| dc.description.abstract | The cost of replacing worn parts in mining equipment, cement plants, thermal power plants and machinery poses a continuous challenge to materials technologists. Components designed for such applications must possess adequate abrasion resistance coupled with the ability to withstand impact, often in corrosive atmospheres. Normally, a material that is highly resistant to abrasion is hard and brittle and lacks toughness. Development of new abrasionresistant materials invariably aims at improving toughness in addition to enhancing abrasion resistance.
For several decades, austenitic manganese steels, chrome-moly steels and white irons have been used for applications involving severe wear. Subsequently, attempts were made to enhance the toughness and abrasion resistance of white irons by adding alloying elements. In particular, large additions of chromium were tried to bring about major modifications in the microstructure of white irons and make them suitable candidates for wearresistant applications. Addition of substantial chromium to cast irons leads to the formation of chromium carbides of the type (CrFe)C; these carbides exhibit hardness in the range of 1200-1600 HV, compared to normal iron carbide FeC, whose hardness is around 1000 HV. In addition to enhanced hardness, the morphology of chromium carbides is different-they are more discontinuous and well dispersed in the matrix, resulting in higher toughness and abrasion resistance. Furthermore, through appropriate heat treatments, it is possible to obtain either a soft but workhardenable austenitic matrix or a hard martensitic matrix to suit varied wearresistant applications.
Adequate data exists on the production and properties of highchromium cast irons (containing up to 30% Cr). However, knowledge on the abrasive wear behaviour of such materials and the effect of processing variables on their wear resistance and toughness is limited. Hence, a systematic investigation was undertaken to study the abrasive wear behaviour of two popular varieties of highchromium cast irons containing 15% Cr and 27% Cr, respectively. In particular, the following aspects were studied extensively:
a. Influence of heat treatment on the structure of the above two highchromium irons.
b. Effect of additional alloying elements and grain refiners on the structure, abrasion resistance and toughness of the two varieties of highchromium cast irons (with austenitic and martensitic matrices).
Test castings of 15% Cr iron and 27% Cr iron were produced, maintaining neareutectic composition by balancing the carbon content. Additional alloying elements (Mo, Ni, Cu, V, B) and grain refiners (Ti, Nb) were added in preselected ranges to study their influence on structure and properties.
Test specimens were subjected to hardening treatments with various combinations of austenitizing temperature and time. The samples were then evaluated for:
a. Structural features (optical microscopy, SEM, EDAX, Xray diffraction)
b. Dry abrasion resistance and hardness
c. Wet abrasion resistance
d. Fracture toughness
e. Impact bend tensile strength and impact energy
Results
Microstructure
15% Cr iron:
Austenitizing temperature significantly influences microstructure. Due to low hardenability, soft phases persist unless alloyed with Mo, Ni, or Cu to improve hardenability and obtain a fully martensitic matrix.
27% Cr iron:
Enhanced hardenability is reflected in heattreated microstructures. Austenitizing temperature plays a major role. Alloy additions (Mo, V, B, Ti, Nb) lead to formation of harder carbides and matrix changes. EDAX results show that carbides in alloyed irons are rich in chromium and the respective carbideforming elements. XRD analysis reveals the influence of heat treatment on retained austenite content.
Dry Abrasion Resistance
27% Cr iron clearly exhibits superior abrasion resistance compared to 15% Cr iron. Poor hardenability limits the wear resistance of 15% Cr irons. Alloying with Mo and Ni improves wear resistance, particularly combined Ni + Cu addition. For 27% Cr irons:
Austenitic matrix excellent abrasion resistance
Martensitic matrix (after high austenitizing temperature) even higher abrasion resistance
Additional alloying further enhances resistance
Wet Abrasion Resistance
Both 15% and 27% Cr irons exhibit far superior wet abrasion resistance compared to conventional materials. 27% Cr iron performs better than 15% Cr iron. Mo additions further enhance performance. Tests in alumina and quartz-acid slurries confirmed superiority of highchromium irons.
Fracture Toughness
Fracture toughness of highchromium irons is generally higher than that of white iron and Nihard iron.
15% Cr iron: Austenitizing temperature must be controlled; Mo and B decrease toughness; V and Ni increase it.
27% Cr iron: Higher toughness than 15% Cr iron due to discontinuous, evenly distributed carbides.
Impact Bend Tensile Strength
For 15% Cr iron, lower austenitizing temperatures yield higher strength. Mo and B reduce strength; Ni and V increase it. Similar trends occur in 27% Cr iron.
Impact Energy
Austenitizing temperature significantly influences impact energy. Mo and B decrease impact strength; Ni and V increase it; Cu shows negligible effect. 27% Cr irons exhibit higher impact strength than 15% Cr irons, with austenitic 27% Cr irons outperforming martensitic ones.
Conclusion
An appropriate combination of heattreatment variables and alloying additions must be selected to achieve superior abrasion resistance and toughness in highchromium cast irons containing 15% Cr and 27% Cr. | |
