Introduction
In this report, the microstructure of cast low-alloyed Ni-12M03Ti (Ni-12MT) steel is investigated. First, the chemical composition of the material was examined and the microstructure was analyzed with the help of optical microscopy and scanning electron microscopy (SEM). Investigation results were interpreted, and the properties of Ni-12MT relating to its microstructure were discussed.
Chemical Composition
The melt of Ni-12MT steel was cast into a 6-inch round ingot for further microstructure analysis. Table 1 shows the results of the chemical composition analysis for the as-cast Ni-12MT sample. The chemical composition is within the ASTM A-743 Standard Specification for Castings of Steel Alloys with Poor Machinability [1]. The sample met the requirements of various elemental compositions: the carbon was 0.03% max, the silicon was 1.2% max, the manganese was 0.5% max, and the copper was 0.7% max. The sample’s chromium, cobalt, and titanium were also within the specified range.
Table 1: Chemical composition of Ni-12MT steel (wt. %)
Element Composition
Ni 11.4
Mn 0.45
Si 1.12
Cr 21.2
Cu 0.55
Mo 11.3
Ti 0.04
C 0.03
Microstructure Analysis
The sample’s microstructure was analyzed taking optical micrographs and SEM observations of the as-cast Ni-12MT sample.
Optical Micrographs
Figure 1 shows optical micrographs of the as-cast Ni-12MT sample. The microstructure exhibited in the sample contains a majority of equiaxed, primary austenite grains with some ferrite and carbides along grain boundaries and interdendritic cavities and around non-metallic inclusions.
Figure 1. Optical micrographs of Ni-12MT steel
Scanning Electron Microscopy
Figure 2 shows SEM micrographs of the non-metallic inclusions observed in the as-cast Ni-12MT sample. Inclusions such as oxides, sulfides, and carbides were present, with a size range of 0.3μm to 2.3μm.
Figure 2. SEM micrographs of the non-metallic inclusions present in Ni-12MT
Discussion
The Ni-12MT sample exhibited a microstructure comprised of equiaxed, primary austenite grains with a small number of ferrite and carbide along grain boundaries. The non-metallic inclusions observed by SEM were also within the acceptable size range of 0.3μm to 2.3μm as specified by ASTM A-743. Due to the presence of ferrite and carbide, the sample’s hardness is expected to be greater than that of pure austenite. The presence of non-metallic inclusions further affects the material’s properties, negatively impacting their fatigue and impact strengths.
Conclusion
In this report, the microstructure of cast low-alloyed Ni-12MT steel was reviewed. The chemical composition was found to meet the specifications for ASTM A-743 Standard Specification for Castings of Steel Alloys with Poor Machinability. It was found to have a microstructure composed of equiaxed primary austenite grains with some ferrite and carbide present along grain boundaries and interdendritic cavities as well as around non-metallic inclusions. The non-metallic inclusions also were found to be within the acceptable size range. The presence of ferrite and carbide improves the material’s hardness but decreases its fatigue and impact strengths.
