Correct Welding Method
Correct Welding Method
What is weld?
Joining metallic materials together using heat or pressure or both, with or without the addition of a material of the same type and with the same or similar melting range, is called "metal welding". If an additional material is used in joining the two parts, this additional material is named as “additional metal".
Basic Welding Methods
TIG Weld
TIG (Tungsten Inert Gas) = WIG ( Wolfram Inert Gas) = GTAW (Gas Tungsten Arc Welding)
A non-melting tungsten electrode in TIG weld. While
this can be performed without providing additional metals for the welding area,
this welding process can also be performed with additional metals. As it can be
understood from TIG weld, INERT gas, meaning noble gasesi are used. Most widely
used ones are argon, helium, or their combinations.
Important Reminder : Argon gas is heavier than air while helium gas is
lighter than air. For this reason, as it can be seen from the picture,
regarding welding processes to be realized at 1G position, helium gas wil not
constitute a suitable option. Since it is heavier than air, instead of
protecting the welding metal, it will move away from the weld metal. Due to
this reason for these positions generally argon gas is used. If ceiling welding shall be performed as seen from 4G, it will be suitable to use helium gas.
In our country TIG weld is generally used in the welding of stainless steel and aluminum alloys.
One of the most important points to consider in relation to tube welds is to apply the root pass properly. A suitable root protection gas should be used during root pass welding. This gas can be argon, nitrogen or the mixture of argon and nitrogen.
In stainless steel welds without root protection, the seam does not have a homogeneous structure and rusting may begin because the fluid passing through the tube accumulates in the uneven root weld. This situation creates a significant obstacle regarding tubes to be used in the food market.
Heat input is at minimum level at TIG weld. When compared with other traditional welding methods, weld seam has a homogeneous appeal. Welding is generally done by using DC (Direct Current) apart from nonferrous materials such as aluminum and the tungsten electrode should be connected to the (-) pole.
MIG-MAG Weld
This is the weld type in the form of continuous weld wire feeding. The welding wire wrapped around the spool is continuously fed towards the torch. As long as the torch trigger is pressed, gas protection and continuous wire advancement are provided.
MIG (Metal Inert Gas): It is the under-gas welding method where nonactive gases are used.
MAG (Metal Active Gas): It is the under-gas welding method where active gases are used.
Active gases are suitable for high alloy steels and for non-alloy steels and low-alloy steels. Furthermore, only the carbondioxide gas is used as active gas for non-alloy and low-alloy steels apart from certain restrictions.
As an alternative to shielding gases, cored wires are also used. These wires have a sheath that melts in the arc current and thus creates a shielding gas atmosphere. Cored wires provide a durable shielding gas cover even under air currents.
Elektrik ARK Weld
Covered electrode weld is one of the most widely used welding methods. In covered electrodes, the electrode is covered with powders of various alloys. These powders form the coating.
Benefit of this cover are as follows;
To ensure easy arc formation
To form a stable arc
To create a slag
To provide a protective gas atmosphere by burning during welding process.
Benefits of this welding method are as follows;
Opportunity to perform welding almost in all types of positions
Practical performance
Light and mobile features of welding equipment
Opportunity to perform welding with almost any type of material
Disadvantages of this welding method are as follows;
Welding process should stop each time electrodes are exhausted.This situation can lead to errors.
Particulars to consider for performing correct welding
1. First of all, in order to perform the welding process correctly, samples of parts with similar properties should be welded beforehand and after the welding process is completed, the necessary tests should be performed to check the condition of the weld.
If weld is considered to be suitable as per standards, all of the parameters used during welding process must be recorded and a method approval related with welding performed should be prepared. This is named as PQR. WPSs are prepared whereas welding parameters are recorded in the light of data available at PQR and to constitute a reference for welders. Certified welders should perform welding with previously verified parameters in the light of WPSs.
2. Gas protection at the area to be welded should be smooth. Gas flow at appropriate flow rate must be provided. Gas flow rate should neither be too high nor too low. In welding areas where gas protection is not adequate, problems such as foaming, oxidation, porosity, cracks and non-adhesion may occur during welding. Especially welding errors take place at site facilities since these environments are uncontrolled.
3. Air flow at the place where welding is done should be carefully monitored. Because air flow can prevent gas protection at the area to be welded. For this reason, when welding is done, attention should be paid not to realize welding at a windly area and to provide necessary protection.
4. When tube weld is done, root gas protection should never be neglected. If root gas protection is not correctly done, this may cause weld penetration failure in the root pass. For instance, since weld corresponding to internal surface of tube is not homogeneous, this situation causes accumulation of the fluid at the place where welding is done.
5. Heat input: When welding is done, heat of material increases in a way to melt it. The structure of the HAZ (Heat Affected Zone) formed 1-2 mm from the weld seam varies according to the initial state of the material. The higher the heat input applied for a material, the more the structure of material changes and distortions observed in relation to the material increase as much. For this reason applying heat input at minimum level bears importance with regards to weld quality. In order to keep the heat input at a low level, appropriate methods can be applied as follows;
To work with low current and voltage levels
Not to perform the welding process much slower than normal
To select an electrode with a suitable diameter
One of the welding methods where the heat input is at minimum level is TIG weld method.
6. Type of additional metal: If a welding method where an additional metal is used is applied, additional metal that will be used should be suitable for the base material. It would be better to make selections by reviewing the catalogs of companies producing weld consumables. When selecting the weld consumables to be used, the basic principle should be to prevent disruption of alloys related with base material and even to ensure that weld beam is more resistant with respect to the base material.
Especially regarding stainless steel welds, attention should be given when selecting electrodes. Stainless steels contain Cr and Ni elements providing the characteristics of stainless and they contain very low amount of carbone. For this reaon when selecting wires, amounts of Cr, Ni and C should be considered.
For example, when performing welding for a material having 304 L quality, additional metal to be selected must contain low amount of carbone. Because high amount of carbone can increase situations of rust and this can also cause brittleness at the weld area.
7. Post-welding heat treatment: As high amount of energy is applied to the material during welding, heat level of material increases so as to melt it. Due to this reason structure of base material gets disrupted and/or cracks or deformations can occur due to the tensions that form. For this reason if a heat treatment (such as voltage elimination) is applied to the alloy related with material following welding process, tensions that form due to welding can be prevented.
Attention should be paid to heating and cooling speed of material during heat treatment. Processes that are applied too quickly or too slowly have negative impact on mechanical and corrosion resistances of materials. In order to eliminate the residual stresses that will form on the material after welding, the residual stresses can be overcome by hitting the weld with a hammer after each pass.
8. Post-welding passivization: Following weld process, it bears importance to wash stainless steels with acid to prevent material corrosion. Because chromium oxide layer on top of it provide stainless feature of stainless steel. By washing with acid following welding process this feature is enhanced. In other words a chromium oxide layer is artifically created.
However, it should be paid attention to clean steel with water following this process. If this cleaning process is not performed, permanent dashes form on stainless steel.
Note: Regarding passivization process to be performed with nitric acid, person to perform this process must be well protected in general. Acid should not get in contact with the body of person who will perform this process.
9. Post-welding cleaning: Following welding process, welding wastes named as slags must be cleaned from the weld. For this purpose weld is cleaned with a stainless steel wire brush. It bears importance to use stainless wire brush to clean stainless steel welds.
10. Materials should be fixed before welding to prevent deformations on materials due to welding. Centering can be used for fixing process. If materials are not properly fixed and water input is not well adjusted, deformations can occur on the part that is welded.
11. Preheating: Weld parameters and methods can vary depending on alloys in the content of materials. Among the most important elements affecting weld quality, carbone element ranks as the first. High amount of carbone denotes that welds are harder and that they have tendency to crack. For this reason in general base material with high level of carbone alloy is not recommended for welding. However, if high carbone steel weld will be performed due to mechanical features, additional metal should be selected among low carbone materials. In this way, weld steam that will form shall have lower carbone content and preventions will be taken against cracks.
12. If welding is to be done in more than one pass, the weld should be waited for to cool after each pass and the other pass should be removed after the weld has cooled. The slag layer formed on the weld seam should be cleaned after each pass.
13. Surface of material to be welded must be cleaned from dirt, grease and rust. If welding is done without cleaning, pores and cracks may form.
14. Weld groove: If weld groove will be opened in the material to be welded, it must be opened with smooth form. An irregular weld groove may cause melting deficiencies in the root pass and side walls. Weld groove type determinations should be made according to the requirements of the projects and engineering calculations.
15. Arc Blow: Arc blow is the irregular movement of the welding arc from right to left. The unstable welding arc causes errors and lack of penetration in the weld seam. The main reason for arc blow is the presence of other magnetic metal materials near the weld area. Therefore, materials that will disrupt the magnetic properties should not be kept near the welded areas.
16. If welding will be performed with electrode, electrodes should be purified from moisture. Because moisture in electrodes causes welding errors. For this reason electrodes must be kept in controlled environments in vacuumed packages and electrode thermostates.
17. TIG Weld must be performed with tungsten electrode. The use of thorium alloyed tungsten electrodes is not recommended because thorium is a carcinogenic substance. Especially grinding process that is applied to correct the edge of electrode is hazardous for health. In such situations person to perform this process should take safety measures.
Rays that come out during welding are too dangerous for human health. Parts of body getting in contact with rays should be protected. Most intense ray occurs during TIG weld.
18. Starting and ending parts of weld are places where most errors are seen during welding process. For this reason operator or welder should pay attention while starting with welding and ending the process and he must take measures against crater cracks that may form. He must not promptly remove the electrode from the weld.
19. Hydraulic test: After completion of welding process related with pressurized vessels, water is pumped into the containers at a pressure above the working pressure. This test is named as hydraulic test. Tanks must be emptied following completion of hydraulic test. If they are not comletely emptied, remaining water can cause corrosion around weld and surrounding area.
20. It is recommended to weld with thin diameter electrodes. When welding is done with thin diameter electrodes, arc blow is less. This prevents welding errors.
Potential Errors that may occur during Welding Process and their solutions
4.1. Spatter
Situation where material surface is not clean enough
Current is high
Arc length is high
4.2. Pores
Rapid cooling of the weld metal
Working with damp electrodes
Rapid welding
Situation where material surface is not clean enough
Lack of proper gas protection
4.3. Inadequate Melting and Insufficient Penetration
Low welding current and voltage
Wrong weld groove selection
Rapid welding
4.4. Distortions - Axis Misalignments
Wrong weld groove selection
Excessive heat input
Not centering the parts
Welding in the wrong order - Not applying Method Welding
Method welding has been developed to prevent distortions in welds. As can be seen from the figure above, this weld process prevents the materials from pulling in one direction by causing mutual welding. If welding is done in this way, the bending of the parts due to heat is reduced to a minimum level.
4.5. Crack Formation
Incorrect weld groove opening
Using wrong welding metal
Rapid cooling of the weld
Situation where sulphur and phosphorus content of the material is above the standards
Material surface being damp, dirty and rusty
4.6. Foaming of the Weld
Weld foaming is a term used in the industry and it denotes formation of many pores.
Weld foaming usually occurs due to improper gas protection. Gas flow rate being more or less than desired level causes porosity on the weld pool.
The first thing to do to prevent this situation is to measure the gas flow rate from the welding torch tip. The gas flow rate is adjusted according to the welding torch tip.
Dirty torch tip and inhomogeneous gas flow
Keeping the torch away from the weld
Incorrect torch angle
Wind, air currents in the environment
Torch cooling water leaks
Problems occurring at the Construction Sites
Since construction sites are generally not as organized and closed as factories, problems can sometimes occur.
The welding environment is expected to be windless, moisture-free, and have a controlled temperature. However, it can be difficult to provide these environments in construction sites.
In cases where climatic conditions are unfavorable, such as rain or snow, the welding area must be isolated. Welding should never be done under rain. This can cause inevitable welding errors. (Pores, cracks, etc.).
When working in windy environments, the required gas protection cannot be provided. Failure to provide the desired gas protection causes the material to rust during welding, insufficient penetration, pores, cracks, and foaming. For this reason, it is absolutely essential to protect the welding areas from wind and adverse weather conditions by drawing a kind of protective curtain.
One of the main issues that welders should pay attention to when welding on construction sites is to ensure that the electrodes they use do not get moisture. For this reason, welders should store the electrodes in a thermos which they must keep close to them. Since the electrodes that are kept constantly warm in thermoses will not absorb moisture, there will be no welding errors.
Welders working on construction sites should definitely have side cutters, thermoses, wire brushes, hammers and WPS (document with welding parameters written on it).
Especially in pipeline welding, heat input should be adjusted well and method welding should be applied. Because pulling materials to one side in pipelines can cause major problems. The possibility of lines deviating from linearity is very high.
Calibrations and verifications for the welding machines used by welders must be performed.
After welding, it is usually necessary to wait between 12 and 24 hours for controlling. This is because the hydrogen in the weld metal must be removed from the weld. Errors usually occur 24 hours after the end of welding. For this reason, quality control activities performed immediately after welding can be misleading.
Information about Galvanized substance
Galvanizing by means of hot dip
Hot dip galvanizing, as its name suggests, is a coating method performed by dipping the steel into a molten zinc bath. In this method, molten zinc at 450 ° C adheres to the steel as a result of an alloy reaction that creates a metallurgical bond between zinc-iron layers.
Before the coating process, the material is subjected to surface cleaning processes to eliminate contaminants such as oil, grease residues on the surface and previously formed corrosion products (rust). After the material enters the degreasing, acid and flux baths and dries, it becomes ready for the galvanizing process.
Hot Dip Galvanized Coating Process The factors affecting the coating thickness in hot dip galvanizing are as follows: a. Dipping time affects the thickness of the alloy layer. b. The speed of removal from the bath affects the thickness of the zinc not added to the alloy. c. Bath temperature affects the thickness of both the alloy and the free zinc layer. d. Wiping, shaking or centrifuging operations performed after the dipping process affect the thickness of the zinc not added to the alloy. Coating thicknesses are given according to the EN ISO 1461 standard depending on the material thickness. The average minimum thickness is given as 85 μm and the minimum local thickness is given as 70 μm for materials having thickness of 6 mm or more. Iron-based materials rust in the atmospheric conditions in which they are located. Rusting of iron over time causes it to lose its functions and the material to decay and disappear. Since zinc is the most active protective element found in nature against rust, it protects iron from the negative effects of the atmosphere.
How Galvanized Coating Protects Steel Even if it is damaged at any point and the iron layer underneath is exposed, it will continue to protect the iron it is coated on. Since it is more apt to react to oxygen in the atmosphere than iron, it combines with oxygen and prevents iron from combining with oxygen and therefore rusting. However, since other surface coatings do not have such a feature, the iron material underneath will rust at the damaged point. Relevant advantages and disadvantages are as follows: 1. It forms a thick, highly penetrative and homogeneous layer, critical points that cannot be reached with other methods (internal volumes, sharp corners, cut areas, hole surfaces, gaps etc) are protected. 2. Other methods are very sensitive to minor damages and corrosion generally occurs in these damages. However, galvanized coating is resistant to scratches and even has the ability to repair itself. This is called "cathodic protection". Zinc, which is electro-negative compared to steel, continues to protect the steel by corroding before the steel in the damaged areas. 3. The coated material and zinc form a metallurgical bond and the alloy layers formed on the surface of the material provide a coating resistant to external effects. The slow wear rate gives the coating a long and predictable durability. 4. Galvanization is the longest-lasting among surface coating methods and does not require any maintenance. Therefore, it is low-cost compared to alternative methods. 5. The quality control of the hot dip galvanization process, which is a relatively simple and effective method, is also simple and reliable. The coating thickness can be exactly measured. 6. The application speed is high. Protection against corrosion is obtained instantly, regardless of tonnage and quantity, and the material is ready for use immediately after the process. 7. One of the biggest disadvantages of dip galvanization is that it cannot be “welded”. It is not possible to repair and fix metals coated with the hot dip galvanization method, the part must be re-galvanized as a whole after the renovation / repair. 8. Zinc accumulation may occur in the holes. 9. Some geometries, such as tanks with small openings, are not suitable for hot dip galvanizing. 10. There is a risk of coating peeling off.
Electrolytic Galvanizing
The electrolytic zinc coating (electro galvanization) method constitutes a method whereas two electrodes connected to the material that will be coated are immersed in a cauldron filled with electrolyte, with pure zinc as the anode and the material to be coated as the cathode. In this method, electricity is supplied with a high current, low voltage generator and rectifier, and the electrons separated from the anode are ensured to stick to the cathode, that is, the surface of the material. With the process, passivation is performed and the coating becomes permanent. In the BS EN 10152:1993 standard, it is required to obtain a minimum average coating thickness of 10 μm and a minimum coating thickness of 9.1 μm locally for low carbon steel materials. In an article written by the Department of Mechanical Education of the Faculty of Technical Education of Fırat University, It is stated that coatings up to the thickness of 23 μm are obtained using different parameters.
Its Advantages and Disadvantages
1. There is no risk related with loss of coating.
2. Material surface is brighter with respect to dip galvanization.
3. Since thickness of zinc layer on surface is not high, expected life span is short.
4. Since it is performed by electrolysis method, there is no accumulation inside the holes.
Result
When electro galvanized and dip galvanized parts are placed side by side, the one that looks clean and shiny is the electro-galvanized part. Based on the literature and standards, it would not be wrong to state that coating thicknesses of 25 μm and below are an indication of electrolytic coating.