Surface grain boundary engineering of Alloy 600 for improved resistance to stress corrosion cracking

Abstract

In this paper, we demonstrate a novel method for grain boundary engineering in Alloy 600 using iterative cycles of ultrasonic nanocrystal surface modification (UNSM) and strain annealing to modify the near surface microstructure (~ 250 µm) for improved stress corrosion cracking (SCC) resistance. These iterative cycles resulted in increased fraction of special grain boundaries whilst decreasing the connectivity of random grain boundaries in the altered near surface region. A disrupted random grain boundary network and a large fraction of low CSL boundaries (Σ3-Σ27) reduced the propensity to sensitization. Slow strain rate tests in tetrathionate solutions at room temperature show that surface GBE lowered susceptibility to intergranular SCC. Detailed analysis of cracks using Electron Back-scattered Diffraction showed cracks arrested at J1(1-CSL) and J2 (2-CSL) type of triple junctions. The probability

Intergranular Corrosion Mechanism of Slightly-sensitized and UNSM-treated 316L Stainless Steel

“…Among these remedies, the reduction of residual stress from tensile stress to compressive stress can be obtained by surface treatment. Applicable and studied techniques to induce compressive residual stress on the weldments can be summarized as shot peening [6][7][8] , laser shock peening 9,10) , water jet peening 11) , ultrasonic peening 12) , and UNSM 13,14) . In a typical UNSM technique, a tungsten carIntergranular Corrosion Mechanism of Slightly-sensitized and UNSM-treated 316L Stainless Steel bide (WC) tip is attached to an ultrasonic horn, which strikes the specimen surface up to 20,000 or more times per second, with 1,000 to 10,000 shots per square millimeter in a very short time.…”

“…In a typical UNSM technique, a tungsten carIntergranular Corrosion Mechanism of Slightly-sensitized and UNSM-treated 316L Stainless Steel bide (WC) tip is attached to an ultrasonic horn, which strikes the specimen surface up to 20,000 or more times per second, with 1,000 to 10,000 shots per square millimeter in a very short time. These strikes, which can be described as micro cold forging, bring severe plastic deformation to surface layers, and thus induce nano-crystalline structure 13) . This is very similar to the ultrasonic technique mentioned by Mukhanov 15) , who used a magneto-strictive transducer instead of a piezo-transducer to provide ultrasonic vibratory energy.…”

“…This is very similar to the ultrasonic technique mentioned by Mukhanov 15) , who used a magneto-strictive transducer instead of a piezo-transducer to provide ultrasonic vibratory energy. Compared with other mechanical surface nano-crystal treatments, UNSM produces the best hardness, and good surface roughness 13,14) . The UNSM effects and their anticipated benefits are that the uniformed micro dimple pattern on the surface and nanocrystalline structure induce compressive residual stress in the subsurface layer.…”

“…The UNSM effects and their anticipated benefits are that the uniformed micro dimple pattern on the surface and nanocrystalline structure induce compressive residual stress in the subsurface layer. This change has the potential to simultaneously improve the static and fatigue strength, as well as the surface hardness and surface roughness of the work piece 13,14,16) . The nano-crystalline surface could increase the density of diffusion paths available for alloying elements to migrate and rapidly form a protective passive film 17,18) .…”

“…In addition, residual stress in the surface layer affects the critical current density and the current density for passivation which are closely related to the formation and retention of the passivation layer 19) . The high mechanical energy imparted by UNSM to the steel surface may also induce the diffusion of the alloying elements 13,14,18) and therefore can affect the intergranular corrosion resistance of the stainless steels. We already reported that the intergranular corrosion properties by aging treatment was closely related to the formation of chromium carbides and carbon segregation, and UNSM treatment reduced the residual stress and thus improved the intergranular corrosion resistance 20) , but we couldn't propose the mechanism in detail between its resistance and carbon segregation.…”