Repairing Fractured Cast Iron Features
- Procedure code:
- Hspg Prepared For Nps - Sero
- Metal Materials
- Last Modified:
- Margot Gayle, David Look, John Waite. Metals in America's Historic Buildings. Washington,DC: National Park Service, 1995.
- L. William Zahner. Architectural Metals. New York: John Wiley & Sons, 1995
- This procedure includes guidance on repairing fractured cast iron elements by joining the fractured sections using one of the following methods:
- Using an epoxy resin cement,
- Brazing with brass rods,
- Welding with "Ni-rods" - special nickel alloy welding rods, or
- Using cold repair techniques.
- See 01100-07-S for general project guidelines to be reviewed along with this procedure. These guidelines cover the following sections:
- Safety Precautions
- Historic Structures Precautions
- Quality Assurance
- Delivery, Storage and Handling
- Project/Site Conditions
- Sequencing and Scheduling
- General Protection (Surface and Surrounding)
These guidelines should be reviewed prior to performing this procedure and should be followed, when applicable, along with recommendations from the Regional Historic Preservation Officer (RHPO).
C. For general information on the characteristics, uses and
problems associated with cast iron, see 05010-04-S.
- Brazing: Brazing is a form of soldering that uses a bronze or brass filler metal.
- Soldering: The joining of metals by fusion using an alloy with a lead or tin base.
- Welding: The joining of metals by heating them with or without the use of filler metal.
- Epoxy resin cement:
- Look for a good quality metal replacement material based on steel particles with an epoxy resin binder.
- Epoxy resin alone has a high thermal expansion which is not compatible with that of the iron. If used without a high proportion of metal filler such as in a thick layer of adhesive, problems with differential expansion may occur.
- Brass rods
- Ni rods (Nickel Alloy welding rods) such as 'Chemtron 99' - a rod with a very high nickel content.
- Nickel-arc rod (or nickel-color wire) is the most common type of welding rod used for welding cast iron.
- It is available in several grades, depending on the nickel content of the rod and the inclusion of other metals. The higher the percentage of nickel, the stronger the weld and the easier the rod is to work with.
NOTE: Mild-steel rods may be used. They produce a strong, hard weld, with the mechanical properties close to those of cast-iron, but they are difficult to work with, in that they must be preheated and they are not machinable.
- Stainless steel or non-ferrous plates or straps, threaded and unthreaded pins or dowels
- Wiping cloths
- Eye and skin protection
- Welding equipment
- Electric drill
- Pneunmatic chisel
- Electric Grinder
- Before proceeding with steps to repair cast-iron features, it is important to first determine the cause and extent of the problem. Determine the age of the features and examine the condition of the entire surface. Inspect for:
- Wearing Surfaces - especially moving parts such as hardware.
- Parts - which have failed or which are unsecured, broken, cracked, missing, distorted, or loose (check screws and bolts).
- Paint - coating failures such as chips, losses, peeling, checks, bubbling, and wear.
- Rust corrosion - caused by moisture, sea water and sea air, deicing salts, acids, soils, gypsum plasters, magnesium oxychloride cements, ashes, clinkers, and sulfur compounds.
- Galvanic action - corrosion caused when iron is exposed to cupro-nickels, aluminum bronzes, gun metals, copper, brasses, lead, soft solders, and some stainless steels and chromium.
- Determine the source of the moisture which causes the deterioration.
- Determine if the feature can be salvaged.
- Measure the dimensions of the various metal parts needing replacement.
- Patching with Epoxy Cement (see 05010-12-R).
- Brazing with Brass Rods (follow procedure for soldering; see 05010-07-R for guidance; use brass rods as filler metal).
NOTE: Brazing of cast iron is very difficult to do in the field. It should be executed only by a skilled welder under careful supervision.
- Welding: Successful welding of cast iron can be a relatively expensive operation and so whenever possible, a cold repair is recommended (see Section 3.03 D. below).
NOTE: Welding of cast iron is very difficult to do in the field. It should be executed only by a skilled welder under careful supervision.
CAUTION: During welding the metal becomes very hot and can undergo tremendous thermal shock. It may recrystallize if excessive heat is applied.
- For large sections, welding should take place off site. The piece must be removed and transported to a workshop where it can be preheated before welding and postheated after welding to ensure a gradual temperature change within the metal.
- Advantages of welding:
- Arc welding produces a strong, durable connection and, if properly executed, is at least as strong as the surrounding metal.
- It is faster and less expensive than threaded connections, which require drilling a pilot and then tapping to accommodate screws or bolts.
- Welding is the most preferred for the attachment of the decorative castings and for other non-structural repairs for economic reasons and because it allows the preservation of the original damaged elements, which otherwise would have to be replaced.
- Disadvantages of welding:
- In cases where the original attachments are bolted, the use of this method may result in internal stresses (welds cannot move with seasonal expansion/ contraction cycles) which may in turn lead to further breaks.
- Welding may leave a 'bead' along the surface some restoration projects, eventhough much of the weld may later be ground down, depending on the location and the welding material.
- Metallic bond (gas) welding is more reliable than fusion (arc) welding in repairing large sections of cast iron, because a lower temperature is used and heat is applied and removed at a slower rate.
- Tapping screws may be visully disfiguring in some significant or decorative castings.
- Cold repair techniques:
NOTE: STAINLESS STEEL OR NON-FERROUS METALS SHOULD BE
USED WHENEVER POSSIBLE.
- Straps: These can often be hidden. The plate should be bedded on a suitable medium to prevent a water trap.
- Threaded studs, screwed into both sides of a fracture.
- Dowels or plain pins, with one or both ends threaded and/or glued into prepared recesses.
- Metal stitching and binding: This system of repair is strong, easy to use on site and introduces virtually no heat into the metal.
NOTE: Metal stitching can be used on all cast iron which is over 1/4 inch (6 mm) thick. It can also be used between cast iron and metal of similar hardness, such as steel.
- Realign the fractured section.
- Drill groups of holes across the metal. Using a pneumatic chisel, cut between the drilled holes to form a series of slots.
- With this method, a 'dumb bell' shaped piece of wrought iron is inserted across a fracture.
- Drive locks of work-hardening nickel alloy into the metal.
- Drill holes along the line of fracture between the nickel alloy locks.
- Tap and fill with studs, each stud interlocking with its neighbor.
- Shear off all excess metal. Ground, prime and paint the surface.
END OF SECTION