Solidification microstructure formation in HK40 and HH40 alloys

Abstract

The microstructure formation processes in HK40 and HH40 alloys were investigated through JmatPro calculations and quenching performed during directional solidification. The phase transition routes of HK40 and HH40 alloys were determined as L → L + γ → L + γ + M₇C₃ → γ + M₇C₃ → γ + M₇C₃ + M₂₃C₆→ γ + M₂₃C₆ and L → L + δ → L + δ + γ→ L + δ + γ + M₂₃C₆ δ + γ + M₂₃C₆, respectively. The solidification mode was determined to be the austenitic mode (A mode) in HK40 alloy and the ferritic–austenitic solidification mode (FA mode) in HH40 alloy. In HK40 alloy, eutectic carbides directly precipitate in a liquid and coarsen during cooling. The primary γ dendrites grow at the 60° angle to each other. On the other hand, in HH40 alloy, residual δ forms because of the incomplete transformation from δ to γ. Cr₂₃C₆ carbide is produced in solid delta ferrite δ but not directly in liquid HH40 alloy. Because of carbide formation in the solid phase and no rapid growth of the dendrite in a non-preferential direction, HH40 alloy is more resistant to cast defect formation than HK40 alloy.

Keywords

iron chromium nickel alloys / solidification / phase transitions / carbides

References

[1] Ali N., Reza E.F., Majid A. Evaluation and modification of inclusion characteristics in HK40 heat resistant cast steel. Int. J. Miner. Metall. Mater., 2013, 20(4): 354.

[2] Li X., Zhao J., Shen F.Z., Feng W. Reliability analysis and life prediction of HK40 steel during high-temperature exposure. Int. J. Pressure Vessels Piping, 2006, 83(10): 730.

[3] Wang Z.M. Hot cracking tendency and protection in Cr25Ni12 casting, 1965, 4, 18.

[4] Gu K.C., Cai C.B., Zhang H.J., Chen L.T., Liang W. Oxidation research of heat-resistant steel used to boiler nozzles. Hot Work. Technol., 2006, 35(16): 53.

[5] Fu J.W., Yang Y.S., Guo J.J., Ma J.C., Tong W.H. Formation of a two-phase microstructure in Fe–Cr–Ni alloy during directional solidification. J. Cryst. Growth, 2008, 311(1): 132.

[6] Abe H., Watanabe Y. Low-temperature aging characteristics of type 316L stainless steel welds: dependence on solidification mode. Metall. Mater. Trans. A, 2008, 39(6): 1392.

[7] Di Schino A., Mecozzi M.G., Barteri M., Kenny J.M. Solidification mode and residual ferrite in low-Ni austenitic stainless steels. J. Mater. Sci., 2000, 35(2): 375.

[8] Whittaker M., Wilshire B., Brear J. Creep fracture of the centrifugally-cast superaustenitic steels. HK40 and HP40, Mater. Sci. Eng. A, 2013, 580, 391.

[9] Kaya A.A. Microstructure of HK40 alloy after hightemperature service in oxidizing/carburizing environment: II. Carburization and carbide transformations. Mater. Charact., 2002, 49(1): 23.

[10] Kim Y.J., Lee D.G., Jeong H.K., Lee Y.T., Jang H. High temperature mechanical properties of HK40-type heat-resistant cast austenitic stainless steels. J. Mater. Eng. Perform., 2010, 19(5): 700.

[11] Ding X.F., Lin J.P., Zhang L.Q., Wang H.L., Hao G.J., Chen G.L. Microstructure development during directional solidification of Ti–45Al–8Nb alloy. J. Alloys Compd., 2010, 506(1): 115.

[12] Ding X.F., Mi T., Xue F., Zhou H.J., Wang M.L. Microstructure formation in -–-' Co–Al–W–Ti alloys during directional solidification. J. Alloys Compd., 2014, 599, 159.

[13] Ding X.F., Zhang L.Q., Lin J.P., He J.P., Yin J., Chen G.L. Microstructure control and mechanical properties of directionally solidified TiAl-Nb alloys. Trans. Nonferrous Met. Soc., 2012, 22(4): 747.

[14] Haxhimali T., Karma A., Gonzales F., Rappaz M. Orientation selection in dendritic evolution. Nat. Mater., 2006, 5, 660.

[15] Ding X.F., Liu D.F., Zhen Y.R., Feng Q. Effect of B micro-alloying on micro-porosities in as-cast HK40 alloys. Acta Metall. Sin., 2015, 51(9): 1121.

[16] Fu J.W., Yang Y.S., Guo J.J. Formation of a blocky ferrite in Fe–Cr–Ni alloy during directional solidification. J. Cryst. Growth, 2009, 311(14): 3661.