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Metal Casting Technologies : Dec 2009
METAL Casting Technologies December 2009 29 ● Primary carbides that form during the first stage of solidification in hyper-eutectic alloys. ● Secondary (precipitated) carbides that form during destabilisation heat treatments, during annealing or during slow cooling after casting into thick sections. ● Pearlitic type carbides that form on cooling after casting, in thicker sections and/or when the Cr/C ratio is low, or during annealing heat treatments. ● Tempered carbides that form during tempering of martensitic structures after hardening treatments. Primary carbides are undesirable except for those applications involving erosion wear with no impact e.g. shot blast blades and certain pump parts. Pearlitic carbides are also avoided in final structures since pearlite reduces wear resistance. The carbide types known to be present in the Fe-Cr-C system are M3C, M7C3 and M23C6. Additional carbide forming elements such as molybdenum, manganese and vanadium are soluble in both M3C and in M7C3, and they may also give rise to other types including M2C and M6C. Elements such as nickel and copper have little solubility in the carbides and remain in solution in the matrix. Together with Mo, Cu and Ni are used to increase hardenability during heat treatment or to assist in the formation of as-cast austenitic matrix structures. The effect of chromium content on structures Most commercial wear resisting irons have hypoeutectic compositions so that they solidify as primary austenite dendrites followed by a eutectic of austenite + carbides. In irons containing less than 12% Cr M3C eutectic carbide with a hardness of around 1000Hv is present; below about 6% Cr this carbide is in a continuous form and the irons have reduced toughness. At 8-10% Cr the eutectic carbides become duplex, consisting of M7C3 and M3C, and are less continuous due to the presence of M7C3. Above 12% Cr the eutectic carbide consists completely of M7C3, this is fibrous (rod-like) with reduced continuity and it has a higher hardness of around 1400-1600 Hv providing improved fracture toughness and wear performance when compared to the lower Cr irons. The effects of the presence of up to 36%Cr on general microstructure are illustrated in Figure 1 (a-d). Eutectic carbide morphology, as revealed by deep etching, in a 5% Cr and in a 30% Cr iron can be compared in Figure 2. The coarse nature of primary M7C3 carbide in a hyper-eutectic 25%Cr iron is illustrated in Figure 3. This view shows the hexagonal nature of the M7C3 carbide: both primary and eutectic forms grow as rods with hexagonal cross section and both contain hollow cores containing matrix constituent. Metal Analyzers that Meet Your Demands Swamped with samples? Fighting high analysis costs? In need of extraordinary analytical performance? Profit from the superior capabilities of the SPECTRO metal analyzers for higher speed, lowest argon consumption, virtually no consumables and very little maintenance requirements - as well as unmatched analytical performance. SPECTROMAXx -- Rapid analysis of all common elements in all common bases -- Low maintenance spark stand with very low argon consumption -- No recalibration or profiling required due to ICAL logic system -- Available as benchtop or floor model with several versions to choose SPECTROLAB -- High-performance metal analysis for demanding requirements -- Unique hybrid optic with analog detectors and digital sensors, recording the complete spectrum from 120-780 nm -- Digital plasma generator for exact control of the plasma conditions -- New kind of readout system for detailed and flexible evaluation of each individual discharge Talk with SPECTRO and find out why SPECTRO's analyzers and monitors are an investment in better efficiency and higher profitability. Tel. +852.2976.9162, Fax +852.2976.9542 firstname.lastname@example.org www.spectro.com
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