20CrMnMo Steel metallographic analysis
This report is about the metallographic analysis of 20CrMnMo steel. This steel is classified as a kind of medium-alloyed high carbon steel and is commonly used in the manufacture of cemented carbide and other cutting tools.
The specimens used for the metallographic analysis were obtained from the annealing furnace after the steel had been heated to 1250 °C for 50-60 minutes and then cooled in the furnace. The specimens were then reheated in hydrogen atmosphere to 1050°C and again cooled in the furnace. After this the samples were ground and the grinding process was carried out with SiC emery at several different grit sizes.
The metallographic analysis of 20CrMnMo steel was carried out using a Zeiss Stereo Microscope with a calibrated
Polaroid Camera. The specimens were first examined in order to detect any signs of macro-structure and then the surface was mounted and polished in order to enhance the contrast of the structure. The following steps then followed:
1. The metallographic samples were examined at 10x magnification under the microscope and then photographed. From this examination it was possible to identify any cracks, segregation or excessive porosity on the surface of the specimens.
2. The next step was to examine the cross sections of the samples at higher magnifications. From this examination more information about the microstructure of the steel was obtained. The grain size, distribution and shape were easily observable.
3. The third step was to observe the metallographic samples under polarization. This allowed for detailed inspection of the stress relief behavior and the presence of any residual austenite which would have been revealed by the characteristic purple color.
4. The fourth step was to examine the micrographs. All of the minute details such as grain boundaries, phases and grain morphology could be observed clearly under higher magnifications.
5. The fifth step was to prepare an etched micrograph. After the samples had been etched using Na3PO4, all of the alloys present in the steel were visible, since different elements and compounds reacted with the etchant in different ways.
The metallographic analysis of 20CrMnMo steel revealed a complex microstructure consisting mainly of austenite grains and martensite with some ferrite and carbide precipitation. The grain size was mostly within the range of 6 to 8 μm. The etching micrograph revealed a homogeneous distribution of evenly sized austenite grains, which accounted for the significant amount of plasticity of this steel. The presence of residual austenite and the results of the stress relief test proved that the samples had experienced a significant amount of heat treatment.
Overall, the results of the metallographic analysis of 20CrMnMo steel revealed that it was suitable for use in cutting tools and other applications that require their extreme toughness and plasticity.
