MODULE 3 THE BRAZING PROCESS ISSUED BY REVISION NUMBER REVISION DATE DOCUMENT NUMBER S. GINBEY 0 5 th May 2014 QP74
Module 3 The Brazing Process Preparation of joints and materials The removal of all oxides and surface soil from the tube ends and fitting cups is crucial to proper flow of filler metal into the joint. Failure to remove them can interfere with capillary action and may lessen the strength of the joint and cause failure. The capillary space between tube and fitting is, see table below. Outside diameter Imperial pipe sizes Minimum fitting Clearance in mm (OD) mm depth 5 OD 8 1/4 6 0.05 TO 0.35 8 OD 12 3/8 7 12 OD 16 ½ AND 5/8 8 0.05 to 0.45 16 OD 25 7/8 10 25 OD 35 1.1/8, 1.3/8 12 0.05 to 0.55 35 OD 45 1.5/8 14 Filler metal fills this gap by capillary action. This spacing is critical because it determines whether there is a proper flow of the filler metal into the gap ensuring a strong joint. Lightly abrade (clean) the tube ends using sand cloth or nylon abrasive pads for a distance slightly more than the depth of the fitting cup. Clean the fitting cups by using abrasive cloth, abrasive pads, or a properly sized fitting brush. Copper is a relatively soft metal. If too much material is removed from the tube end or fitting cup, a loose fit may result in a poor joint. Chemical cleaning may be used if the tube ends and fittings are thoroughly rinsed after cleaning according to the procedure furnished by the chemical manufacturer. Do not touch the cleaned surface with bare hands or oily gloves. Skin oils, lubricating oils and grease impair the adherence of the filler metal. Copyright 2016 Tradewinds Engineering Ltd Page 2
Selection of brazing nozzles The table below gives guidance on the correct size of nozzle to use for specific sized refrigeration grade copper. The table is intended to be guidance only and many external factors may affect the size of nozzle selected. Examples of these factors could be oxy/acetylene pressures, brazing or silver soldering, type of base material and even the ambient conditions. Attention should be given to repairs on condenser or evaporator coils. The copper tubing on these coils is very thin and a size 2 or 3 nozzle should be sufficient to make the repair. Selection of brazing rods Various types of solder are available from your wholesalers so it s important to make sure you ask for the right one to suit the job. Choose the wrong type of rod and the brazing process can become very difficult. Copper to copper Copper Phosphorous CuPh or 2-6% silver solder rods. (Note: CuPh rods contain approx. 92% Copper and 7% phosphorous material) Copper to steel or dissimilar metal 35% to 45% silver solder with a flux This material usually melts at 604 C and flows at 618 C. Copyright 2016 Tradewinds Engineering Ltd Page 3
Use of brazing fluxes Try to prevent brazing fluxes from contacting skin. Occasional contact is not dangerous, but all flux should be thoroughly washed off before eating or smoking. Cuts or breaks in the skin must be properly covered with a dressing. Flux, especially if it contains fluorides or chlorides, can delay the healing of wound. Fluxes produce fumes when heated, especially above the temperatures given as their maximum. Ventilate with fans or exhaust hoods to carry fumes away from workers or equipment operators with air-supplied respirators. Consult manufacturers Material Safety Data Sheets for specific health and safety procedures connected with flux use. Caution: Some fluxes may contain cadmium which is toxic when burned. You should where a breathing apparatus to prevent the inhalation of any fumes. Never use a flux that contains Cadmium if you are brazing pipework to be used near food or drink. Check the location You should always check your surroundings prior to the commencement of any brazing. Please consider the following: a) Suitable emergency exits, b) Fire extinguishers (2kg dry powder minimum for solids, liquid, gas and electrical type fires) c) Telephones and first aid facilities should be ascertained before starting to work. d) Area needs to be clear of any combustible materials. e) Remove none essential materials. f) Remove none essential people. g) Cordon off area. h) Post signs. i) All local safety requirements must be checked and met such as work / hot work permits. j) Hot works permit. Copyright 2016 Tradewinds Engineering Ltd Page 4
Scale prevention When brazing refrigeration pipework we must always purge the system with Nitrogen (Oxygen Free Nitrogen) which is supplied in a grey cylinder with a black shoulder. Without it carbon deposits are left on the pipe surface. These carbon deposits can then be stripped off the surface of the pipe when the refrigerant is circulated and it then travels with the refrigerant. As the refrigerant circulates around the system, the carbon deposits can begin to clog the filter driers and other small components such as the orifice in the expansion device or capillary tube. It can also contaminate the compressor oil and make it become acidic. This is likely to result in operational defects and the equipment warranty will become null and void. Using nitrogen during the brazing process Note the scale build up when not purging with nitrogen Copyright 2016 Tradewinds Engineering Ltd Page 5
Setting up Equipment 1. Stand both cylinders in a vertical position. Note: Oxygen cylinders are painted black. Acetylene are painted maroon. 2. Ensure joining surfaces in cylinder valve ports and regulators are free from oil and grease. 3. Open the valve on the oxygen cylinder momentarily in order to purge the cylinder valve port, dislodging dirt or obstructions, close valve. 4. Open a fuel gas cylinder valve as in item 3. 5. Check pressure rating on regulator is suitable for cylinder in use. Screw the oxygen regulator in to the oxygen cylinder valve port. The cylinder valve port, regulator inlet stem and the regulator outlet connection all have a right hand screw thread. 6. Screw the fuel gas regulator in to the gas cylinder valve. The cylinder valve port, regulator inlet and the regulator outlet have left hand screw thread. 7. Tighten the regulator in to the cylinder valve port - Do not use excessive force, but make certain that the joints are gas tight. 8. Connect the hose to the screwed outlets of the regulator by means of screw connections secured in the ends of the hose. 9. Connect the other ends of the hose that fitted with a hose check valve; to the blow pipe, the fuel gas hose to the left hand connections, and the oxygen hose to the right hand connection. Keep the blow pipe control valves closed. 10. Fit the appropriate sized nozzle to the blow pipe. 11. With the regulator spindle fully back seated, slowly open the oxygen cylinder valve by means of the cylinder key. Do not open suddenly or there may be serious damage to the regulator and the possibility of an accident. Open the cylinder valve spindle one turn only. 12. Open the appropriate valve on the blowpipe. 13. Slowly wind in the regulator spindle until the correct pressure is achieved. Usually 0.3-0.5Bar for each gas type but you should always refer to manufacturer s literature for confirmation. When the correct setting has been achieved, close the valve on the blowpipe. Not only have you set the pressure correctly but you have also purged the hoses prior to lighting. 14. Repeat steps 11 13 for the fuel gas. Copyright 2016 Tradewinds Engineering Ltd Page 6
15. You should now have leak checked all fittings with a proprietary spray. If any leaks are found, shut down the equipment and repair the leak. 16. The equipment is now ready for use. Lighting up procedure 17. Open the fuel gas control valve on the blowpipe and light the gas using a spark igniter which is held at right angles to the nozzle. 18. Reduce or increase the acetylene supply to the blow pipe valve until the flame just ceases to smoke. 19. Slowly turn on the oxygen by the blow pipe control valve until the white inner cone in the flame is sharply defined with the merest trace of an acetylene haze. The blow pipe is now correctly adjusted for welding. Closing down procedure 20. Turn off the acetylene first by the blow pipe control valve and then the oxygen. Close the cylinder valve. Open the blow pipe valves one at a time to release the pressure in the hose, i.e. open the oxygen valve and close it; open the fuel gas valve and close it. Unscrew the pressure regulating screws on the oxygen and acetylene regulators. It is most important to emphasise the earlier instructions, that prior to re-lighting the welding blow pipe the hoses must be purged to ensure a pure and adequate supply of oxygen/fuel gas. Back-fires may occur by one of a combination of circumstances, e.g. defective equipment, incorrect gas pressures, incorrect lighting-up procedure or careless handling of the blow pipe in use, such as permitting the nozzle to touch the work, overheating the tip of the nozzle, or working with a loose nozzle. Usually the back-fire is arrested at the check valve or the source where the gases are mixed, e.g. the head of the cutting blowpipe, and if prompt action is taken in turning off first the oxygen, then the blowpipe may be re-lit as soon as the cause of the trouble has been eliminated. Copyright 2016 Tradewinds Engineering Ltd Page 7
In some cases, however, a back-fire may pass beyond the torch and go back into either the oxygen or the fuel gas hoses; it is then termed a flash-back and its effect is more serious in that it may result in immediate damage to hoses and regulators. In extreme cases there is also a possibility of injury to the operator. The outward signs of flash-back my be squealing or hissing noise, sparks coming out of the nozzle; heavy black smoke; or the blowpipe handle may get hot. If the flame burns back far enough it may even burst through the hose. Both blow-backs and flashbacks can be avoided by adherence to recommended procedure in the case of equipment. Investigation shows that such occurrences often occur purely through overfamiliarity leading eventually to neglect of ordinary safeguards. For example, the blowpipe settings, or a light being applied before the flow of fuel gas is properly established. If the flame snaps out when the blow pipe is in use it is because: A. The regulator pressure, and/or gas flow, are incorrect - either too high or too low. B. The nozzle has been obstructed. C. The nozzle is held too close to the work. D. The nozzle has become overheated. When this happens completely shut both the blowpipe valves, check the regulator setting, cylinder pressures, and re-light in accordance with the procedure. In the case of D, close the acetylene valve, reduce oxygen flow to a trickle, and plunge the nozzle and head into cold water. Flame settings and their effects An oxidising flame has a smaller inner cone that is sharply defined and is pale blue. The welder observes this while adjusting the fuel and oxygen valves on the torch to get the correct balance for the job at hand. There is also a difference in the noise the flame makes. Adjusting the flame is not a hard thing to do after a little experience and practice. The size of the flame can be adjusted to a limited extent by the valves on the torch and by the regulator settings, but in the main it depends on the size of the orifice in the tip. In fact, the tip should be chosen first according to the job at hand, and then the regulators set accordingly. A neutral flame has a well-defined inner cone. A reducing flame has a feathery inner cone. An acetylene flame (as is characteristic of most fuel/oxygen flames) has two parts; the light blue to white coloured inner cone and the blue coloured outer cone. The inner cone is where the acetylene and the oxygen combine. The tip of this inner cone is the hottest part of the flame. In the inner cone the acetylene breaks down and partly burns to hydrogen and carbon monoxide, which in the outer cone combine with more oxygen from the surrounding air and burn. Copyright 2016 Tradewinds Engineering Ltd Page 8
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Overheating of base materials Overheating can occur if the following technique is not followed. It involves heating the joint to brazing temperature and flowing the filler metal through the joint. Both metals in the assembly should be heated as uniformly as possible so they reach brazing temperature at the same time which creates a cherry red colour. Therefore, when joining a thick section to a thin section, more heat should be applied to the thick section. Or, when joining a good conductor of heat to a poor conductor, such as copper to stainless steel, more heat will have to be applied to the good conductor (in this case, the copper) simply because they dissipate the heat more rapidly. If the colour of the material changes from red to orange too much heat is being applied to one area and not evenly, therefore allowing the material to overheat and melt, distorting the shape. Also carelessness of technique by means of holding the nozzle tip to close to the material giving a concentrated area too much heat will result in unnecessary hole blowing, again rendering the material useless. REMEMBER: Always purge with Oxygen Free Nitrogen (OFN) when brazing. Make sure you fill the whole socket Summary of brazing techniques Always follow the Safe Working Practices outlined by the HSE Assess and eliminate all risks prior to starting work Especially Phosgene & Fire risks Ensure correct selection of equipment and in good condition Purge pipe work with Oxygen Free Nitrogen (OFN) Choose correct type of solder CuPH or 6% Ag (Copper to copper) 40% Ag (Dissimilar metals) Ensure 80% - 100% infill in swaged socket Remove flux if using 40%Ag Corrosive and causes leaks ALWAYS purge with OFN and BEWARE of PHOSGENE! The most important thing we need to understand is that under no circumstances should anyone attempt to braze unless they are suitably trained. There are simply too many risks involved and unless you are competent, you are putting yourself, others and the equipment in danger. Copyright 2016 Tradewinds Engineering Ltd Page 10