Page 195 - Handbook of Structural Steel Connection Design and Details

Welded Joint Design and Production

180 Chapter Three

the reattachment of nonstructural components, it is highly desirable
to investigative alternative methods of joining when cast iron is
involved. Bolted attachments are generally more easily made than
welded ones.
Although there are several types of cast iron, the following are gen-
eral guidelines to follow when welding cast iron. First, determine
what type of cast iron is to be welded (that is, gray, malleable, ductile,
or white). If the casting is white iron, which is generally considered
unweldable, then an alternative jointing or repairing procedure
should be investigated. However, if the casting is gray, malleable, or
ductile, an appropriate welding procedure can be developed.
Second, preheating is almost always required when welding cast
iron. Preheating can be achieved by heating large parts in a furnace
or by using a heating torch on smaller parts. In general, the mini-
mum preheat temperature will be around 500°F to sufficiently retard
the cooling rate.
Third, any welding processes can be utilized; however, the electrode
used should be a cast-iron electrode as specified in AWS A5.15.
Finally, after welding, the casting should be allowed to cool slowly
to help reduce the hardness in the heat-affected zone. If the casting is
a load-bearing member, caution must be taken to prohibit brittle frac-
ture. The welding procedures should be tested, and a welding expert
should be consulted.
Cast steels. Steel castings are often used for miscellaneous structural
components such as rockers on expansion joints, miscellaneous brack-
ets, and architectural elements. Welding cast steel is generally simi-
lar to welding of rolled steel of similar chemical composition.
However, steel castings are often made of significantly different
chemical compositions than would be utilized for rolled steel, particu-
larly structural steels expected to be joined by welding. For example,
a steel casting made of AISI 4140, a chromium-molybdenum steel
with 0.40% carbon, will have reduced weldability because of the alloy
content and the high carbon level. The producers of the steel castings
often utilize alloys with reduced weldability simply because it may be
easier to obtain the desired mechanical properties with the enriched
composition, and welding of these materials is often overlooked.
Steel castings with poor weldability should be joined by bolting
when possible, or alternative compositional requirements should be
pursued. For example, it may be possible to select material with lower
carbon and/or alloy levels for a specific application, increasing the
weldability. If the chemistry of the casting is similar to that of a rolled
steel, it will have similar weldability characteristics. However, the
casting will have associated flow pattern–dependent properties.

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