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It is ‘the Arc Welding Process’ known to even a layman and can be considered a ‘roadside welding process’ in india. When invented in 1880’s it used bare electrodes, however the subsequent developments led to the use of coated electrodes. This process is also known as stick electrode welding or coated electrode welding or manual metal arc welding. It uses coated electrodes of 2.5 to 6.35 mm diameter and 300-450 mm length held in an electrode holder. The power source used is of the constant current type and both ac and dc supplies can be employed with equal ease and effectiveness in most of the cases.
In arc welding machine when an arc is struck between an electrode and the work piece, the electrode core wire and its coating melt, the latter provides a gas shield to protect the molten weld pool and the tip of the electrode from the ill effects of the atmospheric gases. The temperature in the core of the arc ranges between 6000-70000C. The radiations originating from the welding arc can damage the eyes thus necessitating the use of a protective shield.
In all types of welding machines, arc welding machine process is very versatile and is used for welding in all positions and all metals for which electrodes have been developed. The coated electrodes are presently available for welding low carbon steels, low alloy steels, quenched and tempered (Q&T) steels, high alloy steels, corrosion resistance steels and stainless steel as well as for cast iron and malleable iron. It is also used for welding nickel and nickel alloys and to a lesser extent for welding copper and copper alloys. It finds a limited use in welding aluminium alloys. Typical applications of the process include its extensive use by the industry for fabrication of ships, bridges, pressure vessels and structurals. However, as the process can be used in its manual mode only. It is slowly getting replaced by other welding processes for heavy fabrication where large quantity of metal need be deposited.
On the complex shapes, the surface curvatures may not allow good contact or coupling, the angles of surfaces may prevent back wall echoes with 00 probes and some forgings, simple or complex may be anisotropic in grain structure (different grain size in different directions).
When searching the defects in forgings you should have, as a minimum, the following information, which is usually written on a techniques or instruction sheet.
- The test component identification and area to test.
- Actions to be taken when defects are found.
- The purpose of the test (defects sought and acceptance criteria).
- Equipment required.
- What method and level of test sensitivity to use.
- The method of scanning.
The instruction sheet would also contain sections giving details of any relevant safety procedures such as the cleaning of the test area afterwards. It would also have the company name, a unique technical reference number, the originator’s name and signature and an authorising signature.
The test may involve testing the whole, of a component, or just parts, this must be specified.
Actions to be taken
When defects are found it may be required that the defects are reported, e.g. on a diagram as a written description, or the component, or material, may be accepted or rejected according to the defects found. If defects are to be reported then the defect information that needs reporting would be contained in this section, i.e. Defect type, size, lateral and longitudinal position in relation to datums, etc.
Purpose of the test
This sections tells us the accept/reject criteria for particular defects, i.e. what size and type of defects to report or which defects render the component rejectable.
The section should give information on; the type of flow detector, type, size, and the frequency of probes, type of couplant, calibration blocks and reference block to use.
Method of setting and level of sensitivity need to be quoted for each scan, e.g. Set the bwe from the DGS block to 80% fsh and note the gain setting. Still on the DGS block, maximise the signal from the flat bottom hole at target depth (test material thickness) and set that to 80% fsh and note the difference in dBs between the new gain setting and the previous one. Set the bwe from the test material to 80% fsh and add the difference noted in the first two gain settings to the present gain and scan at this level.
The method of scanning of the material is either a written, step by step, instruction or technique sheet, or involves following the step laid out in the relevant national standard. And example written step by step could be:
- Prepare the material surface by removing any loose scale, rust, dirt or other debris and visually inspect for surface defects or damage.
- Calibrate the screen on the flaw detector, using a 00 probe and A2 calibration block, for a range of 0 to 200 mm.
- Set the sensitivity (as quoted in the relevant section above) and apply couplant to the test area.
- Scan the designated test area, with a probe overlap between scans of at least 20% of the probe’s diameter and at a maximum probe movement rate of 150 mm/sec.
- When defects meeting the criteria in the “purpose of the test” section are found, record the relevant defect data as in the “Action to be taken” section.
- Prepare a neat concise report giving details of the component identification, test area, equipment used, sensitivity method and settings and a drawing with the defect detailsas recorded in section five above.
Post test Procedure
This would involve cleaning any remaining couplant and dirt from the test area and covering the surface with protective coatings according to client’s requirements.