Influence of rapid heating process on the microstructure and tensile properties of high-strength ferrite–martensite dual-phase steel

Three low-carbon dual-phase （DP） steels with almost constant martensite contents of 20vo1% were produced by intercritical annealing at different heating rates and soaking temperatures. Microstructures prepared at low temperature （1043 K, FH1） with fast-heating （300 K/s） show banded ferrite/martensite structure, whereas those soaked at high temperature （1103 K, FH2） with fast heating reveal blocky martensite uniformly distributed in the fine-grained ferrite matrix. Their mechanical properties were tested under tensile conditions and compared to a slow-heated （5 K/s） reference material （SH0）. The tensile tests indicate that for a given martensite volume fraction, the yield strength and total elongation values are noticeably affected by the refinement of ferrite grains and the martensite morphology. Metallographic observations reveal the formation of microvoids at the ferrite/martensite interface in the SH0 and FH2 samples, whereas microvoids nucleate via the fracture of banded martensite particles in the FH1 specimen. In addition, analyses of the work-hardening behaviors of the DP microstructures using the differential Crussard-Jaoul technique demonstrate two stages of work hardening for all samples.

Grain boundary engineering for enhancing intergranular damage resistance of ferritic/martensitic steel P92

Ferritic/martensitic(F/M)steel is widely used as a structural material in thermal and nuclear power plants.However,it is susceptible to intergranular damage,which is a critical issue,under service conditions.In this study,to improve the resistance to intergranular damage of F/M steel,a thermomechanical process(TMP)was employed to achieve a grain boundary engineering(GBE)microstructure in F/M steel P92.The TMP,including cold-rolling thickness reduction of 6%,9%,and 12%,followed by austenitization at 1323 K for 40 min and tempering at 1053 K for 45 min,was applied to the as-received(AR)P92 steel.The prior austenite grain(PAG)size,prior austenite grain boundary character distribution(GBCD),and connectivity of prior austenite grain boundaries(PAGBs)were investigated.Compared to the AR specimen,the PAG size did not change significantly.The fraction of coincident site lattice boundaries(CSLBs,3≤Σ≤29)and Σ3^(n) boundaries along PAGBs decreased with increasing reduction ratio because the recrystallization fraction increased with increasing reduction ratio.The PAGB connectivity of the 6%deformed specimen slightly deteriorated compared with that of the AR specimen.Moreover,potentiodynamic polarization studies revealed that the intergranular damage resistance of the studied steel could be improved by increasing the fraction of CSLBs along the PAGBs,indicating that the TMP,which involves low deformation,could enhance the intergranular damage resistance.

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.

Development of high strength ferrite-martensite stainless steels ( FMSSs) for railway cargo transportation

Two ferrite-martensite stainless steels （FMSSs） were developed by Baosteel based on the T4003 composition, through optimizing the manganese and nickel contents ,reducing silicon, carbon and nitrogen contents, controlling remnant niobium and molybdenum ,adding sufficient titanium and controlling the processing. In this study ,the physical metallurgy of such FMSSs was investigated with the emphasis on the alloying effect on the phase balance during processing and the transformation behavior during welding for different microstructures. In addition, the mechanical behavior and the weldability were investigated. The results indicate that such steels have a good combination of strength and toughness and better weldability compared with the traditional 1. 4003 steel. Such high strength steels are highly suitable for railway cargo transportation where the wall thickness of the wagons can be reduced,resulting in weight savings.

Effects of Orthogonal Heat Treatment on Microstructure and Mechanical Properties of GN9 Ferritic/Martensitic Steel

Microstructure and mechanical properties of GN9 Ferritic/Martensitic steel for sodium-cooled fast reactors have been investigated through orthogonal design and analysis.Scanning electron microscopy(SEM),transmission electron microscopy(TEM),differential scanning calorimeter(DSC),tensile and impact tests were used to evaluate the heat treatment parameters on yield strength,elongation and ductile-to-brittle transition temperature(DBTT).The results indicate that the microstructures of GN9 steel after orthogonal heat treatments consist of tempered martensite,M23C6,MX carbides and MX carbonitrides.The average prior austenite grains increase and the lath width decreases with the austenitizing temperature increasing from 1000°C to 1080°C.Tempering temperature is the most important factor that influences the dislocation evolution,yield strength and elongation compared with austenitizing tempera-ture and cooling methods.Austenitizing temperature,tempering temperature and cooling methods show interactive effects on DBTT.Carbide morphology and distribution,which is influenced by austenitizing and tempering tempera-tures,is the critical microstructural factor that influences the Charpy impact energy and DBTT.Based on the orthogo-nal design and microstructural analysis,the optimal heat treatment of GN9 steel is austenitizing at 1000°C for 0.5 h followed by air cooling and tempering at 760°C for 1.5 h.

Precipitation behavior and martensite lath coarsening during tempering of T/P92 ferritic heat-resistant steel

Tempering is an important process for T/P92 ferritic heat-resistant steel from the viewpoint of microstructure control, as it facili- tates the formation of final tempered martensite under serving conditions. In this study, we have gained deeper insights on the mechanism underlying the microstructural evolution during tempering treatment, including the precipitation of carbides and the coarsening of martensite laths, as systematically analyzed by optical microscopy, transmission electron microscopy, and high-resolution transmission electron mi- croscopy. The chemical composition of the precipitates was analyzed using energy dispersive X-ray spectroscopy. Results indicate the for- mation of M3C （cementite） precipitates under normalized conditions. However, they tend to dissolve within a short time of tempering, owing to their low thermal stability. This phenomenon was substantiated by X-ray diffraction analysis. Besides, we could observe the precipitation of fine carbonitrides （MX） along the dislocations. The mechanism of carbon diffusion controlled growth of M23C6 can be expressed by the Zener＇s equation. The movement of Y-junctions was determined to be the fundamental mechanism underlying the martensite lath coarsening process. Vickers hardness was estimated to determine their mechanical properties. Based on the comprehensive analysis of both the micro- structural evolution and hardness variation, the process of tempering can be separated into three steps.

Dissolution Behavior of Delta Ferrites in Martensitic Heat-resistant Steel for Ultra Supercritical Units Blades

The dissolution behavior of delta ferrites in martensitic heat-resistant steel was studied.And the reason why the dissolution rate of delta ferrites decreased with dissolution time was discussed.The experimental results show that the chemical compositions of delta ferrites negligibly change with dissolution time.The decrease of dissolution rate of delta ferrites with dissolution time should be attributed to the change of shape and distribution of delta ferrites.The shape of delta ferrites tends to transfer from polygon to sphere with dissolution time,causing the decrease of specific surface area of delta ferrites.The distribution position of delta ferrites tends to transfer from boundaries of austenite grains to interior of austenite grains with dissolution time,decreasing the diffusion coefficient of alloy atoms.Both them decrease the dissolution rate of delta ferrites.

Electronic Structures and Alloying Behaviors of Ferrite Phases in High Co-Ni Secondary Hardened Martensitic Steels

The electronic structure of ferrite (tempered martensite phase) in high Co-Ni secondary hardened martensitic steel has been investigated. The local density of states (LOOS) of alloying elements in the steel displays the relationship between solid solubility and the shape of the LDOS. The bond order integral (BOI) between atoms in the steel shows that the directional bonding of the p orbital of Si or C leads to the brittleness of the steel. At last, ΣBOI between atoms demonstrate that C, Co, Mn, Cr, Mo, Si strengthen the alloyed steel through solid-solution effects.

Effect of heat treatment on the behavior ofδ-ferrite in B410D martensitic stainless steel

The morphology and the evolution of δ-ferrite existing in B410D slabs, hot-rolled plates, annealed plates and quenched plates were studied through metallographic observation. Results show that δ-ferrite forms during the solidification process and that it easily grows and increases in quantity during high temperature annealing. Band-shaped δ-ferrite in hotrolled plates is difficult to be eliminated by conventional heat treatment and hard to recrystallize.

Strengthening mechanisms of reduced activation ferritic/martensitic steels:A review

This review summarizes the strengthening mechanisms of reduced activation ferritic/martensitic(RAFM)steels.High-angle grain boundaries,subgrain boundaries,nano-sized M_(23)C_(6),and MX carbide precipitates effectively hinder dislocation motion and increase high-temperature strength.M23C6 carbides are easily coarsened under high temperatures,thereby weakening their ability to block dislocations.Creep properties are improved through the reduction of M23C6 carbides.Thus,the loss of strength must be compensated by other strengthening mechanisms.This review also outlines the recent progress in the development of RAFM steels.Oxide dispersion-strengthened steels prevent M23C6 precipitation by reducing C content to increase creep life and introduce a high density of nano-sized oxide precipitates to offset the reduced strength.Severe plastic deformation methods can substantially refine subgrains and MX carbides in the steel.The thermal deformation strengthening of RAFM steels mainly relies on thermo-mechanical treatment to increase the MX carbide and subgrain boundaries.This procedure increases the creep life of TMT(thermo-mechanical treatment)9Cr-1W-0.06Ta steel by~20 times compared with those of F82H and Eurofer 97 steels under 550℃/260 MPa.

Effects of Austenite Stabilization on the Onset of Martensite Transformation in T91 Steel

The influences of thermal stabilization of austenitic on the onset temperature for a martensite transformation in T91 ferritic heat-resistant steel were studied by high-resolution differential dilatometer. The phase transformation kinetic information was obtained by adopting lever rule from the recorded dilatometric curves. The results show that an inverse stabilization, featured by the damage of ＂the atmosphere of carbon atoms＂ and the increase of the starting temperature for martensite transformation takes place when the T91 ferritic steel is isothermally treated above the Ms point, and it becomes strong with increasing the holding time. While the continued temperature for martensite transformation decreases gradually when isothermally holding at a temperature below Ms point. The observed inverse stabilization behavior could be attributed to the relatively high temperature of Ms point in the explored T91 ferritic heat-resistant steel.

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.

Cavity Swelling in Three Ferritic-Martensitic Steels Irradiated by 196 MeV Kr Ions

We report on cavity swelling at peak damage regions of three ferritic-martensitic(FM)steels(NHS,RAFM and T91)irradiated by 196 MeV Kr ions at different temperatures(450/550℃).Cavity configurations of the irradiated specimens are investigated by transmission electron microscopy with cross-section technique.For home-made reduced activation ferritic-martensitic(RAFM)and T91 steels irradiated at 450℃,both large size and bimodal size distribution of the cavity are found in their peak damage regions,whereas novel high silicon(NHS)steel exhibits good swelling resistance at different irradiation temperatures.Temperature relativity of the cavity swelling in NHS,RAFM and T91 steels is discussed briefly.

Overview of the Research and Development for Reduced Activation Ferritic/Martensitic Steel CLF-1

Recent accomplishment by the SWIP for the reduced activation ferritic/martensitic steel CLF-1 development has been reviewed. It＇s found that CLF- 1 steel has better room temperature tensile properties than Eurofer97 steel and has a fully martensitic microstructure.

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.

Effect of Ni Contents on the Microstructure and Mechanical Properties of Martensitic Stainless Steel Guide Roll by Centrifugal Casting

A novel process based on centrifugal casting was developed to produce martensitic stainless steel for guideroll materials. Centrifugal casting provides a lower production cost and less of the thermal cracking defects which normally occur in the overlaid welding process. In this study, the effects of Ni on the microstructure and mechanical properties of martensitic stainless steel were investigated. The results show that the addition of Ni resulted in a decrease in the volume fraction of delta ferrite and an increase in the volume fraction of the retained austenite, respectively. Moreover, a tensile strength of 1600 MPa with an elongation of 4% were obtained after tempering at 500℃ for 2 h. These values were higher than those obtained by using the conventional overlaid process.

Si合金化对10Cr铁/马钢在液态铅铋共晶环境中的液态金属脆化敏感性的影响

铁素体/马氏体钢是第四代液态铅铋冷却快堆燃料包壳和其他堆内构件的重要候选结构材料。Si合金化是目前国内外改善铁/马钢在铅铋环境中的腐蚀性能的关键技术手段,但Si的加入对应力腐蚀开裂敏感性(即“液态金属脆化”)的影响规律还有待深入研究。本文通过开展慢应变速率拉伸实验,对比研究了4种不同Si含量对10Cr铁素体/马氏体钢在350℃、贫氧和饱和氧铅铋以及氩气环境中的拉伸断裂行为,并结合断口分析,确定了Si对液态金属脆化敏感性的影响规律。结果表明,在所有Si含量下,铅铋对屈服强度和最大抗拉强度均没有显著影响,铅铋的影响主要体现在延伸率出现了明显下降;屈服强度随Si含量的变化基本保持不变,最大抗拉强度则呈现小幅度增大趋势;在氩气环境中,延伸率随Si含量的增加而增大。在贫氧铅铋环境中,延伸率的下降幅度与Si含量大体呈正相关。这说明Si含量越多,10Cr铁素体/马氏体钢的液态金属脆化敏感性越大。

Microstructure Stability of V and Ta Microalloyed 12%Cr Reduced Activation Ferrite/Martensite Steel during Long-term Aging at 650℃

In view of developing novel alloys for applications in supercritical water-cooled reactor fuel cladding and in-core components, a 12%Cr reduced activation ferrite/martensite(RAFM) steel with good corrosion resistance and irradiation performance was developed. V and Ta were added to form fine MX type carbonitrides and enhance the high temperature creep rupture strength. Microstructure stability of the steel during long-term aging at 650 C was studied experimentally combined with the simulation of ThermoCalc and DICTRA software. The results show that the precipitates in the steel during long-term aging contain M23C6, MX and Laves phase. M23C6 carbides play a major role in the stabilization of the tempered martensite lath structure by exerting a large Zener pinning force as compared with MX and Laves phase.Adding V and Ta in the steel can not only promote MX precipitation, but also refine M23C6 carbides and thus improve the thermal stability of lath/subgrains, which is beneficial to the improvement of high temperature microstructure stability of the 12%Cr RAFM steel.

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].

Microstructure failure in ferrite-martensite dual phase steel under in-situ tensile test

To investigate microstructure failure in ferrite-martensite dual phase steel,in-situ observations were performed on multiple plate DP800 specimens during uniaxial tensile tests. Microstructure evolution of the observed region was investigated in details. The experimental data showed that micro-cracks in various regions differed in the initiation time,and micro-failures mainly occurred from the locations with typical characteristics of stress concentration（ i. e. ferrite interiors,the interfaces of ferrite-martensite grains and the martensite-martensite interfaces）. Growth of micro-crack generally experienced the following stages： cracking from martensite boundaries,tiny particles in ferrite interiors,or martensite interiors,propagating in ferrite,bypassing martensite boundaries,or passing through martensite-martensite interfaces,finally ending on martensite boundaries. Martensite was one important source of micro-failure and changed the propagation of micro-cracks significantly. Microstructure deformation was inhomogeneous in the stage of plastic deformation.