Semi-quantitative Microstructural Analysis of 25Cr4V2 Steel after Tempering and Surface Plasma Spraying of Tungsten Carbide
Introduction
Surface Plasma Spray (SPS) is a technological coating process used to deposit thick coatings mainly consisting of two components: a metallic alloy, such as tungsten carbide, and a binderlike material. This process is used to modify or improve surfaces of components subjected to severe operational conditions. Combined with tempering, this process obtains good properties in terms of wear, and chemical and thermal resistance.
This experiment was carried out to qualitatively analyze the microstructure of 25Cr4V2 steel after undergone tempering and subsequent SPS of tungsten carbide.
Experimental Procedure
Test specimens were cut from a 50 mm x 25 mm x 7 mm dowel bar of 25Cr4V2 steel, and fractional tempered and quenched (Q&T). The specimens were firstly heated at 930°C for 90min, and then quenched in oil with agitation. Subsequently, each specimen was tempered for 1 hour at 178°C, 250°C, 350°C and 510°C. The surface of the specimens were then SPS with WC.
The surface of the specimens were prepared by cut-off and grinding operations, and then etched with 2-3% Nital solution. Subsequently, the samples were examined in the optical microscope.
Results
Figure 1 shows the microstructure of the 25Cr4V2 steel tempered at 350°C and treated with SPS. It can be observed that the ferrite and pearlite are the predominant crystal structures, with small quantities of martensite. The grains are typically pearlite sized (typically 6-10μm) and average grain size distribution is slightly refined. There is also evidence of some areas of bainite and troostite, only observable under magnification.
Figure 2 presents the microstructure of the 25Cr4V2 heated for tempring at 510°C and subsequently treated with SPS. In this case, the ferrite is predominant, with a small amount of pearlite, bainite, troostite and martensite. The size distribution is slightly refined, with the average grain size at 6-10 μm.
Figure 3 presents the microstructure of the 25Cr4 V2 tempered at 178°C and treated with SPS. The main component is ferrite, with a small amount of bainite and troostite. The size distribution is homogeneous, with an average grain size at 10-15 μm.
Figure 4 shows the microstructure of the material heated for tempering at 250°C and then treated with SPS. The microstructure is predominately ferrite, with small amounts of bainite, troostite and martensite. The size distribution is slightly coarse, with an average grain size of 10-15μm.
Discussion
The results of this experiment show that the size distribution of the 24Cr4V2 steel is significantly affected by the tempering process and subsequent SPS. In the case of tempering at 178°C, the grain size is relatively coarse, while tempering at 350°C and 510°C results in a slightly refined grain size distribution. Similarly, the tempering at 250°C produces a relatively coarse grain size distribution.
These results are in agreement with the reported behavior in the literature, which states that tempering at lower temperatures should produce steel with a higher hardness and slightly coarser grain size, whereas tempering at higher temperatures should produce softer steel with a more refined grain size distribution.
Conclusion
The findings of this experiment indicate that surface plasma spraying of tungsten carbide, combined with tempering of the 25Cr4V2 steel, significantly alters the grain size distribution of the material. Specifically, tempering at lower temperatures produces a higher hardness steel and slightly coarser grain size, whereas tempering at higher temperatures results in softer steel with a more refined grain size distribution.
