Low Temperature Impact Toughness Research Articles

Regulation Law of Tempering Cooling Rate on Toughness of Medium-Carbon Medium-Alloy Steel.

Temper embrittlement is a major challenge encountered during the heat treatment of high-performance steels for large forgings. This study investigates the microstructural evolution and mechanical properties of Cr-Ni-Mo-V thick-walled steel, designed for large forgings with a tensile strength of 1500 MPa, under different tempering cooling rates. Optical microscopy (OM), scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD) were employed to analyze the microstructural features. The results demonstrate that the embrittlement occurring during air cooling after tempering is attributed to the concentration of impurities near Fe3C at the grain boundaries. The low-temperature impact toughness at -40 °C after water quenching reaches 29 J due to the accelerated cooling rate during tempering, which slows down the diffusion of impurity elements towards the grain boundaries, resulting in a reduced concentration and dislocation density and an increased stability of the grain boundaries, thereby enhancing toughness. The bainite content decreases and the interface between martensite and bainite undergoes changes after water quenching during tempering. These alterations influence the crack propagation direction within the two-phase microstructure, further modifying the toughness. These findings contribute to the understanding of temper embrittlement and provide valuable guidance for optimizing heat treatment processes to enhance the performance of high-performance steels in large forgings.

Refinement mechanism of large heat-input welding CGHAZ microstructure by N addition and its effect on toughness of a V-Ti-N microalloying weathering steel

The large heat input welding performance of uncoated weathering steel remains a great challenge since their coarse grain heat-affected zones (CGHAZs) are usually occupied by coarse and brittle microstructure consisting of granular bainitic ferrite (GBF) and martensite/austenite (M/A) constituents. In this article, a V-Ti-N microalloying weathering steel suitable for large heat input welding was achieved through the N addition, and the mechanism dominating the modified microstructure and the enhanced impact toughness of CGHAZ was deeply elucidated mainly by HRTEM. The results indicated that in the simulated CGHAZ with large heat input of 80 kJ/cm, the increase of N content from 0.0030 wt% to 0.0075 wt%, and 0.0115 wt% elevated the density of nano- and submicron-sized particles of (Ti, V)(C, N) containing VN, which in turn resulted in the refinement of prior austenite grains (PAGs) by the improved grain boundary pinning effect and the enhancement in heterogeneous nucleation of intragranular ferrite (IGF) by the decreased lattice disregistry (δ) between (Ti, V)(C, N) and IGF. The fraction of high-angle grain boundaries (HAGBs) with misorientation tolerance angles (MTAs) higher than 15° was thereby increased while the mean equivalent diameter (MED) of ferritic grains with HAGBs was decreased. Simultaneously, the N addition scattered the M/A constituents, promoted the internal substructure of dislocation-type M/A at the expense of twin-type M/A, and accordingly reduced the hardness difference between the M/A constituents and the matrix. All these factors retarded brittle crack initiation and propagation, and low-temperature impact toughness of simulated CGHAZ increased from 62.8 J to 141.2 J.

Crystallography of transformation products with different cooling rates in low-carbon alloy steel and its effect on low-temperature toughness uniformity of heavy plates

The crystallography of transformation products with different cooling rates has been investigated in a typical low-carbon alloy steel, and its effect on low-temperature impact toughness uniformity in the thickness direction of heavy steel plates has been elucidated. The results show that both the full lath martenite (LM) microstructure transformed at the cooling rate of 45 K s−1 and granular bainite (GB) + lath bainite (LB) + LM microstructure transformed at the cooling rates of 3∼6 K s−1 have higher density of high-angle grain boundaries (HAGBs). The density of HAGBs in the full GB microstructure transformed at the cooling rate of 0.5 K s−1 is the lowest, and that in the LB + LM microstructure transformed at the cooling rates of 15∼22 K s−1 is the middle. In the GB + LB + LM microstructure, the division of austenite by multi-stage transformation reduces the LB and LM transformation temperatures, contributing to a weakening of variant selection and a tendency to close-packed plane grouping. Therefore, the GB + LB + LM microstructure achieves the refinement degree similar to the full LM microstructure, but slightly higher than that of the LB + LM microstructure. For the 60 mm thick heavy steel plate with ultra-fast cooling, the cooling rate decreases from 41 K s−1 at the subsurface to 14 K s−1 at the 1/4-thickness and then to 4 K s−1 at the 1/2-thickness. The full LM microstructure, LB + LM microstructure, and GB + LB + LM microstructure are obtained in sequence, resulting in a more uniform density of HAGBs. Consequently, the relatively uniform impact toughness at 143 K in the thickness direction is achieved with a small difference of ∼30 J.

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.

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.

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.

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.

The determining role of laminar heterostructures on the mechanical properties of low-density steels

The practical application of low-density steels is hindered because of poor low-temperature toughness. To overcome this limitation, this study is focused on a new low-density steel with exceptional low-temperature toughness performance using a combination of hot rolling, intercritical quenching, and tempering. We study here the different intercritical quenching temperatures on both the microstructure and mechanical properties by EBSD, SEM, Charpy impact experiments and tensile experiments. The results showed that the presence of laminar heterostructures significantly enhanced the steel's low-temperature impact toughness, which can lead to the Charpy impact work of 61 J for the experimental steels at −40 °C. By the observation of the fracture morphology and secondary cracking of the impact specimens, it is found that this improvement was primarily attributed to the formation of obvious anisotropic distribution of randomly oriented grain boundaries between ferrite-martensite laminar heterostructures and the ferrite, which had a strong {001} <110> texture. This led to the generation of more and longer secondary cracks, which increased the energy required for crack propagation (Ep). However, the proportion of ferrite had an optimal value, and as the intercritical quenching temperature increased, the ferrite phase gradually disappeared, resulting in an initial increase and then decrease in toughness. Once the quenching temperature was greater than the completely austenite forming temperature, the formation of a single martensite microstructure led to brittle fractures along the prior austenite grain boundaries, resulting in a significant decrease in toughness, especially at low temperatures. The underlying reasons for the observed behavior are interrogated and discussed in terms of process-structure-properties paradigm.

Effect of V, Nb, and Ti microalloying on low–temperature impact fracture behavior of non–quenched and tempered forged steel

In this study, three kinds of non-quenched and tempered forged steels with V, V–Nb and V–Nb–Ti microalloying elements were designed. The microstructure and low–temperature impact fracture behavior of the three steels were comprehensively compared and analyzed. The results showed that the addition of Nb and Ti elements can significantly refine the grains and form precipitated particles to improve the strength of the steel. The fracture behavior of the three steels types at a low temperature of −28 °C was dominated by brittle cleavage fractures. V–Nb–Ti steel exhibited the worst low–temperature impact toughness. The SEM analysis result showed that coarse (V, Nb, Ti) (C, N) particles present in the steel acted as local cleavage initiation sites. No cleavage fracture caused by (V, Nb) (C, N) particles occurred in the V–Nb steel. The results showed that the high impact energy (70.4 J) at room temperature and low ductile–brittle transition temperature (DBTT, −20 °C) of the V–Nb steel were mainly attributable to the grain refinement of Nb. The micron–sized coarse (V, Nb, Ti) (C, N) particles were the main factor responsible for the low impact energy (35.5 J) at room temperature and high DBTT (−7 °C) of V–Nb–Ti steel and can act as the local cleavage initiation sites to dominate the cleavage fracture at low temperature and decrease the impact toughness.

Experimental study on impact toughness of EH36 ship steel in Arctic areas

ABSTRACT Polar ships are subjected to the ice loading. Low temperature impact toughness of ship steels is important for the ice-breaking efficiency and navigation safety of polar ships. In this paper, the impact absorption energy of EH36 steel under different temperatures, impact velocities and plate thickness is analyzed by experimental test, Taguchi and ANOVA method. The impact absorption energy was divided into crack formation and crack propagation absorption energy. The experimental results show that the impact absorption energy of EH36 steel is positively correlated with plate thickness and temperature, and negatively with impact velocity when it was in the toughness state. The order of significance ranking from high to low is thickness, temperature, and impact velocity. The ratio of the crack propagation absorption energy increases with the temperatures and reflects the impact toughness of EH36 steel. Furthermore, the best factor combination of impact toughness of EH36 steel was obtained.

Study on the Control Method of Ferrite Grain Size of Nodular Cast Iron and Its Effect on Low-Temperature Impact Toughness

Study on the Control Method of Ferrite Grain Size of Nodular Cast Iron and Its Effect on Low-Temperature Impact Toughness

Effect of microstructural heterogeneities on variability in low-temperature impact toughness in multi-pass weld metal of 420 MPa offshore engineering steel

Effect of microstructural heterogeneities on variability in low-temperature impact toughness in multi-pass weld metal of 420 MPa offshore engineering steel

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.

Effect of Nb/V/Ti Microalloy on Microstructure and Properties of 16Mn Seamless Steel Pipe

In order to study the effect of Nb, V and Ti microalloys on the microstructure and properties of 16Mn seamless steel tube, the static CCT diagram of tested steel was made on Glecble-3800 thermo-mechanical simulator. Based on the observation of microstructures and precipitates by optical microscopy, SEM and TEM, the effect of different microalloy on mechanical properties of 16Mn steel was analyzed. The results show that Ti treated 16Mn steel is advantageous to both grain refinement and the low temperature impact toughness. The impact toughness of Nb or V microalloyed steel is decreased, which appeared bainite in hot rolling. Normalizing can improve the impact toughness of Nb microalloyed steel, while the microstructure and properties of V microalloyed steel are not improved. 16MnTi is taken to produce hot-rolled pipes with wall thickness between 8 mm to 16 mm, that impact value is more than 80 J at - 40 °C; while 16MnTiNb is taken to produce normalized pipes with wall thickness greater than 16 mm, that impact value reaches 120 J at -40°C.

Effect of shielding gas on MIG welding performance with austenitic wire

Two different shielding gases, Ar + CO2 and Ar + He + N2, were used for MIG welding of a 20 mm 10CrNi3MoV steel plate. The effects of shielding gases on the chemical composition, microstructure, purity, and low-temperature impact toughness of weld metal were studied in detail. N-containing shielding gas can increase the nitrogen content in the weld metal, change the solidification mode of deposited metal, reduce ferrite content in the weld metal, and significantly improve the low-temperature impact toughness of deposited metal. The purity of weld in the ternary inert gas atmosphere is high, which is beneficial to the improvement of low-temperature toughness. The ferrite content of the ternary joint is higher than that of the binary joint, and the strength of the ternary joint is much higher than that of the binary joint.

Parameter Optimization and Mechanical Properties of Laser Cladding of 316L Stainless Steel Powder on G20Mn5QT Steel

G20Mn5QT steel has excellent mechanical properties and is widely used in key components of rail vehicles. However, during the operation of high-speed vehicles, wear and tear will inevitably occur. In this paper, laser cladding technology was selected to successfully prepare 316L stainless steel coating. The optimum processing parameters were obtained with a laser power of 2300 W, a scanning speed of 500 mm/min, and a powder feeding speed of 14 g/min. The microstructure of 316L coating is mainly composed of planar crystals, cellular crystals, columnar crystals, and equiaxed crystals. Through range analysis, it is found that the microhardness, wear resistance, and micro-shear strength of the cladding layers increase with the increase of laser power, while the tensile strength and yield strength increase first and then decrease with the increase of laser power. Under the optimized process parameters, the low-temperature impact toughness, elongation, tensile strength, and yield strength of the cladding layer were 97.6%, 24%, 10.9%, and 32.5% higher than that of the G20Mn5QT substrate, respectively. An excellent combination of strength and toughness was achieved by cladding 316L alloy on the surface of the G20Mn5QT substrate, which can meet the requirements of remanufacturing fractional key vehicle parts.

Low-temperature impact toughness and deformation mechanism of CT20 titanium alloy

The impact toughness and deformation mechanism of near-α-type low-temperature titanium alloy CT20 at different temperatures (20 °C, 0 °C, −50 °C, −100 °C and − 196 °C) are studied. The results show that the impact toughness is as high as 90 J/cm 2 at 20 °C, and does not decrease obviously at 0 °C, −50 °C and − 100 °C, but brittle fracture occurs at −196 °C (29 J/cm 2 ). The tortuosity of the crack path decreases with decreasing temperature. At 20 °C, the grain boundary and α colony interface with high-angle grain boundary deflect the crack propagation direction effectively, forming a tortuous crack path, while crack tends to cut through α lamellae, forming a relatively straight path at cryogenic temperature. The lamellar α undergoes kink deformation at 20 °C, showing severe plastic deformation characteristics. In addition, deformation twins and geometrically necessary dislocations (GNDs) are observed near and far from the fracture surface, indicating that the plastic zone is large. As the temperature decreases, the twin density near the crack path tends to increase. However, few twins are seen in the region far from the fracture surface, and GND densities are low at −196 °C, implying that the plastic zone is small. In a word, the synergistic effect of tortuous crack path, dislocation slip and deformation twinning enable CT20 to achieve high impact toughness at 20 °C. At −196 °C, the plastic deformation capacity of the microstructure decreased significantly, resulting in the formation of a small plastic zone and brittle fracture. • α colony interface deflects crack propagation direction effectively at 20 °C, while crack tends to cut through α lamellae at cryogenic temperature. • The synergistic effect of tortuous crack path, dislocation slip and twinning enables CT20 to achieve the highest impact toughness at 20 °C. • The coordinated deformation of dislocation slip and twins keeps the impact toughness at a high level in the temperature range of 0 °C ~ -100 °C. • The plastic deformation ability decreases significantly, resulting in the formation of a small plastic zone and brittle fracture at −196 °C.

Microstructure and Mechanical Properties of the Ternary Gas Shielded Narrow-Gap GMA Welded Joint of High-Strength Steel

An 80%Ar-10%CO2-10%He ternary gas mixture was used as the shielding gas during the narrow-gap welding of thick Q690E high-strength steel plates. Complete and defect-free welded joints were obtained, and the microstructure and mechanical properties of the welded joint were investigated. The weld zone consists of a solidification area and interlayer zone, and the heat-affected zone consists of a coarse-grain heat-affected zone (CG-HAZ) and a fine-grain heat-affected zone (FG-HAZ). The microstructures of the weld zone are mainly lath bainite (LB), acicular ferrite (AF), and granular bainite (GB). The microstructure of the CG-HAZ is lath martensite (LM) and the microstructure of FG-HAZ is GB. Methods with different heat inputs were used to study their effects on the mechanical properties of the welded joint. It was found that the microstructure and mechanical properties of the welded joints are better with lower heat input. With tandem wire narrow-gap GMA welding, the tensile strength of the joints declined from 795.3 to 718.3 MPa and the impact toughness at −40 °C resulted in a weak position in the weld zone, which declined from 76~81 J to 55~69 J, when the welding speed reduced from 350 to 250 mm/min. With oscillating-arc narrow-gap GMA welding, the tensile strength achieved 853.4 MPa and the impact toughness at −40 °C was around 69~87 J. The results indicated that, under the appropriate heat input, the tensile strength of the joint exceeds 770 MPa and the low temperature impact toughness at −40 °C exceeds 69 J. A 155 mm-thick Q690E steel welded joint was obtained and the mechanical properties of the welded joint meets the requirements of the offshore drilling platforms.