Why to test the content of oxygen, nitrogen and hydrogen in steel?
Why to test the content of oxygen, nitrogen and hydrogen in steel?
The harm of oxygen
Both oxygen and hydrogen have adverse effects on the mechanical properties of steel. It is not only the concentration of oxygen, but also the number, type and distribution of inclusions containing oxygen. These inclusions refer to metal oxides, silicates, aluminates, oxysulfides and similar inclusion compounds. Deoxidation is required for steelmaking, because during solidification, oxygen and carbon in the solution react to form carbon monoxide, which can cause bubbles. In addition, oxygen can be precipitated from the solution as FeO, MnO and other oxide inclusions during cooling, which weakens the hot or cold workability, ductility, toughness, fatigue strength and machinability of steel. Oxygen, nitrogen and carbon can also cause aging or spontaneous increase in hardness at room temperature. For cast iron, when the ingot is solidifying, the oxide can react with carbon, resulting in porosity and embrittlement of the product.
The harm or function of nitrogen
Nitrogen can not be generalized as harmful gas element, because some special steel is purposefully added nitrogen. All steels contain nitrogen. The amount of nitrogen depends on the production method of steel, the type and quantity of alloying elements and their adding methods, the casting method of steel, and whether nitrogen is added purposefully. For some grades of stainless steel, the amount of Cr can be reduced by properly increasing the content of N. Cr is relatively expensive. This method can effectively reduce the cost. Most of nitrogen in steel is in the form of metal nitride. For example, after storage for some time, the steel will undergo strain aging and cannot be deep drawn (such as deep drawing for automobile protection plate), because the steel will tear and can not be evenly stretched in all directions. This is due to the large grain size and the deposition of Fe4N on the grain interface.
Another example is: in stainless steel, the formation of chromium nitride (Cr2N) on the grain interface will deplete the chromium contained in the interface and cause the so-called intergranular corrosion phenomenon. This harmful effect can be prevented by adding titanium and preferentially forming titanium nitride.
Harm of hydrogen
When the hydrogen content in steel is more than 2ppm, hydrogen plays an important role in the so-called “flake off” phenomenon. The spalling phenomenon is more obvious when internal crack and fracture appear during cooling after rolling and forging, and it is more often found in large section or high carbon steel. Due to the existence of internal stress, this kind of defect will cause the large rotor to crack in the process of engine use. When hydrogen in cast iron is more than 2ppm, it is easy to have pores or general porosity, which will cause embrittlement of iron. Hydrogen embrittlement mainly occurs in martensitic steels, which is not prominent in ferritic steels, but is not clear in austenitic steels. In addition, hydrogen embrittlement generally increases with hardness and carbon content.
Existing forms of oxygen, nitrogen and hydrogen in iron and steel
Forms of oxygen
Oxygen coexists in the form of combined state and free state. Generally, there are few free states, mainly in the form of Fe2O3, Fe3O4, FeO, metal oxide inclusion, silicate, aluminate, oxygen-containing sulfide and similar inclusion compounds. The total oxygen content measured by instrument is generally expressed by T [O].
Forms of nitrogen
Part of nitrogen in steel is in the form of metal nitride or carbon nitride; most of the elements added in special alloy steel can form nitride under appropriate conditions. These elements include manganese, aluminum, boron, chromium, vanadium, molybdenum, titanium, tungsten, niobium, tantalum, zirconium, silicon and rare earth. Considering that many nitride forming elements have several simple or complex nitrides, up to 70 nitrides may be formed in steel. The other part of nitrogen is dissolved in steel in the form of nitrogen atoms. In rare cases, nitrogen is incorporated into the bubbles in the form of molecules or adsorbed on the surface of steel.
The existing form of hydrogen
Hydrogen exists in the form of hydrogen atom in steel. At high temperature, two hydrogen atoms can easily form a hydrogen molecule. Hydrogen atoms are very active, naturally placed state will form hydrogen molecules, slow release.
Sources of oxygen, nitrogen and hydrogen in iron and steel
Sources of oxygen
Oxygen exists in molten steel in a certain amount at the end of smelting in various steel-making furnaces. Oxygen is supplied in the production process, because the oxidation process is the first in the steel-making process. Oxygen is supplied to the molten iron for the removal of [P], [S], [Si] and [C]. However, with the progress of steelmaking process, the relationship between [C] and [O] of molten steel in the molten pool in the steelmaking furnace has a common regularity despite the ever-changing process. That is to say, with the gradual decrease of [C], the [O] is gradually increased, and there is a corresponding equilibrium relationship between [C] and [O].
Sources of nitrogen
The partial pressure of nitrogen in the furnace gas is very high, and the partial pressure of nitrogen in the atmosphere is generally maintained at 7.8 Χ 10 ^ 4pa, so the nitrogen in steel is mainly absorbed and dissolved in the process of molten steel exposure. Electric furnace steelmaking, including arc heating in secondary refining, accelerates the dissociation of gas, so [n] content is high; long open hearth smelting time increases nitrogen content; improper converter combined blowing control and delayed nitrogen argon switching will also increase nitrogen content; nitrogen in ferroalloy, scrap steel and slag will also be brought into molten steel with furnace charge.
Sources of hydrogen
The partial pressure of hydrogen in the furnace gas is very low, and the partial pressure of hydrogen in the atmosphere is 0.053 PA. Therefore, the hydrogen content in the steel is mainly determined by the partial pressure of water vapor in the furnace gas. The main ways for hydrogen to enter molten steel are as follows: rust on the surface of scrap steel (xfeo · yfe3o4 · 2H2O); hydrogen in ferroalloy; water in carburizing agent, deoxidizer, covering agent, insulating agent, slagging agent (Ca (OH) 2), asphalt and tar; UN dried ladle, tundish and mid injection pipe; spray coating of ingot mould; water penetration in mould, water in atmosphere and molten steel or slag Into the steel.
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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