RE) additions of 0.015 wt% in HSTPM steel highly effective in refining inclusions for changing the morphologies of strip MnS and Al2O3 to tiny spherical RE2O2S, and the number of coarse inclusions (diameter exceeding 5 µm) is significantly decreased in the same statistical volume (4.69 × 108 μm3) by three–dimensional X–ray microtomography statistical technique. The purification of molten steel and modification of inclusions by RE treatment are responsible for the improvement of impact toughness. Simultaneously, the lower carbon (C) content in HSTPM steel (high–end 718 H, 0.34 wt%; HSTPM, 0.23 wt%) also plays a large role in improving impact toughness. Finally, the synergistic effect of multiple elements delayed and eventually reduced the transformation range in pearlite compared with that in 718H steel.

Toward Super-Tough Poly(l-lactide) via Constructing Pseudo-Cross-link Network in Toughening Phase Anchored by Stereocomplex Crystallites at the Interface

We demonstrated a novel strategy to toughen poly(l-lactide) (PLLA) by constructing pseudo-cross-link networks based on chain entanglements of long-chain branched structure in the toughening phase, which were anchored by stereocomplex (SC) crystallites at the interface. The formation of pseudo-cross-link network was achieved by simple blending of the copolymer of long-chain branched polycaprolactone and poly(d-lactide) (LB-PCL- b-DLA) with PLLA without introducing any chemical cross-linking structure or nonbiodegradable component. The microscopic morphology analysis suggests that the interface-formed SC crystallites not only enhanced the interfacial interaction between LB-PCL and PLLA but also obviously increased the matrix crystallization rate. Different from those blends without SC crystallites or long-chain branched structures, nano-microgels were observed in chloroform solution of the PLLA/LB-PCL- b-DLA blend, suggesting the formation of pseudo-cross-link network. The pseudo-cross-link network in LB-PCL toughening phase endows PLLA a significantly improved impact toughness (49.5 kJ/m2), which is almost 13 times than that of neat PLLA. Moreover, matrix crystallinity and spherulite size of the PLLA matrix also play significant roles in toughening. Only sufficiently crystallized PLLA with proper spherulite size can effectively trigger the matrix shear yielding, meanwhile, facilitate the energy dissipating.

Simulation Kinetics of Austenitic Phase Transformation in Ti+Nb Stabilized IF and Microalloyed Steels

In the present study, the influence of cooling rates (low to ultrafast) on diffusion controlled and displacive transformation of Ti-Nb IF and microalloyed steels has been thoroughly investigated. Mechanisms of nucleation and formation of non-equiaxed ferrite morphologies (i.e., acicular ferrite and bainitic ferrite) have been analyzed in details. The continuous cooling transformation behavior has been studied in a thermomechanical simulator (Gleeble 3800) using the cooling rates of 1-150 °C/s. On the basis of the dilatometric analysis of each cooling rate, continuous cooling transformation (CCT) diagrams have been constructed for both the steels to correlate the microstructural features at each cooling rate in different critical zones. In the case of the IF steel, massive ferrite grains along with granular bainite structures have been developed at cooling rates > 120 °C/s. On the other hand, a mixture of lath bainitic and lath martensite structures has been formed at a cooling rate of ~ 80 °C/s in the microalloyed steel. A strong dependence of the cooling rates and C content on the microstructures and mechanical properties has been established. The steel samples that were fast cooled to a mixture of bainite ferrite and martensite showed a significant improvement of impact toughness and hardness (157 J, for IF steel and 174 J for microalloyed steel) as compared to that of the as-received specimens (133 J for IF steel and 116 J for microalloyed steel). Thus, it can be concluded that the hardness and impact toughness properties are correlated well with the microstructural constituents as indicated by the CCT diagram. Transformation mechanisms and kinetics of austenitic transformation to different phase morphologies at various cooling rates have been discussed in details to correlate microstructural evolution and mechanical properties.

Enhanced Impact Toughness at Ambient Temperatures of Ultrafine-Grained Al-26 wt.% Si Alloy Produced by Equal-Channel Angular Pressing

Overcoming general brittleness of hypereutectic Al-Si alloys is in urgent need for expanding their application in automotive, aerospace and construction industries. A unique phenomenon was observed that bulk ultrafine-grained Al-26 wt.% Si alloy, produced by severe plastic deformation via equal-channel angular pressing, exhibited higher toughness at the impact temperature of − 196 ~ 100 °C than the coarse-grained casting alloy. The improvement in impact toughness at all testing temperatures was mainly due to the homogeneous ultrafine-grained structure with the breakage of brittle primary silicon crystals, which generated more and deeper fracture dimples that consumed much higher fracture energy. It indicates the advantage of bulk ultrafine-grained Al-Si alloys and spurs their application interest at various ambient temperatures.

Development of hot stamping grade steel with improved impact toughness by Nb microalloying

Low carbon alloy steels, one alloyed with B and Cr and other alloyed with B, Cr and Nb were made and cast into pencil ingots. These ingots were hot-rolled to 2 mm sheets suitable for hot stamping application in an experimental rolling mill and a detailed study was made on the effect of various alloying elements on the microstructure and mechanical properties of these steels. A significantly higher impact toughness (42 J) could be achieved in the as-rolled and water-quenched Nb-Cr-B steel in comparison to Cr-B steel under similar processing condition. Microstructural refinement of martensite must be the reason for achieving high impact toughness.

Повышение ударной вязкости металла комбинированных сварных соединений легированных бейнитных сталей

Повышение ударной вязкости металла комбинированных сварных соединений легированных бейнитных сталей

Comparison of impact-abrasive wear characteristics and performance of direct quenched (DQ) and direct quenched and partitioned (DQ&P) steels

The recently developed method of direct quenching and partitioning (DQ&P) was utilized to produce ultra-high strength martensitic steels with retained austenite. The DQ&P steels have high surface hardness while retaining good impact toughness and elongation values. The toughness and elongation properties are attributed to the retained austenite which is stabilized in the DQ&P process. The aim was to study if DQ&P processing could be utilized for improved abrasive wear resistance. Two medium-carbon (0.3%) chemical compositions were selected with varying amounts of silicon, aluminum and chromium. The processing route for DQ&P involved interrupted water quenching with two different quench stop temperatures (TQ) (175 and 225°C). Direct quenched (DQ) variants were also produced for comparison of both mechanical properties and wear characteristics. Compared to the DQ treatment, improved impact toughness and elongation to fracture were achieved with the DQ&P treatment while initial strength and hardness was reduced. An impeller-tumbler testing device was used to measure the impact-abrasive wear performance of the different experimental microstructures and compared with that of a reference commercial 500 HB steel. No advantage of the increased ductility of the DQ&P steels was apparent; wear resistance was shown to only correlate with the initial surface hardness of the steels.

Significant toughness improvement in iPP/PLLA/EGMA blend by introducing dicumyl peroxide as the morphology governor

Isotactic polypropylene/poly(l-lactide) blends were toughened by blending polyethylene-glycidyl methacrylate (EGMA) as compatibilizer and dicumyl peroxide (DCP) as compatibilization accelerator. Isotactic polypropylene (iPP)/poly(l-lactide) (PLLA)/EGMA/DCP blends showed remarkably improved impact toughness and excellent flexibility, moderate levels of strength, and modulus. Reactive compatibilization between iPP and EGMA as well as PLLA and EGMA was studied using Fourier transform infrared spectroscopy. The “salami”-like phase structure of the dispersed domains in the iPP/PLLA/EGMA ternary blends was observed. With the further addition of DCP, the EGMA phase evolved from occluded sub-inclusions into an interface boundary. The significant increase in impact strength of iPP/PLLA/EGMA/DCP blends was ascribed to the effective interfacial compatibilization. This study provides a new route to prepare a cost-effective material with partial biodegradability, good impact toughness, and tensile strength. Meanwhile, a deep insight into the optimization of properties of multicomponent blends by morphology transition was proposed.