An Unusual Structure of an As-cast 30 % Cr Alloy White Iron

Abstract

An unusual microstructure of an as-cast 30 wt% Cr and 2.26 wt% C iron has been examined by optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the microstructure varied with positions in the cast bar. In the upper part of the cast bar where the cooling rate was faster, the matrix was essentially austenite with some patches of ferrite-plus-precipitated carbides as dendritic regions. Whereas, the central and the lower parts of the cast bar where cooling rates were slower, there was less austenite with greater amount of ferritic zones. The microstructure of core regions at the centre of the dendritic ferrite-plus-precipitated carbides zones contained interconnected carbides that are believed to be the product of a peritectic reaction. TEM examination confirmed that these peritectic carbides were M 7 C 3 type. Bainite and martensite were also observed in the transition zones close to the ferrite-plus-precipitated carbides zones. These were believed to result from solid-state decomposition of the dendritic austenite in the later stage of cooling.KEY WORDS: high chromium cast iron; microstructure; electron microscopy; peritectic reaction.was sectioned transversely to give specimens with an approximate thickness of 15 mm. Sample Preparation for Microstructural Investi-gation Metallographic specimens for optical microscopy (OM) and scanning electron microscopy (SEM) were mounted in resin, ground on silicon carbide papers to 1 200 grit and then polished to 1 mm diamond finish. The etchants used were (1) 50 mL of HCl and 10 g of Na 2 S 2 O 5 in 100 mL distilled water, to reveal the ferritic and austenitic microstructure, and (2) 10 % HCl in methanol for 6 h for deep etching to examine the morphology of carbides. The microstructures were observed using a JEOL JSM-5410LV scanning electron microscope. Chemical microanalysis was performed using an Oxford Instruments ATW2 X-ray Spectrometer. The SEM was operated at 20 kV using a working distance of 17 mm from the objective lens.Thin foils for transmission electron microscopy (TEM) were prepared by sectioning specimens, perpendicular to the axis of the as-cast cylindrical rod, with a cubic boron nitride saw to obtain slices with a thickness of about 200 mm. These were then ground manually on silicon carbide papers (down to 1 200 grit) to reduce their thickness to about 80 mm. Discs, 3 mm in diameter, were then punched out and thinned with a Struers, Tenupol-3 twin-jet electropolisher operated at 15 V and approximately 20 mA, using a solution containing 10% perchloric acid and 30% of 2-butoxyethanol in absolute ethanol maintained at Ϫ15°C. The thin samples were argon-ion-beam-thinned using a Gatan model 600 DIF Duo Mill ion beam thinner for final stage thinning. They were then examined using a JEOL JEM-2010 scanning-transmission electron microscope, operated at 200 kV. Hardness MeasurementThe specimens prepared for SEM were also used for hardness determination. A Galileo microhardness tester was used to measur...

Dynamic Precipitation Behavior of Secondary M7C3 Carbides in Ti-alloyed High Chromium Cast Iron

“…13,14) Furthermore, the volume fraction values (~5%) in this study agree with the Wiengmoon et al results. 6,10) In summary, the present results and the Wiengmoon et al results on volume fraction of secondary carbides indicate that the volume fraction of secondary carbides is not obviously affected by the holding time and temperature when the holding temperature is reached. …”

“…1,2) Therefore, the research has focused on the type, size, volume, morphology and distribution of these secondary carbides. Many previous studies [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] have been carried out to determine the types of secondary carbides (M7C3 type or M23C6 type) after heat treatment by using Light Optical Microscopy (LOM), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Transmission Electron Microscopy (TEM), as shown in Table 1.…”

“…Also, Pearce et al 4,5) and Wiengmoon et al [6][7][8][9][10][11][12][13][14] discussed the effect of the Cr content on the type of secondary carbides. Their reports show that in HCCI alloys containing 15-20%Cr, the secondary carbides are M7C3 type, and in HCCI alloys containing 25-30%Cr, they are M23C6 type.…”

Effect of Heat Treatment on Microstructure and Mechanical Properties of Ti-alloyed Hypereutectic High Chromium Cast Iron

“…Most of the previous studies are focused on identifying the new carbides by using transmission electron microscopy (TEM). 1,[4][5][6][7][8][10][11][12] They have furthermore studied how the new carbides influence the final properties. The mechanical properties are to a large extent determined by the shape, size distribution and volume fraction of martensite and carbides formed in the HCCI.…”

Microstructure, Mechanical Properties, and Abrasive Wear Behaviour of Si‐Mn‐Cr‐B Cast Steel as a Function of Carbon Concentration

“…Abrasion resistant white cast irons, such as high chromium white cast irons, Ni-hard cast irons, high tungsten white cast iron and high vanadium cast iron etc. [7][8][9][10][11][12] have higher abrasion resistance because the eutectic carbide can strongly resist abrasive wear, but they have poor strength and toughness, and they are prone to fracture under impact wear [13][14][15]. The development of high hardness and sufficient strength and toughness abrasion resistant material is thus of great importance.…”