Nitrogen—A Boon to the Metals Industry

The history of mankind has been largely influenced by the evolution and growth of tools, instruments, and machines. Nearly everything we need for our present civilization depends on metals and newer alloys. Quite often the crucial turning points of our civilization have been the emergence of new metals and materials. In a century charged with metallurgical revolutions, the development of high-nitrogen steel (HNS) stands out as a remarkable technological achievement. The impressive and brief historical evolution of HNS is highlighted in this article.     

Characterization of crystallization kinetics of a Ni- (Cr, Fe, Si, B, C, P) based amorphous brazing alloy by non-isothermal differential scanning calorimetry

The thermal stability and crystallization kinetics of a Ni- (Cr, Si, Fe, B, C, P) based amorphous brazing foil have been investigated by non-isothermal differential scanning calorimetry. The glass transition temperature T_g, is found to be 720 ± 2 K. The amorphous alloy showed three distinct, yet considerably overlapping crystallization transformations with peak crystallization temperatures centered around 739, 778 and 853 ± 2 K, respectively. The solidus and liquidus temperatures are estimated to be 1250 and 1300 ± 2 K, respectively. The apparent activation energies for the three crystallization reactions have been determined using model free isoconversional methods. The typical values for the three crystallization reactions are: 334, 433 and 468 kJ mol⁻¹, respectively. The X-ray diffraction of the crystallized foil revealed the presence of following compounds Ni₃B (Ni₄B₃), CrB, B₂Fe₁₅Si₃, CrSi₂, and Ni_4.5Si₂B.     

Investigation of fatigue failure of a stainless steel orthopedic implant device

An orthopedic implant (rush nail) fractured in a patient at a location that corresponded to the site of a prior fracture of the bone (right femur). The crack propagation in the implant proceeded from both sides of the nail, and the final fracture occurred by ductile shear in the midsection of the nail. Dimple structures and tear ridges between fatigue striation patches were observed on the fractured surface. Moreover, the device fractured within a short period of use. Contrary to post-procedure instructions, the patient placed the body’s full weight on the implanted leg at least once, perhaps twice, causing overload-induced fatigue failure of the implant.     

In vitro corrosion evaluation of nitrogen ion implanted titanium in simulated body fluid

Surface modification of commercially pure (CP) titanium was attempted by nitrogen ion implantation to investigate corrosion resistance in simulated body fluid. Nitrogen ion was implanted at 70 keV energy for different doses ranging from 5 × 10¹⁵ to 2.5 × 10¹⁷ ions/cm². In Vitro Open Circuit Potential (OCP-time measurements and cyclic polarization studies were carried out to evaluate the corrosion resistance of the implanted specimens with reference to the unimplanted one. Specimens implanted at 4 × 10¹⁶ and 7 × 10¹⁶ ions/cm² showed optimum corrosion resistance, and implantation beyond this dose deteriorated the corrosion resistance. Gracing Incidence X-ray diffraction (GIXD) was employed on implanted specimens to understand the phases formed with increasing doses. The results of the present investigation indicated that nitrogen ion implantation can be used as a viable method for improving corrosion resistance of titanium. Nature of the surface and reason for the variation and improvement in corrosion resistance are discussed in detail.     

Improving intergranular corrosion resistance of sensitized type 316 austenitic stainless steel by laser surface melting

An attempt was made to modify the surface microstructure of a sensitized austenitic stainless steel, without affecting the bulk properties, using laser surface melting techniques. AISI type 316 stainless steel specimens sensitized at 923 K for 20 hr were laser surface melted using a pulsed ruby laser at 6 J energy. Two successive pulses were given to ensure uniform melting and homogenization. The melted layers were characterized by small angle X- ray diffraction and scanning electron microscopy. Intergranular corrosion tests were carried out on the melted region as per ASTM A262 practice A (etch test) and electrochemical potentiokinetic reactivation test. The results indicated an improvement in the intergranular corrosion resistance after laser surface melting. The results are explained on the basis of homogeneous and nonsensitized microstructure obtained at the surface after laser surface melting. It is concluded that laser surface melting can be used as an in situ method to increase the life of a sensitized component by modifying the surface microstructure.     

Production of (S)-2-chloropropionate by asymmetric reduction of 2-chloroacrylate with 2-haloacrylate reductase coupled with glucose dehydrogenase

(S)-2-Chloropropionate is a synthetic intermediate for phenoxypropionic acid herbicides. We constructed a system for asymmetric reduction of 2-chloroacrylate to produce (S)-2-chloropropionate with recombinant Escherichia coli cells producing 2-haloacrylate reductase from Burkholderia sp. WS and an NADPH regeneration system. The system provided 37.4 g/l (S)-2-chloropropionate in more than 99.9%e.e.     

Dissimilar Joining of Zircaloy-4 to Type 304L Stainless Steel by Friction Welding Process

Zirconium- and titanium-based dissolver vessels containing highly radioactive and concentrated corrosive nitric acid solution needs to be joined to the rest of fuel reprocessing plant made of AISI type 304L stainless steel (SS), which demands high integrity and corrosion resistant dissimilar joints. Solid-state joining process of friction welding was proposed in the present work to join zircaloy-4 and type 304L SS since fusion welding processes produce brittle intermetallic precipitates at the interface which reduce the mechanical strength as well as the corrosion resistance of the joint. The present study attempts to optimize joining parameters, without and with thin Ta and Ni interlayers that can prevent brittle intermetallic formation. Tensile test, three-point bend test, and microhardness measurements were performed on the joints. Characterization techniques such as optical microscopy, scanning electron microscopy (SEM), and x-ray diffraction (XRD) were employed. A good friction weld joint of zircaloy-4 to 304L SS was achieved with the joint strength (~540 MPa) greater than that of the base of zircaloy-4, without using any interlayer. A bend ductility of 5° was only obtained without using any interlayer. However, XRD patterns indicated the presence of intermetallics in the friction-welded joints without interlayers. Corrosion test carried out on zircaloy-4 to 304L SS friction joint in boiling 11.5 M nitric acid exhibited corrosion rate of 225 μm/year after 240 h. SEM examination of the corroded joint indicated severe intergranular corrosion attack over stainless steel and preferential dissolution at the interface.     

Effect of nitrogen ion implantation on the localized corrosion behavior of titanium modified type 316L stainless steel in simulated body fluid

Nitrogen ion implantation on titanium-modified type 316L stainless steel (SS) at the energy of 70 keV was carried out at different doses ranging from 1×10¹⁵ to 2.5×10¹⁷ ions/cm². These samples were subjected to open circuit potential (OCP)—time measurement, cyclic polarization, and accelerated leaching studies—in order to discover the optimum dose that can provide good localized corrosion resistance in a simulated body fluid condition. The results showed that the localized corrosion resistance improved with an increase in doses up to 1×10¹⁷ ions/cm², beyond which it started to deteriorate. The results of the accelerated leaching studies showed that the leaching of the major alloying elements was arrested upon nitrogen ion implantation. Gracing incidence x-ray diffraction studies showed the formation of chromium nitrides at a dose of 2.5×10¹⁷ ions/cm². X-ray photoelectron spectroscopy studies revealed the presence of these chromium nitrides in the passive film, which was attributed to the decreased corrosion resistance at a higher dose. Secondary ion mass spectroscopy studies on the passive film showed the variation in the depth profile upon nitrogen ion implantation. Thus, nitrogen ion implantation can be effectively used as a method to improve the corrosion resistance of the orthopedic implant devices made of titanium-modified type 316L SS. The nature of the passive film and its influence on corrosion resistance are discussed in this article.