The effect of ageing temperature and time on the mechanical properties of Fe-NiCrMo alloys with different contents of δ ferrite

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Abstract

Laboratory cast alloys with 2–27% of δ ferrite were aged for up to 17,520 h in the temperature range 290–350 °C. Tensile and Charpy tests were performed at 22 and 290 °C on specimens aged for different times, and the microhardnesses of both constituents of the microstructure were determined for the alloy with 27% of δ ferrite. The effects of the content of δ ferrite, the ageing and testing temperature, and the ageing time on mechanical properties and notch toughness are presented and discussed.

Introduction

Iron–nickel–chromium–molybdenum cast alloys have been used in the manufacturing of vital parts of older nuclear-power plants. A non-equilibrium solidification in these alloys produces a two-phase microstructure of austenite and δ ferrite. The content of both phases depends on the conditions of solidification and the chemical composition of the melt, whereas ferrite formation is enhanced by the elements chromium, molybdenum, silicon and some impurities, e.g., phosphorus. The instability of mechanical properties and notch toughness of these alloys was established back in the 1980s. Researchers found (Slama et al., 1983) that by ageing in the temperature range 300–400 °C, the change in properties was much greater for the alloy with 14.5% of ferrite than for the alloy with 8.5% of ferrite, and that the decrease of the Charpy toughness and of the J-integral was significant, whereas ageing for 7500 h at 400 °C did not affect the number of loadings-to-fracture during low-cycle fatigue testing.

These findings were later confirmed in several references, and the following observations were made: (Jannson, 1990) that the content of ferrite determined for the same specimen according to the Schaeffler diagram differs significantly from that determined with magnetic measurements or assessed from the microstructure; that the distribution of ferrite in the same cast part is very inhomogeneous and it was, e.g., in the same elbow, in the range 1.5–22.5%; that the Charpy toughness decreased significantly only in the elbow part operating in the temperature range 303–325 °C; that the initial toughness was achieved again after 1 h of annealing at 550 °C; and that with an average content of ferrite up to 20% the degradation of the properties remained acceptable.

Several processes in austenite and ferrite could theoretically induce the embrittlement of alloys with a microstructure consisting of austenite and ferrite (Chung and Leax, 1990)). The conclusion in this paper is that the main embrittlement process is the spinodal decomposition of ferrite, which is a process of demixing of the initially homogenous solid solution of all the elements in the ferrite to domains enriched in nickel and domains enriched in chromium, while the effect of other processes, e.g., carbide-, nitride- and intermetallic-phase precipitation in both phases was only of minor importance.

Section snippets

Experimental work

Three alloys with 1.5–2.5% (alloy A), 10–12% (alloy B) and 26–28% (alloy C) of δ ferrite were melted in a vacuum induction furnace and cast into blocks of section 60 mm × 60 mm. The casting moulds were heated to approximately 150 °C to produce a lower solidification rate and to decrease the internal stresses induced by the difference in the temperature dilatation of both constituents of the microstructure, which could affect some results of the mechanical tests. The chemical compositions in Table 1

Tensile properties

The results, as an average value of two tests, are shown for the as-cast alloys at both testing temperatures in Table 2. The scatter of both parallel tests was, with rare exceptions, in the range of ±5% of the average value, and acceptable for tests on as-cast specimens. This confirms, at least for the alloy with the highest content of δ ferrite, a sufficiently homogeneous distribution of this phase in the cast block shown by magnetic measurements. The large amount of experimental data allowed

Hardness and microhardness

In Fig. 9 the effect of ageing time and temperature is shown for all three alloys. The initial hardness of the as-cast alloys was higher by greater content of ferrite, as result of the smaller grain size. With 2% of ferrite the ageing time and temperature did not affect the hardness. With a higher ferrite content of 11% the hardness started to increase already after short ageing. However, after the longest ageing time the hardness was only for aproximately 10 units above the initial level.

Charpy notch toughness

The effect of ageing time and temperature on the Charpy notch toughness is much greater than the effect on the tensile properties, and for all alloys greater for higher ageing temperature, longer ageing time and higher content of ferrite. Notch toughness starts to decrease with shorter ageing times at higher ageing temperatures and larger content of ferrite.

During ageing at 290 °C the toughness of alloy A with 2% of ferrite remains virtually unchanged for both testing temperatures, also after

Discussion

The ageing affects all the mechanical properties of the investigated alloys in a similar way and to an extent that depends on the content of δ ferrite and of the ageing temperature. The extent of the ageing effect is different for tensile properties, for the Charpy notch toughness and for the microhardness of both constituents of the microstructure—austenite and δ ferrite. The microhardness of austenite is not affected by the ageing, thus, it is clear that all the changes of alloys properties

Conclusions

The empirical results of this investigation confirm the earlier findings found in references and are related to the effect of the instability of ferrite in the FeCrNiMo stainless alloys with a duplex microstructure of ferrite and austenite. The following additional conclusions are proposed on the basis of the findings in the reported investigation:

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    Yield stress and tensile strength of as-cast alloys are higher because at larger content of δ ferrite the linear grain size is smaller. After ageing,

Acknowledgements

The authors are indebted to the Ministry of High Education, Science and Technology and the nuclear power plant NE Krško for the support of the investigation.

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