Why test the oxygen, nitrogen and hydrogen content in steel?
Why test the oxygen, nitrogen and hydrogen content in steel?
Why test the content of oxygen, nitrogen and hydrogen in steel: the harm or effect of oxygen, nitrogen and hydrogen on steel products.
Why test the content of oxygen, nitrogen and hydrogen in steel: the harm or effect of oxygen, nitrogen and hydrogen on steel products.
1. The harm of oxygen
Oxygen, like hydrogen, has an adverse effect on the mechanical properties of steel. Not only the concentration of oxygen, but also the number, type and distribution of oxygen-containing inclusions also have a very important influence. Such inclusions refer to metal oxides, silicates, aluminates, oxygen-containing sulfides and similar inclusion compounds. Steelmaking requires deoxidation, because during solidification, oxygen and carbon in the solution react to produce carbon monoxide, which can cause bubbles. In addition, oxygen can precipitate out of the solution as FeO, MnO and other oxidized inclusions during cooling, thereby weakening its hot work or cold workability, as well as ductility, toughness, fatigue strength and steel machining performance. Oxygen, nitrogen and carbon can also cause aging or a spontaneous increase in hardness at room temperature. For cast iron, when the ingot is solidifying, oxides and carbon can react, thus causing product pores and product embrittlement.
2. The harm or effect of nitrogen
Nitrogen cannot be attributed to harmful gas elements in general, because some special steels are purposely added with nitrogen. All steel contains nitrogen, the amount of which depends on the production method of the steel, the type and quantity of alloying elements and the way of adding them, the method of casting steel, and whether nitrogen is added purposefully. For some grades of stainless steel, appropriately increasing the content of N can reduce the amount of Cr used. Cr is relatively expensive. This method can effectively reduce the cost. Most of the nitrogen in steel is in the form of metal nitrides. For example: after storage for some time, the steel undergoes strain aging and cannot be deep-drawn processing (for example, deep-drawn processing for car protection panels), because the steel will tear and cannot be stretched uniformly in all directions. This is caused by 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. Adding titanium to preferentially form titanium nitride can prevent this harmful effect.
3. The harm of hydrogen
When the hydrogen content in steel is greater than 2 ppm, hydrogen plays an important role in the so-called "flaking off" phenomenon. This kind of spalling is generally more obvious when internal cracks and fractures occur during the cooling process after rolling and forging, and this phenomenon 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 engine use. When the hydrogen in cast iron is greater than 2ppm, pores or general porosity are prone to appear, and the porosity caused by hydrogen will cause the embrittlement of iron. "Hydrogen embrittlement" mainly occurs in martensitic steel, not very prominent in ferrite steel, and it is actually not clear in austenitic steel. In addition, hydrogen embrittlement generally increases with hardness and carbon content.
The existence of oxygen, nitrogen and hydrogen in steel
1. The existence of oxygen
Oxygen coexists in a compound 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, oxygen-containing sulfides and similar inclusion compounds. The total oxygen content measured by the instrument is generally represented by T[O].
2. The existence form of nitrogen
Part of the nitrogen in steel is in the form of metal nitrides or carbonitrides; most of the elements added to special alloy steels can form nitrides under appropriate conditions. These elements include manganese, aluminum, boron, chromium, vanadium, molybdenum, titanium, tungsten, niobium, tantalum, zirconium, silicon and rare earths. Considering that many nitride forming elements have several simple or complex nitrides, more than 70 nitrides may be formed in the steel at this time. The other part of nitrogen is dissolved in steel in the form of nitrogen atoms. In rare cases, nitrogen is trapped in bubbles in molecular form or adsorbed on the surface of steel.
3. The existence of hydrogen
Hydrogen in steel exists in the form of hydrogen atoms. At high temperatures, two hydrogen atoms can easily form a hydrogen molecule. Hydrogen atoms are very active, and hydrogen molecules will be slowly released when placed naturally.
The source of oxygen, nitrogen and hydrogen in steel
1. The source of oxygen
Oxygen exists in the 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 firstly an oxidation process, de[P], de[S], and de[Si] , De [C] all need to supply oxygen to the molten iron. However, with the progress of the steelmaking process, despite the ever-changing processes, the relationship between [C] and [O] of the molten steel in the steelmaking furnace has a common law. That is, as [C] gradually decreases, [O] is gradually increasing, and [C] and [O] have a corresponding equilibrium relationship.
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 inhaled and dissolved during the exposure of molten steel. Electric furnace steelmaking, including the arc heating of the secondary refining, accelerates the dissociation of the gas, so the content of [N] is too high; the long open-hearth smelting time increases the nitrogen content; improper control of the converter double blowing, and the 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 steel is mainly determined by the partial pressure of steam in the furnace gas. The main ways for hydrogen to enter molten steel are: through the rust on the surface of scrap steel (xFeO?yFe3O4?2H2O); hydrogen in ferroalloys; recarburizers, deoxidizers, covering agents, heat preservation agents, slagging agents (Ca(OH)2) , Moisture in asphalt and tar; unbaked ladle, tundish, middle injection pipe; spray coating of steel ingot mold; mold water seepage and atmospheric water and molten steel or slag into the steel.
