Mechanism of structure formation in a metastable melt and estimation of its spatial scale

An estimate of the dendritic parameter based on the assumption of the formation of the spatial structure of the solid phase in a metastable supercooled melt (MSM) is proposed. The solidification of MSM is considered in the quasichemical approximation, which assumes a transition state corresponding to the ideal solution system. It is shown, that the dimensions of the structural cell depend on the ratio of the diffusion rate within the cell and the rate of heat removal. The theoretical estimate of the structure parameter is consistent with the experimental dependence. Keywords: metastable melt, dendritic parameter, cooling rate, segregation zone, diffusion. [email protected]

Homogenization and Extrudability of 5056 Aluminum Alloy Billet

Through a series of homogenization and extrusion experiments, the extrusion efficacy of 5056 seamless tubes is improved. Although better extrusion practice was apparent with better homogenization parameters for 5056 alloy, a little over-burning structure and phenomena were found on and in the surface and segregation zone of homogenized billet. The extrudability is worse than theoretical level with no aluminum and magnesium oxide particles on the billet surface. Additional, another quality problem in this study is the segregation zone in 5056 larger billets. The over-heat structure was smoothly flowed into the seamless tube by indirect extrusion, and that makes surface bubbles.

Assessment of the Fitness for Service of the Flamanville EPR Reactor Pressure Vessel Closure Head and Bottom Head Domes Containing a Segregation Zone Characterized by a High Carbon Content

To complete the manufacturing process qualification of the closure head and bottom head domes of the Flamanville EPR RPV in 16MND5 ferritic steel (similar to A508 cl3), AREVA NP performed in 2014 material testing on a sacrificial dome so-called UA Sup. These tests revealed that the Charpy impact energy at 0°C — in the positive segregation zone at ¼ thickness — was lower than 60 J, the minimum value required by the French regulation. Thereafter, this result was explained by the high carbon content close to 0.3 % — against 0.18 % in the ladle — due to the forging process. The positive segregation was insufficiently eliminated to avoid high carbon content in certain location. As the level of Charpy impact energy is an indicator of the level of fracture toughness, a thorough material characterization of the segregation zone, as well as a brittle fracture analysis have been proposed and carried out by AREVA NP to demonstrate that the Flamanville EPR RPV domes are tough enough for a safe operation. The principles of the demonstration approach were reviewed by ASN and IRSN in 2015 and the final safety case was submitted to the ASN and IRSN review in December 2016. The latter led to the conclusion that, even if the segregation zone fracture toughness is lowered in comparison to that of the acceptance test zone, this does not put into question the Flamanville EPR RPV fitness for service as long as adequate in-service inspection of the domes is periodically performed. This paper aims at presenting the key elements of the IRSN assessment from part of which this conclusion was drawn, namely the material testing program, the description of the loadings in normal and accidental conditions, and the brittle fracture analysis.

Metallographic Methods for Oxide Skin Segregation Studies

This work is a study of the segregation of alloying elements and salts to the surface of Direct Chilled-cast (DC-cast) ingots of alloy: (i) AA5083; and (ii) AA5182. The ingots only have slightly different chemical compositions. Alloy AA5182 contains Cu, while alloy AA5083 is grain refined, contains Cr, and is lower in Si and Fe compared to AA5182. The ingot microstructures were investigated using Optical Microscopy, Scanning Electron Microscope (SEM) and Secondary Ion Mass Spectrometry (SIMS). The results showed that the surfaces contained aluminum dendrites, large intermetallic crystals (Al6(MnFe) and Al3(Fe,Mn)) and impurities, and small crystals between dendrite arms. Between the dendrites small salt crystals containing Na, K, Ca, Mg, F and/or Cl were found. The conclusion is that salt and minority impurities are mainly located to grain boundaries in the segregation zone.

Development of High Strength Line Pipe for Sour Service and Full Ring Evaluation in Sour Environment

To establish mass production technology of high strength line pipe up to API-X70 grade for sour service, the HIC (Hydrogen Induced Cracking) phenomena in line pipe has been carefully examined. The micro-segregation zone originated at final solidification front, which has MnS inclusion and hard phase, was identified as the location of the initiation site of HIC. The proper procedures of Ca treatment to eliminate elongated MnS and TMCP application to reduce maximum hardness have proved to increase the HIC resistance in such segregation zone. In addition to the standard HIC test, full ring test under applied stress was conducted to assure the performance of high strength line pipe in sour service. The clean steel in mass production, which has enough HIC resistance, was attained for high strength line pipe up to API-X70.