Ni Stainless Steel Research Articles

Steady state creep of 20% Cr-25% Ni stainless steel with and without a dispersion of tin particles

The steady state creep behaviour of both single phase and TiN dispersion strengthened 20% Cr-25% Ni stainless steel is examined with the object of determining the role of the second phase particles on dislocation network creep. A correlation between the stress/strain rate variation in the two materials can be made by writing. ε ss = A( σ − σ 0) p , where σ is ∼0 and ~80 MPa for single-phase and dispersion hardened steels respectively, and p is ∼5. An expression of this form leads to the prediction of threshold behaviour at σ 0, but the creep rate data indicate that steady state creep also occurs below this stress. Microstructural observations are described which suggest that σ 0 in the TiN bearing alloy can be identified with σ p , the Orowan stress. The Orowan mechanism cannot operate below σ p , but an alternative mode of dislocation bypass around particles is available, which leads to a mechanistic transition at a stress σ gt where the rates of the two processes are the same. The stress/dislocation density relationship is investigated, and it is found that the density p in the dispersion hardened alloy becomes constant below the Orowan stress at a density ( p 0) such that the average network link length is equal to the interparticle spacing. Further, the strain rate/dislocation density variation in single phase material at high stress appears to be such that ε α σ − σ s pα p p 2 while at low stresses there is an excess of dislocations. It is argued that writing ε α ρ − ρ 0 p 2 with ρp 0 the annealed-in density, allows the strain rate to be self-consistent function of the active dislocation content over the entire stress range. Moreover, treating the data from the two-phase steel in the same manner reduces the ε/( ρ − ρ 0) points from both alloys to a single curve. The density ρ 0 is suggested to arise as a result of stress relaxation where dislocations are held up at hard parts of the microstrucure. The stress relaxation produces an increase in the total dislocation line length, but a reduction in the stored energy, so that the driving force for recovery is reduced at a given total density. The relaxation structures themselves must recover, and this is likely to happen over the microstructural wavelength of the plastic inhomogeneity which is not only responsible for the presence of the geometrically necessary dislocations, but which also determines the content of annealed material. The spatial inhomogeneity of the dislocation density can be described in terms of a steady-state internal stress distribution which acts so as to compensate for variations in local flow stress. The threshold behaviour, however, is suggested to arise because the TiN particles are undeformable. The Orowan stress always therefore has to be exceeded before the network model of dislocation creep can be at all appropriate. However, the measured variations in dislocation density with stress imply that the creep rate can be correlated with the dislocation density, as in the network model, if the effects of plastic relaxation are taken into account.

Microstructure-strength relations in a hardenable stainless steel with 16 pct Cr, 1.5 pct Mo, and 5 pct Ni

Metallographic studies have been conducted on a 0.024 pct C-16 pct Cr-1.5 pct Mo-5 pct Ni stainless steel to study the phase reactions associated with heat treatments and investigate the strengthening mechanisms of the steel. In the normalized condition, air cooled from 1010 °C, the microstructure consists of 20 pct ferrite and 80 pct martensite. Tempering in a temperature range between 500 and 600 °C results in a gradual transformation of martensite to a fine mixture of ferrite and austenite. At higher tempering temperatures, between 600 and 800 °C, progressively larger quantities of austenite form and are converted during cooling to proportionally increasing amounts of fresh martensite. The amount of retained austenite in the microstructure is reduced to zero at 800 °C, and the microstructure contains 65 pct re-formed martensite and 35 pct total ferrite. Chromium rich M23C6 carbides precipitate in the single tempered microstructures. The principal strengthening is produced by the presence of martensite in the microstructure. Additional strengthening is provided by a second tempering treatment at 400 °C due to the precipitation of ultrafine (Cr, Mo) (C,N) particles in the ferrite.

Consequences of slip-twinning interactions on the monotonic and cyclic deformation of b.c.c. stainless steels

The monotonic and cyclic properties of b.c.c. FeCrNi stainless steels which deform by both twinning and slip are examined. A strain recovery effect is observed. The evolution during cyclic deformation of both strain recovery and twinning is discussed in terms of slip-twinning interactions in the b.c.c. lattice.

Alloy depletion profiles during non-protective oxidation

The theory developed in this paper considers the depletion produced by the selective oxidation of an alloy which oxidizes under non-protective conditions and exhibits linear oxidation kinetics. The development is applicable to the case where a pre-existing depletion profile exists within the alloy prior to the advent of non-protective conditions and, thus, is of relevance to the behaviour of a metal substrate after spallation of a protective oxide. Detailed calculations are presented which demonstrate the perturbation to this profile as a function of the recession rate of the oxide-metal interface. The size of the perturbation zone is found to be independent of the velocity of the oxidation front at low values but approaches zero at higher velocities. A limited comparison of the theory is made on a 20 Cr-25 Ni stainless steel in which a pre-existing chromium depletion profile had been produced by vacuum annealing.

Influence of nonsteady state behavior on superplastic deformation of a 25.7 Cr-6.6 Ni stainless steel

Superplastic behavior of a microduplex stainless steel (IN744) was studied by deforming specimens with initially elongated grains. As a result of deformation, grain growth and breaking up of elongated grains were noticed to occur concurrently. These microstructural instabilities result in strain hardening or strain softening, respectively. This then leads to nonuniqueness in the flow behavior. Some examples of nonuniqueness in stress-strain rate relation include the presence of: strength anisotropy, increase in strain rate sensitivity, constant stress level for varying grain size, and two distinct stress-strain rate curves over comparable test condition for the same initial microstructure. These have been explained by the concomitant microstructural changes and the influence of such changes on instantaneous flow stress. Emphasis is made on the need to eliminate microstructural instability in order to characterize the deformation phenomena under steady state.

Carburization and tensile behavior of alloy 800 in liquid sodium

The carbon transfer has been analyzed in the Alloy 800/sodium/stainless steel system by determining the carbon-uptake of Alloy 800 foils, which were exposed in liquid sodium of known carburizing potential. p ]Under equilibrium conditions between 650 and 550°C the measured total carbon concentrations in the Alloy 800 labs were found to be roughly related to the carbon activities of the sodium environment by the equation already stated for the 18Cr- 8 10 Ni stainless steels, extrapolated to the chemical composition of the Ni-rich austenitic alloy. However the Alloy 800 was not found to undergo any decarburization in low-carbon activity environments. The carbon diffusion kinetics was determined as a function of temperature, it was found to be similar to that reported for the AISI-304 type of steel. p ]The effects of sodium exposure on microstructural and mechanical properties of several Alloy 800 heats were examined at 550°C as a function of the active carbon concentration in the sodium and of the Ti C ratio in the alloys.

Trapping and Surface Recombination of Deuterium in Fusion Reactor Metals

Ion implantation and ion beam analysis were used to examine bulk trapping and surface recombination for deuterium in Fe, Ni, and austenitic stainless steel. Irradiation-defect traps were characterized in the three metals, and substantially stronger traps were shown to be associated with small He bubbles. The rate of deuterium release at stainlesssteel and Fe surfaces was measured as a function of temperature and solution concentration. Trapping and surface recombination parameters were extracted by applying established theory for solute diffusion within a field of traps.

Steady-state creep in a 25 wt % Cr-20 wt % Ni austenitic stainless steel

Steady-state creep behaviour of a 25 wt % Cr-20 wt % Ni stainless steel without precipitates was studied in the stress range 9.8 to 39.2 MPa at temperatures between 1133 and 1193 K. The results of stress-drop tests indicate that, in the steady-state creep region, diffusion-controlled recovery creep is dominant. Such recovery creep can be accounted for in terms of the composition of the internal stress,σi=σs+σc, except in the case of fine-grained specimens where d<80 μm, whered is the mean grain diameter,σs is possible to reduce easily and is comparable to the driving stress for creep, andσc is the persistent stress field due to metastable substructure. In the fine-grained specimens, it is suggested that the steady-state creep is dominantly controlled by grain boundaries.

Development of long continuous Nb&amp;lt;inf&amp;gt;3&amp;lt;/inf&amp;gt;Ge tape conductors with high current density at elevated temperature and field

Superconductors based on Nb 3 Ge have been fabricated by the chemical vapor deposition (CVD) process in continuous lengths up to 20 m. This material exhibits a transition temperature T_{c} > 21.0 K and a self field critical current density J_{c} \geq 2.5 \times 10^{6} A/cm2@ 14 K and offers the possibility of high current operation at temperatures > 14.0 K. Values of J c measured in high magnetic fields exceed 8 \times 10^{4} A/cm2at 18T @ 4 K. The material development program that resulted in reproducing the excellent short sample superconducting properties on long continuous lengths of tape is described. The tapes consist of 3-6 μm of Nb 3 Ge coated completely around 0.64-cm-wide, 25-μm-thick substrates of Cu, Ni, or Cu-clad stainless steel. More than 150 m of this tape has been fabricated for use in winding a 1-m prototype transmission cable. AC-loss measurements on short samples of the long tapes show complete shielding of the substrate and losses which are acceptably low for power transmission applications.

Complex investigation on Fe (111) and Cr Ni stainless steel (100) surfaces by surface analysis methods

Complex investigation on Fe (111) and Cr Ni stainless steel (100) surfaces by surface analysis methods

Decomposition of ferrite to austenite in 26%Cr-5%Ni stainless steel

The decomposition of ∆-ferrite to austenite has been studied in a 26%Cr-5%Ni stainless steel. The decomposition takes place via two mechanisms: by nucleation and growth in the high temperature range (650–1200°C) and by an athermal martensitic process in the low temperature range (300–650°C). The high temperature reaction exhibits growth kinetics which are intermediate between volume diffusion control and growth by a ledge mechanism. The morpho-logical variants of the austenite mirror those described in the Dubé morphological classification for low-alloy steels, and the steel was used as a ‘model system‘ for the study of bcc ⇄ fcc transformations. In the low temperature range (400–600°C) the formation of α′, which gives rise to 475°C embrittlement, is shown to occur by a nucleation and growth process, and the subsequent dissolution of the α′ is explained by the precipitation of austenite. Finally, it is shown by STEM microanalysis that Ni partitions to the austenite and Cr partitions to the ferrite during the high temperature reaction and that there is no composition difference between the parent and product phases in the low-temperature martensitic formation of austenite.

The development of localized pits during stainless steel oxidation

A study of the localized pitting attack of 20% Cr/25%Ni stainless steel and a similar alloy containing a dispersion of titanium nitride particles has been made over the temperature range 1023 to 1173 K. Pitting is initiated when localized spoiling of the protective chromic oxide film occurs. Rapid oxidation of the chromium-depleted substrate then proceeds with the formation of an iron-rich oxide mound on the alloy surface and spinels containing nickel, chromium, and iron within the pit itself. A silica layer which, in general, remains on the alloy surface acts as a diffusion barrier during this stage of the reaction. With increasing depth of attack the local chromium concentration in the alloy at the base of the pit attains a critical value (∼16%) for a protective chromium oxide film to reform; the pitting attack then effectively ceases, although a a subsequent slow rate of growth continues through the protective film at the base. The observed maximum depth of pitting as a function of time is consistent with the parabolic variation predicted by the proposed mechanism. There is no significant difference in the kinetics of attack between the alloys examined.

The influence of grain size, stress and temperature on the steady state creep of a 25 Cr-20 Ni austenitic stainless steel without precipitates

This work was performed in order to study the steady state creep behaviour of a modified 25 Cr-20 Ni stainless steel which has no precipitates. The test temperature range was 1171 to 1211 K, the stress range 4.9 to 19.6 MPa, and the grain size was 40 to 600μm. The steady state creep rate,\(\mathop {\dot \in }\nolimits_S \), decreases with increase in grain size, especially at the lowest stress;\(\mathop {\dot \in }\nolimits_S \) is proportional to 1/d2 at 4.9 MPa, whered is a mean grain diameter. The variation of\(\mathop {\dot \in }\nolimits_S \) with grain size is smaller in the middle and coarse-grained specimens than in the fine-grained specimens, the stress exponent,n, gradually decreases from ~ 4 to ~ 1.5 with reducing stress, but in the middle- and coarse-grained specimens, a discontinuous point appears on a ∥ log\(\mathop {\dot \in }\nolimits_S \) versus logσ plot. The activation energy for the steady state creep of the coarse-grained specimens tends to be larger than that of the fine-grained specimens, and the tendency is remarkable in the higher stress level. It is indicated that the creep mechanisms in the fine-grained specimens are essentially different from those in the coarse-grained specimens, and that the creep at the lowest stresses and smallest grain size is similar to that predicted by a vacancy creep model involving grain-boundary diffusion.

The influence of a titanium nitride dispersion on the oxidation behavior of 20%Cr-25%Ni stainless steel

The oxidation behavior of 20%Cr-25%Ni stainless steels and a similar but stronger alloy containing a dispersion of titanium nitride particles has been studied over the temperature range 1023 to 1173°K. The dispersion-strengthened steel oxidized slightly more slowly than its dispersion-free counterpart at 1023°K, but above this temperature it was inferior in terms of gross weight gain. The parabolic growth kinetics of the chromic oxide layer in each alloy were similar, however, with an activation energy compatible with the rate-controlling step being chromium ion diffusion across the oxide. The evidence suggests that the role of the dispersed phase is to produce an oxide of finer grain size than that of the standard steel, which increases the rate of oxygen transport inward. The consequent formation of internal oxide increases the tendency for oxide rupture and spalling with subsequent enhanced local attack. This leads to the observed higher overall weight gains compared with the dispersion-free alloy.

オーステナイトステンレス鋼の孔食および間隙腐食におよぼすMo, SiおよびNの影響

Chemical pitting corrosion tests in neutral 1.0 M NaCl solution containing 0.5 M K3Fe(CN)6 and measurements of pitting potentials Vc′ and Vcre′ for stainless steel open surface and creviced specimens in 0.5 M NaCl solution were conducted to study the resistance to pitting and crevice corrosion of 18 Cr-14 Ni stainless steels to which the alloying elements Mo, Si and N had been added. The following results were obtained. (1) Mo and N increased the resistance to both pitting and crevice corrosion, whereas Si increased only the pitting resistance and had no effect on the crevice corrosion tendency. The beneficial effects of Si and N were found to be pronounced especially when 1∼3%Mo had been alloyed together with them. (2) In many cases, the pitting potentials of creviced specimens Vcre′ were found to be 0.3∼0.4 V more negative than those of open surface specimens. However, in the case of 18 Cr-14 Ni-3.5 Si-0.2 N stainless steel, Vcre′ was about 0.9 V more negative than Vc′, indicating a greater susceptibility to crevice corrosion. For practical purposes of assessing the susceptibility and/or resistance of an alloy to localized corrosion, it is essential that the pitting potential Vcre′ of the creviced specimen as well as that of the open surface, Vc′, be measured.

Study of the interaction between manganese and nitrogen in Cr?Ni stainless steels by the internal friction method

1. It was shown by experiments with four austenitic Cr−Ni stainless steels that with increasing manganese concentrations the solubility of nitrogen increases. 2. With increasing concentrations of manganese the shape of the F-R peak on the TDIF curves is distorted—broadened, transformed into a plateau, or divided into two subpeaks (for steel with 10% Mn). 3. Using the theory of pseudopotentials, we attempted to explain the anomalous form of the TDIF curves and also the character of the interaction between manganese and nitrogen atoms in austenitic Cr−Ni steels.

Internal attack during the oxidation of nitrided stainless steels

Oxidation tests have been performed at 1123°K in CO2on a 20 wt.% Cr-25 wt.% Ni stainless steel containing a dispersion of titanium nitride particles produced by prior nitriding. Two forms of internal attack are distinguished: inger-like, grain boundary penetrations of internally oxidized silicon and globular, discrete voids extending to greater depths within the alloy. The latter are the result of the dissociation of chromium-rich particles in the depleted zones formed near the specimen surface during the oxidation process. The kinetics of this globular attack are parabolic and are closely related to the growth of the depleted zone. The theoretical expectation of a linear relationship between the depth of attack and the thickness of the surface chromic oxide is confirmed. The effect of variation in the chromium and residual nitrogen contents on the depth of attack is discussed in detail.

The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions

In order to clarify the mechanism for increased resistance to pitting in acid chloride solutions by addition of Me to CrNi stainless steels, the anodic polarization curves, a.c. electrode impedances, ellipsometric parameters and X-ray photo-electron spectra have been measured on the Mo-containing steels passivated in 1N HCl. The results showed that the presence of an adequate amount of Cr is indispensable for the improvement of pitting resistance by the Mo addition. The passive films of the Mo-containing steels were found to be composed of a complex oxyhydroxide containing Cr 3+ Fe 2+, Ni 2+, Mo 6+ and Cl − and showed a rather higher d.c. resistance in HCl solution than in H 2SO 4 solution. The thickness of the passive film increases with increase in Mo content.

Characteristics of passivity of extremely corrosion-resistant amorphous iron alloys

The amorphous FeCrPC alloy was compared with the crystallized alloy having the same composition in potentiodynamic polarization curves and with an 18Cr8Ni stainless steel in current decay after abrading the specimens under anodic polarization. Through these results along with the previous ESCA study, the extremely high corrosion-rseistance of the amorphous iron alloys containing 8 at.% or more chromium has been interpreted in terms of the rapid formation of thick, uniform, highly corrosion-resistant passive film due to the characteristics of the amorphous alloys.

Effect of Manganese and Nitrogen on the Mechanical Properties of Fe-18%Cr-1O%Ni Stainless Steels

Effect of Manganese and Nitrogen on the Mechanical Properties of Fe-18%Cr-1O%Ni Stainless Steels