Rafael Colás

Damage in hot rolling work rolls

Samples from a couple of work rolls employed in a hot rolling steel strip mill were examined by scanning electron microscopy in order to study their surface, as well as the wear mechanisms responsible for their damage. It was found that a series of cracks develop within the rolls at the beginning of their operational cycle, these cracks follow the primary carbide network and are attributed to be caused either by thermal fatigue or contact stresses or by a combination of both. Work rolls heat up as a result of contact with the steel strip and an oxide layer forms on them. It is observed that oxidation proceeds through the cracks, isolating healthy material with brittle layers, a characteristic which might explain the high wear rate encountered in some stands of the mill.

A model for heat conduction through the oxide layer of steel during hot rolling

Abstract A heat conduction model developed to predict the temperature distribution within the oxide layer of carbon steel being rolled is presented. This model takes into account the different physical properties of the three oxide species, and the parabolic growth of the layer. The thickness of the layer is divided into 40 nodes or elements of which 36 are considered to be of wustite, three of magnetite and only one of hematite to comply with their proportions. It is found that this particular model is too complicated when the aim is to evaluate the effect of the oxide crust in thin oxide layers, such as those encountered during strip rolling, because similar results can be obtained using a single node model based on the properties of wustite; whereas with the behaviour of thick layers, such as those encountered after reheating or during roughing passes, is modelled, the present model becomes valuable. The thermal gradients predicted by the model can be employed to predict the integrity of the oxide layer.

A metallographic study of aging in a cast heat-resisting alloy

Abstract Microstructural evolution of a heat-resisting cast alloy was studied by light and electron optical methods. Samples from a cast pipe, used in a reformer furnace, were cut and aged for up to 6 weeks at 750°C. The material was prepared for metallographic examination in the standard way, and electrolytically etched with an oxalic acid solution. The samples were examined with a light optical and a scanning electron microscope. Examination with the electron microscope was carried out with secondary and backscattered electron detectors; X-ray analysis was conducted at selected areas of the samples. It was found that the microstructure in the as-cast samples consisted of an austenitic matrix and a network of two types of primary carbides (chromium and niobium), whereas that of the aged materials also exhibit a fine dispersion of small chromium carbide precipitates; decomposition of primary niobium carbides was found to occur as aging progressed.

Precipitation in a heat-treatable aluminum alloy cooled at different rates

A series of cylindrical bars from an extruded heat-treatable type 6063 aluminum alloy were solubilized for 4 h at 520 °C and were cooled to room temperature at different rates. Samples cut from these bars were aged at 130, 180 and 230 °C for periods of time ranging from 20 min to 96 h. The characteristic behavior of precipitation-hardened alloys was observed, and it was found that the peak hardness at a given aging temperature was reduced in samples cooled at rates below 10 °C/s. Selected samples were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). No precipitation was observed to occur in unaged samples cooled at rates of around 60 °C/s; the matrix of these samples was that of the face centered cubic type with a lattice parameter of 4.055 A, which is slightly bigger than that reported for pure aluminum. In contrast, a series of small precipitates were observed in unaged samples cooled at a slower rate. Changes in lattice spacing in aged and unaged samples confirmed precipitation when the cooling rate was reduced.

Modelling heat transfer in hot rolling work rolls

Abstract Work rolls in a hot rolling mill are subjected to successive heating and cooling cycles, with the net result of a positive heat influx. This sort of heating might cause damage, as well as shape defects in the rolled strips, so it is therefore necessary to know how the temperature of the rolls increases. This particular phenomenon has been modelled following different approaches. Three models have been developed to calculate the transient heat flow (two using the finite-difference method and one integrating the heat flow to the roll). A fourth model, based also in the integration of the heat flow, was developed to compute the temperature distribution at steady-state. It is concluded that each model has its own value, and that all of them have to be used when a comprehensive analysis of the rolling conditions is required.

On the variation of grain size and fractal dimension in an austenitic stainless steel

Samples of an AISI type 316L stainless steel were subjected to different treatments to promote changes in their microstructure. The specimens were heated in a box furnace set at four different temperatures for 30 min and cooled in air to room temperature by placing them in water after the cycle was completed. The samples were prepared following standard metallographic procedures, the microstructure was revealed with an electrolytic etchant, and the average grain size in each sample was determined by the mean line intercept technique. Images from the microstructures were digitized and fed into a personal computer for their fractal analysis by box counting. Two different approaches were used to obtain the fractal dimension of the structure, yielding to similar values in both cases. It was found that the fractal dimension of the microstructure increased with the reduction in grain size.

A model for the hot deformation of low-carbon steel

Abstract The high-temperature deformation behaviour of low-carbon steel can be described by the superposition of work hardening and the combination of dynamic recovery and recrystallization. This las mode of restoration takes place once a critical strain, which depends on the strain rate and the temperature at which the test is conducted, is surpassed. Allowance is made for the grain size and the chemical composition of the steel in order to fit the experimental data. The model is employed to predict the separation forces recorded during the industrial production of low-carbon steel strip in a six-stand continuous mill.

Modelling and control of coiling entry temperature using interval type-2 fuzzy logic systems

Abstract The set-up of the cooling water applied to the strip as it traverses the runout table in order to achieve the coiler entry temperature was made by an intelligent model implemented using interval type-2 fuzzy logic systems. The model uses as inputs the targets for coiling entry temperature, strip thickness, finish mill exit temperature and finishing mill exit speed. The experiments of this application were carried out for three different types of coil in a real hot strip mill. The results proved the feasibility of the system developed for coiler entry temperature prediction. Comparison with the online type-1 fuzzy logic based model shows that the proposed interval type-2 fuzzy logic system improves performance in coiler entry temperature prediction under the tested condition.

Solidification analysis of aluminium engine block

Abstract Two V-8 engine blocks with cast-in grey iron liners were cast from a heat-treatable aluminium alloy. These blocks were instrumented by the insertion of 24 type K thermocouples placed at different points in the casting to record the thermal evolution during solidification. One of the blocks was cast after heating up the liners by induction, whereas the second was cast without the use of induction heating. The temperature–time readings from the thermocouples were obtained for thermal analysis. The solidification reactions detected were the formation of primary pre-eutectic aluminium dendrites, and two different eutectics that corresponded to Al–Si and complex Al–Si–Mg–Cu aggregates. It was found that the different solidification reactions were affected by the use of induction to heat up the liners, and by the chilling effect caused by a grey iron inset located at the bottom of the engine block moulds. The results indicate that solidification starts either at the window (bottom) or deck (top) positions in the interliners depending on whether the block was cast with or without the use of induction heating.

Study of weldability of a Cr-Si modified heat-resisting alloy

A cast heat-resistant alloy able to withstand carburization and metal dusting was developed to be used in the conducting pipes of reformed gas heaters for the direct reduction process. However, this alloy exhibits serious weldability problems in its as-cast condition and during the repair of aged pieces. Study of the microstructural and technological factors that affect weldability indicate that the use of low heat input (in the range of 630–950 kJ m −1 ) and a maximum interpass temperature of 150 ◦ C favors the welding repair of the high Cr–Si modified HK40 alloy. A solution annealing treatment prior to welding was necessary to obtain acceptable weldments. The commonly known buttering technique was not successful in this type of alloy. A series of microstructural phenomena, such as precipitation of secondary carbides, formation of intermetallic compounds (sigma phase and nickel silicide), as well as transformation of primary carbides, were observed to occur during aging, and they are suspected to be responsible for reducing the weldability of aged parts.