Phase Transition Kinetics in Austempered Ductile Iron (ADI) with Regard to Mo Content

Abstract

The phase transformation to ausferrite during austempered ductile iron (ADI) heat treatment can be significantly influenced by the alloying element Mo. Utilizing neutron diffraction, the phase transformation from austenite to ausferrite was monitored in-situ during the heat treatment. In addition to the phase volume fractions, the carbon enrichment of retained austenite was investigated. The results from neutron diffraction were compared to the macroscopic length change from dilatometer measurements. They show that the dilatometer data are only of limited use for the investigation of ausferrite formation. However, they allow deriving the time of maximum carbon accumulation in the retained austenite. In addition, the transformation of austenite during ausferritization was investigated using metallographic methods. Finally, the distribution of the alloying elements in the vicinity of the austenite/ferrite interface zone was shown by atom probe tomography (APT) measurements. C and Mn were enriched within the interface, while Si concentration was reduced. The Mo concentration in ferrite, interface and austentite stayed at the same level. The delay of austenite decay during Stage II reaction caused by Mo was studied in detail at 400 °C for the initial material as well as for 0.25 mass % and 0.50 mass % Mo additions.

A Unique Quenching and Deformation Dilatometer for Combined In Situ Neutron Diffraction Analysis of Engineering Materials

“…Between stages I and II, there is a processing window period, which lasts from 0 min to 3 h, depending on the austempering temperature and alloying elements content (i.e., Cu, Ni, and Mo). [32][33][34][35] The change of the phase fractions of austenite and ferrite as well as the carbon redistribution, as encoded in the crystal lattice expansion of austenite, can be extracted from the diffraction profiles and length change curves (Figure 4 and 5). [20,21] At the beginning/end of stage I (Figure 4a,c), the reflections are symmetric and can be fitted using a single Gaussian peak shape function.…”

“…Its result Reproduced with permission of the CC-BY license. [33] Copyright 2020, The Authors, published by Licensee MDPI, Basel, Switzerland.…”

“…Between stages I and II, there is a processing window period, which lasts from 0 min to 3 h, depending on the austempering temperature and alloying elements content (i.e., Cu, Ni, and Mo). [ 32–35 ]…”

“…At the higher austempering temperature of 400 °C, the maximum of carbon enrichment is reached again after the end of stage I phase transformation, as shown in Figure 5b. [ 33 ] This shows that it is impossible to determine whether the phase transformation or carbon diffusion has been completed using dilatometry alone and additional informations from diffraction are essential. At the same time, the length change curve cannot be used directly for quantitative phase calculation, as it is additionally affected by carbon diffusion.…”

“…The phase fraction, lattice constant change of austenite, and TTL curves of ADI during austempering, a) at 350 C for 3 h and b) at 400 C for 3 h. ND: results from neutron diffraction. DIL: length change signal from dilatometer.Reproduced with permission of the CC-BY license [33]. Copyright 2020, The Authors, published by Licensee MDPI, Basel, Switzerland.…”