Fe-4.2%Cr合金α/γ相变中相边界移动和晶粒长大的原位观察

In-situ observations on α/γ phase transformation were made to study the effects of grain boundary microstructures on the formation of a new phase and the migration of α/γ interphase boundary in an iron-4. 2%Cr alloy. It was found that triple junctions with more random boundaries could be the primary nucleation sites for a new phase, while triple junctions with low angle or low ∑ coincidence boundaries did not play any role as preferential sites. The migration of α/γ interphase boundary during heating over the transformation temperature range showed the two stage behaviour characterized by a stage with a migration velocity of 0. 33～0. 75 mm/s and secondly by a stage with 3. 7～7. 6 mm/s. It was also found that abnormal grain growth and a high density of ∑3 coincidence boundaries could occur in a phase with bcc structure after cycling of α/γ phase transformation. A new mechanism of nucleation and growth of a new phase in α/γ phase transformation is proposed on the basis of roles of plane-matching interphase boundaries, as previously discussed on the origin of anisotropy of grain growth due to the migration of {110} plane-matching boundaries in Fe-3%Si alloy. The most recent theoretical work on the distribution of plane-matching boundaries in solids with different crystal structures was found to be useful for the understanding of nucleation and growth during α/γ phase transformation.     

织构对Cu-Al-Mn系形状记忆合金内耗的影响

In-situ observations on α/γ phase transformation were made to study the effects of grain boundary microstructures on the formation of a new phase and the migration of α/γ interphase boundary in an iron4. 2%Cr alloy. It was found that triple junctions with more random boundaries could be the primary nucleation sites for a new phase, while triple junctions with low angle or low ∑ coincidence boundaries did not play any role as preferential sites. The migration of α/γ interphase boundary during heating over the transformation temperature range showed the two stage behaviour characterized by a stage with a migration velocity of 0. 33-0. 75 mm/s and secondly by a stage with 3. 7-7. 6 mm/s. It was also found that abnormal grain growth and a high density of ∑3 coincidence boundaries could occur in a phase with bcc structure after cycling of α/γ phase transformation. A new mechanism of nucleation and growth of a new phase in α/γ phase transformation is proposed on the basis of roles of plane-matching interphase boundaries, as previously discussed on the origin of anisotropy of grain growth due to the migration of {110} plane-matching boundaries in Fe-3z%Si alloy. The most recent theoretical work on the distribution of plane-matching boundaries in solids with different crystal structures was found to be useful for the understanding of nucleation and growth during α/γ phase transformation.     

Unexpected Effect of Nb Addition as a Microalloying Element on Mechanical Properties of δ-TRIP Steels

The concept of microalloying was applied to the δ-TRIP(transformation-induced plasticity)steel to investigate the feasibility of increasing the mechanical properties and understanding the effect of microalloying on the morphology and structure of the steel.A hot rolled δ-TRIP steel with three different contents of Nb(0,0.03,0.07 mass%)was subjected to the microstructural and mechanical examination.The high Al and Si concentration in these steels guaranteed the presence of the considerable δ-ferrite phase in the microstructure after the casting and the subsequent hot rolling.The obtained results showed that Nb dramatically affects the microstructure,the dynamic recovery and recrystallization behavior,as well as the grain shape and thus the stability of austenite after the thermomechanical process of hot rolling.The results also revealed an unexpected effect of Nb on the mechanical properties.The addition of Nb to theδ-TRIP steel led to a significant decrease in the ultimate strength(from 1 144 to 917 MPa)and an increase in ductility(from 24% to 28%).These unconventional results could be explained by the change in the steel microstructure.The work-hardening behaviors of all samples exhibit three stages of the work-hardening rate evolution.At the stage 2,the work-hardening rate of the studied steels increased,being attributed to the TRIP effect and the transformation of austenite to martensite.     

Ultrafine Grained Duplex Structure Developed by ART-annealing in Cold Rolled Medium-Mn Steels简

The microstructural evolutions of the cold rolled Fe-0.1C-5Mn steel during intercritical annealing were ex- amined using combined advanced techniques. It was demonstrated that intercritical annealing results in an ultrafine granular ferrite and austenite duplex structure in cold rolled 0.1C-5Mn steel. The strong partitioning of manganese and carbon elements from ferrite to austenite was found during intercritical annealing by scanning transmission elec- tron microscopy （STEM） and X-ray diffraction （XRD）. Strong effects of boundary characters on the austenite for- mation were indicated by austenite fast nucleation and growth in the high angle boundaries but sluggish nucleation and growth in the low angle boundaries. The ultrafine grained duplex structure in 0.1C-5Mn was resulted from the the sluggish Mn-diffusion and the extra high Gibbs free energy of ferrite phase. Based on the analysis of the micro- structure evolution, it was pointed out that the intercritical annealing of the medium Mn steels could be applied to fabricate an ultrafine duplex grained microstructure, which would be a promising approach to develop the 3rd genera- tion austomobile steels with excellent combination of strength and ductility.     

Microstructural changes during heating of super duplex stainless steel and its thermal deformation behavior

Microstructural changes during heating of highly alloyed Cr26Ni7 type super duplex stainless steel( SDSS2607) and its thermal deformation behavior were investigated. At different heating rates,the mechanism of phase transition from γ phase to δ phase and grow th modes of δ phase differed. Variations in microstructures for ascast SDSS2607 during heat preservation at 1 220 ℃ indicated two kinds of transformations from γ phase to δ phase.In-situ observations of microstructural changes during the tensile process at 1 050 ℃ showed a mutual coordination betw een γ and δ phases. When the true strain increased,the mutual coordination between γ and δ phases was damaged. Subsequently,cracks nucleated at the γ/δ interface. With the increase in temperature,the strength of ascast SDSS2607 decreased while its plasticity increased. Its thermoplasticity was poor,and the reduction in area of tensile specimens was less than 80%. When the deformation strain of hot compression increased,the stable deformation zone in the heat processing maps enlarged gradually. Moreover,the unstable deformation zones were extended.     

The Evolution of Ultrafine Grained Structure Formation Through Isothermal Static Phase Transformation

In the current study,a 0.3C-2Si-2Mn-0.28Mo (in wt%) steel with high hardenability was deformed at a relatively low temperature followed by isothermal static phase transformation.This novel thermomechanical processing made it possible to successfully produce an ultrafine ferrite grained structure (～2 μm) in the absence of both dynamic phase transformation and controlled cooling.The use of a model Ni-30Fe austenitic alloy showed that the low temperature deformation induced very fine intragranular defects throughout the microstructure,which would then act as fine spaced ferrite nucleation sites at an early stage of phase transformation.As a result,the coarsening of ferrite was extremely limited during isothermal phase transformation,resulting a very fine ferrite grained structure;even nanoscale in the region of the prior austenite grain boundary.     

Orientation gradient on surface of non-oriented electrical steel annealed by γ → α transformation

The microstructures and textures of the non-oriented electrical steel sheets under various annealing conditions were investigated.The orientation gradient in the interior of grains appeared on the surface of steel sheets after α→γ→α transformation in various atmospheres.The orientation gradient that appeared on the surface was inherited into the center of the steel sheet in H2 atmosphere,while that appeared only in the grains of the surface layer during γ→α transformation with N2 atmosphere.The thin oxide scale was generated in various atmospheres containing traces of O2 and H2O.The formation mechanism of orientation gradient is related to the thermal stress caused by the difference of thermal expansion coefficients between the oxide scale and the thin ferrite on the surface.The combined effect of phase transformation stress and thermal stress caused the plastic deformation of the thin ferrite grains on the surface in the initial stage of γ→α transformation,and then the orientations of the thin ferrite grains gradually rotated and caused the orientation gradient in the interior of grains.     

Corrosion Behavior of S450EW Low-alloy Weathering Steel in Cyclically Alternate Corrosion Environments

Weathering steel is widely used in various fields due to its excellent mechanical properties and high corrosion resistance. The effect of chromium content on the S450 EW weathering steel in cyclic immersion test was studied. The results indicated that the corrosion resistance of S450 EW weathering steel is closely related to chromium content. The addition of chromium significantly inhibited the weathering steel corrosion. The corrosion rate of experimental steel after 96 h immersion was 1.101 g·m-2·h-1. The rust of S450 EW weathering steel was mainly constituted of Fe OOH and Fe3O4 phase, and the elevation of chromium content promoted the formation of α-Fe OOH. The fine precipitates of the two phases contributed to the formation of dense dust layer of test steel. Furthermore, the increase of chromium is beneficial for the cure of original defects and cracks of the rust layer via the enrichment of chromium. The corrosion potential and the resistance of corrosion process were thus increased, protecting the experimental steel from further corrosion. A S450 EW steel with corrosion resistance more than 1.5 times of Q450NQR1 steel was prepared.     

Experimental Study and Kinetic Calculation on Migration of γ/α Interface Boundary During High Temperature Aging for 2205 Duplex Stainless Steel

Aging treatments in the temperature range of 1000 to 1200 ℃ for different time intervals(10,30,and 60 min)in a solution-treated 2205 duplex stainless steel(DSS)are carried out respectively.The migration of γ/α interface boundary after different aging treatments is measured by optical metallograph and calculated by DICTRA software.The experimental results are in agreement with calculated ones.The migration of γ/α interface boundary with increased keeping time depends on the chemical potential of chromium in each phase.At aging temperature of 1100 ℃,the back diffusion effect of chromium results in the opposite direction migration of γ/α interface.Therefore,the ferrite volume fraction is lower at first and then it is higher after 1800 s diffusion.     

Growth Kinetics of Proeutectoid Ferrite in an Fe-0.09C-1.5Mn-0.2Si Steel

Growth kinetics of proeutectoid ferrite, including grain boundary face nucleated ferrite, grain boundary edge nucleated ferrite allotriomorph and intragranular ferrite idiomorph, were experimentally measured in an Fe- 0.09C-1.5Mn-0.2Si steel and compared with theoretical calculation in local equilibrium and paraequilibrium modes. Grain boundary edge nucleated ferrite exhibited larger growth rate than grain boundary face nucleated ferrite and in- tragranular ferrite idiomorph. Experimental kinetics of proeutectoid ferrite was within the window defined by the lo- cal equilibrium and paraequilibrium limits. A transition of growth kinetics from paraequilibrium to local equilibrium was observed in the temperature range of 650--750 ℃, which can be explained in terms of solute drag.     

Microstructural change during superplastic deformation of δ-ferrite/austenite duplex stainless steel

Superplasticity of a 25 pct Cr-6.5 pct Ni-3 pct Mo-0.14 pct N δ/γ duplex stainless steel has been studied with particular emphasis on the microstructural change during deformation. Two large superplastic elongations are obtained at temperatures around 1323 K in δ/γ duplex phase region and 1173 K where σ phase particles precipitate dynamically at a strain rate of ~10^?3 s^?1. During deformation in the higher temperature region, fine Widmanstätten γ particles coarsen and coarse γ grains formed during the prior treatments are broken into spherical particles, resulting in a homogeneous dispersion of γ particles within the σ-ferrite matrix. The dynamic recrystallization of soft σ-ferrite matrix occurs locally in the region where the strain reaches some critical value, and the final microstructure consists of equiaxed σ and γ grains. In the case of lower temperature deformation, a eutectoid decomposition of δ-ferrite into γ and σ phases occurs. The relatively soft γ grains which are severely deformed by hard σ particles recrystallize dynamically, and these processes lead to the γ/σ equiaxed duplex structure. The extremely large superplasticity of this alloy can mainly be explained in terms of the above microstructural change during deformation.     

Fe-5Mn-2Al-0.15C中锰钢连铸凝固偏析及粒子析出行为

为研究Fe-Mn-Al-C系中锰钢连铸凝固偏析及粒子析出特性,采用光学显微镜观察Fe-5Mn-2Al-0.15C中锰钢的显微组织,并通过Thermo-Calc热力学软件研究了其凝固模式、溶质元素偏析及粒子析出行为。结果表明,该中锰钢铸锭的显微组织主要为板条状马氏体,且含有少量铁素体;其凝固模式为L→L+δ→L+δ+γ→δ+γ→γ;Al元素的平衡分配系数大于1,发生负偏析,偏聚到δ-铁素体枝晶内部;而Mn、Nb、V、S等溶质元素发生正偏析,均偏聚到枝晶间。AlN主要在枝晶内析出,其析出温度为1 448 ℃;MnS、Nb和V的富集物主要在枝晶间析出,且MnS和Nb的富集物均在1 400 ℃以上开始析出,而V的富集物的析出温度为760 ℃。     

δ/δ Grain Boundary Movement in Fe-0.2%C-0.8%Cr During Solidification Process

 The moving process of δ/δ grain boundary during the solidification process of Fe-0. 2%C-0. 8%Cr has been observed in-situ with confocal scanning laser microscope (CSLM), and the establishment of δ/δ grain boundary, the moving mode, the moving speed and the influence of correlative factor were explored in detail. The results showed that the clear grain cannot be formed when δ phase merged during solidification process of the steel if the actual cooling rate was 0. 045 ℃·s-1. The δ/δ grain boundary with shape of flat will be formed if there is no essential conjunction between δ phase in a long period of time. The movement of δ grain boundary cross point drives other δ/δ grain boundaries to move. The moving process is not stable, and the moving rate variation ranges from 0. 12 to 1. 65 μm ·s-1. Temperature of the system is the chief factor for influencing the moving rate of δ/δ grain boundary during the solidification. When the temperature fluctuation of system appears, the solidified grain boundary is re-melted, which results in the catastrophic fluctuation of moving rate for δ/δ grain boundary. When the solidification process reaches its end, the average moving rate of δ/δ grain boundary is quicker than that of L/δ interface. The coarsening rate of crystal grain in the melt is slower than that of solidified solid phase crystal grain.     

Phase transformations in a corrosion-resistant high-chromium nitrogen-bearing steel

The structure, phase composition, and mechanical properties of an austenitic corrosion-resistant high-chromium nitrogen-bearing (～0.5% N) steel are studied in the as-cast state and after homogenizing heat treatment (HT) followed by quenching. The main structural constituents of the as-cast steel are austenite and the σ phase (12%), which forms as an interdendritic metal during solidification, and δ ferrite and M ₂₃C₆-type chromium carbides are absent. Homogenizing HT at 1100–1200°C leads to the σ → γ transformation through the stage of the formation of intermediate δ ferrite via the restructuring of the tetragonal into the bcc lattice. Upon long-term homogenizing HT, the chromium concentration in ferrite decreases due to diffusion chromium redistribution and the δ → γ transformation takes place. The austenite in both the as-cast steel and the steel subjected to homogenizing HT followed by water quenching contains numerous (Cr, V)N nanoparticles. The twofold yield strength of this steel (～400 MPa) as compared to nitrogen-free 18Cr-10Ni-type steels can be explained by not only the solid-solution hardening of austenite by nitrogen but also by precipitation hardening.     

薄带连铸耐大气腐蚀钢高温相变过程的原位观察

对不同含磷量的耐大气腐蚀钢薄带的高温相变过程进行了原位观察。结果表明，磷对薄带耐大气腐蚀钢高温相变过程有显著影响，随着含磷量的提高，高温δ→γ相变起始温度和相变终了温度都明显降低，且相转变温度区间增大，相变过程持续时间延长，使高温的6相滞留至较低的温度；残余的δ相能有效阻碍奥氏体晶粒长大，为γ→α理的转变提供有利的形核核心，从而细化奥氏体晶粒。     

Effects of heating and cooling rates on δ↔γ phase transformations in duplex stainless steel by in situ observation

Ultra high temperature confocal laser scanning microscope is applied to observe the characteristics of the γ?δ phase transformations at different heating and cooling rates on the surface of duplex stainless steels. It is found that the migration of the δ/γ IB is always the main form of the phase transformation and leads to the continuous decline and final disappearance of the retained γ-phase during the γ→δ phase transformation at different heating rates. Heating rate does not radically later the mode of transformation under the current composition system. Interestingly, the δ-phase inside the γ-phase acts as the nucleation cores and grows more rapidly at the slow heating rate than at the rapid heating rate. During the δ→γ phase transformation, the γ-cells prefer to precipitate along the δ/δ grain boundaries with a flaky pattern, and their fronts are jagged in shape at the slow cooling rate, but in needle-like feature at the rapid cooling rate. The γ-cells also nucleate inside the δ-ferrite grains with a flaky pattern at the slow cooling rate and with a needle-like pattern at the rapid cooling rate. Furthermore, at different cooling rates, the growth speed in the longitudinal direction is always much faster than that in the lateral direction. More importantly, the effects of cooling rates on the precipitating morphologies of γ-phase are clarified based on the experimental results and the diffusion controlled growth theory.     

850级钢熔化焊焊缝的组织特征

利用金相显微镜、扫描电子显微镜及附带EDS系统和透射电子显微镜研究850 MPa级焊缝金属的微观组织,并通过分析焊缝金属凝固和相变过程,研究组织形成机制。发现原δ铁素体柱状晶晶界附近的锰和镍含量高于其心部含量。原δ铁素体柱状晶晶界附近组织由平行板条马氏体组成,板条宽度约为300 nm,原δ铁素体柱状晶心部组织由"交织状"板条马氏体组成,板条宽度约为400 nm。分析认为造成原δ铁素体柱状晶晶界附近和心部组织差异的重要原因是锰和镍的偏析,而焊缝金属良好的冲击韧性是因为存在30%"交织状"马氏体和一定量残余奥氏体。     

Morphological Stability of <Emphasis Type="Italic">δ</Emphasis>-Ferrite/<Emphasis Type="Italic">γ</Emphasis> Interphase Boundary in Carbon Steel

The morphological changes of the δ-ferrite/γ interphase boundary have been observed in situ with a high-temperature confocal scanning laser microscope (HTCSLM) during δ/γ transformations (δ → γ and γ → δ) of Fe-0.06 wt pct C-0.6 wt pct Mn alloy, and a kinetic equation of morphological stability of δ-ferrite/γ interphase boundary has been established. Thereafter, the criterion expression for morphological stability of δ-ferrite/γ interphase boundary was established and discussed, and the critical migration speeds of δ-ferrite/γ interphase boundaries are calculated in Fe-C, Fe-Ni, and Fe-Cr alloys. The results indicate that the δ-ferrite/γ interphase boundary is very stable and nearly remains absolute planar all the time during γ → δ transformation in Fe-C alloy. The δ-ferrite/γ interphase boundary remains basically planar during δ → γ transformation when the migration speed is lower than 0.88 μm/s, and the interphase boundary will be unstable and exhibit a finger-like morphology when the migration speed is higher than 0.88 μm/s. The morphological stability of δ-ferrite/γ interphase boundary is primarily controlled by the interface energy and the solute concentration gradient at the front of the boundary. During the constant temperature phase transformation, an opposite temperature gradient on both sides of δ-ferrite/γ interphase boundary weakens the steady effect of the temperature gradient on the boundary. The theoretical analysis of the morphological stability of the δ-ferrite/γ interphase boundary is coincident with the observed experimental results utilizing the HTCSLM. There is a good agreement between the theoretical calculation of the critical moving velocities of δ-ferrite/γ interphase boundaries and the experimental results.     

SUS410半马氏体不锈钢厚壁无缝管内表面裂纹缺陷分析及工艺改进

SUS410半马氏体不锈钢(0.08% ~0. 15%C,12.5% ~ 13. 5%Cr)在生产热轧厚壁无缝钢管时极易产生内表缺陷。通过对φ139.7mm x 38.1 mm管内表面裂纹缺陷进行定位分析,结果表明裂纹附近δ-铁素体大量析出造成钢的热塑性下降,在变形过程中,由于两相区变形不协调,最终在奥氏体和δ-铁素体相界出现裂纹。根据 Schaeffler相图,对化学成分进行重新设计,将Cr当量/Ni当量比值由原来缺陷管的4.51降至≤3.0,管坯加热温度由1200~1300℃降至1180~1280 ℃,内表面裂纹缺陷得到有效解决,超声探伤合格率提高至97.9%。