Page 12 - Jan-Feb 2018
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Technology
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Different parts of the Sarvatra bridge such as (a) Web plate, (b) Ramp assembly, (c)   (a) Manufacture of Sarvatra bridges by
Lifting hook, and (d) Pier bracket as fabricated from the indigenised RDE40M-T 7x51      utilising indigenised RDE40M-T 7x51

                         sheets and plates having different thicknesses                sheets & plates, and (b) Load testing of
                                                                                                       Sarvatra bridge
aluminium alloys, and such alloys         (involving over aging treatments) are
could be made immune to Stress            known to improve the SCC resistance         cracks were also detected in regions
Corrosion Cracking (SCC) by suitable      of the copper-free aluminium–zinc–          away from hinge locations involving
heat treatments. However, such alloys     magnesium alloys.                           components fabricated out of plates.
are non-weldable.
                                             Accordingly, a suitable compo-              The analysis carried out by DMRL
   On the other hand, the copper-free     sition and an overaging treatment           showed that the cracks are of stress
aluminium-zinc-magnesium alloys           were designed for a copper-free             corrosion cracking origin. Specimens
are weldable and could be utilised        aluminium–zinc–magnesium–                   made out of RDE40 materials from
for application such as light military    manganese–zirconium alloy for this          forged and plate components were
bridges where joining by welding is       application. The resultant alloy was        subjected to SCC tests at CECRI,
an essential step. However, such alloys   designated RDE40M, i.e., a modified         Karaikudi. These specimens could
suffer from poor SCC resistance.          form of the previous alloy RDE40.           not qualify the ASTM G47-98 SCC
                                                                                      tests implying that the materials were
   Suitable changes in compositions          The alloy RDE40 developed                indeed susceptible to stress corrosion
(such as maintaining specific             about two decades ago has now been          cracking. This alloy was successfully
zinc: magnesium ratios and zinc+          discontinued because in 2012, in as         produced in the form of sheets and
magnesium contents, and additions         many as twenty (20) Sarvatra bridges,       plates, in various thicknesses (t = 4
of suitable recrystallisation inhibiting  cracks were observed. Such cracks           mm to 20 mm) at BALCO.
elements (such as manganese,              were initially detected in hinge (forged
zirconium, etc. and heat treatments       product) locations, but later on such          The resultant materials have
                                                                                      electrical conductivity values of ≥ 40
                                                                                      per cent IACS and 0.2 per cent PS
                                                                                      values of ≥ 300 MPa.

RDE40M-T7x51 sheets of thickness (a) 6 mm and plates of thickness (b) 20 mm produced at BALCO, Korba

12 January-February 2018                  Aluminium Alloy Technologies for Advanced Defence Systems
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