Microstructure Evolution and Mechanical Properties of HR3C Steel during Long-term Aging at High Temperature

Microstructure evolution and the changes in mechanical properties of HR3 Csteel during long-term aging at650,700 and 750℃ were investigated.The precipitated phases of the aging steel included M23C6 carbides,Z-phase and a trace amount of Nb(C,N).The M23C6 carbides were distributed mainly at the grain boundary,while Z-phase was mainly inside the grains.Amounts of both M23C6 carbides and Z-phase during the aging process increased with increasing aging period and temperature.Coarsening of M23C6 carbides was influenced significantly by aging time and temperature,while the size of the Z-phase was relatively less affected by the aging time and temperature,which had a steady strengthening effect.Coarsening of the M23C6 carbides was the main reason for the decline in high temperature yield strength during long-term aging at 750℃.The M23C6 carbides were linked into a continuous chain along the grain boundary which accounted for the decrease of toughness during aging.     

Ripening behavior of M_(23)C_6 carbides in P92 steel during aging at 800°C

The rapid coarsening of the M_(23)C_6 carbides has been held responsible for the creep fracture in 9-12Cr martensitic heat resistant steels.A commercial P92 steel was subjected to thermal aging at a high temperature of 800°C to investigate the ripening behavior of the M_(23)C_6 carbides.Scanning electron microscopy(SEM)and transmission electron microscopy(TEM)were employed to characterize the microstructure evolution,especially the ripening process of the M_(23)C_6 carbides.The new concept of the effective mean size,dependent on the critical radius,was introduced to correct the measured mean size and then the Ostwald theory was applied to describe the ripening behavior of the M_(23)C_6 carbides.The ripening of the M_(23)C_6 carbides was revealed to be grain boundary diffusion controlled.     

Effect of long-term service exposure at elevated temperature on microstructural changes of 5Cr-0.5Mo steels

Effects of long-term service exposure at elevated temperature on microstructural changes have been studied for both virgin and service-exposed process heater tube pipes of 5Cr-0.5Mo steels used in oil refineries. Samples selected for this study had experienced a nominal temperature range of 450 °C to 500 °C for about 20 to 25 years. Two different initial virgin microstructures were taken and designated by steel A and steel B. The virgin microstructure of steel A exhibited fine platelets of fibrous or hairlike M₂C carbides within the ferrite grains and occasionally irregularly shaped M₂₃C₆, both along the grain boundaries and at the grain interiors, and very few spheroidally shaped M₃C, either along the grain boundaries or at the grain interiors. The size, shape, position, distribution, and type of carbides in virgin steel A changed significantly due to 220,000 hours of service exposure in the temperature range of 450 °C to 500 °C. Massive M₂₃C₆ carbides precipitated along the grain boundaries. In addition, regular geometrically shaped M₂₃C₆ carbides, such as hexagonal, square, and triangular type, were observed to form at the grain interiors. The virgin steel B microstructure exhibited predominantly M₂₃C₆ carbides, either along the grain boundaries or at the lath boundaries. Occasionally, fine platelets of M₂C carbides were also observed within the laths. The position, shape, distribution, and type of carbides did not change significantly due to 172,000 hours of service exposure in the temperature range of 450 °C to 500 °C. The average interparticle spacings of the carbides increased from 0.35 to 1.2 μm due to 172,000 hours of exposure.     

Phase Transformation of a Cold Work Tool Steel during Tempering

The hardness and microstructure evolution of a 8% Cr cold work tool steel during tempering for 40 h were investigated. Transmission electron microscope examinations showed that M_3C carbides precipitated from supersaturated martensite after tempering at 350 ℃. When the tempering temperature was higher than 520 ℃,the M_(23)C_6 carbides precipitated to substitute for M_3C carbides. After ageing at the temperature of 520 ℃ for 40 h,it was observed that very fine and dense secondary Mo_2C precipitates were precipitated. Thus,it can be concluded that the early stage of Mo_2C-carbide precipitation is like to be Gunier-Preston( G-P) zone formed by [Mo-C] segregation group which is responsible for the secondary hardening peak at 520 ℃. Overageing at 700 ℃ resulted in recovery of martensitic microstructure and precipitation of M_(23)C_6 carbides.When ageing at 700 ℃ for more than 20 h,recrystallization occurred,which resulted in a change of the matrix morphology from martensitic plates to equiaxed ferrite. It was noticed that the size of recrystallized grain / subgrain was very fine,which was attributed to the pinning effect of M_(23)C_6 precipitates.     

Effect of Hf on Stress Rupture Properties of DD6 Single Crystal Superalloy After Long Term Aging

 The specimens of the second generation single crystal superalloy DD6 with different Hf contents were prepared in the directionally solidified furnace with a high temperature gradient. The long term aging of the specimens after full heat treatment was performed at 1040 ℃ for 800 h. The effect of Hf on the microstructure and stress rupture properties under 980 ℃/250 MPa of the alloy after long term aging was investigated. The results show that the γ′ coarsening and rafting and no topologically close packed phase (TCP) are observed in the microstructures of DD6 alloy with different Hf contents after aged at 1040 ℃ for 800 h. It indicates that DD6 alloy with different Hf contents all possesses good microstructure stability. With increasing Hf content the rupture life after long term aging turns shorter and the elongation represents the increasing first and decreasing afterwards. The fracture mechanism of the alloy with different Hf contents at 980 ℃/250 MPa all shows dimple model. The influence of the microstructures on the stress rupture properties of the alloy is also discussed.     

Influence of Solution Treatment on Microstructure of AEREX350 Alloy

The microstructure of hot rolled AEREX350 alloy under various solution treatments was investigated by scanning electron microscope (SEM) and transmission electron microscope (TEM). It was found that with increasing solution temperature,η phase precipitated firstly at grain boundaries in parallel at 800℃ and then within grains at about 980℃. The η phase precipitated at grain boundaries played a role in preventing the migration of grain boundaries,and the η phase precipitated within grains would form Widmannsttten structure with further increasing solution temperature. η phase precipitated at grain boundaries was dissolved at temperature ranging from 1080℃ to 1095℃,resulting in grain growth promptly owing to release of the pinning effect on grain boundaries. γ' phase precipitated during the process of air cooling after hot rolling and dissolved at temperature ranging between 1010℃ and 1020℃.     

Oxide evolution in NdDy-Fe-B magnet during aging process

Sintered NdDy-Fe-B magnets were made from strip cast flakes by powder metallurgy method. For the magnet with 6wt.% Dy substituted Nd, a remarkable coercivity increase of 70% was obtained after aging at 900°C for 2h and a high coercivity of 23.5 kOe was achieved after aging at 520°C for 2h. The average diameter of the oxide particles decreased during the following 900°C aging treatment. Dy and O diffusion from oxide particles to Nd-rich phase during the aging process was observed. The oxide evolution and Dy element diffusion were helpful to the coercivity enhancement.     

Microstructures and strengthening mechanisms of Mg-8.2Gd-4.6Y-1.5Zn-0.4Zr alloy containing LPSO,β' and γ type phases

The microstructures and strengthening mechanisms of the Mg-8.2 Gd-4.6 Y-1.5 Zn-0.4 Zr(wt%) alloy with long-period stacking ordered(LPSO),β' and γ type phases were systematically studied.The results show that the LPSO with lamellar and block structures forms near the grain boundaries.The grains are clearly refined,and the 18 R LPSO phase is oriented along the extrusion direction after extrusion.Some particles also precipitate from the Mg matrix dynamically.The extruded alloy exhibits a remarkable agehardening response,and mechanical properties,with a tensile strength(TS) of 449 MPa,yield strength(YS) of 362 MPa,and elongation of 7.9% obtained in the peak-aged alloy.The strengthening mechanisms of the alloy in different states are discussed.Grain boundary and precipitation strengthening are the main strengthening mechanisms for the peak-aged alloy.     

Evolution of microstructure,mechanical and magnetic properties of electrodeposited 50%Ni-Fe alloy foil after thermal treatment

In order to expand the application of the electrodeposited Ni-Fe alloy foil,their mechanical and magnetic properties were studied after heat treatment.The development of grain growth during annealing was in-situ online investigated using a heating stage microscope,and the texture was analyzed via X-ray diffraction(XRD)and electron back-scattered diffraction(EBSD).The results indicated that abnormal grain growth usually occurred during annealing at 1000-1050°C.The{111}oriented grains preferentially grew as the annealing temperature and holding time increased.The plasticities of the electrodeposited Ni-Fe alloy foils after heat treatment were better than those of the original samples.The excellent ductility was obtained without a loss in magnetic properties after annealing at 1100°C for 6h.     

Strength of Al-Zn-Mg-Cu matrix composite reinforced with SiC particles

The AA7075 alloys reinforced with SiC and without SiC particles were fabricated by a pressureless infiltration method, and then, their tensile properties and microstructures were analyzed. The spontaneous infiltration of molten metal at 800 °C for 1 hour under a nitrogen atmosphere made it possible to fabricate 7075 Al matrix composite reinforced with SiC, as well as a control 7075 Al without SiC. A significant strengthening even in the control alloy occurred due to the formation of in-situ AlN particle even without an addition of SiC particles. Composite reinforced with SiC particles exhibited higher strength values than the control alloy in all aging conditions (underaged (UA), peak-aged (PA), and overaged (OA)), as well as a solution treated condition. Spontaneous infiltration was further prompted owing to the combined effect of both Mg and Zn. This may lead to an enhancement of wetting between the molten alloy and the reinforcement. Consequently, strength improvement in a composite may be attributed to good bond strength via enhancement of wetting. The grain size of the control alloy is greatly decreased to about 2.5 μm compared to 10 μm for the commercial alloy. In addition, the grain size in the composite is further decreased to about 2 μm. These grain refinements contributed to strengthening of the control alloy and the composite.     

Microstructure characteristics of spray-formed high vanadium and cobalt high speed steel

The microstructure of high vanadium and cobalt high speed steel(high-V/Co HSS) and the morphology of its carbides were analyzed by optical microscope(OM),scanning electron microscope(SEM),electron probe microanalysis(EPMA),X-ray power diffraction(XRD),atomic force microscopy(AFM) and single phase erosion(SPE).The results suggest that the as-sprayed high-V/Co HSS has fine equiaxed grains(about 20 μm in size),which were homogeneously distributed.The carbides have two classical morphologies:one is fine particles(about 2 μm in size) distributed along the grain boundaries and the other is needle-like one,distributed on the grain boundaries.There are MC carbides,M2C carbides,M6C carbides and Cr2WC2 carbides in the as-sprayed high-V/Co HSS samples,however,in the as-cast high-V/Co HSS,there are MC carbides and Cr2WC2 carbides only.The SPE results show that there are two types of MC carbides in the as-sprayed HSS:the sphere one and the particle-like one.The former is about 2 μm in size and the latter is less than 1 μm,dispersed inside the grains,quite different from the MC carbides in the as-cast HSS.According to the AFM results,the skeleton-like M6C carbides of the as-sprayed high-V/Co HSS are embedded in the matrix along the grain boundaries.It is found that there are sharp membrane pieces on the carbides.Some small bamboo-shoot-like MC carbides grow from the matrix and are dispersed inside the grain.Those larger MC carbides are spherical particles embedded at the grain boundary junctions.     

Grain refinement of AZ31 magnesium alloy by AI-Ti-C-Y master ahoy

Al-Ti-C-Y master alloy was prepared by combining SHS technique and melting-casting method. The microstructure of master alloy and its grain-refining effect on AZ31 alloy were investigated by means of OM, XRD, SEM and EDS. Experimental results indicated that the prepared master alloy consisted of α-Al, TiAl3, TiC and Al3Y phases, and exhibited good grain-refining performance of AZ31 alloy. Morphology of α-Mg changed from coarse dendritic to free equiaxed and the average grain size of α-Mg matrix reduced from the original 580 to 170 μm after adding 1.0 wt.% master alloy. The grain refining efficiency of Al-Ti-C-Y master alloy on AZ31 alloy was mainly attributed to heterogeneous nucleation of TiC particles and grain growth restriction of Al-Y compound or TiC at grain boundaries.     

Microstructure evolution and corrosion properties of Mg-Dy-Zn alloy during cooling after solution treatment

Microstructure evolution and corrosion properties of Mg-2Dy-0.5Zn(at.%) alloy during cooling after solution treatment were investigated.The microstructure of alloy in the solid solution state(530 oC,12 h) was composed of α-Mg and small amounts of(Mg,Zn)_x Dy phases.During cooling at a cooling rate of 2 oC/min,the 14H-type LPSO phase gradually precipitated in the grain interior and its volume fraction increased with increasing cooling time.The alloy cooled for 20 min exhibited the highest hardness value.In addition,electrochemical and immersion test results indicated that the alloy cooled for 5 min exhibited small corrosion current and low corrosion rate.The good corrosion resistance of alloy was mainly attributed to the continuous distribution of LPSO phase along the grain boundary.     

Effects of Yttrium and Cerium on Compression Properties of Ni_3Al-Base Alloys

The effects of yttrium and cerium on the compression properties of Ni_3Al-base alloys have beeninvestigated.The results reveal that the addition of about 0.1 wt% Y to Ni_3Al-B alloy is effective for improvingthe ductility at 1100℃.A ductility increase of about 100% is observed for this Y doped alloy.The yttrium re-fines grains of the alloy.An YNi_5 phase is found to be precipitated on the grain boundaries in the alloy con-taining 0.3 wt% Yor more.Adding about 0.1 wt% Ce to Ni_3Al-B-Cr-Zr alloy obviously improves the ductilityof the alloy at 1100℃.A ductility increase of about 50% is obtained for this alloy.The added cerium also pre-vents the formation of γ+γ' eutectic and refines it.The bulk phase of CeNi_4 appeares at the front of the eutecticin the alloys containing 0.1 wt% Ce or more.     

Chi Phase after Short-term Aging and Corrosion Behavior in 2205 Duplex Stainless Steel

Correlation between pitting corrosion behavior and chi( χ )phase formed after a short-term aging(5,10 and 15 min)at 850 ℃ of 2205 duplex stainless steel(DSS)was investigated using potentiodynamic polarization tests,optical microscopy,and scanning electron microscopy equipped with energy-dispersive spectrum system.Results showed that after aging for 5min,the χ phase initially precipitated at ferrite grain boundaries,developed and then became linked with prolonging aging time.The χ phase was rich in Cr and Mo,resulting in formation of depleted zones nearby.The χ phase could reduce corrosion resistance of DSS and slightly influence its stability,but the specimens still displayed the capacity for repassivation.Some lines of evidence showed that stable pitting corrosion initiated at the boundaries of precipitates.The χ phase was selectively corroded during the first stage of corrosion and then the depleted zones nearby were attacked.In addition,the grain size and volume of precipitates also affected pit nucleation and progress,and suitable size and distribution of χ phase could aggravate pit initiation at precipitate boundaries.The χ phase with considerably low volume fraction and small size was not sensitive position for pit initiation.     

The microstructural evolution,crystallography, and thermal processing of ultrahigh carbon Fe-1.85 pct C melt-spun ribbon

The microstructural evolution, mechanisms of grain refinement, crystallography, and thermal processing of a rapidly solidified Fe-1.85 pct C alloy have been studied by transmission electron microscopy (TEM). Melt-spun ribbons quenched in liquid nitrogen consist of carbide-free highly twinned martensite plates between 0.5-and 2.0-μm long and 0.1-and 0.5 -μm thick, with approximately 40 pct retained austenite also present. Ribbons tempered at 600 °C for 10 seconds consist of ferrite of approximately the same grain size and both intragranular and intergranular cementite precipitates. The intragranular cementite particles are about 0.1 /um or less in size and exhibit a single variant of the Bagaryatskii orientation relationship with respect to a given ferrite grain; the intergranular particles are about 0.1 μm in thickness and can be as long as 0.5 μm due to growth and/or coalescence along ferrite grain boundaries. A heat-treatment cycle investigated with a view toward generating structures suited for superplastic consolidation of the rapidly solidified ribbons consists of quenching the ribbon in liquid nitrogen, tempering at 600 °C for 10 seconds, “upquenching” to 750 °C (austenitizing) for 10 seconds, and subsequently quenching again in liquid nitrogen. This treatment results in martensite grains highly misoriented with respect to one another and typically 0.5 μm or less in both length and thickness and cementite particles 0.4 μm or less in size. (Occasionally, longer martensite plates were observed; but they never exceeded 1 μm in length.) The microstructures produced here offer the potential for producing fine-grained ultrahigh carbon steels of very high strength without the brittleness associated with the formation of coarse carbide particles or the loss of strength due to graphite formation. This investigation has thus provided the basis for follow-on studies currently underway in ultrahigh carbon Fe-C-Cr and Fe-C-Cr-Si steels, with the intent of producing similar microstructures which will also exhibit enhanced high-temperature stability.     

Solidification behavior, microstructure and tensile properties of ZK60-Er magnesium alloys

ZK60-Er (erbium) alloys were made by melting ZK60 and Mg-Er magnesium alloys (20 wt.% Er) in an electric resistance furnace. The contents of Er were 0, 0.5, 1, 2, 3 wt.%, respectively. The influence of Er on solidification behavior, microstructure, corrosion resistant and mechanical properties of ZK60 magnesium alloy was studied. The results showed that long rod-like γ phase (ErZn5) formed during solidification increased with increasing Er content in the range investigated, which resulted in the decrease of the amount of galvanic couplings between phase particles and alloy matrix and the marked improvement of corrosion resistant. It was also found that elongation of the alloys decreased with increasing Er content, but tensile strength of the alloys were improved by the addition of Er due to the strengthening effect of γ phases distributing along grain boundaries.     

Investigation on the hot ductility of Fe-36Ni invar alloy with cerium addition

The hot ductility of Fe-36 Ni invar alloy with different additions of the element cerium was investigated using a Gleeble-3800 thermal-mechanical simulator over the temperature range 850- 1 050 ℃,and the improvement mechanism of the hot ductility was analyzed using a combination of SEM,EDS,and OM. The results indicated that Fe-36 Ni invar alloy exhibited poor hot ductility below 1 050 ℃,which was mainly attributed to weak grain boundaries and the action of grain boundary sliding. However,the alloys with cerium contents of 0. 016% and 0. 024% both demonstrated substantial improvement in the hot ductility over the entire testing temperature range. The observed improvement of the hot ductility of the alloy with 0. 016% cerium at 950 ℃ and the alloy with 0. 024% cerium at 900℃ was associated mainly with the grain boundary strengthening and the restriction of the grain boundary sliding because the addition of cerium reduced the segregation of sulfur at grain boundaries and refined the grain structure.The occurrence and acceleration of dynamic recrystallization were found to be responsible for the high hot ductility of the alloy with 0. 016% cerium at 1 000 ℃ and the alloy with 0. 024% cerium at 950- 1 000 ℃ as a result of the refinement of the grain structure by addition of cerium.     

Relationship Among Microstructure and Properties and Heat Treatment Process of Ultra-High Strength X120 Pipeline Steel

 The variation of heat treatments including directed quenching and tempering off-line after controlled rolling (DQT) and quenching off-line and tempering off-line after controlled rolling (RQT) with microstructure and mechanical properties of a low-carbon microalloyed steel was compared and analyzed. For DQT, the quenched steel was obviously banded microstructure, with increasing tempering temperature, lath martensite coarsened, the cusp carbide precipitated at grain boundaries, the yield strength fluctuated slightly, and the fracture-separation was obvious. The impact toughness was better in the steel tempered at 500 ℃ for 1 h. In RQT, with increasing tempering temperature, lath martensite degenerated, intragranular and intergranular finer precipitations with smaller than 30 nm precipitated and grew up and were distributed dispersedly, the stripe-like carbides were distributed at grain boundaries, and the yield strength and tensile strengthen decreased obviously. The impact toughness of RQT process was much better than that of DQT process, and the comprehensive mechanical properties were better for the steel tempered at 500 ℃ for 1 h of RQT process.     

Effect of Cooling Rate on Microstructure and Mechanical Properties of K465 Superalloy

 K465 superalloy, as a material for production of turbine nozzle, shows high mechanical properties as well as microstructure stability in critical and severe service conditions. The alloy gains its appropriate microstructure and strength through solid solution strengthening mechanism. Heat treatment parameters such as: time and temperature of homogenization, partial solution and aging temperatures, and cooling rate from solid solution affect the microstructure of the alloy. Among these parameters cooling rate from solid solution is the most effective. Therefore, in this study the effect of cooling rate on microstructure and mechanical properties (tensile and stress properties) were investigated. For this purpose, three different cooling rates were applied on the cast K465 specimens after solution treatment at 1210℃ for 4 h. Microstructures of the specimens then were studied using optical and electron microscopy. Also, tensile tests were performed at room temperature and stress rupture tests were performed under the condition of 975℃ and 230 MPa. It was found out that with increasing cooling rate the size of the γ' precipitates decreases and the mechanical properties of specimens increases. Also, it was shown that the shape and volume fraction of primary γ' particles are largely influenced by the cooling rate following solution treatment at 1210℃ for 4 h.