Year

2014

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronics Engineering

Abstract

This thesis aims to investigate the tribological behaviours of the rubbing steel/steel pairs with a proposed polyphosphate lubricant at the temperatures of ferrite rolling and the effect of lubrication on the texture of interstitial free (IF) steel which is rolled in the ferrite temperature range. As complex tribological reactions at the sliding interface depend on the high load and temperature conditions, the thesis will provide a better insight into the possible tribochemical reactions of the given polyphosphate lubricant system which can reduce friction and wear, minimise oxidation, improve surface quality and texture for better drawability.

The flow stress and pyroplastic behaviour of two kinds of IF steels were investigated systematically by the compression tests on Gleeble 3500 Simulator in the ferrite (warm) temperature range. Two different constitutive equations were developed from the experimental results and they can be used to reliably predict the flow stress and pyroplastic deformation behaviour of IF steel under ferrite rolling. The rolling of IF steel indicates a better lubrication performance of the polyphosphate lubricant can be achieved over lubricating oil and unlubricated condition.

The influence of polyphosphate lubricant during ferrite rolling process was studied in the ferrite temperature range. Another two different lubrication conditions of lubricating oil and dry conditions were used to compare the lubrication performance with proposed lubricant. The rolling force, friction coefficient, roll and strip roughness and strip oxidation can be improved under lubricated conditions. The polyphosphate lubricant showed a better micro surface roughness than under lubricating oil and dry conditions. The mechanism of inorganic polyphosphate lubricant effect on the steel surface was studied to explain the improved lubrication performance during ferrite rolling of IF steel.

A series of high temperature tests which included the in-situ thermal evolution test and isothermal reaction test of the polyphosphate lubricant on the steel surface were carried out from 100 to 850 °C by the laser confocal microscope (high temperature microscope). The in-situ thermal evolution test indicated the liquid-like lubricating behaviour of the proposed lubricant. It formed a uniform film covering on the steel surface to further protect the steel surface from the oxidation at elevated temperatures. A homogeneous film was formed which included iron atoms. This phosphate layer adhered well to the steel surface through the chemical reactions at the interface. The IR spectra indicated that for all testing temperatures, the thermal chemical reaction film showed no change of their structure due to its highly thermal stability.

High temperature ball-on-disc tribological tests were carried out to simulate the contact, tribological behaviours, chemical conversion evolution and oxidation resistance of polyphosphate lubricant in the roll bite from 600 to 800 °C The polyphosphate reduced the friction between the tribo-pair and increased the wear resistance because of the formation of layered tribo-induced zone. It also presented good lubrication persistence in the long duration tribotests. The IF steel disc outstandingly showed the reduction of wear volume up to 80%, and the corresponding steel ball showed the reduction up to 60%. The micro roughness of worn surfaces under lubricated condition was also improved.

The dendritic pattern of polyphosphate was observed via SEM/EDS from worn surface. It showed the fluid-like character during the sliding process at elevated temperatures, providing the viscous lubricating ability of molten polyphosphate. The microstructure evolution of the tribo-induced interface was investigated by TEM at the elevated temperatures, as well as the structural change of the lubricant in the molten state and its effect on the oxidation. It clearly showed the inhomogeneously multi-layered structure, including four distinct sublayers from surface to iron oxide base. Higher load always presents a better layer density during the formation of this sublayer. It also provides the motive power for the chemical reaction at high temperatures.

The sublayer formation mechanism was presented in the tribo-induced zone. It was mainly due to the molten glassy state, ions exchange and chemical reaction at high temperature. Different sublayer showed various effective functions during contact and relative motion at ferrite temperature range. It showed the load supporting capability, as well as covered cracks and pores to reduce further oxidation. The whole layered tribo-induced zone showed good wear and friction resistance property at ferrite temperatures.

The lubrication effect on both macrotexture and microtexture of ferrite rolled strips was analysed with various temperature and rolling reduction by means of XRD and EBSD. Compared with dry rolling condition, the polyphosphate lubricant and lubricating oil pronouncedly improved the surface macrotexture with a higher fraction of γ-value of {111} orientation intensity and decreased {100} orientation. As a consequence, good drawability can be obtained after ferrite rolling. These lubricant effects mainly acted on the top surface while the center of thickness cross-section showed less effect by the lubricant. The polyphosphate lubricant can effectively suppress the shear texture even at a higher reduction and reduce in-grain shear bands.

FoR codes (2008)

091006 Manufacturing Processes and Technologies (excl. Textiles), 091205 Functional Materials, 091209 Polymers and Plastics, 091309 Tribology

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.