Effect of Austenitizing Treatment on Structure and Hardness of Bainite DuctileCast Iron

By continuous quenching process, the effect of austenitizing temperature and time on the structure and hardness of bainiteductile cast iron was studied. It was found that (l) low austenitizing temperature would result in scrap ferrite existing in matrix, whichreduces the macro-hardness of bainite ductile cast iron; (2) high austenitizing temperature would make carbide decomposed, which alsoinduces the macro-hardness of bainite cast iron, and (3) austenitizing time has little effect on the structure of bainite ductile cast iron, butas it increases, the macro-hardness ofbainite ductile cast iron and micro-hardness of bainite increases. To the ductile cast iron, as a result,the suitable austenitizing temperature and time are recommended as 880 and 120 min respectively.

INFLUENCE OF AUSTENITIZING TEMPERATURE ON IMPURITY SEGREGATION ALONG GRAIN BOUNDARIES IN STEEL 4330M

The effect of austenitizing temperature on segregation of impurities along grain boundaries in steel 4330M has been examined by AES.The impurity segregation was computed quantitatively.Results showed that the quantity of impurity segregation changed with the austenitizing temperature.The limitation of the McLean′s expression for equilibrium segrega- tion was discussed.

Microstructural evolution of GCr15 steel during austenitizing and quenching considering C and Cr content

Microstructural evolution of GCr15 steels with different C and Cr contents during austenitizing and quenching was studied. Thermodynamic analysis of cementite dissolution was implied to obtain the critical temperature. The coordination number x in Fe_xCr_(3-x)C and the volume fraction of undissolved cementite were computed according to element conservation and equilibrium phase diagram. The M_S(martensite transformation temperature) was calculated by using empirical formula. The retained austenite content was calculated with further consideration of quenching temperature. The results showed that the coordination number and the undissolved cementite content were promoted by the austenitizing temperature and carbon content of the steel. Increasing Cr element reduced the coordination number.GCr15 steels with different components had nearly the same M_S when austenitization at 830 °C to 860 °C. The interaction of C and Cr complicated the evolution of M_S and retained austenite content. The results were in good agreement with the literature, which could guide to obtain specified retained austenite and/or carbides.

Effect of Cast Thickness and Austenitizing Temperature on Microstructure and Mechanical Properties of ADI and IADI Castings

This investigation studies the impact strength, tensile strength, hardness, and wear behavior of thin wall austempered and intercritically austempered ductile iron samples with a chemical composition of 3.37% C, 2.7% Si, 0.30% Mn, 0.01% S, and 0.01% P. The austempered samples were austenitized at 900?C for 1 h and rapidly quenched in a salt bath furnace at 375?C for 1 h. The intercritically austempered samples were sub-austenitized at 810?C for 1 h and rapidly quenched in a salt bath furnace at 375?C for 1 h. The properties of the austempered and intercritically austempered thin wall plates of 5, 10, and 15 mm thickness were evaluated and compared to the as-cast samples. Austempering process affects greatly the tensile properties of all cast thicknesses where ultimate strength reached 1004 MPa for 5-mm thickness. Optimum impact toughness of 40 J was obtained for the austempered samples of 10- and 15-mm thicknesses. The intercritically austempered samples showed properties between the austempered and as-cast samples. Maximum wear resistance was also reported for the austempered samples due to containing retained austenite in the structure which in turn transformed into martensite that increases well the wear resistance. Maximum ultimate strength (1056) MPa and hardness (396 HV) were obtained for 5 mm ADI sample. Maximum impact toughness (43 J) was achieved for 15 mm IADI sample due to existing of pro-eutectoid ferrite in matrix. For all As-cast, ADI and IADI irons, wear resistance decreased with increasing sample thickness. Minimum wear rate (2.22 × 10?6 g/s) was reported for 5-mm ADI sample and maximum one (15.8 × 10?6 g/s) was registered for 15-mm as-cast DI sample, at a sliding speed of 2 m/s.

Effect of austenitizing condition on mechanical properties,microstructure and precipitation behavior of AISI H13 steel

The effects of austenitizing temperature(1223,1303,and 1373 K)and holding time(1-1500 s)on the microstructure,mechanical properties,and precipitation behavior of the H13 hot work die steel were investigated.The results indicate a softening phenomenon when H13 steel is austenitized at 1303 K beyond 900 s and 1373 K beyond 600 s,respectively.For the sample held for 1200 s,the tensile strength is found capable of reaching up to 2.2 GPa when quenched from a temperature above 1303 K.Meanwhile,prior-austenite grain size increases with the increase in austenitizing temperature.The kinetic behavior of the precipitates(mainly MC-type carbides)in H13 steel could be elaborated through the principles set forth by the Arrhenius and Avrami equations.Finally,the comprehensive strengthening of the H13 steel was discussed in detail.The results show that the activation energy of the transformed fraction of carbides is higher than that of the diffusion process for common alloying elements(Cr,V,Mo,and Ni)found in the austenite.This suggests that it would be difficult for precipitates to dissolve into the matrix when H13 steel is austenitized at high temperatures.With the increasing austenitizing temperature,the precipitation fraction decreases,and the dislocation density increases.The dislocation strengthening is regarded as the dominant strengthening contributed to yield strength in as-quenched H13 steel.

Reheating Austenitizing Temperature of Spring Steel 60Si2MnA for Railway

The microsturctural transformation of austenite grain, pearlite interlamellar spacing, and lamellar cement ite thickness of spring steel 60Si2MnA for railway were studied in the hot-rolled and reheated states. Furthermore, the effect of microstructural characterization on its final mechanical properties was discussed. The results showed that as far as 60Si2MnA, the pearlite interlamellar spacing determined the hardness, whereas, the austenite grain determined the toughness. Compared with microstructure and mechanical properties in the hot rolled state, after reheating treatment at 950 ℃, its average grain sizes are apparently fine and the pearlite interlamellar spacing and lamellar cementite thickness coarsen to some extent, but both hardness and impact toughness increase to HRC 48 and 8.5 J, respectively. In the course of making spring, the optimum reheating austenitizing temperature for the 60Si2MnA steel is 950 ℃.

Influence of Austenitizing Temperature on the Microstructure and Corrosion Resistance of 55Cr18Mo1VN High-Nitrogen Plastic Mould Steel

The influence of austenitizing temperature on the microstructure and corrosion resistance of 55Cr18MolVN high-nitrogen plastic mould steel was investigated. The microstructure, elemental distribution and Cr-depleted zone of different heat-treated samples were investigated by X-ray diffraction, electron probe microanalyzer analysis, and trans- mission electron microscopy. The corrosion resistance was evaluated using electrochemical measurements, and the analysis of passive film was carded out by X-ray photoelectron spectroscopy. The results indicated that the volume fraction of precipitates decreased, and the homogeneity of elements was improved with increasing austenitizing temperature. The degree of Cr-depleted zone around coarse M23C6 was severer than that around M2N, and pitting corrosion initiated preferentially around M23C6. The corrosion resistance of the samples increased with the austenitizing temperature. With the increase in austenitizing temperature, the passive film was thickened and Cr（III）cr2O3 in the inner layer of passive film was enriched, which enhanced the corrosion resistance of the steel. The higher content of nitrogen in solid solution at higher austenitizing temperature contributed to the increased intensity of CrN and NH3, leading to the increase in pH value in the pit, and promoting the repassivation of 55Cr18Mo1N steel.

Review on the plastic instability of medium -Mn steels for identifying the formation mechanisms of Lüders and Portevin -Le Chatelier bands

Plastic instability,including both the discontinuous yielding and stress serrations,has been frequently observed during the tensile deformation of medium-Mn steels(MMnS)and has been intensively studied in recent years.Unfortunately,research results are controversial,and no consensus has been achieved regarding the topic.Here,we first summarize all the possible factors that affect the yielding and flow stress serrations in MMnS,including the morphology and stability of austenite,the feature of the phase interface,and the deformation parameters.Then,we propose a universal mechanism to explain the conflicting experimental results.We conclude that the discontinuous yielding can be attributed to the lack of mobile dislocation before deformation and the rapid dislocation multiplication at the beginning of plastic deformation.Meanwhile,the results show that the stress serrations are formed due to the pinning and depinning between dislocations and interstitial atoms in austenite.Strain-induced martensitic transformation,influenced by the mechanical stability of austenite grain and deformation parameters,should not be the intrinsic cause of plastic instability.However,it can intensify or weaken the discontinuous yielding and the stress serrations by affecting the mobility and density of dislocations,as well as the interaction between the interstitial atoms and dislocations in austenite grains.

Review on synergistic damage effect of irradiation and corrosion on reactor structural alloys

The synergistic damage effect of irradiation and corrosion of reactor structural materials has been a prominent research focus.This paper provides a comprehensive review of the synergistic effects on the third-and fourth-generation fission nuclear energy structural materials used in pressurized water reactors and molten salt reactors.The competitive mechanisms of multiple influencing factors,such as the irradiation dose,corrosion type,and environmental temperature,are summarized in this paper.Conceptual approaches are proposed to alleviate the synergistic damage caused by irradiation and corrosion,thereby promoting in-depth research in the future and solving this key challenge for the structural materials used in reactors.

Microstructure and mechanical properties of a cast TRIP-assisted multiphase stainless steel

Stainless steels are used in a wide range of complex environments due to their excellent corrosion resistance.Multiphase stainless steels can offer an excellent combination of strength,toughness and corrosion resistance due to the coexistence of different microstructures.The microstructure and mechanical properties of a novel cast multiphase stainless steel,composed of martensite,ferrite,and austenite,were investigated following appropriate heat treatment processes:solution treatment at 1,050℃ for 0.5 h followed by water quenching to room temperature,and aging treatment at 500℃ for 4 h followed by water quenching to room temperature.Results show reversed austenite is formed by diffusion of Ni element during aging process,and the enrichment of Ni atoms directly determines the mechanical stability of austenite.The austenite with a lower Ni content undergoes a martensitic transformation during plastic deformation.The tensile strength of the specimen exceeds 1,100 MPa and the elongation exceeds 24%after solid solution,and further increases to 1,247 MPa and 25%after aging treatment.This enhancement is due to the TRIP effect of austenite and the precipitation of the nanoscale G-phase pinning dislocations in ferrite and martensite.

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.

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.

Development and microstructure characterization of BG110E high-strength expansion pipe

The BG110E high-strength expansion pipe was developed using medium manganese steel and subjected to a two-phase zone heat treatment process.Mechanical properties and microstructure analysis results have proven that the BG110E expansion pipe exhibits uniform elongation of more than 19%.Moreover,after undergoing expan-sion deformation,its strength,toughness,and plasticity are found to meet the stringent requirements of the P110 pipe.The microstructure of this high-strength expansion pipe,which has a strength of 110 ksi(1 ksi=6.895 MPa),consists of tempered martensite,ferrite,retained austenite,and granular bainite.The propotion of retained austenite reaches up to 12%,ensuring high plasticity and the occurrence of the transformation-induced plasticity effect during the deformation process.Consequently,it enhances the coordinated deformation ability between different phases,which significantly improves the internal yield pressure of the BG110E high-strength expansion pipe in turn.

Research on Heredity of Coarse Ferrite Grains

The changes in austenite grain size of the specimens with coarse ferrite grains under different heat treatment process were investigated.The focus was on studying the effect of annealing on refining coarse ferrite grains,as well as the influence of the ferrite grain size on the main technical indicators of gas carburizing.The results show that coarse ferrite grains may not necessarily cause the coarse austenite grains,but may result in mixed austenite grains.After annealing treatment,the coarse ferrite grains can be significantly refined and homogenized.Moreover,the coarse ferrite grains have no significant effects on hardnessand intergranular oxidationof gas carburizing.

Microstructure evolution and mechanical properties influenced by austenitizing temperature in aluminum-alloyed TRIP-aided steel

The Fe-0.21C 2.2Mn 0.49Si-1.77A1 transformation induced plasticity （TRIP） aided steel was heat trea- ted at various austenitizing temperatures under both TRiP-aided polygonal ferrite type （TPF） and an- nealed martensite matrix （TAM） processes. The microstructure evolution and their effects on mechanical properties were systematically investigated through the microstructure observation and dilatometric analysis. The microstructure homogeneity is improved in TPF steel heated at a high temperature due to the reduced banded martensite and the increased bainite. Compared with the mechanical properties of the TPF steels, the yield strength and elongation of the TAM steels are much higher, while the tensile strength is lower than that of TPF steels. The stability of intercritical austenite is affected by the heating tempera- ture, and thus the following phase transformation influences the mechanical properties, such as the bain- ite transformation and the precipitation of polygonal ferrite. Obvious dynamic bainite transformation occurs at TAM850, TAM900 and TAM950, More proportion of polygonal ferrite is found in the sample heated at 950 ℃. The bainite transformation beginning at a higher temperature results in the wider bainitic ferrite laths. The more proportion of polygonal ferrite and wide bainitic ferrite laths commonly contribute to the lower strength and better elongation. The uniform microstructure with lath-like morphology and retained austenite with high average carbon content ensures a good mechanical property in TAM850 with the product of strength and elongation of about 28 GPa ·%,

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.

Effect of heat treatment on microstructure and mechanical properties of Ti-containing low alloy martensitic wear-resistant steel

Effects of quenching temperature and cooling conditions(water cooling and 10%NaCl cooling)on microstructure and mechanical properties of a 0.2%Ti low alloy martensitic wear-resistant steel used for die casting ejector plate were investigated.The results show that lath martensite can be obtained after austenitizing in the range of 860-980℃and then water cooling.With an increase in austenitizing temperature,the precipitate content gradually decreases.The precipitates are mainly composed of TiC and Ti4C2S2,and their total content is between 1.15wt.%and 1.64wt.%.The precipitate phase concentration by water-cooling is higher than that by10%NaCl cooling due to the lower cooling rate of water cooling.As the austeniting temperature increases,the hardness and tensile strength of both water cooled and 10%NaCl cooled steels firstly increase and then decrease.The experimental steel exhibits the best comprehensive mechanical properties after being austenitized at 900℃,cooled by 10%NaCl,and then tempered at 200℃.Its hardness,ultimate tensile strength,and wear rate reach551.4 HBW,1,438.2 MPa,and 0.48×10^(-2)mg·m^(-1),respectively.

Effect of Chromium-enhanced Diffusivity on Reverted Transformation in Metastable Austenitic Stainless Steels:Theoretical Calculation and Experiment

Effects of nucleation sites and diffusivity enhancement of chromium on reverted transformation of AISI 304 stainless steel during annealing process were investigated.Dynamics calculation revealed that the reverted transformation of strain-inducedα’-martensite→γaustenite could were closely associated with active nucleation sites and diffusivity enhancement of chromium in nanocrystallineα’-martensite.The experimental data and the results were in accordance with 2-grain austenite/α’-martensite junctions calculated theoretically,which could result from high chromium diffusion rate in nanocrystallineα’-martensite.In addition,low temperature is not conducive to reversed transformation,while high temperature and long annealing time will lead to inhomogeneous grain size distribution.

Tailoring Carbon Distribution inα/γPhase of Ductile Iron and Its Effects on Thermal Conductivity

The effects of carbon distribution on the microstructure and thermal conductivity of ductile iron were investigated in the present study.The microstructure of as-cast and quenched ductile iron were characterized by OM and SEM.Results showed that the microstructure of as-cast ductile iron was composed of spheroidal graphite,ferrite with the volume of 80%,and a small amount of pearlite,and quenched ductile iron was composed of spheroidal graphite,coarse/fine acicular martensite(α_(M)phase)and high-carbon retained austenite(γphase).The volume fraction of retained austensite and its carbon content for direct quenched ductile iron and tepmered ductile iron were quantitatively analysed by XRD.Results revealed that carbon atoms diffused fromα_(M)phase toγphase during tempering at low temperatures,which resulted in carbon content in retainedγphase increasing from 1.2 wt%for the direct quenched sample to about 1.9 wt%for the tempered samples.Consequently,the lattice distortion was significantly reduced and gave rise to an increase of thermal conductivity for ductile iron.