Creep Rupture Testing of Alloy 617 and A508/533 Base Metals and Weldments. Page: 3 of 36

                                
Executive Summary

The NGNP, which is an advanced HTGR concept with emphasis on both electricity and
hydrogen production, involves helium as the coolant and a closed-cycle gas turbine for power
generation with a core outlet/gas turbine inlet temperature of 750-1000 C. Alloy 617 is a prime
candidate for VHTR structural components such as reactor internals, piping, and heat
exchangers in view of its resistance to oxidation and elevated temperature strength. However,
lack of adequate data on the performance of the alloy in welded condition prompted to initiate a
creep test program at Argonne National Laboratory. In addition, Testing has been initiated to
evaluate the creep rupture properties of the pressure vessel steel A508/533 in air and in helium
environments. The program, which began in December 2009, was certified for quality
assurance NQA-1 requirements during January and February 2010. Specimens were designed
and fabricated during March and the tests were initiated in April 2010.
During the past year, several creep tests were conducted in air on Alloy 617 base metal
and weldment specimens at temperatures of 750, 850, and 950 C. Idaho National Laboratory,
using gas tungsten arc welding method with Alloy 617 weld wire, fabricated the weldment
specimens. Eight tests were conducted on Alloy 617 base metal specimens and nine were on
Alloy 617 weldments. The creep rupture times for the base alloy and weldment tests were up to
~3900 and ~4500 h, respectively. The results showed that the creep rupture lives of weld
specimens are much longer than those for the base alloy, when tested under identical test
conditions. The test results also showed that the creep strain at fracture is in the range of 7-
18% for weldment samples and were much lower than those for the base alloy, under similar
test conditions.
In general, the weldment specimens showed more of a flat or constant creep rate region
than the base metal specimens. The base alloy and the weldment exhibited tertiary creep after
50-60% of the rupture life, irrespective of test temperature in the range of 750-950 C. The
results showed that the stress dependence of the creep rate followed a power law for both base
alloy and weldments. The data also showed that the stress exponent for creep is the same and
one can infer that the same mechanism is operative in both base metal and weldments in the
temperature range of the current study.
SEM fractography analysis indicated that both base metal and weldment showed
combined fracture modes consisting of dimple rupture and intergranular cracking. Intergranular
cracking was more evident in the weldment specimens, which is consistent with the observation
of lower creep ductility in the weldment than in the base metal.

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