1、 The harm of nitrogen
Nitrogen can not be generalized as harmful gas elements, because some special stainless steel is purposefully added nitrogen. All stainless steels contain nitrogen, and its amount depends on the production method of stainless steel, the type, quantity and addition method of alloy elements, the casting method of stainless steel, and whether nitrogen is added purposefully. Some brands of stainless steel, properly increasing the content of N, can reduce the use of Cr, Cr is relatively expensive, this method can effectively reduce the cost.
Most of nitrogen in stainless steel is in the form of metal nitride. For example: after storage for some time, if the stainless steel has strain aging, it can not be processed by deep drawing (for example, deep drawing is processed as automobile protection plate), because the stainless steel will tear and cannot be evenly stretched in all directions. This is due to the large grain size and the deposition of Fe4N on the grain interface.
2、 Harm of hydrogen
When the hydrogen content in stainless steel is more than 2ppm, hydrogen plays an important role in the so-called "flake off" phenomenon. The spalling phenomenon is more obvious in the cooling process after rolling and forging, especially in large section or high carbon stainless 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 the hydrogen content 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 stainless steel, which is not prominent in ferrite stainless steel, but is not clear in austenitic stainless steel. In addition, hydrogen embrittlement generally increases with hardness and carbon content.
3、 The harm of oxygen
The mechanical properties of stainless steel are the same as those of hydrogen and oxygen. It is not only the concentration of oxygen, but also the number, type and distribution of inclusions containing oxygen. This kind of inclusion refers to metal oxide, silicate, aluminate, oxysulfide and similar inclusion compounds. Deoxidation is necessary for stainless steel production, because during solidification, carbon monoxide will be generated from the reaction of oxygen and carbon in the solution, 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 stainless 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.