Low Nitrogen Stainless Steel Alloy Nitrogen Increasing Process
Low nitrogen stainless steel (hereafter referred to as LNS) is a type of stainless steel that contains relatively little nitrogen, typically below 0.008%. Compared to conventional stainless steel, LNS reduces the risk of localized corrosion by reducing the contact between the steel and corrosive environment. In order to make full use of the properties of LNS, it is necessary to increase the nitrogen content to an appropriate level to form a homogeneous nitride layer on the steel surface. This paper will discuss the possible Nitrogen Increasing Processes (NIPs) for low nitrogen stainless steel alloys.
Producing nitrided low nitrogen stainless steel typically involves applying nitrogen at elevated temperature either directly onto the surface of the steel or in a nitrogen-rich atmosphere. This can be done either in a batch process or a continuous process, depending on the size and shape of the steel pieces. The advantage of the batch process is that it can accommodate larger pieces of steel that require more nitrogen, while the continuous process is preferred for smaller pieces because of its efficiency.
When nitrogen is applied to the surface of the steel, the steel must be heated to temperatures of around 800 to 1400 °C and held at that temperature for a predetermined amount of time. During this time, nitrogen atoms diffuse into the steel and combine with the base iron molecrontributes to increased alloy strength. This process, known as nitriding, is often used for protecting wear surfaces and imparting improved corrosion resistance.
If the atmosphere for nitrogen introduction is used, then the steel is heated in a nitrogen-rich atmosphere of up to 600 to 1000kPa, with the nitrogen concentration depending on the desired final product. The temperature should be maintained between 600 to 1000 °C. For shorter nitriding times, higher temperatures and pressures, as well as an additional refining stage to stabilise and homogenise the nitride layer can be used.
In order to reduce or eliminate the risks posed by impurities or preexisting defects in the steel, some pretreatment processes can also be used before or after the nitriding process. These include pickling, surface grinding, and shot blasting. These will remove the impurities that may be present during the production process and help to create smoother, more uniform surfaces. The advantage of these pretreatment processes is that they can provide a uniform nitride layer and homogenize the structure.
These processes can be used either alone or in combination depending on the conditions and the desired result. The advantages of combining processes is that it provides more control over the structure and properties of the alloy, reducing the risk of material defects and improving dimensional accuracy.
In conclusion, there are various Nitrogen Increasing Processes available for use with low nitrogen stainless steel alloys. These processes can be tailored to the specific requirements of the application and optimize the use of the material to ensure improved corrosion resistance and increased alloy strength. Depending on the size and shape of the steel, the process can be a batch nitriding process or a continuous process in a nitrogen-rich atmosphere. Finally, pretreatment processes such as pickling, grinding or shot blasting can be also be applied in order to reduce or eliminate the risks posed by impurities or preexisting defects in the steel.
