Study on the as-cast microstructure and mechanical properties of 35CrMo steel based on electric field control

Herein, the effect of current on the solidification microstructure and properties of 35CrMo structural steel has been studied. The effect of an electric field on the solidification structure of an ingot was investigated by immersing two parallel electrodes into the free surface of molten steel. Using the interaction between the current and melt as well as the Lorentz force generated by its own induced magnetic field, the whole region of the melt was covered with an eddy current. The numerical simulation of the ingot solidification process has been carried out and its influence on the inner flow field during the ingot solidification control process discussed. The results showed that an applied electric field caused turbulence inside the ingot, which drove the molten alloys to rotate and stir, refined the solidification structure, reduced the solidification defects, such as shrinkage cavity and segregation, and increased from 549.9 MPa at the top edge of the ingot and 411.4 MPa at the middle edge to 560.2 and 510.2 MPa, respectively. In addition, the electric field made the hardness and strength of each part of the ingot more uniform and improved the quality of its rigidity for the steel production process.


Introduction
Steel is still the most widely used structural material in the 21st century.Improvement of the microstructure and properties of steel is the traditional field of materials research.However, with the increased steel performance requirements for industrial applications, as well as the environmental protection requirements of low resource consumption, low energy consumption, and process production, the development and production of steel, a traditional structural material, is facing an increasing number of new challenges.The internal quality of the ingot will decrease with an increase in the ingot volume and serious casting defects, such as looseness, shrinkage cavity and mixtures, will be produced during the solidification process, which will affect the quality and performance of the final product.Therefore, regulating the inner quality of castings and reducing defects, such as looseness and shrinkage, are key to improving the yield of steel [1][2][3].
Recently, in the field of solidification control, new technologies such as electric field, magnetic field, ultrasonic wave, and other external physical field treatments have emerged to control the metal solidification process.By controlling the solidification process of steel ingots, a more pure and uniform solidification microstructure can be obtained, so that the steel has improved mechanical properties.External physical field treatment technology involves the application of a physical field to the metal melt before or during solidification and uses the interaction between the metal and physical field to improve its solidification microstructure.This technology has the advantages of environmental friendliness, convenient operation, and relatively low cost.At present, the research hotspots controlling the microstructure and properties of the metal solidification process using an external field mainly focus on: (1) electric current control, (2) magnetic field control, (3) ultrasonic control, (4) microgravity field and high-pressure field control, etc.Among them, electric and magnetic field control technology has gradually attracted researchers' attention due to its low energy consumption, significant effect, convenient application, strong universality, and ability to ensure metal cleanliness [4][5][6][7].
By adding a magnetic field to the solidification process of molten metal, Xu et al [8] found that the magnetic field can refine the grains and improve the solidification microstructure and mechanical properties of the alloy.Gergely et al [9] also reached a similar conclusion and also found that the introduction of solidification defects can be avoided due to the non-direct contact between the magnetic field and molten metal.Zimmermann et al [10] studied the directional solidification of Al-Cu alloy under a rotating magnetic field.It was found that when the magnetic field intensity (B) = 10 mT, the melt was driven by Lorentz forces to form forced convection and the generated dendrites were broken by the flowing melt and flowed with the melt.Li et al [11] studied the solidification of Al-Si alloy in a rotating magnetic field and microgravity environment, which makes the distribution of Si element in the solidified structure more uniform.As early as the early 1990s, Nakada et al [12] studied the effect of an electric field on the solidification process of an alloy using the current size and intensity as variables.A large enough shear force will break the dendrites and provide nucleation particles for the formation of equiaxed crystals.Liao et al [13] found that the crystal size can be greatly reduced by applying an electric current pulse (ECP) in the nucleation stage and proposed that grain refinement occurs due to the effect of 'grain rain'.Li et al [14] carried out directional solidification experiments on MnBi/Bi eutectic alloy using the fixed frequency fluctuation caused by a current pulse.It was found that the average secondary dendritic arm spacing (λ) of the MnBi rod decreased linearly with an increase in the average current intensity.Gao et al [15] carried out an ECP treatment on the solidification process of ZA27 alloy.The results showed that ECP inhibited the growth of alloy dendrites and changed the microstructure of the alloy from coarse dendrites to small dendrites.Zhu et al [16,17] conducted in situ observation experiments on the solidification microstructure of Al-Bi alloys and found that ECP and temperature gradient changes had a significant effect on the directional movement of the alloy melt, changing the microstructure morphology.The electromagnetic force pinning effect could make the Bi droplets move from the melt center to the vicinity of the skin, and a small amount of Al aggregation segregation.Jiang et al [18] applied ECP to a continuous solidification experiment of Cu-Sn-Bi insoluble alloy.When the conductivity of the matrix liquid was higher than that of these few phase droplets, ECP led to an increase in the energy barrier of nucleation and the formation of a phase separation microstructure.Yan et al [19] studied the effect of pulse discharge on the solidification microstructure of Sn-10wt%Pb hypoeutectic alloy.The pulse pressure generated upon pulse discharge in the alloy melt transformed the solidification microstructure from coarse tin-rich dendrites to near-spherical fine equiaxial grains.Wang et al [20] prepared the solidification microstructure of Sn-Bi alloy without applying a temperature gradient DC electric field.Under the DC electric field, the grain morphology evolved in the form of equiaxed dendrites and the dendrite tip morphology gradually tended to become round or flat.Zhang et al [21] applied ECP to the directional solidification process of Al-20.5wt%Si alloy, resulting in higher strength forced flow, thereby promoting the segregation and separation of primary silicon.Li's [22] experimental results showed that the electromagnetic vibration volume force generated by the combined action of an AC electric field and magnetic field significantly improved the grain density of the solidified structure.Xu et al [23] studied the fluid flow in Ga20 wt%-In 12 wt%-Sn liquid alloy caused by sinusoidal wave form ECP entering into the cylindrical melt through two parallel electrodes by means of experimental measurements and numerical simulations.The simulation results showed that the timeaveraged pulse of Lorentz force could be used to explain the change in the flow intensity with the ECP parameters.Zhao et al [24] applied a high voltage continuous discharge method to the unidirectional solidification of pure aluminum.However, when the discharge energy of the capacitor bank was excessively large, the grain structure became coarse.Liu et al [25] applied a pulsed electric field to the continuous casting process of 20CrMnTi gear steel.The results of the solute distribution measured using in situ metal elemental analysis also showed that the distribution of each element in the billet treated by a pulsed electric field was more uniform and the degree of segregation was reduced.Peng et al [26] considered that the radial pressure generated by the pulsed electric field acoustic wave was the main factor leading to the migration of the crystal nucleus into the melt.Räbiger et al [27] applied an electric current during the solidification process to produce refined equiaxed grains and under the same effective current intensity.Their results showed that the grain refinement effect observed in these experiments was completely attributed to the forced melt flow driven by Lorentz force.
In summary, the introduction of an electric field has a great influence on the solidification process of metals.Most of the previous studies have focused on magnesium and aluminum alloys.Research on high melting point melts, such as steel and iron alloys, is still in its initial stages and its influence mechanism is still unclear.The 35CrMo alloy steel is widely used due to its good static strength, impact toughness, and fatigue resistance.It also has good lasting strength at high temperature.It is generally used to manufacture mechanical parts that withstand impact, distortion, and high load.Therefore, it is of great significance to improve the ingot quality and yield of 35CrMo structural steel as well as reduce the production cost.In this study, the influence of the electric field on the microstructure and properties of 35CrMo structural steel was studied by combining finite element numerical simulations and experimental studies.Improvement of the internal quality of the ingots was achieved by regulating the electric field.The application of electric field in ingot manufacturing helps to improve the internal quality of special steel ingot and provide high-quality intermediate products for subsequent rolling and forging processes.

Experimental materials
The experimental material was 35CrMo structural steel and its nominal chemical composition is shown in table 1.

Experimental methods
In this experiment, a 60 kW medium frequency induction furnace was used to smelt the alloy.A steel bar with a mass of ∼25 kg was placed into a magnesia crucible and an induction furnace was used to heat it to 1,600 °C and completely melt the sample.The power supply was turned-off and 70 g of aluminum strip inserted to deoxygenate and cover the protective slag.After closing the power supply of the induction furnace for 5 min, the self-made external electric field device was controlled (figure 1).Two graphite electrodes with a diameter of 15 mm and a spacing of 10 mm were inserted into the center of the molten steel.The insertion depth of the electrode was controlled at 30 mm using the electronically controlled lifting system.After the electrode was inserted, it was energized and timed, and the direct current started.The direct current of 150 A was first introduced and the energization time was 10 min.After 8 min of shutdown, a 150 A direct current was connected for 5 min.The current was then increased to 350 A for 3 min (table 2).After the energization was completed, the electrode was pulled-out and the protective slag covered again until the molten steel was completely cooled.

Microstructural observations
The samples used for microscopic analysis were polished with 200#, 400#, 800#, 1,000#, 1,500#, and 2,000# sandpaper, respectively and corroded with 4% nitric acid alcoholic solution.The microstructure morphology was then observed under an Axio Vert.A1 Zeiss optical microscope.The microstructure, precipitated phase morphology, and precipitated phase composition of the samples were observed using an EVO MA 10 Zeiss scanning electron microscope.Each ingot was taken at the top, middle, and bottom positions.The sampling at each position is shown in figure 2, and the sampling positions of the two ingots were guaranteed to be the same.

Mechanical properties test
The mechanical properties of the samples were tested on a CMT5105 electronic universal testing machine.Figure 3 shows the sampling position and size of the tensile sample.The tensile test was carried out at room temperature (25 °C) and the crosshead speed was 1 mm min −1 .Each group of samples was tested three times and the results were averaged.The fracture morphology was observed using scanning electron microscopy (SEM).The micro-hardness of the test steel sample in different casting environments was tested on a Q10 M micro-Vickers hardness instrument.The applied load was 0.2 Kgf and the holding time was 15 s.In order to ensure the accuracy and dispersion of the detection, 12 points were selected on the surface of the sample for measurement and homogenization treatment.

Numerical simulations
In this paper, ANSYS finite element analysis was used to simulate the process of convection, heat transfer, and mass transfer in the solution under two experimental environments.According to the size of the experimental ingot, the diameter of the ingot was 13 mm, the height was 23 mm, the depth of the electrode was 5 mm, and the distance between the electrodes was 10 mm, and the grid partition shown in figure 4.

Analysis of the solidification defects in steel ingot
Figure 5 shows the experimental and numerical simulation results of the internal defects of the ingot under different experimental conditions.It can be clearly seen that when the ingot was as-prepared (Alloy 1, figure 5(a)), the core defects in the ingot were serious, the internal quality was low, and serious casting defects occurred when it was ∼140 mm from the bottom of the ingot.When the electric field was applied (Alloy 2,  figure 5(b)), the internal defects were obviously improved and defects such as looser structures, shrinkage cavities, and cracks, were almost eliminated.The numerical value simulation of the defect field (figures 10(c) and (d)) also provided similar conclusions.When there was no electric field, the defect field index was higher, and the defect field index was reduced after the electric field was regulated.

Microstructural analysis
Figure 6 shows the microstructural morphology of the different casting positions under different casting conditions.It can be clearly seen that in the non-magnetic field environment, the upper edge of the casting (figure 6(a)) and the center position (figure 6(b)) were composed of pearlite, grain boundary ferrite, and a small amount of widmanstatten ferrite; the middle edge of the casting (figure 6(c)) was mainly composed of pearlite, blocky ferrite, and a small amount of widmanstatten ferrite, and the corresponding center (figure 6(d)) exhibited blockier ferrite and widmanstatten ferrite.The edge of the bottom of the casting (figure 6(e)) was mainly composed of pearlite and widmanstatten ferrite, while the microstructure of the center of the casting was mainly composed of blocky ferrite and widmanstatten ferrite.It can be seen that there was obvious element segregation in the naturally solidified ingot.Figures 6(g)-(l) shows the microstructure of the ingot at different positions under the condition of the inserted electrode.When compared with the natural solidification ingot, the microstructure was more uniform, mainly composed of pearlite, grain boundary ferrite, a small amount of blocky ferrite and widmanstatten ferrite.It is worth noting that under the condition of electrode treatment, the ferrite distribution of the widmanstatten structure was more uniform and finer.Figure 7 shows the SEM diagram of 35CrMo steel under different casting conditions (a).As-prepared ingot; (b).insert electrode ingot).It can be clearly seen that the distribution of Widmanstatten ferrite was more uniform and finer after the electrode was inserted.EDS analysis of pearlite, grain boundary ferrite, blocky ferrite, and widmanstatten ferrite under two casting conditions was carried out.It was found that pearlite and ferrite were mainly composed of C, Cr, and Fe.In figures 7(a-1), A contained much more C than 7(a-2) and 7(a-3), so A was pearlite.After being inserted into the electrode, A-C were grain boundary ferrite, blocky ferrite, and widmanstatten ferrite, respectively.The elemental distribution was more uniform and the segregation phenomenon reduced.

Analysis of the element segregation in steel ingots
Figure 8 shows the distribution of C at different positions of the casting from the edge to the center under the different casting conditions studied.The experimental results showed that the casting took 24 points from the edge to the center at one time.It can be seen that when no electric field was applied (figure 8(a)), the top of the ingot showed an obvious 'W'-type positive segregation, while the bottom showed negative segregation.This led to poor toughness at the top of the ingot, insufficient strength at the bottom, and the poor overall performance of the casting.After the electric field was applied (figure 8(b)), the segregation of each part of the ingot was obviously improved, C was evenly distributed, and the quality gate of the ingot obviously improved.

Mechanical properties analysis
Figure 9 shows the mechanical properties (figures 9(a)-(c)) and hardness (figure 9(d)) under different casting conditions.Under natural solidification conditions, as shown in figure 9(a), the edge of the top of the ingot had the highest strength of 549.9 MPa, while the edge of the middle had the lowest strength of 411.4 MPa.After the electrode was inserted (figure 9(b)), the strength difference at each position of the ingot was smaller and significantly improved with a maximum of 560.2 MPa and a minimum of 510.2 MPa. Figure 9(c) shows that the applied electric field significantly improved the tensile strength of the ingot and the strength of each part of the ingot was more uniform than that without the electric field.As for the yield strength, the electric field had no obvious effect.According to the hardness analysis of each part of the ingot under the two conditions, it was found that the hardness of the ingot after adding the electric field was higher than that without the electric field with the exception of the top of the ingot.

Numerical simulations
Figure 10 shows the inner magnetic field distribution, flow velocity diagram, and flow field diagram of the molten steel melt when the electrodes were applied at 150 and 350 A, respectively.It can be seen that the magnetic field distribution mainly showed a trend of strong core and weak edge, and the magnetic field intensity increased upon increasing the current intensity.Figures 10(c) and (d) are the streamlines cloud atlas maps of the flow velocity of molten steel in the ingot.After the electrode was inserted into the ingot, a vertical downward electromagnetic driver force was formed under the electrode to drive the molten alloys to flow downward due to the effect of electromagnetic force.During the flow process, part of the fluid was diverted due to the viscous downward flow of the fluid itself and an eddy current was formed.This kind of clockwise turbulence group formed on both sides of the flow field made the molten alloys in the central part continuously flow to the walls on both sides and the alloy melt on both sides flows to the middle, which accelerated the process of solute homogenization.With an increase in the energization time, the position of turbulence gradually moved down, and each part of the alloy was also be affected by the stirring action from the flow field.When the turbulence reached the bottom of the ingot, each part of the cavity was fully stirred under the action of the flow field.After the turbulence group reached the bottom, it spread upward along both sides, gradually making the stirring effect cover the whole alloy.In addition, the turbulence intensity of the flow field before and after the electrode increased with an increase in the current intensity, and the downward driving force generated by the electrode will also increase.When the current intensity was increased to 350 A, the degree of upward reflux in the melt at the edge of the ingot was enhanced, which drove the alloy melt to rotate and stir, improved the speed of forced convection of the metal liquid, and had a significant effect on the improvement of the internal quality of the alloy.

Effect of the electric field on microstructure
The main purpose of introducing an AC magnetic field in the solidification process of molten steel was to realize the shape control, electromagnetic levitation, and electromagnetic stirring of molten steel.Among them,   electromagnetic stirring has achieved very good application in the production practice of ingots.Electromagnetic stirring strengthens the movement of molten steel in the liquid phase cavity by the electromagnetic force induced in the liquid phase cavity of the billet, thereby strengthening the convection, heat transfer, and mass transfer process of molten steel, inhibiting the development of columnar crystals, promoting the uniform composition, and floating of inclusions, thereby controlling the solidification structure of the billet, improving the quality of the billet.Figures 6 and 7 show that after the electrode was inserted, the microstructure of the ingot became uniform and the widmanstatten structures became short and fine.This was because a magnetic field was inevitably be generated upon the introduction of an electric field.Under the action of its own magnetic field, the current produced a radial pressure, which improved the fluidity of the melt through the action of force, and broke the dendrites to refine the grains.From the photographs of the solidification microstructure, the widmanstatten structures in the ingot treated by an electric field were obviously refined and the fine structure obviously increased.On the one hand, the magnetic field promoted a reduction in the superheat of molten steel.The instantaneous radial pressure caused the melt to vibrate rapidly so that local overheating was quickly lost to form a sufficiently large degree of undercooling, resulting in an increase in the nucleation rate.
The results of our numerical simulations showed that with the introduction of an electric field, the magnetic field spread from top to bottom, from center to edge, to the whole ingot with an increase in the electric field application time (figures 10(a) and (b)).Under the action of the electric field, turbulence was generated at the center and edge of the ingot, which drove the alloy melt to rotate and stir, and increased the speed of forced convection of the molten metal.In addition, the large amount of heat generated by the contact between the electrode and the molten steel reduced the cooling rate of the edge and promoted the center to solidify preferentially, and the behavior of the magnetic field force generated by the electric field itself caused the broken crystals to gather at the center, resulting in a large nucleation rate at the center, which also promoted the center to solidify preferentially.In the non-equilibrium solidification process, the actual solute distribution coefficient (K) value increased, which reduced the difference between the solute content in the preferentially solidified grains and the residual liquid phase, thereby inhibiting dendrite segregation.At the same time, due to the refinement of dendrites in the central region and the decrease in dendrite spacing, the elemental segregation was further reduced.

Effect of the electric field on mechanical properties
The introduction of the electric field also had an obvious influence on the performance of the ingot.Through our tensile test results (figure 9), it has been found that the strength of each part of the ingot became uniform after the introduction of an electric field, the strength gap between the center and the edge of the ingot was obviously reduced, and the overall strength improved.The fracture morphology is shown in figure 11 and the typical parts (top edge and middle edge) in two experimental states discussed.Figure 11(a) is the fracture morphology of the top edge of the ingot without an electric field, showing a river pattern and small amount of shallow dimples, exhibiting brittle fractures.After applying the electric field (figure 11(b)), the fracture showed a typical ductile fracture with many deep dimples.The fracture morphology of the middle edge of the ingot is shown in figure 11(c).The fracture was a typical brittle fracture and the performance was poor.After the introduction of an electric field (figure 11(d)), obvious dimples appeared.In summary, the fracture surface without the introduction of an electric field was mostly brittle fractures, mainly the cleavage surface of the river pattern [28].After the introduction of an electric field, the number of dimples on the fracture surface significantly increased, showing a typical ductile fracture.It was seen that the introduction of electric field resulted in a significant improvement in the performance of the resulting ingots.

Conclusions
Herein, the 150 A and 350 A DC electric field applied to the solidification process of 35CrMo steel was studied by means of experimental studies and numerical simulations.The influence of the electric field on the microstructure and properties of ingots was discussed.The following conclusions were obtained: 1.The introduction of an electric field improved the solidification microstructure and reduced the solidification defects, such as shrinkage cavity and segregation.2. The electric field was applied to form turbulence inside the ingot, which drove the molten alloys to rotate and stir, and the widmanstatten structures became obviously refined.
3. The electric field significantly improved the hardness and strength of the ingot, and increased the 549.9MPa value of the top edge of the ingot and the 411.4MPa value of the middle edge to 560.2 and 510.2 MPa, respectively.Moreover, under the action of a magnetic field, the hardness and strength of each part of the ingot were more uniform, and the rigid quality improved.

Figure 1 .
Figure 1.Schematic diagram of the experimental device.

2. 4 .
Composition segregation test A GDS-850A glow discharge atomic emission spectrometer was used to test the composition segregation.The excitation gas was argon and the test pressure were 850 Pa.The sample sampling diagram is shown in figure 2.

Figure 2 .
Figure 2. Sampling diagram of the segregation sample.

Figure 3 .
Figure 3. Sampling position and size of the tensile specimen (unit: mm).

Figure 6 .
Figure 6.Optical microstructure of the samples in different casting environments.

Figure 7 .
Figure 7. SEM images of the different casting environments: (a) as-prepared ingots and (b) ingots inserted with electrodes.

Figure 8 .
Figure 8. Distribution of C from the edge to core under different casting conditions: (a) original state and (b) electrode.

Figure 9 .
Figure 9. Mechanical properties of the castings (a)-(c) under different casting conditions and (d) the hardness curve.

Figure 10 .
Figure 10.Magnetic field distribution nephogram (a) and (b) and streamline nephogram of molten steel flow velocity (c) and (d) in the ingot: (a) and (c) I = 150 A and (b) and (d) I = 350 A.

Figure 11 .
Figure 11.Tensile fracture morphology of the typical positions of the ingot: (a) top edge, no electric field, (b) top edge, electric field, (c) middle edge, no electric field, and (d) middle edge, electric field.

Table 1 .
Nominal chemical composition of 35CrMo structural steel.