Toughness Of Steel Research Articles

Production of improved quality steel in highscale basic oxygen furnaces using vacuum treatment

The article presents the results of the converter steel technology development using ladle treatment including portion vacuum degassing at vessel UPVS-350 and pouring at continuous casting machine. In order to improve the quality of metal products, steels of grades 3sp, 08sp, 20k, 20sp, 09G2S, 10G2S1D, 09G2S, 08GT, 17G1SU, 17G1S, 17GS, 10HSND, 15HSND, E36, pipe steels of type 09G2FB were treated by vacuum. Steel was smelted in basic oxygen furnaces with a capacity of 350 ton using technical pure oxygen (99,2–99,5% О2). The article shows that with a UPVS-350 vacuum chamber dilution to133 Pa (1 mm Hg), a metal portion weight of 20–40 tons, the number of vacuum ing cycles less than 25 and a circulation coefficient from 2 to 5, a degree of oxygen removal of up to 40%and hydrogen up to 50% can be achieved. There is an improvement in the pouring ability of vacuum treated steel, a decrease in the axial discontinuity (stratification)at the central part of the slabs from 3 to 2–2.5 grades, decrease of the nonmetallic inclusions content in the billets by 0,2–2,3 grade and their distribution in the volume of metal is more uniform. When exhaustion in a vacuum chamber reach of 1–2 mm Hg level, the impact toughness and elongation of steel in crease. For carbon steel, the production of inappropriate quality metal according to the ultrasonic control decreased by about 3 times (from 0,39 to 0,12%), and the total degree of the rejected metal decreased from 1,06 to 0,65%. During vacuuming of 08GT steel using for chemical reactors, there is a more compact and greater uniformity of the ferrite-perlite structure with the value of the actual grain no. 7–9. The internal structure at the liquation central zone of the sheets produced of this steel is also improved

Influence of Intercritical Annealing on Microstructure, Ductility, and Toughness of Medium Mn Steels

Influence of Intercritical Annealing on Microstructure, Ductility, and Toughness of Medium Mn Steels

Mechanistic understanding of banded microstructure and its effect on anisotropy of toughness in low carbon-low alloy steel

In this study, the relationship between the anisotropy of toughness and microstructure in low carbon low alloy steel treated by quenching and tempering (QT) heat treatment was investigated with the aid of scanning electron microscope, electron back-scattered diffraction and transmission electron microscope combined with energy disperse spectroscopy techniques. Results show that the impact toughness of the quenched steel plate along the longitudinal and transverse directions are little different. However, after tempering, the longitudinal impact toughness of QT steel plate is improved by 88%, while the transverse impact toughness is slightly decreased, leading to a significant anisotropy of toughness. Microstructural characterizations reveal that banded microstructure with coarse grain size exists along longitudinal direction (i.e., rolling direction) of steel plate, which is attributed to co-segregation of Mn and C and the resulting uneven recrystallization during hot rolling. After tempering, fine and dispersed carbides are precipitated in matrix microstructure, but high-density coarse carbides are formed within banded microstructure. It is suggested that the coarse grains and high-density coarse carbides significantly deteriorate the resistance against crack propagation along banded microstructure, leading to the anisotropy of toughness of QT steel plate. The findings of this study will aid to design metallurgical processes including chemical composition design, hot rolling, and heat treatments to eliminate the anisotropy of toughness of low alloy steels with high strength and toughness.

Thermal Stability and Thermal Fatigue Resistance Improvement of New High Toughness 5% Cr Hot Working Die Steel

This work developed a new high toughness 5% Cr martensitic hot working die steel (GYDCK-20), which exhibits better thermal stability and thermal fatigue resistance than AISI H13 steel. The concrete study involves: conducting on the evolution of microstructures and mechanical properties of two steels during the 60-hour holding process at 600°C, as well as the thermal fatigue behavior under the cyclic heating-water quenching conditions. The experimental results showed that in the case of ensuring the same initial hardness of two steels (43-43.5 HRC), GYDCK-20 emerged better thermal stability and superior toughness after 60-hour holding process at 600°C. After thermal cycling was increased from 1000 to 2000 times, the average crack length growth rate of GYDCK-20 was 31.8% lower than that of H13 and the maximum crack width growth rate was 35.1% lower than that of H13. It was found that thermal fatigue cracking was dominated by transgranular propagation at lower cycle numbers, while more inclined to propagate along the grain boundaries at higher cycle numbers. The higher toughness of GYDCK-20 steel promotes the dispersion of thermal stress, thereby enhancing the thermal fatigue resistance. It is mainly attributed to the lower V content of GYDCK-20 steel, which reduces the density of large-sized MC primary carbides. On the other hand, the lower Si content promotes the dissolution of cementite particles, and the secondary carbides precipitate in a smaller and uniformly distributed form, which hinders the propagation of thermal fatigue cracks, while also delays the coarsening of the fatigue-adverse phase M23C6 carbides.

Synergistic regulation of nano-precipitates and reversed austenite in titanium-free maraging steel by low-temperature solution treatment and double aging treatment

To synergistically enhance the strength and toughness of titanium-free maraging steel, a multi-scale characterization method was used to illustrate the effects of low-temperature solution treatment and double aging treatment on the microstructure of titanium-free maraging steel in this paper. After the low-temperature solution treatment and the double aging treatment, the tensile strength of titanium-free maraging steel increased from 1954 MPa to 2160 MPa and the elongation increased by 8.95 %. By the low-temperature solution treatment, the original austenite grain size of the titanium-free maraging steel was refined to 0.69 μm. The double aging treatment promoted the diffusion of Mo and Ni elements, increased the volume fraction of ω phase, Ni3Mo nano-precipitation phase and reversed austenite, and refined the size of ω phase and Ni3Mo by 14.2 % and 7.9 %, respectively. The nanoparticles of titanium-free maraging steel mainly include the ω phase, Ni3Mo and Laves phase. The strengthening mechanism of nanoparticles was quantitatively evaluated from the shear mechanism and Orowan dislocation loop mechanism. The mechanism shows that the ω phase is the main contributor to the overall precipitation strengthening. Therefore, low-temperature solution treatment and double aging treatment provide a potential solution for achieving high strength and high toughness in maraging steel.

Study on Corrosion Damage Behavior of 300M Ultra-high Strength Steel in Marine Atmospheric Environment

Abstract The marine atmospheric environment exposure test of 300M ultra-high strength steel was carried out at Hainan test station. The corrosion damage behavior of 300M ultra-high strength steel in marine atmospheric environment was studied by macroscopic and microscopic corrosion morphology, tensile properties and stress corrosion. The results show that the comprehensive corrosion behavior of 300M ultra-high strength steel occurred in the marine atmospheric environment. The color of the corrosion products changed from red to reddish brown, dark brown and black, and the maximum corrosion depth in the three years was 400μm. The corrosion had a significant effect on the tensile properties during the test. The tensile strength and elongation of 300M ultra-high strength steel decreased by 13.3 % and 81.8 %, respectively, which indicated that the effect of corrosion on the plasticity of the material was much greater than that on the tensile strength. Corrosion also caused a decrease in fracture toughness, and the fracture toughness of 300M ultra-high strength steel decreased by 26% in 2 years. The results of stress corrosion show that the crack propagation of 300M ultra-high strength steel is very rapid in the initial stage of marine atmospheric environment test, and the crack propagation length reaches 10.836mm in 7 days. The stress corrosion threshold KISCC of 300M ultra-high strength steel is 24.48MPa•m1/2, and the relative index of stress corrosion cracking sensitivity is 0.31, which indicates that 300M ultra-high strength steel is very sensitive to stress corrosion in marine atmospheric environment. The results show that 300M ultra-high strength steel has poor adaptability to the marine environment. Therefore, it is necessary to adopt excellent surface treatment process and spray aging resistant coating to effectively protect 300M ultra-high strength steel to ensure its long-term reliable use in the marine environment.

Corrosion Resistance and Fracture Toughness of Cryogenic-Treated X153CrMoV12 Tool Steel

Abstract Cryogenic treatment can be employed as an additional heat treatment step for martensitic steels, particularly high-carbon and high-alloy tool steels, to improve their mechanical properties and wear resistance. This enhancement results from a transformation of retained austenite and precipitation of finely distributed secondary carbides. The present study examines the impact of a shallow cryogenic treatment, performed between the quenching and tempering processes, on the corrosion resistance and fracture toughness of cold work tool steel X153CrMoV12. The influence of the shallow cryogenic treatment was evaluated using potentiodynamic polarization test, salt spray tests and three-point bending tests in various hydrogen charging conditions. In addition, the microstructure and phase transformation of the tool steel were investigated using high-resolution scanning electron microscopy, X-ray diffraction and dilatometer tests. The results suggest that the shallow cryogenic treatment followed by a single tempering cycle can slightly enhance the corrosion resistance of the steel. More importantly, the shallow cryogenic treatment positively affects fracture toughness and reduces hydrogen susceptibility. It is discussed how these improvements in the properties of the X153CrMoV12 steel are linked to the microstructural changes induced by the shallow cryogenic treatment.

Effect of Ti–Mg–Ce Complex Deoxidation on the Inclusions and Mechanical Properties of S355 Steel

Oxide metallurgy technology not only refines the microstructure but also modifies the type, density, and size of inclusions. This study examines the impact of Ti–Mg–Ce composite deoxidation on the inclusions and mechanical properties of S355 steel used for heavy‐duty H‐beams. Compared to base alloy, Ti–Mg–Ce‐added alloy features a higher presence of inclusions such as MgO, TiN (containing Ce), TiOx, and TiS, except for SiO2, Al2O3, MnS, and MnO. Following Ti–Mg–Ce composite deoxidation, the number density of inclusions increases by 16.4%, and the equivalent diameter, length, and width of the inclusions decrease by 41.0%, 41.2%, and 37.9%, respectively. The length of large‐angle grain boundaries in the same area increases by 20%, and the average grain size reduces by 12.5%, resulting in an enhancement of yield strength by 23.6 MPa, tensile strength by 37 MPa, total elongation by 2.1%, and impact energy by 92 J. By employing Ti–Mg–Ce composite deoxidation to optimize inclusions and microstructure, the strength, plasticity, and toughness of the experimental steel are significantly improved.

Evolution of carbides and Charpy toughness in a low alloy bainitic steel during step-up aging process

The low alloy bainitic steel used in reactor pressure vessels deteriorates during thermal service while the macroscopic thermodynamic parameters that cause thermal aging remains unknown. In this work, a thermal aging restructuring scheme was proposed by step-up aging the steel from 350 °C to 490 °C, with a total duration of 7500 hours. Samples from varied thickness of the steel were characterized in terms of carbides evolution and Charpy impact toughness at 20 °C. The carbide size and its fraction were statistically analyzed showing partial coarsening and dissolution during aging, while the carbide fraction was found linearly correlated with the impact energy for the first time. The critical transition temperature parameter of the aging process was found to be 470 °C for the steel. The macroscopic thermodynamic parameters, including the thermal aging time and temperature, facilitate a comprehensive understanding of the material degradation mechanism and provide a basis for long-term safety of equipment.

The evolution and toughening mechanism of austenite in high Co–Ni ultrahigh strength steel

The austenite, as a “soft” phase, has a significant impact on the mechanical properties in ultrahigh strength steel. The morphology, size, and distribution of austenite play a critical role in toughness of steel. This study concentrates on morphology, size, and distribution of austenite subjected to solid solution, cryogenic, and aging treatments and explores its evolution characterized by transmission electron microscopy, scanning electron microscopy, electron backscatter diffraction and x-ray diffraction. The toughening mechanism of austenite with different characterization was investigated through charpy impact testing and fracture morphology analysis. The results indicate that filmy and blocky austenite existed at the lath boundary of martensite. Blocky austenite and some filmy austenite were eliminated after cryogenic treatment, while approximately less than 15 nm filmy reverted austenite was formed after aging treatment. Filmy reverted austenite at the boundary of martensite laths can enhance toughness. Retained austenite without cryogenic treatment can induce the formation of larger blocky austenite, which decreased toughness.

Effect of Zr addition on MnS inclusion characteristics and mechanical properties in medium carbon ferrite-pearlite steel

In this paper, the influence of decreasing Zr content (0.0110wt%, 0.0044wt%) on the microstructure, MnS inclusion characteristics, and mechanical properties of medium carbon ferrite-pearlite steel was studied. The results show that the volume fraction of intragranular ferrite (IGF) in steel increases with a reduction in Zr content. The ductility of the steel is reduced by 1.83%, the impact toughness is increased by 2.5 times, and the yield strength is almost unchanged. Compound MnS inclusions are the main inclusions that induce IGF formation. The proportion of heterogeneous nucleation of MnS inclusions using oxides as nucleation sites is increased, which is the main reason for the increase in the volume fraction of IGF. Furthermore, MnS inclusions change from type Ⅲ to type Ⅰ and Ⅱ, and their distribution is gradually uniform. Thermodynamic analysis reveals that the underlying cause for the morphological transformation of MnS inclusions is the rapid surge in the supersaturation of S element in molten steel. The presence of liquid-phase low-melting inclusions (MnS-Al2O3) promotes the formation of type Ⅰ MnS inclusions. The volume fraction and the dimension perpendicular to the fracture surface of the MnS inclusions are increased by at least 33% and 111%, while their aspect ratio is reduced by a maximum of up to 19%. The change in the volume fraction of MnS inclusions is the primary factor contributing to the decrease of tensile plasticity. The toughness of steel is mainly affected by the aspect ratio and the dimension perpendicular to the fracture surface of MnS inclusions.

Enhancing strength and toughness in a pearlitic rail steel via multi-alloying and accelerated cooling

The demand for higher speeds and heavier loads has emphasized the necessity to tackle the inherent trade-off between strength and toughness in pearlitic rail steels. Herein, the co-additions of Cr, Ni and Cu and accelerated cooling rate on the microstructure and mechanical properties of a medium carbon pearlitic steel were systematically investigated. In comparison with base alloy, the new steel exhibits smaller prior austenite grain size (PAGS) due to the drag effect of Cu and Ni solutes. The finer PAGS and lower C content increased the volume fraction of proeutectoid ferrite. The finer pearlitic nodule size (PNS) and pearlitic colony size (PCS) were attributed to the small PAGS and greater extent of undercooling due to the combined effect of multi-alloying and accelerated cooling. Additionally, the synergistic effects of multi-alloying and accelerated cooling rate decrease the pearlitic phase transformation temperature, accountable for the finer interlamellar spacing (IS). Compared with the base steel, the yield strength and ultimate tensile strength of the new steel increased from 587 MPa and 1069 MPa to 740MPa and 1178MPa, respectively, with a simultaneous increase of ductility from 12.4% to 14.7%. The strength increments are mainly attributed to the finer IS in the new steel. Meanwhile, the new steel possesses a higher impact toughness compared to the base steel due to the increased volume fraction of proeutectoid ferrite, and finer PNS and IS.

Effect of Trace Rare Earth Element Cerium (Ce) on Microstructure and Mechanical Properties of High Strength Marine Engineering Steel

In this paper, FH460 special steel with rare earth element cerium (Ce) was selected, and the control group without Ce was set up. By changing the content of Ce, the microstructure, phase transition point, and mechanical properties of the test steel were observed to study the effect of trace rare earth element Ce on the microstructure and mechanical properties of high-strength marine engineering steel. The morphology and energy spectrum of inclusions in three kinds of test steels were observed by SEM, and the morphological changes in inclusions in FH460 high-strength marine engineering steel after adding Ce were investigated. The fracture morphology and energy spectrum analysis were carried out by combining the tensile test at room temperature and the gradient low temperature impact toughness test, and the effect of trace Ce on the mechanical properties of the test steel was comprehensively analyzed. The results show that the addition of Ce changes the phase transformation temperature of Ac1 and Ac3, and refines the original microstructure of the test steel. SEM observation showed that the addition of Ce changed the long strip MnS and polygonal irregular Al2O3 inclusions into ellipsoids, which reduced the size of inclusions. The gradient low temperature impact test shows that with the decrease in temperature, the fracture dimple depth of the three test sheets of steel decreases, and the Ce-containing test steel forms a deep dimple centered on rare earth inclusions, which hinders the crack propagation and significantly improves the low temperature impact toughness of the test steel.

Dynamic fracture toughness and crack propagation mechanism of a heterogeneous heterostructured material under combined strengthening mechanisms

Developing fracture-resistant high-strength steels is an attractive prospect for various structural applications. In this work, a combination of carburizing heat treatment (CHT) and shot peening (SP) was used to develop combined strengthening (CS) mechanisms and to improve the mechanical strength and dynamic fracture toughness of 18CrNiMo7-6 alloy steel. Standard tensile tests and split Hopkinson pressure bar tests were conducted to investigate the strength and dynamic fracture toughness of the 18CrNiMo7-6 alloy steel. The crack initiation and propagation of samples were studied using scanning electron microscopy and transmission electron microscopy. Microstructure characterization and molecular dynamic simulations indicated that the excellent dynamic fracture toughness of the CS samples could be attributed to the grain refinement after strengthening and the formation of numerous slip bands at the crack tips, reducing the stress concentration at the crack tips. The Cr23C6 precipitates have a positive effect on the strength improvement of 18CrNiMo7-6 alloy steel. The results showed that this research can be used to guide the design of steels with high-strength and high-dynamic fracture toughness.

Direct quenching and tempering to achieve high strength and toughness of GPa-Grade nano-precipitated steel: The effect of precipitation behavior and variant selectivity

In order to satisfy the application requirements for strength and toughness of hydropower steels for large pumped storage power plants. In this study, based on the synergy of deformation and phase transition, the trade-off between strength and toughness was broken, and the GPa-grade nano-precipitated steel was prepared by direct quenching and tempering. The effects of Nb–V–Ti composite nano-precipitation behavior and martensite variant selectivity on the strengthening and toughening mechanisms were highlighted. The results show that directly quenched steel has the optimal comprehensive mechanical properties after tempering at 550 °C. Combining the contributions of various strengthening mechanisms, the increase in the yield strength relative to off-line quenching and tempering is mainly attributed to additional dislocation strengthening (∼33 MPa) and precipitation strengthening (∼174 MPa). Direct quenching enhances the homogeneous nucleation of finer mono- and binary precipitates and their stable growth during tempering, while the ternary precipitates do not suffer from elemental inhomogeneities due to lack of re-dissolution. Furthermore, the high-frequency variants of the dominant close-packed plane group tend to form high-angle grain boundaries with different orientations and planar alignments, derived from the refinement of Bain groups induced by the self-plasticity modulation of martensite transition due to the accumulation of deformed geometrically necessary dislocations and finite dislocation slip.

Effects of different forging ratios on microstructure, mechanical properties and friction and wear behaviour of Q355D steel

ABSTRACT To meet the mechanical performance demands of large oil cylinder barrels and piston rods, an investigation into the forging process of Q355D low-alloy high-strength steel was conducted. This study rigorously examined the impact of various forging ratios (2.6, 4.3, and 6.2) on both the microstructural characteristics and mechanical properties (including tensile strength, hardness, elongation, and impact toughness) of Q355D steel through a comprehensive systematic analysis. The findings revealed that an escalation in the forging ratio resulted in a refinement of the grain size in both ferrite and pearlite, concurrently with the fragmentation of MnS inclusions. The augmentation in both hardness and impact toughness correlated proportionally with the increased forging ratio, primarily attributed to the grain size refinement and augmented dislocation density. The yield strength exhibited a corresponding increase with the forging ratio increments: 303.56 MPa, 324.32 MPa, and 346.69 MPa, while hardness values were recorded as 174.32 HV, 187.31 HV, and 200.95 HV, respectively. Furthermore, grain refinement significantly contributed to enhancing the steel's ductility and toughness. Additionally, the escalated forging ratio notably reduced the duration required for the steel to reach a stabilized friction coefficient and reduced the stabilized friction coefficient, consequently amplifying the steel's resistance to frictional wear.

Effect of welding method and heat treatment on the fracture toughness of low alloy steel deposited metal

The fracture toughness of the weld deposited metal plays an important role in the safe service of the reactor pressure vessel (RPV). Shielded melting arc welding (SMAW) and submerged arc welding (SAW) are two common welding methods in the construction of RPV. In this paper, the effects of the welding method and post-weld heat treatment (PWHT) on the index T0 of low-temperature fracture toughness of deposited metal were studied respectively. Through the characterization and observation of microstructure and cleavage crack initiation, the effect mechanism of the welding method and PWHT on the low-temperature fracture toughness of deposited metal was determined. The results show that the position in which oxide near the hardening phase is easy to become the cleavage crack initiation of deposited metal. The PWHT dissolves the M-A island to improve the fracture toughness of the deposited metal. The SMAW deposited metal has a smaller oxide inclusion diameter and less cementite than SAW, resulting in higher fracture toughness.

Impact Toughness of High-Strength Low-Alloy Steels Joined by Friction STIR Welding

Impact Toughness of High-Strength Low-Alloy Steels Joined by Friction STIR Welding

Influence of secondary peak temperature on simulated ICCGHAZ microstructure and toughness of high-strength low-alloy steels

Abstract The ICCGHAZ specimens of laser-arc hybrid welding under various secondary peak temperatures were prepared using welding thermal simulation technology. Investigations were conducted into how the toughness and microstructure of the simulated ICCGHAZ specimens were correlated with the secondary peak temperature. Results showed that the size, distribution, and morphology of the M-A constituents in the ICCGHAZ specimens were primarily influenced by the secondary peak temperature. With this temperature at 760 °C, the blocky M-A constituents mostly aggregated into chains along the grain boundaries. As the secondary peak temperature reached 800 °C, while the blocky M-A constituents typically stayed close to the grain boundaries, the necklace M-A constituents began dispersing. The former austenite grain boundaries vanished as the M-A constituents migrated outward when the temperature further rose to 840 °C. At secondary peak temperatures of 760 °C and 800 °C, the volume proportion of the M-A constituents was 2.9% and 3.2%, respectively. In comparison, the volume fraction rapidly decreased to 0.9% at 840 °C. The size of the M-A constituents shrank as the secondary peak temperature rose, whereas the crack initiation energy Ei, crack propagation energy Ep, and impact energy Et increased. The toughness of the ICCGHAZ specimen was diminished because of the grain boundary necklace distribution characterization and the large size of the M-A constituents.

Impact toughness of LPBF 316L stainless steel below room temperature

The fracture behavior of LPBF 316L stainless steel in rapid loading conditions was characterized from room to cryogenic temperatures. Instrumented impact toughness tests allow to determine the influence of the microstructural features of this material on its crack initiation and propagation behavior. In the stress-relieved, homogenized/partially recovered and recrystallized states, LPBF 316L exhibit gradual impact toughness decreases between 20 °C and – 193 °C, with close toughness loss ranging from 0.38 to 0.55 J cm−2/°C. These values were similar to those reported by the literature for hot isostatically pressed (0.35–0.58 J cm−2/°C) and wrought (0.45–0.51 J cm−2/°C) 316L stainless steels, meaning that the microstructural features of LPBF 316L (segregations cells, dislocations cells, oxide particles, etc.) did not significantly influence the temperature sensitivity of its fracture properties. Whatever the testing temperature and microstructural state, fully ductile fracture occurred by decohesion at the oxide/matrix interfaces and void growth into dimples. The martensitic transformation during deformation at low temperature did not appears to play a significant role in the temperature sensitivity of the decrease in impact toughness of these materials. The latter is likely a consequence of the loss of ductility of the austenitic matrix, especially, under localized strain conditions.