Cast Iron

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

Terry Sims

Myton Bridge (1868), Myton on Swale, North Yorkshire following structural repairs and restoration work to replace missing and damaged components

Some of Britain’s most impressive structures are made from cast iron, one of the earliest and best known being the bridge at Ironbridge in Coalbrookdale, the Shropshire town often regarded as the cradle of the industrial revolution.

In the 18th century, as cast iron became more widely available, it was quickly adopted for structural and decorative purposes. The beauty of the material was that it could be produced in an enormous variety of shapes and designs and mass-produced, allowing load-bearing columns and elaborate facades to be created at a fraction of the cost 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 Exchange, Dewsbury Market Hall and the grand designs of railway stations throughout the country.

Now over a century old, these buildings are often of considerable beauty, as well as being of great 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 most frequently encountered in Victorian buildings, was generally produced in green sand moulds. (‘Green’ refers to the aggregate’s ability to be shaped, not its colour.) This process allowed for the reproduction of fine detail, but also presented an inherent problem of brittleness in the material.

TYPICAL PROBLEMS

Grey cast iron has excellent properties under compression, but it’s far weaker than modern structural steels where shear loads and tension are involved.

Today, subsidence is one of the most common reasons for structural failure in cast iron structures. Due to the method of construction it may not 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 covering. These covers can hide underlying problems and, in this sense, the magnificence of decorative cast ironwork can also be its downfall.

>If subsidence occurs in a building constructed from columns and cast iron beams, pressure will be exerted on the bolts leading to localised cracking in the beams and eventual failure.

Another common cause of structural failure is water ingress between sections of cast 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 large 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 virtually 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 fire-proof material in textile mills and warehouses, under intense heat it does not perform well as a building material. Where fires have ravaged Victorian cast iron buildings, the Doncaster Corn Exchange being one example, there has often been serious structural failure. Problems are compounded when the blaze is extinguished using fire hoses as the sudden change in temperature causes further embrittlement and cracking occurs in columns and beams. This is often accompanied by the complete disintegration of decorative cast iron features.

Again due to the brittle nature of grey cast iron, impact damage can have devastating consequences. Examples of this are provided by a series of elaborately decorated cast iron fountains situated on roundabouts in the Avenues area 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 according to the exact nature of the cast iron and the circumstances in which the repair must be performed. These processes include specialised welding techniques, cold metal stitching, and various types of reinforcement.

Repairing a small section casting using manual metal arc welding: top left, the broken sections before repair; below, pre-heating the broken sections; top right, welding the sections; and, below, grinding the repair smooth

TYPES OF STRUCTURAL CAST IRON

Cast iron typically has a carbon content of two to four per cent, which is ten times as much as most steels. In addition to grey cast iron there are two other forms; white cast iron, which is mainly used in mechanical engineering applications, and spheroidal graphite (SG) iron. In most instances a thorough investigation is required to determine the exact type of cast iron by removing samples for laboratory analysis.

The name ‘grey’ cast iron derives from its colour when fractured in comparison with the appearance of white cast iron. Its high carbon content results in the formation of graphite, creating planes of weakness, and it is for this reason that its tensile strength and toughness are inferior to that of structural steels.

SG iron is the second most commonly encountered form of cast iron in Victorian buildings. From a historical perspective, this represented an improvement in the mechanical properties of the original grey cast iron. It is produced by treating the cast iron 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 higher tensile strength than grey cast irons.

From a ‘rule of thumb’ approach, one of the ways of telling the difference between grey cast iron and SG iron (although not infallible) is that when drilled, grey iron tends to produce small chips of material and dust, whereas SG iron produces something resembling small turnings, often accompanied by a distinctive smell.

In terms of weldability, grey cast iron generally requires specialised heat treatment before, during and after welding. In comparison, SG iron is more readily weldable in most situations. This is because the coarse graphite flakes in grey cast iron make it less ductile, leading to the creation of brittle 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 iron is particularly vulnerable to the forces of expansion and contraction created during the welding process and as the material begins to cool. To overcome this problem, it is usually necessary to preheat the entire casting slowly and evenly before welding commences, and afterwards it must be allowed to cool gradually.

WELDING CAST IRON

As can be appreciated, all of these factors have a major impact 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. However, 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 heat is employed), reinforcement or complete replacement might have to be considered.

Fortunately, there 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 conditions.

In some cases, a furnace can be 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 connected to a pepper-pot nozzle which diffuses the heat over the widest possible area of the casting. This temperature is maintained throughout the welding cycle and, when the weld is completed, a very slow, even cooling rate must be achieved.

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

In the case of very large structural castings such as 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 30-75mm 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 avoid a localised build-up of heat.

When it comes to welding cracks in thin section castings which cannot be pre-heated, a ‘step welding’ technique is employed. In this method, ‘balanced’ welding is the key to success. First, 3mm holes are drilled at either end of the crack to prevent it from opening further when heat is applied. In preparation for welding, the crack is then gouged and lightly ground. Step welding begins at the centre of the crack using deposits 25mm in length. The first deposit is made to the right of the centre and the second 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 crack 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 joint.

Manual metal arc (MMA) welding is frequently used in the repair of cast iron because the high temperature arc requires lower levels of preheating. High nickel-based electrodes are used in this method because of their ductility. A meticulous approach is required, first by drying the electrodes in an oven, then preheating the welding quiver.
Amperages are set low, but sufficiently high to fuse the welding material to the parent metal. Again, a balanced approach is essential, working in 30 to 40mm sections and never closing the ends of the weld until the middle section is completed.

The author, Terry Sims, inspecting a weld in a fountain bowl

COLD METAL STITCHING

The major advantage of 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 also avoids the need for hot work permits in buildings containing timber.

Another important benefit is that cold metal stitching can generally be carried out in situ, avoiding unnecessary disturbance to the structure of historic buildings and avoiding 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 example, it is more efficient to take the fragments of a badly shattered ornamental casting to a workshop where the work can be undertaken more easily.

The cold metal stitching process begins by drilling a line of holes at right angles to the crack in the casting and then converting them into a slot. To achieve the correct spacing for the holes, a special drilling jig is used with centres at 1∕8", 5∕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 across the broken sections. The locks are made from a high nickel steel with the same co-efficient of expansion as the cast iron. This material is specifically chosen because it is strong enough to take shear loads, but sufficiently ductile to provide the necessary elasticity.

Most repairs will have a series of locks and stitches spaced at regular intervals along the crack. 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 metal is often stronger than the original cast iron material, and when it has been primed and painted the repair can be invisible.

In some cases involving heavy loading stresses, such as the repair of cast iron columns, there is a need to provide extra reinforcement. In these circumstances, a ‘master key’ is inserted in addition to the normal ‘stitches’. The ‘master key’ is a larger section of metal which can 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 as conventional stitching.

In cases where whole sections of material are missing due to the effects of corrosion or mechanical damage, a patch of metal, known as an ‘insert’, can be made to fill the gap and stitched into position. Inserts of this type can be anything from a few inches wide to several feet.

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

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

One of the most 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 damage.

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

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

In every case, the key to successful restoration of cast iron structures is a thorough understanding of the materials under repair, a comprehensive knowledge of the most appropriate repair techniques and the experience to apply them effectively and in the most sympathetic manner.

 

 

This article is reproduced from The Building Conservation Directory, 2008

Author

TERRY SIMS has retired since writing this article. He was at the time technical manager of Casting Repairs Ltd. He had over 30 years of experience on major restoration projects involving cast iron materials and the use of specialised welding and cold metal stitching techniques.

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