Interaction of Hydrogen and Retained Austenite in Bainite/Martensite Dual-Phase High Strength Steel

The hydrogen trapping phenomena in two bainite/martensite dual-phase high strength steels(U20Si and U20DSi)were investigated by electrochemical permeation technique.The hydrogen diffusivity was calculated from data of permeation delay time,and the diffusion coefficient in U20 Si is far less than that in U20 DSi.Moreover,the hydrogen diffusivity decreases as the volume percent of retained austenite increases.The experiment results show that there are different hydrogen trappings and different volume percents of retained austenite in U20 Si and U20 DSi.The retained austenite is precipitated as films.The trap binding energy for the retained austenite and hydrogen is calculated to be 40.4kJ·mol-1.

Effect of Microstructure on Corrosion Fatigue Behavior of 1500 MPa Level Carbide-Free Bainite/Martensite Dual-Phase High Strength Steel

Influence of microstructure of the experimental steels on the corrosion fatigue behavior in 3.5% of NaCl aqueous solution was studied.Experimental results show that compared with the full martensite（FM）steel,the carbide-free bainite/martensite（CFB/M）steel has higher corrosion fatigue strength and corrosion fatigue crack threshold（ΔKthcf）,and lower corrosion crack propagation rate [（da/dN）cf].

Ultra-high cycle fatigue behavior of high strength steel with carbide-free bainite/martensite complex microstructure

The ultra-high cycle fatigue behavior of a novel high strength steel with carbide-free bainite/martensite （CFB/M） complex microstructure was studied. The ultra-high cycle fatigue properties were measured by ultrasonic fatigue testing equipment at a frequency of 20 kHz. It is found that there is no horizontal part in the S-N curve and fatigue fracture occurs when the life of specimens exceeds 10^7 cycles. In addition, the origination of fatigue cracks tends to transfer from the surface to interior of specimens as the fatigue cycle exceeds 10^7, and the fatigue crack originations of many specimens are not induced by inclusions, but by some kind of ＂soft structure＂. It is shown that the studied high strength steel performs good ultra-high cycle fatigue properties. The ultra-high fatigue mechanism was discussed and it is suggested that specific CFB/M complex microstructure of the studied steel contributes to its superior properties.

EFFECT OF SILICON ON PRODUCING DUCTILE IRON WITH COMPLEX STRUCTURE OF BAINITE AND MARTENSITE

This paper deals with an important role of silicon in producing ductile iron with quenched complex structure of bainite and martensite. The samples are cast in permanent mold and quenched in solution of sodium silicate. The result of thc experiments shows that the austenizing temperature should rise with increasing silicon content, otherwise much undissolved ferrite is present in the matrix after quenching. However the undissolvec ferrite can be decreased greatly or even eliminated by adding appropriate amount of ooron. On this condition, the amount of bainite gets increasing and the amount of residual austenite decreasing with the silicon cortent increasing. An approach has also been made to the mechanism of the effect of silicon on the transformation of bainite in ductile iron. The T.T.T. curves measured show that the increase of sllicon content causes the curve to shift to the left. This is quite different from the fact in steel.

EFFECTS OF RETAINED AUSTENITE ON DUPLEX MICROSTRUCTURE OF MARTENSITE AND LOWER BAINITE

The morphology and amount of the retained austenite in the duplex microstructure of martensite and tower bainite of a tow,alloy cold die steel(it is called GD steel for short) with high strength and high toughness have been investigated.The thermal and mechanical stabilities of the retained austenite were analyzed.Furthermore the effects of retained austenite on the mechanical properties of the steel were studied.The results show that the morphology and amount of retained austenite vary with silicon content in the steel.Retained austenite with high stability was responsible for the simultaneous increase in strength and toughness of the steel.The duplex microstructure of martensite and abnormal lower bainite of the steel with the maximum silicon content exhibits a relatively good strength-toughness combination.

Corrosion behavior of tempered dual-phase steel embedded in concrete

Dual-phase （DP） steels with different martensite contents were obtained by appropriate heat treatment of an SAE1010 structural carbon steel, which was cheap and widely used in the construction industry. The corrosion behavior of DP steels in concrete was investigated under various tempering conditions. Intercritical annealing heat treatment was applied to the reinforcing steel to obtain DP steels with different contents of martensite. These DP steels were tempered at 200, 300, and 400℃ for 45 min and then cooled to room temperature. Corrosion experiments were conducted in two stages. In the first stage, the corrosion potential of DP steels embedded in concrete was measured every day for a period of 30 d based on the ASTM C 876 standard. In the second stage, the anodic and cathodic polarization values of these steels were obtained and subsequently the corrosion currents were determined with the aid of cathodic polarization curves. It was observed that the amount of second phase had a definite effect on the corrosion behavior of the DP steel embedded in concrete. As a result of this study, it is found that the corrosion rate of the DP steel increases with an increase in the amount of martensite.

Effect of austempering time on microstructure and properties of a low-carbon bainite steel

The effect of austempering time after the bainitic transformation on the microstructure and property in a low-carbon bainite steel was investigated by metallography and dilatometry. The results showed that by prolonging the austempering time after the bainite transformation, the amount of large-size martensite/austenite islands decreased, but no significant change of the amount and morphology of bainite were observed. In addition, more austenite with a high carbon content was retained by prolonging the holding time at the bainite transformation temperature.Moreover, with a longer holding time, the elongation was improved at the expense of a small decrease in tensile strength. Finally, the Avrami equation B(RF) = 1-exp(-0.0499 × t^0.7616) for bainite reaction at 350℃ was obtained for the tested steel. The work provided a reference for tailoring the properties of low-carbon steels.

Influence of Creep Strength of Weld on Interfacial Creep Damage of Dissimilar Welded Joint between Martensitic and Bainitic Heat-Resistant Steel

The mechanical properties, creep rupture strength, creep damage and failure characteristics of dissimilar metal welded joint （DMWJ） between martensitic （SA213T91） and bainitic heat-resistant steel （12Cr2MoWVTiB（G102）） have been investigated by means of pulsed argon arc welding, high temperature accelerated simulation, mechanical and creep rupture test, and scanning electronic microscope （SEM）. The results show that there is a marked drop of mechanical properties of undermatching joint, and low ductility cracking along weld/G102 interface is induced due to creep damage. Creep rupture strength of overmatching joint is the least. The mechanical properties of medium matching joint are superior to those of overmatching and undermatching joint, and creep damage and failure tendency along the interface of weld/G102 are lower than those of overmatching and undermatching joint after accelerated simulation for 500 h, 1 000 h, 1 500 h, and the creep rupture strength of medium matching joint is the same as that of undermatching joint. Therefore, it is reasonable that the medium matching material is used for dissimilar welded joint between martensitic and bainitic steel.

Mechanism of Bainite Nucleation in Steel, Iron and Copper Alloys

During the incubation period of isothermal treatment（or aging） within the bainitic transformation temperature range in a salt bath （or quenching in water） immediately after solution treatment, not only are the defects formed at high temperatures maintained, but new defects can also be generated in alloys, iron alloys and steels. Due to the segregation of the solute atoms near defects through diffusion, this leads to non-uniform distributions of solute atoms in the parent phase with distinct regions of both solute enrichment and solute depletion. It is proposed that when the Ms temperature at the solute depleted regions is equal to or higher than the isothermal （or aged） temperature,nucleation of bainite occurs within these solute depleted regions in the manner of martensitic shear. Therefore it is considered that, at least in steel, iron and copper alloy systems, bainite is formed through a shear mechanism within solute depleted regions, which is controlled and formed by the solute atoms diffusion in the parent phase.

FATIGUE BEHAVIOUR OF GRANULAR BAINITE STRUCTURE

The strain fatigue,impact fatigue and rotation beam fatigue behaviour of granular bainitic structure has been studied.The results show that the strain fatigue properties and the impact fatigue properties of granular bainite are superior to that of tempered martensite under the condition that the ultimate tensile strength is equal.The impact.fatigue life increases with in- creasing amount of granular bainite,because the M-A islands might retard the propagation of fatigue crack.The rotation beam fatigue properties of granular hainite are similar to that of tempered martensite.The relationship between.fatigue limit S_f,yield strength σ_y and frac- ture strength S_k may be expressed as S_f=4.651+0.1411(σ_y+S_k)

Diffraction and Morphology Study of Surface Structures Accomponying with the Formation of α'Plate(Bainite)in a Cu-Zn-AI Alloy

In the present paper the morphologies of sur- face structures and the anomalous contrast of β_1 phase accompanying with the formation of bainite plate in the Cu-Zn-Al shape memory alloy have been investigated in detail by the transmission elec- tron microscopy and diffraction.The experimental results have proved that the extra diffraction and anomalous contrast results from the surface martensite formed on the β_1 phase.However,the crystal structure and morphology of the surface martensite are varied at different stage of bainite formation.The observation of interfacial disloca- tion structure of bainite/β_1 matrix reveals that there is a large strain in the bainite transformation of β CuZnAl alloy.Based on the experiment results the formation of surface martensite and bainitic transformation were discussed.

Effect of Tempered Martensite on Magnetomechanical Damping of Ferritic Matrix Dual Phase Alloys

The influence of the volume fraction of tempered martensite on magnetomechanical damping of the ferritic matrix in dual-phase alloys was studied.The experimental results showed that(1) dual-phase damping alloy is of good combination of strength and toughness: (2)good damping ca- pacity(Q^(-1)≥5.0×10^(-3))can be obtained in dual-phase alloys only when the volume fraction of tempered martensite was controlled properly.The effect of martensite on magnetomechanical damp- ing of the ferritic matrix was analysed.The damping capacity of dual-phase alloys was related to the volume fraction V_m of tempered martensite by Q_^(-1)=Q_^(-1) (1-V_m/1+BV_m)at small vibration strain amplitudes ε_A≤5.0×10^(-6).

Influence of Martensite Volume Fraction on Mechanical Properties of High-Mn Steel

Elastic-plastic deformation behavior of austenitic, martensitic, and austenitic-martensitic high-Mn steels is investigated by using crystal plasticity theory. The development of expandable pipes made of two-phase steel for oil and gas well applications is needed for improved and efficient recovery of hydrocarbons from difficult reservoirs. The current research is aimed at improving the down-hole post-expansion material properties of expandable pipes. A mathematical model is first developed based on finite-deformation crystal plasticity theory assuming that slip is the prime mode of plastic deformation. The developed model is then numerically implemented by using the finite element software ABAQUS, through a user defined subroutine. Finite element simulations are performed for austenitic, martensitic, and austenitic-martensitic steels having different proportions of martensite in an austenite matrix. Three primary modes of loading are considered: uniaxial tension, compression and simple shear. The variation in yield strength, hardening pattern and dissipated energy is observed and analyzed.

SCM435冷镦钢连续冷却过程中的组织转变及球化退火

利用Gleeble-3800热模拟试验机测定了SCM435冷镦钢的热膨胀曲线。通过金相分析和硬度试验获得了钢的动态连续冷却转变曲线。研究了原始显微组织对钢球化退火效果的影响。结果表明:以小于0.5℃/s的速率冷却后,SCM435钢的组织主要为铁素体和珠光体,以1℃/s及以上速率冷却后组织为贝氏体和马氏体以及少量铁素体,以3℃/s及以上速率冷却后组织为马氏体和贝氏体,以15℃/s及以上速率冷却后组织全部为马氏体。采用相同工艺球化退火时,马氏体和贝氏体的球化效果优于珠光体和铁素体。通过优化轧制工艺试制的原组织为贝氏体和马氏体的SCM435钢盘条球化退火后,组织中碳化物分布均匀,球化效果好,球化退火时间明显缩短。

Effect of Substructure on Toughness of Lath Martensite/Bainite Mixed Structure in Low-Carbon Steels

The quantitative analysis of substructure in the martensite/bainite mixed structure, which is obtained from low-carbon NiCrMoV steels under different cooling conditions, was made by means of optical microscope （OM）, scanning electron microscope （SEM）, electron backscatter diffraction （EBSD）, and transmission electron microscope （TEM）, in order to research the effect on toughness. The test results indicate that the toughness of the steel is en- hanced with the decrease in the packet and block size under the condition of the same prior austenite grain size mixed with different ratios of martensite and bainite while the lath width is about 0.38μm. The calculation shows that both the packet and block boundaries have the same hindering effect on crack extension. Furthermore, the effect of the block width on impact energy is much larger than that of the packet. Therefore, the block can be used as microstruc- tural substructure to affect the toughness in low-carbon martensite steels, suggesting that the block size is ＂the effective grain size＂ for controlling toughness.

Roles of pre-formed martensite in below-Ms bainite formation,microstructure,strain partitioning and impact absorption energies of low-carbon bainitic steel

The roles of pre-formed martensite(PM)in below-Ms bainite formation,microstructure,crystallography,strain partitioning and mechanical properties of a low-carbon bainitic steel were investigated using electron-backscattered diffraction,transmission electron microscopy,micro digital image correlation technique and mechanical tests.It is demonstrated that the pre-formation of martensite eliminates the incubation time for bainite transformation at various austempering temperatures below Ms,indicative of its acceleration effect at the early stage of transformation.This effect is mainly attributed to the surfaces or tips of the PM acting as the nuclei of subsequently-formed bainite,with initial bainite tending to form around the PM.However,the finishing time for below-Ms bainite transformation,especially at even lower temperatures,is retarded,owing to the dividing effect of PM on parent austenite grains,the decreasing effect of lowered isothermal temperature on the diffusion rate of carbon atoms and the strengthening effect of lowered isothermal temperature on supercooled austenite.PM and its adjacent bainitic laths have nearly the same crystallographic orientation and belong to the same block.The pre-formation of martensite largely refines the bainitic blocks/laths and retained austenite.The specimens with PM show relatively uniform strain partitioning among various phases,contrasting with the specimens without PM,for which strains are highly concentrated in the bainite region nearby fresh martensite/austenite(M/A)blocks or between adjacent M/A blocks.The impact absorption energies of the specimens with PM,when austempered at 30-60℃ below Ms,are more than twice higher than those of the specimens without PM,at no expense of tensile properties.

Research and modeling on correlation among microstructure,yield strength and process of bainite/martensite steel

The contributions of different strengthening mechanisms to yield strength of bainite/martensite multiphase rail steel with different finish cooling temperatures in the controlled cooling process were quantitatively investigated.Dislocation density and substructure size of the rail steel were measured by scanning electron microscopy,electron backscatter diffraction and X-ray diffraction.The results show that the dislocation density increases with the decrease in block width in rail steel.Based on the correlation among dislocation density,block width and yield strength,a physical model was proposed to predict the yield strength of rail steel.The variation of block width and dislocation density in different positions of rail head microstructure was integrated with temperature field simulation.Dislocation density and block width reveal significant correlations with the finish cooling temperature.

Bake-Hardening Response of High Martensite Dual-Phase Steel with Different Morphologies and Volume Fractions

Bake-hardening behaviour of carbon steel with different martensite morphologies and volume fraction was investigated. The specimens with fibrous and bulky martensite were prestrained in tension by 4%. After this, they were unloaded and bake hardened at 180 ℃ for 10-160 min. It was found that dual-phase steel samples which were bake hardened at 180 ℃ for 20 rain showed an increase in the yield stress （YS） and ultimate tensile stress （UTS） but a decrease in ductility. Further increase in the bake-hardening time of 80 or 160 min has reduced the YS and UTS, but increased the ductility. △σ （increase in stress due to bake hardening）, YS and UTS values are higher for the microstructure containing fibrous martensite compared to the microstructure-containing bulky martensite. It was also observed that at a given baking temperature Aa, YS and UTS increased by volume of martensite.

P92钢的CCT图及组织转变

研究国外P92钢的过冷奥氏体转变具有一定理论意义和应用价值。用淬火膨胀仪结合组织形貌和硬度,用Origin软件绘制钢的过冷奥氏体连续冷却相转变曲线(CCT),分析不同冷却速度下的组织转变规律和硬度变化。结果表明:P92钢的组织在缓慢冷却速度0.01~0.03℃/s下,发生铁素体、珠光体转变。由于贝氏体转变凸出于CCT曲线左方,在一般冷却条件下即可淬火得到贝氏体、马氏体组织,硬度可达564HV_(1)。

不同冷却速度下25CrNi2MoV轴承钢的组织与性能

采用Gleeble-3500热模拟试验机、OM、硬度测试、磨损试验等手段,研究了不同冷却速度对25CrNi2MoV轴承钢组织与性能的影响。结果表明:冷却速度在0.2~3.0℃/s范围内,25CrNi2MoV钢组织主要为贝氏体+铁素体,随着冷却速度的升高,25CrNi2MoV钢中铁素体含量逐渐减少,晶粒逐渐细化,其硬度值均在250 HV以下,耐磨性较差。当冷却速度为10℃/s时,25CrNi2MoV钢组织为马氏体和少量贝氏体的混合组织。冷却速度达到30℃/s时,组织中开始生成残余奥氏体。当冷却速度在10~60℃/s范围内,25CrNi2MoV钢的硬度值均在430 HV以上,耐磨性较好。