Structural Characterisation of High Chromium Cast Irons

摘要

Alloy white irons containing 12-30% chromium are widely used to combat abrasive wear in applications such as mineral processing, cement manufacture and slurry pumping. Microstruc-tural characterisation of these complex alloys plays a key role in the development of optimum compositions and thermal treatments. This paper describes how the microstructures of high chromium cast irons can be examined using both SEM and thin foil TEM techniques. The value of electron microscopy studies is considered in relation to continued alloy developments and improvements in service performance. Observations of the form of eutectic and secondary carbides and associated matrix structures are described and discussed with reference to the effects of heat treatment in the following areas of study. 1. hardening and tempering 2. annealing before subsequent hardening treatments. 3. examination of the duplex nature of eutectic carbides in 30% Cr irons. 4. improvement of toughness The type and distribution of secondary and pearlitic carbides are dependent on alloy composition and heat treatment conditions. TEM examination revealed that the secondary carbides were M_7C_3 in 15%Cr irons and M_(23)C_6 in 25-30%Cr irons. Both eutectic M_7C_3 and secondary M_7C_3 carbides contain stacking faults whereas the secondary M_(23)C_6 carbides appear to be free from such faulting. No eutectic M_(23)C_6 carbides were identified in the alloys examined. Conventional annealing and destabilisation treatments have no apparent effect on the eutectic carbides in 15 to 25%Cr alloys but are found to cause a M_7C_3 to M_(23)C_6 transition in the eutectic carbide in a 30%Cr iron. Where possible the paper attempts to relate the results of the various structural characterisations to variations, in hardness and wear performance.