ANALYSIS OF A TUBE-GRID OSCILLATORY CONTACT - METHODOLOGY FOR THE SELECTION OF SUPERFICIAL TREATMENTS

To minimize the failure risks (metal loss, cracking) under continuous vibration loadings (fretting) at tube-grid contact of a tubular heat e xchanger, a methodology for the identification and the estimation of t he main damages was developed. Wear testing was based on a specific de vice devoted to fretting analyses. In the case studied a rapid and sim ple test has been set up to establish the so-called running condition fretting map. This methodology was applied to characterize the frettin g behaviour of selected surface treated iron alloys as compared to the reference pair of the base material (SA 213 grade T91). Increment app lied displacement (IAD) and constant normal load are used to determine the boundaries between the three fretting conditions which are stick, partial slip or gross slip. The IAD method is analyzed and validated by the means of a comparison with more sophisticated test techniques a nd surface analyses. From an Hertzian-like approach, several surface t reatments and coatings were studied depending on the nature of the pos tulated damage (wear, cracking...). ''Surface treatment'' (nitriding.. .) or ''coatings'' (TiN, varnishes...) are described using required pr operties such as Young modulus, friction coefficient, residual stresse s.... The simplified IAD method was used to enhance an a priori select ion of surface treatments and to set guidelines for the development of coating well fitted with the industrial objectives.