Solid-State Phase Transformation in Metallic Matrix of High-Chrome Cast Iron in Subcritical Temperature Range
Yu. G. Chabak
Pryazovskyi State Technical University, 7 Universytets’ka Str., 87500 Mariupol, Ukraine
Received: 16.06.2015; final version - 28.09.2015. Download: PDF
The results of study of phase—structural changes’ kinetics in cast high-chromium cast iron during ageing in the subcritical range of temperatures (350—700°C) are presented in a given article. The iron composed of 2.70% C, 2.20% Mn, 0.55% Si, 14.55% Cr, 0.93% Ni, 0.39% Mo, 0.38% V, and 0.11% Ti is studied. Using the microstructural method, the TTT-diagram of transformation of primary (as-cast) austenite is plotted. As found, within 25 hours of soaking, the austenite transformation occurs only in a range of temperatures of 550—700°C to form a fine eutectoid consisting of ferrite and cementite carbides having the fibrous or platelet shapes. Kinetics maximum of transformation corresponds to 650°C; the incubation period at this temperature is 5 minutes; the completion of the transformation is recorded after 8 hours of soaking. Formation of eutectoid at 700°C is accompanied by the precipitation of fine secondary carbides from austenite. Repeated heating of cast iron having fully eutectoid matrix at 650—700°C for up to 25 hours results in complete spheroidization and coagulation of eutectoid carbides. This leads to the formation of ferrite matrix with the granular carbides of mean diameter of 0.23—0.28 $\mu$m. The data regarding the influence of soaking time at 650—700°C on the number of eutectoid carbides of different shapes are presented in a given article. During repeated heating, the cementite type of carbides is not changed. The activation energy of coagulation of eutectoid carbides is calculated to be 73.1 kJ/mol. It indicates that rates of spheroidization and coagulation of eutectoid carbides are controlled by diffusion of carbon in ferrite. Because of doubled subcritical treatment, the microhardness of matrix reduces to 285—365 HV as well as bulk hardness reduces to 38—41 HRC that provides satisfactory machinability of cast iron.
Key words: cast iron, austenite, eutectoid carbides, spheroidization, hardness.
PACS: 61.72.Qq, 62.20.fk, 62.20.Qp, 64.75.Op, 81.30.Mh, 81.40.Cd, 81.40.Ef
Citation: Yu. G. Chabak, Solid-State Phase Transformation in Metallic Matrix of High-Chrome Cast Iron in Subcritical Temperature Range, Metallofiz. Noveishie Tekhnol., 37, No. 10: 1349—1367 (2015) (in Russian)