How Inconel 600 is influenced by dynamic strain aging

Dynamic strain aging (DSA) of Nickel based alloy Inconel 600 is analyzed. Dynamic strain aging is applied on alloys that are used in nuclear power plants.

In austenitic stainless steels carbon and nitrogen at high temperatures can interact with dislocations and result in to occurrence of DSA serrations. In nickel base alloys interstitial and substitutional solute atoms often cause a jerky flow.

DSA occurs in alloy Inconel 600 wires over a wide range of temperature and strain. Interstitials have a special role in DSA of pure nickel that has been found for Nickel-Carbon and Nickel-Hydrogen interstitial alloys. In Nickel-Carbon alloys the lower critical temperature of DSA had an activation enthalpy of half of wide diffusion of carbon and it was related with carbon diffusion in the dislocation core. Moreover it was noticed that serrations were not influenced by cooled vacancies. DSA temperature in Inconel 600 was close to austenitic stainless steel and much higher than Nickel-Carbon binary alloy.

Inconel 600 is commonly used in vessel head penetrations in pressurized water reactors. Primary water stress corrosion cracking and intergranular stress corrosion cracking in parts in addition of steam generators are issues of nuclear power plants.

Various uniaxial tensile tests were performed in lab air in axial displacement control with constant cross head speeds on Inconel alloy 600. A split design furnace with temperature control was used for tests at high temperatures. An elevated temperature. Inconel 600 was used in mill annealed condition and was cut by using water jet cutting and spark erosion methods to develop the samples.

Base material in pre-strained conditions and some samples cooled after the tensile test were evaluated by using internal friction method. The samples utilized for this measurement were strained to fracture and then water cooled from the test temperature to room temperature instantly.

The samples after tensile testing were cut from the gauge length parts of tensile test samples through an abrasive disc-saw. The samples for analyzing the as-supplied condition were cut in the tensile test direction. All IF samples were polished with 1200 grit emery paper to prevent surface effects.

Temperature dependencies of internal friction and pendulum frequency were measured at temperatures from room level to 600oC.The varying natural frequency of pendulum was noticed.

Stable elongation rate tensile tests were performed to receive the map of DSA serration appearance on the basis of strain rate and temperature for commercial grade Inconel alloy 600. The influence of DSA on mechanical characteristics of evaluated alloy is also checked.

It is found that yield stress and ultimate tensile stress reduces with increasing temperature about 200oC and remain stable after it. The strain hardening coefficient increases with increasing temperature limit up to 200oC. The elongation fracture increases for Inconel 600 however its ductility decreases at 400oC. Amplitude of internal friction increases with cold defomation and increases more subsequent to tensile tests at higher temperatures and particularly when the DSA serrations were available. The internal friction in deformed alloy 600 is not stable and it decreases gradually with annealing.