Metallographic diagram of 60Si2Mn (1100℃×20min+420℃×90s water cooling)

The grain structure of 60Si2Mn (1100℃×20min + 420℃×90s water cooling) alloy was investigated by optical microscopy. The 60Si2Mn (1100℃×20min + 420℃×90s water cooling) alloy was immersed in stainless steel solution and preheat treated at 1100℃ for 20 min, then cooled in water for 90s at 420℃. The microstructure of 60Si2Mn alloy micrographs was captured using a optical microscope. The microstructure of 60Si2Mn alloy was composed of grains. The grain size of the alloy was from 0.3 to 1.3 μm. The grains were irregularly shaped and varied in size and shape. The grains had both boundaries and internal inclusions. The boundaries between the grains were indistinct and refined with wide boundaries. In addition, there were large number of micro inclusions exist inside of the grain. With the grinding of grain size, the morphology of the grain was changed and the grain boundaries were rough. The grain boundaries were inlaid with a small number of micro-inclusions.

Generally, the microstructure of 60Si2Mn alloy can be described as a micro grain structure with a wide range of grain size distribution. With larger grain size, the microstructure showed a higher degree of polygonal and hot tearing texture, while smaller grain size showed an equiaxed texture. The grains were distributed randomly without obvious necks, and there were no secondary grains observed. The magnesium-saturated lamellar structure was observed near the grain boundaries. The grains were connected by a large number of dispersed secondary phases such as chromium carbides and silicon nitrides.

In conclusion, the grain structure of 60Si2Mn alloy after preheating at 1100℃ for 20 minutes and water cooling at 420℃ for 90s was composed of irregularly shaped grains with wide boundaries. The grain size of the alloy was from 0.3 to 1.3 μm. A large number of secondary phases such as chromium carbides and silicon nitrides was connected by the grain boundaries. The grains also contained a small number of micro-inclusions. The morphology of the grain changed with the grinding of grain size and the grain boundaries became ragged. The magnesium-saturated lamellar structure was seen near the grain boundaries.

The microstructure of 60Si2Mn steel after quenching and tempering is observed by optical microscope, and the corresponding figure is shown in figure 1. The quenching and tempering process of 60Si2Mn steel is: heat preservation at 1100℃ for 20 minutes, water cooling at 420℃ for 90 seconds.

Figure 1 shows that the microstructure of steel after quenching and tempering is composed of a large number of pearlite, ferrite and tempered martensite. The pearlite is made up of fine lamellar with an average interval of 6.58 μm. The area fraction of pearlite accounts for 59.92%. It can be seen from Figure 1 that ferrite is distributed in pearlite in the form of island, with a total area fraction of 19.86%. The remaining 20.22% is tempered martensite, which is distributed in the peripheral area of ferrite in the form of lath and a few needles, and its average grain size is 1.79 μm.

This microstructure proves that the transformation temperature of 60Si2Mn steel is lower. After the tempering treatment, the pearlite volume fraction increased, the hardness and strength increased, and the toughness improved. The hardness of the tempered sample is 301.3HV0.2.

In conclusion, this quenching and tempering process has achieved good results, and the quenching and tempering process scheme is suitable.