Figure 3-28 Shows flame adjustments from carburizing to a neutral flame
Flame Temperatures
The tip of the inner cone is the hottest part of the flame. The inner cone is where the optimum mixture of oxygen and acetylene burn. The outer envelope where any unburned acetylene burns with oxygen from the atmosphere. A neutral flame is when enough oxygen is present in the flame to be burning all of the acetylene gas and is used for most welding processes. See Figure 3-29.
Figure 3-29 Graph of an oxyacetylene flame temperature profile
TOOL TIP
Lighting a Multi-Flame (Rosebud) Tip
This type of tip produces a large flame for heating metal prior to welding, bending, or brazing. When using a multi-flame tip you first set the acetylene pressure at or just below 15psi (1 bar) and the oxygen pressure at 30psi (2 bar); open the acetylene torch valve far enough to light the acetylene flame. Once the flame is ignited, open the acetylene valve until you have full flow of gas; now you can open the oxygen torch valve and adjust the flame to slightly carburizing. You may now use the multi-flame (rosebud) to heat materials but keep the sharp inner cone flame away from the material and only touch the carburizing flame to the material being heated. A heat sensing device such as a pyrometer or temperature sensing stick can be applied to the material to indicate the temperature of the material being heated.
Figure 3-30 A rosebud tip is often used to preheat materials prior to welding
Photo courtesy of Hobart Welders
How is the equipment shut down?
First turn off the oxygen and then the acetylene with the torch handle valves. Turning off the acetylene first can cause a flashback.
Turn off the oxygen and acetylene cylinder valves at the upstream side of the regulators.
Separately, open and reclose the oxygen and acetylene valves on the torch handle to bleed the remaining gas in the hoses and regulator into the atmosphere. Verify that both the high-pressure and low-pressure gauges on both regulators indicate zero.
Unscrew the regulator pressure adjustment screws on both cylinders in preparation for the next use of the equipment. The regulator screws should be loose but not about to fall from their threads.
What are the joint preparations for welding?
Refer to Figure 3-31.
Figure 3-31 Preparation for OAW butt welds
For butt welds, here are examples of a correct weld, poor penetration weld, excessive reinforcement, undercutting, and excessive root reinforcement. See Figure 3-32.
Figure 3-32 Correct and defective butt weld profiles
Producing a good weld bead is a combination of four main factors: the distance between the torch tip and the work, the angle at which you hold the torch, your speed when moving the torch along the weld area, and the heat produced by the torch. Getting everything right takes practice, so always test your technique on scrap metal first.
Figure 3-33 Create a puddle at the start of the bead. Keep the tip steady until a puddle begins to form. Begin making a circular motion with the torch, slowly moving the torch tip in the direction of the bead. Keep the distance to the work and the speed of your movements consistent.
Figure 3-34 Now try it with a filler rod. The goal is to intermittently dip the end of the rod into the puddle to add material to the weld. Dip the end of the rod into the puddle while making a circular motion with the torch. This will help blend the filler material into the weld. Withdraw the rod from the puddle, but keep it close to the end of the torch to keep it preheated. Don’t directly heat the end of the rod with the torch.
Figure 3-35 The finished bead should be even throughout its length. The ripples created by the circular motion of the torch tip should be consistent.
This process is commonly known as “stick welding” or “arc welding,” but it is officially labeled as “shielded metal arc welding” (SMAW) by the American Welding Society. In this process, an electric circuit is established between the welding power supply, the electrode, the welding arc, the work, the work connection, and back to the welding supply. Electrons flowing through the gap between the electrode and the work produce an arc that provides the heat to melt both the electrode metal and the base metal. Temperatures within the arc exceed 6,000°F (3,300°C). The arc heats both the electrode and the work beneath it. Tiny globules of metal form at the tip of the electrode and transfer to the molten weld pool on the work. As the electrode moves away from the molten pool, the molten mixture of electrode and base metals solidifies and the weld is complete.
HOW DOES THE PROCESS WORK?
WHEN CAN SMAW BE USED?
WHAT IS THE BASIC EQUIPMENT SETUP FOR SMAW?
WHAT ARE THE FUNCTIONS OF SMAW ELECTRODES AND HOW ARE THEY SELECTED?
HOW IS SMAW EQUIPMENT SET UP?
STICK WELDING TIPS FROM WEST COAST CUSTOMS