High nitrogen steels

The article provides a brief overview of the properties and production technology of high-nitrogen steels (HNS), which have several advantages over traditional ones. The main advantages are: up to four times higher yield strength with unique preservation of the remaining characteristics; reduction in consumption or a 100 % elimination of the use of some expensive alloying elements, such as Ni, Mo, Co, W, and others; effective alloying with unconventional elements (Ca, Zn, Pb, etc.). The basics of HNS technology, dependence of the properties on nitrogen content in steels, producing technologies for ferritic-pearlitic, martensitic and austenitic steel, their properties and applicability are discussed. Alloying with nitrogen for ferritic-pearlitic steel requires more precise adherence to the chemical composition in order to prevent the formation of insoluble nitrides during heat treatment (due to its greater solubility compared to carbon). Features of martensitic steels are associated with the possibility of formation of nitrides and carbonitrides during tempering. The possible effect of nitrogen in these steels may be as a decrease in the size of nitride particles as compared with carbide ones. Increased stability temperature of nitrides and carbonitrides provides increased mechanical and physical properties. In austenitic steels, nitrogen, due to the strong γ-forming equivalence to nickel, replaces it in a ratio of 1 kg of nitrogen ≈ 6 – 39 kg Ni. In austenitic-martensitic steels, the main role is played by thermal martensite. Stable austenite is obtained in the process of its aging at operating temperatures. Examples of effective use of HNS in important details are described.

CORROSION AND CAVITATION RESISTANCE IN SEAWATER OF CHROMIUM–NICKEL–MANGANESE HIGH-STRENGTH NITROGEN AUSTENITIC STEELS

Corrosion and cavitation resistance in seawater of high-strength economically alloyed nitrogen chromium–nickel–manganese steels Cr19Mn10Ni6Mo2N and Cr19Mn10Ni6Mo2Cu2N is experimentally studied compared to chromium-nickel steels Cr18Ni9 and Cr18Ni9N. Tests for resistance to pitting corrosion were carried out according to the chemical method in the test solution 100 g/l FeCl3 ·6H2 O. Resistance to general corrosion was assessed by tests in synthetic seawater (3 % NaCl). Test for cavitation resistance in seawater was performed using a research stand of high-intensity cavitation effects with the use of ultrasonic devices UIP 1000 hd Hielscher Ultrasonic in 3 % NaCl solution in water at a frequency of 20 kHz, a power of 1000 W and amplitude of 25 microns for 8 – 36 hours. The extent of damage and change in the surface microhardness, change in the phase composition and mass of the samples were assessed after cavitation. It is shown that steels Cr19Mn10Ni6Mo2N and Cr19Mn10Ni6Mo2Cu2N are more susceptible to pitting in seawater and in solution of ferric chloride, and have the general corrosion rate lower than that of chromium-nickel steels type Cr18Ni9. It is shown that ultrasonic cavitation can not only lead to surface damage due to erosion, enhance local corrosion, but also to changes in their physico-mechanical properties by strain hardening and phase transformations. Steels Cr19Mn10Ni6Mo2N and Cr19Mn10Ni6Mo2Cu2N with thermally and mechanically stable austenite are more resistant to ultrasonic cavitation in the seawater in comparison with chromium-nickel steels, especially those with less strength and less resistant steel Cr18Ni9. So subjected to cavitation in the seawater for 36 hours, samples of chromium-nickel steels Cr18Ni9 and Cr18Ni9N had a significant change in their condition: significant damage (etching) and surface hardening, and there was formation of a small amount of martensite in steel Cr18Ni9. Samples of steels Cr19Mn10Ni6Mo2N and Cr19Mn10Ni6Mo2Cu2N had only minor changes in surface conditions and hardening of the surface layers.

Induced ferrite formation above the Ae3 during plate rolling simulation of a X70 steel

ABSTRACTPartial amount of austenite can be dynamically transformed into ferrite above the Ae3 temperature when it is being deformed. This happens by a displacive mechanism. On removal of the load, it retransforms back into the stable austenite by diffusional processes. Plate rolling simulation under continuous cooling conditions was carried out on a high Nb X70 steel. Pass strains of 0.2 together with interpass times of 10, 20 and 30 s were employed. The initial and final temperatures for the finishing simulation were 920 and 830 °C, respectively. The mean flow stresses (MFS`s) behaviour indicates that dynamic transformation (DT) and recrystallization (DRX) were taking place during straining. It is shown that ferrite is formed during the roughing passes and increases its volume fraction throughout the finishing rolling steps. The ferrite formation is favoured by strain accumulation, shorter time between passes and also when the temperature reaches the Ae3 line. The results obtained here can be used to design improved models for transformation on accelerated cooling.

Regularities of Microstructure Evolution and Strengthening Mechanisms of Austenitic Stainless Steels Subjected to Large Strain Cold Working

The deformation microstructures and their effects on mechanical properties of austenitic stainless steels processed by cold rolling at ambient temperature to various total strains were studied. The cold working was accompanied by the development of strain-induced martensitic transformation because of meta-stable austenite at room temperature. The strain-induced martensitic transformation and deformation twinning promoted the grain refinement during cold rolling, leading to nanocrystalline structures consisting of a mixture of austenite and martensite grains with their transverse grain sizes of 50-150 nm containing high dislocation densities. The rolled samples experienced substantial strengthening resulted from high density of strain induced grain/phase boundaries and dislocations. The yield strength of austenitic stainless steels could be increased to 2000 MPa after rolling to total strains of about 4. The martensite and austenite provided almost the same contribution to overall yield strength. The dislocation strengthening was much higher than the grain boundary strengthening at small to moderate strains of about 2, whereas the latter gradually increased approaching the level of dislocation strengthening with increasing the strain.

Multi length scale characterization of austenite in TRIP steels using high-energy X-ray diffraction

The martensitic transformation behavior of the meta-stable austenite phase in low alloyed TRIP steels has been studied in situ using high-energy X-ray diffraction during deformation. The stability of austenite has been studied at different length scales during tensile tests and at variable temperatures down to 153 K. A powder diffraction analysis has been performed to correlate the macroscopic behavior of the material to the observed changes in the volume fraction of the phases. Our results show that at lower temperatures the deformation induced austenite transformation is significantly enhanced and extends over a wider deformation range, resulting in a higher elongation at fracture. To monitor the austenite behavior at the level of an individual grain a high-resolution far-field detector was used. Sub-grains have been observed in austenite prior to transformation.

Stability of Reversed Austenite in 9Ni Steel

The samples of 9Ni steel were treated by Quenching + Tempering (QT) and Quenching + larmellarizing + Tempering (QLT). The morphology, thermal stability and mechanical stability of the reversed austenite in 9Ni steel were studied by TEM, XRD, EBSD, subzero treatment, three-point bending, uniaxial tension and uniaxial compression. It turns out that blocky reversed austenite occurs in QT-treated steel, while blocky and filmy reversed austenite occurs in QLT-treated steel, additionally, the filmy reversed austenite distributes in the lath boundaries. Subzero treatment shows that certain amount of reversed austenite in QLT-treated samples transforms after dipping into liquefied nitrogen, but it retains a higher level of more stable austenite in QLT-treated sample than that in QT-treated sample. Both tension and compression facilitate the transformation of reversed austenite into martensite, and the retained austenite mainly locates within grains.

The Effect of Texture on the Stability of Retained Austenite in Al-Alloyed TRIP Steels

ABSTRACTSteels with transformation induced plasticity (TRIP) offer an excellent combination of high strength and ductility. The transformation of meta-stable austenite into martensite during straining leads to strong local hardening and prevents early localization of strain. Therefore, the mechanical properties of TRIP steels, including the damage resistance depend to a significant extent on the stability of retained austenite. The aim of this study was to evaluate the effect of texture on the stability of retained austenite. In order to compare the changes in both tension and compression the steel was deformed by a micro 3-point-bending device. The texture development upon bending was followed by electron backscatter diffraction (EBSD) technique. Based on a simple analysis using the relation between face centered cube (FCC) and body centered cube (BCC) shear geometries theoretically expected changes of texture components due to deformation are proposed. Using the results of this analysis the observed changes of the austenite texture due to deformation could be distinguished from those due to transformation, by comparing the experimental results with the theoretically expected behavior. From this comparison, austenite grains with “Brass (B) {011} <211>” and “Goss (G) {110} <100>” texture components were found to transform into martensite much easier than differently oriented grains.

Fatigue Life Curves of NiTi Alloys – The Z Effect

Superelasticity is closely related to shape memory effect. It refers to the property presented by some materials submitted to large strains (usually up to about 8%) to restore their original shape immediately after unloading without the need of heating. This phenomenon results directly from a diffusionless transformation of the material from an austenitic to a martensitic phase (martensitic transformation). The recovering mechanism is the reverse transformation, from martensite to austenite. This paper compares fatigue live curves obtained in bending-rotation fatigue tests carried out on wires of NiTi alloys with three different microstructures, stable austenite, unstable austenite (superelastic), and stable martensite. These curves are also compared to data from the literature. The tests were strain controlled and the wires were submitted to strain amplitudes from 0.6% to 12.0%. To minimize changes in material properties, the wire temperature was monitored using a thermocouple and controlled by its rotation speed. For strain amplitudes up to 4%, the εa-Nf curve for superelastic wires was consistent with those reported in the literature, closely approaching the curve of the stable austenite wire. For higher strain amplitudes, fatigue life of superelastic wires increased with strain until it approached the fatigue life curve of stable martensitic wire. This unusual behavior results in a “Z-shaped” curve for high strain values. It is possibly linked to the changes in microstructure and fatigue properties that occur when the superelastic material is deformed.