Brazing Gases for Torch Brazing

There are about four basic gases for torch brazing.  They are generally used in combination with oxygen.   

• Acetylene (C2H2)
• Propane (C3H8)
• Natural Gas, Methane (CH4)
• MAPP Gass, Liquefied Petroleum gas with Methylacetylene - Propadiene (C3H4)
  

Heating Value is defined as the amount of energy released when a fuel is burned completely in a steady-flow process and the products are returned to the state of the reactants. 

Burning Velocity: The speed of a two-dimensional flame front normal to its surface and relative to the unburned gaseous-and-fuel oxidizer mixture. 

There are many definitions for these terms.  A Google search will give others.   None of them really seem to conflict but, instead, offer different viewpoints.   

Cost Comparisons (2000 approximate figures)

Figures supplied by Gas Company

Acetylene is a manufactured gas.  Propane, Natural Gas and PAPP gas all start with gas from oil wells.  MAPP has an additive.  This affects the cost of the gas. 

Acetylene is the fastest but it is the most unstable and requires the most careful handling.  Natural gas can be the cheapest.  It is often available from a gas line.  The natural gas flame is more diffuse and less intense than acetylene.  It also has a lower temperature.  

Natural gas is considerably cheaper per cubic meter but you need much more gas and oxygen than you do with acetylene to do the same job. 

Propane is roughly equivalent to natural gas.  It is more expensive and makes a cooler flame.  Propane is usually sold in tanks where natural gas is not available. 

MAPP Gas is a natural gas with an additive.  It is generally preferred over propane in small gas/air torches because it produces more heat, faster.

Comments

Mapp gas heats more slowly than acetylene, which makes it easier to control.  It generally requires about twice as much oxygen for a neutral flame. 

Acetylene is easier to use and adjust. 

Torch considerations

A single stage regulator does not work as well as a two-stage regulator but it is a generally close enough.  The big difference will occur when you are cutting with a lot of oxygen or when the oxygen tank is getting very low.  As the oxygen pressure goes down the delivery pressure from a stem type regulator tends to go up.  In a nozzle type single-stage regulator the delivery pressure tends to go down as the oxygen pressure drops. 

A major problem is failure to open the tank valves all the way.  Regulate the gas with the regulator.  Do not try to regulate it with the tank valve. 

As the oxygen in your tank goes down, the flame will change from a neutral flame to a carburizing flame, which will add carbon and remove oxygen.  

Burning temperatures 

Fahrenheit

 Celsius 

 Kelvin 

Carbon subnitride / O2

9,032

5,000

5,300

Cyanogen / O2

8,672

4,800

5,100

Hydrogen / F2

7,232

4,000

4,300

Aluminium / O2

7,592

6,872

3,800

Acetylene / O2

5,684

3,140

3,413

MAPP / O2

5,301

2,977

3,250

n-Butane / O2             

5,252

2,900

3,173

MPS / O2

5,200

2,870

3,143

Propylene / O2

5,200

2,870

3,143

Propane / O2

5,162

2,850

3,123

Methane / O2

4,856

2,680

2,953

Hydrogen / O2

4,820

2,660

2,933

CO / O2   

4,712

2,600

2,873

Acetylene / Air

4,352

2,400

2,673

Cyclopropane / Air   

4,190

2,310

2,583

Decane / Air  

4,109

2,265

2,538

Hydrogen / Air

3,713

2,045

2,318

CO / Air  

3,560

1,960

2,233

Propane / Air 

3,497

1,925

2,198

n-Butane / Air    

3,450

1,895

2,168

Methane / Air 

3,407

1,875

2,148

Brazing with Hydrogen

Letter from Jerry Welch:

Dear Tom,

I took the privilege to cross out acetylene and the kind of flux on your "causes of brazing problems". We have had success in doing it the way listed below for the last 10 years.

Acetylene: it is a very dirty burning gas that discolors the plate when brazing on the teeth. 

Use" hydrogen: it burns very clean and very hot. You can regulate the heat that you need, depending on the size of the tooth being brazed.(see your supplier because you will have to have another gauge for the use of Hydrogen). 

Kind of flux; take the guessing out. Use  high temperature brazing flux "black".  It is the highest temperature flux available and very affordable with the results that you will have. 

Why do it this way? Because in the last year we have put on over 10,000 teeth with not one coming off due to poor Brazing.  In brazing this many teeth 60% were up to 1.250 wide. 

We have been sharpening since 1973, but began using pretinned teeth for the last 10 years. In the preceding years. We have encountered some difficulties which were corrected by the use of hydrogen and hi-temp black flux. 

Your temperature scale of 12 different ways of determining temperature is a good guide, but only time and experience will tell you how to mix your oxygen & hydrogen for the application that you are doing.  

Jerry Welch

Jerry's Saw & Tool Sharpening Service

Sylvania, Ohio 43560

Other Data

Hydrogen diffuses more rapidly than any other gas. It will even diffuse through metals, especially those that adsorb it, as well as through quartz, and probably also glass. It will search out the smallest leak, especially in valves and similar devices, which must be carefully designed to hold hydrogen. This property may well result in occasional surprises for hydrogen users. 

Hydrogen can be burned in a torch with air or oxygen. An air-hydrogen torch flame reaches 2045°C, while an oxyhydrogen flame reaches 2660°C. Flame temperatures are subject to considerable uncertainty, and depend on the mixture used. The hydrogen flame contains no carbon, and so is invisible. A mixture of 45% 1025 Btu/cuft natural gas and oxygen produces a flame temperature of 2930°C, it is reported. Oxyhydrogen flames playing on CaO refractories produced the "limelight" used in theatrical productions before electrical lighting. 

Comparison of Gases

Hydrogen 

This is by far the cleanest gas you can use. Hydrogen is a simple element, with no bound-up carbon at all. The byproducts of burning hydrogen are heat and water--hence the clean burn, a very practical advantage. Hydrogen is also the only gas recommended for melting platinum alloys. All the other flammable gases (compounds all not elements) provide hydrogen with another attached element, such as carbon. 

Hydrogen is a high heat gas and very, very explosive. (High heat and explosive potential always come together.) Hydrogen is extremely light, so it rises away from you quickly. The gas molecule is so small it will flow through almost any opening in the ceiling or roof material, which usually allows it to disperse harmlessly. If hydrogen becomes trapped, however, it is extremely hazardous, so adequate ventilation is essential. You'll use far less oxygen to boost the heat with hydrogen than with other gases. Less oxygen is universally a plus whenmelting precious metals, since fewer oxides will form.  

Hydrogen is the Houdini of elements. As soon as you’ve gotten it into a container, it wants to get out, and since it’s the lightest of all gases, it takes a lot of effort to keep it from escaping. Storage devices need a complex set of seals, gaskets, and valves. Liquid hydrogen tanks for vehicles boil off at 3-4% per day. 

Hydrogen also tends to make metal brittle. Embrittled metal can create leaks. In a pipeline, it can cause cracking or fissuring, which can result in potentially catastrophic failure. Making metal strong enough to withstand hydrogen adds weight and cost.  

Leaks also become more likely as the pressure grows higher. It can leak from un-welded connections, fuel lines, and non-metal seals such as gaskets, O-rings, pipe thread compounds, and packings. A heavy-duty fuel cell engine may have thousands of seals. Hydrogen has the lowest ignition point of any fuel, 20 times less than gasoline. So if there’s a leak, it can be ignited by a cell phone, a storm miles away, or the static from sliding on a car seat.  

Leaks and the fires that might result are invisible, and because of they high hydrogen pressure, the fire is like a cutting torch with an invisible flame. Unless you walk into a hydrogen flame, sometimes the only way to know there’s a leak is poor performance.  

Natural gas

Sometimes called "city gas," natural gas is a mixture of several hydrocarbon family compounds, primarily methane and ethane. This gas is a bit weak on heat, but there is a natural (forgive the pun) safety advantage-the gas comes from the city pipe as needed, so there is not a tank of explosive gas sitting in the room with you. You are, however, still stuck with that tank of oxygen. In some areas such as the very cold northeast, th content of the gas you receive can vary with the season or the availability to your city source. This is the most commonly used gas in small to medium capacity shops. Natural gas is also the only gas commonly used in blow furnaces, which are found in many large silver casting houses. For large melting jobs, a forced-air blow furnace offers a great reducing atmosphere limiting the formation of oxides.

Propane

Propane is a hydrocarbon family compound commonly used in areas where natural gas is not available. This gas burns fairly cleanly but it needs plenty of oxygen to boost the heat. It tends to be a bit hard on the metal, since with higher oxygen use, oxidation becomes a proportionally greater problem. This gas is stored in a small pressurized tank in a liquid state. Regulators are simple and inexpensive, and refills are as close as the nearest gas station that sells propane to the RV and bar- becue crowd. A safety caution: Propane is extremely heavy and explosive, so be sure to ventilate properly.  See our article on Brazing with Propane. 

Butane

For our purposes here, butane is the same as propane, although refills are harder to find than propane. I run across this gas very rarely. 

Acetylene

This gas is the dirtiest gas. Acetylene offers lots of heat--and lots of carbon soot as well, making it very hard on your gold. In addition, acetylene can react with copper and silver alloys, making it less than ideal for use in jewelry manufacturing. Acetylene tends to stay in pockets rather than dissipate, and as with all the flammable gases, acetylene can explode if it builds up. It is also shock sensitive, so cylinders must be handled with extreme care. Acetylene is one of the least expensive gases to refill.