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MATERIALS & WELD TESTING (DT & NDT)
WHAT IS DT & NDT?
DT and NDT are abbreviations for DESTRUCTIVE and NON-DESTRUCTIVE TESTING. Both these terms are somewhat self-descriptive; destructive testing requires the test piece to be prepared, then broken up and inspected, non-destructive techniques allow the tested item to remain intact.
Destructive testing methods include macro etching, micro etching, bend testing and nick breaking. Non-destructive methods include visual inspection, dye penetrant, magnetic particle and radiography. The tests which most readily apply to sculptural and architectural constructions are macro etching (destructive), and visual or dye penetrant (both non-destructive). All these tests are relatively quick, easy and inexpensive to carry out.
WHY & WHEN TEST WELDS & MATERIALS?
Castings, welded joints and construction materials might have to undergo testing for any number of reasons, but usually a test is carried out if a failure of the joint or material in question could have serious safety consequences. Testing might also be done if the art/designwork is destined for a finishing process that demands a very high standard of material surface integrity.
Testing, especially visual inspection, is usually carried out by the founder as part of their everyday quality control process. Sometimes though, a specialist more detailed test process may be demanded by a structural engineer, a typical example might be a test carried out on a welded support structure for insertion into a cast sculpture. A typical test procedure might include the welding of a sample COUPON (TEST PLATE), which is of the same material and thickness as that used in the structure itself. The inspector will cut through a section of the test coupon (see MACRO ETCH below) to reveal a cross section of the weld, they can then visually check the cut section to ensure the weld has penetrated the full depth of the joint. This is best done by polishing the cut cross section, then applying an etching solution (SEE BELOW). The etching solution highlights the entire area of the weld and allows the inspector to check that the weld filler material has properly penetrated into the parent metal of the test piece. If the test piece is satisfactory, the founder can be fairly confident that welding the main structure in the same manner will produce a sound and therefore safe joint.
As well as welded joints, the construction materials themselves can also be tested. Some of the techniques used in industry, such as BATCH SAMPLING (of metal from the furnace), and RADIOGRAPHIC INVESTIGATIONS of casts, are neither readily available to the art founder, nor necessary under most conceivable circumstances. However, some finishing processes (ENAMELLING for instance), require a very high integrity finish over the material’s surface to be sucessful. Flaws in the cast’s surface (minute POROSITY holes and fine cracks), might otherwise lead to serious defects in a costly finish. Testing a cast or fabrication can help identify potential problems areas in the surface or structure, these faults can then be rectified before committing to the final finishing process.
Testing is nowhere near as common a procedure in the art foundry as it is in a commercial engineering environment, though most art foundries capable of casting and constructing large scale artworks in metal will have almost certainly encountered materials or weld testing issues at some point. Testing is not normally a procedure that need overly concern an artist or designer; If testing is appropriate this will normally be decided by the founder themselves, or else specified by a supervisory authority such as a STRUCTURAL ENGINEER or ARCHITECT.
TESTING METHODS
VISUAL (NDT)
All testing methods involve some form of visual judgment on the inspector’s part, but checking by sight alone is the quickest, cheapest and easiest method of assessing a weld, cast surface or fabrication. To successfully test a subject piece in this way the inspector obviously requires some prior knowledge and understanding of what to look for, as well as some experience to then decide what is, or is not acceptable for the circumstances that apply to a particular job. The following is a brief list of the typical items looked for during a visual inspection:
Overall dimensional and general accuracy.
Distortion & scratches.
Porosity & inclusion.
Shrinkage, weld undercut & catering (holes at the ends of weld runs).
MACRO ETCH (DT)
Only so much information can be gained from visually inspecting the surface of a weld or piece of material, many of the more significant problems found in welds and casts are sub-surface, and therefore normally hidden from view. For critical structures, it may become necessary to examine the internal condition of the weld or the cast’s sub-surface. Inspection in these cases usually involves the destruction [or partial damage] of the test piece, unless some non-invasive (usually expensive), investigation is undertaken – such as an ULTRASOUND SCAN or RADIOGRAPHIC PLATE IMAGE.
Destroying part of an artwork just to see if a cast or weld is satisfactory is somewhat counter-productive, so it is more usual to instead produce a small test sample with the same characteristics (ie composition and thickness), that are going to be used for the actual art work. ETCH testing is most usually carried out on welded structures, though it can be adapted to test a cast sample. The founder first creates a TEST COUPON (a sample section), large enough to deposit a length of weld (usually about 150 mm or 6 inches in length).The test coupon is prepared as per the actual piece (for instance, a butt weld joining two cast silicon bronze plates, using the same 60° INCLUDED WELD PREPARATION used for the actual casting). The coupon should be of the same thickness as the proposed structure and welded using the same process, consumables and position (ie flat, vertical, overhead) – though any initial tacking of the plates can be done in any convenient position.
The idea here is to accurately reproduce the material and joint quality of the actual structure/artwork. After welding, the test coupon is first given a careful visual inspection, then cut through to create a transverse cross-section (or series of sections), revealing the underlying weld deposit and parent material. The exposed cross-section is ground flat, filed, and then polished with the aid of increasingly fine wet and dry grits. Once polished, a metal specific etching solution (SEE BELOW), is applied to the area of the weld, this develops to clearly reveal the local structure of the weld and parent metal. The inspector can then check for underlying porosity and inclusions in the parent metal as well as identify poor weld penetration through the joint, any insufficient fusion and evidence of cavities and porosity faults in the weld; all of which might seriously weaken the completed structure if present.
After testing, the founder would be able to adjust the settings on their welding set, or modify their welding technique and procedure until a sound sample could be produced. Testing will later allow the final design to be welded with a greater degree of certainty than might otherwise be the case.
ETCHING SOLUTIONS FOR MACRO ETCHING
COPPER ALLOYS: 25% Nitric acid in water or alcohol solution.
ALUMINIUM ALLOYS:10% Hydrofluoric acid in water solution.
IRON & STEEL ALLOYS:15 % Nitric acid in water solution.
18/8 STAINLESS STEEL:15% Hydrochloric acid + 5% Nitric acid in water solution.
QUALITY CONTROL CERTIFICATION RECORD OF APPROVAL TEST OF WELDING PROCEDURE
Data sheet filled in by the welder/inspector that describes in detail a weld test procedure. Items noted typically include:
Welder’s details.
Welding consumables (ie type of electrode etc).
Test location (site/workshop).
Parent material (ie cast silicon bronze).
Process used (MMA, TIG etc).
Welding conditions (voltages, current, rate of wire feed etc).Welding position (overhead, vertical, flat etc)
Post weld heat treatment.
Weld preparation (60°included ‘V ’butt, or ‘T’ fillet etc).
Any other relevant information.
Preparatory cleaning.
Inspectors remarks, extent of approval given (ie parameter for the variety of materials and processes covered by the test).
CERTIFICATE OF COMPOSITIONAL ANALYSIS BY PERCENTAGE/WEIGHT
Certification issued by ingot manufacturer describing smelt origination, materials standard/specification and composition.
EXAMPLE
Typical compositional analysis by percentage for a leaded gunmetal (LG 3):Cu base+ Sn 6.56, Zn 5.36, Pb 2.52, Fe 0.10, Ni 0.50 P 0.01, As 0.02, Sb0.10, Bi 0.001, Si 0.003, Al 0.009, Mn 0.004, S 0.034
Typical compositional analysis by percentage for a silicon bronze casting alloy: Cu base+ Sn 1.0, Zn 5.0, Pb 0.5, Fe 2.5, Al 1.5, Mn 1.5, Si 1.0-5.0
CERTIFICATE OF CONFORMITY
Document supplied by manufactures/stockholders with materials (especial-ly wrought metal sections), attesting to their conformity with relevantapproved specifications and international standards (BS, EN, ISO, DIN,ASTM etc).
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