Influence of austenitising and austempering temperatures on microstructure and properties of dual phase ADI

Abstract

The present work describes studies about the influence of processing variables on the microstructure and properties of dual phase austempered ductile iron (ADI). The upper and lower critical temperatures of conventional ductile iron melt were determined. Heat treatments involving austenitising within the intercritical interval, followed by austempering, allowed microstructures to be obtained composed of different combinations of free ferrite and ausferrite. Mechanical and fracture toughness tests performed on samples with mixed structures showed interesting combinations of strength and toughness, in comparison with fully ferritic and fully ausferritic matrices, particularly when austempering was carried out at 350uC. The results of the critical crack size, expressed by the relationship (K IC /s YS ) 2 , which indicates the relative toughness of the material, showed the best values for ferritic matrices with y20% ausferrite. This effect is attributed to the location of the ausferrite in the last to freeze regions (the weakest areas in the matrix) where it acted as a reinforcing phase.

High Silicon Ductile Iron: Possible Uses in the Production of Parts with ^|^ldquo;Dual Phase ADI^|^rdquo; Microstructure

“…The methodology employed to establish the intercritical interval for a specific alloy has been detailed in previous works, [4][5][6] and is herein summarized as follows: several specimens of each melt 12 mm in diameter × 25 mm in length were subjected to annealing thermal cycles consisting of: a) austenitizing at 900°C for 3 hours, b) cooling down to 740°C inside the furnace, c) holding at 740°C for 10 hours, and d) cooling down to room temperature inside the furnace. To establish the intercritical interval for each melt, after annealing, the samples were subjected to thermal cycles involving austenitizing stages ranging from 730°C to 900°C at steps of 10°C.…”

“…As already defined by the authors in previous works, [4][5][6] the lower critical temperature (Lct) is the lower temperature at which the austenite transformation starts (detected by the presence of less than 5% of martensite after quenching). The upper critical temperature (Uct), in turn, is defined as the temperature at which a matrix with over 98% of martensite is detected after quenching the samples held at such temperature.…”

“…This improvement is explained by the novelty of this DI microstructure, which is composed of different amounts of free (or allotriomorphic) ferrite and ausferrite (ADI regular structure). [1][2][3][4][5][6][7][8][9][10][11] Special thermal cycles have been developed by different researchers in order to obtain Dual Phase ADI. One of them has been advanced and used by Kilicli et al 7,8) and Basso et al [4][5][6] It consists in subjecting fully ferritic DI parts to an incomplete austenitization stage, at different temperatures within the intercritical interval of the Fe-C-Si diagram (see Fig.…”

As cast high silicon ductile irons with optimised mechanical properties and remarkable fatigue properties

“…The temperature was 21¡2uC during all the experiments. The tests were stopped after breaking the specimen or after a total number of 4610 6 cycles. In order to compare the fatigue results with the ones obtained from a low silicon cast iron alloy (LSI) with high impact properties, a set of 9 EN GJS 400-18-LT grade keel blocks were produced following an analogous methodology to the one described above.…”

“…An alternative is the development of austempered ductile iron (ADI) alloys where alloying methods and subsequent heat treatments are required to form a bainitic or ausferritic matrix. [4][5][6][7] Owing to the extra costs related to ADI, several investigations have been devoted to improve the mechanical properties obtained from as cast cast irons. In this line, silicon strengthened DI alloys become an interesting group of materials due to their good strength/elongation ratio when comparing to the standard DI grades.…”