MECHANICAL STABILIZATION OF DEFORMED AUSTENITEDURING CONTINUOUS COOLING TRANSFORMATION IN AC-Mn-Cr-Ni-Mo PLASTIC DIE STEEL

The influence of prior austenite deformed at different temperature on the subsequent continuous cooling bainitic transformation has been investigated in an C-Ma-Cr-Ni-Mo plastic die steel. The results show that the prior deformation in low temperature region of austenite retards significantly the bainitic transformation. For the same continuous cooling schedule, as austenite deformed at lower temperature, the quantity of the classical sheaf-like bainite becomes less. The present results show that severe deformation leads to mechanical stabilization of austenite and causes the difficulty of bainitic ferrite propagation into the austenite.

Effects of Deformation on Bainite Transformation During Continuous Cooling of Low Carbon Steels

Hot deformation experiments were carried out on Gleeble 1500 thermo-mechanical simulator. The bainite transformation after deformation was investigated by optical microstructure analysis. The results indicated that the deformation accelerated the bainite transformation when the deformation was carried out at high temperature and no or little ferrite was precipitated before bainite transformation; when the deformation was carried out at low temperature, the deformation hindered the bainite transformation because a lot of ferrite precipitated before bainite transformation.

Continuous cooling precipitation diagram of high alloyed Al-Zn-Mg-Cu 7049A alloy

The precipitation behaviour during cooling from solution annealing of high alloyed 7049A aluminium alloy was investigated, covering the complete cooling-rate-range of technical interest. This ranges from slow cooling rates close to equilibrium up to rates above complete supersaturation and is covering seven orders of magnitude in cooling rate （0.0005 to 5000 K/s）. The continuous cooling precipitation behaviour of 7049A alloy was recorded by combining different differential scanning calorimetry （DSC） techniques and microstructure analysis by SEM and Vickers hardness testing. The high alloyed, high strength and quench sensitive wrought aluminium alloy 7049A was investigated during quenching from solution annealing by conventional DSC in the cooling rate range of 0.0005 to 4 K/s. In this range at least two exothermal precipitation reactions were observed： a high temperature reaction in a narrow temperature interval of 450-430℃, and a low temperature reaction in a broad temperature interval down to about 200 ℃. Intensities of both reactions decreased with increasing cooling rate. Quenching from solution annealing with rates up to 1000 K/s was investigated by differential fast scanning calorimetry （DFSC） and the differential reheating method （DRM）. A critical quenching rate to suppress all precipitation reactions of 100-300 K/s was been determined.

Continuous cooling precipitation diagram of cast aluminium alloy Al-7Si-0.3Mg

The precipitation behaviour during quenching of cast Al-7Si-0.3Mg aluminium alloy was investigated by DSC in the cooling rate range of 0.01 K/s to 3 K/s and by quenching dilatometry for higher rates. Two main precipitation reactions were observed during cooling, a high temperature reaction starting almost directly with quenching from 540℃ and a low temperature reaction starting at about 400℃. Quenching with 3 K/s already significantly suppresses precipitation during quenching. Hardness after T6 ageing increases with increasing quenching rate, due to the increasing content of supersaturated solid solution. By dilatometry and hardness results the critical cooling rate can be estimated as about 60 K/s. Quenched Al-7Si-0.3Mg microstructures have been investigated by light microscopy. The microstructures consist of an aluminium-silicon eutectic structure, aluminium solid solution dendrites and precipitates inside the aluminium dendrites, depending on quenching rate.

MODELLING FOR PREDICTION OF CONTINUOUS COOLING TRANSFORMATION KINETICS OF HOT-DEFORMED AUSTENITE STEEL

On the thermodynamics basis of regular solution sub-lattice model and soperelement model, kinetics basis of Cahn's transformation kinetics theory, and according to Scheil's additivity rule and eoperimental results obtained by thermal dilation method,a prediction model of transformations from hot-deformed austenite to ferrite, pearlite and bainite in low alloy steels, which could be applied to continuoas cooling process, is developed. The calculated transformed junctions of each phase based on laboratory controlled rolling and controlled cooling conditions in a low alloy steel are in reasonable agreement with the measured ones.

Grain Size Prediction after Continuous Cooling Transformation from Deformed Austenite to Ferrite

On the basis of transformation thermodynamics and kinetics theories,an algorithm for predicting ferrite grain size after continuous cooling transformation from deformed austenite to ferrite is suggested.The calculated results of computer simulation with the algorithm are in so good agreement with the measured ones in controlled rolling and controlled cooling experiments that the theoretical algorithm is feasible.

Modelling of Phase Transformation of Plain Carbon Steels during Continuous Cooling

A model based on Avrami equation and Scheil's additivity rule was proposed to simulate the phase transformation in plain carbon steels during continuous cooling in hot strip mill. In this model, a wide range of composition, cooling rate, primary austenite grain size and retained strain has been taken into account. It can be used to calculate the phase fraction transformed at different temperatures during continuous cooling. The phase equilibrium and transformation starting temperature can be determined by using Thermo-Calc and DICTRA. The simulated results containing the transformation at starting and finishing temperatures, Ae1, Ae3 and the maximum volume fraction for Q235B, were obtained. The calculated phase volume fractions are in good agreement with .the experimental results.

Effect of Mo on the continuous cooling transformation behavior of Nb-Ti micro-alloyed low carbon steel

The effect of molybdenum on the continuous cooling transformation behavior of the micro-alloyed low carbon steel containing niobium and titanium was investigated by a Gleeble 3800 thermo-mechanical simulator. The phase transformation temperature of the steel at various cooling rates was detected. The microstmcture was observed by optical microscope （OM） and scanning electronic microscope （ SEM）, and its Vickers hardness was tested. Based on these, its dynamic continuous cooling transformation （CCT） diagrams were determined. The results show that the transformation temperature from deformed austenite to acicular ferrite （AF） is decreased when Mo is added, and the formation of pro- eutectoid ferrite （F） and pearlite （P） is either inhabited or postponed. Mo can also enlarge the range of the cooling rate in forming AF, and refine the microstructure effectively.

Effect of Continuous Cooling Conditions on Microstructure and Mechanical Properties of High Carbon Steel Rod

The effect of cooling rate and austenitizing condition on the mechanical properties of high carbon steel (SWRH82B) has beeninvestigated. Specimens were made of high carbon steel rod and heat-treated by Gleeble-2000 to produce a wide variation in prior austenite size. Different cooling rates were carried out, and then pearlite interlaminar spacing and mechanical properties were measured andtested respectively. According to the results, it could be found that under the continuous cooling with the increase of cooling rate, tensilestrength greatly increases and reduction in area exhibits a slightly increase for an equivalent value of prior austenite grain size. Whenprior austenite size increases, reduction in area decreases, and tensile strength increases slightly for an equivalent value of pearlite interlaminar spacing. It is concluded that prior austenite size primarily controls ductility and pearlite interlaminar spacing controls tensilestrength. Mathematical formulae are given for these relations.

Microstructural Transformation and Precipitation of an Ultra-high Strength Steel under Continuous Cooling

We investigated phase transition and precipitation of ultra-high strength steel（UHSS）in a new ＂short process＂ with controlled rolling and controlled cooling.Thermalexpansion test combined with metallographic observation was used to research the continuous cooling transformation（CCT）curve.Moreover,the microstructuraltransformation and precipitation law was revealed by morphologicalobservation and alloying elements by electron probe micro-analyzer（EPMA）.Transmission electron microscopy（TEM）was utilized to analyze the composition and grain orientation of microstructure.The study showed that the measured criticaltransformation temperatures of Ac1 and Ac3 were 746 and 868 ℃,respectively.The CCT curve indicated that the undercooled austenite was transformed into proeutectoid ferrite and bainite with HV 520 in a broad range of cooling rate 0.1^（-1） ℃·s^（-1）.When subjected to a cooling rate of 1 ℃·s^（-1）,the undercooled austenite was divided into small-sized blocks by formed martensite.With further increase of cooling rate,micro-hardness increased dramatically,the microstructure of specimen was mainly lathe bainite（LB）,granular bainite（GB）,lath martensite（LM）and residualaustenite.By diffraction test analysis,it was identified that there was K-S orientation relationship between martensite and austenite for {110}_α//{111}_γ,{111}_α//{101}_γ.EPMA clearly showed that carbon diffused adequately due to staying for a long time at high temperature with a lower cooling rate of 2 ℃·s-1.Phase transition drive force was lower and the residualaustenite existed in the block form of Martensite austenite island（M-A）.With the increase of cooling rate to 10 ℃·s^（-1）,the block residualaustenite reduced,the carbon content of residualaustenite increased and α phase around the residualaustenite formed into a low carbon bainite form.

Phase Transformation under Continuous Cooling Conditions in Medium Carbon Microalloyed Steels

Several 35CrMo4 and 38MnV7 steels with different additions of Ti and V were manufactured by electroslag remelting. The influence of the alloying and microalloying elements on phase transformation at different cooling rates was studied and the continuous cooling transformation diagrams were plotted. In order to optimize the heat treatment and improve the mechanical properties, the range of cooling rates leading to a fully bainitic microstructure （without ferrite, pearlite and especially without martensite） was determined. Bainite and martensite transformation start temperatures （Bs, Ms） were also established and compared with the values predicted by empirical equations. The important role of precipitates （especially V carbonitride particles） on final microstructure and mechanical properties was assessed.

Continuous Cooling Transformation Behavior and Kinetic Models of Transformations for an Ultra-Low Carbon Bainitic Steel

The aim was to investigate transformation behavior and transformation kinetics of an ultra-low carbon bai- nitic steel during continuous cooling. Continuous cooling transformation （CCT） curves of tested steel were measured by thermal dilatometer and metallographic structures at room temperature were observed by optical microscope. Then transformation kinetic equation of austenite to ferrite as well as austenite to bainite was established by analyzing the relationship of lnln]-l/（1--f）] and lnt in the kinetic equation on the basis of processed experimental data. Finally, the measured and calculated kinetic behaviors of the steel during continuous cooling were compared and growth pat- terns of transformed ferrite and bainite were analyzed. Results showed that calculated result was in reasonable agree- ment with the experimental data. It could be concluded that the growth modes of transformed ferrite and bainite were mainly one dimension as the Avrami exponents were between 1 and 2.

INFLUENCE OF HOT DEFORMATION ON CONTINUOUSCOOLING BAINITIC TRANSFORMATION IN A LOWCARBON STEEL

The influence of hot deformation conditions on continuous cooling bainitic transformation has been investigated for a low carbon microalloyed steel. The CCT diagrams show that deformation in unrecrpstallized austcnite ation can accelerate transformation process. Bainitic transformation in intermediate transformation temperature region is prominent, and the proeutectoid polygonal ferrite transformation at evelated high temperature is suppressed. According to optical and TEM analyses, low carbon bainitic ferrite is characterized by granular and lathlike ferrite, based on the cooling rate and deformation conditions. For nondeformation, groaps of coarse parallel ferrite lath form from the prior austenite grain boundaries with the same crystallographic orientation. For heavy deformaton, cell structure within the austenite grains due to the high dislocation density formed, which provides more nucleation sites for bainite ferrite. So deformation can discontinue the growth of ferrite laths and decrease the length of ferrite laths.

Continuous Cooling Bainite Transformation Characteristics of a Low Carbon Microalloyed Steel under the Simulated Welding Thermal Cycle Process

Continuous cooling transformation of a low carbon microalloyed steel was investigated after it was subjected to the simulation welding thermal cycle process and the interrupted cooling test. Microstructure observation was performed by optical microscopy and transmission electron microscopy. On the basis of the dilatometric data and microstructure observation, the continuous cooling transformation （CCT） diagram was determined, which showed that the main microstructure changes from a mixture of lath martensite and bainitic ferrite to full granular bainite with the increase in the cooling time t8/5 from 10 to 600 s, accompanied with a decrease in the microhardness. The interrupted cooling test confirmed that the bainitic ferrite can form attached to grain boundaries at the beginning of transformation even if the final microstructure contains a mixture of granular bainite and bainitic ferrite.

A hybrid machine learning model for predicting continuous cooling transformation diagrams in welding heat-affected zone of low alloy steels

Continuous cooling transformation diagrams in synthetic weld heat-affected zone(SH-CCT diagrams)show the phase transition temperature and hardness at different cooling rates,which is an important basis for formulating the welding process or predicting the performance of welding heat-affected zone.However,the experimental determination of SH-CCT diagrams is a time-consuming and costly process,which does not conform to the development trend of new materials.In addition,the prediction of SHCCT diagrams using metallurgical models remains a challenge due to the complexity of alloying elements and welding processes.So,in this study,a hybrid machine learning model consisting of multilayer perceptron classifier,k-Nearest Neighbors and random forest is established to predict the phase transformation temperature and hardness of low alloy steel using chemical composition and cooling rate.Then the SH-CCT diagrams of 6 kinds of steels are calculated by the hybrid machine learning model.The results show that the accuracy of the classification model is up to 100%,the predicted values of the regression models are in good agreement with the experimental results,with high correlation coefficient and low error value.Moreover,the mathematical expressions of hardness in welding heat-affected zone of low alloy steel are calculated by symbolic regression,which can quantitatively express the relationship between alloy composition,cooling time and hardness.This study demonstrates the great potential of the material informatics in the field of welding technology.

Effects of continuous cooling rate on morphology of granular bainite in pipeline steels

The morphology and characteristics of granular bainite (GB) in pipeline steels at different continuous cooling rates were investigated by scanning electron microscopy, transmission electron microscopy and electron back-scattered diffraction (EBSD). The results show that the morphology of ferrite matrix in GB turned from the lath sheaf structure into the nearly equiaxed large grain with the cooling rate decreasing from high (60℃/s) to low (5-10℃/s). At the medium cooling rate (20-40℃/s), GB consisted of the irregular ferrite matrix, the granular martensite/austenite (M/A) constituents and abundant substructures inside. The formation of the irregular ferrite and substructure was attributed to the high-temperature recovery which occurred at relatively high-temperature stage before phase transformation. The granular morphology of M/A constitu-ents was formed from the carbon-rich triple junctions which were produced by the multidirectional substructure interfaces converged with each other. Particularly, some martensite in M/A constituents was misoriented from the adjacent ferrite by very small misorientation angle, which could be characterized by the mean band contrast function of EBSD qualitatively or semiquantitatively.

Influence of cooling rate on phase transformation and precipitation behavior of Ti-bearing steel in continuous cooling process

The influence of cooling rate on microstructural evolution and precipitation behavior in Ti,Ti–Nb and Ti-Mo low-carbon steels during the continuous cooling process was studied by dilatometer method,optical microscopy,and transmission electron microscopy.The results indicated that austenite transformation temperature decreased with the increasing cooling rate in three steels.The addition of Nb and Mo promoted bainite and martensite transformation and improved the hardenability of steels.In addition,precipitates formed in deformed austenite and ferrite can be observed simultaneously.Deformation in the austenite non-recrystallization zone can introduce a large number of deformation bands,and then,the precipitates preferentially nucleated in these deformation bands.In the following process,randomly distributed precipitates and interphase precipitates will be formed in ferrite.The precipitates formed in deformed austenite obey Kurdjumov-Sachs orientation relationship with the matrix,while the precipitates formed in ferrite obey Baker-Nutting orientation relationship with the matrix.The addition of Nb and Mo in Ti-bearing steels decreased the precipitates size and increased the number density of precipitates and then improved the precipitation hardening.And the effect of Mo addition is more obvious than that of Nb addition.

Role of Solute Rare Earth in Altering Phase Transformations during Continuous Cooling of a Low Alloy Cr–Mo–V Steel

Effects of solute rare earth(RE)on continuous cooling transformation of a low-alloy Cr–Mo–V bainitic steel are investigated in detail by dilatometry,optical microscopy(OM),scanning electron microscopy(SEM)and transmission electron microscopy(TEM).Microstructures appeared in thermal dilatometric samples of both low-alloy Cr–Mo–V(RE)steels are composed of quasi-polygonal ferrite(QPF),degenerate pearlite(DP),granular bainite(GB),lath bainite(LB),and martensite(M)depending on cooling rate.When cooling rate is lower than 2°C/s,the addition of RE suppresses QPF transformation,and thereby inducing a broader transformation region of GB.When cooling rate ranges from 2 to 100°C/s,the addition of RE decreases the start temperature of bainitic transformation distinctly,which results in finer bainitic ferrite grain size and higher dislocation density.The addition of RE can enhance the hardness of the low alloy Cr–Mo–V steel by affecting the aforementioned diffusional and/or partly displacive transformation.However,when cooling rate increases up to 150°C/s,two steels have the same hardness value of about 435 HV due to only martensite obtained by displacive transformation.

Wear behavior of bainite ductile cast iron under impact load

The dry impact wear behavior of bainite ductile cast iron was evaluated under three different impact loads for 30000 cycles. The strain-hardening effects beneath the contact surfaces were analyzed according to the surfaces＇ micro-hardness profiles. Scanning electron microscopy （SEM） and X-ray diffraction （XRD） were used to observe the wom surfaces. The results indicated that the material with the highest hardness was the one continuously cooled at 20℃, which exhibited the lowest wear rate under each set of test conditions. The hardness of the worn surface and the thickness of the hardened layer increased with the increases in impact load and in the number of test cycles. The better wear performance of the sample cooled at 20℃ is attributed to its finer microstructure and superior mechanical properties. All the samples underwent the transformation induced plasticity （TRIP） phenomenon after impact wear, as revealed by the fact that small amounts of retained austenite were detected by XRD.

Mathematical Modelling of Carbonitride Precipitation during Hot Working in Nb Microalloyed Steels

Based on thermodynamics and kinetics, precipitation behavior of microalloyed steels was analyzed. Deformation greatly promotes isothermal carbonitride precipitation and makes C-curve shift leftwards. The position and shape of C-curve also depend on the content of Nb and N. C-curve shifts leftwards a little when N content increases and the nose temperature is raised with increasing Nb content. Deformation shortened precipitation start time during continuous cooling, raised precipitation start temperature, accelerated precipitation kinetics of carbonitrides. With decreasing the finishing temperature and coiling temperature, the precipitates volume fraction increases and strength increment is raised during hot rolling. The simulated results are in agreement with experiment results.