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.

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.

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.

Phase and CCT Diagram of an N-Containing 8%Cr Roller Steel

The pseudo-equilibrium phase diagram and continuous cooling transformation diagram of an N-containing 8% Cr roller steel were investigated by using thermodynamic calculation,differential scanning calorimetry, X-ray diffraction, expansion method, and so on. Under equilibrium conditions, the main carbonitrides are MX,M7C3,and M23C6types. The measured Ac1,Ac3,start temperature of martensitic transformation,and M7C3transformation temperatures are 811,855,324,and 1100 ℃,respectively. Bainite appears at cooling rates ranging from 0. 5 to 5 ℃ / s and ferrite forms at grain boundaries at a cooling rate lower than 0. 5 ℃ / s. Finally,the effects of adding N and lowering the C content on workability and mechanical properties of common 8%Cr steel were discussed.

Effect of cooling rate and composition on microstructure and mechanical properties of ultrahigh-strength steels

The influence of cooling rate on the microstructure and mechanical properties of two new ultrahigh-strength steels(UHSSs)with different levels of C,Cr and Ni has been evaluated for the as-cooled and untempered condition.One UHSS had higher contents of C and Cr,while the other one had a higher Ni content.On the basis of dilatation curves,microstructures,macrohardness and microhardness,continuous cooling transformation diagrams were constructed as a guide to heat treatment possibilities.Cooling rates(CRs)of 60,1 and 0.01°C/s were selected for more detailed investigations.Microstructural characterization was made by laser scanning confocal microscopy,field emission scanning electron microscopy combined with electron backscatter diffraction,electron probe microanalysis and X-ray diffraction.Mechanical properties were characterized using macrohardness,tensile and Charpy V-notch impact tests.UHSS with the higher C and Cr contents showed lower transformation temperatures and slower bainite formation kinetics than that with the higher Ni content.Higher cooling rates led to lower volume fractions and carbon contents of retained austenite together with finer prior austenite grain size,as well as effective final grain size and lath size.These changes were accompanied by higher yield and tensile strengths.The best combinations of strength and toughness were obtained with martensitic microstructures and by avoiding the formation of granular bainite accompanied by proeutectoid carbides at low CR.For the cooling rates studied,UHSS with the higher C and Cr contents showed the higher hardness and strength but at the cost of toughness.

Analysis on hot deformation and following cooling technology of 25Cr2Ni4MoVA steel

The true stress–true strain curves of 25Cr2Ni4MoVA steel were obtained by uniaxial compression experiments at 850–1200℃ in the strain rate range of 0.001–10.0 s^(−1).And the dynamic continuous cooling transformation curves were obtained at the cooling rate range of 0.5–15.0℃ s^(−1) from the austenitization temperature of 1000℃ to the room temperature by pre-strain of 0.2 as well.The power dissipation map and the dynamic continuous cooling transformation diagram were constructed based on the data provided by these curves.Compared with the optical micrographs of the compressed samples,the full dynamic recrystallization region is located between 1000 and 1200℃ and at the strain rate range from 0.01 to 10.0 s^(−1) with the power dissipation efficiency not less than 0.33.In the full dynamic recrystallization region,the power dissipation efficiency increases and the dynamic recrystallization activation energy decreases with the temperature increasing.With the strain rate decreasing,the power dissipation efficiency increases firstly and then starts to decrease as the strain rate is less than 0.1 s^(−1),and dynamic recrystallization activation energy changes on the contrary.According to the dynamic continuous cooling transformation diagram,slow cooling is a better way for the hot-deformed piece with large size or complex shape to avoid cracking as the temperature of the piece is lower than 400℃,and different cooling ways can be used for the hot-deformed piece with small size and simple shapes to obtain certain microstructure and meet good compressive properties.