The design of a carbide-free alloy compsition and heat treatment process for forged rail wheel application in South Africa

Abstract

This study investigates the carbide-free bainitic steels for forged rail wheel applications in South Africa. This project emanated from the need for durable rail steels by the South African industry, particularly for wheel applications. The pearlitic steels have reached a threshold in their development and bainitic steels have emerged as the third generation of rail steels, particularly the carbide-free bainitic alloys. The addition of high silicon has been found to retard the formation of carbides in bainite, forming what is called carbide-free bainite. This thesis gives an in-depth literature review on carbide-free and pearlitic steels, their wear properties and failure mechanisms when applied as rail steels. It also gives details on the experimental procedure followed to manufacture and test experimental alloys, as well as the results found. There is also a discussion on the future work to be completed. In this study a number of carbide-free bainitic alloys were manufactured with the addition of 1 wt% - 2 wt% silicon and other alloying elements, such as carbon, manganese and chromium. A thermodynamic study of the effects of alloying elements showed that the addition of molybdenum, vanadium and boron were beneficial in increasing the hardenability of the alloys. Examination of the bainite using transmission Kikuchi diffraction showed no carbides in the microstructure. The mechanical properties of the experimental carbide-free alloys were ranked against those of the commercial AAR Class C. The yield and tensile strengths of Alloy F were better than those of the reference. Alloy H showed better elongation and impact properties even though the impact properties of experimental carbide-free alloys was generally poor with high ductile to brittle transition temperatures. The sliding wear resistance of Alloy I heat treated at 400°C was similar to that of AAR Class B. In this study a number of alloy compositions and heat treatments were used to produce a carbide-free bainitic microstructure. The three different Alloy Series 1 to 3 studied the carbide-free bainitic microstructure under different test conditions. The carbide-free bainite alloy composition ranges have been developed for forged rail wheel applications for different sets of parameters: • Series 1 alloys (Alloys A to D) were produced to study the effect of alloying elements and heat treatment (isothermal and continuous cooling) on the carbide-free microstructure. • Series 2 alloys (Alloys E to H) were used to study the mechanical properties (hardness, tensile and impact) of the carbide-free bainitic microstructure. • Series 3 (Alloy I) was used to study the wear behaviour of the carbide-free microstructure. The best performing alloys for the different series tested are summarised in the table below.