Nickel-saving or nickel-free austenitic stainless steel refers to austenitic stainless steel with Mn and N-generation Ni. It is a new material development direction that has received widespread attention with the progress of metallurgical technology in recent years. Because nitrogen-containing austenitic stainless steel has many advantages, it has set off a research boom. With the advancement of nitrogen-containing austenitic stainless steel manufacturing technology, manufacturing costs will continue to decrease, performance will be further improved, and the application range of nitrogen-containing austenitic stainless steel will continue to expand.
Therefore, it can be expected that nitrogen-containing, especially high-nitrogen austenitic stainless steel will be widely used in many important fields such as transportation, construction, space, marine engineering, atomic energy, and military industry. In this study, manganese and nitrogen were used to partially replace the role of nickel in the structure and properties of stainless steel. The structure analysis and tensile and impact mechanical properties of two low-nickel stainless steels with different nitrogen content were carried out, and the fracture mechanism was studied.
According to the influence of alloying elements on the structure and properties of austenitic steel, two-component low-nickel stainless steel was designed. After being smelted and forged in a vacuum melting furnace, the main chemical composition of the material is shown in Table 1. Use wire cutting to process the material into a long strip of impact specimen and a round rod-shaped tensile specimen, each of which takes 3 specimens. In the SXZ-10-13 box-type resistance furnace, the sample is solid-solution treated.
The solid solution temperature of the high-nitrogen test steel is 1050℃, and the low-nitrogen test steel is 1100℃. The cooling method is water cooling. After solution treatment, the sample with the size of 10mm×10mm×10mm is tested on the Rockwell hardness tester model HR150A, the load is 1.96N, the pressure holding time is the 20s, and each sample is hit 3 Points and take the average. The room temperature impact test is carried out on the XJ-502 combined impact tester in accordance with GB/T229-2007 “Charpy Pendulum Impact Test Method for Metallic Materials”.
The room temperature tensile test is carried out on an electronic creep testing machine model RDL100 in accordance with GB/T228-2002 “Metallic Materials Room Temperature Tensile Test Method”. Prepare metallographic samples and observe the microstructure using Leica’s DM2500M transflective optical microscope. D/MAX2005PC X-ray diffractometer was used to analyze the structure and a JSM-7001F scanning electron microscope (SEM) was used to observe tensile and impact fractures.
Table 1 Chemical composition of test steel (mass fraction %)
| Test Steel | C | Si | Mn | P | S | Cr | Mo | Ni | N | Fe |
|---|---|---|---|---|---|---|---|---|---|---|
| High Nitrogen | 0.0350 | 0.158 | 14.96 | 0.0233 | 0.0162 | 16.94 | 2.56 | 1.65 | 0.75 | Bal |
| Low Nitrogen | 0.0324 | 0.226 | 19.78 | 0.0248 | 0.0175 | 17.46 | 1.68 | 1.64 | 0.213 | Bal |
The results show:
(1) The matrix of high nitrogen and low nitrogen content and low nickel stainless steel after the solid solution is single-phase austenite and a dual-phase structure with half austenite and ferrite. The tensile strengths of both are relatively high, 840.18 and 688.38MPa respectively. Nitrogen acts as an interstitial atom to achieve good solid solution strengthening in steel.
(2) The room temperature tensile and impact fractures of the high nitrogen content and low nitrogen content test steels have obvious dimple morphologies. The impact absorption energy is 191 and 250J, respectively, and has good impact resistance, and the hardness is greater than 93HRB. The comprehensive mechanical properties of high nitrogen austenite at room temperature are better than duplex stainless steel with low nitrogen content.