Effect of nitrogen on corrosion behavior of 28Cr-7Ni duplex and microduplex stainless steels in air-saturated 3.5 wt% NaCl solution

The corrosion behavior of 28Cr-7Ni-O-0.34N duplex stainless steels in air-saturated 3.5-wt% NaCl solution at pH 2, 7, 10 and 27 °C was studied by the potentiodynamic method. Two types of microstructures were investigated: the as-forged duplex and microduplex (average austenite grain size 5-16 μm) structures. The austenite volume fractions of the tested steels were between 0.35 and 0.64. The nitrogen effect on corrosion behaviors of both duplex and microduplex stainless steels were the same. At pH 2, the corrosion potential increased when the nitrogen content increased, however, corrosion current density as well as corrosion rate decreased. At pH 7 and 10, the effect of nitrogen on corrosion potential and corrosion rate could not be observed. Corrosion potential at pH 10 was lower than at pH 7. Pitting potential increased when the nitrogen content in the tested steels increased at all tested pH. For the nitrogen effect on the passive current density, it seemed that only at pH 2, the average passive current densities reduced when the nitrogen content increased. Nitrogen may have participated in the passive film or has been involved in the reaction to build up passive film. The ammonium formation and nitrogen enrichment at the interface metal/passive film with adsorption mechanism were discussed. The dissolute nitrogen might have combined with the hydrogen ions in solution to form ammonium ions, resulting in increasing solution pH. The steel could then easily repassivate, hence the corrosion potential and pitting potential would increase. However, the ammonium formation mechanism could not explain the decrease of corrosion potential in basic solution. Nitrogen enrichment at the metal/passive film interface with adsorption mechanism seemed to be an applicable consideration in increasing pitting potential. However, this mechanism did not involve the ammonium ion formation. In general, for the duplex and microduplex stainless steels tested, nitrogen increased the general corrosion resistances in acid solution and pitting corrosion resistance at all solution pH. Metallographic observation in both tested duplex and microduplex steels after pitting corrosion at all tested pH revealed that, the corroded structure in the tested steels without nitrogen alloying was austenite, but those with nitrogen alloying was ferrite. Even though ferrite had a higher chromium content than austenite but higher dissolved nitrogen in austenite than in ferrite may have increased the pitting resistance equivalent number (PRE) of austenite to be higher than that of ferrite.     

Grain Refinement by Cyclic Displacive Forward/Reverse Transformation in Fe-High-Ni Alloys

A novel method for grain refinement of martensite structures was proposed, in which transformation strain is accumulated by cyclic displacive forward and reverse transformations. This method can refine martensite structures in an Fe-18Ni alloy because a high density of austenite dislocations is introduced by a displacive reverse transformation in addition to an inheritance of dislocations in body-centered cubic martensite into austenite during cyclic transformation. The addition of a small amount of carbon accelerates structure refinement significantly, which results in the formation of ultra-fine-grained structures after ten cycles.

     

Effect of carbon content on variant pairing of martensite in Fe–C alloys

The effect of carbon content on the variant pairing tendency of martensite formed in Fe–C alloys is investigated by means of electron backscattered diffraction analysis. The method used is based on experimentally determined orientation relationships between austenite and martensite. The results show that the carbon content has a strong effect on the martensite variant pairing tendency. This observed change in variant pairing tendency is discussed in relation to the well-known morphological transition from lath to plate martensite in Fe–C alloys and the formation of packets and plate groups. The results indicate that quantitative analysis of variant pairing, as demonstrated here, may facilitate martensite characterization in Fe–C alloys as well as in other alloy systems.     

Direct measurement of carbon enrichment during austenite to ferrite transformation in hypoeutectoid Fe–2Mn–C alloys

Carbon enrichment in untransformed austenite at the end of Mn partitionless growth of ferrite for Fe–2Mn–(0.05, 0.14)C (mass%) alloys isothermally transformed in the temperature range 873–998 K was measured using field-emission electron probe microanalysis to reveal its dependence on the transformation temperature, nominal carbon content and prior austenite grain size. The PLE/NPLE model gives much better predictions than the PE model for carbon enrichment in untransformed austenite at the end of partitionless growth. Carbon enrichment could be increased by reducing the prior austenite grain size. Furthermore, carbon enrichment shifted from the PLE/NPLE transition line to the T₀ line on lowering the transformation temperature. This shift is probably attributed to the solute drag effect and/or to the finite interface mobility, both of which vary with the transformation temperature.     

Thickening of grain-boundary α allotriomorphs in a Ti-Cr alloy by multiple sets of ledges

A computer model is developed to simulate the growth of grain-boundary allotriomorphs having more than one set of growth ledges at their interfaces. The growth is controlled by the volume diffusion of solute to or from the riser of a ledge. The time dependence of the growth rate of two orthogonal sets of ledges is found to be somewhat different from that of a single set of ledges. However, the operation of multiple sets of ledges is unlikely to alter significantly the growth kinetics of grain-boundary allotriomorphs from those predicted from the disordered growth theory, except at small ledge spacings or at short reaction times. Faster growth kinetics of proeutectoid α allotriomorphs than those of either planar or ellipsoidal disordered boundaries which have been reported in a Ti-6.6 at. pet Cr alloy are not likely to be accounted for with the heights and spacings of double sets of ledges actually observed on the interfaces of allotriomorphs. Hence, the grain-and interphase-boundary diffusion-assisted growth of precipitates, (rejector plate mechanism, RPM) appears to be operative during the growth of a allotriomorphs, as previously proposed on the basis of growth-rate measurements. This paper is based on a presentation made in the symposium “The Role of Ledges in Phase Transformations” presented as part of the 1989 Fall Meeting of TMS-MSD, October 1–5, 1989, in Indianapolis, IN, under the auspices of the Phase Transformations Committee of the Materials Science Division, ASM INTERNATIONAL.     

Variant Selection in Grain Boundary Nucleation of Upper Bainite

The crystallography of bainitic ferrite nucleated at austenite grain boundaries was studied in an Fe-9Ni-0.15C (mass pct) alloy. The relationship between bainitic ferrite orientations (variants) and grain boundary characters, i.e., misorientation and boundary orientation, was examined by electron backscatter diffraction analysis in scanning electron microscopy and serial sectioning observation. Bainitic ferrite holds nearly the Kurdjumov–Sachs (K-S) orientation relationship with respect to the austenite grain into which it grows. At the beginning of transformation, the variants of bainitic ferrite are severely restricted by the following two rules, both advantageous in terms of interfacial energy: (1) smaller misorientation from the K-S relationship with respect to the opposite austenite grain and (2) elimination of the larger grain boundary area by the nucleation of bainitic ferrite. As the transformation proceeds, variant selection establishing plastic accommodation of transformation strain to a larger extent becomes important. Those kinds of variant selection result in formation of coarse blocks for small undercooling. This article is based on a presentation given in the symposium entitled “Solid-State Nucleation and Critical Nuclei during First Order Diffusional Phase Transformations,” which occurred October 15–19, 2006 during the MS&T meeting in Cincinnati, Ohio under the auspices of the TMS/ASMI Phase Transformations Committee.

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Effects of Mn,Si and Cr addition on reverse transformation at 1073 K from spheroidized cementite structure in Fe–0.6 mass% C alloy

The effects of addition of Si, Mn and Cr on the kinetics of reverse transformation at 1073 K from the spheroidized cementite structure obtained by heavy tempering of high carbon martensite are investigated. The rate for reverse transformation is the fastest in the Fe–0.6 mass% C binary material, and becomes slower with the addition of Mn, Si and Cr. In particular, the retarding effect of Cr addition is remarkable, and holding times orders of magnitude longer than for other alloys are necessary to complete the reverse transformation. Based on thermodynamics and TEM/EDS analyses, it is supposed that austenite growth is controlled by carbon diffusion in specimens with Si and Mn added, as well as the Fe–0.6 mass% C binary material, while a decrease in the carbon activity gradient with the addition of these elements results in slower reversion kinetics. However, in the Cr-added specimen, Cr diffusion is necessary for austenite growth, yielding extremely sluggish reaction kinetics.     

Crystallography and Interphase Boundary of Martensite and Bainite in Steels

Grain refinements in lath martensite and bainite structures are crucial for strengthening and toughening of high-strength structural steels. Clearly, crystallography of transformation plays an important role in determining the “grain” sizes in these structures. In the present study, crystallography and intrinsic boundary structure of martensite and bainite are described. Furthermore, various extrinsic factors affecting variant selection and growth kinetics, such as elastic/plastic strain and alloying effects on interphase boundary migration, are discussed.     

Excess Carbon Enrichment in Austenite During Intercritical Annealing

Carbon enrichment in the austenite transformed from martensite during intercritical annealing was measured by electron probe microanalyzer and three-dimensional atom probe microscopy in Fe-2Mn-0.3C and Fe-0.35C alloys. At early stages of the transformation, negligible Mn partitioning occurs, and carbon content in austenite is higher than orthoequilibrium and paraequilibrium predictions. This is presumably attributed to finite intrinsic interface mobility and/or solute drag effect. The resultant free energy dissipation at interface was estimated.     

Synergistic alloying effect on microstructural evolution and mechanical properties of Cu precipitation-strengthened ferritic alloys

We report the influence of alloying elements (Ni, Al and Mn) on the microstructural evolution of Cu-rich nanoprecipitates and the mechanical properties of Fe–Cu-based ferritic alloys. It was found that individual additions of Ni and Al do not give rise to an obvious strengthening effect, compared with the binary Fe–Cu parent alloy, although Ni segregates at the precipitate/matrix interface and Al partitions into Cu-rich precipitates. In contrast, the co-addition of Ni and Al results in the formation of core–shell nanoprecipitates with a Cu-rich core and a B2 Ni–Al shell, leading to a dramatic improvement in strength. The coarsening rate of the core–shell precipitates is about two orders of magnitude lower than that of monolithic Cu-rich precipitates in the binary and ternary Fe–Cu alloys. Reinforcement of the B2 Ni–Al shells by Mn partitioning further improves the strength of the precipitation-strengthened alloys by forming ultrastable and high number density core–shell nanoprecipitates.