$ultra fractal crack$

Ultra fractal crack crack#

For Custom ® 465 † (C465 Fe-11.4Cr-10.9Ni-0.9Mo-0.7W-1.6Ti-0.01C by wt%, UNS S46500), manufacturer’s data suggest that the threshold stress intensity (K TH) for occurrence of SCC is above 108 MPa√m for the H1000 condition (quenched to martensite and aged at 538☌ σ YS = 1,470 MPa), but falls to 58 MPa√m for H900 (σ YS = 1,615 MPa to 1,650 MPa). 1- 2, 6- 7 However, SCC susceptibility at the strength necessary to replace ultra-high strength steel (UHSS) limits UHSSS application. 1- 5 These alloys resist stress corrosion cracking (SCC) when tensile yield strength (σ YS) is restricted below maximum hardening capability. Ultra-high strength stainless steel (UHSSS) alloys, based on low carbon intermetallic-precipitate hardened martensite, are important for applications that require very high strength, fracture toughness, and corrosion resistance without noxious Cd plating or surface coating.

Ultra fractal crack crack#

A quantitative decohesion model effectively predicts the potential dependence of da/dt II using crack tip H solubility reverse calculated from a K TH model. Increased Cr does not degrade HEAC resistance, suggesting that crack passivity dominates cation acidification to reduce H production and/or uptake. The breadth of this protective-potential window increases with decreasing steel strength. PH48S and C465 are nearly immune to HEAC when cathodically polarized by 50 mV to 500 mV, attributed to a minimum in occluded-crack tip overpotential for H production. This diffusivity reduces the Stage II growth rate by 1 to 3 orders of magnitude compared to C465 and carbide strengthened ultra-high strength steels. Nanoscale strengthening precipitates in PH48S reduce effective H diffusivity to the mid-10 −10 cm 2/s range, because of reversible H trapping with a binding energy of 12 kJ/mol. However, a susceptible network of random packet/block interfaces is connected in 3D to limit interface engineering. Martensite transformation produces a large area of coincident site lattice interfaces in the refined microstructure of PH48S. Transgranular HEAC occurs along martensite packet and α′-block interfaces in PH48S, likely a result of H decohesion enabled by localized plasticity.

$ultra fractal crack$

Minimum threshold for HEAC (K TH) is low (8 MPa√m to 17 MPa√m) for each steel under substantial cathodic polarization. HEAC is transgranular, because of increased steel purity and La addition, compared to severe intergranular HEAC in Custom ® 465-H900 without rare earth elements. A modern martensitic stainless steel (Ferrium ® PH48S ™) resists hydrogen environment assisted cracking (HEAC) in aqueous NaCl at ultra-high yield strengths (1,400 MPa to 1,600 MPa).

$ultra fractal crack$