Análisis palinológicos (paleovegetación)

a. No welding is to be undertaken on castings until a concession has been approved in accordance with the requirements of NES 824 Part 1 Section 13. Concessions submitted for consideration are to include:

(1) Full details of the proposed method of repair including the approved welding procedure(s) to be used and confirmation that the weldability test defined in NES 824 Part 1 Clause 1104 has been satisfactory completed and

(2) Sketches of the proposed repair indicating the position, nature, extent and depth of defects, design wall thickness of the area and thickness of the original parent material remaining.

b. The extent of excavations and subsequent weld repair is to be only that necessary to bring the casting within the acceptance standards of NES 824 Part 1 Section 12. The extent of weld repair is to be limited in accordance with the following clauses.

5.1 Welding Limitations

a. The rectification of defects by welding is not permitted on any sea water wetted surface. See also NES 824 Part 1 Clause 1304.

b. Defects may only be rectified when, after removal of the defect, excavation depth does not penetrate to within 10mm of the wetted surface. See also NES 824 Part 1 Clause 1305.

c. Corrections to casting dimensions and machining errors by surface build-up will be considered providing the following finished limits are not exceeded.

(1) 5mm maximum build up for wall thicknesses 25mm and under.

(2) 20% of wall thickness maximum build up for wall thicknesses over 25mm but must not exceed 10mm.

(3) Area of build up on Class I and Class II castings (see NES 824 Pt 1 Section 7 for definitions) is to be restricted to 10% of the inner or outer surface area not including flange thicknesses and webs, subject to any further limitations imposed by Clause 5.1g.

(4) Areas of build up on Class III castings are to be restricted to 20% of the inner or outer surface area not including flange thickness and webs.

d. Unacceptable surface or near surface defects is to be blended out in accordance with NES 824 Clause 1311.

e. Defects which cannot be blended out within the required limits are to be treated as sub-surface defects.

f. Any individual sub-surface repair in a Critical Test Region is not to exceed 5%

of the test area containing the defect. In Test Regions any individual repair is not to exceed 10% of the test area containing the defect.

g. The total combined area of weld repairs from all causes, eg dimensional corrections, machining errors and sub-surface is not to exceed;

(1) 10% of Critical Test Regions and

(2) 20% of Test Regions and non-designated areas of Class III castings.

h. When repairs are required which are more extensive than permitted by Clauses 5.1a. to 5.1g. then the casting is to be either:

(1) Scrapped or

(2) A special concession submitted which defines the extension of weld repair beyond the permitted limits and clearly explains the reasons why such repairs are necessary.

5.2 General Welding Requirements

a. Defects are to be removed by machining or pneumatic chipping and grinding.

Excavation of defects is to commence in sound material at the ends of the defects and proceed from each end towards the centre.

b. Excavations are to be finished smooth, tapered at the sides and rounded at the bottom to facilitate full access for the root run and correct manipulation of the filler wire.

c. Before weld action is undertaken a thorough examination of the excavated area  is to be made using radiography and dye penetrant methods to ensure adequate removal of the defect.

d. Prior to the deposition of weld material, all contaminants resulting from non-destructive examination are to be removed with lint free cloth using  acetone or the approved solvent remover for the dye penetrant system used.

e. Interpass cleaning is to be carried out using a stainless steel wire brush which has not been used on any other material.

f. Interpass grinding, when necessary, is to be carried out with resin bonded alumina grinding wheels or with tungsten carbide burrs.

g. All welding is to be carried out in the ‘Flat Position’ in accordance with the definitions given in BS 499 Part 1.

h. In order to achieve satisfactory results it is essential that the instructions detailed in this NES are followed in all repairs to castings and are incorporated in the appropriate weld procedures.

i. Stop and start porosity or deep end craters are tobe minimized byuse of the‘run back technique’ as appropriate to the welding process being used. This technique is a means of avoiding porosity and crater cracking which can be caused bythe abrupt withdrawal ofthe welding arc atthe end of a weld run. This technique consists of delaying the consumable at the weld end to fill the crater and then welding back along the top of the previously deposited weld for a short distance, thus facilitating the withdrawal of the arc from superfluous weld metal which is subsequently removed.

 j. Where necessary, extension ‘run-on’ and ‘run-off’ plates of the same material as that to be welded are to be securely welded to the component and grooved to conform to the actual weld preparation. For the repair of excavations wedge shaped ‘run-on’ and run-off’ plates may be used.

k. When large areas of weld repair are involved, the weld metal is to be deposited in such a manner that the final weld pass consists of weld metal to weld metal.

l. If a casting repair is unsuccessful and the ‘repair’ requires to be rewelded, not more than two rewelds are permitted.

m. The interpass temperature during welding is not to exceed 150° C.

5.3 Temperature Measurement

a. Interpass temperature is to be determined by contact pyrometers or thermocouples; the use of low melting metallic alloys or any temperature indicating crayons or paints is prohibited.

5.4 Heat Treatment of Weld Repairs

a. After weld repair all castings are to be heat treated by placing them in a furnace with temperature not higher than 100° C, raising to 450° C to 500° C at a  maximum rate of 200° C/hr, holding for 1 hour per 25mm thickness (minimum period 2 hours) and then air cooling. The furnace is to be validated in accordance with NES 746.

NOTE The heat treatment requirement for weld repaired areas described above is in addition to the mandatory ‘Stress Relief’ heat treatment which is required for all Copper Nickel Chromium castings in accordance with NES 824 Part 1.

5.5 Approved Processes

a. Welding may be carried out with TIG, MIG, Pulsed MIG or a combination of  these processes.

5.6 Approved Welding Consumables

a. All approved welding consumables are to be readily identifiable so that they can be positively related to their Quality Assurance documentation.