Wednesday, June 18, 2014

SMAW & FCAW

SMAW  &  FCAW

These two are highly acceptable process of welding in industries as per foundry is concern 
we will discuss more about these two types in details and i will give you some references for more details 

1. SMAW-  It stand for shielded metal arc welding , an arc welding process in which joining takes place between the tip of covered electrode and the surface of the base metal






Shielded Metal Arc Welding (SMAW) or Stick welding is a process which melts and joins metals by heating them with an arc between a coated metal electrode and the work piece. The electrode outer coating, called flux, assists in creating the arc and provides the shielding gas and slag covering to protect the weld from contamination. The electrode core provides most of the weld filler metal.

When the electrode is moved along the work piece at the correct speed the metal deposits in a uniform layer called a bead

The Stick welding power source provides constant current (CC) and may be either alternating current (AC) or direct current (DC), depending on the electrode being used. The best welding characteristics are usually obtained using DC power sources


The power in a welding circuit is measured in voltage and current. The voltage (Volts) is governed by the arc length between the electrode and the work piece and is influenced by electrode diameter. Current is a more practical measure of the power in a weld circuit and is measured in amperes (Amps).

The amperage needed to weld depends on electrode diameter, the size and thickness of the pieces to be welded, and the position of the welding. Thin metals require less current than thick metals, and a small electrode requires less amperage than a large one.

It is preferable to weld on work in the flat or horizontal position. However, when forced to weld in vertical or overhead positions it is helpful to reduce the amperage from that used when welding horizontally. Best welding results are achieved by maintaining a short arc, moving the electrode at a uniform speed, and feeding the electrode downward at a constant speed as it melts

SHIELDED METAL ARC WELDING (SMAW)
PROCEDURE

1. Work piece

Make sure work piece is clean before welding.

2. Work Clamp

Place as close to the weld as possible


3. Electrode

Before striking an arc, insert an electrode in the electrode holder. A small diameter
electrode requires less current than a large one. Follow recommendations of the electrode manufacturer when setting weld amperage or just follow according to your WPS/PQR.


4. Arc Length
Arc length is the distance from the electrode to the work piece. A short arc with correct amperage will give a sharp, crackling sound. Correct arc length is related to electrode diameter. Examine the weld bead to determine if the arc length
is correct.

Arc length for 1/16 and 3/32 in. diameter electrodes should be about 1/16 in. (1.6mm); arc length for 1/8 and 5/32 in. electrodes should be about 1/8 in. (3 mm)


5. Slag

Use a chipping hammer and wire brush to remove slag. Remove slag and check
weld bead before making another weld pass

Striking An Arc − Scratch Start Technique

Drag electrode across work piece like striking a match; immediately lift electrode slightly after touching work. If arc goes out, electrode was lifted too high. If electrode sticks to work piece, use a quick twist to free it.

Striking An Arc − Tapping Technique

Bring electrode straight down to work piece; then lift slightly to start arc. If arc goes out, electrode was lifted too high. If electrode sticks to work piece, use a quick twist to free it

Positioning Electrode Holder

Hold the electrode nearly perpendicular to the work, although tilting it ahead (in the direction of travel) will be helpful

Electrode Movement During Welding
1. Stringer Bead − Steady Movement along Seam

2. Weave Bead − Side To Side Movement along Seam

Conditions That Affect Weld Bead Shape

Electrode Angle


Arc Length

Travel Speed

Poor Weld Bead Characteristics

1. Large Spatter Deposits
2. Rough, Uneven Bead
3. Slight Crater During Welding
4. Bad Overlap
5. Poor Penetration

Good Weld Bead Characteristics
1. Fine Spatter
2. Uniform Beads
3. Moderate Crater During Welding
4. No Overlap
5. Good Penetration Into Base Metal

Typical Weld Joints



Types of Groove (Butt) Joint Welds

1. Tack Welds
Prevent butt joint distortion by tack welding the materials in position before final weld. Work piece distortion occurs when heat is applied locally to a joint. One side of a metal plate will “curl” up toward the weld. Distortion will also cause the edges of a butt joint to pull together ahead of the electrode as the weld cools

 2. Square Groove Weld

3. Single V-Groove Weld

4. Double V-Groove Weld

Materials up to 3/16 in. (5 mm) thick can often be welded without special preparation
using the square groove weld. However, when welding thicker materials it may
be necessary to prepare the edges (Vgroove) of butt joints to ensure good welds.
The single or double V-groove weld is good for materials 3/16 − 3/4 in. (5-19 mm) thick. Generally, the single V-groove is used on materials up to 3/4 in. (19 mm) thick and when, regardless of thickness, you can weld from one side only. Create a 30 degree bevel with oxyacetylene or plasma cutting equipment. Remove scale from material after cutting. A grinder can also be used to prepare bevels.

Groove (Butt) Joint Training Procedure
Practice welding butt joints on 1/8 in. (4mm) or thicker material. (Avoid thinner materials since they require greater skill.) Separate the squared edges of the material about 1/16 in. (1.6 mm) and make a butt weld all the way through with a 1/8 in. electrode. (You may need to adjust the weld current and travel speed to obtain the desired weld.) Perform a similar exercise on 1/4 in. (6 mm) material, depositing a bead on each side of the joint and fusing one to the another (no bevel needed).


Practice making a single V-groove weld on 1/4 in. (6 mm) plate beveled 30°. Start with a 1/8 in. electrode for the first bead and finish with a 5/32 in. (4 mm) electrode. Be sure to penetrate about 1/32 in. (1 mm) beyond the bottom of the “V” or root. Perform a similar exercise on thicker materials. Generally, deposit a bead for each 1/8 in. (3mm) of material thickness, cleaning the joint between layers. On heavier plates, it may be necessary to weave the top layers to fill the groove.

Weld defects and its troubleshooting

Possible cause
Corrective action
 Arc length too long.
Reduce arc length.

Work piece dirty.
Remove all grease, oil, moisture, rust, paint, coatings, slag, and dirt from work surface before welding
Damp electrode.
Use dry electrode.


2. Excessive Spatter

Scattering of molten metal particles that cool to solid form near weld bead.

Possible cause
Corrective action
Amperage too high for
electrode.
Decrease amperage or select larger electrode
Arc length too long or voltage
too high
Reduce arc length or voltage.


3. Incomplete fusion

Failure of weld metal to fuse completely with base metal or a preceding weld bead

Possible Cause
Corrective action
Insufficient heat input.
Increase amperage. Select larger electrode and increase amperage.
Improper welding technique
Place stringer bead in proper location(s) at joint during welding.
Adjust work angle or widen groove to access bottom during welding.
Momentarily hold arc on groove side walls when using weaving technique.
Keep arc on leading edge of weld puddle.
Work piece dirty.
Remove all grease, oil, moisture, rust, paint, coatings, slag, and dirt from work surface before welding.


4 . Excessive Penetration

Weld metal melting through base metal and hanging underneath weld.
Possible cause
Corrective action
Excessive heat input.
Select lower amperage. Use smaller electrode
Improper weld technique.
Adjust travel speed.


 
FCAW-FLUX CORED ARC WELDING

Flux Cored Arc Welding (FCAW) is a welding process by fusion which is widely used on ferrous metal. The consumable electrode can have an interior flux or a mix of flux and metal powder and has a tubular form . It is as semi-automatic or automatic arc welding process












Advantages and applications
·         FCAW may be an "all-position" process with the right filler metals (the consumable electrode)
·         No shielding gas needed with some wires making it suitable for outdoor welding and/or windy conditions
·         A high-deposition rate process (speed at which the filler metal is applied) in the 1G/1F/2F
·         Some "high-speed" (e.g., automotive) applications
·         As compared to SMAW and GTAW, there is less skill required for operators.
·         Less precleaning of metal required
·         Metallurgical benefits from the flux such as the weld metal being protected initially from external factors until the slag is chipped away
Used on the following alloys:
·         Mild and low alloy steels
·         Stainless steels
·         Some high nickel alloys
·         Some wearfacing/surfacing alloys
·         Porosity chances very low

Disadvantages
Of course, all of the usual issues that occur in welding can occur in FCAW such as incomplete fusion between base metals, slag inclusion (non-metallic inclusions), and cracks in the welds. But there are a few concerns that come up with FCAW that are worth taking special note of:
·         Melted Contact Tip – happens when the contact tip actually contacts the base metal, thereby fusing the two and melting the hole on the end
·         Irregular wire feed – typically a mechanical problem
·         Porosity – the gases (specifically those from the flux-core) don’t escape the welded area before the metal hardens, leaving holes in the welded metal
·         More costly filler material/wire as compared to GMAW
·         The equipment is less mobile and more costly as compared to SMAW or GTAW.
·         The amount of smoke generated can far exceed that of SMAW, GMAW, or GTAW.
·         Changing filler metals requires changing an entire spool. This can be slow and difficult as compared to changing filler metal for SMAW or GTAW.
·         Creates more fumes than stick welding.[1]


for more info try those website it will help you to increase your knowledge












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