2850 diploma in engineering fabrication and welding ...· 2850. diploma in engineering. fabrication

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  • 2850

    DIPLOMA IN ENGINEERING

    FABRICATION AND WELDING TECHNOLOGY

    PRE-ATTENDANCE REVISION WORKBOOK FOR

    UNIT 213

    V1

  • Welcome

    This booklet is designed to outline the areas that you will study when you complete unit 213 at the centre.

    Throughout your time in centre you will have a qualified assessor there to help and advise you.

    We want you to achieve the best possible benefits from your time in centre, therefore before attending we would like you to revise by carrying out the work detailed in this booklet.

    Please remember that you should not carry out any welding practical work unless you are in class with one of our qualified assessors.

    Please bring your completed workbook with you when you attend in class.

    We very much look forward to welcoming you at the centre.

    When you have studied this material please contact us to make arrangements for your practical training.

  • Date:

    Title First name Last name College number

    Please complete this page before attending the centre as there will be many other students who have this same workbook.

    You do not need to leave this workbook at the centre. You can take it home with you and retain it for your future reference.

  • SmartScreen Level 2 Certificate/Diploma in Engineering Unit 213 Handout 1

    Unit 213: Welding by manual metal arc process Handout 1: Manual metal arc welding principles and practice

    Heat source: electric arc

    Shielding media: flux, slag and gas generated by electrode covering

    Current range: 25450 A

    Range of electrode sizes: 1.6 to 8 mm diameter (commonly 2.5 to 6 mm)

    Principles of process The welder establishes an arc between the end of the electrode and the parent metal at the joint. The arc melts the parent metal and the electrode to form a weld pool that is protected by the molten flux layer and the gas generated from the flux covering of the electrode. The melted metal is transferred across the arc into the molten pool and becomes the deposited weld metal. As the welding progresses the welder moves the electrode along the joint and towards the weld pool keeping a constant arc gap and leaving a uniform deposit. The solidified slag is removed from the surface and the weld continued with a new electrode.

    ELECTRODE CORE WIRE

    ELECTRODE COVERING

    ELECTRIC ARC

    WELD POOL

    GAS SHIELDSOLIDIFIED SLAG

    WELD METAL DEPOSIT

    MOLTEN DEPOSIT

    PARENT METAL

    ELECTRODE

    Control of arc length

    MMA welding requires the welder to maintain a constant arc length (normally between

    2 and 3 mm), whilst feeding the electrode towards the work, as it is consumed by the

    arc, and at the same time moving along the joint. The consistency of the weld is

    therefore almost entirely dependent on the welder's skill in estimating the arc length

    and manually adjusting the electrode feed rate. Naturally, slight variations in arc

    length are unavoidable and this has a direct affect on the arc voltage and subsequently

    the arc current.

  • SmartScreen Level 2 Certificate/Diploma in Engineering Unit 213 Handout 1

    When the arc length remains constant the arc voltage will also remain constant. An increase in arc length will result in an increase in arc voltage and a decrease in the arc length will reduce the arc voltage. This is true for all arc welding processes and can be expressed in the form of a graph.

    ARC LENGTH

    Striking an arc MMA welding requires the electrode to come into contact with the work in order to establish an arc. This process is known as arc initiation or more commonly as arc striking. Arc striking is very similar to striking a match for the beginner, for the more experienced welder it is usually more refined. First the welder moves the electrode close to the work, in readiness for striking, then it is necessary to position the shield (head or hand) in front of the face. With the shield in place the electrode can then come into contact with the work. This is done by striking the electrode on the work at the point where welding is to be started. Once contact is made the electrode is withdrawn to approximately 2 mm above the work surface, thus establishing the arc. This arc length should be maintained throughout welding.

    1 MOVE THEELECTRODE CLOSE TO THE WORK

    2 MAKECONTACT WITH WORK

    3 WITHDRAW2 mm TO ESTABLISH THE ARC

    WORK WORK

    2 mm

    WORK

    Striking an arc

    Upon completion of the weld or when it is necessary to change electrodes it is necessary to break the arc. Before breaking the arc the welder should pause, with the

  • SmartScreen Level 2 Certificate/Diploma in Engineering Unit 213 Handout 1

    electrode held in position long enough to build up the weld pool, then simply move the electrode away from the work surface quickly. Building up of the weld pool avoids cavities, fine cracks and porosity defects in the solidified weld pool.

    CONCAVE CRATER CONVEX CRATER AFTER BUILD-UP

    Filling a crater

  • SmartScreen Level 2 Certificate/Diploma in Engineering Unit 213 Handout 2

    Unit 213: Welding by manual metal arc process Handout 2: Inter-relationship between voltage, current and arc length

    The following three diagrams show how the arc length has a direct affect on the arc

    voltage that in turn affects the current that can affect the arc length. Note the

    connection of the ammeter and the voltmeter. The ammeter is connected in series

    and so forms part of the circuit, while the voltmeter is measuring the voltage across the

    terminals and is therefore connected in parallel.

  • SmartScreen Level 2 Certificate/Diploma in Engineering Unit 213 Handout 2

    Suggest a suitable electrode size for this welding current.

  • SmartScreen Level 2 Certificate/Diploma in Engineering Unit 213 Handout 3

    Unit 213: Welding by manual metal arc process Handout 3: Arc blow Arc blow is the deflection of an electric arc from its normal path because of magnetic

    forces. Arc blow is frustrating to the welder, it adversely affects the appearance of the

    weld, results in defects such as excessive spatter, incomplete fusion and porosity and

    makes the weld pool and molten slag difficult to control.

    When an electric current passes through a conductor it produces a magnetic field in circles around the conductor with their centres in the conductor. The electric arc being

    a conductor has this magnetic field around it and as long as the field is symmetrical

    (equal all around) and there is no unbalanced magnetic force, there will be no arc

    deflection. Under these conditions the arc is in line with the centre line of the

    electrode. If the magnetic field is disturbed, the forces on the arc are no longer equal

    and the strongest force deflects the arc.

    Arc blow is encountered in corners and in deep groove joints such as vee butts and U butts.

    The most effective method of overcoming the problem of arc blow is to use ac current instead of dc. Alternating current reduces arc blow because the rapidly changing

  • SmartScreen Level 2 Certificate/Diploma in Engineering Unit 213 Handout 3

    polarity sets up other currents (known as eddy currents) that tend to neutralise the magnetic field or greatly reduce its strength. Other methods that can be tried to reduce arc blow are:

    using a smaller electrode

    reducing welding current

    maintaining a short arc

    angling the electrode opposite the direction of arc blow

    REDUCED ARC BLOWARC BLOW

    welding towards a heavy tack or towards a weld already made

    repositioning the return lead

    wrapping the return cable around the work in a direction that will tend to

    neutralise the magnetic field in the work

    any combination of any of two or more of the above.

  • SmartScreen Level 2 Certificate/Diploma in Engineering Unit 213 Handout 3

    Unit 213: Welding by manual metal arc process Handout 4: Electrodes

    An electrode for MMA welding is a coated metal wire of compatible composition to the metal(s) to be welded. During welding an arc is formed between the electrode and the

    base metal. The molten electrode deposits into the weld pool and can form a bond

    between two pieces of metal.

    Electrodes are available for various metals, they come in a range of different types and

    sizes to suit different output currents (dc or ac/dc) and welding positions (flat and horizontal/vertical or all positions). These factors and a number of other factors are considered during electrode selection.

    Factors influencing electrode selection: mechanical properties

    type of coating

    welding position

    electrical requirements

    rate of weld deposition

    surface appearance

    type of penetration

    metal recovery rate.

    These requirements are reflected in BS EN ISO 2560: 2005 that deals with the welding consumables, covered electrodes for manual metal arc welding of non alloy

    and fine grain steels and classification.

    Electrode classification This standard details a system by which electrodes can be given a code number. This

    makes it easier to identify the different types without using trade names.

    The classification is divided into eight parts.

    1 The symbol indicating the product/process used. 2 The symbol indicating strength and elongation of the weld metal. 3 The symbol indicating the impact properties of the weld metal. 4 The symbol indicating the chemical composition of the weld metal. 5 The symbol indicating the

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