Dynamic recrystallization during creep in a 45 Pct Ni-35 pct Fe-20 pct Cr alloy system

A combined 3.5 wt pct Mo + 1.2 wt pct Ti imparted dynamic recrystallization in a 35 wt pct Fe-45 wt pct Ni-20 wt pct Cr alloy system during creep at 700 °C, whereas 3.5 wt pct Mo addition alone did not initiate recrystallization. Dynamic recrystallization substantially increased the creep elongation and produced a high ductile fracture topography in the present alloy system. A subgrain coalescence nucleation mechanism for dynamic recrystallization mechanism was operative during creep. The critical initiation strain requirements are also discussed.     

The effect of dispersed oxides on the oxidation behavior of Al-and Ti-Containing Ni-20 pct Cr alloys

The effect of dispersed Y₂O₃ and La₂O₃ on the isothermal and cyclic oxidation behavior of Al- and Ti-containing Ni-20Cr alloys has been investigated. It has been found that the dispersolids improve the oxidation resistance of these alloys, but this improvement is less than that observed in aluminum- and titanium-free Ni-20Cr alloys. The oxidation resistance imparted by the presence of the dispersoids was found to increase as the aluminum and titanium contents were decreased. It is proposed that these changes in oxidation resistance arise from differences in the extent of subscale attack, oxidation induced alloy porosity and internal oxidation.     

Effect of titanium,niobium and zirconium on creep rupture strength and ductility of cobalt base superalloys

An attempt has been made to develop a cobalt base casting superalloy (30Cr-10Ni-7W-Co) having high creep rupture strength and ductility for first stage nozzles of gas turbines. In cobalt base superalloys, there was found to exist a close correlation between the creep rupture strength and MC type carbide forming elements such as Ti, Nb and Zr. In cobalt base alloys with 0.25 wt pct C, precipitation and coarsening of carbides can be reduced by addition of Ti, Nb and Zr. Therefore, by adding the optimum amount of Ti, Nb and Zr, precipitation of carbides in the alloy reaches such an amount as to give the highest creep rupture strength. Excess addition of Ti, Nb and Zr does not improve the creep rupture strength. By adding C, creep rupture strength of the cobalt alloy with Ti, Nb and Zr can be improved and becomes the highest at 0.40 wt pct. C. According to the experimental results, the creep rupture strength becomes the highest at a value of (Ti + Nb + Zr)/C (atomic ratio) of about 0.3. Contrary to the expectation, it was found in this experiment that the ductility in creep rupture tests increases with increasing carbon content up to 0.6 wt pct.     

The relationship between austenite strength and the transformation to martensite in Fe-10 pct Ni-0.6 pct C alloys

The effect of austenite yield strength on the transformation to martensite was investigated in Fe-10 pct Ni-0.6 pct C alloys. The strength of the austenite was varied by 1) additions of yttrium oxide particles to the base alloy and 2) changing the austenitizing temperature. The austenite strength was measured at three temperatures above theM _s temperature and the data extrapolated to the experimentally determinedM _s temperature. It is shown that the austenite yield strength is determined primarily by the austenite grain size and that the yttrium oxide additions influence the effect of austenitizing temperature on grain size. As the austenite yield strength increases, both theM _s temperature and the amount of transformation product at room temperature decrease. The effect of austenitizing temperature on the transformation is to determine the austenite grain size. The results are consistent with the proposal¹ that the energy required to overcome the resistance of the austenite to plastic deformation is a substantial portion of the non-chemical free energy or restraining force opposing the transformation to martensite.     

The effect of nitrogen additions upon the pitting resistance of 18 pct Cr, 18 pct Mn Stainless Steel

The effect of nitrogen additions upon the pitting resistance of 18 pct Cr, 18 pct Mn stainless steel has been investigated by potentiokinetic techniques in a 1000 ppm NaCl solution. Nitrogen additions increased the pitting resistance of the steel irrespective of structure, however, the ferritic steel was less pit resistant than the (duplex) steels containing both austenite and ferrite which, in turn, were less pit resistant than the totally austenitic steels. For steels having a duplex structure, the effect of nitrogen on the pitting resistance was observed to follow a linear function of the relative amount of austenite in these steels due to the area effects of the austenite and ferrite which are galvanically coupled in these steels. The addition of nitrogen was found to increase the amount of austenite at a rate of approximately 200 times the percent nitrogen addition from 36 pct austenite for the 0.02 pct N steel to 100 pct for the 0.40 pct nitrogen steel. The addition of nitrogen to the totally austenitic steels increased the pitting resistance at the rate of approximately 0.31 volts per pct nitrogen added, but no mechanism was found for the increased resistance. This paper is based on a presentation made at a symposium on “New Developments in Ferritic and Duplex Stainless Steels,” held at the Fall Meeting in Cleveland, Ohio, on October 19, 1972, under the sponsorship of the Corrosion Resistant Metals Committee of TMS-IMD and the Corrosion and Oxidation Activity of the ASM.     

The effect of sample size on the steady state creep characteristics of Ni-6 pct W

The effects of sample size and grain morphology on the steady state creep properties of Ni-6 pct W in the temperature range of 0.55T _m to 0.74T _m have been studied. It is shown that a decrease in sample thickness results in a corresponding decrease in the number of grains per thickness and gives rise to a decrease in the measured values of bothQ _creep andn when compared to existing data on thick samples with many grains per thickness. The observed effects of sample configuration on the creep properties are explained with a model for creep deformation which is based on the interaction of free surfaces with grain boundary sliding and grain deformation. Using this model, an expression for the stress and temperature dependence of the total steady state strain rate is obtained as a function of the grain matrix strain rate and the grain boundary sliding strain rate. The results of this model are shown to correlate well with the observed deformation characteristics of the thin samples and to explain the variations ofQ{creep} andn with sample morphology. Formerly Graduate Student, Department of Materials Science and Engineering, Stanford University     

Characterization and High-Temperature Oxidation Behavior of Cold-Sprayed Ni-20Cr and Ni-50Cr Coatings on Boiler Steels

Microstructure and mechanical properties of cold-spray coatings are usually required in order to explore the potential industrial application of the latter. This article demonstrates the successful formulation of Ni-20Cr and Ni-50Cr coatings on two boiler steels, namely, SAE 213-T22 and SA 516 steel by cold-spray process. The microstructure, coating thickness, phase formation, and microhardness properties of the coatings were evaluated. The coatings were subjected to cyclic heating and cooling cycles at an elevated temperature of 1173.15 K (900 °C) to ascertain their high-temperature oxidation behavior. Moreover, these cyclic exposures can give useful information regarding the adhesion of the coatings with the substrate steels. Of all the coatings, the Ni-50Cr coating on SA 516 steel had a maximum average hardness value of 469 Hv. As observed from the surface field emission–scanning electron microscopy (FE-SEM) analysis, the coatings were found to have nearly dense microstructure with the sprayed particles in interlocked positions. It was concluded that the cold-spray process is suitable for spraying the preceding powders onto the given boiler steels to produce nearly dense and low oxide coatings. The coatings, in general, were found to follow the parabolic rate of oxidation and were successful in maintaining their surface contact with their respective substrate steels.     

Erosion-oxidation interaction in Ni and Ni-20Cr alloy

The solid-particle erosion of metals and alloys at elevated temperatures is characterized by different mechanisms of material removal, depending on the temperature of exposure, impact velocity, impact angle, and flux rate of the eroding particles. The objective of this article is to delineate the prevalent erosion mechanisms in nickel and a nickel-20 chromium alloy over a large range of temperatures, impact velocities, impact angles, and particle feed rates. For this purpose, a specialized elevated-temperature erosion rig has been utilized. Nickel and a Ni-20Cr alloy have been chosen as the test materials, in view of their substantially different oxidation behaviors. Results from the present study indicate that while the erosion rate generally increases with increasing temperature, an increased particle feed rate causes a reduction in the erosion rate, especially at higher temperatures beyond 650 K. On the basis of a detailed examination of the morphology of the eroded surfaces and the subsurface features beneath the eroded surfaces, four different material removal mechanisms have been identified in the nickel, while only three material removal mechanisms were operative in the Ni-20Cr alloy. Utilizing the aforementioned information, erosion-oxidation (E-O) interaction mechanism maps (herein, termed E-O maps) delineating the regions of dominance of the various erosion mechanisms in an impact velocity-test temperature space have been constructed for Ni and a Ni-20Cr alloy. Finally, the differences in erosion behavior between the Ni and Ni-20Cr alloy have been identified and rationalized.     

Diffusion of chromium and aluminum in Ni-20Cr and TDNiCr (Ni-20Cr-2ThO2)

Diffusion coefficients have been measured for⁵¹Cr in fine- and coarse-grained TDNiCr (Ni-20Cr-2ThO₂) and in fine-grained Ni-20Cr in the temperature range of 1038° to 1200°C. Selective diffusivities have also been determined for specimens of these alloys which were aluminized to give an initial surface concentration of 5.8 wt pct Al. Finally, diffusion coefficients for interdiffusion of aluminum in TDNiCr and Ni-20Cr have been obtained from electron probe microanalysis of the aluminized specimens. For a given grain size and temperature there is no difference in diffusivities for chromium diffusion in TDNiCr or Ni-20Cr. Diffusion coefficients increase with decreasing grain size for both alloys. Comparison of aluminum diffusion data obtained from electron microprobe profiles with radiotracer chromium diffusivities suggests that aluminum diffuses approximately three times faster than chromium in TDNiCr and Ni-20Cr.     

Effect of carbon content and helium gas environment on creep crack growth properties of Ni-26 pct Cr-17 pct W-0.5 pct Mo alloy at 1273 K

Creep crack growth tests were conducted on Ni-26 pct Cr-17 pct W-0.5 pct Mo alloys with different carbon contents in air and in helium gas environment at 1273 K using the compact-type (CT) specimen, and the effects of carbon content and environment on creep crack growth rate are discussed. Creep crack growth rateda/dt is evaluated by theC* parameter. Theda/dt is faster in higher-carbon alloys than in lower-carbon alloys in each environment. This effect of carbon content is attributed to the lower creep ductility due to the increase of fine trans-granular carbides in higher-carbon alloys. The environmental effect on theda/dt vs C* relations is scarcely observed in higher-carbon alloys. In the 0.003 pct C alloy, however,da/dt is much lower in the He gas environment than in air. Carburization is observed ahead of the crack tip in the He gas environment at 1273 K. The intergranular carbides precipitated due to carburi-zation have a granular configuration and are considered to prevent the grain boundary sliding in lower-carbon alloys.     

Microstructure and creep behavior of a niobium alloyed cast heat-resistant 26 pct Cr steel

The effects of niobium additions to a “straight chromium” alloy were investigated by optical metallography and creep testing at 1033 K. The results indicate that by increasing the amount of interdendritic constituent, niobium adds to creep strength and fracture ductility. Optimum alloy design requires balancing niobium, chromium, and carbon content.     

Effects of shock loading and cold rolling on the structure and high temperature creep properties of γ′ strengthened Ni-18.6 pct Cr-4.3 pct Al

The effects of shock loading and cold rolling on the structure and high temperature creep properties of precipitation strengthened Ni-18.6 pct Cr-4.3 pct Al have been investigated. The creep properties of this alloy depend on whether the testing is conducted in forming gas (90 pct N₂ + 10 pct H₂) or in vacuum. It is found that cold rolling invariably increases the creep rate and decreases the creep life when testing is conducted in forming gas. This unexpected result is believed to be caused by strain concentrations at grain boundaries which are associated with cold rolling and which contribute to creep by promoting premature intergranular fracture. In order to minimize the effects of intergranular fracture and to determine the intrinsic effects of prestraining it is necessary to conduct creep tests in vacuum. Taking this precaution, we find that the creep strength of this alloy can be enhanced only slightly by cold rolling a few percent. Cold rolling by a greater amount or shock loading to about 70 kbar (6.99 × 109 Pa) results in higher creep rates than the asaged condition. It is shown that these unexpected weakening effects can be correlated with, and are probably caused by, the presence of misshaped γ′ precipitate particles. The irregular shapes of these precipitates are believed to be caused by the line tension effects of surrounding dislocations.     

The effect of aging on the tensile mechanical properties of high-purity binary Ni-10, -20, and-30 Wt pct Cr alloys

The variation of the tensile mechanical properties and hardness (at 25‡C) of three Ni-Cr alloys has been determined as a function of aging time (up to 4 months) in the range 290 to 530‡C. Aging has a negligible effect on the 10 pct Cr alloy, and only a slight effect on the 20 pct Cr alloy. However, the 30 pct Cr alloy showed a marked sensitivity to aging; for example, at 479‡C the yield strength doubled after about 1 month, then decreased slightly at 4 months. The effect in the 30 pct Cr alloy is due to the formation of the Ni₂Cr superlattice.     

Improved creep strength and creep ductility of type 347 austenitic stainless steel through the self-healing effect of boron for creep cavitation

Composition of type 347 austenitic stainless steel was modified with the addition of boron and cerium. An improvement of creep strength coupled with creep ductility of the steel was observed with boron and cerium additions. The observation of enhanced precipitation of carbonitrides in boron-containing steel over that of boron-free steel may in part contribute to the increase in creep strength. Both grain boundary sliding and nucleation and growth of intergranular creep cavities were found to be suppressed in steel-containing boron. This results in an increase in creep strength and creep ductility. Auger electron spectroscopic analysis of the chemistry of creep cavity surfaces (exposed by breaking the creep-exposed steel specimen at liquid nitrogen temperature under impact loading) revealed the segregation of elemental boron on the creep cavity surface. Boron segregation, on the creep cavity surface in the absence of sulfur contamination, suppressed the cavity growth and provided the steel with a self-healing effect for creep cavitation. Cerium additions enabled boron to segregate on the cavity surface by effectively removing the traces of free sulfur in the matrix by the formation of ceriumoxysulfide (Ce₂O₂S).     

On the microstructure and mechanical behavior of melt-spun Fe-24 pct Ni-0.5 pct C Ribbons: Effect of austenite grain size

The role of carbon on the retention and decomposition of austenite in a melt-quenched Fe-24 wt pct Ni-0.5 wt pct C alloy made by the melt-spinning method has been investigated, using a combination of X-ray diffractometry, optical and TEM metallography, microhardness measurements, and tensile tests. It is found that the addition of 0.5 wt pct C to Fe-24 wt pct Ni alloy leads to retention of austenite to a temperature close to -196 °C, when the alloy is quenched from the melt. The austenite grain size varies from ∼0.2 μm to ∼2 μm on going from the wheel to the gas side. The cooling rate, accordingly, changes from 5 × 10⁷ to 4 × 10⁴ Ks^-1. The changes in the mechanical properties have been correlated with the accompanying changes in the ribbon microstructure. The Central Metallurgical Research and Development Institute, National Research Centre, Dokki, Cairo, Egypt     

High temperature creep of Ni-20Cr-2ThO2 single crystals

The high temperature creep properties of single crystals of Ni-20Cr-2ThO₂ have been investigated in the temperature range 650–1300°C. Single crystal tensile specimens were cut from plates having a large elongated grain structure and constant stress vacuum creep tests were conducted at various stresses and temperatures.By including the temperature dependence of the elastic modulus in the analysis of the data, the creep activation energy was found to be very nearly equal to that for self diffusion of Ni-20Cr. The stress dependence of the creep rate was observed to be large and variable, with power law exponents ranging from 9 to 75 over the temperature range studied. The ductility of these crystals as measured by percentage reduction of area was very high, typically ranging from 40 to 60%. The percentage elongation varied inversely with the stress exponent and ranged from 1 to 28%.The creep properties of these dispersion strengthened crystals can be described accurately by considering the creep strength to be given by a simple sum of the creep strength of unthoriated polycrystalline Ni-20Cr and the Orowan strength, as calculated from the measured ThO₂ particle distribution. This simple model describes the measured creep rates to within a factor of three at all stresses and temperatures.