1. How is the decarburization of steel pipes caused?
Decarburization of steel pipes is a phenomenon that occurs when the oxidation rate of the steel surface is smaller than the diffusion rate of carbon. Both oxidation and carbon diffusion occur strongly at high temperatures above the A1 phase transition 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 serious. Also, during the heat treatment of blank forgings, shot blasting is used to remove the oxide scale in advance. If shot peening is excessive, oxidation and decarburization will occur, so attention should be paid to this.

2. How is the oxidation of steel pipes caused?
During heat treatment, steel combines with oxygen in the air to form iron oxide, a phenomenon called oxidation. The film formed due to oxidation becomes an oxide scale. Oxide scale can hinder rapid cooling during quenching and cause so-called quenching soft spots. Generally, when the oxidation rate of oxygen is greater than the expansion rate of carbon, an oxide scale is produced; on the contrary, decarburization occurs. Therefore, thick oxide scales can be produced without decarburization in strongly oxidizing furnace gases where the oxidation rate is significantly greater than the diffusion rate. Therefore, it has become common knowledge 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: and above the As phase transformation point, the order of the oxide scale from the outside to the inner layer is Fe2O3, Fe3O4, FeO,

3. Others
(1) Alligato skin: alligato skin is a pattern formed by the oil attached to the surface of the steel pipe that is not completely burned when heated, and remains on the surface. Because it looks like crocodile skin, it is called crocodile skin-like oil marks. When heating in a heavy oil furnace, heavy oil dripping on the surface of quenched parts is prone to produce crocodile skin-like oil marks, so you must pay attention.
(2) Sweating: Sweating is a phenomenon in which a composition with a low melting point appears as small particles on the steel surface when heated or cooled. Because it is like a person sweating, this phenomenon is called sweating. For example, if high-speed steel is heated to a temperature close to melting (1320°C~1350°C) and quenched, the surface will sweat. Based on the size and number of the sweat particles, it can be known whether it was heated and quenched at the appropriate temperature.
(3) Wrinkled surface: Wrinkled surface is a phenomenon in which the melting temperature of steel decreases due to carburization during heating, resulting in wrinkles on the surface. It is a surface melting phenomenon that often occurs in high-speed steel during quenching and heating.
(4) Warls: The so-called warls are when the steel is heated in a salt bath furnace, to prevent decarburization, charcoal powder is sprinkled on the surface of the salt bath. This charcoal sticks to the surface of the part and melts, often forming small, wart-like particles. It is often called a wart. In short, warts are evidence of carburization.
(5) Blister: After the steel pipe is heat treated, due to the expansion of the gas below the surface (mainly hydrogen), bubbling occurs on the surface. This bubbling is also called a blister. This phenomenon often occurs when pickling parts are heated.