P. Rodriguez

Activation energy for serrated flow in a 15Cr5Ni Ti-modified austenitic stainless steel

Serrated flow in a 15Cr15Ni titanium-modified austenitic stainless steel has been investigated at a wide range of temperatures (300–1023 K) and strain rates (10−5−10−2s−1). Three regimes of serrated flow have been identified as low temperature type A (LT-A), high temperature type A (HT-A) and high temperature type C (HT-C). Different methods suggested in the literature for the determination of activation energy, Q were employed and Q was obtained aas 115 ± 9, 140 ± 7 and 178 ± 7 kJ/mol for LT-A, Ht-A and HT-C regimes respectively. Based on the Q values, the mechanisms responsible for the three regimes of serrated flow were identified as vacancy migration for LT-A, C (or N) as controlling diffusing species to form a pair with vacancy causing Schoeck-Seeger locking for HT-A regime and interaction of Ti with C (or N) for HT-C regime. Titanium was found to influence the serrated flow in all regimes.

Transition metal joints for steam generators—An overview

Abstract The transition metal joint (TMJ) between an austenitic stainless steel and a chromium-molybdenum (Cr-Mo) ferritic steel used widely in steam generators of power plants has for a long time presented problems relating to premature failures in service. The direct (bimetallic) TMJ presently in use is designed for a service life of about 200,000 h; but such TMJs with iron-base weld metals have been failing in service within about one-third of their design lifetime, while their counterparts with nickel-base weld metals fail within about one-half of their design lifetime. The causes for such premature service failures of these TMJs are discussed in detail, leading to the development of improved TMJs. One of the improved TMJs with a trimetallic configuration of austenitic stainless steel/Alloy 800/Cr-Mo ferritic steel is discussed in detail, covering its development, characterisation and evaluation. Accelerated performance tests in the laboratory have indicated a four-fold improvement in the service life of the TMJ with this trimetallic configuration compared to the bimetallic configuration. The metallurgical details of these studies are also discussed in this paper.

Effect of ageing on the microstructural stability of cold-worked titanium-modified 15Cr-15Ni—2.5Mo austenitic stainless steel

A titanium-modified 15Cr-15Ni-2.5Mo austenitic stainless steel conforming to ASTM A 771 (UNS S 38660), commercially called Alloy D9, is being indigenously developed for application as material for the fuel clad and the hexagonal wrapper for fuel subassemblies of the Prototype Fast Breeder Reactor. As this material would be used in the cold-worked condition and would be subjected to prolonged exposure to elevated service temperatures, the effect of ageing on the microstructural stability was studied as a function of the amount of cold work. The material was given 2.5–30% prior cold work and then aged at temperatures in the range 923 to 1173 K for times ranging from 0.25 to 1000 h. Hardness measurements made before and after ageing were correlated with the Larson-Miller parameter to determine the highest stable prior cold-work level. Optical microscopy was used to study the microstructural changes. The influence of prolonged exposure for two and three years at the operating temperatures of clad and wrapper, on the elevated temperature tensile properties of a 20% prior cold-worked Alloy D9 was also studied through accelerated ageing treatments based on the present parametric approach.

Effect of composition on the transformation of δ-ferrite TO σ in type 316 stainless steel weld metals

A study of the effect of Cr on {delta}-ferrite transformation kinetics has yielded a relationship between the kinetics parameter n{sub 2} and weld metal chromium content. The amount of {sigma}{sub 0.9} formed from {delta}-ferrite has been correlated with weld metal composition. A nomogram is proposed, to predict the amount of {sigma}{sub 0.9} formed after 90% ferrite transformation in a given weld metal, as a function of C, Cr and Mo contents. The nomogram can be employed to optimize the composition for reducing high temperature embrittlement resulting from {sigma} precipitation in type 316 stainless steel weld metals. The nomogram has been developed from data obtained at 650 C but has been shown to be valid in the temperature range 600--700 C.

A Comparative Study of the Creep Rupture Properties of Type 316 Stainless Steel Base and Weld Metals

Creep rupture properties of a modified grade of type E316-15 stainless steel weld metal have been studied in the temperature range 823 to 923 K at different stress levels. The results are compared with the creep properties of the base metal under identical test conditions. Rupture lives of the weld metal were found to be lower than those of the base metal by a factor of five to ten. Minimum creep rates of the weld metal were usually higher than those of the base metal whereas the rupture ductility of the weld metal was generally higher at 823 and 873 K and lower at 923 K. Differences in the microstructural changes, creep damage and fracture behavior between the weld and the base metals have also been investigated.

Dynamic fracture toughness and Charpy impact properties of an AISI 403 martensitic stainless steel

Abstract Dynamic fracture toughness and Charpy impact properties of a normalised and tempered AISI 403 martensitic stainless steel obtained from instrumented impact tests are presented. Procedures for estimating dynamic fracture toughness ( K Id ) from the load-time traces obtained in instrumented tests of unprecracked Charpy V-notch (CVN) specimens are considered. The estimated K Id values show reasonable agreement with those obtained from instrumented drop-weight and precracked Charpy tests. Also, except in the upper transition and uppershelf regions, the ASME K IR curve is generally conservative (i.e. gives lower K Id values) when compared to the above K Id estimates. The conservatism of the ASME K IR at the upper transition and uppershelf temperatures needs verification/validation. The lowest K Id values estimated at the lower shelf temperatures for the above steel, namely, 33–42 MPa√m are in good agreement with the reported values of 35–50 MPa√m for the same steel in the literature.

Evaluation of the stress corrosion resistance of cold rolled aisi type 316 stainless steel using constant load and slow strain rate tests

The effect of cold rolling (5 to 15% reduction in thickness) of an AISI Type 316 stainless steel on stress corrosion cracking (SCC) has been studied in boiling magnesium chloride of 45% concentration (boiling point = 428 K) using both constant load and slow strain rate methods. The constant load test results indicated increasing SCC susceptibility with increasing degree of prior deformation, whereas the ranking by the slow strain rate technique was in the reverse order. The reasons for this discrepancy have been discussed. A modified procedure of slow strain rate testing, involving the interruption of the test after a chosen duration of time and measuring average crack growth is suggested as an alternative.

Evaluation of ageing-induced embrittlement in an austenitic stainless steel by instrumented impact testing

Fracture properties of a thermally aged Type 316 stainless steel have been investigated at room temperature by an instrumented impact test. The impact energy is found to depend on the heat treatment conditions. Several alternative estimates for toughness are evaluated and compared with the conventional Charpy impact energy, Cv, to assess the degree of embrittlement. Sensitivity of these parameters to monitor the ageing-induced embrittlement in comparison with Cv is discussed.

Microstructural aspects of creep-rupture life of Type 316L(N) stainless steel in liquid sodium environment

Abstract The influence of flowing sodium on creep-rupture properties of AISI Type 316L(N) stainless steel base material has been investigated at 550 and 600°C. In sodium test results were compared with reference creep-rupture data generated in air. The creep-rupture lives were longer in air than in sodium environment at 550°C, however, at 600°C, creep-rupture lives were longer in the latter than in the former environment. Microstructural studies showed the presence of sensitization and χ phase on longer duration test specimens at both temperatures. Surface cracks in sodium tested specimens were sharp and relatively more in numbers than in air where cracks were blunted. Cracks seem to follow the intergranular mode. Cavities were formed in long duration tests and propagated ahead of the χ phase.

Prediction of creep parameters of type 316 stainless steel under service conditions using the π-projection concept

Abstract The creep behaviour of type 316 stainless steel has been studied under constant loading conditions in the temperature range from 823 to 923 K and for rupture lives up to 50 000 h. The θ-projection concept has been applied to the creep curves and was found to yield accurate predictions when interpolating and predicting creep data under service conditions. The shape of the creep curves, as well as the minimum creep rate, the time to reach a limiting strain, and the time to fracture, were considered.