51313-02284-Development of Crack-Resistant Alloy 617B for Components in 700 °C Power Plants

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Product Number: 51313-02284-SG

ISBN: 02284 2013 CP

Author: Jutta Kloewer

Publication Date: 2013

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High temperature nickel alloy 617 (UNS N06617) is considered as a candidate material for the 700°C Power Plants due to its combination of creep strength and good fabricability. The alloy has been investigated in numerous publicly sponsored and privately financed programmes with regard to its use in USC boilers and it is currently being tested in several 700°C test loops in Europe. Based on the gained experience a modified version of UNS N06617 with additions of Boron and narrowly tolerated alloying elements was created; this alloy is known as 617B (UNS number not yet assigned). This alloy shows a creep rupture strength at 700 °C about 25 % higher than that of VdTÜV – Sheet 485 and ASME.

However like most high strength alloys alloy 617B is susceptible to so-called stress-relaxation cracking at intermediate temperature. It may furthermore suffer from hot cracking and ductility dip cracking during welding of large components. To tailor a crack-resistant alloy 617B for application in 700°C-USC boilers a research project was carried out to investigate the effect of alloying elements of heat treatment and cooling rate on stress relaxation cracking and hot cracking. The susceptibility to stress relaxation cracking was measured in slow strain rate tests at a strain rate of 10-6 /s. To study the effect of environmentally-assisted cracking some tests were also performed in vacuum. The susceptibility to hot cracking was determined in a so-called PVR-test (Programmierter Verformungsriss-Test ) where welding is carried out under deformation. All tests are followed by microscopy with a certain focus on carbide distribution.

The results show that boron has a beneficial effect on the susceptibility to stress relaxation cracking. However above a certain limit of circa 40 ppm it increases the proneness to hot cracking. Both heat treating temperature and cooling rate strongly influence the sensitivity to cracking. Stress relaxation cracking can completely be avoided by relief annealing at 980 °C for 3 hours. A comparable effect is gained by slow cooling from solution annealing temperature to room temperature. The presence of oxygen has no detrimental effect on stress relaxation cracking; evidence of the so-called SAGBO-effect (stress-assisted grain boundary oxidation) was not found. The effect of heat treatment on the weldability is extremely complex; its effect on the different types of hot cracks will be discussed. However the test results demonstrate that crack-resistant alloy 617B can be designed if alloy composition welding parameters and heat treatment are appropriately adjusted.