Precession Electron Diffraction Detection and Phase Mapping of Retained Austenite and Carbides in a Heat Treated Low Alloy Carbon Steel Using a JEOL ARM 200 TEM with an AppFive Topspin System for Synchronized Beam Scanning and Precession
Retained austenite in heat treated steels is usually measured using X-Ray diffraction, with protocols outlined in ASTM Standard E975 [1]. Retained austenite has been measured in high alloy steels with large grains of retained austenite using EBSD [2]. However, low alloy steels heat treated to reduce retained austenite may have small grains of retained austenite that cannot reliably be detected by EBSD [3] or X-Ray diffraction.
Characteristics of Retained Austenite in Quenched High C-High Cr Alloy Steels
In this paper, the morphologies of martensite and retained austenite for 1.5 mass%C-12 mass%Cr and 1 mass%C-8 mass%Cr steels were observed by means of optical microscopy, XRD, SEM/EBSD and TEM. The amount of retained austenites was quantitatively investigated and compared with XRD, EBSD and TEM observation methods. The retained austenites were distributed in the form of a block type and a film type in the martensite structures. For the 1.5 mass%C-12 mass%Cr steel, the amount of block type retained austenite and film type one are almost equal in three distinct regions of carbide; primary carbide, secondary carbide and without carbide. In the case of 1 mass%C-8 mass%Cr steel, as to film type retained austenite there are not so much differences among those regions. However, block type retained austenites are distributed with much amount, especially in the region around primary carbide. [doi:10.2320/matertrans.MRA2008161]
Effect of the Distribution of Primary Carbide on Fatigue Strength of Cold Work Die Steels
The effect of distribution of primary carbides on fatigue strength has not been fully clarified for cold work die steels. Eight percent Cr type steel and JIS SKD11 have been studied by changing carbides area fraction. We have tested low cycle fatigue properties and discussed with comparing tensile properties and microfractographs. The test results are summarized as follows. 1) Tensile strength of 8Cr type steel and JIS-SKD11 is decreased by increasing carbide area fraction. But σ0.2 and σ0.02 is same in tested steels. independent to carbide area fraction. 2) Fatigue strength is decreased by increasing carbide area fraction and fatigue strength of 8Cr type cold die steel is higher than that of JIS-SKD11. 3) Fatigue crack is initiated from primary carbide in all specimens. 4) Fatigue life is determined by initiation life of fatigue crack in these cold work die steels. 5) Small crack is initiated at primary carbide in early stage of fatigue test and this small crack is connected to neighborhood carbide, finally this crack changes to propagation mode of fatigue phenomena.