Thermal Simulation Study on the Solidification Structure and Segregation of a Heavy Heat-Resistant Steel Casting

Abstract

The prediction and controlling of the solidification structure and macro-segregation in heavy steel casting, which is usually produced in limited quantities, was a conundrum in the foundry field. In this work, the cooling and solidification processes of a 16 t CB2 ferritic heat-resistant steel (FHRS) valve casting were reproduced by studying the solidification behavior of three typical units through a thermal simulation method. The results indicate that the types of casting without chilling have the most uneven distribution of solutes and hardness, while those types of casting in which parts are solidified by chilling are much more uniform. The macro-segregation degrees of B, C, Nb, P, Cr, Mo, Si, V and Mn decrease gradually during heavy casting of CB2 ferritic heat-resistant steel. Of them, B, C, Nb, and P are solutes prone to segregation, and the maximum macro-segregation index of B can even reach 15. The macro-segregation tendencies of Cr, Mo, Si, V, and Mn are relatively small. Further studies on the last solidification portion of samples taken by electron microprobe reveal that large-sized precipitates such as MnS and NbxC are easily formed due to solute enrichment, and the sizes of these precipitates were distributed from dozens to hundreds of micrometers.

Advanced Simulation Technologies of Metallurgical Processing

“…Five articles address casting [1][2][3][4][5] and its inherently multi-physics aspects that involve a strong interaction between thermodynamics, multiphase fluid dynamics and electromagnetic effects.…”

Metallurgical Risk Factors in Grade 91 Steel

“…The impact of chemical segregation on the precipitation of phases in Gr.91 steel has yet to be studied in detail, however recently it has been associated with a loss in creep rupture strength [9]. The segregation behavior of a ZG12Cr9Mo1Co1NiVNbNB ferritic steel has recently been reported [12]. It showed a high enrichment in Cr, Mo, and Mn during the end of the solidification process of a cast due to inhomogeneous cooling through its thickness.…”

“…It showed a high enrichment in Cr, Mo, and Mn during the end of the solidification process of a cast due to inhomogeneous cooling through its thickness. This segregation also caused a hardness variation in the cast, with the parts near chills (fast cooling) having a more consistent hardness due to the suppression of segregation [12]. Furthermore, a link between the degree of chromium segregation in a T91 (Tubular Gr.91) steel and creep rupture life has been recognized, showing a greater variation of chromium concentration through the thickness of the tube correlating with a loss of creep strength [5].…”

“…Furthermore, a link between the degree of chromium segregation in a T91 (Tubular Gr.91) steel and creep rupture life has been recognized, showing a greater variation of chromium concentration through the thickness of the tube correlating with a loss of creep strength [5]. This is because it is recognized that these segregated regions may be the location of the notoriously harmful Z-phase [10] and other preexisting phases such as inclusions and NbC particles [12].…”