MEM05 Metal and Engineering Training Package MEM05048B Perform advanced welding using flux cored arc welding process Learner guide Version 1

Transcription

1 MEM05 Metal and Engineering Training Package MEM05048B Perform advanced welding using flux cored arc welding process Learner guide Version 1 Training and Education Support Industry Skills Unit Meadowbank Product Code: 5811

2 Acknowledgments The TAFE NSW Training and Education Support, Industry Skills Unit Meadowbank would like to acknowledge the support and assistance of the following people in the production of this resource package: Avesta Polarit BOC Gases Australia CIGWELD Lincoln Australia OneSteel Australia Standards Australia Writers: Ed Harkness (Illawarra Institute) Stephen Davies (TES, Industry Skills Unit) Reviewer: John Anderson (Hunter Institute) TAFE NSW Project Manager: Stephen Davies Education Programs Manager Training and Education Support, Industry Skills Unit, Meadowbank TAFE NSW Enquiries Enquiries about this and other publications can be made to: Training and Education Support, Industry Skills Unit Meadowbank Meadowbank TAFE Level 3, Building J, See Street, MEADOWBANK NSW 2114 Tel: Fax: TAFE NSW (Training and Education Support, Industry Skills Unit Meadowbank) 2014 Copyright of this material is reserved to TAFE NSW Training and Education Support, Industry Skills Unit Meadowbank. Reproduction or transmittal in whole or in part, other than subject to the provisions of the Copyright Act, is prohibited without the written authority of TAFE NSW Training and Education Support, Industry Skills Unit Meadowbank. ISBN TAFE NSW (Training & Education Support Industry Skills Unit, Meadowbank) 2014

3 Topic 1: FCAW safety Electrical safety Welding operators commencing study of this advanced welding unit must be familiar with the dangers and risks associated with working with electrical welding equipment. A brief summary of these safety issues are as follows: Electric shock - low voltage Electric shocks are possible on the secondary (low voltage) side of the welding circuit. They may be caused by: Working on wet floors - a shock may be felt when contact is made with machine parts, wire electrode or return lead clamps. Always stand on insulated mats or wooden boards to reduce the risk Always wear dry leather gloves or gauntlets Working in a very humid climate or rainy weather Keep wire, spools and gloves dry at all times. Electric shock - high voltage High voltage shocks can be avoided by: Having power sources checked and tested on a regular basis by licensed electrical repairers Never interfere with the internal electrical components of a welding power source. Symptoms of electrical shock The severity of an electric shock can range from a tingling sensation, muscle spasms, burns, or in extreme cases, death. When electricity passes through the body, particularly through the arms or legs it causes muscles to contract when the heart is in this current path it can stop the heart beating. Factors that can affect the severity of an electric shock include: The open circuit voltage (OCV) of the power source Whether the power source supply is alternating (a.c.) or direct current (d.c.) The amount of moisture generated from perspiration, humidity or other source How well the operator is insulated from the gun and the workpiece The parts of the body in contact with the work and/or the wire electrode. Current flow from hand to hand or hand to foot via the torso (heart) are the most dangerous Susceptibility of the victim to shock which is dependent on general health and wellbeing of the operator as well as other factors. Weld fumes Compared to other welding processes flux cored arc welding is the largest fume producer. The volume of fumes generated by the process will vary depending upon such factors as the type of wire, size and level current used. Self-shielding gasless and metal cored wires have a tendency to generate more fumes than other types of wire. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014 Page 15 of 110

4 Additional precautions should be taken when FCAW stainless steels due to concentrations of chromium fumes. To minimise fume inhalation an approved fume extraction system must be used to protect the operator and others working in the vicinity from fumes, dusts and metal particles. Welding fumes consist of: Particulate fume, the part you can see and Gaseous fume, the invisible part which you usually smell. Particulate fume is largely formed from vaporisation of the welding consumable. More than 90% of the fume arises from vaporisation of the wire electrode as weld metal is transferred across the arc. The composition of particulate fume is influenced by the composition of the consumable used. To a lesser extent, particulate fume will also depend on the composition of the material being welded, such as chromium and nickel compounds within stainless steels. Gaseous fumes present when gas metal arc welding include ozone (O 3 ), oxides of nitrogen and carbon monoxide. The intense ultraviolet light emitted by the FCAW process reacts with the oxygen in the surrounding atmosphere to form toxic ozone. Welding aluminium using aluminium-silicon wires have a tendency to produce high ozone levels. When using carbon dioxide gas to weld plain carbon steels, concentrations of carbon monoxide are formed, whilst high levels of nitrogen dioxide are also produced in the arc. Because these gases are found in the fume column, adequate ventilation is required to protect the operator. The mode of metal transfer used has a significant effect on fume levels produced by the FCAW process. For example, spray transfer uses high current densities and longer arc lengths compared to short arc transfer. As a result, spray transfer will generate much higher levels of fume. Due to the close proximity of the operator to the welding arc, fume generated presents an immediate danger to the health and safety of the operator and those working in close proximity. A summary of fume generation sources include: Elements present in the wire electrode, i.e. chromium, nickel etc Elements in the parent metal Metal transfer mode used (spray, short arc) The production of toxic gases such as ozone, nitrous gases and carbon dioxide/ monoxide. Over-exposure to ozone is most likely to occur during GMAW of aluminium and stainless steels or when welding in a confined space. Surface coatings on steel such as: - Galvanising - Cadmium plating - Chrome plating - Paints and solvents - Paints (red oxide, lead based paints) - Degreasing solvents. Page 16 of 110 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014

5 General wire information As with all welding consumables, operators should follow approved welding procedures or consult and follow the manufacturers recommendations for correct operation of each type of flux cored electrode. Ø1.2 and 1.6 mm FCAW wires are common, although larger diameters such as Ø2.4 mm gas-assisted wires are better suited for downhand welding applications. Smaller sizes such as Ø0.8 and 0.9 mm self-shielding wires are also available. 15kg spools are common, however small handi-spools are available with self-shielding wires. Heavy duty contact tips and knurled drive rolls are recommended for FCAW to prevent the tubular wire being crushed during feeding. Refer to MEM05047B for more details. Gases for carbon and low alloy steels When using gas shielded wires 100% carbon dioxide (CO 2 ) or a mixture of argon + CO 2 are used for welding carbon, manganese and low alloy steels. Argon based gas mixtures normally contain between 20-25% CO 2. By comparison, gas-shielded wires use both a slag system and an external shielding gas to protect the arc from the atmosphere. Both CO 2 and argon/co 2 mixtures offer some advantages and disadvantages as both behave differently in the welding arc. Selecting straight CO 2 or an argon mixture is dependent on such factors as: Type/composition of material being welded Type of wire (some are designed for use with CO 2 or a specific mixture or both) Properties of deposited weld metal (tensile strength, impact properties etc) Weld profile and penetration levels required Mode of metal transfer required (spray, short arc, pulse) Fume emissions and spatter levels Cost of gas verse s productivity. Carbon dioxide (CO 2 ) A CO 2 welding arc has a tendency to produce a globular type of arc transfer with larger droplet sizes (typically larger than the diameter of wire) which results in a harsher, more erratic arc along with increased spatter levels. Costs associated with using straight CO 2 are less than for argon/co 2 mixtures while at the same time producing good quality welds. CO 2 produces a deep penetration shaped bead which is ideal for welding thick sections. Argon + CO 2 mixtures Argon/CO 2 mixtures have a tendency to produce more of a spray type arc transferring smaller droplets of weld metal. Despite being more expensive than CO 2, blended gas mixtures offer smoother, softer arcing characteristics, lower spatter levels and operator appeal. Argon/CO 2 mixtures have a tendency to keep the weld hotter and more fluid with increases in productivity compared to using CO 2. Increasing CO 2 levels to an argon based gas improves depth of penetration, heat of the arc, weld width and welding speed but may also give an increase in spatter levels. Argon/CO 2 mixtures tend to produce a wider finger like weld profile. Page 30 of 110 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014

6 After heating, the job needs to be left for a short time to allow heat to distribute evenly in the section before measuring the temperature. A range of crayons may be needed to accurately determine the temperature during various stages of the heating cycle. Further heating due to heat loss may need to be applied before the desired temperature is reached. To prevent contamination, crayon marks should not be made on weld joint surfaces. Temperature indicating crayons Contact thermometers These are small thermometers capable of measuring preheat temperatures up to 400ºC and are held in place by small magnets. The measuring device is a small coiled strip which is designed to unwind as it heats up and the temperature is shown on its dial face. The recommended placement of these thermometers is approximately 75 mm from the centre of the weld. Contact thermometer Contact pyrometers (thermocouples) Contact pyrometers are virtually instant temperature measuring devices. A probe is placed on the surface of the heated metal and the temperature is read from a dial or chart that is attached to the device. For large and/or critical work requiring continuous monitoring of a heat treatment procedure, probes are attached to multiple points and the heat energy recorded on a continuous time + temperature recording device. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014 Page 47 of 110

7 Review questions These questions have been included to help you revise what you have learnt in Topic 4: High alloy stainless steels. Q1 What is the minimum chromium content required in a steel for it to be classified as a stainless steel? Q2 Explain how the corrosion resistant properties of stainless steels are achieved. Q3 List four (4) types of stainless steel. Q4. List five (5) precautions to consider in your procedure when welding stainless steel. Q5. List three (3) industrial applications for austenitic stainless steels. TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014 Page 59 of 110

8 Q.6 Explain what is meant by the terms passivation and pickling in relation to stainless steels. Passivation: Pickling: /false questions (circle the correct answer). Q7 A 304 grade stainless steel is better suited for welding than a 304L. Q8 Compared to low carbon steel, the frequency of tack welds should be increased for welding stainless steel. Q9. The co-efficient of linear expansion for austenitic stainless steel is 50% lower than for low carbon steel. Q10. It is recommended to use slightly lower welding currents when welding stainless steel. Q11. Convex weld profiles are recommended when welding stainless steel. Q12. The chromic oxide film on stainless steel surfaces stops the metal from rusting. Page 60 of 110 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014

9 Q5. List three (3) internal defects that can be exposed by destructive weld tests. _ / questions (Circle the correct answer) Q6. Tensile tests are a common workshop testing method. Q7. Test plates must be made of the same material as the finished job. Q8. Destructive testing is used to qualify welding operators and procedures. Q9. Destructive test pieces should be selected from a higher tensile material than the structure being welded. Page 72 of 110 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014

10 Topic 7: Structural welding standard A number of Standards have been developed specifically for welding applications. One of the most prominent being AS/NZS :2011 Part 1: Welding of steel structures. The purpose of this Australian/New Zealand Standard is to provide fabricators with a set of rules suited for welding a range of steel structures. Although the main purpose of AS/NZS :2011 applies to statically loaded welds, it may also be relevant for machine frames and other steel constructions including some welded structures subject to fatigue loading. Note: Statically loaded structures refers to those in a fixed, nonmoving position. Fatigue loaded structures are those subject to a repetitive loading and unloading of stress. The scope of AS/NZS 1554 Part 1: 2011 Welding of steel structures provides details relating to the welding of steel sections such as plate, sheet and hollow sections using a range of welding processes such as MMAW, GMAW, FCAW, GTAW and SAW. AS/NZS provides extensive details and rules pertaining to: Section 2: Materials of construction. Includes details relating to parent/backing materials and welding consumables. Section 3: Details of welded connections. Butt, fillet, compound, seal, plug, slot welds and combining steel sections. Section 4: Qualification of weld procedures and personnel. Methods used, prequalified procedures, qualification of welding procedures, consumables, testing, recoding of tests etc. Section 5: Workmanship. deals with edge preparation, assembly, tack welding, preheating, controlling distortion, backgouging, temporary attachments, cleaning and finishing etc. Section 6: Quality of welds. Weld categories, methods of inspection, and permissible levels of imperfections, weld defects and reporting. Section 7: Inspection. Qualification of inspectors, visual and non-destructive methods of examination. Appendices Note: AS/NZS 1554 comprises of seven (7) parts covering the welding of studs, reinforcing steel, high strength and stainless steels etc. When welding stainless steel use part 6 AS/NZS 1554: Steel fabrications typically constructed to AS/NZS include: Building framework Road, rail and pedestrian bridges Chimneys, stacks, masts and towers Tanks, bins and bunkers TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014 Page 73 of 110

11 JOB 4: Fillet - Vertical (Pipe to plate) Cut and relocate Note: Rotate job to ensure welds are made in the vertical (3F) axis. Objective: Deposit fillet welds in the vertical position using the FCAW process. Suggested material: 1 piece low carbon steel plate 100 x 10 x 100 mm or available stock 1 piece low carbon steel pipe Ø 80NB or available stock. Work method advice: Thoroughly clean joint surfaces before welding Align and securely tack weld plate sections in position Deposit multiple fillet welds as demonstrated/advised by your teacher Thoroughly clean the weld joint of slag and spatter after each run Evaluate the finished job for appearance, weld size, contour and external defects Submit completed job and weld procedure for assessment. Important operating data: Wire speed setting Coarse and fine voltage settings Gas flow (if used) Arc length - stick-out distance Position of weld joint in relation to job requirements Welding speed and uniformity of travel Gun angles Monitor spatter levels. Assessment criteria: (AS/NZS 1554 SP) Clean and prepare material ready for welding Joint to be correctly aligned and assembled Uniform weld profile, no unfilled craters Angular distortion 0 to 5 Weld size mm to required dimensions Even starts and restarts Completed weld to be free of slag and spatter Maximum of 2 surface defects per 250 mm of weld Economical use of materials and consumables Apply safe welding practices. Page 88 of 110 TAFE NSW (Training & Education Support, Industry Skills Unit Meadowbank) 2014