How does steel form oxide scale
Oxide scale; degree a; heating time; furnace gas medium; chemical composition of steel
As a result of oxidation, an oxide film-iron oxide scale is formed on the surface of the metal. Due to the diffusion of oxygen and iron during heating, iron oxide scale is formed as a result. Iron diffuses into the surface layer, while oxygen diffuses into the metal through a thin layer (film) with iron oxide scale. As a result, the iron oxide scale layer becomes thicker. At the beginning of the process, iron oxide Fe0 is formed. At a temperature of about 900°C, the oxide scale is composed of three distinct layers. The upper layer of the oxide scale is iron oxide Fe 03. This layer accounts for 2% of the thickness of the oxide scale. The middle layer is composed of magnetic oxide F3 04 and occupies about 18% of the thickness of the oxide scale. The inner layer of Fe0 adhered to the metal accounts for about 80% of the thickness of the oxide scale. At higher temperatures, the oxide scale consists of two layers.
The formation of oxide scale is determined by the following basic factors: a degree, heating time, furnace gas medium and the chemical composition of steel.
In order to determine the effect of heating temperature on the formation of oxide scale, the following experiment was done. Take a sample of carbon steel (0.45%C) with a diameter of +18mm and a length of 60mm, and heat it in an electric muffle furnace at 700, 750, 800, 850, 900, 950, and 1000°C for 0.5h. Put four samples in the furnace at the same time. The edge of the sample has a hot-rolled metal surface lightly brushed with a metal brush. Two samples on the right are stripped 0.5mm in diameter, and the second sample on the left is pickled. To measure the temperature, a thermocouple is placed in the middle of the hole with the sample. All samples are placed on the bottom of the muffle furnace with ceramic mats at equal distances from each other. After heating, they are cooled in the air and brushed gently to remove the oxide scale. Burning loss (formation of oxide scale) is determined by the quality of the sample before and after the test.
The data obtained show that the scale of iron oxide increases with the increase of temperature. At 700-750 ℃, the least oxide scale is observed. When the temperature rises to 800-850℃, the scale of iron oxide does not increase significantly. Starting from 850°C, the scale increases significantly, while starting from 900°C it increases strongly.
The influence of temperature in the range of 900-1420℃ on the formation of iron oxide scale, the iron oxide scale increases sharply from 1200-1300°C. The formation of iron oxide scale at 1400°C and 1350qC is 5 times and 2.75 times that at 1200°C, respectively, and 28 times and 16 times that at 900°C.
The iron oxide scale formed at 900°C is i.6 times more than that at 700°C. As the temperature increases, the inner layer of Fe0-containing iron oxide scale increases. Water vapor and C07 have the most significant effect on the formation of iron oxide scale starting from low temperature. Oxygen and air have a strong influence on the formation of scale in the temperature range of 900-1000 ℃.
The formation of iron oxide scale increases with the increase of heating time, but the degree of increase of iron oxide scale gradually decreases.
The formation of iron oxide scale largely depends on the composition of the steel. Increasing the content of m, Cr. Si, W and Cu in the steel can reduce the iron scale on the metal surface.
Co. NIo and Ni have opposite effects on the formation of metallic iron oxide. As the content of these elements increases under the heating conditions indicated above, the penetration of oxides into the deep metal also increases.
The formation of effective iron scale is related to the physical properties of oxides. Porous oxides are not easy to adhere and cannot resist the diffusion of oxygen into the metal. On the contrary, oxides with dense structure strongly adhere to metals and can well resist the diffusion of oxygen into the deep metal: such oxides have protective properties. Si, Al, and Cr promote the formation of oxide films that are dense and adhere well to metals. For this reason, some of these elements are used as the alloy composition of the hot-strength steel, and some of the steel grade content reaches 2.9%, Si. 20% CrO
Burning occurs mainly during heating in a heating furnace, which is a widely popular opinion. The burning loss during rolling is considered to be not too large. In fact, the damage tip of the oxide scale formed outside the furnace is sometimes even extremely large, and in some cases exceeds the burning loss in the furnace.
Since a large amount of the surface of the metal heated to a relatively high temperature is in contact with the air, the scale loss produced by the rolling on the wire and small-scale rolling mill is even greater than the burning loss by heating in the furnace.
Edit: Yuan Hui