High nitrogen steel is one kind of stainless steel whose nitrogen content exceeds the limit nitrogen content that can be achieved in steel under conventional conditions. According to the different amount of nitrogen added, the following classification is roughly made, that is, the steel with nitrogen content >1% is ultra-high [SCALE STEEL]nitrogen, the steel with nitrogen content 0.3 ~ 0.5% is high nitrogen, and the steel with nitrogen content below this range is nitrogen. The effect of nitrogen in stainless steel is mainly reflected in three aspects: the microstructure, mechanical properties and corrosion resistance of stainless steel. The results show that nitrogen is a very strong element that forms and stabilizes austenite and enlarges the austenite phase region. It can replace part of nickel in stainless steel, reduce the ferrite content in steel, make austenite more stable, prevent the precipitation of harmful intermetallic phases, and even avoid the occurrence of martensite transformation under cold working conditions.
Used to think that nitrogen in steel will be brittle and must be processed to remove elements as, but during the period of 1910 ~ 1930 in nitrogen in steel can improve the strength of the research, and use it to improve the toughness, fatigue strength and corrosion resistance, such as performance, found that the addition of[SCALE STEEL] nitrogen quantity the more the more the performance improving trend, so as to carry out the related research of the amount of the expanded to join.
The biggest reason for interest in nitrogen is that it can substitute nickel. In the 1930s and 1940s, in order to save nickel from wartime supplies in Japan, nitrogen as a substitute for nickel to form austenite phase was paid [SCALE STEEL]attention to. However, until now, how to improve the properties of steel by nitrogen solution and its mechanism are still unknown, which need to be solved urgently.
Mechanical Properties of High Nitrogen Content Stainless Steel:
With the increase of nitrogen element in stainless steel, the hardness, yield strength, tensile strength and fatigue resistance of stainless steel have been significantly improved. The introduction of nitrogen can effectively improve the strength of stainless steel and stabilize the austenite phase, which can be said to kill two birds with one stone. Especially the introduction of nitrogen can significantly improve the yield strength and tensile strength of alloy materials.
Local Area Corrosion Resistance:
There is no doubt that the introduction of nitrogen greatly improves the corrosion resistance of the material. This can be clearly [SCALE STEEL]seen from The calculation formula of The Pitting resistance Equivalent number:
Formula 1: PREN=%Cr+3.3%Mo+16%N
According to the above formula, the pitting corrosion equivalent of stainless steel is mainly determined by the nitrogen content in stainless steel. Its calculation factor reaches 16. Nitrogen content also significantly[SCALE STEEL] changed PREN values. This is just a calculation formula for the PREN value of nitrogenous stainless steel materials. In the high nitrogen stainless steel materials composed of different formulas, the calculation factor of nitrogen element even reaches more than 25.
Because of high strength stainless steel’s excellent matching and corrosion resistance, strong toughness in aerospace, it is widely usded in the field of national economy and people's livelihood application like marine engineering ,energy equipment manufacturing, such as the plane of the main bearing components, fasteners, [SCALE STEEL]satellite gyroscope, ship shell, offshore oil platform, automobile industry, nuclear industry, gear and bearing manufacturing, etc., It is the preferred material for lightweight design, energy saving and emission reduction of future equipment components. As one of the important candidate materials for load-bearing and corrosion-resistant structural parts, how to combine ultra-high strength and toughness with excellent service safety is the key development direction of this kind of steel in the future.
Carnegie Illionois developed the first generation of martensitic precipitation-hardened stainless steel in 1946 to meet the needs of high performance corrosion resistant structural steels for aerospace and Marine engineering. On the basis of Stainless W steel alloy system, Cu and Nb elements were added and Al and Ti elements were removed. American Arm‐ CO Steel Company developed 17-4pH steel in 1948 . Due to its good strength, toughness and corrosion resistance, it is widely used in manufacturing fasteners and engine parts besides landing gear components of F-15 aircraft, but its cold deformation ability is poor. In order to reduce the high temperature δ -ferrite which is unfavorable[SCALE STEEL] to the transverse mechanical properties, a 15-5pH steel [5-7] was developed by reducing the content of Cr and increasing the content of Ni. This steel overcomes the disadvantage of 17-4pH steel in transverse plasticity and toughness, and has been used in the manufacturing of ship and civil aircraft bearing parts. In the early 1960s, Inco invented martensitic aging steel and introduced the concept of martensitic aging strengthening for the development of high strength stainless steel, thus opening the curtain of the development of martensitic aging stainless steel. In 1961, the American company first developed the maraging stainless steel Custom450 containing Mo. Later, Pyromet X-15 and Pyromet X-12 were developed in 1967 and 1973 respectively. During this period, the United States has also developed AM363, In736, PH13-8MO, Unimar CR, etc. Martin et al. [8,9] obtained the invention patents of Custom465 and Custom475 steel in 1997 and 2003 respectively, and applied them in civil aviation aircraft. British developed FV448, 520, 520(B), 520(S) and other high strength stainless steel brands. Germany developed the Ultrafort401 and 402 in 1967 and 1971. In addition to copying and improving American steel grades, the Former Soviet Union independently researched a series of new steel grades. In 2002, QuesTek undertook the pollution prevention project of THE STRATEGIC Environmental Research and Development Program (SERDP) of the U.S. Department of Defense. Through the Material Genome Project, QuesTek designed and developed Ferrium®S53, a new type of ultra-high strength stainless steel for aircraft landing gear , and published [SCALE STEEL]the AMS5922 aerospace standard at the end of 2008. Ferrium®S53 has A strength of about 1,930mpa and fracture toughness (KIC) of 55 MPa·m1/2 or more. It has been added into the MMPDS trunk Material Manual of the United States in 2017, and has been successfully applied to THE A-10 fighter aircraft and T-38 aircraft of the United States. It is the preferred material for the landing gear of the next generation of carrier-based aircraft.
China began to develop high strength stainless steel in the 1970s. In 2002, CIRON and Steel Research Institute designed and developed a new type of ultra-high strength and toughness stainless steel material, which is the ultra-high strength stainless steel USS122G of Cr-Ni-Co-Mo alloy system independently developed by China with independent intellectual property rights. Its strength is more than 1900 MPa and KIC is more than 90 MPa·m1/2 . At present, the material has broken through the key technology related to the preparation of bar with a diameter of 300 mm, and has a wide application prospect in the field of Aerospace equipment manufacturing in China.
Stress Corrosion Cracking of Ultra High Strength Stainless Steel:
According to the failure investigation report of Aircraft parts in The United States, stress corrosion cracking is one of the main forms of [SCALE STEEL]sudden failure accidents occurred in the service of key load-bearing parts of aircraft, and most landing gear is finally broken due to stress corrosion or fatigue crack propagation . At present, stress corrosion occurs not only in aviation, aerospace, energy, chemical and other high-tech industries, but also in almost all commonly used corrosion resistant steel and metal. Therefore, it is of great scientific value and practical significance to analyze the stress corrosion cracking mechanism of ultra-high strength steel and the factors affecting the stress corrosion of ultra-high strength steel.
The corrosion resistance of materials becomes an important factor to limit the stress corrosion cracking of high strength steel, and pitting corrosion is the most common and harmful form of corrosion. Most stress corrosion cracking [SCALE STEEL]originated from pitting pits. In the process of aging treatment, the microstructure of ultra-high strength stainless steel is not uniform due to the precipitated phase from supersaturated martensite matrix, which is the main source of pitting corrosion of ultra-high strength stainless steel. The passivation film near the precipitated phase is weak, and the invasion of Cl- leads to the destruction of the passivation film, and the formation of microbatteries between the precipitated phase and the matrix, so that the matrix is dissolved, the precipitated phase spares off, and pitting corrosion is formed. For example, cr-rich carbides M23C6 and M6C and intermetallic compounds Laves phase equal σ are prone to form cr-poor zone around, resulting in pitting phenomenon. Luo et al.  and Yu Qiang  studied the effect of aging time on the microstructure and electrochemical behavior of 15-5pH ultra-high strength stainless steel by using THREE-DIMENSIONAL atomic probe chromatography. Cu-rich clusters and (Cu,Nb) nanoparticles were observed when aging time was 1-240 min. After long-term aging treatment, the sample surface is more susceptible to Cl- erosion. After aging for 240 min, The Cr content around the [SCALE STEEL]precipitates also decreased, and Cr poor zone was easily formed in these parts. The decrease of Cr/Fe ratio in passivated film leads to the decline of pitting resistance of passivated film. In addition, the continuous precipitation of Cr-rich carbides at grain boundaries reduces the intergranular corrosion resistance of steel. For example, the study  found that AISI 316Ti stainless steel has higher intergranular corrosion resistance than AISI 321 stainless steel, because the precipitation of Ti C reduces the formation of Cr-rich carbides, which is one of the precipitates leading to intergranular corrosion.
As the most important ductile phase in high strength stainless steel, the content, morphology, size and stability of austenite also affect the stress corrosion sensitivity of steel. Under the condition of the same size, morphology and stability, the stress corrosion cracking threshold value (KISCC) increases with the increase of austenite content, and the stress corrosion cracking sensitivity of steel decreases. The reason is that the thin-film austenite structure formed on the martensitic slat boundary improves the toughness of steel and reduces the hydrogen-induced crack growth rate. There are two main reasons for the decrease of crack growth rate. One is: When the crack expands from martensitic matrix to thin-film austenite, either it continues to expand into the austenite or changes the direction of propagation to bypass the austenite structure, it will consume more energy, resulting in the decrease of crack growth rate and the increase of stress corrosion resistance sensitivity. Second: as I mentioned earlier, H in austenitic organization have higher solid solubility, low [SCALE STEEL]partial tendency, and the rate of diffusion of H in austenite is far smaller than in the martensite structure, is beneficial in high strength stainless steel hydrogen trap, results in the decrease of hydrogen embrittlement sensitivity of the crack front, the crack propagation rate reduce, improve the stress corrosion sensitivity. It should be noted that the stability of austenite is also a key parameter determining the stress corrosion sensitivity of steel. After the stress or strain-induced martensitic transformation, the fresh martensite transformed from austenite can not only suppress the crack propagation, but also improve the sensitivity of steel hydrogen embrittance as a new source of hydrogen diffusion.
In conclusion, the strength and toughness, stress corrosion and hydrogen embrittlement sensitivity of steel are affected by the complex multistage and multiphase structure, and the design and preparation of ultra-high strength stainless steel with excellent service performance by traditional trial and error method is difficult, long cycle and high cost. Compared with the trial-and-error method, the rational design method, such as establishing a series of multi-scale analysis models of strength and toughness, stress corrosion properties and hydrogen brittleness, will be more purposeful. The results of simulation analysis can be used to establish the design standard of high strength stainless steel, optimize the morphology, size and content of precipitated phase, martensite and austenite structure in steel, and further combine the multi-scale simulation with the actual material development process, which will greatly reduce the difficulty of material development, reduce the cost and shorten the development cycle.
Ultra High Strength Stainless’ Future Development:
As a metal structure material with excellent strength, toughness and service safety, high strength stainless steel has a broad application prospect in aviation, aerospace, Marine engineering and nuclear industry. In view of the harsh [SCALE STEEL]application environment of this kind of steel, the exploration of a new generation of high-strength stainless steel should not only focus on breaking the bottleneck of matching ultra-high strength and excellent plasticity and toughness, but also take into account the excellent service safety. In the process of alloy design and heat treatment process formulation, the traditional trial-and-error method is gradually transferred to thermal/dynamic assisted alloy design, artificial intelligence mechanical learning and other rational design methods, in order to greatly improve the research and development cycle of new high-strength corrosion resistant alloy and save the research and development cost. The mechanism of strengthening and toughening in high strength stainless steel is still to be further studied, especially the understanding of the precipitation behavior of the second phase and the superposition of the strengthening contribution value. The effect of austenite content, size, morphology and stability on the toughness[SCALE STEEL] of high strength stainless steel has been studied extensively, but no effective mathematical model has been established to quantitatively estimate the contribution of austenite content, size, morphology and stability to the toughness of high strength stainless steel.
In addition, it is urgent to solve the stress corrosion fracture mechanism and hydrogen embrittlement sensitivity of ultra-high [SCALE STEEL]strength stainless steel under complex strengthening system, so as to provide a theoretical basis for the durability design of ultra-high strength stainless steel.
FROM SCALE STEEL