Views: 28 Author: Site Editor Publish Time: 2018-08-02 Origin: www.fuchun-casting.com
Enquiries are often received about welding wrought iron; for example, information is sought on repairing structures built as long ago as the middle of the 19th century.
Notable examples are the Palm House at Kew Gardens, restored under the direction of Posford Pavry and Partners, and the G-MEX Exhibition Centre in Manchester.
Originally, wrought-iron structures were either bolted or riveted and, in addition to general corrosion, more serious rusting is often found in the region of the connections.
Wrought iron is a duplex material consisting of low-carbon iron which is rolled into plate form together with some of the slag used in the refining process. The resulting composite material consists of iron containing drawn-outstringers of slag.
The chemical composition of wrought iron will give some indication of its weldability. If the carbon and sulphur contents are low (typically 0.035% and 0.050% maximum respectively), the iron was correctly manufactured in the furnace.
Rolling or hammering applied to the iron after it comes out of the furnace elongates the slag particles to form stringers which, for good weldability, should be low in volume - preferably with low width or depth, i.e. they should be more or less one-dimensional.
Most repairs consist of either bolting or welding on reinforcing plates of mild steel; sometimes replacing entire wrought-iron members by steel. Bolts resembling rivets are often used for aesthetic purposes in repairs of this type. Welding should only be used where there is no practical alternative and should be avoided in areas of high stress.
Small quantities of reclaimed wrought iron are occasionally available for use in restoration work. There is therefore a need to weld wrought iron either to itself or to mild steel.
This is possible using manual metal arc or shielded metal arc (MMA or SMAW)) welding with rutile-coated electrodes complying with BS EN 499 E38 X R XX or AWS E6013, or by MAG (GMAW) welding with a filler wire complying with BS EN440: G2Si or G3Si, or AWS ER 70S-2.
Because of the high oxygen content of wrought iron, the MAG (GMAW) welding filler wires recommended contain aluminium as well as silicon and manganese as de-oxidisers, and there is some evidence that this gives sounder welds than other filler wires. However, results also depend on the quality of the wrought iron being welded, and weld tests using samples of the actual iron to be welded are recommended.
Rutile electrodes are chosen for MMA (SMAW) welding because of their low penetration and their ease of use for positional welding, and the lower tensile properties reduce the stress on the heat affected zone (HAZ).
If mechanical working of the wrought iron has not been thorough and the slag stringers are large in volume and extent, welding can be difficult as large slag inclusions form in the weld and at the fusion boundary. Those at the fusion boundary may be oriented normal to the direction of stress and could lead to premature failure if the structure is subjected to alternating stresses. Welding that results in a through thickness stress must be approached with great caution since the wrought iron can be very weak in this direction and a form of lamellar tearing can easily occur.
High-quality wrought iron, i.e. with low carbon and sulphur content and well-distributed slag stringers, gives fewer problems in welding. However, while the chemical composition may suggest good weldability, it will give no indication of the distribution or the volume of slag; this will be revealed only by microsections.
It should be bourn in mind that due to the manufacturing methods and the abscence of national Standards the mechanical properties of wrought iron may vary widely even in two plates from one manufacturer, hence a degree of variability may be encountered if a weld repair is attempted and satisfactory results from one test plate may not be representative of other plates to be welded
The procedure for welding wrought iron is, in general, the same as that used for welding structural steel. However, excessive penetration into the parent plate should be avoided to reduce the risk of slag inclusions in the weld metal. When MMA (SMAW) welding, this is achieved by using small-diameter electrodes, which need low current. For MAG (GMAW) welding, dip transfer or pulsed arc should be used.
Again, microsections should be obtained wherever possible before welding. A welding procedure test should be carried out if a large enough sample can be obtained.
Techniques to reduce shrinkage stresses due to welding should also be adopted. These could include the use of stringer beads, buttering of the wrought iron prior to completing the structural weld, keeping the weld preparation to a minimum, back-stepping and cooling-out between runs.
It must be recognised that the microstructure of wrought iron may mean that volumetric non-destructive examination of the welds is difficult. It is, therefore, important that structural repairs are only undertaken if there is a high probability of sound welds being produced.
An additional problem frequently encountered when repairing old structures, whether they be wrought iron or steel, is the presence of lead oxide based paints. To reduce the risk of welders breathing lead fume, these coatings should be fully removed prior to any welding repair being undertaken. This must be done with due regard to health & safety issues.
Tests carried out by TWI have shown that when subject to shear or bending loads welded wrought iron can fail at unexpectedly low loadings so it is advised that welding should wherever possible be restricted to cosmetic applications or addition of stiffeners, rather than main structural joints.
The metallurgical characteristics of wrought iron, identified above, together with the methods used when originally assembling this material as structural sections, mean that repair/strengthening of wrought iron structures by welding should only be carried out following consultation with companies experienced in this field and with all site work completed by companies with the necessary know-how, skills and experience.
Article Source: www.twi-global.com/technical-knowledge/faqs/faq-can-wrought-iron-be-repaired-by-welding