Hazards or effects of oxygen, nitrogen and hydrogen on steel products

1. The hazards of oxygen
Oxygen, like hydrogen, can have an adverse effect on the mechanical properties of steel. Not only the concentration of oxygen, but also the number and type of inclusions containing oxygen and their distribution have a very important influence. Such inclusions are metal oxides, silicates, aluminates, oxygen-containing sulphides and similar inclusions compounds. Deoxidation is required in steelmaking because the reaction of oxygen and carbon in solution during solidification produces carbon monoxide, which can cause air bubbles. In addition, oxygen can precipitate out of solution as FeO, MnO and other oxidation inclusions during cooling, thus weakening its hot or cold workability, as well as ductility, toughness, fatigue strength and the mechanical workability of the steel. Oxygen with nitrogen and carbon can also cause ageing or a spontaneous increase in hardness at room temperature. For cast iron, when the cast block is solidifying, the oxide and carbon can react and therefore cause porosity and embrittlement of the product.

2. The hazards or effects of nitrogen
Nitrogen cannot be generalised as a harmful gaseous element, as some special steels are purposefully added with nitrogen. All steels contain nitrogen and the amount present depends on the method of steel production, the type and amount of alloying elements and the way they are added, the method of casting the steel and whether nitrogen is purposefully added. With some grades of stainless steel, an appropriate increase in N content can reduce the amount of Cr used, which is relatively expensive, and this method can be effective in reducing costs. Most of the nitrogen in steel is in the form of metal nitrides. For example, after some time in storage, the steel undergoes strain aging and cannot be deep-drawn (e.g. deep-drawn as a car protection plate) 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 at the grain interface.
Again: in stainless steel, the formation of chromium nitride (Cr2N) at the grain interface will deplete the chromium contained at the interface and cause the
the so-called intergranular corrosion phenomenon. The addition of titanium, preferentially formed titanium nitride, will prevent this harmful effect.

3、Harmful effects of hydrogen
Hydrogen plays an important role in the phenomenon known as "scale flaking" when the hydrogen content of the steel is greater than 2 ppm. This flaking is generally more pronounced when internal cracks and fractures occur during the cooling process after rolling and forging, and is more often found in large sections or in high-carbon steels. Due to the presence of internal stresses, this defect can cause large rotors to break apart during engine use. Cast iron with hydrogen greater than 2ppm is prone to porosity or general porosity, and this hydrogen-induced porosity will cause embrittlement of the iron. "Hydrogen embrittlement" mainly occurs in martensitic steels, is not very prominent in ferrite steels, and is actually unknown in austenitic steels. In addition, hydrogen embrittlement generally increases together with hardness and carbon content.

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Second, the presence of oxygen, nitrogen and hydrogen in steel in the form of
1, the existence of oxygen form
Oxygen is coexisting in the chemical and free state, generally the free state is very little, mainly in the form of Fe2O3, Fe3O4, FeO and metal oxide inclusions, silicates, aluminates, oxygen-containing sulphides and similar inclusions compounds, instrumentation to test the total oxygen content, generally expressed as T[O].
2、The presence of nitrogen in the form of
Part of the nitrogen in steel is in the form of metal nitride or carbon nitride; nowadays, most of the elements added to special alloy steel can form nitrides under the right conditions. These elements include manganese, aluminium, 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, up to 70+ nitrides may be formed in steel at this time. Another part of the nitrogen is solidly dissolved in the steel in the form of nitrogen atoms. In a very few cases, nitrogen is entrapped in molecular form in bubbles or adsorbed on the surface of the steel.
3. The presence of hydrogen
Hydrogen is present in steel in the form of hydrogen atoms and at high temperatures two hydrogen atoms can easily form a hydrogen molecule. The hydrogen atoms are very active and the natural state of placement results in the slow release of hydrogen molecules.

III. Sources of oxygen, nitrogen and hydrogen in steel
1、The source of oxygen
Oxygen is present in the 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], de[Si], de[C] all need to supply oxygen to the iron. However, as the steelmaking process proceeds, despite the many variations in the process, there is a common regularity in the relationship between [C] and [O] in the molten pool of the steelmaking furnace. That is, as [C] gradually decreases, [O] is gradually increasing, [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 remains roughly at 7.8 Χ 10^4 Pa
Therefore, the nitrogen in steel is mainly inhaled and dissolved in the bare steel process. Electric furnace steelmaking, including the electric arc heating of the secondary refining, accelerates the dissociation of the gas, so the [N] content is high; the long smelting time of the flat furnace increases the nitrogen content; improper control of the converter re-blowing, nitrogen-argon switching is not timely will also increase the nitrogen content; ferroalloys, scrap steel and slag in the nitrogen will also be brought into the steel with the furnace charge.
3、The source of hydrogen
The partial pressure of hydrogen in the furnace gas is very low, the partial pressure of hydrogen in the atmosphere is 0.053 Pa. Therefore the hydrogen in the steel is mainly determined by the partial pressure of the water vapour in the furnace gas. The main ways in which hydrogen enters the steel are: through rust on the surface of the scrap (xFeO-yFe3O4-2H2O); hydrogen in the ferroalloy; water in the carbidizer, deoxidizer, covering agent, insulation agent, slagging agent (Ca(OH)2), asphalt and tar; water in the unbaked dry ladle, intermediate ladle, middle injection tube; ingot moulds Spray coating; water seepage from the crystalliser and water from the atmosphere entering the steel by interaction with the steel or slag.
Source: Pressure Vessel Man
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