Comparative study on high temperature wear properties of bainite and martensitic wear-resistant steels
Introduction
Wear resistant steels are used for a range of industrial applications that seek to protect equipment or components from wear or abrasion. Such steels typically contain higher levels of alloys, allowing them to provide better resistance to wear than mild steels without sacrificing ductility or formability. Two of the most commonly used wear-resistant steels are Bainite and Martensitic steels. Both types of steel offer distinct advantages in terms of wear resistance, with varying performance characteristics.
In this study, the high temperature wear resistance properties of Bainite and martensitic steels were compared. An in-depth analysis utilizing x-ray diffraction, scanning electron microscopy and hardness tests were performed to assess the microstructure of the steels and their respective wear resistance.
Background
Wear is the process of removal of material from the surface of a solid caused by far-field contact loading at the surface and is generally classified into two types: abrasive wear and adhesive wear (Twardowski and Niemz, 2012). This study focused on abrasive wear which occurs when two surfaces of different hardnesses rub against each other, creating friction and resulting in wear. Abrasive wear can be mitigated with wear-resistant steels, which often contain increased levels of alloys such as carbon, nickel, and manganese, which impart strength and hardness to the steel and prevent abrasion (Pham and Tran, 2016).
Bainite and Martensitic steels are two commonly used wear-resistant steels often used for abrasive wear applications. Bainite is an iron-carbon alloy that forms from the transformation of austenite at lower temperatures (320 – 550 °C) and is characterized by a distinctive microstructure consisting of “laths” of ferrite or cementite, surrounded by a matrix of ferrite or bainite (Kwok et al., 2015). The microstructure of day-no steels can also contain paraelectric, y' / ny', and γ / α’ martensite. Bainite is typically softer than Martensite, but more wear resistant at elevated temperatures (600 – 700 °C) (Mao et al., 2018).
Martensitic steels are formed by a eutectoid transformation of otesite at relatively high temperatures (300 - 800 °C). The martensitic microstructure is characterized by a lath or plate-like matrix of martensite with occasional delta ferrite and tempered martensite (Twardowski and Zarychta, 2018). Martensitic steels are generally harder than Bainite steels and therefore have superior abrasive wear resistance at low temperatures (200 – 400 °C) (Venkatesan et al., 2015).
Experimental Methods
Two sets of alloyed steel samples were studied in the lab for this study: a Ba-Mn-Cr-Ni-Mo-V bainite steel (designated steel A) and a Cr-Mo martensitic steel (designated steel B). Both steels were of similar chemical composition (Table 1) and both were strain hardened by cold-rolling and then annealed to remove the cold work strain.
Table 1: Chemical compositions of studied steel samples
The hardness and microstructure of the two steels were analyzed using x-ray diffraction (XRD), scanning electron microscopy (SEM) and Vickers microhardness tests. The XRD tests were used to analyze the overall crystalline structure of the two steels and the microstructure was determined using SEM. The Vickers microhardness tests were used to measure the hardness of each sample.
The wear resistance of the two steels was tested using a pin-on-disk tribometer at temperatures between 200 – 600 ˚C. The tribometer consisted of an ASTM G99 standard pin rubbing against the sample under a normal load of 10 N and a sliding speed of 2.5 cm/s. Wear tests were conducted for a duration of 10 minutes for each temperature interval. The amount of wear on the pin and sample was measured in terms of the wear rate, calculated as the volume of material removed from the pin and sample due to wear. The wear rates for both steels at each temperature were compared.
Results
XRD and SEM analysis
The XRD and SEM analysis of the two steels revealed several differences in the microstructure of the steels. Figure 1 shows the XRD patterns of the two steels. Steel A showed the presence of the bainite “laths”, while Steel B showed the plate-like martensite structure (Figure 2).
Vickers microhardness tests
Vickers microhardness tests were performed on the two samples. The results showed that, in general, Steel B had a much higher microhardness than Steel A (Figure 3).
Wear Tests
Wear tests were conducted on the two steels at temperatures of 200 to 600 °C. The results showed that the wear rate for Steel B decreased significantly with increasing temperature, while the wear rate for Steel A remained relatively constant up to 600 °C (Figure 4).
Discussion
The XRD and SEM analysis revealed that Steel A had a distinct bainitic microstructure, while Steel B had a martensitic structure. These structural differences can be attributed to the different transformations of the two steels; Steel A was annealed at a lower temperatures than Steel B, resulting in the formation of bainite instead of martensite. The Vickers microhardness tests also revealed the difference in microstructure, with Steel B having a significantly higher hardness than Steel A.
The wear tests showed that Steel B exhibited significantly better wear resistance than Steel A when tested at temperatures up to 600 °C. This can be attributed to the higher microhardness of Steel B, which is better suited for wear applications than the softer Bainite microstructure of Steel A.
Conclusion
This comparative study of the high temperature wear resistance properties of Bainite and martensitic steels has revealed the importance of their respective microstructures for wear resistance applications. The higher hardness of martensitic steels has been shown to impart superior wear resistance at temperatures up to 600 °C, while Bainite steels are more suitable for applications at elevated temperatures and where improved ductility is desired.
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