Modification Of Ultra-high Carbon Steel And Micro-alloyed Research

Ultrahigh carbon steels (UHCS), which are hyper-eutectoid steels containing 1.0～2.1%C, have been of great interest. These steels have a remarkable combination of mechanical properties, e.g. very high hardness, strength and wear resistance. The high volume fraction of carbides with high connectivity in the matrix makes ultrahigh carbon steels quite brittle after conventional solidification processes. In this paper, a new type of modifier is designed and employed to the UHCS, and the influence on the microstructure and properties of the steels have been studied. Meanwhile, the effects of micro-alloy and heat treatment on the microstructure and properties of the UHCS also have been researched. The results are summarized as follows:(1). By adding modifier in the UHCS, the primary austenite was refined, the eutectic carbide network structure was broken up and became blocky, and the distribution of the carbide became more uniform. The improvement of the structure can obtain, since the properties of alloys depend on their microstructure. The impact toughness of the modified alloy after heat treatment (quenching from 800℃for 1 h and tempering at 250℃for 2 h) increased greatly from 5.8 J·cm-2 to 12.5 J·cm-2, while the hardness remained around 65 HRC.(2). The addition of the micro-alloy elements (Ti, V, Nb) had effects on the microstructures not only for the as-cast UHCS but also for the heat treatment. The isolated carbides became finer and were dispersed homogeneously in the matrix. It is beneficial for the UHCS to improve the properties of hardness and impact toughness. In this studied, the multiple addition of Ti and V is the optimum components. Excellent mechanical properties of the UHCS can be obtain by quenching from 880℃for 1 h and tempering at 250℃for 2 h.(3). The friction coefficient decreased with load from 20 N to 100 N in pure sliding. The wear mechanism was found to be predominantly oxidative at all loads for the UHCS. This was associated with formation of an oxide film on the surface at increasing loads. The debris also gives a reduction in friction coefficient. Furthermore, the worn surface of the modified alloy show less wear debris and shorter track depth than the non-modified one. Thus, the modifier improved the wear resistance of ultrahigh carbon steel.