How fretting occurs on metal and its effects

To design and manufacturing practices, a clearance between tube and tube supporting device in steam generators and heat exchangers is needed. Vibration in these tubes makes them sensitive to impacting and rubbing with the supporting system or adjacent tubes resulting in local wear damage. The tube vibration may be activated by cross-flow perpendicular to the tube centerline and longitudinal flow along the tube or tubes. Although the real flow in the practical conditions is mixed, hence the tube oscillates in different directions causing some type of combined sliding and impact motion between the tube and supporting equipment and feasibility between adjacent tubes.

Considering the vibratory nature of the impact and rubbing motions, the damage is usually accounted a result of fretting, however fretting in its ordinary manner is featured by nominal reciprocating motion between the wear materials held together by a normal force.

Fretting corrosion was corrosion to include a chemical factor, oxidation and mechanical factor, welding and shearing of metal asperities. However it has been discovered that a corrosive media is not essentially for fretting to occur and that some materials that do not oxidize do fret. Three mechanisms by which fretting corrosion can arise-

a.       Eradication of metallic particles by grinding or by the development of welds at the points of contact after tearing. Subsequent oxidation of the particles is assumed to have no contribution in causing wear.

b.      The elimination of metal particles that subsequently oxidized develop an abrasive powder. An abrasive action is then considered to be more severe factor causing wear.

c.       Direct metal oxidation and continuous removal of oxide layer by the scraping of one surface over the other.

The mechanism of fretting refers that fretting is three stage process. At first, a surface developed oxide layer prevents metallic interaction, it is distributed by an oscillatory motion, then adhesion, plastic deformation and metal flow occurs. The transferred particles can get oxidized and dislodged to become discrete wear particles or the moved particles can create into surface forming a moderate zone, partially oxidized surface region preventing further transfer, the fretting action then develops loose wear particles. Eventually a steady state reaches that is attributed by a general disintegration and dispersal of zones influenced by the early stages. Shortly, three stages are adhesion and metal transfer, development of oxidized debris and eventually attainment of steady wear rate.

Damage caused by fretting varies from discoloration of the mating surface to the damage of large magnitude of materials. The frequency, total count of cycles, amplitude of motion, normal pressure and physical properties of interacting materials and environmental conditions all add to the results. The slip amplitude is normally considered as the major parameters have an impact on fretting.

Monel 400 wire and tube are found to be more resistant to fretting wear as compare to plain steel. The wear rate by oxidized wear debris developed on the damaged surface is more severe and it reduces with time.