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Abstract

The influence of copper on the structure and the mechanical properties of low carbon steels has been examined. The investigation included three sets of heat treated steels as follows.

• Three very low-C, Mn-AI steel plates have been quenched in ice-brine and aged at 60 and 100°C for various periods of time in order to obtain the age-hardening behaviour. The purpose of this exercise has been the examination of a simple agehardening system before proceeding to the more complex Cu-containing steels. The mechanical properties and structure of the steels have been examined for the as- quenched, the quenched and fully-aged and the quenched and over-aged conditions. In all conditions the steel microstructure consisted of polygonal ferrite the size of which did not vary with ageing. The yield strength of the steels has been found to increase after ageing at peak hardness and then reach a minimum value after over-ageing. There have not been any significant changes on the impact transition temperatures after ageing the steel. The relative insensitivity of impact transition temperatures to ageing has been attributed to the presence of coherent iron-carbide precipitates.

• The age-hardening behaviour and structure-property relationships of three simple low-C, Mn-AI steel plates with 0%, 1.22% and 2.02% Cu content have been examined. The steels were austenitised at 910°C for 1/2 h and slowly cooled (5°C/min) to produce a ferrite-pearlite microstructure and then tempered at various temperatures for 1 h. The copper-free steel did not show any structural or property changes on tempering whereas the two copper steels showed peak strength and hardness after tempering at 525-500°C. The impact transition temperatures were observed to rise, but analysis of the results has indicated that the increase was much lower than expected. The best combination of yield strength and impact properties was given by the 2.02% Cu steel in the over-aged condition (ageing at 650°C for 1 h). SEM and TEM examination has confirmed the presence of a coarse copper precipitate which is incoherent with the matrix in all the steel conditions. Numerical analysis of the yield strength results has shown that there is a constant vector of precipitation strengthening due to the presence of incoherent precipitation and this is in accord with the Ashby-Orowan strengthening model.

• A 1.26% Cu-containing steel plate of commercial composition has been examined. The steel was austenitised at 920°C for 1/2 h, cooled either in air or in oil and tempered as in the case of the simple copper containing steels. The prime phase of the microstructure was ferritic with 10-20% martensite and retained austenite. TEM examination has confirmed the presence of coarse incoherent copper precipitates and finer niobium-carbide precipitates in all the steel conditions. The mechanical properties of this steel were largely influenced by the presence of martensite, however when this was accounted for, the analysis of the results showed that as in the case of the simpler copper containing steels, the presence of copper precipitates leads to good impact performance.

The relative improvement of the impact transition temperatures of the copper containing steels has been attributed to the ability of the copper precipitates to deform. This has been verified by straining an over-aged 2.02% Cu sample at 15% plastic deformation. Finally, it has been observed that for a given copper content, a fine dispersion of the incoherent copper precipitate is more preferable than a coarse one. The presence of the coherent copper precipitation has also been found to enhance the favourable effect of the incoherent precipitate on the impact transition temperatures.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TJ Mechanical engineering and machinery
Departments:	School of Science & Technology > Engineering > Mechanical Engineering & Aeronautics School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

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