Effects of B on the Hot Ductility of Fe-36Ni Invar Alloy

Yaxu Zheng,Yutian He,Changrong Li,Fuming Wang,Zhanbing Yang

doi:10.1515/htmp-2018-0076

Yaxu Zheng, Yutian He + Show 3 more

Open Access

https://doi.org/10.1515/htmp-2018-0076

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Abstract

Abstract This work conducted systematic studies on the effect of B on the hot ductility behavior of Fe-36Ni alloy over the temperature range of 900–1,200 °C by use of Gleeble-3500 thermal simulator, Thermo-Calc software, transmission electron microscopy and secondary ion mass spectroscopy. The influencing factors and mechanisms are also discussed in the present work. Results show that all the values of area reduction of the investigated alloy samples are below 60 % in the temperature range of 900–1,000 °C, indicating the poor hot ductility of the investigated alloys in this temperature range. When the grain boundary sliding occurs during the hot tensile processes, the fine secondary phase particles at grain boundaries prevent the occurrence of dynamic recrystallization and promote the nucleation and propagation of cracking simultaneously, resulting in the poor hot ductility of the investigated alloys in this temperature range. In the B bearing alloy, the segregation of B atoms around austenite grain boundaries promotes the solute dragging effects at grain boundaries and strongly inhibits the occurrence of dynamic recrystallization, which increases the brittle temperature to 1,000 °C. When the temperature exceeds 1,050 °C, the occurrence of dynamic recrystallization improves the hot ductility significantly. However, the coarsening of recrystallized grains and the formation of inter dendritic cracks decrease the hot ductility of the alloy gradually when the temperature increases from 1,100 °C to 1,200 °C.

Highlights

Fe-36Ni invar alloy is widely used in liquefied natural gas containers owing to its uniquely low coefficient of thermal expansion below the Curie point [1]
Results show that all the values of area reduction of the investigated alloy samples are below 60 % in the temperature range of 900–1,000 °C, indicating the poor hot ductility of the investigated alloys in this temperature range
When the grain boundary sliding occurs during the hot tensile processes, the fine secondary phase particles at grain boundaries prevent the occurrence of dynamic recrystallization and promote the nucleation and propagation of cracking simultaneously, resulting in the poor hot ductility of the investigated alloys in this temperature range

Summary

Introduction

Fe-36Ni invar alloy is widely used in liquefied natural gas containers owing to its uniquely low coefficient of thermal expansion below the Curie point [1]. The inhibition of segregation of sulfur atoms to grain boundaries effectively improves the hot ductility of Fe-36Ni alloy. Many previous researches [9,10,11] reported that the addition of boron in carbon steels improved the hot ductility in ferrite and austenite temperature region and single austenite temperature region simultaneously by the acceleration of dynamic recrystallization and inhibition of ferrite transformation. Yu contained 0.14 % Mn and 0.002 % S It was reported [14] that when the actual Mn:S ratio is higher than the critical Mn:S ratio (Mn:S)c, the free sulfur is prevented to segregate to grain boundaries due to the formation of MnS. The Mn fixes the free sulfur atoms Under this condition, the segregation behavior of boron and precipitation behavior of boron containing particles around grain boundaries in Fe-36Ni alloy have been studied. The reduction of area (RA%) was used to evaluate the hot ductility of the experimental alloys

Materials and methods

Results and discussion

Conclusions