How To Repair Cast Iron

Cast Iron

Specialist welding and common cold metal stitching techniques for repairing historic cast iron structures

Terry Sims

	Myton Bridge (1868), Myton on Swale, Due north Yorkshire following structural repairs and restoration work to supercede missing and damaged components

Some of United kingdom's most impressive structures are fabricated from bandage iron, ane of the earliest and best known being the bridge at Ironbridge in Coalbrookdale, the Shropshire town oft regarded equally the cradle of the industrial revolution.

In the 18th century, every bit cast atomic number 26 became more widely available, it was quickly adopted for structural and decorative purposes. The beauty of the fabric was that it could be produced in an enormous variety of shapes and designs and mass-produced, allowing load-begetting columns and elaborate facades to be created at a fraction of the price of traditional stone carved ones.

This development coincided with a period of rapid growth in trade and commerce, reflected in magnificent buildings such as the Doncaster Corn Commutation, Dewsbury Marketplace Hall and the thou designs of railway stations throughout the state.

At present over a century quondam, these buildings are often of considerable beauty, as well equally being of smashing architectural and historic importance, and many are listed, necessitating a meticulous approach to repair and restoration if structural failure occurs.

Grey cast iron, the form nigh frequently encountered in Victorian buildings, was generally produced in green sand moulds. ('Green' refers to the aggregate'southward ability to exist shaped, non its colour.) This procedure immune for the reproduction of fine item, just besides presented an inherent problem of brittleness in the cloth.

TYPICAL PROBLEMS

Grey bandage iron has excellent properties under compression, merely it's far weaker than mod structural steels where shear loads and tension are involved.

Today, subsidence is one of the most common reasons for structural failure in bandage iron structures. Due to the method of construction it may non be noticed until the failure has reached quite an advanced stage. The reason for this is that in structures consisting of cast iron columns and beams, it was usually necessary to disguise the bolted joints with some kind of decorative roofing. These covers can hide underlying problems and, in this sense, the magnificence of decorative cast ironwork tin can as well be its downfall.

If subsidence occurs in a edifice constructed from columns and bandage fe beams, pressure will exist exerted on the bolts leading to localised cracking in the beams and eventual failure.

Another mutual crusade of structural failure is water ingress between sections of bandage iron, particularly if previous remedial work has been carried out using stainless steel bolts which have not been isolated using nylon washers. In the presence of moisture, this rapidly leads to bimetallic corrosion and subsequent corrosion and cracking in the area surrounding the bolts.

In one case, during the redecoration of an Edwardian market hall, a big section of decorative casting fell out as paint was being scraped off it. To determine the extent of the problem, engineers working from cherrypickers removed small squares of material from some of the decorative castings to reveal the bolted connections between the columns and the steelwork. They found that most all the bolts they inspected had failed: new sections of beams were required and the existing columns had to be reinforced.

Although cast iron was originally introduced as a burn down-proof textile in material mills and warehouses, under intense heat information technology does not perform well as a building material. Where fires have ravaged Victorian bandage iron buildings, the Doncaster Corn Exchange being one example, there has often been serious structural failure. Problems are compounded when the bonfire is extinguished using burn down hoses equally the sudden alter in temperature causes further embrittlement and cracking occurs in columns and beams. This is often accompanied by the complete disintegration of decorative cast fe features.

Again due to the brittle nature of grey cast fe, touch harm can have devastating consequences. Examples of this are provided by a series of elaborately decorated cast iron fountains situated on roundabouts in the Avenues expanse of Kingston upon Hull. Over a period of time all were shattered as the result of road traffic accidents and had to be repaired.

Cast iron can be repaired using a variety of processes co-ordinate to the exact nature of the cast atomic number 26 and the circumstances in which the repair must be performed. These processes include specialised welding techniques, common cold metal stitching, and various types of reinforcement.

Repairing a pocket-size section casting using manual metal arc welding: top left, the broken sections earlier repair; below, pre-heating the broken sections; top right, welding the sections; and, beneath, grinding the repair smoothen

TYPES OF STRUCTURAL Bandage IRON

Cast iron typically has a carbon content of two to iv per cent, which is ten times as much as most steels. In addition to grey bandage iron there are two other forms; white cast iron, which is mainly used in mechanical applied science applications, and spheroidal graphite (SG) iron. In about instances a thorough investigation is required to determine the verbal blazon of bandage iron past removing samples for laboratory assay.

The proper name 'grey' cast iron derives from its colour when fractured in comparison with the advent of white cast fe. Its high carbon content results in the germination of graphite, creating planes of weakness, and it is for this reason that its tensile strength and toughness are junior to that of structural steels.

SG atomic number 26 is the second most commonly encountered class of cast fe in Victorian buildings. From a historical perspective, this represented an improvement in the mechanical properties of the original grey bandage iron. Information technology is produced by treating the cast fe with magnesium or cerium additions before casting. This creates castings in which the graphite is in spheroidal form instead of in flakes, resulting in greater ductility and college tensile force than grey bandage irons.

From a 'dominion of thumb' approach, i of the means of telling the departure between grey cast iron and SG iron (although not infallible) is that when drilled, grayness iron tends to produce small chips of material and dust, whereas SG atomic number 26 produces something resembling small turnings, often accompanied by a distinctive smell.

In terms of weldability, grey cast atomic number 26 generally requires specialised oestrus treatment before, during and after welding. In comparison, SG fe is more readily weldable in most situations. This is because the fibroid graphite flakes in gray cast fe arrive less ductile, leading to the creation of breakable microstructures in the heat-affected zone. In contrast, the nodular graphite in SG irons provides greater ductility, reducing the occurrence of brittle microstructures.

Grey cast fe is specially vulnerable to the forces of expansion and contraction created during the welding process and every bit the material begins to cool. To overcome this problem, it is ordinarily necessary to preheat the unabridged casting slowly and evenly earlier welding commences, and later it must be allowed to cool gradually.

WELDING CAST Iron

Equally can exist appreciated, all of these factors have a major bear on on the practicality of repairing structural and decorative castings. Minor defects in decorative castings or cracks of not more than 25mm in length can usually be welded effectively in situ. Still, in some instances, welding is ruled out entirely due to the problems of removing very large structural members and heat-treating them in the appropriate manner. In this case, alternative forms of repair such as cold metal stitching (where no oestrus is employed), reinforcement or complete replacement might have to exist considered.

Fortunately, at that place are many instances where sections of structural and decorative castings can be transported to a workshop where welding can be carried out under carefully controlled weather.

In some cases, a furnace tin exist used for preheating the casting, but in many instances it is more effective to make an improvised enclosure using refractory materials. This allows the casting to be preheated to a temperature of 200-300°C using oxyacetylene equipment continued to a pepper-pot nozzle which diffuses the rut over the widest possible area of the casting. This temperature is maintained throughout the welding cycle and, when the weld is completed, a very dull, even cooling rate must be achieved.

One of the ways in which the pre-heat temperature is maintained is by the apply of a thermal coating to cover the casting. This involves considerable skill on the office of the welder who must complete the weld while adjusting the thermal blanket both to incorporate the heat and to shield himself.

In the case of very large structural castings such equally the columns and beams found in Victorian railway stations, pre-heating may be impracticable due to the overall size of the component or the difficulty of removing it from site. In these circumstances, the welding technique involves selecting a low amperage setting to reduce heat input and avoid local overheating. Weld beads approximately xxx-75mm in length are applied and the area is immediately 'peened' by light hammering to reduce the stresses in the welded joint. This procedure is repeated making sure that the weld beads are spaced sufficiently far apart to avert a localised build-up of estrus.

When it comes to welding cracks in thin department castings which cannot be pre-heated, a 'pace welding' technique is employed. In this method, 'balanced' welding is the key to success. Starting time, 3mm holes are drilled at either end of the crack to preclude it from opening further when heat is practical. In grooming for welding, the crack is so gouged and lightly ground. Step welding begins at the centre of the scissure using deposits 25mm in length. The first eolith is made to the right of the centre and the 2nd to the left, allowing each to cool completely before proceeding to the next. In this way, the amount of heat applied is carefully balanced throughout the welding cycle.

When welding cracks in larger sections, the scissure is prepared in a 'U' shape to allow for steel studs to be inserted at 75mm intervals to reinforce the joint. During welding, these studs fuse with the material which is being applied to strengthen the articulation.

Manual metal arc (MMA) welding is frequently used in the repair of cast fe because the loftier temperature arc requires lower levels of preheating. High nickel-based electrodes are used in this method because of their ductility. A meticulous arroyo is required, starting time past drying the electrodes in an oven, then preheating the welding quiver.
Amperages are set low, but sufficiently high to fuse the welding textile to the parent metal. Again, a counterbalanced approach is essential, working in 30 to 40mm sections and never endmost the ends of the weld until the heart department is completed.

	The writer, Terry Sims, inspecting a weld in a fountain basin

Common cold METAL STITCHING

The major reward of common cold metal stitching is that it does not involve the application of heat, avoiding the problems of expansion and contraction and the resultant stresses on the material. Therefore all of the complex heat treatment processes mentioned above are circumvented. It as well avoids the demand for hot work permits in buildings containing timber.

Another important do good is that cold metal stitching can generally be carried out in situ, fugitive unnecessary disturbance to the construction of historic buildings and fugitive the cost and complications which are involved in dismantling structures, transporting them to a workshop and then reassembling them on completion.

Of course, this is not practical or desirable in every case. For instance, it is more than efficient to take the fragments of a desperately shattered ornamental casting to a workshop where the work can be undertaken more hands.

The cold metal stitching procedure begins past drilling a line of holes at right angles to the scissure in the casting and and so converting them into a slot. To achieve the right spacing for the holes, a special drilling jig is used with centres at one∕eight", v∕32", 3 ∕16" or ¼" depending on the thickness of the material which is being repaired.

Preformed locks are then fitted into the slots to create a bridge beyond the cleaved sections. The locks are made from a high nickel steel with the aforementioned co-efficient of expansion every bit the cast iron. This material is specifically called because it is potent plenty to have shear loads, but sufficiently ductile to provide the necessary elasticity.

Most repairs volition have a series of locks and stitches spaced at regular intervals along the scissure. When this is completed, holes are drilled along the line of the fracture between each stitch. These are tapped to receive special screws which fill the crack and ensure that it is completely watertight.

Finally, the area which has been stitched is ground level to the surface of the original material to create a seamless repair. In this way, when the crack has been repaired the area of metallic is oft stronger than the original bandage iron material, and when it has been primed and painted the repair can exist invisible.

In some cases involving heavy loading stresses, such equally the repair of cast fe columns, there is a demand to provide extra reinforcement. In these circumstances, a 'master key' is inserted in addition to the normal 'stitches'. The 'master cardinal' is a larger section of metal which tin be varied in shape or size to suit the particular requirements of the repair. It involves a larger bridging section which is fitted using the same technique every bit conventional stitching.

In cases where whole sections of fabric are missing due to the furnishings of corrosion or mechanical damage, a patch of metal, known equally an 'insert', can be made to fill the gap and stitched into position. Inserts of this type tin can be annihilation from a few inches wide to several feet.

Cold metal stitching is an uncommonly versatile technique which can be used in the repair of structural columns, beams, spandrel brackets and other structural members. Information technology can also be used for the restoration of elaborate decorative castings which may have cracked or shattered due to touch on damage or the result of water ingress.

In a restoration project, cold metal stitching can exist combined with specialised welding techniques, depending on the circumstances and the nature of the fabric which is being repaired.

One of the almost impressive examples of large scale restoration using cold metal stitching in association with specialised welding techniques is the Doncaster Corn Exchange where thousands of fragments of decorative casting were reassembled after extensive fire harm.

Another example is provided by the Ferguson Gallery in Perth, Scotland, a cast iron building erected in 1832 equally a waterworks, but more than recently converted into an art gallery. Over the years, the 192 bandage iron panels which surround the gallery had go severely corroded and many of the fixings had failed. In 2003 a restoration program for the Category A listed building was commenced with the aim of preserving every bit much of the original material equally possible.

In this case it was necessary to ship the panels to a workshop where they could be individually assessed and repaired. This involved a combined approach using cold metallic stitching and specialist welding techniques. Every bit a result, simply nine of the panels had to exist completely renewed while the majority were repaired and re-installed.

In every instance, the key to successful restoration of cast fe structures is a thorough agreement of the materials under repair, a comprehensive knowledge of the about appropriate repair techniques and the experience to utilise them effectively and in the most sympathetic manner.

This article is reproduced from The Building Conservation Directory, 2008

Writer

TERRY SIMS has retired since writing this commodity. He was at the time technical managing director of Casting Repairs Ltd. He had over xxx years of feel on major restoration projects involving cast iron materials and the apply of specialised welding and common cold metal stitching techniques.

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