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Acta Metall Sin  2016, Vol. 52 Issue (7): 769-777    DOI: 10.11900/0412.1961.2015.00532
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EFFECT OF TiN PRECIPITATES ON SOLIDIFICATION MICROSTRUCTURE OF MEDIUM CARBON Cr-Mo WEAR RESISTANT STEEL
Wenying GUO1,2,Xiaoqiang HU2(),Xiaoping MA2,Dianzhong LI2
1 School of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230022, China
2 Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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Abstract  

As an important type of wear-resistant material, the low-alloyed medium carbon wear resistant steel has been widely used in mining, power and metallurgical industries due to its low cost and excellent mechanical properties. However, the coarse as-cast microstructure tends to form in large wear resistant castings because of the long solidification time. As a result, spalling wear resulting from the preferential initiation and propagation of cracks along interdendrite will occur during service process, which severely degrades the wear resistance and service life. In this work, Ti is added to improve the mechanical properties of medium carbon Cr-Mo wear resistant steel. The precipitation behavior of TiN in the solidification process and its effect on the solidification microstructure were investigated by thermodynamic calculation, constant temperature solidification experiment at solid-liquid two phase region and continuous cooling solidification experiment by using OM, SEM, EDS and EPMA. The results show that TiN precipitation temperature gradually increases at solid-liquid two-phase region with the increase of contents of Ti and N. TiN precipitates directly in the liquid region when Ti and N contents (mass fraction) are 0.090% and 0.014%, respectively. Holding at different temperatures of solid-liquid two-phase region, a very small amount of TiN precipitates are present within the dendritic arm, and a large number of TiN precipitates are present at the interdendritic positions and frontiers of dendrites. After quenching, in the remaining liquid most of TiN are present at the boundaries of equiaxed grain and a little amount of TiN stay within the equiaxed grain. During the continuous cooling solidification, TiN precipitation temperature is the main factor affecting the refinement of solidification microstructure. With the increase of Ti content, TiN precipitation temperature increases. At the same time, the actual solidification temperature of liquid steel rises, the solidification temperature range broadens and the local solidification time extends, which results in the increase of secondary dendrite arm spacing. When Ti content exceeds 0.066%, TiN precipitation temperature is near or above the liquidus line. The actual solidification temperature of liquid steel remains unchanged. Therefore, the secondary dendrite arm spacing becomes stable.

Key words:  wear-resistant steel      TiN precipitate      solidification microstructure      secondary dendrite arm spacing     
Received:  15 October 2015     
Fund: Supported by National Natural Science Foundation of China (No.51301175)

Cite this article: 

Wenying GUO,Xiaoqiang HU,Xiaoping MA,Dianzhong LI. EFFECT OF TiN PRECIPITATES ON SOLIDIFICATION MICROSTRUCTURE OF MEDIUM CARBON Cr-Mo WEAR RESISTANT STEEL. Acta Metall Sin, 2016, 52(7): 769-777.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2015.00532     OR     https://www.ams.org.cn/EN/Y2016/V52/I7/769

Fig.1  Schematic of constant temperature (a) and continuous cooling (b) solidification processes (TL—liquidus temperature, TS—solidus temperature, W.Q.—water quenching, F.C.—furnace cooling)
Sample C Si Mn P S Cr Mo Cu Ni Ti N Fe
J0 0.35 0.69 1.04 0.009 0.003 2.25 0.31 0.27 0.27 0.090 0.014 Bal.
J1 0.36 0.71 1.02 0.008 0.006 2.24 0.28 0.25 0.26 0.066 0.014 Bal.
J2 0.34 0.72 1.00 0.007 0.006 2.23 0.27 0.25 0.26 0.041 0.011 Bal.
J3 0.35 0.72 1.00 0.006 0.006 2.25 0.30 0.25 0.26 0.021 0.010 Bal.
J4 0.34 0.70 1.02 0.006 0.006 2.40 0.28 0.25 0.28 0 0.011 Bal.
Table1  Chemical composition of samples
Fig.2  Mass fraction of phases (a) and TiN (b) as a function of temperature in experimental steels at thermodynamic equilibrium state (L—liquid phase)
Sample Mass fraction
of Ti / %
Mass fraction
of N / %
TiN precipitation
temperature / ℃
Fraction of
solidification
J0 0.090 0.014 1500 0
J1 0.066 0.014 1483 0.10
J2 0.041 0.011 1471 0.50
J3 0.021 0.010 1451 0.80
Table 2  Precipitation parameters of TiN in samples
Fig.3  OM images of solidification microstructures of J2 steel after holding at 1510 ℃ (a), 1480 ℃ (b), 1450 ℃ (c) and 1410 ℃ (d) for 20 min followed by water quenching
Fig.4  SEM image of TiN precipitates (a) and EDS analysis of TiN (b), Al2O3 (c) and Ti(C, N) (d) in J2 steel after holding at 1450 ℃ for 20 min
Fig.5  SEM images (a, c) and EPMA analysis (b, d) of TiN precipitate in interdendritic (a, b) and within dendrite frontier and residual liquid (c, d) in J2 steel after holding at 1450 ℃ for 20 min
Fig.6  Statistical results for mass fraction of TiN in solidification microstructure of J2 steel after holding at 1450 ℃ for 20 min (A—within dendrite, B—interdendritic, C—dendrite frontier, D—equiaxed grain boundary, E—within equiaxed grain)
Fig.7  Solidification microstructures of J0 (a), J1 (b) and J4 (c) steels continuously cooled to 1425 ℃ at cooling rate of 2.83 ℃/min
Fig.8  Effect of Ti contents on secondary dendrite arm spacing of experimental steels continuously cooled to 1350 ℃ at cooling rate of 2.83 ℃/min
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