The first heattreatment is heating the high chromium cast iron to a temperature between 300and 400°C. This temperature range produces no change in the microstructure ofthe material. There is no precipitation of secondary carbides and notransformation of the matrix.
However, hardness of material can be slightly affectedand reduced. This caused by relieving of internal residual stresses partialtempering of martensite. This can lead to have different microstructures, suchas epsilon-carbides formation and presence of cementite. The change in phase isa function of many parameters such as time of heating, composition of materialand temperature range 11. 3.
7.2 Heat treatment at 500-600°C: Hardness ofas-cast irons increased to withstand aggressive environments in slurry pumps.Heating the high chromium white iron to 550°C for 2-6 hours will relieve thestresses in the iron. Literature results show that heating in a range of 2-6hours will slightly increase hardness. Heating the material for more than 6hours can cause a significant change where the microstructure change to amixture of ferrite and cementite, causing the drop of hardness.
At thistemperature range, there is no possibility for any secondary carbides to form,enhancing significantly the hardness 11. 3.7.3 Heat treatment at 600-850°C: Experiments showedthat heat treatment above 600 °C results in a decrease of hardness.
This isbecause of the transformation of the matrix (austenite and/or martensite) todifferent microstructures with lower hardness, such as ferrite and pearlite11. Figure 10 illustrates that quenching the material at 850oCwill increase its hardness; hence, its wear resistance will increase. Incontrast quenching the media at 750oC before its fully formingaustenite will cause the material to have low hardness. Literature wasconducted to improve the abrasive wear resistance of the high chromium castiron ASTM A-532 through thermal treatment cycles.Figure 10 The change of temperature with hardness 3.7.4 Heat treatment at 900-1000°C (Destabilizing):In order to obtainmaximum hardness of high chromium cast iron and highest wear performance,as-cast cast irons are subjected to destabilizing heat treatment. The ideabehind it is to heat the cast iron in the range of 950-1065°C for 2-4 hours in orderto get secondary chromium carbides.
This will lead the austenitic matrix tolose carbon becoming less stable at room temperature, while martensiticstarting temperature (Ms) is been raised. Upon cooling the unstable austenitematrix will transform into martensite, which is harder and have a betterperformance in abrasive applications such as slurry pumps. It’sbeen known that high chromium cast iron in as-cast condition can have a matrixof either austenite or martensite. Destabilization in cast iron with austeniticmicrostructure has been explained. In as-cast martensitic microstructure thereare retained austenite and they have a negative effect on hardness levels. Toquench the material after destabilization procedure.
Different quenching mediacould be applied to show the change of the mechanical properties anddetermining the increase of hardness. The quenching media are water, oil, airand air & dry air as shown in the figure 8.