Cracking in 475°C embrittlement of Stainless Steels- a short critique of API 571, Third Edition

Hamed Mirabolghasemi, Ph.D, P.Eng

A specific kind of cracking in some stainless steels, known as 475°C embrittlement, refers to a reduction in ductility and fracture toughness resulting from (a) metallurgical change(s). This phenomenon is typically observed in stainless steels containing a ferrite phase, such as 400 series stainless steels, Duplex stainless steels, and Austenitic stainless steel weld metals with up to approximately 10% ferrite content. It occurs when these materials are exposed to temperatures ranging from 600°F to 1000°F (315°C to 540°C). Additionally, prolonged heat treatment within or slow cooling through this temperature range can also lead to embrittlement. This embrittlement can be reversed through heat treatment, typically performed at temperatures of 1100°F (595°C) or higher, followed by rapid cooling [1].

?

The primary cause of this embrittlement is the formation of α' Chromium rich domains within the ferrite phase within the abovementioned temperature range. At these temperatures, a spinodal decomposition of supersaturated solid ferrite into two phases occurs: an iron-rich nanophase and a chromium-rich nanophase. This process results in a spinodal structure shown in Figure 1. However, detecting the presence of α' domains or precipitates in this temperature range is challenging, even under a microscope, due to their small size, which ranges from 20 to 200 ? (angstroms) [2].

?

Despite the difficulty in direct observation, the presence of chromium-rich α' domains/precipitates has an important effect on the material. They pin the dislocations, reducing the number of active sliding planes and promoting the appearance of straight-line deformation patterns on the polished surface of the ferrite. This deformation is primarily caused by twinning, and the straight lines observed are actually a result of deformation twinning.

?

In certain alloys with higher nickel (Ni) and silicon (Si) content, another phase called G-phase may precipitate. Some researchers have suggested that additional precipitation and/or growth of existing carbides and nitrides at the ferrite/austenite phase boundaries may occur. Smaller carbides and nitrides precipitated at interfaces and grain boundaries can provide easier paths for crack propagation [4]. However, research by Gang Liu [5] and coworkers has shown that the blocking effects of spinodal decomposition precipitates and G-phase precipitates on dislocation movement are stronger than the influence of grain boundaries and phase boundaries, and Micro-cracks preferentially initiate in the ferrite phase.

An interesting point worth noting is a change in the caption of a micrograph of this failure mechanism between API 571 2nd edition and 3rd Edition (Figure 4). In the 2nd Edition, the caption mentions cracks running "through" the grains, while in the 3rd Edition, it refers to "intergranular" cracks. However, as explained earlier, cracking in this type of failure initiates and start to propagate inside the grains and is of an "intra/trans-granular" nature. Intergranular cracks, if observed, may be the result of other effects like precipitation but are considered a secondary phenomenon. Therefore, the 2nd Edition's caption appears to be more accurate in describing the primary nature of the cracks.

?

References:

[1] API 571, Damage mechanisms affecting fixed equipment in the refining industry, ANSI / API recommended practice, Third edition, March 2020

[2] C.R.F. Azevedo, A.F. Padilha, The most frequent failure causes in super ferritic stainless steels: are they really super? Procedia Structural Integrity, Volume 17, 2019, Pages 331-338

[3] Abdalrhaman Koko et al., In situ characterisation of the strain fields of intragranular slip bands in ferrite by high-resolution electron backscatter diffraction, Acta Materialia, Volume 239, 2022, 118284

[4] Byun, T.S., Yang, Y., Overman, N.R. et al. Thermal Aging Phenomena in Cast Duplex Stainless Steels. JOM 68, 507–516 (2016)

[5] Liu, G., Li, SL., Zhang, HL. et al. Characterization of Impact Deformation Behavior of a Thermally Aged Duplex Stainless Steel by EBSD. Acta Metall. Sin. (Engl. Lett.) 31, 798–806 (2018)