Intensive Interstitial Strengthening of Stainless Steels

Abstract

The high chromium content of stainless steel impairs the interstitial solubility of carbon in austenite at solution annealing or hardening temperature. Replacing carbon by nitrogen improves the interstitial solubility which is raised most, if carbon and nitrogen are added jointly. Respective thermodynamic equilibrium calculations have led to a new group of austenitic and martensitic steels as well as to a thermochemical surface treatment, that make use of the C + N concept to intensively strengthen stainless steels with about 0.5 to 1 mass% of these interstitials. Pressurized electro slag remelting (PESR) is required to raise the N content in the melt and to avoid degassing during solidification of martensitic stainless steels with C + N known as CRONIDUR®. For austenitic CrMn steels the C + N concept affords to dissolve about 1 mass% of these elements in the melt at atmospheric pressure and keep them in solid solution during solidification of the new CARNIT® steels. Case hardening of stainless martensitic steels with nitrogen instead of carbon, called SolNit®, combines the effect of C in the steel and N dissolved in a surface zone via heat treatment in N2 gas of controlled pressure. The key properties and respective microstructures of these three versions of the C + N concept are discussed along with some applications.

Austenitic high interstitial steels vs. CoCrMo – Comparison of fatigue behavior

“…It needs to be recognized that the amount of phase transformation is smaller with higher C content and smaller grain size of the CoCrMo0.25. C decreases D F and increases the SFE [33,37]. The same is valid for SFE in case of grain size reduction [38].…”

Effect of nitrogen and cold working on structural and mechanical behavior of Ni-free nitrogen containing austenitic stainless steels for biomedical applications

“…Besides using nitrogen alone, very substantial interstitial strengthening may be achieved by the combined use of carbon and nitrogen, and so the (C + N) concept has been the subject of investigation in a series of recent studies [49]. Moreover, nitrogen and carbon of the strengthening elements have a strong effect on the deformation strengthening.…”

“…In order to accommodate the deformation, the multiplication of dislocation is necessary, which leads to the increase of the flow stress. Several new high-strength stainless steels have been fabricated and Berns et al [49] have recently explained the impressive (C + N) interstitial strengthening in terms of the large increase in free electrons associated with the addition of C, N or (C + N) and short-range ordering. More strengthening may be achievable by the combined use of carbon and nitrogen.…”

Influence of the PM-Processing Route and Nitrogen Content on the Properties of Ni-Free Austenitic Stainless Steel

“…[14] Furthermore, the non-magnetic behavior makes this steel grade suitable for applications in environment of strong magnets as are present in magnetic resonance imaging or alternating electric fields. [14,15] Detailed information about the concept of combined alloying of C and N (C + N concept) and the influence of N on the mechanical properties can be extracted from. [8][9][10][11][12][13] The steel investigated in this work Fe-19Mn-18Cr-0.4C-0.3N was developed in previous studies using the C + N concept.…”

“…Furthermore, because precipitation starts at lower temperatures, a lower critical cooling rate is required to obtain a precipitate-free material leading to higher producible cross sections. [15,17] This higher nitrogen content of 0.825 mass pct can be obtained if a powder-metallurgical (PM) production route is applied. During sintering under N 2 atmosphere, a gas-solid interaction results in a nitrogen uptake of the steel powder.…”