Case hardening of plain carbon and nickel alloy steels

Abstract

One of the most established methods of enhancing the mechanical properties of machine parts such as gears, cams spindles and shafts with respect to their surface wearing resistance and core toughness, is Case-hardening by Pack Carburising. though this process is nowadays somewhat superseded by other new case-hardening techniques, it is still preferred for individual processing of small parts, being efficient and cost effective at this scale. The aims of this dissertation are to: (i) Investigate the physical and mechanical properties of heat treating a selection of four case-hardening steels which are currently being used at the Malta Dry Docks. Such steels possess sufficient difference in chemical composition and hardenability to cover most of the variations in case and core properties likely to be encountered in the case-hardening process. (ii) Establish the optimum carburising temperature together with case and core re-heat temperatures, in order to obtain the complete heat treatment cycle for each type of steel to give maximum surface hardness and a tough core. (iii) Forward any recommendations likely to be required to improve the mechanical properties and to consider other mechanical tests representative of specimens behaviour.

The dissertation is divided into 7 chapters. CHAPTER 1 is an introduction covering techniques of surface hardening of steels, other than case hardening by carburising. It includes a discussion on the type of steels suitable for case-hardening, particularly alloyed steels, the effect of alloying elements on such steels and their consequent effect of rendering these steels coarse and fine grained.

CHAPTER 2 includes aspects of metallurgy of steels giving further details about the behaviour of the alloying elements and the effect of heat treatment on the equilibrium diagram such as the displacement of the eutectoid point and eutectoid temperature. CHAPTER 3 deals with the process of case-hardening by pack carburising, followed by the application of heat treatments. It also includes comments on typical heat treatments, the type of compounds suitable for pack carburising and the advantages and disadvantages of this process.

CHAPTER 4 deals with the machining and preparation of the specimens and of the carburising boxes to be used in this project. The experimental procedures adopted in various heat treatments applied and in their respective mechanical tests are presented in CHAPTER 5, while the mechanical results obtained from each test are presented in graph form and commented upon in CHAPTER 6. CHAPTER 7 discusses the metallographical examinations carried out on each specimen followed by comments and conclusions.