Several tests have been conducted to determine the effect that length has on the Incoloy 800 screws length. These tests include torsion simulation, the effect of mechanical cycling on strength, and grain boundary cracking characteristics.
Grain boundary cracking characteristics
In the study, small pieces of Incoloy 800H screws were metallographically prepared. They were then heat-treated in an oxygen atmosphere at 982 deg C for three hours. In the heat-treatment process, oxygen is released from the alloy, which enhances its brittleness of the alloy.
Cracking is evident along the cracked grain boundaries and around the axial crack in the fracture surface. However, these fractures are not associated with fatigue or stress corrosion. Internal defects cause them. These defects will likely be produced during the hot forging process and later in the cold working process.
During the heat-treatment process, chromium carbides are seen at grain boundaries. However, the extent of these particles depends on the corrosive environment. These particles are likely to be MC-type carbides.
Brittleness of Incoloy 800 screws caused by internal defects
While assembling a batch of TB3 alloy screws, a couple of them broke. This was not a random occurrence. Rather, it resulted from a rapid manufacturing process wherein there was little time to allow creep to develop.
Various measures were taken to determine what caused the screw to fail. For starters, the Incoloy 800 screws were inspected in an ultrasonic washing machine for five minutes. The specimens were cleaned using a proprietary solvent. The next step was to measure the resulting torque. The results showed that the screw's torsion moment was on the low end of the acceptable range. This was corroborated by a torsion simulation test performed on fifty screws of the same production batch. The results also showed that the Incoloy 800 screws was not susceptible to hydrogen embrittlement. The best part was that the resulting torque was slightly less than the initial value.
The procedure mentioned above was repeated on the remaining Incoloy 800 screws. The most significant changes were that the previous test was performed under room temperature conditions, the maximum torque was reduced from 3.5 N*m to 2.5 N*m, and the pitch was increased from 0.7 to 0.8 mm.
Impact of mechanical cycling on strength
Several investigations have been done to determine the impact of mechanical cycling on the strength of Incoloy 800 screws. The study consisted of a series of thermal fatigue tests spanning 300 cycles from room temperature to 800 AdegC. These tests were carried out with the aid of IPS e-max ZirCAD.
The results of these tests revealed that the alloy 800H is a temperature-dependent material. Moreover, the low cycle fatigue life of the alloy was shortened as the strain rate decreased. This was attributed to the presence of borocarbide particles at grain boundaries. The high carbon content in the alloy also positively influenced the life of the LCF.
The microstructure of the alloy specimens was studied using scanning electron microsmbrace micros. In addition, the cross-section microstructure of the alloy specimens was studied after the thermal fatigue tests. The metallographic study also included a segregation analysis of Mo and Cr. Lastly, a masticatory simulator was used to simulate the forces of compression. This device exerted 50 N at the central fossa of each pontic.
Torsion simulation test
Several Incoloy 800 screws produced from a single production batch were used in a torsion simulation test. These screws were subjected to sustained tensile load at room temperature for 200 hours. In addition, torque was applied at different rates - 1, 3, or 5 rpm. The maximum torsional moment, Tmax, was determined as the rupture angle at the surface. The results demonstrate that the durability limit of metallic screws is a function of the core diameter and axial direction.
The main failure mechanism of the screws was a brittle fracture. The microscopy observation of the fracture surfaces showed no evidence of plastic deformation or corrosion products. A large number of non-reflective facets are observed at the fracture center of the Incoloy 800 screws. This indicates that the grain boundary cracking characteristic is observed around the axial crack.
