Health & Safety

The brazing process has been used to successfully join dis-similar metals for over 3000 years and is still widely regarded as probably the most versatile metal joining technique. If the correct alloy is used for the application, the joint designed and heated correctly the process is as safe as any other industrial process.

Normally it is only when there are variations to the above eg incorrect alloy selection or poor heating technique that problems arise. It is our view that only by providing correct data re alloy composition, potential health risks etc that a customer can make an informed selection. As a responsible supplier, we take great efforts to provide all our customers with the information required. It beggars belief that there are so many sources of materials at exhibitions etc that provide no obligatory information whatsoever - particularly to the small user eg model engineer. It is not only irresponsible but probably contravenes HSE regulations.

There are three aspects worthy of mention;
Metal Fume
Fluxes
Heating

Metal Fume

All silver solders contain silver copper and zinc. As a group of metals they form a range of highly versatile brazing alloys. Lowering the melting point of these alloys, improving fluidity, joint penetration and their intrinsic cost were seen in the 1930's to be highly desirable characteristics. Cadmium was added to achieve this. About 1970 it came to light that there was a downside to the use of cadmium, as with other heavy metals when used, with hindsight, incorrectly.

Cadmium can enter the body orally or by inhalation. The most common form being as cadmium oxide as a fume or as dust. The dust is formed from condensed fume on the surfaces of the brazing station.

Cadmium is a white metal that in its solid form alloyed with other metals presents little danger. Potential hazards start when the alloy melts. Cadmium melts at 321 deg C. It must be realised that to some degree potentially dangerous cadmium oxide will be formed as the silver solder melts and exposure to high levels could prove fatal. Once the alloy has melted and flowed remove the heat. Continued heating will not produce a better joint, on the contrary, it is likely to have the opposite effect.

The alloys melt quickly, the joints are usually small in volume and surface area. In well ventilated conditions their use presents no real problems.

However it is extremely unlikely that a hazardous set of circumstances will arise unless the alloy is being grossly abused in use.

This includes
attempting to braze without using flux
brazing in a close poorly ventilated area
prolonged heating with an intense heat source eg oxy-acetylene
or a combination of the above or other bad brazing practices.

Because of the cumulative effect, the HSE has set an exposure limit of 0.05mg of cadmium oxide per cubic metre based on 8 hours/day brazing activity, 5 days/week, over an unspecified number of years.

Assume
a workshop of 40 cubic metres
with 40 air changes per hour eg from a 12 inch fan
using an alloy containing 25% cadmium

According to the HSE, the amount of cadmium bearing solder that can be used is

(TLV Value) x Volume of workshop x Number of air changes/hour divided by weight loss of cadmium per gram of alloy used

0.05 x 40 x 40/ 4 = 20gm in a 10 minute period.

20 gm = 2.5 rods of 42% silver alloy at 1.5mm dia x 500 or 5 rods @ 1.0mm.

Simple Precautions
Braze in well ventilated areas.
Do not braze with the head directly over the workpiece.
Do not overheat the alloy.
Avoid building up large fillets. This is also wasteful and in most cases contributes nothing to joint integrity or strength.
Maintain good flux coverage in the joint areas.

If this is not possible then consideration should be given to using cadmium free silver solders.

Fluxes

Most silver solder fluxes contain fluorides and fluoro-borates. These products should not be ingested or inhaled. If overheated they will produce noxious fumes. Fluxes should be kept in airtight containers preferably with screw caps to avoid spillage. Should spillage occur, it should be swept up immediately and the surface wiped down thoroughly with a damp cloth.
Keep away from all foodstuffs.
Keep out of reach of children.

Torches

Always light torches from underneath or from the side.
Do not point the torch towards any combustible materials.
Switch the gas supply off at the cylinders.
Store the cylinders in a safe and stable manner.

Generally the safest way to braze is to use common sense.
Keep the brazing station free of debris and dust.
Wear gloves.
Do not eat, drink or smoke when brazing.
Use only clean parts free of oil and grease.
Select the correct alloy and flux for the job.
Use sufficient flux and treat it carefully.
Heat the work evenly taking account of the weights of the components.
Watch the flux, it will indicate the joint temperature.
Don't heat the alloy - heat the joint.
Help prevent overheating by avoiding brazing in strong or bright sunlight.

IF IN DOUBT - STOP BRAZING AND SEEK ADVICE

Latest news
Supplies of Cadmium Bearing Silver Solders Continue Tue 20/01/2009 After 17 months, in May 2010, we still know of no european manufacturer about to cease production of cadmium bearing silver solders. These alloys...