Use of electron microscopy on microstructure characterization of high chromium cast irons

The physical metallurgy underlying the development of cast microstructures in abrasion resistant high chromium cast irons, and their structural modification by thermal treatments is relatively complex. Structural characterisation via electron microscopy therefore has a key role to play in furthering our understanding of the phase transformations that control the microstructures and hence the service performances of these irons as wear parts. This paper shows how both scanning and especially transmission electron microscopy can provide valuable information on the nature of eutectic and secondary carbides and on the matrix structures in these irons. Particular attention is given to current characterisation research on conventionally cast 30%Cr irons that are used for applications involving corrosive wear e.g. slurry pumps and on a semi-solid cast 27%Cr iron that has a potential for applications in industry.     

Primary particle movement and change of property of cast iron by centrifugal effect in semi-solid processing

The particle distribution in semi-solid slurry under centrifugal field was simulated and the main factors such as fraction of solid, rotation speed, and holding time effecting the particle distributions are discussed. The simulation results showed that primary particles rich zone is produced in radially outer area and these results are in good agreement with the experiment. The centrifugal effect produces the primary particles distribution along the radial direction. Denser particles are concentrated in outside than inner side. For high fraction of solid samples, ‘wall’ appear in the middle of samples because of high viscosity region making particle difficult to move. Longer holding time gives denser primary particles concentrated more in outside than inner side. Higher rotation speed gives increased gradient of hardness in the radial direction. It is due to that number of primary austenite at inner side at higher rotation speed is less than that at lower rotation one. At higher rotation speed, ledeburite forms more at inner side of specimen than at the other one. It is also shown that semi-solid processing at lower fraction of solid gives higher hardness because smaller number of primary austenite, namely more ledeburite forms in microstructure. The present study gives useful information on producing material with locally changing property by semi-solid processing.     

Effects of Heat Treatment on Hardness and Dry Wear Properties of a Semi-Solid Processed Fe-27 wt pct Cr-2.9 wt pct C Cast Iron

EfFects of heat treatments on hardness and dry wear properties of a semi-solid processed Fe-26.96 wt pct Cr- 2.91 wt pct C cast iron were studied. Heat treatments included tempering at 500℃, destabilisation at 1075℃ and destabilisation at 1075℃ plus tempering at 500℃, all followed by air cooling. Electron microscopy revealed that, in the as-cast condition, the primary proeutectic austenite was round in shape while the eutectic M7C3 carbide was found as radiating clusters mixed with directional clusters. Tempering did not change the microstructure significantly when observed by scanning or transmission electron microscopy. Destabilisation followed by air cooling led to a precipitation of secondary M23C6 carbide and a transformation of the primary austenite to martensite. Precipitation behaviour is comparable to that observed in the conventionally cast iron. Tempering after destabilisation resulted in a higher amount of secondary carbide precipitation within the tempered martensite in the eutectic structure. Vickers macrohardness and microhardness in the proeutectic zones were measured. Dry wear properties were tested by using a pin-on-disc method. The maximum hardness and the lowest dry wear rate were obtained from the destabilisation-plus-tempering heat treatment due to the precipitation of secondary carbides within the martensite matrix and a possible reduction in the retained austenite.     

Primary particle distribution in cast iron by semi-solid processing under centrifugal field

The particle distribution in semi-solid slurry under a centrifugal field was simulated and the main factors affecting the particle distribution are discussed. The simulation results showed that a zone rich in primary particles is produced radially in the outer area and these results are in good agreement with experimental results. IJCMR/478     

Effect of alloying elements on structure and mechanical properties of semi-solid processed cast iron

Semi-solid metal processing of alloys is one of the key technologies for producing advanced materials. Through semi-solid processing, it is possible to produce high quality cast components from grey cast iron. A series of experiments were carried out to clarify the effect of the alloying elements copper, chromium, molybdenum, and nickel on the properties of cast iron. A comparison was made of the microstructure and mechanical properties in semi-solid processed cast iron and ordinary cast iron. This showed that an increase in the level of alloying elements in cast iron gave a higher level of hardness. However, the tensile strength of alloyed semi-solid cast iron did not exceed that of grey iron, for every composition, as a result of the characteristic microstructure produced by semi-solid processing. Thus, the alloying elements had little effect on the tensile strength of semi-solid cast iron, but did have an effect on elongation. The tensile strength of semi-solid cast iron, however, still depends on the cooling rate.     

Electron microscopy and hardness study of a semi-solid processed 27 wt%Cr cast iron

A semi-solid processed 27 wt%Cr cast iron was studied by electron microscopy and its microstructure was related to the hardness. In the as-cast condition, the primary proeutectic austenite was round in shape while the eutectic M₇C₃ carbide was found as radiating clusters mixed with directional clusters. Growth in the [0 0 1]M₇C₃ with planar faces of {0 2 0}M₇C₃ and was usually observed with an encapsulated core of austenite. Destabilisation heat treatment followed by air cooling led to a precipitation of secondary M₂₃C₆ carbide and a transformation of the primary austenite to martensite in the semi-solid processed iron. Precipitation behaviour is comparable to that observed in the destabilisation of conventional cast iron. However, the nucleation of secondary M₂₃C₆ carbide on the eutectic M₇C₃ carbide was observed for the first time. Tempering after destabilisation led to further precipitation of carbide within the tempered martensite in the eutectic structure. The maximum hardness was obtained after destabilisation and tempering heat treatment due to the precipitation of secondary carbides within the martensite matrix and a possible reduction in the retained austenite.