Characterization of microstructures and mechanical properties of Inconel 617/310 stainless steel dissimilar welds

The microstructure of the Ni-base superalloy IN617 that had undergone prolonged aging (approximately 65,000 hours) at a series of temperatures from 482 °C to 871 °C has been characterized by microhardness measurements, optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) Cr23C6, Mo-rich eta-M6C, and Ti(C,N) constitute the major primary coarse precipitates both within the grains and along the grain boundaries The secondary carbides were mostly fine Cr23C6, which had a cube-on-cube orientation relationship (OR) with the fcc matrix, and at long times were present in cuboidal and plate-shape forms within the grains and as films along the grain boundaries Fine, eta-M6C carbides were also observed at low to intermediate temperatures with an OR given by [011] carbide//[011] matrix, \( \left( {\bar{1}\bar{1}1} \right) \) carbide//\( \left( {\bar{1}\bar{1}1} \right) \) matrix The coarse eta-M6C carbides increased in extent at 871 °C, whereas the counterpart fine carbides were absent The γ′ phase was found to be present at all aging temperatures up to 871 °C, with a volume fraction ranging from very low to approximately 5 pct at 593 °C, where the peak in microhardness occurs The observations have also suggested that the presence of a very small amount of γ′ at temperatures as high as 871 °C at long times may be associated with a reaction between the fine eta-carbides and the γ matrix Ultrafine precipitates of the intermetallic phase Ni2(Cr,Mo) with the Pt2Mo-type structure was observed in addition to γ′ in samples aged for 28,300 hours at the lowest aging temperature of 482 °C These precipitates were absent in samples aged at higher temperatures The various observations made have suggested that the long-term thermal stability of the IN617 alloy is reasonably good over a wide temperature range of 538 °C to 704 °C, whereas at higher temperatures (871 °C), the substantial decrease in the volume fraction of γ′ and coarsening and clustering of the carbides lead to a large drop in the microhardness A modified time-temperature-transformation (TTT) diagram was constructed based on the results of this study and comparison with previous reports

Microstructure of Long-Term Aged IN617 Ni-Base Superalloy

Creep tests have been correlated with microstructural changes which occurred during creep of Inconel 617 at 1000 °C, 24.5 MPa. The following results were obtained: 1) Fine intragranular carbides which are precipitated during creep are effective in lowering the creep rate during the early stages of the creep regime (within 300 h). 2) Grain boundary carbides migrate from grain boundaries that are under compressive stress to grain boundaries that are under tensile stress. This is explained in terms of 1 the dissolution of relatively unstable carbides on the compressive boundaries, 2 the diffusion of the solute atoms to the tensile boundaries and 3 the reprecipitation of the carbides at the tensile boundaries. The rate of grain boundary carbide migration depends on grain size. 3) M23C6 type carbides, having high chromium content, and M6C type carbides, having high molybdenum content, co-exist on the grain boundaries. M23C6 type carbides, however, are quantitatively predominant. Furthermore, M6C occurs less frequently on the tensile boundaries than on the stress free grain boundaries. This is attributed to the difference of the diffusion coefficients of chromium and molybdenum. 4) The grain boundaries on which the carbides have dissolved start to migrate in the steady state creep region. The creep rate gradually increases with the occurrence of grain boundary migration. 5) The steady state creep rate depends not so much on the morphological changes of carbides as on the grain size of the matrix.

Morphological changes of carbides during creep and their effects on the creep properties of inconel 617 at 1000 °C

Oxidation behaviour of an Alloy 617 in very high-temperature air and helium environments

Strain controlled low cycle fatigue tests have been conducted in air to ascertain the influence of strain rate(e = 4 × 10-6'to 4 × 10-3 s-1) and temperature(T = 750/850/950 °C) on LCF behavior of Alloy 617. A strain range of 0.6 pct and a symmetrical triangular wave form were employed for all the tests. Crack initiation and propagation modes were studied. Microstructural changes that occurred during fatigue deformation were evaluated and compared with the results obtained on isothermal aging. Deformation and damage mechanisms which influence the endurance have been identified. A reduction in fatigue life was observed with decreasing e at 850 °C and with increasing temperature at e = 4 × 10-5 s-1. Cyclic stress response varied as a complex function of temperature and strain rate. Fatigue deformation was found to induce cellular precipitation of carbides at 750 and 850 ‡. Dynamic strain aging characterized by serrated flow was observed at 750 °C (e = 4 × 10-5 s-1) and in the tests at higher e at 850 °C. Strengthening of the matrix due to dynamic strain aging of matrix dislocations by precipitation of M23C6 carbides led to fracture of grain boundary carbide films formed at 750 °C, producing brittle intergranular crack propagation. At 850 °C transgranular crack propagation was observed at the higher strain rates e≥4× 10-4 s-1. At 850 and 950 °C even at strain rates of 4 × 10-5 s-1 or lower, life was not governed by intergranular creep rupture damage mechanisms under the symmetrical, continuous cycling conditions employed. Reduction of endurance at lower strain rates is caused by increased inelastic strain and intergranular crack initiation due to oxidation of surface connected grain boundaries.

Influence of time and temperature dependent processes on strain controlled low cycle fatigue behavior of alloy 617

INCONEL alloy 617 (54 Ni, 22 Cr, 12.5 Co, 9 Mo, 1 Al, 0.07 C) is a solid-solution alloy with good corrosion resistance and an exceptional combination of high-temperature strength and oxidation resistance. A laboratory study was performed to determine the effects of long-time (215 to over 10,000 h) exposure to temperatures up to 2000°F (1093°C) on the microstructure and phase stability of the alloy. To investigate the strengthening response exhibited by the alloy during high-temperature exposure, microstructures were correlated with mechanical properties. The major phase present in the alloy after exposure to all temperatures from 1200 to 2000°F (649 to 1093°C) was found to be M23C6. The phase precipitated as discrete particles and remained stable at au temperatures. No MC or M6C carbides were found. A small amount of gamma prime was found in samples exposed at 1200°F (649°C) and 1400°F (760°C). A PHACOMP analysis indicated 0.63 pct gamma prime could form. No topological close-packed phases such as sigma, mu, and chi were found. Strengthening of the alloy during exposure to temperature was found to result primarily from the precipitation of M23C6. The phase provides effective strengthening because it precipitates in discrete particles and remains stable at temperatures to 2000°F (1093°C). The amount of gamma prime formed is not sufficient to cause appreciable hardening, but it does provide some strengthening at 1200 to 1400°F (649 to 760°C).

T. H. Bassford

Papers

Microstructure and phase stability of INCONEL alloy 617