Effects of Isothermal Aging on Microstructure and Mechanical Property of Low-Carbon RAFM Steel

In order to investigate the microstructure and mechanical property evolution of low-carbon reduced activation ferritic/martensitic(RAFM) steel during isothermal aging, the normalized and tempered specimens were aged at 600 °C for 500,1000, and 3000 h, respectively. The microstructural evolution with aging time was analyzed, including the precipitation and growth of M_(23)C_6 and MX-type carbides as well as the formation of Laves phase. The results indicate that the coarsening of M_(23)C_6 is more obvious than that of MX with increase in aging time. During the long-term thermal exposure, the Fe_2 W Laves phase precipitates adjacent to M_(23)C_6 along the prior austenite grain boundaries and packet boundaries. Lower carbon content can delay the precipitation of Laves phase compared to the steel containing higher carbon. In addition, the Laves phase precipitated along boundaries can provide the precipitation strengthening, slightly increasing the tensile strength of low-carbon RAFM steel after aging for 3000 h.     

Effect of Aging Heat Treatment on the Microstructure and Creep Properties of the Cast Ni-Based Superalloy at Low Temperature

Effect of aging heat treatment on the grain boundary microstructure and creep properties of a cast Ni-based superalloy was investigated. With increasing aging temperature from 750 to 1000 ℃, M_(23)C_6 carbides along the grain boundaries evolve from fine distributed block, continuous film into the coarse discrete block. Moreover, the M_(23)C_6 carbides are mainly enveloped within γ’ layers along grain boundaries during 1000 ℃ aging. Creep rupture lifetime and elongation at 760 ℃ and 645 MPa are improved with increasing the aging temperature. In particular, the creep rupture lifetime of the specimens aging at 1000 ℃ is one order of magnitude higher than that of the specimens aging at 750 ℃. The enhancement of ductility induced by the γ’ envelopes plays a significant role in the improvement of creep rupture lifetime.     

Precipitation Behavior and Mechanical Properties of a 16Cr-25Ni Superaustenitic Stainless Steel Weld Metal During Post-weld Heat Treatment

A 16Cr-25Ni superaustenitic stainless steel weld metal for austenitic stainless steel/ferrite heat-resistance steel dissimilar metal weld was designed and prepared through tungsten inert-gas welding. The precipitate evolution and its correlation with mechanical properties were investigated during post-weld heat treatment (PWHT) at 690 °C for up to 12 h. The primary precipitates in the as-welded weld metal were identified as Mo-rich M₆C carbides in the interdendritic region and semicontinuous fine-sized M₂₃C₆ carbides along grain boundary. After PWHT, three types of precipitates coexisted in the interdendritic region: primary M₆C carbides, newly precipitated Mo-rich M₂X carbonitrides and some of the secondary M₂₃C₆ carbides. Additionally, mass secondary M₂₃C₆ carbides formed and coarsened along grain boundary. No undesirable intermetallic phases formed during the whole period. The M₂X and interdendritic M₂₃C₆ improved the strength of the weld metal after PWHT, but the elongation and impact toughness degraded, which were mainly owing to the intergranular M₂₃C₆ carbides that changed the fracture mode from ductile transgranular mode to mixed mode of transgranular and intergranular fracture. Meanwhile, the coarsening of M₂X carbonitrides may lead to the elongation loss during 8 h to 12 h. Evolution of impact toughness was also related to the M₂X carbonitrides, which made the crack easier to propagate compared with austenitic matrix and contributed to the decline of impact toughness. However, due to the sluggish precipitation of M₂X carbonitrides with longer holding time, the decreasing trend became slow from 4 to 12 h. The results showed that PWHT should be controlled within 8 h to obtain better combination of strength and ductility.     

Effect of Co on Microstructure and Stress Rupture Properties of K4750 Alloy

The effects of substituting Co for Fe on the microstructure and stress rupture properties of K4750 alloy were studied.The microstructure of the alloy without Co(K4750 alloy) and the alloy containing Co(K4750-Co alloy) were analyzed.Substitution of Co for Fe inhibited the decomposition of MC carbide and the precipitation of η phase during long-term aging treatment.In K4750-Co alloy,the morphology of MC carbide at the grain boundary(GB) remained dispersed blocky shape and no η phase was observed after aging at 750℃for 3000 h.However,in K4750 alloy,almost all the MC carbides at GBs broke down into granular M_(23)C_6 carbide and needle-like η phase.The addition of cobalt could delay the decomposition of MC carbides,which accordingly restricted the elemental supply for the formation of η phase.The stress rupture tests were conducted on two alloys at 750℃/430 MPa.When Co is substituted for Fe in K4750 alloy,the stress rupture life increased from 164.10 to 264.67 h after standard heat treatment.This was mainly attributed to increased concentration of Al,Ti and Nb in γ' phase in K4750-Co alloy,which further enhanced the strengthening effect of γ' phase.After aging at 750℃for 3000 h,substitution of Co for Fe can also cause the stress rupture life at 750℃/430 MPa to increase from 48.72 to 208.18 h.The reason was mainly because MC carbide degradation and η phase precipitation in K4750 alloy,which promoted the initiation and propagation of micro-crack during stress rupture testing.     

Hot Deformation Behavior and Hardness of a CoCrFeMnNi High-Entropy Alloy with High Content of Carbon

A CoCrFeMnNi high-entropy alloy with a high content of carbon was synthesized, and its hot deformation behavior was studied at the temperatures 800–1000 ℃ at the strain rates ranging from 0.001 to 0.1 s~(-1).As-prepared alloy is a face-centered cubic-structured solid solution, with a large amount of carbides residing at grain boundaries.True stress–strain curves were employed to develop the constitutive equation of apparent activation energy.The apparent activation energy( Q) was found to be 423 kJ mol~(-1), indicating a dynamic flow softening behavior.The size of dynamic recrystallized(DRXed) grains increases with increasing the temperature or decreasing the strain rate.A processing map was sketched on the basis of the flow stress.The temperature range of 900–1000 ℃ and 10~(-3)–10~(-2.6) s~(-1) strain rate were found to be the optimum hot-forging parameter.With increasing temperature or decreasing strain rate, the volume fraction of fine carbides(≤ 1 μm) increases.A lot of coarse carbides can be found in the matrix after deformation at 800 ℃, which leads to a high hardness value of 345 HV.The carbides after deformation at 1000 ℃ are mainly nano-sized M_7C_3 and M_(23)C_6, which can promote the nucleation of DRX.     

Effects of Phosphorus and Iron on Microstructures and Mechanical Properties in NiCrFe-Based Alloys

The microstructures and mechanical properties, especially creep properties, of the NiCrFe-based alloys with various contents of phosphorus and iron were investigated. The results showed that the tensile yield strength decreased with increasing iron contents while had no obvious change with the addition of phosphorus. For creep properties, the alloy with 15.8 wt% iron and 0.09 wt% phosphorus possessed the longest creep life (679 h) among all alloys. Only M₂₃C₆was formed in the alloys with low phosphorus contents, while both intergranular M₃P and M₂₃C₆ precipitated with the increment of phosphorus, which enhanced the strength of grain boundary by hindering the movements of dislocations during creep tests. The reasons for the enhancement of creep life were mainly related to the solid solution strengthening effect of phosphorus and optimization of grain boundary precipitations by phosphorus.     

Precipitate Phases in an 11% Cr Ferritic/Martensitic Steel with Tempering and Creep Conditions

Precipitates in an 11% Cr ferritic/martensitic steel containing Nd with tempering and creep conditions were investigated using transmission electron microscope with energy-dispersive X-ray spectroscopy. The precipitates in the steel with a tempering condition were identified to be Cr-rich M23C6 carbide, Nb-rich/V-rich/Ta–Nb-rich MX carbides, Nbrich MX carbonitride, and Fe-rich M5C2 carbide. Nd-rich carbonitride, which is not known to have been reported previously in steels, was also detected in the steel after tempering. Most of the Nb-rich MX precipitates were dissolved, whereas the amount of Ta-rich MX precipitates was increased significantly in the steel after a creep test at 600 °C at an applied stress of180 MPa for 1,100 h. No Fe2 W Laves phase has been detected in the steel after tempering.(Fe, Cr)2W Laves phase with a relatively large size was observed in the steel after the creep test.     

Effect of Long-Term Thermal Exposures on Tensile Behaviors of K416B Nickel-Based Superalloy

The effect of long-term thermal exposure on the tensile behavior of a high W content nickel-based superalloy K416B was investigated. The microstructure and the deformation characteristics were observed by scanning electron microscopy and transmission electron microscopy, and the phase transformation of the alloy during long-term thermal exposure was analyzed by X-ray diffraction patterns and differential thermal analysis. Results showed that after thermal exposure at 1000 °C, the MC carbides in the K416B alloy decomposed into M₆C. During tensile deformation, dislocations slipping in γ matrix crossed over the M₆C by Orowan bowing mechanism. With the increase of thermal exposure time, the secondary M₆C reduced greatly the yield strength of the alloy at room temperature. Meanwhile, the continuous distribution of the secondary M₆C with great brittleness in the grain boundary could become the main source of crack, which might change the fracture characteristic of the alloy from trans-granular to intergranular.     

Effect of Indirect Transformation of Retained Austenite During Tempering on the Charpy Impact Toughness of a Low-Alloy Cr-Mo-V Steel

A modified tempering treatment has been designed in order to avoid the direct transformation of retained austenite (Ar) during tempering of a low-alloy Cr-Mo-V steel. Instead of the direct transformation of Ar into ferrite and M₂₃C₆ carbides during conventional tempering at 700 °C, transformation into aggregate of ferrite and cementite has been forced by a pre-tempering at 455 °C before conventional tempering. Experiments have been performed on specimens quenched with cooling rates 1.5, 3 and 12 °C/s, providing different types of Ar within the as-quenched microstructures. The results show that the tempering modification does not improve the Charpy impact toughness at the highest quenching rate of 12 °C/s, where the specimens cannot incur cleavage cracking induced from fine and discontinuous M₂₃C₆ carbides along lath interfaces. For the lowest quenching rate 1.5 °C/s, the Charpy impact toughness can be improved, and the failure is dominated by carbide aggregates, which originate from the decomposed products of blocky Ar. This is because the tempering modification effectively suppresses the formation of coarse M₂₃C₆ carbides at interfaces between the carbide aggregate and bainitic matrix, thereby resulting in a relatively homogeneous distribution of M₂₃C₆ carbides inside carbide aggregates. Therefore, the tempering modification is recommended for large-scale forgings, in which relatively high quenching rates are difficult to achieve.     

Phase Evolution and Thermal Expansion Behavior of a γ′ Precipitated Ni-Based Superalloy by Synchrotron X-Ray Diffraction

The phase evolution and thermal expansion behavior in superalloy during heating play an essential role in controlling the size and distribution of precipitates, as well as optimizing thermomechanical properties. Synchrotron X-ray diffraction is able to go through the interior of sample and can be carried out with in situ environment, and thus, it can obtain more statistics information in real time comparing with traditional methods, such as electron and optical microscopies. In this study, in situ heating synchrotron X-ray diffraction was carried out to study the phase evolution in a typical γ′ phase precipitation strengthened Ni-based superalloy, Waspaloy, from 29 to 1050 °C. The γ′, γ, M₂₃C₆ and MC phases, including their lattice parameters, misfits, dissolution behavior and thermal expansion coefficients, were mainly investigated. The γ′ phase and M₂₃C₆ carbides appeared obvious dissolution during heating and re-precipitated when the temperature dropped to room temperature. Combining with the microscopy results, we can indicate that the dissolution of M₂₃C₆ leads to the growth of grain and γ′ phase cannot be completely dissolved for the short holding time above the solution temperature. Besides, the coefficients of thermal expansions of all the phases are calculated and fitted as polynomials.     

Effect of Intergranular Precipitation on the Internal Oxidation Behavior of Cr–Mn–N Austenitic Stainless Steels

The effect of intergranular precipitation on the internal oxidation behavior of Cr–Mn–N austenitic steels at 1000 °C in dry air atmosphere was investigated using scanning electron microscope, transmission electron microscope, and X-ray diffraction analysis. The results show that intergranular M23C6 carbide morphologies play an important role on the internal oxidation behavior of Cr–Mn–N steels. During the period of the oxidation, both discontinuous chain-shaped and continuous film-shaped intergranular M23C6 carbides precipitated along the grain boundaries. Internal oxides of silica preferentially intruded into the matrix along grain boundaries with discontinuous M23C6 carbide particles, while silica was obviously restricted at the interfaces between the external scale and matrix on the occasion of continuous film-shaped M23C6 carbides. It is seemed that reasonable microstructure could improve the oxidation resistance of Cr–Mn–N steels.     

GH3230合金M23C6型碳化物的析出行为

研究了固溶态GH3230合金在800~1100 ℃时效不同时间下的碳化物析出行为。结果表明:GH3230合金固溶态组织主要为γ相+初生粒状碳化物M₆C+少量晶界粒状碳化物M₂₃C₆。试验合金在800~1100 ℃短时时效后,晶界和晶内析出的碳化物主要为M₂₃C₆型。其中晶界粒状M₂₃C₆型碳化物有沿着晶内长大的倾向,并逐渐变成胞状碳化物。在同一时效温度下,晶内碳化物析出数量会随着时效时间的增加而增加,此后会逐渐回溶,回溶开始的时间会随着时效温度的提高而逐渐提前。     

Influence of Fe on Microstructure and Mechanical Properties in Pdoped Ni–Cr–Fe Alloys

The influence of Fe on the microstructure and mechanical properties of P-doped Ni–Cr–Fe alloys has been investigated.Results showed that increasing Fe content refined the dendrite microstructure and enhanced the solubility of P in as-cast alloys. The change of microhardness in different dendrite regions was attributed to the segregation of P atoms in solid solution state, which had strengthening effects. Increasing Fe contents from 15.2 to 60.7 wt% reduced the yield strength and tensile strength but had little influence on the elongation of alloys. The stress rupture life of alloys after heat treatment decreased with the increment of Fe contents, and the failure fracture modes transferred from transgranular to intergranular fracture mode. The change of fracture modes was due to the weakness of grain boundaries caused by the increment of Fe.In addition, the precipitation of M_(23)C_6 was believed to be related to the segregation of P toward grain boundaries, which led to the fluctuation of carbon and chromium atoms near the grain boundaries in alloys with low Fe contents. Consequently, the increment of Fe decreased the strength of matrix and changed the existence of P atoms and the precipitates at grain boundaries.     

Evolution of Microstructures and Mechanical Properties of Zr-Containing Y-CLAM During Thermal Aging

The thermal stability and mechanical properties of China low activation martensitic steel with Zr and Y were investigated via thermal aging at 550 °C for 8000 h. The Laves phase content monotonically increased with thermal aging, and the volume fraction of the Laves phases stabilized in the alloy after 3000 h of thermal aging. The observed degradation in mechanical properties was because of the coarsening of M_(23)C_6 carbides and matrix grains during the earlier stages of thermal aging. The precipitation of Laves phases and V_3Zr_3C particles increased the strength and hardness of the alloy. Grain coarsening was the primary reason for the decrease in impact properties, and the ductile-to-brittle transition temperature increased from-71 to -48 °C after 8000 h of thermal aging.     

Investigation of the effect of the types and densities of grain boundary carbides on grain boundary cavitation resistance of AISI 321 stainless steel under creep–fatigue interaction

The effect of MX (where X=C, N) and M₂₃C₆ and their densities at the grain boundaries on creep–fatigue behavior of AISI 321 stainless steel are investigated. The creep–fatigue lives of fine MX and coarse MX aged alloys were longer than those of M₂₃C₆ aged alloy under the same test conditions. In order to better understand the difference in the creep–fatigue lives between the tested alloys, microstructural observations are conducted by scanning electron microscope (SEM) and transmission electron microscope (TEM). The differences in the creep–fatigue lives of the alloys are due to the stronger cavitation resistance of MX carbides compared with that of M₂₃C₆ carbides. From the microstructural observations, it is verified that formation and growth of grain boundary cavities in MX carbides are more retarded than in M₂₃C₆ carbides. Therefore, it is suggested that the types of carbides are a more prominent factor than the density of carbides for grain boundary cavitation in austenitic stainless steels.     

Phase Transformation and Carbide Precipitation of Functional Gradient Semi-solid 9Cr18 Steel

The unique phase transformation and carbide evolution in 9Cr18 steel were investigated during semi-solid forming and subsequent heat treatment. The functional gradient thixoforging 9Cr18 component was divided into inner area and edge area. Microstructure evolution was different at each area. After semi-solid cooling, the solid particles in the inner area were retained as meta-austenite. During annealing, M_(23)C_6 carbide began to precipitate when temperature reached 700 °C.Martensite transformation occurred when temperature reached 800 °C. The occurrence of M_(23)C_6 carbide and martensite structure would be harmful to the mechanical properties of inner area. In the edge area, the liquid underwent eutectic transformation to form bar-shape M_7C_3 carbide and secondary austenite after semi-solid cooling. The width of bar-shape carbide would decrease during annealing. By controlling the carbide evolution, we could tailor the functional gradient material with required property.     

Effect of Post-weld Tempering on the Microstructure and Mechanical Properties in the Simulated HAZs of a High-Strength-High-Toughness Combination Marine Engineering Steel

The effects of tempering temperatures on the microstructure and mechanical properties of the simulated coarse-grain heataffected zone(CGHAZ) and inter-critical heat-affected zone(ICHAZ) were investigated for a high-strength-high-toughness combination marine engineering steel.The results demonstrate that the microstructure of the simulated CGHAZ and ICHAZ after tempering is characterized by tempering sorbites and coarse grain in the simulated CGHAZ.As tempering temperature increases,the tensile strength of the simulated CGHAZ and ICHAZ decreases and the Charpy absorbed energy of the simulated ICHAZ at-50℃increases remarkably,but the impact toughness of the simulated CGHAZ is not improved.After tempering at 550℃,the coarse flake carbides,which distribute at the prior austenite grain and martensite lath boundaries,deteriorate the impact toughness of the simulated CGHAZ.With the increase in tempering temperature,the morphology and the size of the carbides gradually change from coarse flake to fine granular,which is beneficial to the improvement of impact toughness.However,the coarse-grain size of the simulated CGHAZ and the M23 C6-type carbide precipitated along the grain boundaries weakens the enhancing effect of carbides on impact toughness.     

Effect of Short-Time Aging on the Pitting Corrosion Behavior of a Novel Lean Duplex Stainless Steel 2002

The effect of short-time aging in the temperature range between 400 and 1000 °C on the pitting corrosion behavior and mechanical property of a novel lean duplex stainless steel(LDSS) 2002 was investigated through the potentiostatic critical pitting temperature(CPT) tests and the Charpy impact tests. Both the pitting corrosion resistance and the toughness of aged specimens degraded due to the precipitation of detrimental secondary phases and the most significant reduction of CPT and impact energy emerged at 650 °C concurrently. The CPT of LDSS 2002 specimen aged at 650 °C decreased by 28 °C, and the impact energy dropped from 69 to 29 J/cm~2 compared with the solution-annealed sample. Transmission electron microscopy characterization showed that the main precipitates in LDSS 2002 were Cr_2N and M_(23)C_6 along the ferrite–austenite grain boundaries.     

High-temperature Tensile Behavior in Coarse-grained and Fine-grained Nb-containing 25Cr-20Ni Austenitic Stainless Steel

In this study, tensile behavior of Nb-containing 25Cr-20Ni austenitic stainless steels composed of coarse or fine grains has been investigated at temperatures ranging from room temperature to 900 °C. Results show that the tensile strength of fine-grained specimens decreases faster than that of coarse-grained specimens, as the test temperature increases from 600 °C to 800 °C. The rapidly decreasing tensile strength is attributed to the enhanced dynamic recovery and recrystallization, because additional slip systems are activated, and cross-slipping is accelerated during deformation in fine-grained specimens. After tensile testing at 700-900 °C, sigma phases are formed concurrently with dynamic recrystallization in fine-grained specimens. The precipitation of sigma phases is induced by simultaneous recrystallization as the diffusion of alloying elements is accelerated during the recrystallization process. Additionally, the minimum ductility is observed in coarse-grained specimens at 800 °C, which is caused by the formation of M₂₃C₆ precipitates at the grain boundaries.     

Formation Mechanism of Lamellar M 23C6 Carbide in a Cobalt-Base Superalloy During Thermal Exposure at 1000 °C

The precipitation of the lamellar-shaped M ₂₃C₆ carbide within the dendritic matrix of a cobalt-base superalloy during thermal exposure at 1000 °C has been investigated. Such a precipitation is not commonly observed in cobalt-base superalloys. It is found that M ₂₃C₆ particles nucleate preferentially at stacking faults (SFs) in the dendritic matrix and grow along the SFs to develop a lamellar character. Additionally, a Cr depletion zone is observed in the vicinity of the lamellar M ₂₃C₆ carbide, which strongly supports the presence of Suzuki segregation.