Inconel 617 alloy – Effect of heat processing on mechanical characteristics

Inconel 617, a high temperature nickel based alloy is a fit for use as a construction material for 700oC power plants as it offers superior creep strength and adequate fabrication characteristics. This alloy has been tested in various programs for use in the USC boilers. Depending on the received experience, the alloy is customized to fit the special application of USC boilers.

In this post Inconel 617 is evaluated for service in the elevated temperature gas cooled reactors (HTGR). Methods were created for the developed of sound welds and tests were conducted on base metal and metal welds. Samples of alloy were used for aging to 20,000 hours to determine the heat stability. Short term tensile tests were performed that have showed that aging widely decreased strain at fracture at ambient and high temperatures. The impact energy at ambient temperature was terribly decreased by aging. Creep tests describes that cracking is noticed at 593 – 704oC after 1 -2% strain and higher strains were observed at the elevated temperatures. The creep properties were same in air and reactor helium conditions.

Preface

Inconel alloy 617 was developed for service at the high temperatures. It is a solid solution alloy that features high strength at the elevated points. In the elevated temperature gas cooled reactor service, structural alloys are subjected to a gaseous media comprising of helium with nominal magnitudes of hydrogen, methane, carbon monoxide, carbon dioxide, nitrogen and water.

Various test specimens were included in this program. Inconel 617’s three heats were observed in addition of its weld metal heat. The base of alloy 617 received general coarse grains. A sample was aged at 593oC, and its grain size was not changed. This heat processing has the maximum grain size among the three heat processing of the metal.

The tests were conducted in stainless steel 304l retorts and in aluminium oxide. The specimens aged in the inert media at 538oC, 704oC and 871oC were gathered in the metal chambers. The samples were aged for 20,000 hours in HTGR helium in the steel retorts. Aging is continued up to 704oC.

Test conditions

The test gas is contained in pressurized cylinders and supplied to various chambers maintained at 83 kPa. The pipes and valves are organized to allow the parallel supply to all test chambers.

Outcomes

The tensile characteristics of the heated samples were varied significantly. The strength of sample heated from 600 – 750oC show an erratic nature. The cracking strain and reduction percent in area are unlike particularly above 600oC when the ductility increases with raising temperature whilst it decreases in other samples with increasing temperature. The yield stress increased by 20% by aging in inert condition at 538oC to 704oC and reinforcing level on the base of slight aging time was analyzed. Aging in HTGR in helium at 593 – 704oC raised the yield stress by 70%.

The aging time creates a wide effect on samples heated at 593oC with nominal strengthening for 10,000 hours aging whilst aging for 20,000 hours increased strength by 70%. The aging time has no noticeable effect at 704oC. Aging at 871oC for 10,000 hours in reactor containing helium gas showed 30% increased yield strength/

Aging significantly alters the ultimate tensile strength by 10%. Aging for 10,000 hours at 593oC in the reactors didn’t show any effect whilst aging for 20,000 hours increased the ultimate tensile strength above 20%. In aging at 871oC, the eventual tensile strength reduced for aging time above 2500 hours. The strength reductions were higher for samples aged in HTGR containing helium tan for samples aged in an inert media. After 20,000 hour aging in HTGR-helium, the ultimate tensile strength was decreased by above 30%.

Aging at 538oC created erratic influence however the elongation values lied in 53 – 69%, that was very large. The cracking length reduced with increasing aging time and temperature. In an inert media, the least cracking strain was 20% for a specimen aged at 20,000 hours at 871oC. Aging in reactor always caused smaller fracture strain than the contextual aging processing in an inert media. The least value observed after aging in the reactor was 6% subsequent 20,000 hours at 871oC.

Aged samples were also observed at the aging temperature and the outcomes have similar behaviour for specimens observed at 25oC. Aging in the limit about 500-700oC improved the yield and tensile strengths and the enhancement was higher in the reactor aging condition as compare to in the inert. Aging in both conditions at 871oC had nominal influence on the mechanical strength of alloy. Aging in inert media decreased the fracture elongation nominally at 704oC and create unnoticeable effect at other aging limits. Aging in reactor containing helium gas widely decreased the cracking length at all the given temperatures.

Inconel 617 welds

The eventual tensile strength of the weld metal was nominally higher than the base metal at the same temperature limit. The cracking extension of the weld metal was about half of the base metal the whole temperature limits. The area reduction for weld was larger at 25oC and smaller at the high temperatures as compare to the base metal. Although the weld was highly ductile under the whole test media.

Creep tests

Creep tests were performed on Inconel 617 base metal. It was found that the surrounding condition has no overall influence on the nominal creep rate. At 760oC to 871oC the cracking strains were very high however the effect of surrounding on the fracture strain was not evident.

Discontinued creeping
Alloy 617’s specimens were subjected to a creep load for long time length and short term tensile test at the ambient temperature. The initial test included alloy’s sample that was creep tested up to 871oC for 26,117 hours and received strain about 0.3%. The specimen was widely carburized and was discovered to comprise of 0.233% carbon. The yield stress of the creep specimen is larger and the ultimate tensile stress and elongation smaller. Carbon concentration of the creep sample is larger than sample aged in the reactor condition. It resulted into reduction of the ultimate tensile stress and fracture elongation.