Machining marks

It is interesting to note that the iron is not removed but is transformed in situ to iron oxide. What is even more remarkable is that the volume change anticipated when iron is converted to iron oxide does not occur. Consequently, surface contours of the corroded metal remain intact, even to the point of retaining fine detail such as machining marks. 

Internal and external surfaces of castings meet requirements of MSS SP-55. For internal surfaces, pull the caps and look down bores with dental mirror for evidence of damage, such as heavy machine marks, deep pitting or cracks. 

Pressure vessels are also subject to cyclic stress. Cyclic stress arises from pressure and/or temperature cycles on the metal. Cyclic stress can lead to fatigue failure. Fatigue failure, discussed in more detail in Module 5, can be initiated by microscopic cracks and notches and even by grinding and machining marks on the surface. The same (or similar) defects also favor brittle fracture. 

The ram extrusion process for UHMWPE powder was described in Section 5.4.4, and the effects of diffusion on the strength of the particle boundaries in Section 6.4.3. Ram extruded rods are machined on a lathe into the required cup shape. It is not essential to remove all machining marks, since the smooth metal counter-face polishes the PE in contact with it. 

An example of machining marks in an as-machined (never implanted) GUR 1050 UHMWPE component. 

Fig. 5. The exposed sample retains many of the machining marks seen in the unexposed sample. However, a slight change in color and reflectivity indicate some level of reactivity, although the attack is not extensive. After 420 hours the machining marks have disappeared and after 698 hours there is little change in the macroscopic surface appearance. At higher magnification, however, needle-like grains are clearly visible. Fig, 6. There is no evidence of the formation of corrosion products or corrosion layer which would result in a decreasing corrosion rate with exposure time. Therefore, it appears this material degrades by simple dissolution and the corrosion rate can be expected to be nearly constant and independent of exposure time.

The main reasons for preparing the wood surface before bonding are (1) to produce a close fit between the adherends and a bond-line with uniform thickness (2) to produce a freshly cut or planed surface, free from machine marks and other irregularities, extractives and contaminants and (3) to produce a mechanically sound surface, without crushing or burnishing it, which would inhibit adhesive wetting and penetration. 

Post-irradiation examination from the MSRE showed no interactions (erosion or corrosion) between the salt and the graphite. The original machining marks were still clearly visible. Out of reactor tests were conducted to 1400°C with no interactions between the salt and graphite. Experiments show the non-wetting behaviour of the fluoride salts of interest and demonstrate that liquid fluoride salts will not penetrate small cracks in the graphite. For the AHTR, this has the practical implication that the clean coolant will not contact the fuel micro spheres that are embedded in a carbon matrix. 

On the other hand, the 50kGy irradiated and stabilized GUR 1050 surface after wear testing exhibited signs of abrasive and adhesive wear and the complete loss of machine marks at 1.98 mUhon cycles relative to the soak and untested pins. 

The process has the ability to correct geometrical inaccuracies, for example, bell-mouthing, barreling, tapers and waviness in holes, as well as removing machining marks. 

Machine marks should be in line with the flow of solder. 

---