ASME boiler and pressure vessel codeの許容応力の改訂

To prevent hot-cracking, austenitic stainless steel welds generally contain a small percent of delta ferrite. Although ferrite has been found to effectively prevent hot-cracking, it can lead to embrittlement of welds when exposed to elevated temperatures. The aging behavior of type-308 stainless steel weld has been examined over a range of temperatures 475--850 C for times up to 10,000 hrs. Upon aging, and depending on the temperature range, the unstable ferrite may undergo a variety of solid state transformations. These phase changes creep-rupture and Charpy impact properties.

https://digital.library.unt.edu/ark:/67531/metadc741815/m2/1/high_res_d/79733.pdf

Embrittlement of austenitic stainless steel welds

The microstructure of type-308 austenitic stainless steel weld metal containing {gamma} and {delta} and ferrite is shown. Typical composition of the weld metal is Cr-20.2, Ni-9.4, Mn-1.7, Si-0.5, C-0.05, N-0.06 and balance Fe (in wt %). Exposure of austenitic stainless steel welds to elevated temperatures can lead to extensive changes in the microstructural features of the weld metal. On exposure to elevated temperatures over a long period of time, a continuous network of M{sub 23}C{sub 6} carbide forms at the austenite/ferrite interface. Upon aging at temperatures between 550--850 C, ferrite in the weld has been found to be unstable and transforms to sigma phase. These changes have been found to influence mechanical behavior of the weld metal, in particular the creep-rupture properties. For aging temperatures below 550 C the ferrite decomposes spinodally into {alpha} and {alpha}{prime} phases. In addition, precipitation of G-phase occurs within the decomposed ferrite. These transformations at temperatures below 550 C lead to embrittlement of the weld metal as revealed by the Charpy impact properties.

/pdf/embrittlement-of-austenitic-stainless-steel-welds-123hmam67z.pdf

Austenitic stainless steel microstructures produced by directed energy deposition (DED) are analogous to those developed during welding, particularly high energy density welding. To better understand microstructural development during DED, theories of microstructural evolution, which have been established to contextualize weld microstructures, are applied in this study to microstructural development in DED austenitic stainless steels. Phenomenological welding models that describe the development of oxide inclusions, compositional microsegregation, ferrite, matrix austenite grains, and dislocation substructures are utilized to clarify microstructural evolution during deposition of austenitic stainless steels. Two different alloys, 304L and 316L, are compared to demonstrate the broad applicability of this framework for understanding microstructural development during the DED process. Despite differences in grain morphology and solidification mode for these two alloys (which can be attributed to compositional differences), similar tensile properties are achieved. It is the fine-scale compositional segregation and dislocation structures that ultimately determine the strength of these materials. The evolution of microsegregation and dislocation structures is shown to be dependent on the rapid solidification and thermomechanical history of the DED processing method and not the composition of the starting material.

Microstructural development in DED stainless steels: applying welding models to elucidate the impact of processing and alloy composition

Controlled Residual Element (CRE) additions of Ti, P, and B to type 308 stainless steel filler metal were previously shown to improve the elevated-temperature creep-rupture properties of welds The solidification and aging behavior of types 308 and 308CRE stainless steel welds were examined in an effort to identify the mechanism responsible for the improved mechanical properties Analysis of the as-welded microstructure and thermal analysis of the solidification behavior did not reveal any major differences that could be responsible for the creep-rupture improvement However, extensive aging studies at 550 to 850/sup 0/C for times up to 10,000 h revealed significant differences The transformation of ferrite in the as-welded duplex structure to sigma phase was not retarded by CRE additions However, the extensive precipitation of M/sub 23/C/sub 6/ at the austenite-ferrite interfaces in the aged type 308 stainless steel welds was replaced by more homogeneous precipitation of titanium-rich carbides, nitrides, and sulfides in aged type 308CRE stainless steel The results are summarized in the form of time-temperature-transformation diagrams It is proposed that the elimination of a continuous network of carbides is responsible for the improved elevated-temperature properties The different type of precipitate formed may also play a role

Solidification and aging behavior of Types 308 and 308CRE stainless steel welds

A shielded metal arc (SMA) Type 308 stainless steel pressure vessel test weld with the controlled residual elements (CRE) boron, titanium, and phosphorus was tensile tested and creep tested at temperatures to 650/sup 0/C (1200/sup 0/F). Improved ductility in tests lasting several thousands of hours, compared to that of standard welds, is associated with the absence of interphase separation. Systematic variations in microstructure and mechanical properties throughout the weld and in the heat-affected zone are due to thermal and mechanical cycling during the welding process. Anisotropic deformation, that is related to local substructure and preferred crystallographic orientation, occurs.

G.M. Goodwin

Papers

Relation between mechanical properties and microstructure in CRE Type 308 weldments