The Effect of Nitrogen on Mechanical Propertiesof Type 316L Austenitic Stainless Steels

• Main Authors: Hung-Yi Lee, 李宏毅
• Other Authors: Chi-Shang Huang
• Format: Others
• Language: zh-TW
• Published: 2002
• Online Access: http://ndltd.ncl.edu.tw/handle/17202483598812127039

碩士 === 義守大學 === 材料科學與工程學系 === 90 === Austenitic stainless steels have excellent ductility, toughness and weldability , but are low in yield strength as compared with other structural steels. The methods include grain refinement, solid solution hardening , precipitation hardening, and work hardening. For solid solution hardening , strengthening with interstitial solute elements is efficient . Carbon and nitrogen, which are interstitial elements are found to be most effective. But, nitrogen has a greater effect on tensile strength than carbon.It is shown that nitrogen is a powerful strengthening element except when strong nitride forming elements are also present .Apart from its strengthening effect, nitrogen also delays the formation of carbides by reducing the diffusivity of carbon .Carbon cannot be considered, due to corrosion problem. On the other hand, as nitrogen is soluble enough in asutenite it is not related to the susceptibility to intergranular corrosion .In addition, nitrogen is known to improve the resistance of austenitic stainless steel to pitting corrosion and crevice corrosion , in particular .Thus , nitrogen becomes the most suitable solid solution strengthener. In order to investigate the influence of nitrides and carbides precipitates on mechanical properties of type 304L and 316L stainless steel in the sensitizing temperature range , type 304L and 316L specimen are heated at 550、650、750、850℃ for 24 hours. Meanwhile , the hardness test , tensile test , pitting potential measurement , and the observation of microstructure and fracture surface are used. In order to study the martensite transformation type 304L and 316L specimen are strained at different reduction. X-ray diffraction analysis and δ-ferrite measure instrument are used. The results obtained are summaries as follows. 1. The results from tensile tests show that the yield and tensile strength of type 304L and 316L stainless steels increase with the increasing of nitrogen content, and that of type 304L are higher than those of type 316L stainless steels. 2. With increasing nitrogen content and sensitizing temperature the elongation of type 304L and 316L stainless steel will decrease. 3. Type 304L and 316L stainless steels with nitrogen 0.16% have better toughness, yield and tensile strength, but over this value strength will not clearly increase. 4. From the microstructure observation sensitized at 650℃ to 850℃ ×24 hours, type 316L stainless steels have no clear intergranular corrosion phenomenon. 5. Pitting corrosion is promoted by manganese addition .