1. How is Decarburization Caused in Steel Pipes?
Decarburization occurs when the oxidation rate on the steel pipe surface is slower than the carbon diffusion rate. Both oxidation and carbon diffusion occur intensely at temperatures above the A1 transformation point. Decarburization can be controlled by the furnace’s heating atmosphere. An oxidizing atmosphere can cause both decarburization and oxidation. Decarburization is particularly severe in the presence of water vapor. Furthermore, during heat treatment of forgings, shot peening is used to remove oxide scale. Excessive shot peening can lead to increased oxidation and decarburization, so this should be carefully considered.
2. How is Oxidation Caused in Steel Pipes?
During heat treatment, steel combines with oxygen in the air to form iron oxide, a phenomenon known as oxidation. The film formed by oxidation is called scale. Scale can hinder rapid cooling during quenching, causing so-called quenching soft spots. Generally, scale forms when the oxygen oxidation rate is faster than the carbon diffusion rate; conversely, decarburization occurs. Therefore, in highly oxidizing furnace atmospheres where the oxidation rate is significantly faster than the diffusion rate, thick scale can form without causing decarburization. Therefore, it’s common knowledge that high-carbon steel (SK) should be annealed in a strongly oxidizing atmosphere. Generally, the oxide scale of high-carbon steel is dense, while the oxide scale of low-carbon steel is loose and easily flaked. The composition of the oxide scale differs between temperatures above 570°C and below 570°C. Below 570°C, the outermost layer of the oxide scale is Fe2O3, the next layer is Fe3O4, and the innermost layer is Fe. Above the As transition point, the order of the oxide scale from outermost to innermost is: Fe2O3, Fe3O4, FeO.
3. Other problems with the surface condition of steel pipes
(1) Alligator skin-like oil marks: Alligator skin-like oil marks (alligator; skin; the oil attached to the steel surface is not completely burned during heating, and the pattern formed on the surface. Because it looks like crocodile skin, it is called alligator skin-like oil marks. When heating in a heavy oil furnace, heavy oil drips on the surface of the quenched part and easily causes alligator skin-like oil marks, so it must be paid attention to.
(2) Sweating: Sweating (sweating) is the phenomenon in low-melting-point components that appear as small particles on the surface of the steel pipe when heated or cooled. Because it is like the state of sweating, this phenomenon is called sweating. For example, if high-speed steel is heated to a temperature close to the melting point (1320℃~1350℃) and quenched, the surface will sweat. The size and number of these sweat particles can be used to determine whether the heating and quenching are at the appropriate temperature.
(3) Small ripples: Small ripples (wrinkled surface) are a phenomenon in which the melting temperature of steel decreases during heating due to carburization, resulting in wrinkles on the surface. It is a surface melting phenomenon that often occurs during the quenching of high-speed steel.
(4) Warts: Warts are caused by charcoal powder being sprinkled on the surface of the salt bath to prevent decarburization when steel is heated in a salt bath. This charcoal powder adheres to the surface of the part and melts, often forming small wart-like particles. It is usually called a wart. In short, warts are evidence of carburization.
(5) Blisters: After heat treatment, bubbles form on the surface of steel due to the expansion of subsurface gas (mainly hydrogen). This blister is also called a blister. ● This phenomenon often occurs when pickled parts are heated.
Post time: Sep-17-2025