The use of nitrogen in martensitic stainless steels is limited by its solubility. Nitrogen solubility can be increased by alloying with elements such as Cr, Mn, and Mo and the use of pressure, such as in Pressurized ElectroSlag Remelting (PESR). Furthermore, the joint addition of C þ N increases their solubility. Solid-state nitriding can be used for case hardening or N-enrichment of steel powders before sintering. However, the resulting stabilization of austenite can be a drawback for martensitic steels. Besides cryogenic treatment below the martensite finish temperature, ausforming, that is, metal working above M s , could be promising. This contribution gives an overview about latest developments in N-rich martensitic stainless steels.
In this work, the corrosion resistance of X190CrVMo20‐4‐1 martensitic stainless steel and the hardness are correlated with the tempering temperature. The steel was hardened from a typical austenitisation temperature, quenched in oil and tempered up to 600 °C. The corrosion resistance was investigated by means of electrochemical tests, in both 5% sulphuric acid and in 3% sodium chloride. Static immersion tests were performed in H2SO4 only. The best compromise for a high hardness and corrosion resistance is found for tempering between 100 and 200 °C. The data obtained can be used for designing and application of this steel.
Optimal treatment of bone fractures with minimal complications requires implant alloys that combine high strength with high ductility. Today, TiAl6V4 titanium and 316L steel are the most applied alloys in bone surgery, whereas both share advantages and disadvantages. The nickel-free, high-nitrogen austenitic steel X13CrMnMoN18-14-3 (1.4452, brand name: P2000) exhibits high strength in combination with superior ductility. In order to compare suitable alloys for bone implants, we investigated titanium, 316L steel, CoCrMo and P2000 for their biocompatibility and hemocompatibility (according to DIN ISO 10993–5 and 10993–4), cell metabolism, mineralization of osteoblasts, electrochemical and mechanical properties. P2000 exhibited good biocompatibility of fibroblasts and osteoblasts without impairment in vitality or changing of cell morphology. Furthermore, investigation of the osteoblasts function by ALP activity and protein levels of the key transcription factor RUNX2 revealed 2x increased ALP activity and more than 4x increased RUNX2 protein levels for P2000 compared to titanium or 316 steel, respectively. Additionally, analyses of osteoblast biomineralization by Alizarin Red S staining exhibited more than 6x increased significant mineralization of osteoblasts grown on P2000 as compared to titanium. Further, P2000 showed no hemolytic effect and no significant influence on hemocompatibility. Nanoindentation hardness tests of Titanium and 316L specimens exposed an indentation hardness (HIT) of about 4 GPa, whereas CoCrMo and P2000 revealed HIT of 7.5 and 5.6 GPa, respectively. Moreover, an improved corrosion resistance of P2000 compared to 316L steel was observed. In summary, we could demonstrate that the nickel-free high-nitrogen steel P2000 appears to be a promising alternative candidate for applications in bone surgery. As to nearly all aspects like biocompatibility and hemocompatibility, cell metabolism, mineralization of osteoblasts and mechanical properties, P2000 was similar to or revealed advantages against titanium, 316L or CoCrMo.
The chromium content of standard martensitic stainless steel X65Cr14 is raised to 17 mass% to enhance the solubility of nitrogen. Up to 4 mass% of nickel are added to suppress a partially ferritic solidification. This combination increases the nitrogen content soluble at 1 bar pressure from 0.14 to 0.24 mass%, which allows to reduce the carbon content to about 0.4 mass% without a loss of hardening capacity. The lower carbon level prevents the precipitation of coarse eutectic carbides in segregated areas encountered in X65Cr14. Instead of nickel pressure is applied to raise the nitrogen content up to 0.45 mass%. Calculations are verified by melts. As the hardening temperature is increased the CrMoN solute content of austenite is raised and so is the pitting resistance after hardening. However, retained austenite reduces the hardness and ausforming at 200°C is applied to transform it during subsequent deep freezing and raise the hardness.
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