Gray cast Iron

               GRAY CAST IRON                   

Gray iron, or grey cast iron, is a type of cast iron that has a graphitic microstructure. It is named after the gray color of the fracture it forms, which is due to the presence of graphite.^[1] It is the most common cast iron and the most widely used cast material based on weight.

ADVANTAGES AND DISADVANTAGES

Gray iron is a common engineering alloy because of its relatively low cost and good machinability, which results from the graphite lubricating the cut and breaking up the chips. It also has good galling and wear resistance because the graphite flakes self lubricate. The graphite also gives gray iron an excellent damping capacity because it absorbs the energy and converts it into heat. Grey iron cannot be worked (forged, extruded, rolled etc.) even at temperature.

Gray iron also experiences less solidification shrinkage than other cast irons that do not form a graphite microstructure. The silicon promotes good corrosion resistance and increase fluidity when casting. Gray iron is generally considered easy to weld. Compared to the more modern iron alloys, gray iron has a low tensile strength and ductility; therefore, its impact andshock resistance is almost non-existent.

APPLICATIONS

It is used for housings where the stiffness of the component is more important than its tensile strength, such as internal combustion engine cylinder blocks, pump housings, valve bodies, electrical boxes, and decorative castings. Grey cast iron's high thermal conductivity and specific heat capacity are often exploited to make cast iron cookware and disc brake rotors

       STRUCTURE

A typical chemical composition to obtain a graphitic microstructure is 2.5 to 4.0% carbon and 1 to 3% silicon by weight. Graphite may occupy 6 to 10% of the volume of grey iron. Silicon is important to making grey iron as opposed to white cast iron, because silicon is a graphite stabilizing element in cast iron, which means it helps the alloy produce graphite instead of iron carbides; at 3% silicon almost no carbon is held in chemical form as iron carbide. Another factor affecting graphitization is the solidification rate; the slower the rate, the greater the time for the carbon to diffuse and accumulate into graphite. A moderate cooling rate forms a more pearlitic matrix, while a fast cooling rate forms a more ferriticmatrix. To achieve a fully ferritic matrix the alloy must be annealed.^[1][4] Rapid cooling partly or completely suppresses graphitization and leads to the formation of cementite, which is called white iron.

The graphite takes on the shape of a three-dimensional flake. In two dimensions, as a polished surface, the graphite flakes appear as fine lines. The graphite has no appreciable strength, so they can be treated as voids. The tips of the flakes act as preexisting notches at which stresses concentrate and it therefore behaves in a brittle manner. The presence of graphite flakes makes the Grey Iron easily machinable as they tend to crack easily across the graphite flakes.

Crystallinity, Microstructure and Mechanical Strength of Yttria-Stabilized Tetragonal Zirconia Ceramics for Optical Ferrule    (USN:4JC15ME026)

INTRODUCTION

It is well known that yttria (Y2O3)-stabilized tetragonal zirconia (ZrO2) polycrystal (Y-TZP) possesses excellent mechanical properties and represents toughened zirconia ceramics [1,2]. The relationship between microstructure and mechanical properties in Y-TZP ceramics has been studied extensively over the past decade [3-5]. Generally, it has been found that Y-TZP ceramics have high strength and fracture toughness, making them attractive candidates for a number of demanding structural applications. It is also well established that these desirable mechanical properties are strongly influenced by grain size. For example, maximum strength is usually achieved with a small grain size (< 1 μm). Their excellent mechanical properties are derived from the stress-induced martensitic transformation of the metastable tetragonal to the monoclinic phase [6]. The synthesis of fully tetragonal, pure, nano-crystal, uniformly aggregated, agglomerate free zirconia powder is the main emphasis in the production of advanced ceramics with a desirable microstructure and properties. The small grain size of the nanomaterials has a pronounced effect on many physical properties, such as increased strength and hardness. Throughout our previous work [7], we studied crystallinity and microstructure of Y-TZP ceramics as a function of the variation of raw materials provided by different suppliers with different particle-properties. From previous results, we confirmed that the raw materials with fine particle size and high tetragonality could be sintered to dense Y-TZP at 1400˚C. In this work, in order to compare commercially available sample A with two raw materials (denoted B and C), which exhibited relatively good sinterability in our previous work [7], were selected for practical usage. Crystallinity, microstructure and mechanical strength were examined as a function of various sintering temperature.

 show the FE-SEM images of fractured cross section of the sintered specimens. As clearly all lowered by high-temperature sintering. We assume that hardness was probably decreased by existence of some amorphous-like phases in appearance, as shown in  Conclusively speaking, for the specimen B, low-temperature sintering was favorable for the sintered body with high tetragonality and good mechanical properties.

shows surface morphology (a) and photo-graph (b) of prepared optical ferrule annealed at 1350°C by using raw material B. As shown well-defined small grains with below ~1μm were densely formed. For our specimen B, it is difficult to find pores after sintering, which generally exhibited at the grain or at the grain boundary of sintered zirconia specimens. A smooth specimen surface was obtained by polishing the scratches.

the specimens had well-crystallized grains with 0.2 ~ 0.4 μm in size. Break down was concurrently occurred at the inside and at the interface of the grains for all the specimens. However, for the specimen C, some pores were found 

 shows the bending strength of the specimens. The highest bending strength was obtained for the standard sample A. Specimen C, which possessed some pores as shown exhibited low bending strength, while the specimen B performed relatively high bending strength. Low bending strength of the specimen C was quite reasonable, since observable pores probably corresponding to weak mechanical strength were recognized in sintered body, as shown

Vickers’ hardness is plotted as.It is found that specimen B sintered at 1350°C and specimen C sintered at 1450°C showed higher values. It should be pointed out in this work that, for the specimen C, some unclear problems, such as pores in sintered body and low bending strength, were still existed, although their hardness above ~1200 was sufficient for practical usage to optical ferrule. Furthermore, for the specimen B, hardness was rather

4. Conclusions

In this work, two raw materials (B and C), which exhib-ited relatively good sinterability in our previous work were selected in order to compare with commercially available sample A. Low-temperature sintering was ef-fective to increase tetragonality of the specimens B and C. For all the specimens, microstructures contained well- crystallized grains with 0.2 ~ 0.4 μm in size were char-acterized by FE-SEM. For practical usage, characterized by FE-SEM. For practical usage, low-temperature sintering at 1350°C was favorable for the specimen B because of high tetragonality and mechanical strength. While, for the specimen C, some unsoluble problems, such as pores in the sintered body and low bending strength, was still occurred.