PSI - Issue 14

Neeta Paulose et al. / Procedia Structural Integrity 14 (2019) 649–655 Neeta Paulose etal. / Structural Integrity Procedia 00 (2018) 000–000

650

During solidification, the δ phase forms first and consumes almost 60% of the liquid, then ϒ and δ consume most of the remaining liquid, and finally a small amount of M(C,N) precipitates in the final liquid. The alloy is expected to have almost 75% ferrite and 25%austenite at the temperature where solidification is complete. Therefore majority of the ferrite in the CF8C steel transforms to austenite during cooling to room temperature. δ ferrite present in the CF8C makes it ferromagnetic. F. Padilha and P. R. Rios (2002) have indicated that the properties and performance of stainless steels are strongly related to its microstructure, especially the amount and distribution of δ ferrite. Angelo Fernando Padilha etal. (2013) have reported that amount and distribution of δ ferrite is strongly affected by the chemical composition and the cooling rate during solidification. Further the alloy may undergo microstructural changes during heat treatment or welding process, when exposed to elevated temperature for shorter or longer time (S Kozuh et al. (2009)). The presence of ferrite in the austenite may be beneficial or detrimental, depending on the application. The ferrite phase in austenitic matrix is known to play an important role in the prevention of hot cracking in both as-cast and as welded structure. In case of Stress Corrosion Cracking the presence of ferrite pools in the austenite matrix is thought to block or make more difficult the propagation of cracks (G. SUI E.et al. (1996)). Ferrite can be detrimental in some application one concern may be the reduced toughness form ferrite, although this is not a major concern, given the extremely high toughness of the austenite matrix. At higher temperatures (550-900°C), ferrite may transform to hard and brittle sigma phase and reduce impact energy, ductility and corrosion resistance (Adrian P. Mouritz (2012)). The authors have not found much information on effect of CF 8C microstructure on tensile property of the alloy at various temperatures. Therefore the present investigation was taken up. 2. Materials and experimental methodology CF 8C in as cast and two heat treated conditions (Cycle A and Cycle B) was used for microstructural, characterization and tensile testing. The detail of heat treatment cycle is given below: Cycle B: 1080°C/45 min/Water Quench The chemical composition of the alloy used for the study is given in Table.1. For microstructural study specimens were polished and etched using kallings reagent. Microsructural study was done using Optical microscope (Model Clemex) andTabletop Scanning Electron Microscope (SEM). (Phenom XL). Semi quantitative elemental analysis of various phases was carried out using Energy Dispersive Spectroscopy (EDS) attached with Tabletop SEM. Tensile specimen of gauge dia 4 mm and gauge length 22 mm was used for the study. Tests were carried out as per ASTM E8 standard at a constant strain rate 10 -3 sec -1 over a temperature range 25-550°C.   Cycle A: 1066°C/45 min/Furnace Cool

Table.1 Chemical Composition

Element

Wt (%)

Carbon

0.045

Manganese

1.07 1.06

Silicon

Phosphorus

0.030 0.006 18.34

Sulfur

Chromium

.

Nickel

9.23 0.51 0.18 0.27

Columbium (Niobium)

Molybdenum

Copper