In this article we will discuss about:- 1. Principle of Welding of Steel 2. Standard Welding Processes of Steel 3. Defects 4. Inspection 5. Welding Positions 6. General Considerations 7. Economy 8. Advantages and Disadvantages.
Principle of Welding of Steel:
Welding is a process of connecting metal parts by fusion. Arc welding and oxy-acetylene welding are the two usual methods adopted. Molten or Fused metal is deposited between the metal parts which are to be connected. The metal parts are also fused to a specified depth. When the fused material is deposited, the metal parts get joined by new metal.
The ends of the metal parts to be connected and the tip of the weld rod are fused by electric arc which causes a high temperature of about 3300°C. The electrode i.e., the weld rod is coated with a flux which melts and vaporizes under the action of heat producing shield which stabilizes the arc and prevents the molten metal from absorbing atmospheric gases.
In addition to this, the molten coating forms a slag which reaches the upper surface of the molten metal forming a protective shield against atmospheric action. In the Oxy-acetylene method a jet of burning oxygen and acetylene is used as a source of heat.
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It is important to realize the effect of welding on material properties. When steel is heated to a temperature of 815°C, its structure is practically uniform and crystalline. If it is allowed to cool fast it will result in a brittle structure where as slow cooling results in a ductile structure.
Thick metals absorb heat rapidly and also cool quickly tending to become brittle. To avoid this condition it is good to preheat the members so that the rate of flow of heat from the weld zone is minimized.
The position in which the electrode is placed at the work spot is also important, since it affects the quality of the weld. The downward on the flat position is no doubt the best position for placing the electrode. The worst position is the overhead position. Overhead welding is possible since the magnetic field present in the arc carries the molten metal to the joint.
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The metal surfaces and edges to be welded should be free from cracks, tears and other defects. In addition, the surfaces at and close to the welds should be free from loose scale, grease, rust moisture and any material that may prevent proper welding.
The parts to be fillet welded should be brought very close to each other. The gap between the parts should not exceed 1.5 to 2 mm. In case the gap is more than this limit the fillet weld size should be increased by the amount of separation.
Parts to be joined at butt joints should be properly aligned. As the welding progresses the welds should be deposited in such a way, the heat applied is balanced. The process of welding may progress from the position where the parts are relatively fixed in position.
Standard Welding Processes of Steel:
Some of the processes of welding in use are briefly described below:
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(i) Shaded Metal Arc Welding (SMAW):
In this method of welding coalescence or fusion is produced by the heat of an electric arc struck between a coated metal electrode and the metal parts to be connected i.e., base metal. The electrode delivers a filler metal to make the weld, a gas to shied the molten metal and a flux also to refine this metal. The process is generally called manual, or hand or stick welding. Pressure is not applied on the parts to be joined.
As a consequence of the arc struck between the electrode and the base metal the intense heat creates a small molten pool on the base metal’s surface. The arc also decomposes the coating on the electrode and melts the metal at the tip of the electrode. The stream of electrons from the electrode carries this metal in globules through the gap and deposits and mixes it into the molten pool over the surface of the base metal.
As deposition of the material from the electrode does not depend on gravity, arc welding is conveniently done in various positions including overhead position. The decomposed coating of the electrode becomes a gas shied around the molten metal thus preventing contact with the air and absorption of impurities.
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The electrode coating also promotes electrical conduction across the arc and adds flux, slag forming material to the molten pool to refine the metal and provides the material to provide the shape of the weld.
In some types the coating adds some alloy material. Following the movement of the weld the molten material left behind solidifies to form the homogeneous weld. The electric power meant for this work may be by direct current or alternating current.
The size of the electrode depends mainly on joint detail and the position of welding. The commonly used sizes of electrodes are 3, 4, 5, 6, 7, 8 mm. Small size electrodes may be 350 mm long. The large size welds may be 450 mm long.
About 60% to 65% of the weight of the welding electrode forms the weld and the rest of the electrode matter is lost by spatter, coating and stub end losses. This method of welding is widely used for manual welding of low carbon steels.
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(ii) Submerged Arc Welding (SAW):
This process produces coalescence by the heat of an electric arc struck between a bare metal electrode and the base metal. The weld is shielded by a flux, a cover of granular fusible material deposited over the joint. Pressure is not applied on the parts to be joined. The necessary filler material is provided either from the electrode or from a separate welding rod.
The electrode is thrust through the flux to strike an arc. The heat generated by the arc melts the base metal and flux. With the progress of welding, the molten flux forms a protective shied covering the molten metal. On cooling this flux solidifies below the unutilized flux which forms a brittle slag which can be easily removed. Any unfused flux recovered can be put to subsequent use.
This method of welding needs high current with high heat inputs. Accordingly rate of deposition and speed of welding are higher than for manual welding. In this method deep weld penetration occurs, and hence edge preparation of the material to be joined required is minimized.
This method of welding can be done using direct current or alternating current. Equipment with 4000 A current rating is needed.
This method of welding is widely used for welding low carbon steels.
(iii) Gas Metal Arc Welding (GMAW):
This is a type of metal inert gas (MIG) welding which produces coalescence by the heat of electric arc struck between a filler metal electrode and base metal. The necessary shielding is obtained from a gas or gas mixture. There is no slag formation in this method. Smoke and fume are reduced. This method can be used for welding all metals. This is very suitable for welding stainless steels and non-ferrous metals.
Defects in Welding of Steel:
While welding is a very effective and specialized technique for structural connections, it is very important that the deposited weld should be free from various types of imperfections and defects.
Many factors are likely to affect the quality of the weld. The material of the weld electrode, the equipment used for welding, the process of welding and besides all the skill of the operators will all influence the weld quality.
Defects that are likely to occur in the process of welding are briefly described below:
(i) Undercutting:
Due to excessive current and very long arc, the base metal gets burnt. This defect can be easily detected visually, and can be rectified by depositing additional weld metal at the place affected by undercutting. Proper maintenance of specified current and voltage can prevent this defect.
(ii) Inadequate Fusion:
Due to the presence of some foreign material on the surface of the base metal like mill scale, slag etc. proper fusion of the base metal does not take place in the groove. This defect can be avoided by cleaning the surfaces to be fused. Proper selection of electrode size, speed and current also can prevent inadequate fusion.
(iii) Inadequate Penetration:
This is a defect caused due to inability of the weld metal to penetrate right up to the root of the joint. This defect is mainly due to faulty groove made, inadequate current and using large size electrodes and fast welding rate. This defect can lead to stress concentration.
(iv) Slag Formation:
During the welding process, as a consequence of chemical reactions metal oxides are formed in the form of a slag, which reaches the surface of the molten metal and can be removed easily after the mass has cooled. But if subjected to quick cooling the joint is liable to hold some slag within the weld preventing it to rise to the surface. The strength of the weld is affected by the presence of the slag within the weld.
(v) Defect due to Porosity:
Sometimes gas pockets get trapped within the weld mass in the process of cooling. The metal when fused gives out gases which can get entrapped in the molten mass resulting in pores. Formations of such pores become stress concentration zones. This defect is caused due to defective welding, use of excessively high current, moisture content in electrodes etc.
(vi) Cracks:
Cracks are liable to occur in welds and also in parts of members being connected. Two types of cracks have been noticed. Cooling leads to contraction which may cause cracking of the weld. Absorption of hydrogen is the main cause of hydrogen induced cracking in the heat affected zone and laminar tearing is likely to occur along a slag in plates.
Inspection of Welds for Welding of Steel:
Welded fabrications should be checked tested and approved before accepting them.
The tests applied to welding are the following:
(i) Visual inspection for uniformity of weld.
(ii) Magnetic particle testing is done to detect defects close to the surface. Magnetic powder is applied to the magnetized joint.
(iii) Penetrant testing involves application of a red dye to detect cracks in the weld.
(iv) By ultrasonic testing voids in a weld can be detected by recording the amplitude of a high frequency sound wave transmitted through the material.
(v) Radiographic testing can be made to record presence of cracks in a X-ray film.
For fillet welds only visual surface inspections are made. Internal examinations can be made for butt welds.
Defects in Welding of Steel:
Failures are unlikely in properly made welds of adequate designs. In case a fracture occurs, it starts as a notch like defect. For various reasons notches may occur, like undercutting, or poor weld profile etc. A natural notch may be formed at the toe of the weld.
Flaws like slags, porosity, cracks, may be present in the weld which act as notches. If a welding arc strikes the metal base without depositing weld material it can produce an embrittling effect. Once a crack is formed at such a notch it will propagate further.
It is good to preheat the metal base before welding as it will reduce the risk of brittle failure. In such a treatment the temperature gradient between the weld and the adjoining metal is reduced. Thus there is less likelihood of cracking in the process of cooling.
Incidentally, in this treatment the entrapped hydrogen which is a source to cause embrittlement is made to exit. Moreover preheating enhances ductility and also notch toughness of base and weld metal.
Welding processes where the weld metal with low hydrogen content is deposited, can eliminate the need for preheating. Such processes include low hydrogen electrodes and submerged arc welding.
Thorough fusion of the weld, the base metal and the successive layers of the weld metal is a basic requirement of all welds. Welds should not become defective due to undercutting, crater formation, overlap porosity or cracks. In case craters, objectionable concavity or under sized weld are noticed in the effective length of the weld, these should be cleaned and filled to full cross-section of the weld.
Any undercutting (removal of base metal at toe of weld) should be made good by depositing weld metal so as to restore the original profile. Stress concentration and excessive concavity are likely due to the defect, overlap (rolling over of the weld surface due to inadequate Fusion at the edge). This defect can be minimized by grinding out excess material.
Welding Positions for Steel:
The electrode’s position with respect to the joint where the weld is being made affects the quality of the weld.
The standard welding positions are the following:
(i) Flat:
The weld face is nearly horizontal. The electrode is nearly vertical.
(ii) Horizontal:
Axis of weld is horizontal. For groove welds the weld face is nearly vertical. For fillet welds the weld face is about 45° to horizontal.
(iii) Vertical:
Axis of weld is nearly vertical (welds are made upward).
(iv) Overhead:
Weld face is nearly horizontal. The electrode is nearly vertical. Welding is done from below the joint.
General Considerations in the Welding Procedure for Steel:
Welds should be allowed to be made by qualified welding operators.
Welding should not be allowed in the following circumstances:
(i) When the temperatures are very low.
(ii) When the surfaces to be welded are wet or exposed to rain or high speed winds.
(iii) When the welders are exposed to stormy or severe inclement conditions.
All surfaces and edges meant to be welded should be free from cracks, tears and other defects. The surfaces at and close to the welds should be free from rust, loose scales, moisture, grease etc. which affect the quality of welding.
The parts which are to be fillet welded should be in close contact. The gap between parts should not exceed 4 mm. In cases where the gap is more than 1.5 mm, the weld size should be increased at least by the amount of separation. In the case of butt joints the parts to be connected should be carefully aligned.
The sequences and procedures in welding should avoid unnecessary distortion and should minimize shrinkage stresses. In the process of welding the welds may be deposited so as to balance the heat applied. It is advisable to progress the welding of a member from positions where the components are relatively fixed in position towards the regions where the components appear to have greater freedom for relative movement.
The effective length of fillet weld shall be taken as only that length which is of the specified size and required throat thickness. In practice the actual length of the weld is made of the effective length shown in drawing plus two times the weld size, but not less than four times the size of the weld.
The effective length of butt weld shall be taken as the length of the continuous full size weld, but not less than four times the size of the weld.
The effective area of a plug weld shall be considered as the nominal area of the hole in the plane of faying surface. These welds shall not be designed to carry stresses.
Unless otherwise specified, the intermittent fillet welding shall have an effective length of not less than four times the weld size with a minimum of 40 mm.
The clear spacing between the effective lengths of intermittent fillet weld shall not exceed 12 and 16 times the thickness of the thinner plate joined, for compression and tension joints respectively and in no case be more than 200 mm.
Unless otherwise specified, the intermittent butt weld shall have an effective length of not less than four times the weld size and the longitudinal space between the effective length of welds shall not be more than 16 times the thickness of the thinner part joined. The intermittent welds shall not be used in positions subject to dynamic, repetitive and alternating stresses.
The design stress for a fillet weld fwd shall be based on the throat area and shall be given by-
Economy in Selection of Weld Type:
While selecting a weld, the designer has to consider the type of joint to be made and also the type of weld that need the minimum amount of weld metal. Such consideration results in saving material as well as time.
It should be noted that while the strength of a fillet weld varies with the size of the weld, the volume of the metal in the weld varies with the square of the weld size. For instance, a 10 mm fillet weld has four times as much metal per mm length as a 5 mm weld but is only twice as strong. It is worth realizing that a longer smaller size weld would cost less than a larger size weld of the same quality to provide the same strength. Small size welds are easily deposited in a single pass while large size welds need multiple passes. They take longer time, need more metal and cost more.
(i) Relative Strength of Longitudinal and Transverse Fillet Welds:
For design purposes we assume that longitudinal and transverse fillets are equally strong. In reality the transverse fillet welds offer uniform resistance and are stronger than the longitudinal fillet welds whose resistance along its length is not uniform. In fact the strength of the transverse fillet weld is 1.25 to 1.30 times that of longitudinal welds. However in our designs we will consider both these fillet welds to be equally strong.
(ii) Groove or Butt Welds:
These welds are made between the edges of two parts to be connected. These welds are used to connect two plates in the same plane. Single V, double V, Single U, double U butt welds are used. Butt welds are treated as parent metal with a thickness equal to the throat thickness and the stresses are not to exceed those permitted in the parent metal. For single grooved welds the throat thickness is taken equal to 5/8 the thickness of the thinner plate.
For double grooved welds the throat thickness is taken equal to the thickness of the thinner plate.
The strength of a welded joint depends on both the strength of the weld and the strength of the base metal. In the case of grooved welds, weld metal whose properties are matching or comparable with the base metal is used. In fillet welds the weld metal strength may be one classification lower than the matching weld metal.
As per IS 800 code, design strength of a fillet weld = fwd = fu/√3 γmw
For butt welds, the weld will be treated as parent metal with a thickness equal to the throat thickness and the stresses shall not exceed those permitted in the parent metal.
... fwd = fy/γmw
When the length of the welded joint I, of a splice or end connection in a compression or tension element is greater than 150 tt, the design capacity of the weld shall be reduced by the factor,
where Lj = length of the joint in the direction of the force transfer, tt = throat size of the weld.
The generally followed specifications for fillet welded connections are shown in Fig 5.7.
Advantages and Disadvantages of Welded Connections in Steel Structure:
The various advantages of welded connections are the following:
(i) Since the process does not involve driving holes, the gross-sectional area of a member is effective. In the case of riveted or bolted tension members deductions have to be made for the area due to punching or drilling holes.
(ii) The welding process offers an air tight and water tight joint.
(iii) Welded structures are comparatively lighter than corresponding riveted or bolted structures.
(iv) Welding eliminates the need for splice plates.
(v) Welded connections provide rigidity. Welded trusses deflect less than bolted trusses. Welded members are subjected to smaller bending moments.
(vi) A welded joint has a great strength. Often a welded joint has the strength of the parent metal itself.
(vii) Repairs and further new connections can be done more easily and faster than in bolting or riveting.
(viii) Often welded joints are economical to bolted or riveted joints. For a welded structure, maintenance and painting are easy.
(ix) Members of such shapes that offer difficulty for bolting or riveting can be very easily welded.
(x) A welded structure has a better finish and appearance than the corresponding riveted or bolted structure.
(xi) Connecting angles, gusset plates, splicing plates can be minimized and in many cases avoided in welded structures.
(xii) Steel bars in reinforced concrete structures may be welded easily. Lapping of bars may be avoided if welding is resorted to.
(xiii) It is possible to weld at any point at any part of a structure. But bolting or riveting requires enough clearance.
(xiv) The welding process is less noisy compared to riveting.
Welded connections have some shortcomings too as given below:
(i) Welding requires skilled labour and supervision.
(ii) Testing a welded joint is difficult. An X-ray examination alone can enable us to study the quality of the connection.
(iii) Due to uneven heating and cooling the welded members are likely to get warped at the welded surfaces.
(iv) Internal stresses in the weld zone are likely to be set up.