Charpy V-notch Impact Toughness Research Articles

Microstructural Dependence of the Impact Toughness of TP316H Stainless Steel Exposed to Thermal Aging and Room-Temperature Electrolytic Hydrogenation.

This work deals with the effects of two individual isothermal aging experiments (450 °C/5000 h and 700 °C/2500 h) and the subsequent room-temperature electrolytic hydrogen charging of TP316H stainless steel on its Charpy V-notch (CVN) impact toughness and fracture behavior at room temperature. Microstructural analyses revealed that aging at 700 °C resulted in the abundant precipitation of intermediary phases, namely, the Cr23C6-based carbide phase and Fe2Mo-based Laves phase, whereas aging at 450 °C resulted in much less pronounced precipitation of mostly intergranular Cr23C6-based carbides. The matrix phase of 700 °C-aged material was completely formed of austenitic solid solution with a face-centered cubic (FCC) crystal structure, whereas an additional formation of ferritic phase with a base-centered cubic (BCC) structure was detected in 450 °C-aged material. The performed microstructure observations correlated well with the obtained values of CVN impact toughness, i.e., a sharp drop in the impact toughness was observed in the material aged at 700 °C, whereas negligible property changes were observed in the material aged at 450 °C. The initial, solution-annealed (precipitation-free) TP316H material exhibited a notable hydrogen toughening effect after hydrogen charging, which has been attributed to the hydrogen-enhanced twinning-induced plasticity (TWIP) deformation mechanism of the austenitic solid solution. In contrast, both aging expositions resulted in significantly lowered hydrogen embrittlement resistance, which was likely caused by hydrogen trapping effects at the precipitate/matrix interfaces in thermally aged materials, leading to a reduced TWIP effect in the austenitic phase.

Carbon distribution in lath martensite and quench embrittlement

Effect of water quenching and low-temperature tempering (LTT) at T≤280 °C on fracture mechanisms upon tension and during the Charpy V-notch (CVN) impact test of a 0.53%C-1.6%Si-0.9%Mn-0.76%Cr-0.14%V-0.05%Nb steel was studied. The steel exhibits quench embrittlement (QE). Tensile specimens fractured in elastic region after quenching due to easy crack initiation by intergranular fracture mechanisms. CVN impact toughness was ∼5 J/cm2. Low fracture toughness is attributed to superposition of intergranular fracture and transgranular quasi-cleavage fracture within packets during crack propagation; crack initiation occurs locally at the notch surface. QE is attributed to carbon segregations on high-angle boundaries if a sufficient portion of bulk carbon could not be consumed for the formation of Cottrell clouds on dislocations. LTT provides precipitation of transition carbides that leads to depletion of carbon from martensite and boundary segregations that decreases severity of QE due to suppression of intergranular fracture, increase of fracture brittle stress and contribution of dimple fracture to overall fracture process. The onset of fracture takes place after yielding in tension and fracture toughness increases by a factor of ∼2 after LTT.

General investigations on the heat treatment and thermal fatigue behavior of an experimental hot work tool steel tailored for laser powder bed fusion

An experimental hot work tool steel with a leaner chemistry compared to the standard AISI-H13 grade, aimed at enhanced processability, was gas atomized and processed using laser powder bed fusion (L-PBF) to >99.8 % relative density. The lower carbon content (∼0.25 wt% vs. ∼0.4 wt% in H13) resulted in a softer as-built (AB), and quenched martensite (∼485 HV1 vs. 625 HV1 in H13), higher martensite-start temperature (∼360 °C vs. 280–320 °C), and about ∼18 % reduction in dilatation, ascribed to the volume expansive martensitic transformation. Charpy V-notch impact toughness in AB condition (>30 J) was higher than those reported for AB H13. These features account for the improved L-PBF processability. The tempering response in comparison with a wrought H13, was characterized by a larger precipitation of thermally stable Mo- and V-rich secondary hardening alloy carbides at the expense of the easy-to-coarsen Cr-rich ones, attributed to the higher Mo and lower Cr content in experimental alloy. The combination of lower C- and V-contents in experimental alloy, reduced the driving force for the precipitation of coarse vanadium rich carbides during austenitization, as confirmed by electron diffraction spectroscopy (EDS) and synchrotron X ray diffraction. This led to a stronger fine secondary hardening alloy carbide precipitation during tempering, and enhanced temper resistance. As elaborated by isochronal tempering, as well as tempering curves, reduction of the elements Si, and Cr shifted the secondary hardness peak to higher tempering temperatures. Consequently, tempering, and thermomechanical softening resistance was significantly enhanced in the new steel with a maximum hardness of ∼550 HV1 in over-tempered condition (i.e., slightly above secondary hardness peak). Thermal fatigue (TF) tests revealed a denser and finer TF crack network with a larger oxidation in experimental steel. Thereby, the TF crack penetration depth scaling with the local plastic yielding of the matrix was evidently shorter in the new steel compared with wrought H13, thanks to significantly improved hot hardness, thermal conductivity, and enhanced thermomechanical softening resistance. Finally, a proof of concept is demonstrated by processing a relatively large injection molding die (150 mm × 110 mm × 30 mm) with promising tensile and impact toughness properties after direct double tempering to 51 HRC, and 45 HRC hardness levels.

Postweld Heat Treatment on Toughness of Electric- Resistance Welded X70 Steel

The effect of postweld heat treatment (PWHT) on Charpy V-notch impact toughness of a highfrequency electric-resistance welded (HF-ERW) grade X70 pipeline steel is investigated. PWHT thermal cycles are simulated using the Gleeble on the as-welded specimens. Microstructure, along with crystallographic texture and microhardness, is characterized in specimens that are impact tested at –5°C (23°F), –30°C (–22°F), and –45°C (–49°F). The impact toughness values show a wide scatter band and decrease with the increasing peak temperature of the PWHT. For higher peak temperatures, the microstructure of the heat-affected zone (HAZ) gradually changed from equiaxed ferrite to bainitic ferrite. Furthermore, the prior austenite grain size (PAGS) increases with the increasing peak temperature. The density of high-angle grain boundaries decreases, and the fraction of cleavage planes {100} parallel to the impact fracture plane increases for higher peak temperatures of the PWHT.

Comparative study about the results of HAZ physical simulations on different high-strength steel grades

With continuous improvements, structural steels are available in even higher strength grades above 1000 MPa yield strength. As the great majority of these steels are used in welded structures, their weldability needs to be taken into account. Several factors can cause difficulties during welding of these steels, but in this paper the softening behavior and the toughness characteristics of the heat-affected zone (HAZ) are examined. As the critical parts of the HAZ in a real welded joint are relatively small, their investigating ability is limited. However, the physical simulation provides a way of evaluating specimens made from a given material to produce the specified HAZ areas in a suitable size range for subsequent testing. In this research work, three different strength categories of high-strength structural steels (with yield strength of 960 MPa, 1100 MPa, and 1300 MPa) are investigated by physical simulation. In the case of different technological variants of gas metal arc welding (GMAW) process, the effect of the cooling time t8/5 is investigated in different HAZ subzones considered to be critical. The thermal cycles were determined according to the Rykalin 3D model. The investigated cooling times were t8/5 = 5 s, 15 s, and 30 s. The properties of the selected coarse-grained, intercritical and intercritically reheated coarse-grained zones are analyzed by laser scanning microscope, scanning electron microscope, hardness test, and instrumented Charpy V-notch impact toughness test. Furthermore, additional investigation like JMatPro calculations, electron backscatter diffraction measurements, and prior austenite grain size calculation were carried out. As a result of the tests, the investigated heat-affected subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Overall, the results of the tests show that the application of shorter t8/5 cooling time can be beneficial for the investigated high-strength steel grades, since significant toughness reduction and the risk of softening occur in the whole cooling time range.

Relationships between Strength, Ductility and Fracture Toughness in a 0.33C Steel after Quenching and Partitioning (Q&P) Treatment

The effect of quenching and partitioning (Q&P) processing on strength, ductility and fracture toughness is considered in a 0.33% C-1.8% Si-1.44 Mn-0.58% Cr steel. The steel was fully austenitized at 900 °C and quenched to 210 °C for 30 s. Partitioning at 350 °C for 600 s produces a martensitic matrix with transition carbides, bainitic ferrite and film-like retained austenite (RA) that is stable against transformation to strain-induced martensite under tension. This processing provided the highest strength and fracture toughness but the lowest ductility and product of strength and elongation (PSE), σB·δ (MPa·%). Partitioning at 500 °C produced RA with a relatively low carbon content and low volume fraction of carbides. The steel after this Q&P processing exhibits the highest ductility and PSE but low YS and Charpy V-notch (CVN) impact toughness. High ductility and PSE correlate with the ability of RA to transform into strain-induced martensite, while high strength and impact toughness are associated with the high-volume fraction of transition carbides in the carbon-depleted martensitic matrix and a lack of transformation of RA to strain-induced martensite. The highest CVN impact energy was attained in the steel exhibiting transgranular quasi-cleavage fracture with the lowest effective grain size for brittle fracture. No correlation between strength, ductility and fracture toughness is observed in Q&P steels if these materials have distinct structural constituents.

Effect of quenching and tempering on structure and mechanical properties of a low-alloy 0.25C steel

Relationships between the microstructure including the dispersed particles and the mechanical properties of a 0.25%C-1.6%Si-1.5Mn-0.5%Cr-0.3%Mo steel was examined after quenching and tempering at 200, 280, 400, and 500 °C. The lath martensite containing the dislocation and twin substructures provides a combination of 1350 MPa yield strength with 9% elongation-to-failure and Charpy V-notch impact toughness of about 60 J/cm2. The transition η-Fe2C carbide precipitates at tempering temperature of 200–400 °C, whereas cementite precipitates at tempering temperature of 500 °C. An increase in tempering temperature in the range of 200–400 °C leads to insignificant decrease in the strength and an increase in the impact toughness, while further increase in tempering temperature to 500 °C results in remarkable decrease in the yield and ultimate tensile strengths to 1090 MPa and 1180 MPa, respectively, and an increase in the impact toughness to 116 J/cm2. An attractive combination of the yield strength and the impact toughness is attributed to the large contribution of the solid solution strengthening to the overall yield strength and the high value of the fracture stress.

Effect of Heat Treatments on Microstructure and Mechanical Properties of Low-Cost Ti-6Al-4V Alloy Produced by Thermomechanical Powder Consolidation Route

This paper investigates the level of properties enhancement achievable by heat-treating Ti-6Al-4V alloy produced from a blended powder mixture using a thermomechanical powder consolidation route involving warm uniaxial pressing and vacuum sintering followed by extrusion at super transus temperature (1150 °C). The as-extruded material with a higher oxygen content of 0.55 wt.% was subjected to two different sub-transus annealing treatments: HT-A: 955 °C/1 h-furnace cooling and HT-B: 925 °C/4 h-cooling @ 50 °C/h to 760 °C-furnace cooling. Room temperature Charpy v-notch impact toughness tests and tensile tests were performed to ascertain the effect of microstructural changes during post-extrusion annealing treatments. After impact tests, analysis of microstructures and fracture surfaces of samples was carried out using optical and scanning electron microscopy. The as-extruded material displayed mean impact toughness of 4 J along with a yield strength of 956 MPa, an ultimate tensile strength of 1150 MPa, and an elongation to fracture of 2.4%. The annealing treatments gave a noticeable enhancement in the impact toughness (average values 5–6 J obtained) while maintaining a yield strength and ultimate tensile strength level of about 992 MPa and 1164–1181 MPa, respectively. Additionally, the level of change in ductility was limited for each sub-transus annealing treatment, and HT-A has given only a 30% increase compared to as-extruded material.

A single-phase Ti3Zr1.5NbVAl0.25 refractory high entropy alloy with excellent combination of strength and toughness

A Ti3Zr1.5NbVAl0.25 refractory high entropy alloy (RHEA) was prepared by vacuum melting and subsequently subjected to hot forging and annealing at elevated temperatures. The as-prepared alloy has a single body-centered cubic (BCC) phase structure with a balanced matching of strength and toughness. It is shown that tensile yield strength and fracture elongation of this alloy are 899 MPa and 24.50%, respectively. In addition, the Charpy V-notch impact toughness of the Ti3Zr1.5NbVAl0.25 alloy was measured to be 119. 5 J/cm2, which is better than most traditional alloys and HEAs. The superior quasi-static and dynamic mechanical properties synergy makes this RHEA attractive for advanced structural applications.

Metallurgical Design Rules for High-Strength Steel Weld Metals

A review of a dilatometric analysis of selected Fe-C-Mn high-strength steel shielded metal arc weld metals showed that balanced Ti, B, Al, O, and N additions reduced the austenite-to-ferrite transformation-start (TS) temperature. These microalloy additions must match the following aim levels for composition control: Ti at 400 ppm (0.04 wt-%), B at 40 ppm (0.004 wt-%), Al at 200 ppm (0.020 wt-%), O at 400 ppm (0.04 wt-%), and N preferably below 80 ppm (0.008 wt-%) to ensure effective deoxidation, form complex inclusions, and distribute them to enable development of highly fracture-resistant refined weld metal microstructures. It may be wiser to avoid the rich and lean ends for these microalloy additions, except N, which should be held at the lean end, preferably much below 80 ppm (0.008 wt-%). The balanced Ti, B, Al, O, and N additions offered nearly a 100°C shift in lowering the Charpy V-notch (CVN) test temperature for either 28 or 100 J absorbed energy. Dilatometric evaluations of reheated weld metals showed that 1) the balanced Ti, B, Al, O, and N additions lowered the actual TS temperature by about 60°C compared to the calculated austenite-to-ferrite transformation temperature obtained from the constitutional equation; 2) N with more than 100 ppm (0.010 wt-%) effectively nullified the beneficial effects of Ti, B, and Al additions in lowering the transformation temperature; and 3) at N content much below 80 ppm (0.008 wt-%), both a lower TS temperature and a narrow start-to-finish (TS–Tf) temperature range helped in achieving exceptional weld metal CVN impact toughness.

Effect of weld microstructure on brittle fracture initiation in the thermally-aged boiling water reactor pressure vessel head weld metal

Effects of the weld microstructure and inclusions on brittle fracture initiation are investigated in a thermally aged ferritic high-nickel weld of a reactor pressure vessel head from a decommissioned nuclear power plant. As-welded and reheated regions mainly consist of acicular and polygonal ferrite, respectively. Fractographic examination of Charpy V-notch impact toughness specimens reveals large inclusions (0.5–2.5 µm) at the brittle fracture primary initiation sites. High impact energies were measured for the specimens in which brittle fracture was initiated from a small inclusion or an inclusion away from the V-notch. The density, geometry, and chemical composition of the primary initiation inclusions were investigated. A brittle fracture crack initiates as a microcrack either within the multiphase oxide inclusions or from the de-bonded interfaces between the uncracked inclusions and weld metal matrix. Primary fracture sites can be determined in all the specimens tested in the lower part of the transition curve at and below the 41-J reference impact toughness energy but not above the mentioned value because of the changes in the fracture mechanism and resulting changes in the fracture appearance.

Assessment the Partial Welding Influences on Fatigue Life of S700MC Steel Fillet Welds

Fine-grained steels belonging to the HSLA group (High-Strength Low-Alloy steels) of steels are becoming increasingly popular and are used in both statically and dynamically stressed structures. Due to the method of their production, and thus also the method use to obtain the required mechanical properties, it is really necessary to limit the heat input values for these steels during welding. When applying temperature cycles, HSLA steels in highly heated heat-affected zones (HAZ) reveal intensive grain coarsening and also softening behaviour. This subsequently results in changes in both mechanical and brittle-fracture properties, and the fatigue life of welded joints. While grain coarsening and structure softening have a major effect on the change of strength properties and KCV (Charpy V-notch impact toughness) values of statically stressed welded joints, the effect of these changes on the fatigue life of cyclically stressed welded joints has not yet been quantified. The paper is therefore conceived so as to make it possible to assess and determine the percentage impact of individual aspects of the welding process on changes in their fatigue life. To be more specific, the partial effects of angular deformation, changes that occur in the HAZ of weld, and the notch effect due to weld geometry are assessed.

Mitigating Scatter in Mechanical Properties in AISI 410 Fabricated via Arc-Based Additive Manufacturing Process.

Wire-based metal additive manufacturing utilizes the ability of additive manufacturing to fabricate complex geometries with high deposition rates (above 7 kg/h), thus finding applications in the fabrication of large-scale components, such as stamping dies. Traditionally, the workhorse materials for stamping dies have been martensitic steels. However, the complex thermal gyrations induced during additive manufacturing can cause the evolution of an inhomogeneous microstructure, which leads to a significant scatter in the mechanical properties, especially the toughness. Therefore, to understand these phenomena, arc-based additive AISI 410 samples were fabricated using robotic gas metal arc welding (GMAW) and were subjected to a detailed characterization campaign. The results show significant scatter in the tensile properties as well as Charpy V-notch impact toughness data, which was then correlated to the microstructural heterogeneity and delta (δ) ferrite formation. Post-processing (austenitizing and tempering) treatments were developed and an ~70% reduction in the scatter of tensile data and a four-times improvement in the toughness were obtained. The changes in mechanical properties were rationalized based on the microstructure evolution during additive manufacturing. Based on these, an outline to tailor the composition of “printable” steels for tooling with isotropic and uniform mechanical properties is presented and discussed.

High impact toughness of CrCoNi medium-entropy alloy at liquid-helium temperature

We show that face-centered-cubic CrCoNi medium-entropy alloy sustains a Charpy V-notch impact toughness (AK) as high as 340 J at 4 K. Deformation twinning (DT) is rendered exceptionally profuse by this alloy's low stacking fault energy, combined with liquid-helium temperature and high strain rates that both incur high stresses to favor DT across a large plastic zone. This efficiently dissipates imposed mechanical energy, accumulates defects to sustain high strain hardening, and restrains strain localization events from evolving into major shear bands. Martensitic transformation to hexagonal phase is insignificant. CrCoNi is therefore a strong competitor to the best existing cryogenic alloys.

Microstructure and properties of thermomechanically treated and bake hardened AISI 4340 steel

Abstract In the present work, response of AISI 4340 steel towards high temperature thermomechanical treatment (HTMT) and bake hardening (BH) studied. During HTMT, steel was austenitized to 1100°C and plastically deformed up to 30% with the help of rolling and immediately quenched in water, followed by tempering at 310±5°C for 2 hours. Optical microstructure, toughness and tensile strength, hardness were evaluated and compared with conventionally hardened and tempered (CHT) steel samples. Observations of HTMT process clarified no alterations in hardness, but a significant improvement in UTS in comparison to CHT samples. Charpy V-Notch (CVN) impact toughness drastically decreased in HTMT samples. Impact toughness of HTMT samples was 13 Joules, but 199 Joules for CHT samples. In HTMT samples, effect of deformation source on process investigated and found that rolling is an effective in improving properties than hammer forging. In addition, significant isotropic properties developed in the case of rolling, compared to hammer forging. Effect of amount of deformation studied; observed 30% deformation was the optimum deformation to achieve the best properties. During bake hardening (BH) flat tensile AISI 4340 steel specimens were pre-strained to 2% and baked at 170°C for 20 minutes a headed with uniaxial tensile testing in which there was no improvement in yield strength after bake hardening observed. This might be because of the low amount of pre-straining or may be the lack of availability of interstitial atoms near to the dislocations generated during pre-straining. Bake hardened specimens were tested at different strain rates; it was observed that yield strength decreases with increasing strain rate, while UTS remains unaffected. FESEM images showed no significant structural changes.

Microstructural Evolution and Wear Behavior Involving the K-Carbides Precipitation of a Fe-25Mn-6.6Al-1.3C Austenitic Steel

In this paper, the microstructural evolution and mechanical properties of the as-cast Fe-25Mn-7Al-1.3C austenitic steel after different heat treatment were investigated. After solution treatment and subsequent aging treatment, the κ-carbides with perovskite structure were found to precipitate coherently within the austenite matrix, which improved the initial hardness and mechanical strength. The experimental steel exhibited an optimal comprehensive performance after being solution treated at 1050 °C for 1 h and then aged at 550 oC for 2 h. The tensile strength was 751 MPa, the yield strength was 581 MPa, the elongation was 48%, the hardness was 252 HB, and the Charpy V-notch impact toughness was 168 J, respectively. The impact wear test was carried out on MLD-10 abrasive wear testing machine, and the worn out surfaces under different heat treatment were characterized by scanning electron microscopy (SEM). The results indicated that the abrasion resistance of the steel under the additional aging treatment was better than that of the as-solutionized steel. The optimal abrasion resistance was obtained after being soluted at 1050 °C for 1 h and then aged at 550 oC for 2 h. However, with the aging time increasing, the coarse κ-carbides precipitating around the grain boundaries would deteriorate toughness, which lead to increase of the abrasive wear volume loss. Besides, obvious micro-cracking and relatively larger peeling pit were observed.

Ageing Tests of Alloy 617 to Simulate Service Embrittlement

Abstract Alloy 617 is a solid solution-strengthened, wrought, nickel base superalloy. It is extensively used in the chemical industries, whenever superior corrosion resistance is required, and in piping applications of the most highly advanced thermal power plants, where creep resistance is important for piping and steam turbine disk applications, and in hot going casings of gas turbine engines, both heavy-duty and industrial scale. It is known from numerous studies that the relatively high chromium and carbon contents of this alloy gives rise to chromium carbide precipitation at well determined temperature ranges. This applies to both the precipitation of primary carbides on solidification, which may be rich in both chromium and molybdenum and are mostly of the MC and M6C types, and the precipitation of secondary chromium carbides of the Cr23C6 type, which are precipitated preferably at grain boundaries, twin boundaries, and slip planes. This precipitation might embrittle the alloy, which could manifest itself in a marked drop of ductility and toughness. In long-term high-temperature applications, this may lead to a failure mechanism called stress relaxation cracking, which may occur when high residual or service stresses cannot be relieved by plastic deformation, because of simultaneous precipitation of secondary carbide precipitates. The onset of this embrittling mechanism is reported to begin as early as after only 500 service hours, if affected components are operated within the carbide precipitation range of approximately 750 – 875 °C. In a more recent development, there were concerns in the fabrication department of the authors' organization that tooling, used for heat treatment of such components, could be affected not only by embrittlement, but also by a drop in strength levels, this after long-time service in industrial furnaces at heat treatment temperatures in the order of 850 °C. Because this is somewhat counter-intuitive, a long term study was launched to test the sensitivity of this alloy for a decrease in strength after service. This study was performed for ageing times between 500 and 3000 h, at the relevant temperature of 850 °C. The findings suggest, as expected, that the onset of embrittlement by secondary carbide precipitates occurs as early as after 500 h in service, as also reported in the literature. This is evidenced by a marked drop in both tensile ductility and Charpy V-notch (CVN) impact toughness, and can also be seen microfractographically by an increase of the fraction of intergranular cleavage fracture. Also as expected, a drop in strength could not be determined. On the contrary, the marked drop in ductility and toughness goes along with a marked increase in ultimate tensile strength (UTS) and yield strength (YS).

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Fe–Mn–Al–C lightweight steels have great potential to replace Cr–Mo quenched and tempered steels where weight reduction is an important design variable. Previous research has centered around a fully austenitic cast composition of Fe–30Mn–9Al–0.9C–1Si–0.5Mo that achieves high strength through homogenous precipitation of κ-carbide during age hardening. However, the effect of varying Mo content and the corresponding effects on age hardening kinetics and resulting mechanical properties are not well documented. The current study investigates Mo additions in the range of 0–0.7% Mo on the microstructure, age hardening response, and toughness of a Fe–30Mn–9Al–0.8C–1Si steel. Molybdenum was shown to decrease the age hardening response at 530 °C. Higher concentrations of Mo resulted in an increase in the Charpy V-notch impact toughness of aged specimens. Optical microscopy revealed that alloys with Mo additions had much less κ-carbide along grain boundaries in the aged condition and this decreased the propensity of intergranular fracture and increased toughness.

Toughness and ductility improvement of heavy EH47 plate with grain refinement through inter-pass cooling

In this study, a novel inter-pass cooling technology was applied during rough rolling (R-IPC) to process a 60 mm ultra-heavy EH47 steel plate with excellent toughness and ductility. The plate was compared to a plate processed through conventional thermo-mechanical controlled processing (TMCP). The microstructure evolution at 1/4-thickness and 1/2-thickness of the plates was studied, while the relationship between microstructure and mechanical properties, through tensile and Charpy V-notch impact toughness, was established. In this study, it was reported that the R-IPC steel deformation penetration was higher compared to the TMCP steel. The ferrite fractions of the R-IPC and TMCP steels were 47% and 10% at 1/4-thickness, as well as 35% and 0.5% at 1/2-thickness, while the acicular ferrite fractions at 1/2-thickness of the R-IPC and TMCP steels were 50% and 9.5%, respectively. The granular bainite width at 1/2-thickness of the R-IPC steel (14 ± 4 µm) was significantly lower than the TMCP steel (27 ± 10 µm). The M/A island volume fraction in R-IPC steel was higher than the TMCP steel at 1/4-thickness, whereas it was reduced dramatically at 1/2-thickness. The high intensity of the desired texture {112}<110> was obtained at 1/2-thickness of the R-IPC steel, while the undesired {411}<148> was obtained at 1/2-thickness of the TMCP steel, which was responsible for the R-IPC steel superior toughness compared to the TMCP steel. The tensile and yield strengths of the two rolled plates were similar, while the elongation at 1/4-thickness of the R-IPC steel was higher compared to the TMCP steel. The low temperature impact energy of the R-IPC steel was significantly higher compared to the TMCP steel, resulting from finer grain size, higher ferrite fraction and higher area of the {110} texture parallel to the impact direction.

Effect of Aging Isothermal Time on the Microstructure and Room-Temperature Impact Toughness of Fe–24.8Mn–7.3Al–1.2C Austenitic Steel with κ-Carbides Precipitation

The microstructure and impact toughness of the as-cast Fe–24.8Mn–7.3Al–1.2C austenitic steel after solution treatment and subsequent aging treatment were investigated in the present work. Research on the κ-carbides precipitation behavior was carried out by transmission electron microscope. The results show that nano-sized coherent κ-carbides were obtained in the as-solutionized steel after aging treatment, which produced precipitation hardening. After being aging treated at 550 °C for 1 h, the steel with regular hexagonal grain structure exhibited a good combination of yield strength (~ 574 MPa) and room-temperature impact toughness (~ 168 J). In the present steel, the typical cube-on-cube orientation relationship between austenite and κ-carbides was observed. However, due to the long aging isothermal time and high C content, the coarse intergranular κ′-carbide was formed and grew along the austenite grain boundary, which caused this orientation relationship to be destroyed and a dramatical increase of the coherency strain energy at grain boundary. Furthermore, serious embrittlement of grain boundaries caused that cleavage cracks trend to propagate along the grain boundaries. Accordingly, the room-temperature impact toughness decreased sharply. After aging isothermal time prolonging to 13 h, the Charpy V-notch impact toughness was only ~ 5 J and fracture mode turned to fully brittle fracture accompanied with flat facets, shear cracks and well-developed secondary crack.