• SkillCat Team

How to Braze a Joint in 7 Steps

Updated: Jun 23

Brazing and Soldering: Chapter 4


Brazing Joint


In this module we will learn about steps to carry out brazing process. We will also learn about heating methods for brazing. Skip to quiz!


Brazing Set Up


Recall that brazing is a metal-joining technique used to join both similar metals and different types of metals. Much like welding, brazing produces a strong joint.


Unlike welding, brazing is carried out at much lower temperature. The base metals are heated to about 840 °F in brazing. This makes them suitable for metals that cannot handle higher heat and temperatures of joining.


Brazing is less expensive than other metal joining processes. We often use brazing to join parts manufactured as a single unit. For example, copper tubings in evaporator and condenser coil are brazed.


Recall that a brazing torch consists of the following main components:

  • Cylinder,

  • Regulator,

  • Hose, and

  • Torch

For producing flame at the torch tip, we need to first set up the torch. To set up a torch, start by opening the cylinder valves by turning it counter-clockwise by 1.5 turns. This will allow us to quickly close it in case of emergency.


Recall that regulator is a knob on the cylinder that adjusts the pressure of the fuel gas. We need to adjust the pressure of oxygen and other fuel gas according to manufacturer’s recommendations for the torch tip.


The torch handle has two knobs on it, one for oxygen and one for fuel gas. The fuel gas and oxygen from different cylinders is sent to torch tip by adjusting the knob. This is just like the knob used to adjust a gas stove.


First we need to open fuel gas knob on torch handle. The fuel gas at the torch tip is then ignited using torch lighter. This is similar to lighter for gas stove.


Think about a gas stove burner we use for cooking. Sometimes the burner flames become uneven and yellow in color. This won’t cook the food properly so we adjust the flame.


Similarly, when we light up the torch, it also produces smoky yellow flame. We need to ensure that it is neutral, meaning even and blue in color. The flame should burn the fuel completely and not produce any chemical effect on metal to be brazed.


A neutral flame is a even flame having two cones. The inner cone has white color and temperature about 3100℃. The outer cone is blue in color and has a temperature about 1275℃.


Recall that oxygen and other settling gas such as acetylene or MAPP gas torch is preferred for brazing. Neutral flame has equal amount of oxygen and other fuel gas fuel that helps the flame to burn completely.


A neutral flame can be produced by adjusting the supply of oxygen. When we bring in oxygen, the yellow flame turns into blue. We need to keep adjusting amount of fuel and oxygen to get a proper sharp pointed flame. Let us look at the video that shows how to get neutral flame.


As seen in the video, a star pattern of approximately 2 to 2.5 inches ensures a neutral flame. We need to keep adjusting the knobs on torch handle to achieve this.



Brazing the Joint


Recall that a joint is the gap formed between two pieces of the base metal when we push them together to join. Recall that in brazing, the base metals are heated to about 840 °F. Neutral flame is used to heat them quickly and evenly.


While heating the metals, we should run nitrogen through the system. This is because oxygen present in the atmosphere react with metal at high temperature. Reaction of metals with oxygen forms oxide impurities on the surface of the metal and inside the pipes.


Recall that refrigerant lines used in HVAC system are made of copper tubing. Such impurities can be formed inside copper tubings while brazing them. This contaminants can restrict the flow through refrigerant lines.


Nitrogen carries impurities along with it and escapes from other end of pipe. Nitrogen gas is used because it is dry, non reactive, and non-combustible. It will not react with impurities and does not catch fire.


Recall that filler metal melts and fills the narrow spaces of joint during brazing. The strength of joint depends on the capillary action of filler metal. Capillary action is the ability of liquid to flow in a narrow space.


Capillary action draws molten filler metal through the gap between the metals to be joined.

This is just like when we touch the paper towel with water. The paper towel soaks the water and the water rises throughout the towel.


A proper capillary action ensures good fit between the joint. Similarly, a proper clearance is necessary to create strong joint. Clearance is the radial distance between the joints to be brazed. The strongest joint is achieved when the joint clearance is 0.0015”.


When the clearance is narrower than 0.0015”, capillary action is reduced. If the gap is wider than necessary, filler metal may fail to fill the joint completely. This will reduce the joint strength. Let us look at the video that shows capillary action.


Capillary action depends on wetting property of the surface. Wetting is the ability of a liquid to maintain contact with a solid surface. A clean surface is required to maintain the contact of liquid.


If the surface is dirty, the capillary action does not work properly. So, we need to clean the surface to avoid disruption of filler metal during brazing. Next video shows how capillary action gets affected due to dirty surface.


Once the metal is cleaned and the torch is set up, it is time to braze the joint. We need to ensure that the base metal is heated uniformly. Improper heating ruins capillary action and filler metal piles up outside the joint.


To ensure proper capillary action, first heat the base metal to be joined. Then apply heat to the joint. Keep heating the base metal and the joint by constantly moving the torch until it reaches brazing temperature.


Recall that we need to apply filler metal when we reach brazing temperature. Always apply filler metal to the opposite side of joint from flame. If the flame is on left side keep filler metal on right side. Capillary action takes place and joint is formed.


Brazing is used to join both similar metals and different types of metals. The strength of joint depends on the capillary action of filler metal. A proper clearance is required for good capillary action.


A neutral flame is used for brazing to heat the base metals. We need to ensure that the joint is heated uniformly. In the next module, let us look at different heating methods for brazing.




Heating Methods


In this module we will learn about heating methods for brazing. We will also discuss their applications in brief. Skip to quiz!


Heating the Joint


We can use several methods to heat the base metals to brazing temperature. They are as follows:

  • Torch brazing,

  • Furnace or Vacuum brazing,

  • Induction brazing, and

  • Resistance brazing

Torch brazing is carried out using a gas torch. Recall that the torch is used to produce a flame. This flame is used for heating the base metals.


Recall that we need to manually ensure that brazing temperature is reached in torch brazing. In furnace brazing, a constant brazing temperature is maintained using control systems.This is similar to how thermostat controls temperature of air conditioner.


Furnace brazing is also called vacuum brazing. Recall that metal reacts with air while brazing. In vacuum brazing, no air is present inside the furnace. This prevents oxidation and rust formation.


Furnace brazing is best suited for high and medium production lines. We have to apply filler metal to the base metal before the parts are sent in the furnace. The filler metal applied to base metal melts due to furnace temperature and joint is formed.


In this method, we use induction heater to heat the base metal. Think about an induction stove we use for cooking. It works on electricity to heat the cooking pan. Similarly, induction brazing uses electricity to produce heat in the base metals.


Induction heating is flame-free and contact-less method of heating the base metal. It helps to heat the base metal uniformly that improves the capillary action required for strong joint.


The advantages of induction brazing makes them suitable for commercial HVAC systems. They are used for brazing coils of evaporator, condenser, and heat exchangers.


Think about an electric oven. It has a coil that is in contact with the food plate. As electricity flows through the coil, the resistance to the flow of electricity generates heat. Similarly, in case of resistance brazing, brazing electrodes are used to heat the base metals.


As the electrode has low resistance, it allows the electricity to flow through the electrode. When this electrode is connected to base metal, it resists the flow of electricity. This resistance will generate heat. Electrodes are then pressed on base metals to fuse them.


We use several heating methods for heating base metals.

  • Torch brazing,

  • Furnace or Vacuum brazing,

  • Induction brazing, and

  • Resistance brazing

Torch brazing produces flame using gas fuel.


Furnace brazing, also called vacuum brazing, prevents oxidation of base metals while brazing.

Induction heating is flame-free and contact-less method of heating the base metal. In resistance brazing, resistance by the base metals to flow of electricity generates heat.




Checking the Joint


In this module, we will learn about how to check pipe joints for leaks. We will also learn about how to prevent formation of impurities during brazing. Skip to quiz!


Checking the Joint


The next step after successfully brazing the joint is checking the joint. We need to carefully inspect the joint for any leakage before it is used for application.


Recall that filler metals have flux in them. After we braze the base metals, the flux residues are left on the joint. Base metals also form oxide scales when they react with oxygen.


We need to perform post-braze joint cleaning to remove residual flux and oxide scales. This step is crucial because we cannot inspect a joint that is covered with flux. Also, flux has a corrosive nature that can lead to joint failure.


Most brazing fluxes are water-soluble. So, we can remove the residues by dipping the brazed metal in hot water about 120 °F or hotter. We need to make sure that the filler metal has solidified completely before dipping in hot water.


We can also clean the residue from brazed joints with a wire brush. Acids react with base metals and remove flux without deteriorating base metals. We need to soak the base metals into acids for a short time to ensure flux is removed completely.


The oxide scales can be removed by using pickling solution. Pickling solution is a mixture of hydrochloric acid and water. We should only use the pickling solution recommended by the manufacturer of the brazing materials.


The next step after cleaning the joint is inspecting the joint. We need to examine the joint visually for any cracks and pinholes. Nitrogen leak test is carried out to check the pipe joints for leaks using the nitrogen gas.


Before starting nitrogen leak detection test, the system must be isolated. For isolating the system, turn OFF the system and close all the valves connected to the piping.


Leak checking is carried out using number of components such as:

  • Nitrogen cylinder,

  • Regulator,

  • System gauge set, and

  • Hoses

We need to fit the regulator to the nitrogen cylinder. Regulator is a knob that adjusts the pressure of the gas. Gauges are present on the regulator to verify the pressure of nitrogen that will be inserted into the piping.


We also need to fit gauges to the system that we will test. They are called system gauges. For example, these gauges are connected to inlet of compressor and outlet of evaporator. If any leak is present in the system that gauge pressure will drop down to indicate leakage.


Fit the common manifold hose of the gauge set of the system to the nitrogen regulator. Through this, the nitrogen gas is inserted to the system.


We need to test the system at the minimum allowable system pressure to avoid damage to components. Pressure limit for testing is specified by the manufacturer. The least pressure limit in HVAC system is evaporator pressure and it is 450 psi as per manufacturer recommendation.



Purging


Recall that we need to run nitrogen through the system while brazing. Purging is a process of removing impurities inside the pipe using a gas. Recall that we use nitrogen gas because it is dry, non reactive and non-combustible.


Recall that while installing an air conditioning system, the refrigerant lines from the indoor unit to the outdoor unit have to be connected. We use brazing process to connect them. If impurities are formed during brazing, they can get carried by the refrigerant through the system. This will damage the system.


Recall that the regulator is used to control pressure of gas. We need to connect the regulator to nitrogen cylinder and insert the regulator hose to one end of the metal pipe. Once the hose is connected to the pipe, we can insert nitrogen into the pipe.


Recall that we need to first close the nitrogen handle and open the regulator knob completely. This will ensure that nitrogen is not leaking from cylinder. To pressurise the system, we need to regulate the pressure using regulator. Rotate the nitrogen regulator in clockwise to get 2 psi pressure.


We need to run nitrogen through the system before starting brazing process. Allow nitrogen to escape from other end of pipe to remove impurities. We can control the amount of nitrogen flowing inside the pipe using knob on the hose.


Avoid an excessive flow rate. A high flow rate will tend to cool the tube. This will reduce brazing temperature. A poor braze joint is created at low temperature.


The flux residues that are left on the joint after brazing can be removed by hot water and acids. Nitrogen leak test is carried out to detect leak present in the pipe joint. Nitrogen purging is used to prevent impurities formed inside the tubing during brazing.




Question #1: When do you use brazing process?

  1. To heat the base metal above melting point

  2. For metals that cannot handle higher heat of other processes.

  3. When we need to join metals to non-metals

  4. For joining non-metals

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Answer: For metals that cannot handle higher heat of other processes.

Brazing is carried out at much lower temperature. This makes them suitable for metals that cannot handle higher heat and temperatures of joining.



Question #2: Fuel gas cylinder valves should never be opened more than _____ turns?

  1. 3 turns

  2. 1.5 turns

  3. 1 turn

  4. 2 turns

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Answer: 1.5 turns

Recall that cylinder valves should never be opened more than 1.5 turns so they can be quickly closed in the event of an emergency.



Question #3: What is the color of inner and outer cone?

  1. Blue and orange

  2. Orange and blue

  3. colorless

  4. White and blue

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Answer: White and blue

Recall that The inner cone has white color and temperature about 3100℃.

The outer cone is blue in color and has a temperature about 1275℃.


Question #4: What does neutral flame consists of?

  1. Equal amount of oxygen and other fuel gas

  2. 60% oxygen and 40% other fuel gas

  3. 100% oxygen

  4. 30% oxygen and 70% other fuel gas

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Answer: Equal amount of oxygen and other fuel gas

Recall that neutral flame has equal amount of oxygen and other fuel gas fuel that helps the flame to burn completely.


Question #5: A square pattern signifies it is a neutral flame.

  1. False

  2. True

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Answer: False

False!

Recall that a star pattern of approximately 2 to 2.5 inches ensures a neutral flame.


Question #6: Nitrogen gas is run through the system while brazing.

  1. False

  2. True

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Answer: True

True!

Recall that nitrogen carries impurities along with it and escapes from other end of pipe.


Question #7: The strength of a joint depends on capillary action.

  1. True

  2. False

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Answer: True

True!

Recall that the strength of joint depends on the capillary action of filler metal.

Capillary action is the ability of liquid to flow in a narrow space.


Question #8: A proper clearance is necessary for strongest joint.

  1. False

  2. True

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Answer: True

True!

Recall that The strongest joint is achieved when the joint clearance is 0.0015”.


Question #9: What happens if the proper clearance is not maintained between joints while brazing?

  1. Joint becomes more stronger

  2. Joint will fail

  3. Joint can’t be brazed

  4. Joint gets heated

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Answer: Joint will fail

Recall that when the clearance is narrower than 0.0015”, capillary action is reduced.

If the gap is wider than necessary, filler metal may fail to fill the joint completely.


Question #10: A clean surface is not required for wetting.

  1. False

  2. True

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Answer: False

False!

Recall that wetting is the ability of a liquid to maintain contact with a solid surface.

A clean surface is required to maintain the contact of liquid.


Question #11: A clean surface is required for good capillary action.

  1. False

  2. True

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Answer: True

True!

Recall that if the surface is dirty, the capillary action does not work properly.

So, we need to clean the surface to avoid disruption of filler metal during brazing.


Question #12: Furnace brazing is carried out in air for proper heating.

  1. False

  2. True

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Answer: False

False!

Recall that Furnace brazing is called vacuum brazing.

In furnace brazing, no air is present inside the furnace. This prevents oxidation and rust formation.


Question #13: In furnace brazing we have to check for brazing temperature manually.

  1. False

  2. True

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Answer: False

False!

Recall that in furnace brazing, a constant brazing temperature is maintained using control systems


Question #14: Induction heating method involves brazing base metal using a flame.

  1. False

  2. True

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Answer: False

False!

Recall that induction heating is flame-free and contact-less method of heating the base metal.


Question #15: Resistance brazing method is contact-less method of heating base metals.

  1. False

  2. True

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Answer: False

False!

Recall that when this electrode is connected to base metal, it resists the flow of electricity. This resistance will generate heat.

Electrodes are then pressed on base metals to fuse them.



Question #16: In resistance brazing, resistance is offered by filler metal to flow of electricity.

  1. False

  2. True

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Answer: False

False!

Recall that in resistance brazing resistance is offered by base metal to flow of electricity.


Question #17: What are the methods to remove soldering flux? (select all that apply)

  1. Cold water

  2. Hot water

  3. Acid solution

  4. Pickling solution

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Answer: Hot water

Acid solution

Recall that flux can be removed using hot water and acid solution


Question #18: Hot water is used for removing oxide scales.

  1. False

  2. True

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Answer: False

False!

Recall that the oxide scales can be removed by using pickling solution.

We should only use pickle solution recommended by the manufacturer of the brazing materials.


Question #19: Why regulator is installed on nitrogen cylinder?

  1. To control air inside the nitrogen

  2. To control density

  3. To control temperature

  4. To control pressure

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Answer: To control pressure

Recall that he regulator controls pressure of nitrogen.

Gauges are present on the regulator to verify the pressure of nitrogen that is going to be inserted into the piping.



Question #20: The regulator gauges can indicate temperature change.

  1. False

  2. True

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Answer: False

False!

Recall that gauges indicate pressure.

They are present on the regulator to verify the pressure of nitrogen that is going to be inserted into the piping.


Question #21: The nitrogen leak test is performed using nitrogen and oxygen mixture to find leakage.

  1. False

  2. True

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Answer: False

False!

Recall that in nitrogen leak test, only nitrogen gas is inserted to the system.

Introduction of oxygen can lead to oxide formation.


Question #22: How do you recognize the leakage during nitrogen leak test?

  1. Using temperature sensor

  2. Bubbles coming out after applying soapy water

  3. Measuring nitrogen volume

  4. Measuring nitrogen mass

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Answer: Bubbles coming out after applying soapy water

Soapy water is used to identify the leak point.

Leaks present in the material will cause gas bubbles to form at defective spots.


Question #23: Which gas is used for purging?

  1. Oxygen

  2. Nitrogen

  3. Propane

  4. Methane

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Answer: Nitrogen

Recall that Nitrogen gas is used for purging because it is dry, non reactive and non-combustible.


Question #24: What is the need of purging?

  1. To remove liquid nitrogen

  2. To remove burr

  3. To remove oxide layer and impurities

  4. To check leakage

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Answer: To remove oxide layer and impurities

The oxide layer and impurities are formed inside the pipe during brazing.

To avoid this, purging is carried out on newly laid piping.


Question #25: We need to purge the nitrogen inside the piping with higher flow rate. This will improve brazing efficiency.

  1. False

  2. True

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