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Dear oh dear oh dear!
I leave this forum for a bit and it goes to pot!
Opel Fruit only touched on soldering and brazing, in his invaluable treatise on French plumbing and heating and after reading some of the nonsense lately, I thought it was time for some clear and cogent information.
Soldering:
This is a process where metals are joined, not for mechanical strength but mainly to promote an effective seal. The solder forms an amalgam (Def. An homogeneous mixture or solid solution of two or more metals, the atoms of one replacing or occupying interstitial positions between the atoms of the other) and becomes an alloy of the solder filler and parent metal. The bonding effect is thus molecular.
Mechanical strength of solder is low: solder mixtures are designed for low melting point, not strength. (Excepting “Hard” solders and certain specialised solders – see below - such as Aluminium Solders, Stainless Steel Solders and etc. which are outside the cope of this item).
Copper plumbing pipe, correctly soldered, will withstand significant pressure, far above that experienced in even French mains water head pressure systems. Typically. 200-300 PSI/13.78-20.68 Bar. Which is far beyond the point of Failure of the pipe itself.
Brazing:
Brazed joints using Copper-Phosphorus or Silver-Copper Phosphorus filler rods provide far greater mechanical strength, since the tensile strength of the filler rod – which is the critical aspect – is far higher than that of the parent metal.
Most brazing, per se is carried out using Silicon-Bronze filler rods, or exceptionally Manganese-Bronze filler rod. MB rod goes in at a much lower temperature and does not cause excessive changes to the crystalline structure at the edge of the joint: always a weak point for all brazed and welded metal joins.
Copper cannot be successfully brazed with Silicon-Bronze filler rod.
Swaging copper pipe to create a join whilst easy, promotes a weak point at the neck of the swage, since the metal here has been stretched, thus introducing strain lines in the molecular structure: additionally as the metal has been stretched, it is thinner and thus weaker than was originally.
One benefit for brazed copper pipe joints is their resistance to breakage under abnormal pressures: when frozen for example. However for all practical purposes, the application of brazed joints requires Oxy-Acetylene or Oxy-Propane or Oxy-MAPP equipment, able to promote and maintain far higher temperatures than single gas air torches. Normal torches (or blowlamps if you like) gain their oxygen for the combustion process from the surrounding air: a useful free draft effect is gained by their construction. Mixed gas torches combine the more effective combustion possible from using oxygen, with the pressure force effect of the oxygen gas being energetically pushed through the mixing chamber by pressure; typically 5-6 PSI/0.344-0.41Bar upwards.
General:
Solder is normally described as "Hard" and "Soft". This descriptor normally denotes the melting point of the solder itself. Hard solders contain silver, in varying proportions; this is normally called Silver Solder and is commonly used where greater mechanical strength is required: the melting point of Silver Solder is far higher and requires better equipment than a simple single gas powered torch.
Silver solders are used for a variety of demanding applications, which include the jewellery trade, medical applications and joining Carbon Tungsten tool tips to lathe and milling tooling. They are also used for extremely high pressure hydraulics assembly, for controls systems in (e.g.) aircraft, where a robust joint is essential capable of resisting thousands of pounds per square inch, without compromising the innate strength of pipe work.
Lead free soft solders (Typically 90% + Tin - <8% Silver - <2% Copper), are now mainly specified for potable water installations.
In any metal jointing process, cleanliness is a paramount key to a strong and pressure tight joint.
Soft metals (typically Copper, aluminium etc), oxidise from bright, immediately on exposure to air. If the surface of a metal is even slightly oxidised, then an amalgam joint will be faulty.
Soldering fitted joints relies on the capillary effect of all fluids. Once the solder is molten it “Creeps” into the joint. A good bright joint evidences completion by the tiny ring of solder, which appears on the end of the fitting. Adding excess solder past this point is not only useless, it carries the risk of over-heating the work and destroying the amalgam and thus the bond. It is also unsightly.
The correct temperature of the work is reached when the solder melts easily and runs controllably, into the joint. If the solder melts instantly and drops off the work, then the work is far too hot and will compromise the integrity of the joint.
Normally, sufficient heat can be carried by conduction into the pipes by simply heating the fitting. Over-heating is the classic mistake of the beginner. It tends to burn the flux (Oxidation) and this creates slag which makes a good solder bond to the parent metals all but impossible. The joint may look alright, but invariably is "Dry" and patchy.
A good solder joint should be bright and shiny.
FLUXES:
These serve two disparate purposes. The first and simplest is the exclusion of air from the joint whilst being heated.
Heating soft metals causes a far greater rate of free oxidation. The chemical combustion of the fuel gas/gases also adds further impurities to the surface of the metal.
Years ago, the simplest flux used for electronic soldering was Rosin: ideal if you were also a violinist or cellist! (It is still used on certain demanding work, such as steel ropes used in (e.g.) mines for lifting skips and buckets)
Joints were cleaned, prior to fluxing, using Baker's Fluid (A proprietary cleaner) also known as Killed Spirit: Hydrochloric Acid with Zinc dissolved into the acid to neutralise it. It become Zinc Chloride.
The second use of flux.
A modern soldering flux paste cleans, excludes air (it changes its chemical composition on the application of heat and forms a harder air-resistant coating), and facilitates the amalgam process additionally, rather as a catalyst speeds and enables a chemical reaction.
To Clean Or Not To Clean?
I am most unhappy with the suggestion that pipe work and joints need no cleaning.
In manufacture a number of processes are used to produce continuous seamless copper tube: same with all fittings. Inevitably, these employ various chemicals.
Additionally, foreign matter often finds its way inside tubes and fittings.
Copper is a very energetic element and will ally with a vast range of chemical compounds and other elements, and in doing so, produces a raft of salts: sulphates, sulphites, chlorides, chlorates etc.
Personally, I would ALWAYS clean outside and inside mating surfaces, in order to avoid unwanted products of the chemical reaction between the copper tube and fittings and the flux being carried, unseen, inside the joint by (normally) the capillary action of the solder filler.
Such foreign matter in any metal and heat process be it smelting, foundry, welding, brazing and soldering is generically called “Slag”.
Slag creates weakness in all metal jointing processes and invariably causes failures: pinholes in solder are very difficult to see with the naked eye: but they make themselves felt when you turn on the water and a tiny jet blows water into a small fountain!
Perhaps worse is the joint with included slag, which suddenly fails a year or two later when you are not watching!
As with so many craft skills, people who do this every day develop an “Eye”: with solder, one’s eye tells you precisely when stick filler is about to run, if for example, working on soldering zinc guttering: or joining lead pipe to lead pipe: or filling car bodywork. There is a tiny margin between solder being workable and dropping on your toes! And this is where the eye comes in.
Essential for “Wiping” lead joints. Too cold and it is not a cogent joint: too hot and it’s all over your boots!
However, here we are dealing with mainly amateurs: skilled amateurs many of us, who are turning our hands to a wide range of artisan’s jobs.
Therefore clean, ideally with OO grade wire wool. Then flux: then heat: and then join.
Finally, and this is important, after the joint has cooled a little, wipe off ALL excess and visible flux with a clean damp rag.
Flux is highly corrosive, for weeks after the pipe work is installed and forgotten.
If necessary, because you have been over-enthusiastic with the flux, clean the cold surface with clean wire wool, which has been wetted slightly: and wipe down with a clean damp rag afterwards.
Don’t forget to also run a reasonable quantity of water through the finished work and straight into the drain, to wash out flux in side the pipes. Be even more thorough with tap feeds.
Happy Plumbing!
"Yes, but that apart, Mrs Lincoln, did you enjoy the play?"
Gluestick
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