Doctor of Philosophy
School of Mechanical, Materials and Mechatronic Engineering
Zamri, Wan Fathul Hakim, A study of the mechanical properties and wear behaviour of the oxide layer formed on high speed steel work rolls, Doctor of Philosophy thesis, School of Mechanical, Materials and Mechatronic Engineering, University of Wollongong, 2013. http://ro.uow.edu.au/theses/3978
This research is part of a larger study conducted to understand the wear of hot rolling rolls. A significant cost of hot rolling is associated with the consumption of rolls; hence a comprehensive understanding of how the roll material wears is important. As the surfaces of rolls are covered by an oxide layer it is important that the mechanical and tribological properties of these oxides be known. The mechanical properties of the oxide layer are an important aspect that leads to a better prediction of its behaviour during hot rolling operations. Due to the complex nature of the phenomena that occur in, and the practical difficulty of accessing the hot strip and work roll interface, computer based simulations coupled with experimental study are needed to understand and predict the wear of the oxide layers.
A combined FE simulation and nanoindentation experiments approach was developed and used to characterise the mechanical properties of the oxide scale formed on the HSS sample. The main procedure consisted of two approaches: first, a nanoindentation experiment was carried out to determine the load-displacement curve, and second, the load-displacement curve was validated against the simulation based curve. In this thesis, nanoindentations were performed on the cross section of the oxide layers. The experiments clearly revealed the variation of mechanical properties in the inner and outer sub-layer. The XRD pattern and SEM-EDS analysis of the oxide layer sample indicated that these layers consist of two sub-layers, namely Fe2O3 and (Fe, Cr)3O4 in the outer and inner sub-layers respectively. The result of the load-displacement curves indicated that the outer sub-layer of Fe2O3 is generally harder, as demonstrated by the smaller penetration depths on the outer sub-layer compared to the inner sub-layer. It was hypothesized that the mechanical properties of the outer sub-layer were significantly influenced by the hard grains and small pores.
From the FE simulations, the output load-displacement curves were obtained. The simulated load-displacement curves were then analysed in terms of the unloading slope (dP/dh) and the maximum load (Pmax) and compared with the experimental curves. Each simulation was refined iteratively until the values of the mechanical properties used in the simulation yielded an average unloading slope and maximum load difference with the experiments of less than 1 percent.
FE simulations were carried out to simulate the load-displacement curves of the nanoindentation experiments to give the properties of the oxide layer. In this application the use of a nanoindentation experiment alone to measure the properties was hindered by the unknown value of the Poisson’s ratio and porosity effect of the sample. In a porous media, the load displacement relationship depends on the porosity and grain sizes, which are largely heterogeneous. To account for the porosity effect, the finite element model adopted Gurson’s model of plasticity for porous material. The mechanical properties of the oxide sub-layers were determined from the FE simulations input parameters e.g. elastic modulus, E, yield strength, σy, Poisson’s ratio, υ and pore fraction, ƒ, which could not be measured directly from the nanoindentation experiments. It was found that the outer sub-layer has a higher E of around 200-240 GPa compared to the inner sub-layer’s E of 90-220 GPa. The large variation of E in the inner sub-layer was probably due to the influence of voids and a non-uniform porosity of the sub-layer. The smaller size grains of the inner sub-layer increased the sensitivity of the indentations to variations in porosity. The E and H for the 20 mN load tests are lower than for the 5 mN load. This phenomenon was probably caused by the hardening which arises from the interaction between geometrically necessary dislocations (GNDs) and grain boundaries.
To understand how the oxide layer deforms when scratched by asperity, a three-dimensional (3D) finite element (FE) simulation was carried out to investigate an indenter scratching on the oxide layers/high speed steel substrate system. Both ductile and brittle failure models were used to study the deformation and failure modes of the oxide layer. The coefficient of friction increased in an approximately linear function with the depth of the scratch. The oxide layer formed on HSS can be considered as ductile rather than brittle material based on the better agreement between the FE simulations and the scratch experiments when the ductile model was used. In fact the nanoindentation results on the oxide scales showing cracking free deformation, which further corroborated a ductile behaviour of the oxide layers.
An investigation on the deformation of the oxide layers and indenter-carbide particle interactions during a scratch test was also carried out using a threedimensional finite element method. The interaction between the indenter and the carbide particle reinforcements were investigated after three different possible scenarios: particle on an oxide free surface, particle on an oxide-substrate interface, and particle in the substrate. The results showed that the magnitude and distribution of stresses/strains in the oxide layer and interaction of the particle with the tip are the main reason for the particle de-bonding during scratching. The von Mises stress at room temperature was significantly higher than at high temperature. This situation was probably due to the difference between the mechanical properties such as elastic modulus and yield strength of the oxide layer in both cases.
In summary, this thesis has presented 6 major contributions to the study of oxide layers formed on high speed steel hot rolling material, namely:(i) the development of combined FE simulations and nanoindentation experiments to study the mechanical properties of the oxide layer, (ii) finding the variation in the oxide layer morphology, (iii) finding the significant relationship between the mechanical properties of the oxide layer and the nanoindentation parameters, (iv) development of FE simulation to predict the oxide layer wear, (v) suggestion on how and when to treat the oxide layer as brittle and as ductile material, and (vi) development of an FE model of oxide layer deformation and wear with carbide inclusion.