Characteristics of Wrought Iron

Characteristics of Wrought Iron:

-    Soft

 

-    Ductile

 

-    Magnetic

 

-    Strong - high elasticity and tensile strength

 

-    Malleable – wrought iron can be heated and reheated and worked into various

     shapes.  Note:  Wrought iron becomes stronger the more it is worked.

 

-    Wrought iron is suitable for members in tension OR compression; whereas, cast

     iron is suitable for members in compression ONLY.

 

Wrought iron was typically produced by the indirect reduction

process.  Stages in the production of wrought iron include the

following:

 

1.   Stage 1:  Preparation of puddled iron. 

 

     a.  Pig iron was first smelted in blast furnaces and

          subjected to a reducing condition when it apparently

          'boiled'. 

 

     b.  The boiling iron was worked continuously by the puddler

          and more impurities were removed the iron, making the

          iron stiffer. 

 

     c.  The iron was boiled until virtually no carbon remained,

          leaving a pasty mass of iron.  This was evident when the

          carbon monoxide would stop bubbling through the iron. 

 

     d.  The iron was then formed into balls to be molded. 

 

2.   Stage 2:  Iron balls were hammered with a shingling hammer, to

     expel surplus slag or cinder (shingled). 

 

     a.   Shingling was completed in minutes and the finished

          product was a bloom approximately 5 inches x 5 inches x

          3 feet. 

 

     b.   The bloom, still at bright red heat, was then passed

          through rolling mills, becoming more elongated and

          thinner in section after each pass, and finished as

          puddled iron bar. 

 

3.   Stage 3:  The bars were reheated and reworked as required to

     achieve the desired grades. 

 

     a.   This stage increased the ductility and tensile strength

          of the puddled iron. 

 

     b.   The more times the metal was reheated and reworked, the

          stronger were its mechanical properties.

 

TYPICAL USES

 

Historical uses of Wrought Iron during the 17th and 18th centuries were typically

decorative and include:

 

-    Wrought Iron Fences, Iron gates and Iron railings

 

-    Iron Balconies

 

-    Iron Furniture

 

-    Wrought Iron Porches and Iron verandas

 

-    Iron Canopies and Gazebos

 

-    Iron Roof cresting

 

-    Wrought Iron Lamps

 

-    Wrought Iron Grilles

 

-    Wrought Iron Hardware

 

Historically, uses for Wrought Iron during the middle of the nineteenth century

became more structural and include:

 

-    Wrought Iron Nails

 

-    Iron cramps (i.e. to secure masonry veneer building frames)

 

-    Structural members in tension such as tie rods (or strapwork),

     bulb-tees and I-beams.  The standard sections of wrought iron

     included bar iron, angle and T irons, channel iron (half H

     iron), rolled girder iron (rolled joist iron, beam iron, I

     iron, or H iron), various special sections (sash bar, beading

     iron, cross iron, quadrant iron), iron bars, rivet iron, chain

     iron, horseshoe iron, nail iron, plate iron, coated iron (tin

     or lead), and corrugated sheet iron (generally galvanized). 

 

     Note:  By the end of the nineteenth century the use of wrought

     iron for structural purposes had been superseded by steel. 

 

PROBLEMS AND DETERIORATION WITH WROUGHT IRON

 

Problems may be classified into two broad categories:  1) Natural

or inherent problems based on the characteristics of the material

and the conditions of the exposure, and 2) Vandalism and human-

induced problems.    

 

Although there is some overlap between the two categories, the

inherent material deterioration problems generally occur gradually

over long periods of time, at predictable rates and require

appropriate routine or preventive  maintenance to control.

Conversely, many human induced problems, (especially vandalism),

are random in occurrence; can produce catastrophic results; are

difficult to prevent, and require emergency action to mitigate.

Some human induced problems, however, are predictable and occur

routinely. 

 

NATURAL OR INHERENT PROBLEMS WITH WROUGHT IRON

 

Chemical corrosion can attack decorative and structural wrought

iron features in several ways:

 

1.   Uniform Attack:  Corrosion attacks the metal surface evenly

 

2.   Pitting:  Attacks the metal surface in  selected areas

 

3.   Selective Attack:  When a metal is not homogenous throughout,

     certain areas may be attacked in preference to others

 

4.   Stress corrosion cracking:  Attacks areas in a metal which

     were stressed during metal working and were later exposed to

     a corrosive environment.  Old, hand wrought iron items are

     more likely to be affected than are machine rolled wrought

     iron pieces. 

 

5.   Rust:  Probably the most common form of chemical corrosion of

     wrought iron.  It occurs when unprotected metal is exposed to

     oxygen in the atmosphere in the presence of moisture.

     Moisture can be in the form of normal humidity, rain, dew,

     condensation, etc.  Other gases, such as carbon dioxide,

     sulfur compounds, soot and fly ash will exacerbate the

     corrosion of the iron, as will airborne salts.

 

6.   Galvanic (or Electro-Chemical) Corrosion:  Galvanic corrosion

     occurs when two dissimilar metals are in contact with one

     another and an electrolyte, such as rainwater, condensation,

     dew, fog, etc. is present.  Such a reaction will cause one or

     the other of the metals to corrode.  In the case of wrought

     iron, direct contact with copper or zinc, and to a lesser

     extent galvanized iron or steel, will cause galvanic

     corrosion.

 

VANDALISM OR HUMAN-INDUCED PROBLEMS WITH WROUGHT IRON

 

Mechanical or physical deterioration of Wrought Iron:

         

1.   Fatigue:  Failure of metal that has been repeatedly stressed

     beyond its elastic limit. 

 

     a.   Wrought iron is generally fatigue resistant because it is

          so tough.  It will deform considerably, within its

          elastic limit, without failure. 

 

     b.   Even if past overloading has caused deformation, wrought

          iron fixings will usually continue to function. 

 

     c.   Defects in the wrought iron itself, or stress points can

          cause a feature to fracture.

 

2.   Heat:  Usually in the form of fire, will cause wrought iron

     features to become plastic, distort, and fail.

 

3.   Distortion:  Permanent deformation or failure may occur when

     a metal is overloaded beyond its yield point because of

     increased live or dead loads, thermal stresses, or structural

     modifications altering a stress regime.

 

Connection failure of Wrought Iron: 

 

1.   Chemical and mechanical processes can cause the breakdown or

     reduced effectiveness of structural metal fixings such as

     bolts, rivets, and pins.

 

2.   Stress failure in wrought iron is often a contributor to breakdown situations.

     Iron connections which are water traps are particularly

     Susceptible.