Welding,Filler Metal,Rods and Fluxes

a)Filler metal:
-The material which is added in the weld pool to assist in filling the gap (or grooves).It forms an integrated part of the weld.
-Filler metal is usually available in stick or in a rod form.This rod is called filler metal.
-Filler rod have the same or nearly the same chemical composition as the base metal.
-Welding filler rods are available in a variety of composition(for welding different materials) and sizes.
Some of them are given in table below.

b)Fluxes:-
-During welding if the metal is heated/melted in air,
oxygen from the air combines with the metal to form oxides
which results in poor quality,low strength welds or in some cases may even make welding impossible. In order to avoid this difficulty, a flux is employed during welding.
-A flux is material used to prevent dissolve or facilitate
removed of oxides and other undesirable substance. A flux prevents the oxidation of molten metal.
-A flux is fusible and non-metallic
-During welding flux chemically reacts with the oxides and a slag is formed that floats and covers the top of the molten puddle of metal and thus helps keep out atmospheric oxygen and other gases.
-Fluxes are available as powders, pastes or liquids.
-Flux may be used either by applying  directly on to the surface of the base metal to be welded or by dipping the heated end of the filler rod in it. The flux sticks to the filler rod end.
-After welding the slag from over the welded joint can be
removed by chipping,filing, or grinding.
-No flux is used in the gas welding of Steel
-Fluxes are used in the gas welding of cast iron,stainless steel,and most non-ferrous metals other than lead,zinc and some precious metals.

   

ADVANTAGES OF RIGHTWARD TECHNIQUE

Rightward techniques has certain advantages over the leftward techniques.

-Up-to 8.2mm plate thickness no bevel is necessary. This saves the cost of preparation and reduces the consumption of filler rod.

-To weld thicker metal, where beveling of plate edges becomes necessary, the included angle of V is only 60 degree which require less filler metal against 80 degree V preparation used in leftward welding technique.

-The welders view of the weld pool and the sides and bottom of the V groove is unobstructed. This result in better control and higher welding speed.

-The smaller volume for deposited metal as compared to leftward welding reduces shrinkage and distortion.

-The weld quality is better than that obtained from leftward technique.

-Due to less consumption of filler metal, the rightward technique involves lower cost of welding than leftward
technique.

WELDING TECHNIQUES

-There are two usual methods depending upon the ways in which welding rod and welding torch are used.
1-Leftward technique or forehand welding method.
2-Righward technique or Backhand welding technique.

1)Leftward Technique
-The welder holds welding torch in the right hand and filler rod in the left hand.
-The welding flame is directed away from the finished weld i.e. towards the un-welded part of the joint.filler rod when used is directed towards the welded part of the joint.(fig-)
-The weld is commenced on the right hand side of the seam, working towards the left-hand side.The blowpipe or welding torch is given small side way movement, while the filler rod is moved steady across the seam. The filler rod is added using the backward and forward movement of the rod allowing the flame to melt the bottom edge of the plate just ahead of the weld pool.
-Since the flame is pointed in the direction of the welding, it preheats the edges of the joint.
-Good control and a neat appearance are characteristics of
the leftward method.
-Leftward technique is usually used on relatively thin metals, i.e having thickness less than 5mm.
-When workpiece thickness is over 3mm bevel the plate edges to produce a V-joint so that good root fusion may be achieved. The included angle of V-joint is 80-90 degree. This large volume weld is uneconomical in terms of time, weld metal deposited and quantity of gases used and may
also over-distort weldmentt and welding thick materials.
Long welding time also leads to overheating of the weld area and thus the weld metal may have coarse grain.
-When welding materials over 6.5mm thick, it is difficult to attain even penetration at the bottom of the V and therefore the quality of the weld decreases as plate thickness increases.
-The leftward technique requires careful manipulation to
guard against excessive melting of the base metal and filler metal. The influence of the base metal on the properties of the weld metal can be very deep.
-Another disadvantage associated with leftward technique
is that the look of the joint edges is interrupted and it
is necessary to remove the end of the rod,(which slow down the welding process) this itself resulting in the oxides formed on the tip of the rod being deposited into the weld pool when the weld is recommenced.

2)Rightward Technique:
-The welder holds the welding torch in the right hand and the filler rod in the left.
-Welding begins at thew left hand end of the joint and proceeds towards the right, hence the name rightward technique.
-The direction of welding is opposite to that when employing the leftward technique.
-The torch flame in rightward technique is directed towards the completed weld and the filler rod remains between the flame and the completed weld section.
-Since the flame is constantly directed towards the V ahead of the weld puddle no side-wise motion of the welding torch is necessary. As a result a narrower V groove can be utilized than in leftward welding.This provide a greater control and reduced welding cost.
-During welding the filler rod can be moved in circles(within the puddle) or semi-circles (back and
forth around the puddle).
-In rightward welding the weld puddle is less fluid and this result in a slightly different appearance of the weld surface. The ripples are heavier and spaced further apart.
-The rightward techniques is used on heavier or thicker
(above 5mm) base metal.Weld with penetration of approximately12mm can be achieved in a single pass.

GAS WELDING TECHNIQUE

Base Metal Preparation:

Joints used in gas welding are:

i)  Butt joint
ii) lap joint
iii) Edge joint
iv)  T-joint
 v)  Corner joint
Either fillet or groove welds are used depending on the work-piece and on strength requirement.
-Workpiece edges may be prepared by
i)gas cutting
ii)Plasma cutting
iii)Milling
iv)Shaping
v)Planning etc.   
fig--
-In joint preparation it should be ensured that the plate edges are free from rust or oil.this prevents excessive fumes and helps improves the appearance of finished weld.
a strong wire brush is used to dispose of rust and small burns.The parts to be welded may be pre-set to counteract distortion. 

EXTINGUSHING THE FLAME AND STOP WELDING

1)When the welding or cutting operation is finished, close the torch acetylene valve and then turn off the torch oxygen valve.

2)Close the oxygen cylinder valve.

3)Release the pressure in the hose and regulator by opening the oxygen control valve on the torch.

4)Release the pressure in the oxygen regulator diaphragm by turning the regulator to the minimum pressure position.

5)Close the oxygen control valve on the torch.

6)Repeat the same procedure for acetylene.

FLAME ADJUSTMENT

1)To start with, when the oxyacetylene gas welding torch is ignited, it gives an acetylene flame (fig-) in which enough oxygen is drawn in from the atmosphere to burn acetylene partially.From acetylene flame excess of free carbon is released into the air. Air acetylene flame may be used to apply carbon to mold faces in the foundry, because the carbon acts as an insulator between the molten metal and the mold face.

2)As the oxygen valve in the torch is progressively opened, the flame becomes luminous.Then the luminous portion contracts towards the welding tip forming a distinct bright zone within a blue outer envelope. This is Carbonizing flame and has large excess of acetylene.

3)With further increase of oxygen content, the bright zone of the flame contracts farther and is seen to consists of two parts: a brighter inner cone and a pale green feather trailing off its end into the blue envelope. This is a reducing flame.(fig)

4)If at this stage oxygen flame is increased gradually a certain point will reach where one will notice a distinct change in the sound of the flame and a well defined white cone will appear near the tip surrounded by a bluish envelope that is faintly luminous.This is a neutral flame.There is an approximate one to one mixture of acetylene and oxygen to result a neutral flame.

5)Further increase of oxygen content into the mixture will give rise to an oxidizing flame.

LIGHTNG THE WELDING TORCH

                                                                 

 1) Crack the cylinder valve on oxygen and cylinder by opening the valve and closing it quickly in order to blow out foreign matter.

2)Attach the oxygen and acetylene pressure regulator to the respective oxygen and acetylene cylinders.

3)Attach the hoses to the pressure regulators.

4)Attach the other ends of the two hoses to the welding torch.

5)Select the proper tip(nozzle) and mixing head and attach it to the welding torch.

6)Back off the regulator screws on both units until the screws turns freely.This is necessary to eliminate a
sudden surge of excessive pressure on the working side of the regulator when the cylinder is turned on.
7)Make sure that both torch needle valves are turned off  (clock-wise).

8)Open the acetylene cylinder valve 1/4 to 1/2 turn. Open the oxygen cylinder valve all the way.

9)Open the acetylene (torch) needle valve one full turn.Turn the adjusting screw on the acetylene pressure regulator clockwise until acetylene gas comes from the torch tip.

10)Light the welding torch using a spark lighter.

11)The acetylene pressure regulator screw should then be adjusted until there is a gap of about 6mm between the torch tip and the flame.This is the proper pressure for the size of the tip being used regardless for the gauge pressure shown on the working pressure gauge on the regulator.

12)Oxygen needle valve on the torch is then opened to adjust the welding flame.