Changes

Ag/C (GRAPHOR) contact materials are usually produced by powder metallurgywith graphite contents of 2 – 5 wt% ''(Table 2.32)''. The earlier typicalmanufacturing process of single pressed tips by pressing - sintering - repressing(PSR) has been replaced in Europe for quite some time by extrusion. In NorthAmerica and some other regions however the PSR process is still used to someextend mainly for cost reasons.

The extrusion of sintered billets is now the dominant manufacturing method forsemi-finished AgC materials ''(Figs. 2.126 – 2.129)''. The hot extrusion processresults in a high density material with graphite particles stretched and oriented inthe extrusion direction ''(Figs. 2.130 – 2.133)''. Depending on the extrusionmethod in either rod or strip form the graphite particles can be oriented in thefinished contact tips perpendicular (GRAPHOR) or parallel (GRAPHOR D) to theswitching contact surface ''(Figs. 2.131 and 2.132)''.

Since the graphite particles in the Ag matrix of Ag/C materials prevent contacttips from directly being welded or brazed, a graphite free bottom layer isrequired. This is achieved by either burning out (de-graphitizing) the graphiteselectively on one side of the tips or by compound extrusion of a Ag/C billetcovered with a fine silver shell.

Ag/C contact materials exhibit on the one hand an extremely high resistance tocontact welding but on the other have a low arc erosion resistance. This iscaused by the reaction of graphite with the oxygen in the surroundingatmosphere at the high temperatures created by the arcing. The weld resistanceis especially high for materials with the graphite particle orientation parallel to thearcing contact surface. Since the contact surface after arcing consists of puresilver the contact resistance stays consistently low during the electrical life of thecontact parts.

A disadvantage of the Ag/C materials is their rather high erosion rate. In materialswith parallel graphite orientation this can be improved if part of the graphite isincorporated into the material in the form of fibers (GRAPHOR DF), ''(Fig. 2.133)''.The weld resistance is determined by the total content of graphite particles.

Ag/C tips with vertical graphite particle orientation are produced in a specificsequence: Extrusion to rods, cutting of double thickness tips, burning out ofgraphite to a controlled layer thickness, and a second cutting to single tips.Such contact tips are especially well suited for applications which require both,a high weld resistance and a sufficiently high arc erosion resistance ''(Table 2.33)''.For attachment of Ag/C tips welding and brazing techniques are applied.

welding the actual process depends on the material's graphite orientation. ForAg/C tips with vertical graphite orientation the contacts are assembled withsingle tips. For parallel orientation a more economical attachment starting withcontact material in strip or profile tape form is used in integrated stamping andwelding operations with the tape fed into the weld station, cut off to tip form andthen welded to the carrier material before forming the final contact assemblypart. For special low energy welding the Ag/C profile tapes GRAPHOR D and DFcan be pre-coated with a thin layer of high temperature brazing alloys such asCuAgP.

In a rather limited way, Ag/C with 2 – 3 wt% graphite can be produced in wireform and headed into contact rivet shape with low head deformation ratios.

The main applications for Ag/C materials are protective switching devices suchas miniature molded case circuit breakers, motor-protective circuit breakers,and fault current circuit breakers, where during short circuit failures highestresistance against welding is required ''(Table 2.34)''. For higher currents the lowarc erosion resistance of Ag/C is compensated by asymmetrical pairing withmore erosion resistant materials such as Ag/Ni and Ag/W.

Fig. 2.126:Strain hardeningof Ag/C 96/4 Dby cold working

Fig. 2.128127: Softening of Ag/C 96/4 D after annealing[[File: Strain hardeningof Ag C 96 4 D.jpg|right|thumb|Strain hardening of Ag/C DF 96/4 D by cold working]]

Fig. 2.128: Strain hardening of Ag/C DF by cold working[[File:Strain hardening of Ag C 96 4 D.jpg|right|thumb|Strain hardening of Ag/C 96/4 D by cold working]] Fig. 2.129: Softeningof Ag/C DF after annealing[[File:Strain hardening of Ag C 96 4 D.jpg|right|thumb|Strain hardening of Ag/C 96/4 D by cold working]]

'''Table 2.32: Physical Properties of Silver–Graphite (GRAPHOR) Contact Materials Table 2.33: Contact and Switching properties of Silver–Graphite (GRAPHOR) Contact Materials'''

'''Table 2.3433: Application Examples Contact and Forms of Supply Switching properties of Silver–Graphite Silver–Graphite (GRAPHOR) Contact Materials'''

Pre-Production '''Table 2.34: Application Examples and Forms of Supply of Silver– Graphite (GRAPHOR) Contact Materials(Bild)'''