Process Control for Structure of Martensite/Austenite Islands of 15.3 mm X100 Pipeline Steel in BX Steel

This paper mainly studies the influence of deformation amount and cooling rate,which existed in X100 pipeline steel rolled at 2300mm rolling mill in BX STEEL,on morphology,size relations and distribution of M/A islands of steel plate structure.Experimental results show that structures of M/A islands are homogenized and refined under the conditions of appropriately increasing the deformation amount in non-recrystallization region on basis of rational deformation amount in recrystallization region and cooling rate after deformation,respectively.Finally,according to the actual situation of 2300mm hot rolling mill,this paper gives rational deformation amount and cooling rate used to obtain ideal structure of M/A islands.

Effect of deformation parameters on the austenite dynamic recrystallization behavior of a eutectoid pearlite rail steel

Understandings of the effect of hot deformation parameters close to the practical production line on grain refinement are crucial for enhancing both the strength and toughness of future rail steels.In this work,the austenite dynamic recrystallization(DRX)behaviors of a eutectoid pearlite rail steel were studied using a thermo-mechanical simulator with hot deformation parameters frequently employed in rail production lines.The single-pass hot deformation results reveal that the prior austenite grain sizes(PAGSs)for samples with different deformation reductions decrease initially with an increase in deformation temperature.However,once the deformation temperature is beyond a certain threshold,the PAGSs start to increase.It can be attributed to the rise in DRX volume fraction and the increase of DRX grain with deformation temperature,respectively.Three-pass hot deformation results show that the accumulated strain generated in the first and second deformation passes can increase the extent of DRX.In the case of complete DRX,PAGS is predominantly determined by the deformation temperature of the final pass.It suggests a strategic approach during industrial production where part of the deformation reduction in low temperature range can be shifted to the medium temperature range to release rolling mill loads.

Interface Migration between Martensite and Austenite during Quenching and Partitioning (Q&P) Process

An Fe-0.2C-1.5Si-1.67Mn steel was subjected to quenching and partitioning （Q＆P） process, and the interface migration between martensite and austenite at an elevated partitioning temperature was observed. The interface migration is excluded in constrained paraequilibrium （CPE） model. Based on ＂endpoint＂ predicted by CPE model the thermodynamic condition of interface migration is analyzed, that is, the difference in the chemical potential of iron in both ferrite （martenisite） and austenite produces the driving force of the iron atoms to migrate from one phase to the other phase. In addition, the interface migration can change the austenite fraction; as a result, the austenite fraction at partitioning temperature may be higher than that at quenching temperature through the interface migration, but this phenomenon cannot be explained by CPE model.

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.

Effect of lower bainite/martensite/retained austenite triplex microstructure on the mechanical properties of a low-carbon steel with quenching and partitioning process

We present a study concerning Fe-0. 176C-1.31Si-1.58Mn-0.26Al-0.3Cr （wt%） steel subjected to a quenching and partitioning （Q＆P） process. The results of scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and tensile tests demon- strate that the microstructures primarily consist of lath martensite, retained austenite, lower bainite （LB）, and a small amount of tempered martensite; moreover, few twin austenite grains were observed. In the microstrucmre, three types of retained austenite with different sizes and morphologies were observed： blocky retained austenite （-300 nm in width）, film-like retained austenite （80-120 nm in width）, and ul- tra-fine film-like retained austenite （30-40 nm in width）. Because of the effect of the retained austenite/martensite/LB triplex microstructure, the specimens prepared using different quenching temperatures exhibit high ultimate tensile strength and yield strength. Furthermore, the strength effect of LB can partially counteract the decreasing strength effect of martensite. The formation of LB substantially reduces the amount of retained austenite. Analyses of the retained austenite and the amount of blocky retained austenite indicated that the carbon content is critical to the total elongation of Q＆P steel.

HREM STUDY OF THE AUSTENITE/MARTENSITE INTERFACES IN AN Fe-9Ni ALLOY

The atomic level structures of fcc/bcc interfaces in an Fe-9Ni alloy have been examined by means of HREM. It has been found that the orientation of γ prime /α interface had great effects on its atomic structure. These interfaces with different orientations may be composed of only structural ledges, structural ledges with misfit dislocations, superledges with misfit dislocations or only misfit dislocations. A structural model of growth ledge was suggested. The terrace of growth ledge was composed of structural ledges or misfit dislocations. The atomic structure of the riser of growth ledge was the same as that of the side facet of γ prime lath, which is composed of superledges with misfit dislocations.

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.

RESIDUAL AUSTENITE AND STRAIN-INDUCED MARTENSITE IN LASER HARDENING LAYER ON GRAY IRON

The distribution of residual austenite in the laser hardening laver on the gray cast iron and the change in the amount of residual austenite during sliding wearing have been investigated by X-ray diffractometer.The thin foils of the hardening layer worn down have been observed by electron microscopy.It was revealed that two types of martensite are strain-induced by slid- ing wearing under load of 1.72 MPa on the hardening layer of residual austenite.The strain induced martensite is profitable to improve the sliding wearing 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.

Physical Simulation of Thermally Induced Martensite Formation from Retained Austenite in TRIP Steels

The present work analyses the total free energy of the material during the martensitic transformation. A general expression for the martensite fraction as a function of temperature is derived, assuming that the nonchemical free energyis proportional to the volume of martensite. This expression indicates that the temperature-dependent martensitefraction can be predicted once the characteristic transformation temperatures and the relation between the chemicalfree energy and temperature of the martensite and austenite are known. An advantage of this development is thatthe proposed equation is valid for all types of relations between the chemical free energy and temperature. Thissimulation is successfully applied to the martensitic transformation upon further cooling of retained austenite in alow-alloyed TRIP steel, in which the relation between chemical free energy and temperature is quadratic and thefraction is determined from a thermo-magnetic measurement.

Analysis of phase transformation from austenite to martensite in NiTi alloy strips under uniaxial tension

Phase transformation from austenite to martensite in NiTi alloy strips under the uniaxial tension has been observed in experiments and numerically simulated as a localized deformation. This work presents an analysis using the theory of phase transformation. The jump of deformation gradient across the interface between two phases and the Maxwell relation are considered. Governing equations for the phase transformation are derived. The analysis is reduced to finding the minimum value of the loading at which the governing equations have a unique, real and physically acceptable solution. The equations are solved numerically and it is verified that the unique solution exists definitely. The Maxwell stress, the stresses and strains inside both austenite and martensite phases, and the transformation-front orientation angle are determined to be in reasonably good agreement with experimental observations.

Effect of Austenite Pre-deformation on Isothermal Martensite Transformation in an Fe-20.5Ni-4.8Mn Alloy

With electron microscopy the investigation on isothermal martensite transformation in an Fe20.5Ni-4.8Mn alloy has been carried out to clarify the effect of austenite state on the transformation, by applying pre-deformation to austenite before isothermal holding. Under the condition without pre-deformation, the isothermal martensite products are lath martensite with {111}fhabit planes. Dislocations in austenite seem to contribute to nucleation of martensite, and in this nascent Stage austenite substructure has no obvious effect on martensite growth. The consequent thickening of martensite laths is apparently influenced by local austenite states, resulting in the changes in orientation, morphology as well as substructure of martensite lath. The kinetics of isothermal martensite transformation is controlled by intedece dislocation determined nucleation of martensite in primary stage, but to a larger extent, by the austenite accommodation for the shape strain of martensite in the thickening Stage

Effect of Si on austenite stabilization,martensite morphology,and magnetic properties in Fe-26%Ni-x%Si alloys

The effect of Si on the austenite stabilization, martensite morphology, and magnetic properties in Fe-26%Ni-x%Si （x=3.5, 5, and 6） alloys have been studied by means of transmission electron microscopy （TEM） and M？ssbauer spectroscopy techniques. TEM observations reveal that the martensite morphology is closely dependent on the Si content. The volume fraction changes of martensite and austenite phases, the hyperfine magnetic field, and isomer shift values have been determined by Mssbauer spectroscopy. The M？ssbauer study reveals that the hyperfine magnetic field, the isomer shift values and the volume fraction of martensite decrease with increasing Si content.

Kinetics of austenite growth and bainite transformation during reheating and cooling treatments of high strength microalloyed steel produced by sub-rapid solidification

First,strip cast samples of high strength microalloyed steel with sub-rapid solidification characteristics were prepared by simulated strip casting technique.Next,the isothermal growth of austenite grain during the reheating treatment of strip casts was observed in situ through confocal laser scanning microscope(CLSM).The results indicated that the time exponent of grains growth suddenly rise when the isothermal temperature higher than 1000℃.And the activation energy for austenite grain growth were calculated to be 538.0 kJ/mol in the high temperature region(above 1000℃)and 693.2 kJ/mol in the low temperature region(below 1000℃),respectively.Then,the kinetics model of austenite isothermal growth was established,which can predict the austenite grain size during isothermal hold very well.Besides,high density of second phase particles with small size was found during the isothermal hold at the low temperature region,leading to the refinement of austenite grain.After isothermal hold at different temperature for 1800 s,the bainite transformation in microalloyed steel strip was also observed in situ during the continuous cooling process.And growth rates of bainite plates with different nucleation positions and different prior austenite grain size(PAGS)were calculated.It was indicated that the growth rate of the bainite plate is not only related to the nucleation position but also to the PAGS.

Predicting the Volume Fraction of Martensite in Welded Mild Steel Joint Reinforced with Titanium Alloy Powder

Welded mild steel is used in different applications in engineering. To strengthen the joint, the weld can be reinforced by adding titanium alloy powder to the joint. This results in the formation of incomplete martensite in a welded joint. The incomplete martensite affects mechanical properties. Therefore, this study aims to predict the volume fraction of martensite in reinforced butt welded joints to understand complex phenomena during microstructure formation. To do so, a combination of the finite element method to predict temperature history, and the Koistinen and Marburger equation, were used to predict the volume fraction of martensite. The martensite start temperature was calculated using chemical elements obtained from the dilution-based mixture rule. The curve shape of martensite evolution was observed to be relatively linear due to the small quantity of martensite volume fraction. The simulated result correlated with experimental work documented in the literature. The model can be used in other powder addition techniques where the martensite can be observed in the final microstructure.

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.

INFLUENCE OF AUSTENITE STRENGTHENING ON MARTENSITIC TRANSFORMATION IN Fe-Mn-C ALLOYS

The M_s temperature and the yield strength of austenite at M_s temperature have been meas- ured for five Fe-Mn-C alloys.The experimental results show that there is a linear relation- ship between them.The effect of the solution strengthening of austenite on martensite morphology is also studied.It is pointed out that there is a characteristic temperature T_c in austenite strengthening.Martensite morphology is mainly of dislocated laths when M_s>T_c, and is mainly of twinned plates when M_s<T_c.A theoretical analysis is given which is in good agreement with experimental results.

Pre-existing orthorhombic embryos-induced hexagonal-orthorhombic martensitic transformation in MnNiSi_(1-x)(CoNiGe)_x alloy

The thermal-elastic martensitic transformation from high-temperature Ni_(2)In-type hexagonal structure to low-temperature TiNiSi-type orthorhombic structure has been widely studied in MnMX(M=Ni or Co,and X=Ge or Si)alloys.However,the answer to how the orthorhombic martensite nucleates and grows within the hexagonal parent is still unclear.In this work,the hexagonal-orthorhombic martensitic transformation in a Co and Ge co-substituted MnNiSi is investigated.One can find some orthorhombic laths embedded in the hexagonal parent at a temperature above the martensitic transformation start temperature(M_(s)).With the the sample cooing to M_(s),the laths turn broader,indicating that the martensitic transformation starts from these pre-existing orthorhombic laths.Microstructure observation suggests that these pre-existing orthorhombic laths do not originate from the hexagonal-orthorhombic martensitic transformation because of the difference between atomic occupations of doping elements in the hexagonal parent and those in the preexisting orthorhombic laths.The phenomenological crystallographic theory and experimental investigations prove that the pre-existing orthorhombic lath and generated orthorhombic martensite have the same crystallography relationship to the hexagonal parent.Therefore,the orthorhombic martensite can take these pre-existing laths as embryos and grow up.This work implies that the martensitic transformation in MnNiSi_(1-x)(CoNiGe)_(x) alloy is initiated by orthorhombic embryos.

Effect of Mn addition on microstructure and mechanical properties of GX40CrNiSi25-12 austenitic heat resistant steel

Three types of steels were designed on the basis of GX40CrNiSi25-12 austenitic heat resistant steel by adding different Mn contents(2wt.%,6wt.%,and 12wt.%).Thermodynamic calculation,microstructure characterization and mechanical property tests were conducted to investigate the effect of Mn addition on the microstructure and mechanical properties of the austenitic heat resistant steel.Results show that the matrix structure in all the three types of steels at room temperature is completely austenite.Carbides NbC and M_(23)C_(6)precipitate at grain boundaries of austenite matrix.With the increase of Mn content,the number of carbides increases and their distribution becomes more uniform.With the Mn content increases from 1.99%to 12.06%,the ultimate tensile strength,yield strength and elongation increase by 14.6%,8.0%and 46.3%,respectively.The improvement of the mechanical properties of austenitic steels can be explained by utilizing classic theories of alloy strengthening,including solid solution strengthening,precipitation strengthening,and grain refinement.The increase in alloy strength can be attributed to solid solution strengthening and precipitation strengthening caused by the addition of Mn.The improvement of the plasticity of austenitic steels can be explained from two aspects:grain refinement and homogenization of precipitated phases.

Microstructure and martensitic transformation in quaternary NiTiHfV alloy

The effect of age hardening on the microstructure,martensitic transformation behavior,and shape memory properties of the(Ni_(50)Ti_(30)Hf_(20))_(95)V_(5)alloy was investigated by scanning electron microscopy,transmission electron microscopy,X-ray diffraction,differential scanning calorimetry,microhardness,and bending tests.The results demonstrate a significant influence of V addition on the microstructure of the alloy.V addition leads to the formation of a(Ni,V)_(2)(Ti,Hf)-type Laves phase,which coexists with B19'martensite at room temperature.Aging at 550℃results in precipitation hardening due to the formation of nano-scale orthorhombic H-phase,with the peak hardness observed after 3 h of aging.The alloy at peak hardness state exhibits higher transformation strain and lower unrecovered strain compared to the solution-treated sample.The aged sample achieves a maximum transformation strain of 1.56%under 500 MPa.