Investigating the thermally grown oxides and attributed effects of scales on heat transfer and interfacial friction between the ingot and die during the open-die forging process

Bulk metal forming is indicated for the deformation techniques for materials with a high volume to surface ratio. The classic bulk metal deformation methods continue to survive despite the emergence of novel deformation techniques. As one of the oldest bulk metal forming methods, open-die forging is a classic method, commonly employed to deform largesize ingots. In this method, the ingot is plastically deformed between two flat platens, where a post-forging step like machining will be needed afterward. Open-die forging can be realized as cold, warm, and hot deformation. During the hot deformation of steels, it is inevitable to experience the forming of thermally grown oxides. The steel surface will react with the interactive gaseous environment to form different oxide layers. The formed oxide layers are a cause of material waste. The scale can detach and penetrate ingot by deformation loads causing cracks and failures. The formed oxide layers have very low conductivity, which makes them act as insulators. Thus, the heat transfer of ingot is remarkably affected by the presence of oxide layers. Oxide layers can affect the interfacial friction between anvils and ingot. The anvils are costly to repair or replace. Hence, the effect of oxide layers on interfacial friction is critical to be investigated. 
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\nGrown over the past 70 years, Finkl Steel Co. is the largest producer of open-die forgings in Canada, with in-house melting, forging, heat treating, machining, testing, and inspection facilities. Finkl Steel Co. employs open-die forging to serve the industries of plastic injection molds, automotive, aerospace, oil and gas, construction, and mining. However, the company has experienced over hundred kilograms of material waste for each ingot due to thermal oxidation. The ingot temperature gradient, which is affected by scale layers, has caused undesired material characteristics. Furthermore, the interfacial friction between die and ingot is influenced by the presence of oxide. The deformation loads are significantly crucial for the company, and the anvils are costly to be repaired due to wear. This study was defined in this context and had aimed to experimentally and numerically assess the thermal oxidation and its effects on heat transfer and interfacial friction of large-size ingots. This study has resulted in three publications, which can be explained as follows: 
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\nIn the first article, the thermal oxidation of two high-strength steels was investigated. The chemical composition of both steels was the same, except Ni's concentration, which was almost six times (2.92 wt%), to investigate Ni's effect on oxidation. Four oxidation temperatures of 1000, 1100, 1150, 1200 °C, and oxidation times of 3,10, 20, 30, and 60 min were allocated to evaluate the effect of oxidation time and temperature. Since the sample's weight increases during the formation of oxide layers, the weight variation can indicate the steel's oxidation rate. So, differential thermal analysis equipped with thermogravimetry (TG) was employed. The samples were heated to the desired oxidation temperature under argon flow, where a unit switches the argon to air at that temperature. The sample is exposed to air for desired oxidation time. The outcomes for both steels demonstrated that by increasing the oxidation temperature and oxidation time, the oxidation increased. However, two steels' weight changes showed that the steel with the higher amount of Ni had less thermal oxidation than steel with lower Ni. The obtained activation energy for steel with higher Ni was 275 KJ mol−1 compared to 238 KJ mol−1 for steel with lower Ni, which shows higher oxidation resistance by the addition of Ni. To investigate Ni's underlying mechanism for less oxidation, X‐ray diffraction (XRD) analyses were conducted to investigate the different layers. In addition to three general layers of wustite, magnetite, and hematite, two layers of spinel (FeOCr2O3) and chromite (Cr2O3) were present. Energy‐dispersive X‐ray spectroscopy (EDS) showed the concentration of Ni on the top side of oxide layers. Also, there was a Cr rich layer at the interface of oxide-metal. This Cr rich layer was thicker for steel with higher Ni. All these layers acted as obstacles against diffusion of oxygen and Fe, suppressing the accessibility of cations and anions to each other and consequently decreasing oxidation at high temperatures. So, the material waste was less by the addition of Ni. 
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\nIn the second study, the study aimed to assess the effect of oxide growth on large-size ingots' cooling. The samples were heated to oxidation temperature under argon gas and then exposed to air for a determined time employing a radiative furnace. A confocal laser microscope measured the formed oxide thickness, where a parabolic behavior was seen for both steels. The formed oxide layers on high Ni steel was thinner compared to low Ni steel. It was found out that the thickness fraction of the wustite layer increased by increasing the oxidation temperature, and that of the hematite decreased. However, the thickness fraction of magnetite stayed the same for all testing conditions. Also, the relative thickness fraction of layers was the same for formed oxide layers on both steels. In other words, although the formed oxide layers were thinner for high Ni steel, they had the same fractions of low Ni steel. For the ingot’s cooling simulation, the steels' thermal properties were acquired by both the flash laser method and JMatPro software. The cooling rates, h-values, were obtained by Holman’s equations for finite element simulations. The finite element results demonstrated that the ingot with an oxide layer tends to cool faster from higher temperature than the ingot without an oxide layer, despite the oxide layers' insulation effect. The is due to the higher radiation coefficient of the oxide layer compared to the steel surface, which facilitates heat loss through vast ingot surfaces. However, sometimes the ingot stays more than hours in the furnace in an industrial situation, leading to thicker oxide layers. For an industrial case, a 7.8 mm oxide layer was observed. This oxide layer was employed in finite element simulation, and it was found that the ingot with this oxide layer has 200 K more temperature compared to the ingot without oxide. So, the oxide layer's insulation effect comes after a certain thickness is attained, where before that thickness, it causes higher heat loss. Also, in contact with the anvils, the thicker oxide layer prevented a temperature rise of 194 K on anvils. The developed finite element models were verified by experimental measurement of ingot surface temperature by a thermal camera. 
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\nIn the final article, the effect of thermally grown oxides on the interfacial friction between the ingot and die are evaluated. The ring test is a commonly employed method for assessing interfacial friction. Prior to the ring test, indentation tests were conducted to investigate the mechanical properties of formed oxide layers on different steels and use them for the ring test's finite element simulation. An indentation tool is penetrated to the oxide surface up to a determined load and kept for a dwell time, and then unloaded. The indentation outcomes showed that the hardness and young modulus of formed oxide layers on steel with the higher concentration of Ni were lower than oxide formed on steel with lower Ni. Friction calibration curves were obtained by finite element simulations. The rings were deformed at high temperatures of 1273, 1373, 1423, and 1473 K between two flat platens. The variation of internal diameter was recorded concerning the variation of height for different friction coefficients. The developed simulations were verified by experimental ring tests. For ring tests, rings with geometrical dimensions of 18:9:6 mm were machined from blocks. These rings were oxidized in a radiative furnace for a determined time and then plastically deformed. The outcomes showed that the oxide layer can act as an additional lubricant at high temperatures.

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