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Historic Preservation - Technical Procedures

Spectitle:

Repairing Fractured Cast Iron Features

Procedure code:

0501013R

Source:

Hspg Prepared For Nps - Sero

Division:

Metals

Section:

Metal Materials

Last Modified:

11/12/2014

Details:

Repairing Fractured Cast Iron Features



REPAIRING FRACTURED CAST IRON FEATURES

 

REFERENCES:

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.

PART 1---GENERAL

1.01 SUMMARY

    A.   This procedure includes guidance on repairing fractured
         cast iron elements by joining the fractured sections
         using one of the following methods:

         1.   Using an epoxy resin cement,

         2.   Brazing with brass rods,

         3.   Welding with "Ni-rods" - special nickel alloy
              welding rods, or

         4.   Using cold repair techniques.

    B.   See 01100-07-S for general project guidelines to be
         reviewed along with this procedure.  These guidelines
         cover the following sections:

         1.   Safety Precautions

         2.   Historic Structures Precautions

         3.   Submittals

         4.   Quality Assurance

         5.   Delivery, Storage and Handling

         6.   Project/Site Conditions

         7.   Sequencing and Scheduling

         8.   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.

1.02 DEFINITIONS

    A.   Brazing:  Brazing is a form of soldering that uses a
         bronze or brass filler metal.

    B.   Soldering:  The joining of metals by fusion using an
         alloy with a lead or tin base.

    C.   Welding:  The joining of metals by heating them with or
         without the use of filler metal.


PART 2---PRODUCTS

2.01 MATERIALS

    A.   Epoxy resin cement:

         1.   Look for a good quality metal replacement material
              based on steel particles with an epoxy resin
              binder.  

         2.   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.  

    B.   Brass rods

    C.   Ni rods (Nickel Alloy welding rods) such as 'Chemtron 99'
         - a rod with a very high nickel content.

         1.   Nickel-arc rod (or nickel-color wire) is the most
              common type of welding rod used for welding cast
              iron.

         2.   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.

    D.   Stainless steel or non ferrous plates or straps, threaded
         and unthreaded pins or dowels

2.03 EQUIPMENT

    A.   Wiping cloths

    B.   Eye and skin protection

    C.   Pliers

    D.   Hammer

    E.   Screwdriver

    F.   Wrench

    G.   Welding equipment

    H.   Electric drill

    I.   Pneunmatic chisel

    J.   Electric Grinder


PART 3---EXECUTION

3.01 EXAMINATION

    A.   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:

         1.   Wearing Surfaces - especially moving parts such as
              hardware.

         2.   Parts - which have failed or which are unsecured,
              broken, cracked, missing, distorted, or loose
              (check screws and bolts).

         3.   Paint - coating failures such as chips, losses,
              peeling, checks, bubbling, and wear.

         4.   Rust corrosion - caused by moisture, sea water and
              sea air, deicing salts, acids, soils, gypsum
              plasters, magnesium oxychloride cements, ashes,
              clinkers, and sulfur compounds.

         5.   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.

         6.   Determine the source of the moisture which causes
              the deterioration.

         7.   Determine if the feature can be salvaged.

         8.   Measure the dimensions of the various metal parts
              needing replacement.

3.02 ERECTION/INSTALLATION/APPLICATION

    A.   Patching with Epoxy Cement (see 05010-12-R).

    B.   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.

    C.   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.  

         1.   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.  

         2.   Advantages of welding:

              a.   Arc welding produces a strong, durable
                   connection and, if properly executed, is at
                   least as strong as the surrounding metal.  

              b.   It is faster and less expensive than threaded
                   connections, which require drilling a pilot
                   and then tapping to accommodate screws or
                   bolts.  

              c.   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.

         3.   Disadvantages of welding:

              a.   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.

              b.   Welding may leave a 'bead' along the surface
                   of the connection which may be unacceptable in
                   some restoration projects, eventhough much of
                   the weld may later be ground down, depending
                   on the location and the welding material.

              c.   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.

              d.   Tapping screws may be visully disfiguring in
                   some significant or decorative castings.

    D.   Cold repair techniques:

         NOTE:  STAINLESS STEEL OR NON-FERROUS METALS SHOULD BE
         USED WHENEVER POSSIBLE.

         1.   Straps:  These can often be hidden.  The plate
              should be bedded on a suitable medium to prevent a
              water trap.

         2.   Threaded studs, screwed into both sides of a
              fracture.

         3.   Dowels or plain pins, with one or both ends
              threaded and/or glued into prepared recesses.

         4.   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.

              a.   Realign the fractured section.

              b.   Drill groups of holes across the metal.  Using
                   a pneumatic chisel, cut between the drilled
                   holes to form a series of slots.

              c.   With this method, a 'dumb bell' shaped piece
                   of wrought iron is inserted across a fracture.

              d.   Drive locks of work-hardening nickel alloy
                   into the metal.  

              e.   Drill holes along the line of fracture between
                   the nickel alloy locks.

              f.   Tap and fill with studs, each stud
                   interlocking with its neighbor.  

              g.   Shear off all excess metal.  Ground, prime and
                   paint the surface.

                         END OF SECTION
 


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