How to Prevent Weld Delayed Cracking
Delayed Cracking also known as Hydrogen Induced Cracking or Cold cracking, occurs when the following three factors are present at the same time; high strength steel, high stress level (residual or mechanical/structural loading) and a source of hydrogen. Moisture in welding filler material or environment and joint contamination all are the typical source of hydrogen in a weld metal. Elimination of one or more of the factors can prevent the delayed cracking to occur. So all you need to do is to control these factors to make crack free welds.
High Strength Steels Steels with high strength level may be susceptible to hydrogen induced cold cracking. Example of this includes; Precipitation Hardened Steels, high strength nickel base alloys, 400 Series Stainless Steels. Welding of above mentioned materials may prone to hydrogen embrittlement in their microstructure consequently consequently results in cold cracks.
High Stress Level A weld joint with high stress level is more likely to prone to delayed cracking. It may include residual stresses from cold working or shop fabrication and mechanical or structural loading (for example: wind loadings). These stresses can be reduced by using streamline joint designs and elimination of any sharp edges, corners or notches. Sharp joints and notches are stress raisers where stress concentration may cause catastrophic failure. Hydrogen Entrapment As states above hydrogen comes from high moisture content in atmosphere, welding consumables and joint. Thorough cleaning of weld joint at least 25mm on either side, inside and outside if joining a pipe removes grease, oil, coatings, water etc. Preheat the base metal at the minimum of 200 F degrees and maintain the preheat temperature throughout the whole process. It must be ensured that coated consumables are dry stored and baked in ovens prior to use. The Use of low hydrogen rods (H8 or less) helps to prevent cracking. If using SMAW process, the weldment after completion of welding must maintained to about 500 F (260 C) degrees for about two hours without allowing it to cool down to specified minimum pre-heat temperature. Prevention of Delayed (Hydrogen Induced) Cracking 1. Joint must be thoroughly cleaned, dried and free from any kind of rust, scale, mill, oil, grease, coating etc. About one inch from either sides of joint must be cleaned using specific type of cleaning process. 2. Make sure to complete at the minimum of 1/3 length of weld deposit without any interruption to avoid any thermal stresses to develop due to high thermal gradient. Before restart, make sure the metal temperature should not below the minimum preheat temperature. 3. Rods must be oven baked according to manufacturer’s specific product details, procedure and recommendation or sealed pack prior to use. Opened rods must be discarded after 7-8 hour of exposure. Use diffusible-hydrogen designator H8 or less filler metals. 4. Use of low strength materials with good weldable composition and properties, is a best way to avoid cracking.
Find 10 more ways to control Hydrogen Induced Cold Cracking If you found a welder using AWS 5.1 EXX10 rods for root pass disallow him. Because it is a general practice in several fabrication shop to use EXX10 rods for roots pass and EXX18 for subsequent hot, fill and cap passes. For this I must say that it’s not a good practice as EXX10 rods are not low hydrogen and not suitable for welds subjected to cold cracking. Try to use rods with low hydrogen content and with essential pre-heat and post-heat requirements.
10 ways how to control Hydrogen Induced Cracking Hydrogen induced cracking (also referred to as cold cracking), is caused by the diffusion of hydrogen in the crystal lattice of HAZ near the fusion boundary. Atomic hydrogen is dispersed throughout the weld material, making it weak in mechanical properties. The entrapped hydrogen makes cavities in metal that results in cracking due to weak ductility and tensile strength. When metal is subjected to tensile stress with high level of hydrogen content, hydrogen induced cracking occurs. Hydrogen cracking is also named as toe cracking or under bead cracking due to its location of occurrence. It primarily occurs in High strength Low alloy steels. During welding hydrogen is induced due to the moisture and organic compounds by the usage of cold cellulosic electrodes. Proper cleaning of metal surface and pre-heating are the preventive steps to avoid alarming hydrogen induced cracking. Below listed are the ways to reduce Hydrogen cracking in welded items.
1. Pre-Heat Pre-heating is one of the best step to achieve good solidify microstructure as it slows down the cooling rate to facilitate solidification. In this way controlled cooling provides the desired microstructure that is not prone to hydrogen cracking. In worst the rapid cooling may result in sensitive microstructure to HIC. 2. Interpass Temperature Welder associates must define the peculiar interpass temperature. Once this temperature is specified, the base metal must be pre-heated to that temperature before depositing the other weld pass. In this way cooling rate is controlled and the mechanical properties of both weld metal and HAZ are secured, avoiding hydrogen cracking. 3. Post-Heat After welding is finished, apply post heat to eliminate any remaining hydrogen. On post weld heating, the metal gives off the hydrogen that lessens the risk of hydrogen induced cracking. Moreover post weld heattreatment (PWHT) is also applied for the removal of harmful residual stresses, in order to achieve the desired microstructure as per requirement of service. Also see the usefulness of Post Weld Heat Treatments. 4. Suitable arc process Metals that are prone to hydrogen cracking must not be welded by those arc processes that induce hydrogen in the weld metal. MIG welding with surface tension technology is the recommended process for welding of high strength low alloy steels, rather than applying stick welding with cellulosic electrodes. In latter case high diffusion of hydrogen occurs that leads to serious cracking in steels. 5. Suitable filler metal As mentioned above cellulosic stick electrodes are the mean of hydrogen embrittlement. Moisture and the organic compounds are the source of hydrogen. Such electrodes provide hydrogen gas shielding. For this, basic electrodes (minerals like calcium and magnesium carbonates) are available that give low hydrogen shielding, reducing the risk of cracking in low alloy steels.
6. Multipass technique Multipass method renders tempering effect, modulating the microstructure in order to get rid of hydrogen content. Every next pass tempers the previous deposit pass and protects it from cracking. On the other hand hot pass is also applied for same purpose. We know hot pass is used to clean the root pass by the removal of slag content. Moreover it is also applied to render heat treatment effect so that root may not be subjected to hydrogen cracking. 7. Temper bead technique Temper bead technique is introduced to provide the heat treatment effect in repair welds where post weld heat treatment is difficult. In this technique overlapping of beads, assists the self-heat treatment that refines the course grain zones, due to which desired microstructure with improved properties in both weld metal and HAZ, is obtained. Furthermore it also minimizes the residual tensile stresses. All these features encourage the reduction of hydrogen induced cracking. 8. Shielding gases As it is stated above cellulosic electrodes offer hydrogen gas shielding. For this moist free (low hydrogen) shielding gas must be used in case of metals that are prone to hydrogen cracking. Use of baked electrodes, is the recommended practice in order to lower the hydrogen content. 9. Rust and scale removal Before welding, cleaning of joint with removal of rust and scale, is always endorsed. Since the presence of moisture or grease on parent metal, is the major source of hydrogen. So making the joint rust free and proper cleaning are the paramount activities before the commencement of weld. 10. Fusion Perfect fusion is achieved by the merging the weld metal at weld toes, to provide reinforcement. As stress concentration is one of the cause for hydrogen cracking/toe cracking. Reinforcement at weld toes reduces the stress concentration and thus avoiding the toe cracking. Comments In brief residual stresses, high hydrogen content and sensitive micro structure are the primary causes for hydrogen cracking. Control of these elements gives a cracking free weld. In order to achieve quality welds, following practices must be exercised; Proper cleaning of joint Pre-heating of joint Use of nickel and austenitic filler materials Use multi-pass method in case of those weldable materials that are sensitive to HIC.
How to Prevent Weld Solidification Cracking
The formation of cracks during the solidification of a weld metal, is commonly termed as Solidification cracking. The reason behind that the hot Weld metal lose the required ductility and may be subjected to cracking. Since the solidification starts from fusion line to centerline, the low melting components in composition of molten weld puddle remains liquid in the center even at low temperature and as a result stresses are developed due to contraction of remaining solid zone. These stresses are the major cause to make splitting of a weld bead. Learn What is a Fusion line in Weldment? Normally in Fabrication industries, this problem is known by three common names; hot, crater and centerline cracking.
Types of Cracking Hot cracking – happens when the weldment is at high temperature. Crater cracking – the appearance of small cracks at the end of bead. Centerline cracking – the appearance of cracks at the centerline of bead.
Causes Following are the common reasons for above mentioned types to happen in a weld metal. A weld metal is more subjected to solidification cracking when three conditions occur simultaneously. 1. 1. If the parent metal has chemical composition sensitive to cracking. 2. If the welding parameters produce irregular size and shape of weld bead. 3. If the weld has unfavorable stress level. How to Prevent Solidification Cracking? The prevention requires, the reduction of above mentioned conditions to an acceptable level. Below are the preventive actions that are practicable to reduce the trigger causes of cracking. 1. Avoid the Contamination of Weld pool The contamination of metal (in case of C-Mn/C steels) with elements like copper and sulphur, makes the weld composition sensitive to cracking as the entrapment of these constituents lead to the formation low melting compounds. For example sulphur makes iron sulphide that remains liquid even at the last stages of solidification. 1.1 Sources Contamination of parent metal with oil and grease Less refined material delivered by the supplier Contaminated welding rods 1.2 Prevention Before welding make sure the joint is properly prepared by cleaning and grinding. Welding consumables having copper content must be maintained at good conditions. Ensure the material testing report of supplied parent material. 2. Avoid the unfavorable Welding Parameters Improper welding parameters may result in irregular size and shape of a weld bead. For this a width to depth ratio is an important factor to determine the soundness of a weld bead which is totally insensitive to cracking. If the width to depth ratio is less than a specific number*, the weld bead will be sensitive to cracking. If the width to depth ratio is greater, the weld bead will completely insensitive to SC cracking. *You can find more details about width to depth ratio in CSWIP 3.1 21.4.3 So the welding parameters must be selected carefully in case of those welding procedures which are liable to produce beads with SC cracking. 2.2 Prevention Use low welding current settings – to produce the shallower bead. Slow Travel – to produce the wider bead at comparatively lower pace. 3. Avoid the crater in a weld bead Crater is a shallow cavity, produced at the end of weld bead . It is also another trigger to impart cracking in the weldment. In order to avoid crater, one must follow the below mentioned practices. In modern TIG welding machine, the function of ‘slope -out’is introduced to help filling the craters. In this case the welding current is reduced along with the lengthening of arc manually by a welder to produce weld bead of required width to depth ratio. In case of stick welding the crater is filled by reversing the direction of weld run at the end of bead.
7 Effective Ways to Prevent Weld Undercutting
The Undercut is an imperfection which is characterized by a groove formation at the weld toe due to overheating at the free edges of base metal or weld metal (of the previous run). This happens when a welder tries to make weld either with high welding current or with fast travel speed. Also, the use of incorrect angle directs the arc towards thinner edges where overheating can burn the metal. See what are the angles which help to eliminate welding defects? Moreover, the incorrect usage of gas shielding, wrong welding technique, and position are other primary causes of undercut imperfection. Undercutting always has been a serious concern in steel welding industries and efforts have been made to resolve this problem in recent years.
Following are the preventions that effectively assist, in reducing the concentration of undercutting in your weld.
1. Correct Heat Input One of the major causes of undercut in the weld is high heat settings while making a run near to the free edges. This may lead to the overheating and melting of near parent metal or previous weld metal. In order to prevent this, one must take great care of weld heat by reducing the current, when approaching to thinner and free edges.
2. Correct Electrode Angle As we know welding angles always play a vital role in producing defects free runs. If one makes a run with an incorrect angle which directs more heat to free edges, here in this case weld is more prone to undercuts. For this one must use correct angles (lead, work, electrode, travel, and slope) in order to direct more heat towards thicker components as compared to the free edges.
3. Moderate Travel Pace Making a weld with high travel pace is another big cause of weld undercutting. In doing so, some portion of parent metal is drained into the molten metal and never came back due to fast solidification, leaving a depression along the edges. It is recommended to travel a weld run with moderate pace because too slow travel also does not give satisfactory results.
4. Right Selection of Gas Shielding In case of MAG (metal active gas) process, wrong selection of gas shielding is also one major cause of undercuts in a weld. Welding associates must ensure the correct composition of gas according to the requirement of material type and thickness. Use of Carbon dioxide in conjunction with inert gases provides quality results in welding of carbon steels.
5. Correct welding Technique Problem of undercut also arises when a welder endeavors to make a weld run with excessive weaving style. Codes and standards call for the recommended width of weaving according to the electrode size. Size of weaving should not be exceeded to the acceptable limit, otherwise, the weld may prone to undercut grooves. In order to prevent this one must endeavor either to reduce the size of weaving or to produce the weld with multipass technique.
6. Correct Welding position Sometimes making a fillet weld in horizontal-vertical position, also results in weld undercuts. In this case, a depression is formed due to inadequate metal filling along the joint edges. In order to prevent this, try to weld in the flat position to ensure the adequate electrode consumption.
7. Use of multi-run technique This one is the best option to adopt, in order to reduce the level of undercuts in welding. Multi-run technique supports all above-mentioned preventions and helps to produce fine welds with improved mechanical properties.
7 Effective Ways to Prevent Lack of Fusion Failure to complete amalgamation with parent metal or between the weld runs, is referred to as lack of fusion in welding. The weld metal fails to blend with parent metal or sometimes two weld beads are not completely fused to make as a whole. The problem occurs when the work piece not receives adequate heat for fusion. Normally there are three locations where lack of fusion may occur. Along the joint wall Between the weld runs At the root Following are the possible causes of lack of fusion and the practicable ways to control them.
1. High Heat Input As it is stated earlier that l ess heat supply results in poor fusion. We know heat input i s the relation of welding voltage, current and travel speed. It has direct relation with current and voltage and that of
inverse with travel speed. Low voltage/current settings don’t supply enough heat. On the other hand too faster travel induces the same problem. Adjust the maximum allowable current and blend weld metal with moderate pace. Here great skills are expected from a welder. 2. Scale/oxide removal Weld joints can be subjected to atmospheric contamination. Any unwanted particle or presence of oxide film may impede the process of fusion and may harm weld properties. Therefore it is recommended to remove any foreign overlay on the joint. Joint cleaning prior to welding makes it easier to eliminate lack of fusion. 3. Correct Angles Electrode and work angles play an important role in achieving best quality results. Make sure the right electrode angle with moderate pace because an incorrect electrode handling makes liquid weld to interrupt arc activity. See what are the angles that may improve your weld . 4. Bead Orientation Wrong positioning of weld beads along the joint wall creates slot and fissure between them. The development of slot may lead to slag inclusions and consequently lack of fusion takes place. A welder must be trained enough, to put the beads in right place and ensure the complete blend of weld along the side wall. 5. Position welding Root pass with vertical down position creates disruption in the process and may result in partial fusion. In order to prevent this, vertical up position is preferred. 6. Edge Preparation Well prepared edge is an essential part of perfect welding. If the joint pieces are not aligned in the same plane, variation occurs in root attributes. Furthermore if the root face is too large, weld metal fails to fuse completely at root. Fix this problem by preparing small root face edge. On the other hand, small root opening also results in unfused output by influencing the aspects of fusion and it is recommended to confirm the correct root opening before commencing the welding procedure. 7. Correct voltage and Inductance During short circuiting mode in MIG/MAG process, inductance may exceed up to the certain level which can cause severe fusion problems. It is only the matter of fact that the magnetic field produced due to immoderate inductance, resists the welding current to rise. In this case metal at the wire tip is not heated adequately and fails to fuse with parent metal. Appropriate adjustment of voltage and inductance settings are required to prevent spattering and lack of fusion, as too low inductance also causes severe spatter. Use of modern equipment with advance power source (that provides regulated voltage and inductance values) is appreciable.
Mechanics of Crack Tip Opening Displacement Crack tip opening displacement is kind of a destructive testing that is applied to measure the fracture toughness of the material. In shortened form the method is known as CTOD. In this technique a notch is made on the test material and subjected to bending at specific temperature (usually at low temperature) until the crack initiates. Bending is applied in such a way that the crack tip opens widely and the extent of opening without brittle fracture is measured as toughness. The technique proves quite useful in repair works when the crack is appeared during service . Following is the demonstration on mechanics of crack tip opening displacement.
Specimen Specimens are taken across the axis of weld metal or heat affected zone. The dimensions of test specimen are demonstrated in codes and standards. A rectangular piece is cut in 10T x 1T x 2T (L x W x H) where T is total thickness of weld. From the middle of rectangular specimen a V-cut is indented inside. It is to be noted that V-cut is applied inside the notch, not at the surface of specimen. See the figure. Technique Test technique is as simple as bending a long bar. The specimen with V-cut is inserted in bending equipment and subjected to cyclic bending until a crack initiates. A bending former is pressed at the bottom so that notch is in tension. Stress concentration at the tip of crack, causes the crack to open widely. A gauge is connected to the opening which measures the length of gap created by the widening of crack tip. The experiment provides the statistics about the increase in opening against the load and the required CTOD value in millimeter is calculated. Test Conditions Temperature – The experiment is performed normally at low temperature. For this the specimen is put inside the cooled liquid or gas to achieve the required temperature. Position – location of notch on weld metal or HAZ. What information it gives? CTOD value assists to determine the toughness and brittleness of weld metal. Tougher the test weld, wider will be the tip opening. In this case considerable tip opening is observed when applied load is increased. Bending load pulls the crack edges apart. On the other hand slight loading may cause fracture in brittle test piece. Here brittle crack i s developed without any widening. It also assists in determination of maximum crack size that can cause fracture. This information is quite helpful when a crack is inspected in weld metal during service. It gives an idea about the extent to which the crack can withstand particular conditions of temperature and load. Moreover determination of maximum load/stress that can cause failure for specific crack size at particular temperature hence, making it easier to make judgments accordingly to prevent failure.
Solid Inclusions that impair Weld characteristics The entrapment of unwanted solid particles impairs weld metal strength. Such weld imperfections are developed by the inclusion of solid substances like slag, flux, foreign metal or oxide. These inclusions are appeared singly or in clustered form. On the basis of shape they are categorized in linear and rounded forms. Common causes are defected consumables, welder’s negligence, inadequate shielding gas flow and joint contamination. They are liable to occur in multi-pass welding method when proper slag removal is not exercised prior to new weld bead. Unlike cracks the solid inclusion have no wedge shape spots for crack propagation. These imperfections are primarily linked with flux using welding processes like Stick welding, submerged arc welding and Flux cored arc welding. So proper joint cleaning, gas shi eld, well maintained consumables and welder’s proficiency are foremost elements in order to avoid unwanted inclusions in weldment. Below is the brief description about possible causes of solid inclusions and their practicable prevention.
Flux Inclusion The entrapment of flux in weld metal due to incomplete fusion and melting of flux, primarily in manual metal arc welding, SAW and FCAW. Common causes are defected consumables (damage electrode coatings, flux core wires and spoiled SAW flux) and incorrect welding parameters voltage, arc length, current etc. To fix flux inclusion problems, trained welder must; Use well maintained consumables Deposit good profile weld by correct configuration of welding parameters. Slag inclusion The entrapment of slag appears in linear form along the weld metal length when spotted with the help of radiography. Slag is the product produced by the de-oxidation reaction of flux during the weld bead deposition and must be removed before the next weld bead. In worst situation poor overlapping of beads allows cavities to develop. On the other hand rough surface of previous beads can also cause cavity formation. These cavities are the primary source of slag inclusions in your weld. Moreover, incorrect electrode angle creates slag overflow, impairing the arc characteristics. In order to prevent this; well maintained flux coated consumables must be used and welder must have good experience to tackle slag inclusion problems. Moreover correct current, voltage and good arc characteristics are essential elements, to produce smooth and quality welds with complete fusion among multi-pass runs. Metallic Inclusion The absorption of other metals in liquid weld, is referred to as metal inclusion. This problem frequently occurs in Tungsten Inert Gas welding where tungsten metal may entrap due to a slight negligence in welding. Tungsten inclusion looks white when observed on radiography film. (where the procedure and identification is same as the X-ray examination of a broken bone). Besides tungsten, other metals like copper can also be captured in weld deposits. Tungsten inclusion is primarily caused by welder’s incompetency, inappropriate welding parameters, improper gas shield
and defected consumables. In order to fix it, one must prevent above mentioned causes. TIG welder must use accurate welding variables and ensure the proper gas shield during bead deposition. Oxide Inclusion The entrapment of oxides in weld metal (especially in case of welding aluminum alloys). Oxide inclusion occurs due to the presence of mill scale and rust on the joint surface. Such imperfection can harm the mechanical characteristics of weldment. To prevent this, welder must prepare rust and scale free joint surface by suitable procedures like grinding, chipping and brushing. Last Comments Solid inclusions genesis is mainly promoted by inept welders. Only the qualified welder must be authorized for specific arc process. Pre-cleaning and post cleaning must be employed. On the other hand consumables must be stored in good condition to avoid any damage.
10 causes of Weld Porosity and their practicable prevention Weld Porosity is referred to as cavities produced in a weld due to the entrapment of gas. The introduction of gas in a weld is supported by some mistakes that ought to be eliminated. In weld metal porosity is categorized according to shape and location of occurrence. Some gas cavities occur in the spherical shape, some in elongated (like in case of wormholes) while on the other hand, some cavities occur throughout weld metal and some on the weld surface. These gas cavities appear either singly called voids or uniformly distributed throughout the weld and sometimes developed in clustered form. These voids and cavities are produced primarily by gas entrapment during the solidification procedure of weld. So making the prediction about mistakes that may cause porosity and their removal is an essential part of welding.
Given below are the 10 reason for weld porosity and their solution.
1. Moist electrodes
It is recommended to use baked electrodes in Stick welding process in order to avoid involvement of moisture content in the weld metal. In worst, steam can be driven out, during the heat of welding which creates small cavities in the weld metal. Porosity eventuates when stainless steel electrodes or low hydrogen electrodes are not properly kept in dry condition. Although in low hydrogen electrodes some moisture is required within a limit, for better performance but is some case if moisture exceeds from the limit, the weld metal will prone to porosity. For prevention following are the steps to follow; Parent metal must be preheated in order to remove any moisture. The electrodes must be stored in dry conditions. Apply final backing before use. Electrodes must not be oven baked before use because a little amount of moisture is required for better performance and weld-ability of the electrode. 2. Contaminated surface
Parent metal or electrodes may be subjected to atmospheric contamination. Any grease, oil or moisture content on surface contribute gas formation when exposed to welding temperature and may cause porosity problems in your weld. In order to fix it, clean the material by specified surface preparation procedures. 3. Improper Gas Shield The possibility of air entrapment due to inconsistent gas shield may also cause porosity problems. For this, the welder must check gas hose attachment with the equipment and remove hose contamination if any. Correct connection prevents any loss of gas shield and consequently no air entrapment. Moreover, airflow and draughts in the shop can cause the restriction in gas flow. Some sort of screening is required to avoid such kinds of restrictions in the gas flow. 4. Too high gas flow The High flow rate of the gas shield may cause turbulence due to which air can be drawn into the weld metal. Optimize gas flow, provides quality welds with no gas loss. Moreover, it saves cost on the other side. 5. Inadequate electrode deoxidant During solidification, the excess oxygen comes out from weld metal due to reduced solubility and may undergo carbon monoxide formation that may cause porosity too. For this, some deoxidants are added in electrodes, filler metals and even in parent metals to remove the oxygen content (which is so-called deoxidation). Therefore inadequate deoxidants may result in poor deoxidation and are responsible for weld defects. In order to prevent this, use electrodes with adequate deoxidants. 6. Too high arc length Too much longer arc length (in other words high voltage) is also a cause of weld porosity. If the welding gun is held away from the joint keeping arc length longer, the magnitude of shielding is reduced which may lead to air entrapment from atmosphere. Correct arc length is recommended primarily to avoid draughts/breezes.
7. Incorrect surface treatments Release of gases, during painting/surface treatments, can impair weld characteristics. Treatments like zinc coating or galvanizing produce gas and other unwanted particles. In an endeavor to fix this problem, make predictions about the outcome of chemical reactions before specific surface treatments. 8. Openwork surface Any surface which is open to atmospheric air can be contaminated. Air introduction from the back side through root opening gets entrapped in the weld puddle. Make sure that weld joint is protected from such kinds of air crevices. 9. Laminated surface Laminated surface also creates disturbance in welding. In case of welding a laminated workpiece, prepare un-laminated parent metal to conduct quality weld procedure. 10. In-appropriate flux
Welding flux must be treated with great care as they have the capability to absorb moisture (granular SAW flux). They must be supplied in dry condition. On the other hand, use of low activity flux results in surface porosity. That is why it is recommended to use high activity flux which offers quality welds with no porosity.
