1. Why test the oxygen, nitrogen and hydrogen content in steel: that is, the harm or effect of oxygen, nitrogen and hydrogen on steel products.
1. The danger of oxygen
Oxygen, like hydrogen, can adversely affect the mechanical properties of steel. Not only the concentration of oxygen but also the amount, type and distribution of oxygen-containing inclusions also have an important influence. Such inclusions refer to metal oxides, silicates, aluminates, oxysulfides and similar inclusion compounds. Steelmaking requires deoxidation because during solidification, oxygen and carbon in the solution react to form carbon monoxide, which can cause gas bubbles. In addition, oxygen can precipitate out of solution as FeO, MnO, and other oxide inclusions during cooling, thereby impairing its hot or cold workability, as well as ductility, toughness, fatigue strength, and machinability of the steel. Oxygen along with nitrogen and carbon can also cause aging or a spontaneous increase in hardness at room temperature. For cast iron, when the ingot is solidifying, the oxides can react with the carbon, thereby causing porosity and embrittlement of the product.
2. Hazards or effects of nitrogen
Nitrogen cannot be generally attributed to harmful gas elements, because some special steels are purposefully added with nitrogen. All steels contain nitrogen, and its presence depends on the method of production of the steel, the type and amount of alloying elements and how they are added, the method of casting the steel, and whether nitrogen is intentionally added. For some grades of stainless steel, appropriately increasing the N content can reduce the amount of Cr used. Cr is relatively expensive, and this method can effectively reduce the cost. Most of the nitrogen in steel is in the form of metallic nitrides. For example, after some time in storage, steel is strain-aged and cannot be deep-drawn (such as deep-drawing into automotive protective panels) because the steel will tear and cannot be stretched uniformly in all directions. This is due to the large grain size and the deposition of Fe4N on the grain interface.
Another example: in stainless steel, the formation of chromium nitride (Cr2N) on the grain interface will deplete the chromium contained on the interface and cause
The so-called intergranular corrosion phenomenon. The addition of titanium, which preferentially forms titanium nitride, prevents this detrimental effect.
3. The hazards of hydrogen
When the hydrogen content in the steel is greater than 2 ppm, hydrogen plays an important role in the so-called “flaking” phenomenon. This spalling is generally more pronounced when internal cracks and fractures occur during cooling after rolling and forging and is more often found in large sections or high carbon steels. Due to the existence of internal stress, this defect will cause the large rotor to crack during the use of the engine. When hydrogen in cast iron is greater than 2ppm, pores or general porosity are prone to appear, and the porosity caused by this hydrogen will cause embrittlement of iron. “Hydrogen embrittlement” occurs mainly in martensitic steels, is not very prominent in ferritic steels, and is actually unclear in austenitic steels. Additionally, hydrogen embrittlement generally increases along with hardness and carbon content.
2. Existence form of oxygen, nitrogen and hydrogen in steel
1. Existence form of oxygen
Oxygen coexists in a combined state and a free state. Generally, there are few free states, mainly in the form of Fe2O3, Fe3O4, FeO and metal oxide inclusions, silicates, aluminates, oxysulfides and similar inclusion compounds. The instrument tests the total oxygen content, generally expressed as T[O].
2. Existence form of nitrogen
A portion of the nitrogen in steel is in the form of metallic nitrides or carbonitrides; most elements added to special alloy steels today can form nitrides under the right conditions. These elements include manganese, aluminum, boron, chromium, vanadium, molybdenum, titanium, tungsten, niobium, tantalum, zirconium, silicon, and rare earth, among others. Considering that many nitride-forming elements have several simple or complex nitrides, as many as 70 nitrides may be formed in the steel at this time. Another part of the nitrogen is dissolved in the steel in the form of nitrogen atoms. In rare cases, nitrogen is entrapped in bubbles in molecular form or adsorbed on the surface of the steel.
3. Existence form of hydrogen
Hydrogen in steel exists in the form of hydrogen atoms. At high temperature, two hydrogen atoms can easily form a hydrogen molecule. Hydrogen atoms are very active, and they will form hydrogen molecules and release them slowly in the natural state.
3. The source of oxygen, nitrogen and hydrogen in steel
1. The source of oxygen
Oxygen exists in molten steel in a certain amount at the end of smelting in various steelmaking furnaces. Oxygen is supplied during the production process, because the steelmaking process is the first oxidation process, removing [P], removing [S], and removing [Si]. , De[C] needs to supply oxygen to molten iron. However, with the progress of the steelmaking process, despite the ever-changing processes, the relationship between [C] and [O] of molten steel in the molten pool in the steelmaking furnace has a common regularity. That is to say, as [C] gradually decreases, [O] is gradually increasing, and [C] and [O] have a corresponding equilibrium relationship with each other.
2. The source 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 7.8×10^4Pa, so the nitrogen in the steel is mainly absorbed and dissolved during the exposure process of molten steel. Electric furnace steelmaking, including arc heating in secondary refining, accelerates the dissociation of gas, so the [N] content is high; long open hearth smelting time increases nitrogen content; improper control of converter re-blowing and untimely switching of nitrogen and argon will also increase Nitrogen content; nitrogen in ferroalloys, scrap steel and slag will also be brought into molten steel with the charge.
3. The source of hydrogen
The partial pressure of hydrogen in the furnace gas is very low, and the partial pressure of hydrogen in the atmosphere is 0.053Pa. Therefore, the hydrogen in the steel is mainly determined by the partial pressure of the water vapor in the furnace gas. The main way for hydrogen to enter molten steel is: through rust on the surface of scrap steel (xFeO•yFe3O4•2H2O); hydrogen in ferroalloy; carburizer, deoxidizer, covering agent, heat-insulating agent, slag agent (Ca(OH)2), water in asphalt and tar; unbaked ladle, tundish, and injection pipe; spray coating of steel ingot mold; water seepage in mold and water in the atmosphere and molten steel or slag into steel.