Hard Soldering an Alternative to Brazing

Hard soldering (silver soldering) is an alternative to brazing. It specially implies the use of the filler materials containing silver, which have lower melting point than brazing spelter and therefore associated with greater ductility and often greater strength. Hard soldering is general term used to cover brazing and silver soldering. However, hard soldering can be defined as an intermediate stage between low temperature soft soldering and high temperature brazing.

The operation of brazing and hard soldering are almost identical and the two terms are used synonymously. Hard soldering normally means silver soldering, which has a lower melting temperature than brazing. Silver soldering is used principally for fine work and in case where joint is required to be stronger than soft soldering, but where, at the same time it is not desirable to raise the work to the brazing temperature.

Groups of Hard Solders

Hard solders can be divided into following two groups:

(a) Low Temperature Silver Brazing Alloys.  These contain copper, zinc and cadmium with up to 50% silver and have a melting point ranging from 600C – 800C. These alloys are used for most general engineering applications where strength is required.

(b) Silver Solders. Silver solders contain silver from 50 to 80%, copper up to 30% and zinc up to 20%. These have a melting point ranging from 700°C to 780°C. These are known as hard, medium or easy grade solders. Hard grade consists of 80% silver and 20% copper. Medium grade consists of 75% silver, 20% copper and 5% zinc. Easy grade consists of 50% silver, 30% Copper and 20% Zinc. Since all silver solders look alike, it is usual to scratch the whole length of solder bar as per markings.

Also Read  Hand Shearing Tools

Composition and Melting Temperature of Silver Solder

The various compositions and their respective melting points are given in the table below:

Composition and Melting Temperature of Silver Solder

Composition Melting Range (in C)  

Uses

Element Percentage
Silver

Copper

Zinc

Cadmium

49 – 51

14 – 16

15 – 17

18 – 20

620-640 Very fluid and easy to use (Grade C)
Silver

Copper

Zinc

60 – 62

27.5 – 29.5

9-11

690-735 Particularly suitable for electric work (Grade A)
Silver

Copper

Zinc

42-44

33-38

18.5-20.5

700-775 Used for general engineering work

Silver Soldering Procedure

The procedure to carry out silver soldering is as follows:

(a) The joint gaps for silver soldering should be between 0.05 mm and 0.15 mm.

(b) The parts must be clean and fit properly.

(c) Flux and secure the parts. Borax is the suitable flux.

(d) Heat the parts slowly.

(e) Apply the solder.

(f) Allow the parts to cool down.

(g) Cut and remove binding wire.

(h) Pickle the parts in caustic soda solution.

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