3 questions about the surface condition of steel pipes

1. How is decarburization caused
Decarburization is a phenomenon that occurs when the oxidation rate of the steel surface is lower than the diffusion rate of carbon. Both oxidation and carbon diffusion occur strongly at high temperatures above the A1 phase transformation point. Decarburization can be controlled by the heating atmosphere in the furnace, and an oxidizing atmosphere can cause decarburization or oxidation. Especially when there is water vapor in the furnace, decarburization is very serious. In addition, during the heat treatment of the blank forging, the oxide scale is knocked off by shot peening in advance. If the shot peening is excessive, there will be more oxidation and decarburization, so pay attention to this.

2. How is oxidation caused
During heat treatment, steel combines with oxygen in the air to form iron oxide, a phenomenon called oxidation. The film generated by oxidation is called the oxide scale. The oxide scale can hinder the rapid cooling during quenching, causing the so-called quenching soft spot. Generally, the oxide scale is produced when the oxidation rate of oxygen is greater than the diffusion rate of carbon; conversely, decarburization is produced. Therefore, a thick oxide scale can be produced without decarburization in a strong oxidizing furnace gas where the oxidation rate is significantly greater than the diffusion rate. Therefore, it has become common sense that high-carbon steel (SK) should be annealed in a strong oxidizing atmosphere. Generally, the oxide scale of high-carbon steel is dense, while the oxide scale of low-carbon steel is loose and easy to peel off. The composition of the oxide scale above 570℃ and below 570℃ is different. Below 570℃, the outermost layer of the oxide scale is Fe2O3, the lower layer is Fe3O4, and the innermost layer is Fe: Above the As phase transition point, the order of the oxide scale from the outer to the inner layer is Fe2O3, Fe3O4, FeO,

3. Others
(1) Alligator skin-like oil wave marks: Alligator skin-like oil wave marks (alligato; skin) are the patterns formed on the surface of the steel when the oil attached to the surface is not completely burned during heating. Because it looks like alligator skin, it is called alligator skin-like oil wave marks. When heating in a heavy oil furnace, heavy oil drips on the surface of the quenched part, which is easy to causes alligator skin-like oil wave marks, so it must be paid attention to.
(2) Sweating: Sweating is the phenomenon that when heating or cooling, low melting point components appear in small particles on the steel surface. This phenomenon is called sweating because it is similar to the state of sweating in humans. For example, if high-speed steel is heated to a temperature close to the melting point (1320℃~1350C) and quenched, the surface will sweat. The size and number of sweat particles can be used to determine whether the quenching is performed at an appropriate temperature.
(3) Wrinkled surface: Wrinkled surface is the phenomenon that when steel is heated, the melting temperature decreases due to carburization, resulting in wrinkles on the surface. It is a surface melting phenomenon that often occurs in high-speed steel during quenching.
(4) Warls: Warls are when steel is heated in a salt bath furnace and charcoal powder is sprinkled on the surface of the salt bath to prevent decarburization. This charcoal powder adheres to the surface of the part and melts, often forming small particles like warls. It is usually called warls. In short, warls are evidence of carburization.
(5) Blistering
After heat treatment, the steel will produce bubbles on the surface due to the expansion of gas (mainly hydrogen) under the surface. This kind of blistering is also called a blister. This phenomenon often occurs when pickling parts are heated.