Effects of sintering process on preparing iron-based friction material directly from vanadium-bearing titanomagnetite concentrates

Highlights

• A novel utilization of the vanadium-bearing titanomagnetite concentrates is presented.

• Increasing sintering temperature can facilitate formation of laminated pearlites.

• Increasing sintering time can also facilitate formation of laminated pearlites.

• The economical sintering time should not be more than 3 h.

Abstract

In order to optimize an innovative two-stage process for preparing an iron-based friction material directly from vanadium-bearing titanomagnetite concentrates, this paper focuses on the effects of sintering process on the microstructures and properties of an iron-based friction material. On one hand, the samples were sintered at 900 °C, 950 °C, and 1000 °C for 3 h respectively; On the other hand, the samples were sintered at 1000 °C for 1 h, 2 h, 3 h, and 4 h respectively. As a result, after the samples were sintered at above 950 °C for more than 3 h, a lot of laminated microstructures appear in these samples owing to the formation of a large number of pearlites. Besides, the density, the hardness, and the friction coefficient of this material are positively correlated to the sintering temperature or the sintering time, and the wear rate of this material is negatively related to the sintering temperature or the sintering time. This study can contribute to the attainment of much clearer insight into the effects of sintering process and lay the foundation of practical application of this innovative two-stage process.

Introduction

As an important metal matrix composite, iron-based friction material has been applied widely in clutch or brake mechanisms of aeroplanes, tanks, trucks, ships, tractors, engineering machinery and metal-cutting machines. Conventionally, based on pure substance powders, iron-based friction material is produced by means of powder metallurgy. Generally, it is costly and time-consuming to prepare various pure substance powders.

At present, some ferro-matrix composites can be prepared directly from natural minerals by means of in-situ synthesis technology, which have caught great interests of researchers because of great process simplification and cost decrease. Welham et al. discussed the carbothermic reduction of ilmenite and rutile in detail [1], and made successful attempts to produce TiN/TiC–Fe composites cheaply from ilmenite concentrate [2]. Jayasankar et al. succeeded in synthesizing Fe–TiC composites from cheap raw materials such as mild steel scrap, ilmenite, and petroleum coke [3]. Razieh Khoshhal et al. used cheap ilmenite to synthesize Fe–TiC/Al₂O₃ composite [4]. Based on ilmenite, carbon black, and aluminum powder, Mansour Razavi et al. prepared Fe–TiC–Al₂O₃ hybrid nano-composite via carbothermic reduction caused by mechanical activation [5].

However, up until now, seldom documents are concerned with the material preparation from the abundant vanadium-bearing titanomagnetite by means of in-situ synthesis technology. As a special and complex iron ore, the vanadium-bearing titanomagnetite mainly consists of ferrous oxides, titanium oxides, vanadium oxides and other oxides such as Al₂O₃, SiO₂, CaO, and MgO [6]. After the beneficiation process, vanadium-bearing titanomagnetite concentrates are produced with a high content of total iron. At present, these concentrates are mainly applied in the ironmaking process. During this process, most of the iron and part of the vanadium can enter the hot metal, but almost all of the titanium remains in the slag to form the high titanium slag. Hitherto, no an appropriate and economical method has been found to utilize this kind of slag [7]. At the same time, these oxides such as Al₂O₃, SiO₂, CaO, and MgO in the vanadium-bearing titanomagnetite concentrates are regarded as impurities and must be removed during this ironmaking process. Consequently, this application in the ironmaking process gives rise to an enormous waste of precious elements in the vanadium-bearing titanomagnetite concentrates.

On the other hand, theoretically, based on selective in-situ carbothermic reactions of the vanadium-bearing titanomagnetite concentrates in a specific condition, the ferrous oxides can be reduced to metal iron, the titanium oxides can be converted into TiC and the vanadium oxides can be converted into VC, yet Al₂O₃, SiO₂, CaO, and MgO cannot be reduced by carbon because of their chemical stability. As a matter of fact, TiC, VC, Al₂O₃, SiO₂, CaO, and MgO are useful hard particles in iron-based friction material. Consequently, it is possible to find an alternative method to prepare an iron-based friction material by means of selective in-situ carbothermic reactions of the vanadium-bearing titanomagnetite concentrates.

Based on the above backgrounds, Guangming Zhang and Keqin Feng have recently invented a two-stage process, consisting of a selective pre‐reduction stage and a final sintering stage, to prepare iron-based friction material directly from the vanadium-bearing titanomagnetite concentrates [8]. In general, as a key step of powder metallurgy, sintering process plays an important role in the quality of final product. Both the sintering temperature and the sintering time have great effects on the microstructures and the properties of the material. If the sintering temperature and the sintering time are not proper, the properties of the material cannot be guaranteed. Besides, an appropriate sintering temperature and sintering time can contribute to the attainment of economical sintering process since they can help to reduce the energy consumption of sintering process. Thus, this paper focuses on the effects of the sintering temperature and the sintering time on the microstructures and properties of an iron-based friction material with the main chemical compositions of 74 wt.% Fe, 8 wt.% Cu, and 5 wt.% C. The corresponding results are helpful to optimize the sintering process and lay the foundation of the practical application of this two-stage process.