Effects of Ultrasonic Treatment during the Solidification Process

on the Structure Formation of Low Carbon Steel

W. Kong*, D. Q. Cang and J. H. Song

School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China

This paper studied effects of ultrasonic treatment during the solidification process on structure formation of low carbon steel. Resultsshowed that with ultrasonic treatment grains were refined and the steel ingot avoided the cast widmanstatten structure. When ultrasonictreatment was employed, pearlite was broken and the average length of pearlite was decreased from 550mm to 140mm. The correspondingaspect ratio was reduced from 12 to 1. At high magnification the treated pearlite was cloud-like, rather than plate-like, as normal. Ultrasonictreatment also can remove the gas from low carbon steel. These phenomena of low carbon steel generated by ultrasonic treatment wereexplained. [doi:10.2320/matertrans.M2011091]

(Received March 28, 2011; Accepted June 21, 2011; Published August 3, 2011)

Keywords: ultrasonic treatment, low carbon steel, solidification, crystal structure

1. Introduction

In prior work it was found that external ultrasonictreatment applied in the molten metal could produce a seriesof beneficial effects. For instance, The structure and proper-ties of ingot and extruded shapes from high-strength Al-Zn-Mg-Cu alloys were improved after ultrasonic treatment.1)

When the ultrasonic power was 1000 W, the surface qualityof horizontal continuously cast Al-1%Si alloy was evidentlyimproved and the grains of cast ingots were refined. And itsboundary segregation was suppressed with the increase ofultrasonic power.2) There were advantages of ultrasonictreatment of aluminum melts for degassing and removingnonmetallic inclusions.3) The ultimately refined aluminiumalloy ingot structure with non-dendritic grains obtained dueto ultrasonic treatment increased plasticity of ingots, reducedprobability of cracks and was inherited in wrought semisfrom aluminum alloys with improvement of resource proper-ties.4) Good degassing and refinement results of 7050aluminium alloy could be obtained after the melt wasultrasonically treated.5) The morphology of the Al primaryphase of A356 was refined and changed shape from dendriticto nondendritic and the eutectic phase of A356 was alsorefined and changed morphology from dendritic to plateshape.6) Tensile strength of AZ91 magnesium alloy wassignificantly improved by ultrasonic treatment of the melt.7)

Ultrasonic treatment could obviously refine the solidificationstructure of high carbon steel containing rare earth.8) Ultra-sonic treatment in molten high carbon steel with rare earthcould disperse, refine and remove the inclusions.9) Duringcrystallization with ultrasonic treatment metals, it could befound the decrease of grain size, elimination of acicularstructure in most cases, the increase in uniformity of structureand the decrease in size of nonmetallic inclusions and insolimpurities.10) The effects of ultrasonic degassing on thequality of deformed semiproducts of aluminum alloy ingotssubjected to ultrasonic treatment during casting were highstrength and ductility and no lamination defects.11) High-

intensity ultrasound had been applied to metal shaping,thermal and thermochemical treatment, welding, cutting,refining, and surface hardening.12) Ultrasonic treatment ofmelt during its crystallization of magnesium alloy was anefficient method. A large amount of nuclei was formed beforesolidified front with ultrasonic. This leads to formation offine-grained structure.13) Ultrasonic could remove gas andinclusion of liquid steel, improve surface quality and interstructure of casting block.14) A good degasification could beobtained when the ingot was treated by ultrasonic vibrationat an appropriate time. However, the blowhole of an ingotwould be increased if the ultrasonic treating time was toolong.15) However, only a few of these prior researches werebased on steel. So as an expansion of previous studies, thepresent one proposed that ultrasonic treatment also exertsinfluence on structure evolution and degassing of low carbonsteel. The objectives of the present investigation were two-fold: (1) to find out the effects of ultrasonic vibration onmodification of the steel specimen; (2) to explain thesephenomena generated by ultrasonic.

2. Experimental

Compositions of steel employed in the present study isshown in Table 1. The melting point of specimens is 1530�Cmeasured by the melting point tester. Ultrasonic treatmentwas applied by a metallurgical generator (300 W and25.96 kHz measured). The toolhead (Mo-Al2O3-ZrO2) con-nected by the end of amplitude transformer (made of titaniumalloy) was used to insert into molten steel.

The specimen of 1000 g was melted in a corundumcrucible of �55 mm� 120 mm. The crucible was put into acarbon tube furnace, which was protected by argon. Aftersteel in the crucible was melted completely at 1590�C, it washeld for 8 min. Then ultrasonic treatment was applied for 30 sfrom the surface of melt, and the insertion depth was 15 mm,while maintaining the cooling rate at 1.5�C/s. After ultra-sonic treatment, the crucible was removed to a carbon drumand it was cooled to room temperature (18�C�24�C). Thecorresponding test was repeated without ultrasonic treatment.*Corresponding author, E-mail: kongwei0668@126.com

Materials Transactions, Vol. 52, No. 9 (2011) pp. 1844 to 1847#2011 The Japan Institute of Metals RAPID PUBLICATION

After cooling, the density of specimens was measured byHAK-D (an electronic density meter). Specimens were cuttransversely at the same point. They were then furtherpolished and etched for observation of their macrostructureusing high pixel digital camera (DC), metalloscope andscanning electron microscopy (SEM) with light elementenergy dispersive spectroscopy (EDS). The quantitativemetallographic analysis was studied by QuantLab-MG (aquantitative metallographic analysis software).

3. Results and Discussion

The optical micrographs (�50) of the specimens fromcastings made with and without ultrasonic treatment areshown in Fig. 1. The casting specimen made withoutultrasonic treatment exhibited coarse grain (grain boundarymarked in black) and Widmanstatten structure. In addition,the grain size shown in the middle of Fig. 1(a) was in therange of a few millimeters. In contrast, the ultrasonicallytreated specimen displayed a microstructure of the ferritematrix interspersed with the broken pearlite (Fig. 1(b)). The

size of individual pearlite was not distinguishable at such amagnification. Grains were refined with ultrasonic treatment,while the grain boundary (marked in black in Fig. 1(b)) wasnot as clear as that without ultrasonic treatment.

Results of quantitative metallographic analysis of pearlitemorphology are illustrated in Fig. 2. Without ultrasonictreatment, the average length of pearlite was about 550 mmand the aspect ratio was about 12. With ultrasonic treatment,the average length and width were all about 140 mm, with anaspect ratio of slightly more than 1. A comparison of thepearlite aspect ratio of untreated and ultrasonically treatedlow carbon steel implies that pearlite morphology in lowcarbon steel treated with ultrasonic is not simply a fine formof pearlite in untreated low carbon steel.

Both specimens were characterized with DC at a lowmagnification of �1, as shown in Fig. 3 and Fig. 4. The fineequiaxed grain zone took up almost all the cross section ofthe treated specimen and the coarse dendrite zone (outsidethe black line) occupied about two third of the cross sectionof the untreated specimen, indicating that the structurewith ultrasonic treatment is refined. This agreed with thatof metalloscope observation above. Densities of untreatedand treated ingots were respectively 7.65 kg�m�3 and7.83 kg�m�3, and there were much porosity in the untreatedspecimen edge shown in Fig. 3(a), which demonstrates thatultrasonic treatment can remove the gas in low carbon steel.

Figure 5 shows the comparison of pearlite morphologyobserved by SEM. Figure 5(a) shows pearlite in the specimennot subject to ultrasonic treatment. Pearlite exhibited atypical plate-like form. Thinner and thicker plates wererespectively cementite and ferrite. Pearlite morphology in thespecimen subject to ultrasonic treatment is shown inFig. 5(b). There were some distinct features of pearlite inthe ultrasonically processed specimen. Ultrasonically treatedpearlite was cloud-like, rather than obvious plate-like, andcementite was dispersedly distributed in the matrix of ferrite.These features also suggest that modified pearlite usingultrasonic treatment is not simply a finer form of theunmodified specimen.

The mechanism by which the finer grains form withultrasonic treatment could be that: since the cavitation effect(local transient high temperature, high pressure and shockwave) and acoustic streaming by ultrasonic, grains, whichhave been solidified, are split into smaller particles. In

(a)

(b)

Fig. 1 Microstructure observed by metalloscope (a) without ultrasonic

treatment and (b) with ultrasonic treatment (�50).

0

2

4

6

8

10

12

14

TreatedUntreated0

100

200

300

400

500

600A

spect Ratio(%

)

Average LengthAspect Ratio

Ave

rage

Len

gth

,L/m

m

Fig. 2 Pearlite morphological analyses of low carbon steel without and

with ultrasonic treatment.

Table 1 Compositions in mass% of steel employed.

C Si Mn S P Al

0.068 0.12 0.36 0.026 0.024 0.012

Effects of Ultrasonic Treatment during the Solidification Process on the Structure Formation of Low Carbon Steel 1845

addition, high melting point compounds such as MnS areformed in steel. On the basis of the disregistry theory of B. L.Bramfitt, etc.16) Whether the high melting point compoundbecomes a nucleus for heterogeneous nucleation depends onlattice disregistry between two phases, i.e.:

�ðhklÞsðhklÞn ¼X3

i¼1

ðd½uvw�is cos �Þ � d½uvw�ind½uvw�in

� 100

3ð1Þ

Where, � is disregistry mentioned above, ðhklÞs is the low-index lattice plane of the compound, ½uvw�s is the low-indexdirection of ðhklÞs, ðhklÞn is the low-index lattice plane ofthe new crystalline phase, ½uvw�n is the low-index directionof ðhklÞn, d ½uvw�n is the lattice plane distance in the directionof ½uvw�n, d ½uvw�s is the lattice plane distance in thedirection of ½uvw�s and � is the angle between ½uvw�s and½uvw�n.

Lattice disregistry (�) between MnS and �-Fe is 8.8%.17)

Lattice disregistry (�) between MnS and intragranular ferriteis 3.1%.18) These are both lower than 12% (the lowestpercentage under which the heterogeneous nucleus can beformed). Then MnS is able to act as the heterogeneousnucleus.17,19–22) And MnS particles (verified by EDS) are alsoable to be split into smaller ones, which can be proved by thecomparison between the average MnS particle size (about

(b)

(a)

Fig. 3 Macrostructure observed by DC (a) without ultrasonic treatment

and (b) with ultrasonic treatment (�1).

(a)

(b)

Fig. 4 Macrostructure observed by DC (a) without ultrasonic treatment

and (b) with ultrasonic treatment (�1) with a higher resolution.

(a)

(b)

Fig. 5 Pearlite observed using SEM (a) without ultrasonic treatment and

(b) with ultrasonic treatment (�6000).

1846 W. Kong, D. Q. Cang and J. H. Song

5 mm) with ultrasonic and that (about 2 mm) without ultra-sonic. Two categories of particles split, as mentioned above,can improve nucleation and the grain refinement.

The gas produced by ultrasonic cavitation is defined as Ci;The cavitation bubble removed by buoyancy and acousticstreaming is defined as Cd which is negative; Because vaporpressure of the original gas in molten steel is bigger than thatin the cavitation bubble, the original gas in molten steelenters the cavitation bubble and is removed. The original gasremoved is defined as Sd which is negative too. Algebra sumof Ci, Cd and Sd is defined as S (S ¼ Ci þ Cd þ Sd). If S < 0,the total gas in molten steel decreases. S is getting smaller,and then the degassing degree is higher.

Further research is needed to investigate the ultrasoniceffect on the nucleation and growth of pearlite in order tofully understand the mechanism of the pearlite variation withultrasonic treatment.

4. Conclusion

In summary, results showed that: (1) when ultrasonictreatment was employed, grains were refined and the steelingot can avoid the cast widmanstatten structure. (2) withultrasonic treatment pearlite was broken and the averagelength of pearlite was decreased from 550 mm to 140 mm. Thecorresponding aspect ratio was also reduced by ultrasonictreatment from 12 to 1. At the high magnification pearlitesubject to ultrasonic treatment was cloud-like, rather thanobvious plate-like. (3) ultrasonic treatment can remove thegas from low carbon steel.

Acknowledgements

This work has been financially supported by the National

Natural Science Foundation of China (Grant No. 51034008)and Major Project of the National Twelfth-Five YearResearch Program of China (Grant No. 2011BAC06B10).

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