Welding Technology

استعرض الموضوع السابق استعرض الموضوع التالي اذهب الى الأسفل

Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:08 pm



1. INTRODUCTION

Welding offers a means of making continuous, load bearing, metallic joints between the components of a structure.
In structural work, a variety of welded joints are used; these can all be made up from the basic configurations shown in Figure 1, which are classified as follows:


  • butt joints.
  • tee joints.
  • lap joints.
  • corner joints.

As illustrated in Figure 2, a welded joint is made by fusing (melting) the steel plates or sections (the parent metal) along the line of the joint. The metal melted from each member at the joint unites in a pool of molten metal which bridges the interface. As the pool cools, molten metal at the fusion boundary solidifies, forming a solid bond with the parent metal, see Figure 3. When the solidification is complete, there is continuity of metal through the joint.


2. METHODS OF MAKING A WELDED JOINT

Two types of weld are in common use: butt welds and fillet welds. In the former the weld metal is generally contained within the profiles of the welded elements; in the latter, deposited weld metal is external to the profile of the welded elements.
Obviously the complete length of joint cannot be melted simultaneously. In practice a heat source is used to melt a small area and is then moved along the joint line, progressively fusing the parent metal at the leading edge of the weld pool, as shown in Figure 4. At the same time, the metal at the trailing edge of the pool solidifies. The most commonly used heat source, in structural work, is a low voltage (15 to 35 volt), high current (50 to 1000 amp) arc. As shown diagrammatically in Figure 5, the arc operates between the end of a steel electrode (rod) and the work piece. It melts both the parent metal and the electrode; molten metal from the electrode is thereby added to the weld pool.


The molten steel in the pool will readily absorb oxygen and nitrogen from the air, which could lead to porosity in the solidified weld and possibly to metallurgical problems. Figure 6 shows how this is avoided by covering the pool with a molten flux, as in Manual Metal Arc (MMA) and Submerged Arc Welding (SAW), or by replacing the air around the arc by a non-reactive gas, as in Metal Active Gas (MAG) Welding or cored wire welding.

__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:10 pm

3. STRUCTURE AND PROPERTIES OF WELDS

The solidified weld metal has a cast structure and has properties characteristic of cast steel, i.e. higher ratio of yield to ultimate strength than structural steel. The weld metal is a mixture of parent metal and steel melted from the electrode. In structural work the composition of the electrode is usually chosen so that the resultant weld metal is stronger than the connected elements. Occasionally, specific conditions may override this chocie. For example, when joining stainless steel to carbon-manganese steel, a highly alloyed electrode must be used to avoid cracking in the weld metal.
When the weld pool is cooling and solidifying, the majority of the heat flows through the parent metal alongside the joint. The steel is thus subjected to heating and cooling cycles similar to those experienced in heat treatment practice. As shown in Figure 7, the structure of the steel will be changed in this region (called the heat affected zone, HAZ). This must be taken into account in the design in terms of notch toughness (Charpy value), etc.

The structure of the HAZ will be controlled by:


  • the composition of the steel (carbon equivalent).
  • the cooling rate in the HAZ.
In turn the cooling rate is determined by:


  • arc energy, i.e. heat input to the joint.
  • type of joint.
  • thickness of steel.
  • temperature of steel plate or section prior to welding, e.g. preheat.
A method of determining the interaction of these factors in relation to the avoidance of cracks in the HAZ is given in the sample chart shown in Figure 8.

In addition to its effect on the cooling rate, preheat is used to:


  • Disperse hyrodgen from the weld pool and HAZ. Hydrogen in the HAZ increases the risk of cracking if hardening has occurred. The hydrogen comes principally from the flux. An appropriate electrode, correctly stored, will reduce the risk of hydrogen pick-up.
  • Remove surface moisture in high humidity conditions or on site.
  • Bring the steel up to 'normal' ambient conditions (20°C).

__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:11 pm

4. EDGE PREPARATION FOR BUTT WELDS

For square edge preparations the depth of melting into the plate is called the Depth of Penetration, see Figure 9a. As a very rough guide, the penetration is about 1mm per 100 amp. In manual welding the current is usually not more than 350 amp; more commonly 150-200 amp. This means that the edges of the plate must be cut back along the joint line for continuity through the thickness to be achieved (Figure 9b). The groove so formed is then filled with metal melted from the electrode (Figure 9c). Various edge profiles are used and are illustrated in Figure 10; the edges may be planed, sawn, guillotined or flame cut.


The first run to be deposited in the bottom of the groove is called the root run. The root faces must be melted to ensure good penetration, but at the same time the weld pool must be controlled to avoid collapse, as seen in Figure 11. This task requires considerable skill. The difficulties can be eased by using a backing strip.

The choice of edge preparation depends on:


  • type of process.
  • position of welding (Figure 12).
  • access for arc and electrode.
  • volume of deposited weld metal which should be kept to a minimum.
  • cost of preparing edges.
  • shrinkage and distortion (Figure 13).


حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:13 pm

WELDING PROCEDURES

The term welding procedure is used to describe the complete process involved in making a weld. It covers choice of electrode, edge preparation, preheat, welding parameters (voltage, current and travel speed), welding position, number of weld runs to fill the groove, and post-weld treatments, e.g. grinding or heat treatment. Welding procedures may be devised to meet various needs, e.g. to minimise costs, control distortion, avoid defects or achieve good impact properties. Specific aspects of the weld procedure are worth detailed comment.
5.1 Current

The current controls heat input. The minimum value is fixed by the need to fuse the plate and to keep the arc stable; the specified minimum, however, may be higher to avoid HAZ cracks. The maximum current depends on operating conditions. Usually, as high a current as possible is used to achieve faster welding, and hence lower costs. The use of maximum current may be restricted by position; in the overhead position, for example, currents above 160 amps cannot be used. High currents usually give low impact properties. Note that the current used is chosen to match the electrode diameter.
5.2 Welding Position

The effect of position on current is noted above. Welding in the overhead position requires greater skill to avoid defects, such as poor profile, and should only be used when absolutely necessary. Vertical welding is slower than welding in the flat position but requires less skill than the overhead position.
5.3 Environment

If on site welding is necessary the following points must be considered:

  • in cold weather the steel may need to be heated to bring it up to 20°C.
  • overnight condensation and high humidity can lead to porosity.
  • care must be taken to ensure the electrodes are kept dry in the stores.
  • it is often difficult to achieve accurate fitting of the joint; variable and/or large gaps may result in defective welds, distortion and increased costs.
__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:13 pm

6. SHRINKAGE

During cooling, the hot metal in the weld zone contracts, causing the joint to shrink. The contraction is restrained by the cold metal surrounding the joint; stresses are set up which, being in excess of the yield stress, produce plastic deformation. This can lead to the distortion or buckling shown in Figure 13. Distortion can be reduced by choice of edge preparation and weld procedure; examples are shown in Figure 14.

When the plastic deformation has ceased, the joint is left with the residual stress pattern of Figure 15 with tension in the weld metal and HAZ, and compression in the surrounding steel. The significance of these residual stresses is discussed in other lectures.

7. CONCLUDING SUMMARY


  • A welded joint is made by fusing parent metal from both components being joined, usually with added weld metal.
  • The properties of both the weld metal, which has melted and solidified, and the surrounding heat affected zone may differ from those of the parent metal.
  • Welding procedures should be properly specified to give a satisfactory welded joint. The major parameters are: welding position, electrode type, edge preparation, preheat, voltage, current, travel speed, number of runs and post-weld heat treatments.
  • Hot metal in the weld zone contracts during cooling causing residual stresses. Distortion will occur if appropriate control is not exercised.

__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:15 pm

1. INTRODUCTION

Lecture 3.2.2 discusses, in detail, the technical aspects of steelwork erection including the requirements for bolted connections. In some cases it may not be possible to use bolts and site welding may then be necessary. Where welding is used careful pre-planning is required, as outlined in Section 2 below.
It is always necessary to have quality control and safety procedures in force on site to ensure the successful completion of the project with minimum risk to the workforce. This lecture discusses these matters in detail, outlining the basis for a Quality Control Programme and giving guidelines on how risk to the workforce may be minimised.
2. WELDING CONNECTIONS ON SITE

It should be the aim of the designer to ensure that site connections are bolted wherever possible. There will be occasions, however, when site welding is necessary. In such cases, careful pre-planning will be required as follows:


  • it will be necessary to provide for temporary alignment of the adjacent components which are to be welded together, and to hold them in position until they are welded. The methods adopted for alignment may have to be able to carry the weight of the components and in some cases a substantial load from the structure.
  • safe means of access and a secure working platform must be provided for the welder and his equipment. The working platform may also have to incorporate weather protection, since wind, rain and cold can all adversely effect the quality of the weld.
  • the design of the weld and the preparation of the components to be joined must take into account the position of those components in the structure; the method statement for the erection and the welding procedure for each joint must take all these factors into account.
  • all the welding must be carried out by qualified welders in accordance with approved procedures.
  • a detailed welding plan must be made for the more important structural joints as well as for structures to which special specifications apply.
  • the earth return of the electric current must never be made through the steel frame of buildings, cranes, or through metallic parts of installations but must be directly connected to the construction part being welded.
  • the surface of the zone to be welded must be clean and dry.
3. QUALITY CONTROL

3.1 Quality Assurance Manual

The Quality Assurance Manual defines the appropriate procedures required to ensure that the finished product is up to specification. The staff responsible for the erection must be informed of all the variables affecting the quality of the assembly, so that they can be monitored.
3.2 Quality Control Programme

The Quality Control Programme is the particular programme, that has been specifically written for the job in hand.
It is based on the following:


  • the Quality Assurance Manual.
  • the General Contract for the Project.
  • the general standards, applicable to the Project.
  • the manufacturer's standards, procedures and specification.
The Quality Control Programme will consist of:


  • the site-organisation (in relation to Quality Control).
  • the (written) procedures.
  • the inspection programme.
3.3 Inspection Programme

The Inspection Programme is based on the erection plan and is complemented by written procedures and points of inspection. Its aim is to ensure good standards of workmanship.
It may incorporate the following:


  • Reference numbers for the work procedures.
  • Revision numbers.
  • Written procedures for:


welding
heat-treatment
non-destructive testing
alignment and plumbing
tolerances
HSFG bolting.

4. SAFETY AT THE ERECTION-SITE


By its very nature, erection of a structural frame is a process involving a certain amount of risk. The work is carried out at height and, until it has progressed to a certain point, there is nothing to which a safe working platform can be attached. In fact, it is true to say that the process of establishing a safe working platform can be as hazardous as the erection process itself; a possible solution may be to provide mobile access platforms if ground conditions permit.
The object of a Safety Procedure is to ensure that everything possible is done to eliminate the risk of an accident. In order to achieve this objective the following procedures should be adopted:


  1. The safety procedure should be communicated to all concerned with its implementation by, for example, issuing abstracts or running courses. In practice, awareness of safety aspects can best be maintained by continually monitoring hazardous areas of the site (restricted areas, scaffolding, plant, etc.) to check that the appropriate restrictions are in force, and by informing the person in authority if potential hazards exist.
  2. The necessary equipment should be made available on the site and maintained in good condition. This equipment ranges from safety helmets and belts, to ladders, working platforms and properly selected tools.

  3. The work should be organised so that as little as possible is done at height. The danger can be minimised as follows:

    × by the use of sub-assembly techniques.
    × by fixing ladders and working platforms onto the steelwork before it is lifted into place.
    × by the early provision of horizontal access walkways.
    × by the provision of temporary staircases or hoists, where appropriate.
  4. It should be ensured that all portable equipment such as gas bottles and welding plant, is firmly anchored while it is being used. Care should be taken to ensure that there are no flammable materials below on which sparks could fall.
  5. Finally, and fundamentally, the design should be done with safety in mind as follows:


× Splices should be positioned to give simple site connections, bearing in mind that these may have to be connected at height.
× Lifting cleats and connections for heavy and complex components should be incorporated, as far as possible, in the fabricated elements to be connected.
× Consideration should be given, at design stage, to incorporating cleats, brackets or holes in the fabrication to facilitate fixing of safety belts, safety nets and working platforms.

5. CONCLUDING SUMMARY


  • Careful pre-planning is required if site welding is necessary.
  • A Quality Control Programme, involving inspections, should always be implemented.
  • All site personnel should be made aware of the safety procedures.
  • Safety procedures should be strictly enforced.

__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:47 pm

. INTRODUCTION - HEAT SOURCES AND METHODS OF SHIELDING

There are three principal methods used to generate the heat required for welding:


  • oxygen-acetylene flame.
  • resistance to the passage of a current.
  • electric arc.
Each method produces a pool of molten steel which must be protected against atmospheric contamination. The method used to achieve this, i.e. the shielding technique, has a major influence on the characteristics of the process. For constructional steelwork, the processes used are usually based upon the electric arc.
In arc welding, a flux or a non-reactive (inert) gas can be used to 'blanket' the weld pool and thus exclude air. This lecture is particularly concerned with the four arc welding processes commonly used in structural work.
2. MANUAL METAL ARC WELDING

This manual method is one of the most widely used arc welding processes (see Figure 1). It requires considerable skill to produce good quality welds. The electrode consists of a steel core wire and a covering flux containing alloying elements, e.g. manganese and silicon. The arc melts the parent metal and the electrode. As metal is transferred from the end of the core wire to the weld pool, the welder moves the electrode to keep the arc length constant. This is essential as the width of the weld run is largely governed by the arc length. The flux melts with the core wire and flows over the surface of the pool to form a slag, which must be removed after solidification.

MMA has many advantages as follows:




  • Low capital cost.
  • Freedom of movement; it can be used up to 20m from the power supply (useful on site).
  • It can be used in all positions.
  • It is suitable for structural and stainless steels (but not aluminium).
Its main drawback is a low duty cycle, i.e. only a small volume of metal is deposited before the welder has to stop and insert another electrode. This is not a problem on short welds but becomes a consideration on long welds, especially when labour costs are high.
The operating characteristics of the electrode are controlled by the composition of the flux covering. A variety of electrodes are available to suit different applications. The current used is chosen to match the diameter of wire being used. When low hydrogen *******s in the weld pool are necessary to avoid cracks in the heat-affected zone (HAZ) on cooling, MMA electrodes must be baked and stored at temperatures and times recommended by the manufacturer. These procedures ensure that the electrodes deposit weld metal with appropriate low levels of diffusible hydrogen.
3. METAL ACTIVE GAS (MAG) WELDING

This process is sometimes referred to as Metal Inert Gas (MIG) Welding, although strictly speaking the term MIG should be limited to the use of pure argon as a shielding gas, which is not used for carbon steel.
MAG is a semi-automatic process where the welding gun at the end of a flexible conduit can be hand held and manipulated, but all other operations are automatic (see Figure 2).


The arc and weld pool are shielded by a gas which does not react with molten steel; in current practice the shielding gas is carbon dioxide, or a mixture of argon and carbon dioxide. No flux is necessary to shield the pool since the alloying elements are in the electrode wire, but sometimes a flux-cored electrode is used to produce a slag which controls the weld profile and reduces the liability of lack of fusion defects and the incidence of porosity. The arc length is controlled by the power supply unit. Although MAG welding is somewhat easier to use than MMA, skill is required to set up the correct welding conditions.
The way in which metal is transferred from the electrode wire to the molten pool depends upon current, voltage and shielding gas composition. As the current is increased the form of the transfer changes abruptly to a stream of fine drops which are propelled across the arc gap by the electro-magnetic forces in the arc. This is called spray transfer and it enables welding to be carried out against gravity. Changing the shielding gas to carbon dioxide (assuming steel electrodes) causes the transfer to become more globular and less well directed; however, the situation can be reversed by using a mixture of inert gas and carbon dioxide.
When using steel electrodes, decreasing the arc voltage markedly and also reducing current (by reducing the wire feed rate) results in a form of transfer known as dip transfer or short-circuit transfer. In this mode of transfer metal is fused directly into the pool without passing freely across the arc gap. At slightly higher voltages the transfer is across a gap but is in larger globules without the pronounced directionality of the spray transfer. The globular to spray change is less marked with steel than with certain other metals. Welds in steel are sometimes made in which this type of transfer predominates. It is also possible to control the type of metal transfer at low to medium currents by using a special power source which delivers pulsed current to the arc.
For 'positional' welding, i.e. vertical and overhead, the current must be kept below 180 amp (so that welding takes place in the 'Dip Transfer' mode) and welding speeds are comparable with MMA. Overall times for a joint, and hence productivity, are better since there is no need to deslag or change electrode. In the flat position, currents up to 400 amp ('Spray Transfer') can be used to give high welding speeds. MAG welding is especially suitable for fillet welded joints, e.g. beam to column and stiffener to panel connections. It is not easy to use on site because of problems of equipment movement and the need to provide screens to avoid loss of the gas shield in draughty conditions.
4. SUBMERGED ARC WELDING (SAW)

This is a fully mechanised process in which the welding head travels along the joint automatically (Figure 3). The electrode is a bare wire which is advanced by a governed motor. The voltage and current are selected at the beginning of the weld and are maintained at the pre-selected values by feed-back systems which, in practice, vary in sophistication. The flux is in the form of particles and is placed on the surface of the joint. The arc operates below the surface of the flux, melting a proportion of it to form a slag. Unfused flux is collected and may be re-used for the next weld.

Submerged arc welding is generally operated at currents of between 400 and 1000 amps. This means that weld pools are large and can only be controlled in the flat position, although fillets can be deposited in the horizontal-vertical position up to 10mm leg length in one run. Where it is difficult to control penetration in a root run a backing strip may be used; alternatively, the root run can be made by MMA or MAG and the groove filled with SAW. SAW offers considerable advantages when welding long joints (i.e. those in excess of one metre in length). The high welding speeds and continuous operation lead to high productivity. An accurate joint fit-up is, however, a prime requirement.
5. STUD WELDING

This is a variation of arc welding in which studs are welded to plane surfaces automatically (Figure 4). The stud, which may be a plain or threaded bar (if plain it will have a head) is the electrode and it is held in the chuck of a welding gun which is connected to the power supply. The stud is first touched onto the surface of the steel plate or section. As soon as the current is switched on, the stud is moved away automatically to establish an arc. When a weld pool has formed and the end of the stud is molten, the latter is automatically forced into the steel plate and the current is switched off. The molten metal which is expelled from the interface is formed into a fillet by a ceramic collar which is placed around the stud arc at the beginning of the operation. This ferrule also provides sufficient protection against atmospheric contamination.

Stud welding offers an accurate and fast method of attaching shear connectors, etc., with the minimum of distortion. Whilst it requires some skill to set up the weld parameters (voltage, current, arc time and force), the operation of the equipment is relatively straightforward.
6. CHOICE OF PROCESS

When choosing a welding process a number of factors must be taken into account:




  • Thickness of the material to be welded.
  • Where the welding is to be carried out. SAW and MAG are best carried out in the protected environment of the fabrication shop. MMA may more readily be used on site.
  • Accuracy of fit-up and possibility of misalignment. SAW and Spray Transfer MAG require good fit-up; they are particularly sensitive to variation in root gap and/or root face dimensions.
  • Access to joint. It is necessary to ensure that both the welding plant and the welding torch or head can be properly positioned.
  • Position of welding. SAW and Spray Transfer MAG are not suitable for vertical or overhead positions. Dip transfer MAG is acceptable for vertical and overhead welding, but MMA is probably best for overhead work, especially on site.
  • Steel composition. Steels with lower carbon equivalent values are more readily welded and require lower preheat levels.
  • Comparative cost. The cost per unit length of weld can be calculated, but depends upon the burn-off rate of the process and must allow for differences in duty cycle (idle time between electrodes for MMA, etc.), Figure 5.

7. CONCLUDING SUMMARY




  • The welding processes commonly used in constructional steelwork are: Manual Metal Arc Welding, Dip and Spray Transfer Metal Active Gas Welding, Submerged Arc Welding and Stud Welding.
  • Stud welding is used for attaching shear connectors and other studs to structural steelwork.
  • The correct choice of process depends on: situation, fit-up, access, position, steel composition and economic factors.
__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:47 pm

1. INTRODUCTION - FABRICATION

The fabricator's role is to convert rolled steel into finished goods with added value. This is achieved by selling workmanship and machine utilisation on a competitive basis where costs are directly related to time.
Fabricators rely increasingly upon production engineering techniques. Their continued success in this direction depends upon better standardisation. Time and therefore labour costs can be cut significantly by the repetition of dimensions and geometry, member sizes and shapes, centres and diameters of bolts, etc. All of these are amenable to rationalisation. Further economy is derived by reducing the number of detailed components, which tend to be labour intensive to produce, even when this results in heavier parent members. The cardinal rule is that, relatively, labour is expensive but material is cheap (Figure 1).
2. COST STRUCTURE

Fabrication costs are estimated by separating the various activities into categories such as cutting, drilling and welding which enables man hours to be allocated and valued to arrive at a total price.
Relying upon a combination of historical data and practical experience, the cost build-up bears little relationship to the weight of steel involved, although cost references in ECU/tonne can be a useful index for rapid comparison of different classes of work.
A typical breakdown in costs, in the light to medium category, shows that over 50% of the fabricator's cost is absorbed by labour charges and overhead expenses (Figure 1).
It is customary to recover such expenses as a contributory factor to labour. If the ratio between labour and overheads is 1: 2½, it is significant that for every 100 ECU of labour cost incurred, the amount chargeable would be 100+250=350ECU.
3. PRODUCTION NETWORK

Fabricating companies differ widely in layout, capacity and scope. Whilst the extent and nature of the services available is influenced by policy and resources, the basic flow of activities tends to follow a similar pattern. This can be visualised as a tunnel for the main flow or Primary Operations, supported by branches or Secondary Operations (Figure 2).

This network forms the basis for production control, which is time related to cost standards. Output must be geared to the sequence of the construction programme. This rarely coincides with the most effective use of all resources. The system has to be extremely flexible to respond to changes in demand whilst minimising disruption or costly delays.
3.1 Primary/Secondary Production

The planning objective is to schedule production so that raw material is transformed into a finished state within an allocated time.
Since most of the important machine tools, such as saws, are sited at the start of the primary production line, the flow of material has to be sustained by an independent supply of essential components such as brackets, cleats and plates in the correct quantities and in the correct order.
This is the task of secondary production together with sub-assembly of detailed fabrications in suitable cases. Bought-in (BI) items or services of a specialised nature such as forgings, pressings or even non-destructive testing have to be available at the correct time.
3.2 Workshop Layout - Material Preparation

Steel framed buildings are mainly constructed as a series of linear elements using standard sections. The preparation area for these is typified by a group of fixed work sections consisting of (Figure 3):

A. Blast cleaning
B. Sawing
C. Drilling
D. Cropping/Punching
The initial step is to pass the steel through a blast cleaning cabinet at "A" to remove any surface rust and mill scale. Various levels of surface treatment are available, but for most buildings, a standard of SA 2½ to the Swedish specification SIS 055900 is adequate. This requires at least 95% of the surface to be clean.
The next stage is to transfer the material to the sawing station at "B" for cutting to length followed by drilling of holes at "C". In a number of workshops, sawing and simultaneous 3 axis drilling may be combined as one activity. Alternatively, angle sections and flats of suitable thickness for cropping and punching would be routed directly to "D".
For speed and ease of handling, sections are transported increasingly by a system of powered conveyors fed by cross transfers. The latest automation now allows all operations and material flow to be conducted from a central numerically controlled console.
Because plates are less stiff, these tend to be more awkward to handle. Lifting and handling is usually carried out by an overhead magnetic crane for subsequent cutting by flame or guillotine in a separate plate working area.

__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:48 pm

3 Workshop Layout - Assembly/Finishing

At this stage the main elements on the primary flow are joined by secondary components, end plates, stiffeners, etc. for fitting and assembly, mostly by welding. Depending upon the nature and purpose of the structure, some bolting may be used, if only for trial alignment. However, as a general rule, shop connections are welded and site connections bolted.
Due to variations in the size and nature of the work carried out in any period, the assembly area has to be extremely flexible and well serviced by cranes. Output must be geared to the sequence of the construction programme. As a result designated areas may have to be switched rapidly from beams and columns to bulky lattice girders.
Further planning complications arise because the most cost effective use of workshop labour and equipment rarely coincides with site requirements. It is for this reason that seemingly simple modifications are costly to execute once production has commenced.
Where priming paint is required, elaborate specifications, which are not necessary for steelwork contained within a normal building environment, can easily add 20% to fabrication costs. The function and future maintenance requirements should be considered in each case, rather than adopting a blanket philosophy.
Paint coatings for structural steelwork should "flash off" fairly rapidly to allow further handling and to minimise congestion. Whilst brushing is suitable for touching up minor damage, large surfaces can only be covered economically by spraying. Spraying can be carried out manually or automatically where the work is conveyed through an enclosed cabinet containing the spray nozzles. The process may also be supplemented by a drying kiln.
After assembly, inspection concentrates mainly upon overall dimensions, position of cleats, holes and so on, to ensure proper alignment during site erection. Framed elements, such as latticed girders, are self checking to a certain extent by virtue of the fit of members during assembly. This principle is often used to prove complex structures by trial erection prior to despatch.
Where in-depth weld examination is required, it should be conducted at the appropriate stage determined by the nature of the work, and to the level specified by the Engineer. In the interests of economy however, it should be noted that radiographic and allied techniques are, not only expensive operations, but attract additional costs due to their disruptive influence upon production. Judgement should be exercised to confine the programme of examination to those areas critical to structural performance.
The aim of inspection is to ensure that the steelwork complies with the contract documents. For the majority of building structures the inspection pattern outlined is practical and economic. Where more precise tolerances or accuracy are required, the frequency and intensity of inspection may need to be higher. For this reason inspection procedures need to be clearly identified in the tender documents so that appropriate provisions may be made by the fabricator.
Following an itemised numerical check together with application of identification marks, the steelwork is transferred to the finished stockyard unless it is due for immediate transport. There it is stacked ready for consignment, together with any loose fittings wired together and attached to the parent member.
Transport operating costs are not based upon load factor. A vehicle loaded to a fraction of its rated capacity will cost exactly the same as one which is fully laden. Framed elements occupy considerable space but it may be possible to mitigate the consequences by the number and disposition of splices.
In addition to the site programme, due regard must be given to limitations of off-loading and handling facilities, to access restricted to particular timings, to clearance under low bridges, and to police authority requirements concerning the transport of wide loads.
4. DESIGN/DETAILING ECONOMIES

In considering possible structural options, an overall compromise has to be achieved which recognises the links between related cost areas. Unless this consideration extends from material specification to site erection, cost perceptions may become distorted. Details are largely dictated by the basic design concept which is the key factor in determining how the structure will be made, how it will be transported and ultimately assembled on site.
Whilst it is not possible to lay down hard and fast rules, the following examples are intended to be illustrative.
Column Bases (Figure 4): Detail (a) uses no fewer than eleven separate plate components with extensive welding. Not only has this work to be conducted during primary assembly but considerable manipulation will also be necessary not only for access but to control weld distortion.

By comparison, the base detail (b) using channels would probably be longer with thicker base plates but the number of components is reduced to six and workmanship is drastically cut down. Note also that the inner edge of the two base plates is welded to the column flange eliminating any need for separate stiffeners.
Multi-storey Columns (Figure 5): Based on the philosophy of lowest weight, the columns involve three changes of section profile with two splices. It will be noted that the latter require packing pieces either laminated or solid machined to accommodate the difference in depth.

The saving in material costs by reducing the shaft above the 4th floor will be overtaken by the cost of the splice and, if the total material requirement is less than 20 tonnes, further costs will be incurred by quantity premiums on the basic rate.
The change in section depth also varies the geometry and therefore the lengths of the bracing members will vary with consequent adjustment to the skew of the end connections.
Consider the column shaft from ground to 2nd floor. Clearly the loads will be greatest here. A possibility is to investigate the use of high strength steel to match the upper section in low strength steel. Although high strength steel is more expensive, the result will be consistency of details, beam lengths and connections throughout.
Finally, it should be noted that the bracing connection is attached to the bracing member rather than the column. The benefits are as follows:


  1. Primary production is faster because operations on the column are kept to a minimum.
  2. The column lengths are less obstructed and therefore easier to nest for transport.
  3. Welded projections are vulnerable to transit damage and costly to rectify on site.
Lattice Trusses (Figure 6): Whilst there may be sound reasons for adopting bolted joints, a variety of differently shaped gusset plates are required. They all have to be punched or drilled in addition to the framing members and then individually bolted up.

Because the inside faces of the boom members will become permanently inaccessible, these components and the gussets would need to be painted individually in advance of assembly. This work is expensive and disruptive. Therefore, although the material cost of T-sections is up to 20% higher than angles, the welded truss may still prove to be a cheaper proposition, except for girders with fairly short spans.
__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:51 pm

5. GENERAL - ERECTION

Whilst steelwork erection may be regarded as the final stage of fabrication, it differs from the latter in two principal ways: firstly, there is the added dimension of height and the time occupied by vertical movement of materials, equipment and labour; secondly, the fact that work has to be carried out in the open means that progress may be hampered by adverse weather.
By its nature, work done on site can become unduly expensive. The primary aim of the programme should be to minimise costs by condensing the time scale realistically. Options and alternatives need to be carefully examined at the preliminary design stage otherwise the scope for reducing the time scale may be unduly restricted.
Clearly the significance of the various issues will vary according to the type of building and any limitations which the site and its environment may impose. Even when structures possess marked similarities, different erection methods and procedures may need to be adopted. For this reason, only the broad principles concerning erection can be stated.
5.1 Site Planning

Invariably, erection of structural steelwork has to be closely integrated with other major trades such as flooring, cladding and services. Operations on site where there may be competition for limited resources, are potentially difficult to control. A far-sighted strategy has to be developed and maintained.
Key objectives and, most importantly, starting and finishing dates must be clearly established and progress reviewed on a regular basis. Failure to meet commitments can result in substantial cost penalties. Further complications may easily arise which are totally disproportionate to the cause.
5.2 Site Organisation

The maximum size and weight of the various steel members which can be delivered may be restricted on a site with limited and restricted access.
Narrow streets in a busy town centre may cause difficulties with space to manoeuvre. Waiting time to off-load may also be restricted to specific periods. Matters of this kind must be investigated well in advance and decisions made accordingly.
Within site, movement may often be hampered by a variety of obstructions such as scaffolding, shoring, pile caps, excavation, and so on. Service roads and off-loading areas need to be hard cored and adequately drained to support heavy vehicles during the severest winter conditions. The steelwork has to be erected in the general sequence determined by the construction programme. Each consignment of steel has to be strictly regulated to this timetable. Whilst in some instances, a few key components can be lifted directly from the vehicle into position, most of the material will need to be off-loaded and stacked temporarily until needed.
The area of the site allocated for this purpose has to be orderly and well managed, particularly where space is limited. To compensate for minor interruptions in delivery, for example due to traffic delays, a small buffer stock is usually held in reserve.
Space is also required for laying material out and for assembly of frames or girders prior to hoisting into position.
5.3 Setting Out

Before commencement of erection, the plan position and level of the column bases should be verified by the erection contractor. This needs to be carried out as soon as possible to ensure that any errors can be corrected in good time or, at least, alternative measures approved and introduced.
Checks should include not only the centres of the foundation bolts relative to the reference grid lines, but also the projection of the bolts above the base level.
To compensate for minor discrepancies, a limited amount of deviation of the column from its true vertical and horizontal position is provided for by the grout space under the baseplate and by leaving a movement pocket around each bolt during pouring of the concrete. Normally this will allow latitude of about ±25mm in any direction.
5.4 Operations

Steel erection may appear to be a series of distinct operations when in reality they overlap and merge. Nevertheless, each complete stage of the work has to follow a methodical routine which consists of:


  • Hoisting
  • Temporary Connections
  • Plumbing, lining and levelling
  • Permanent connections.
Because minor dimensional inaccuracies can accumulate during fabrication and setting out, it would be impractical to complete the entire structure before compensating for these by adjustment. The work is therefore sub-divided into a number of phases which may be controlled by shape or simply by an appropriate number of bays or storeys. For stability, each phase relies upon some form of restraint to create a local box effect. This effect may be achieved in various ways, such as employment of temporary or permanent diagonal bracing.
Initially, end connections and base anchorages are only secured temporarily. After completion of plumbing, lining and levelling, all connections are then made permanent by tightening up all nuts or inserting any bolts initially omitted to assist adjustment. This process allows substantial areas to be released quickly for grouting and following trades are able to proceed much earlier than would otherwise be possible.
5.5 Single-Storey Buildings

Under normal circumstances, single-storey buildings are quickly and easily erected. A high proportion of industrial buildings are rigid jointed. It is common practice to bolt, assemble or weld these joints on the ground and then lift the complete frame upright using a mobile crane.
Lattice girders and trusses are also erected in a similar manner but temporary stiffening may be required to prevent lateral buckling. Care should also be taken, by provision of lifting eyes or similar at specific positions, to ensure that slender members are not subjected to undue compressive stresses.
Ideally, erection should commence at an end which is permanently braced. When this is not possible, temporary bracings should be provided at regular intervals as a safeguard against collapse or deformation (Figure 7).

Space frames are designed to span in two directions. Because of the number of connections required, it is much more economical to assemble the modules at ground level where the joints are readily accessible and then hoist the complete framework. Two or possibly four cranes may be needed depending on the size of the building. Meticulous co-ordination is essential.
5.6 Multi-storey Buildings

In most cases, multi-storey buildings are erected storey by storey enabling the lower floors to be completed earlier, offering access, overhead safety and weather protection. Depending upon the site, a single tower crane may be the sole lifting facility. In this case use of the crane has to be shared between a number of sub-contractors, thereby limiting available "hook" time for any given trade.
Since the position of a tower crane is fixed (Figure Cool, it is completely independent of any obstructions, such as basements or ground slabs, which could deny access to a mobile crane. This independence allows useful freedom in overall planning. However, the fixed location also means a fixed arc of lifting capacity where the load will be minimum at the greatest reach. As a result the steelwork may have to be provided with site splices simply to keep the weight of the components within such limits.

One of the major virtues of a mobile crane (Figure 9) is its flexibility and independence which enables it to keep moving with the flow of the work. These cranes are generally fitted with telescopic jibs which allow then to become operational very quickly. The vehicles are stabilised during lifting by extended outriggers equipped with levelling jacks.

Whilst permanent stability in the completed building may be introduced, in a number of ways, including braced bays, rigid joints and stiff service cores (Figure 10) and via diaphragm action of the floors, stability must also be ensured throughout the entire construction programme. It may therefore be necessary to install temporary bracings solely for this purpose, which must not be removed until the permanent system has been provided and has become effective.
5.7 Timing

The rate of steelwork erection is governed by a wide range of factors some of which are beyond the influence of the design engineer. The factors which he can control include:


  • type of end connections.
  • extent/type of bolting or welding.
  • number of separate pieces.
Simple connections for shear force are straightforward and employ Grade 4.6 or 8.8 bolts. The bolt diameter should be selected with a degree of care. For example, whilst a single M30 bolt has more than twice the shear capacity of two M20's, the effort required to tighten an M30 bolt is some 3½ times greater. An M20 bolt can be tightened without difficulty using ordinary hand tools, a considerable advantage when working at height.
Joints which are required to transmit bending moments are inherently more robust and may require stiffening ribs and haunches; if this is the case careful attention is required to ensure access for the bolts. For such applications pre-tensioned bolts are often used. They are normally tightened to a minimum torque using a power operated wrench.
Compared to bolting, the site welding of joints is time-consuming and expensive for conventional structures. There may be occasions, however, when site welding is the only realistic way to form a joint, as, for example, in alterations or remedial work. In this case, joint preparation, fitting, inspection and the provision of purpose made enclosures (for access and weather protection) are additional cost factors that must be taken into account.
As a rough guide, about 50% of erection man hours are occupied with lining, levelling, plumbing and final bolting and the remainder of the time is spent hoisting members into position. However, in suitable cases, beam and column elements may be pre-assembled at ground level and lifted directly on to their foundations.
5.8 Safety

The erection of a building framework is potentially hazardous. Many serious and fatal accidents occur each year on construction sites and most of these are caused by falling from, or whilst gaining access to, heights; handling, lifting and moving materials, however, are also hazardous.
Risks can be minimised considerably by measures such as adequate provision for stability throughout construction, accessibility of splices and connections, guard rails and attachments for safety harnesses and so on.
In addition, safety, need not be compromised on grounds of cost. For example, it will prove cheaper to assemble frames at ground level (Figure 11) rather than bolt them together in mid-air. Metal decked floor systems are not only economical but offer rapid access for all trades whilst providing overhead protection. Safer access is also promoted by the immediate provision of steel stair flights at each floor level as steelwork erection proceeds.

Current and future legislation may place greater responsibilities upon the design engineer because of the influence of design and details on the method and sequence of erection.

__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:57 pm

. CONCLUDING SUMMARY

  • Steelwork erection normally occupies a relatively short period in the construction programme, but considerable activity occurs during this time which is vital to the performance of the contract as a whole.
  • The steel framework should not be seen in isolation but as a key link in the construction chain where the time saved can have considerable impact in lowering overall costs.
  • Early consideration should be given to erection during design and detailing so that the full benefits of steel construction may be realised and, the need for late changes and subsequent compromise can be substantially reduced.
__________________






Welding Calculations





For more information on Mohrs Circle got to efunda.com

__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

رد: Welding Technology

مُساهمة من طرف حسن المهندس في الخميس أبريل 03, 2008 4:57 pm

T Fillet Welds

__________________

حسن المهندس
مشرف تكنولوجيا اللحام
مشرف تكنولوجيا اللحام

عدد الرسائل : 52
العمر : 40
تاريخ التسجيل : 08/03/2008

معاينة صفحة البيانات الشخصي للعضو

الرجوع الى أعلى الصفحة اذهب الى الأسفل

استعرض الموضوع السابق استعرض الموضوع التالي الرجوع الى أعلى الصفحة

- مواضيع مماثلة

 
صلاحيات هذا المنتدى:
لاتستطيع الرد على المواضيع في هذا المنتدى