Cavitation in creep

159

The principal limitation of today's Ni-and Fe-based superalloys continues to be their susceptibility to intergranular degradation arising from creep, hot corrosion, and fatigue. Many precipitationstrengthened superalloys are also difficult to weld, owing to the formation of heat-affected zone (HAZ) cracks during postweld heat treatments (PWHTs). The present work highlights significant improvements in high-temperature intergranular degradation susceptibility and weldability arising from increasing the relative proportion of crystallographically ''special'' low-⌺ CSL grain boundaries in the microstructure. Susceptibility to intergranular degradation phenomena is reduced by between 30 and 90 pct and is accompanied by decreases in the extent and length of PWHT cracking of up to 50-fold, with virtually no compromise in mechanical (tensile) properties upon which the functionality of these specialty materials depends. Collectively, the data presented suggest that ''engineering'' the crystallographic structure of grain boundaries offers the possibility to extend superalloy lifetimes and reliability, while minimizing the need for specialized welding techniques which can negatively impact manufacturing costs and throughput.

Section: Mechanical Testingmentioning

confidence: 84%

Improving the weldability and service performance of nickel-and iron-based superalloys by grain boundary engineering

Lehockey

Palumbo

Ping

1998

159

“…[12,13] The precipitates can inhibit grain boundary sliding and prevent cracking. Conversely, they can also act as the nucleation site of the crack.…”

Section: Effect On the Stress Rupture Propertiesmentioning

confidence: 99%

Effect of phosphorus on the microstructure and stress rupture properties in an Fe-Ni-Cr base superalloy

Sun

Guo

et al. 1997

This article describes the effect of phosphorus on the microstructure and stress rupture property at 650 ЊC in an Fe-Ni-Cr base superalloy. The results showed that phosphorus markedly improved the intergranular precipitation in the range of 0.0005 to 0.016 wt pct, which facilitated M 23 C 6 and M 3 B 2 precipitation but inhibited the formation of MC carbide. A too high phosphorus addition (0.051 wt pct P) resulted in an excessive precipitation at grain boundaries, while a too low phosphorus content (0.0005 wt pct P) led to many precipitate-free grain boundaries. Phosphorus also enlarged the size of the ␥' particles and lowered its stability, that -Ni 3 Ti preferred to form in the alloy with 0.051 wt pct P. Due to the improvement of the microstructure, appropriate amount of P content significantly prolonged the rupture life of the alloys in the range of 0.0005 to 0.016 wt pct. The peak value was 660 hours at 0.016 wt pct, more than 4 times that of the alloy with 0.0005 wt pct phosphorus, but phosphorus reduced the fracture elongation. The mechanism by which phosphorus influenced the alloy is discussed.

“…[9,10] Among the interfacial properties believed to strongly influence nucleation and growth of cavities are the energies of, and diffusivities along, both grain boundaries and cavity surface, as well as the resistance of grain boundaries to sliding. [11] These interfacial properties are in turn expected to be influenced by any trace element segregation that should take place. [12] The segregation of impurity elements such as O, S, As, and Sb on the grain boundary greatly lowers the interfacial energy of the boundary and enhances the creep cavitation.…”

Section: Introductionmentioning

confidence: 99%

Improved creep strength and creep ductility of type 347 austenitic stainless steel through the self-healing effect of boron for creep cavitation

Laha

Kyono

Sasaki

et al. 2005

Composition of type 347 austenitic stainless steel was modified with the addition of boron and cerium. An improvement of creep strength coupled with creep ductility of the steel was observed with boron and cerium additions. The observation of enhanced precipitation of carbonitrides in boron-containing steel over that of boron-free steel may in part contribute to the increase in creep strength. Both grain boundary sliding and nucleation and growth of intergranular creep cavities were found to be suppressed in steel-containing boron. This results in an increase in creep strength and creep ductility. Auger electron spectroscopic analysis of the chemistry of creep cavity surfaces (exposed by breaking the creepexposed steel specimen at liquid nitrogen temperature under impact loading) revealed the segregation of elemental boron on the creep cavity surface. Boron segregation, on the creep cavity surface in the absence of sulfur contamination, suppressed the cavity growth and provided the steel with a self-healing effect for creep cavitation. Cerium additions enabled boron to segregate on the cavity surface by effectively removing the traces of free sulfur in the matrix by the formation of ceriumoxysulfide (Ce 2 O 2 S).