Creeping resistance of Nickel Base Alloys at 700oC in Steam Turbines

Nickel base alloys are crucial for proposed steam turbines serving at 700oC or above. As suitable materials special versions of super alloys Inconel 706 and Inconel 617. The objective is to find the creeping resistance of these alloys at temperature limits from 600oC to 750oC and slightly higher about 800oC. Creeping tests are conducted for 15,000 hours. Their outcomes alloy comparisons among various alloys. In the annealing tests conducted for 15,000 hours shrinkage is noticed.

The mechanical prolong term performance of forged nickel base alloys is crucial for their applicaton as rotor material in steam turbines at temperatures of 700oC to 720oC for application times about 200,000 hours. Various nickel base alloys were chosen as candidate materials. There are alloy 617 showing a solid solution hardened material, Inconel 706 shows a gamma double prime strengthened material. Alloy 706 is tested in two different heat treatments.

The creep and creep rupture characteristics of different testing materials are found. They are under consideration of a connected DFG research program combining the efforts of various partners to further enhance these materials with an objective to receive recommendations of suitable nickel base alloys for service around 700oC and above.

Creep Rupture Tests and their Outcomes

Creep rupture tests were performed on the chosen materials. These are tested in two various conditions in a new condition after heat processing and in aged form after 5000 hours annealing at 750oC. The later condition approximates a mid life condition of 100000 hour at 700oC after a Larson Miller parameter. The creep rupture analysis following EN 10 291:2000 is primarily conducted by intermittent tests and by continuous tests. Few intermittent tests are  conducted in the interrupted way as long as rupture after a specific time or plastic strain is noticed. Inconel 706 test for a temperature range of 600oC to 750oC, analysis on Inconel alloy 617 is performed at temperature limits between 600oC to 800oC.

Additionally the tests conducted interrupted annealing analysis with strain evaluation are performed. In many cases, the density based strain Ed from these tests results into contraction. Although, on alloys Inconel 706, no contraction is noticed after prolong test times. In earlier cases, Inconel 617 showed a contraction. Although, a material density based strain Ed is noticed with a saturation effect at around 0.06% after 5000 hours. This contraction is taken into consideration when creep is modeled, whilst creeps for Inconel 706 can be directly modeled depending on the plastic strain Ep.

Moreover, the creep tests hot tensile analyses following EN 10 002:1991 were conducted. These were conducted with strain rates of about 0.5% to 1% plastic strain and thereafter 5% as long as it ruptures.

The comparison of tensile strength values of test materials describes a decreased tensile strength for aged conditions of alloy Inconel 706 however an increase in tensile strength for alloy Inconel 617 that is resulted by gamma prime precipitation during ageing. An outcome of a earlier assessments on alloys such as Inconel 617 and Inconel 718 the Larson Miller parameter with a constant of C= 22 that can be preferred for alloys of the kinds considered here. This was noticed by the latest evaluations conducted. 

At the main temperature of 700oC and 650oC, a perfect agreement is found for alloys Inconel 617. For this alloy the parameter based predictions appear to be optimistic. If the creep rupture strength values of the materials in the new condition are evaluated to the correspondent values of the materials in an aged form, alloy 617 performs well. Although, Inconel 706 describe a strong loss of creep rupture strength in an aged form. This tells about the prolong application of this alloy at temperatures above 650oC.

In the frame of a joint DFG- research program the alloys were chosen as the suitable materials for rotors for service up to 720oC in the steam turbines. These candidate materials are two kinds of Inconel 706 with various heat processing and on kind of alloy Inconel 617. The material is tested in two conditions – new and aged that was produced by extra annealing at 750oC for 5000 hours to around a mid of life status of 700oC for 100,000 hours. An essential criterion for the prolong use of these alloys is the creep rupture behavior. Hence creep rupture analyses are conducted on material alloy 706 in a temperature limit of 600oC to 750oC and on material Inconel 617 in the temperature limit of 600 – 800oC. The test lengths of 15,000 hours have reached to now. As a prime outcome, Inconel 617 shows lower values with 100,000 hour value of around 140 MPa at 700oC.

An evaluation of creep rupture strength of the conditions new and aged shows a good stability of this alloy. The creep rupture nature of Inconel 706 describes that the highest service temperature of 650oC is preferred for this material. At such temperature 100,000 hour creep rupture strength of 230 MPa is anticipated.

Traditional creep formulations can be made for the first candidate materials with an aim to support creep crack measurements.