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.

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.

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.

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.

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.

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.

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.

Effect of cooling rate on microstructure and mechanical properties of CB2 tempered martensitic steel

The cooling rate of casting has a significant effect on microstructure and mechanical properties of castings.The 9Cr-1.5Mo-1Co cast steel,referred to as CB2,is one of the most promising alloys for various cast components to be used under ultrasupercritical conditions.In this study,HRTEM,SEM,and XRD methods were used for qualitative and quantitative analyses of contents,phases,and sizes of the inclusions and precipitates,as well as microstructure observation of the tempered martensitic steel at different cooling rates.Traditional tensile tests were conducted to characterize the material mechanical properties.Results show that the size of the boron nitride and precipitate,the width of the martensite lath and the content of theδ-ferrite are reduced as the cooling rate increases from 5-8℃·min^-1(CB2-S steel)to 50-60℃·min^-1(CB2-F steel).The precipitates are mainly composed of M23C6 and a small amount of M3C.The average diameters of the M23C6 precipitates in CB2-F and CB2-S are 202 nm and 209 nm,respectively.The inclusions are mainly composed of BN,Al2O3 and MnO2,and the inclusion ratios are 0.1969%for the CB2-F and 0.06556%for CB2-S steel.The average martensite lath widths of CB2-F and CB2-S steels are 289 nm and 301 nm,respectively.Furthermore,the M3C having a diameter of about 150 nm and a thickness of 20 nm is observed in theδ-ferrite of the tempered CB2-S steel.The presence of theδ-ferrite reduces the precipitation strengthening and dislocation density in CB2-S steel.In addition,the lower cooling rate stabilizes theδ-ferrite structure at room temperature.

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 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.

Atomistic study on the microscopic mechanism of grain boundary embrittlement induced by small dense helium bubbles in iron

The helium bubbles induced by 14 MeV neutron irradiation can cause intergranular fractures in reduced activation ferritic martensitic steel,which is a candidate structural material for fusion reactors.In order to elucidate the susceptibility of different grain boundaries(GBs)to helium-induced embrittlement,the tensile fracture processes of 10 types of GBs with and without helium bubbles in body-centered cubic(bcc)iron at the relevant service temperature of 600 K were investigated via molecular dynamics methods.The results indicate that in the absence of helium bubbles,the GBs studied here can be classified into two distinct categories:brittle GBs and ductile GBs.The atomic scale analysis shows that the plastic deformation of ductile GB at high temperatures originates from complex plastic deformation mechanisms,including the Bain/Burgers path phase transition and deformation twinning,in which the Bain path phase transition is the most dominant plastic deformation mechanism.However,the presence of helium bubbles severely inhibits the plastic deformation channels of the GBs,resulting in a significant decrease in elongation at fractures.For bubble-decorated GBs,the ultimate tensile strength increases with the increase in the misorientation angle.Interestingly,the coherent twin boundary∑3{112}was found to maintain relatively high fracture strength and maximum failure strain under the influence of helium bubbles.

Stress-strain partitioning analysis of constituent phases in dual phase steel based on the modified law of mixture

A more accurate estimation of stress-strain relationships for martensite and ferrite was developed, and the modified law of mixture was used to investigate the stress-strain partitioning of constituent phases in dual phase （DP） steels with two different martensite volume fractions. The results show that there exist great differences in the stress-strain contribution of martensite and ferrite to DP steel. The stress-strain partitioning coefficient is not constant in the whole strain range, but decreases with increasing the true strain of DP steel. The softening effect caused by the dilution of carbon concentration in martensite with the increase of martensite volume fraction has great influence on the strain contribution of martensite. The strain ratio of ferrite to martensite almost linearly increases with increasing the true strain of DP steel when the martensite volume fraction is 22%, because martensite always keeps elastic. But the strain ratio of ferrite to martensite varies indistinctively with the further increase in true strain of DP steel above 0.034 when the martensite volume fraction is 50%, because plastic deformation happens in martensite. The stress ratio ofmartensite to ferrite decreases monotonously with increasing the true strain of DP steel whether the martensite volume fraction is 22% or 50%.

Effect of deformation and cooling rate on the transformation behavior and microstructure of X70 steels

The effects of the deformation in the non-recrystallization region of austenite and the cooling rate on the transformation behavior and microstructure of low-carbon low-alloy steel for pipeline application were studied on the thermal-mechanical simulator Gleeble-1500. It was shown that an increase in deformation amount can greatly increase the nucleation site of ferrite when deformed in the non-recrystallization region of austenite, and an increase in nucleation ratio can greatly refine grains. When the cooling rate is accelerated, the driving force of nucleation is increased and the nucleation rate also improves. Ultra-refine grains can be obtained by controlled rolling. The high density of ferrite nucleus, which forms along the austenite grain boundary, twin interface, and deformation band are introduced in the matrix of austenite by the control of hot rolling, after which the microstructure can be refined. It was found that the acicular ferrite has a very fine sub-structure, high dislocation density, and a thin slab with ultra-fine grains. Small M/A islands and cementite are precipitated on the matrix of the slabs by the analysis technique of TEM and SEM.

Technical Issues for the Fabrication of a CN-HCCB-TBM Based on RAFM Steel CLF-1

Reduced activation ferritic/martensitic steel （RAFM） is recognized as the primary candidate structural material for ITER＇s test blanket module （TBM）. To provide a material and property database for the design and fabrication of the Chinese helium cooled ceramic breeding TBM （CN HCCB TBM）, a type of RAFM steel named CLF-1 was developed and chaxacter^zed at the Southwestern Institute of Physics （SWIP）, China. In this paper, the R＆D status of CLF-1 steel and the technical issues in using CLF-1 steel to manufacture CN HCCB TBM were reviewed, including the steel manufacture and different welding technologies. Several kinds of property data have been obtained for its application to the design of the ITER TBM.

Microstructure and Property of Coarse Grain HAZ X80 Pipeline Steel

The coarse grain HAZ microstructure and property of X80 pipeline steel with different carbon content was investigated. The weld thermal simulation test was carried out on Gleeble 1500 thermal mechanical test machine. The Charpy tests were completed at --20 ℃ for evaluating the toughness of coarse grain heat affected zone （CGHAZ）. The microstructure was examined by optical microscope （OM） and transmission electron microscopy （TEM）, and the austenite constituent was quantified by X-ray diffraction. The results showed that the ultra-low carbon can improve the toughness of CGHAZ by suppressing the formation of carbide, decreasing the martensite austenite （M-A） constituent and increasing the residual austenite in the M A.

Effect of Nb on the transformation kinetics of low carbon(manganese) steel during deformation of undercooled austenite

The hot compression tests using Gleeble 1500 were performed by varying the true strain up to 1.6 （80% reduction） in Nbfree and Nb-microalloyed steels. The effect of Nb addition on the transformation kinetics during deformation of undercooled austenite was investigated. It was found that as compared with Nb-free steel, the transformation incubation period of Nb-bearing steel was prolonged and the transformation kinetics curves parallelly moved to higher strain because of the solute Nb drag effect. Studies on kinetics also showed that the deformation-enhanced ferrite transformation （DEFT） of the two steels were composed of three stages, which can be expressed by the J-M-A equations individually. However, the parameter n related to the mode of nucleation and growth is somewhat different in the first and second stages of the two steels, and the same in the third stage for both the steels corresponding to the nucleation Of retained austenite.