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Metal Stitching Stitching Products Services Training Technical Support Cast Iron Welding Repair Examples Special Repairs Spark Plug Thread Repair Inserts ArticlesPreheating & Welding Procedures

 

Here are some very specific preheating and welding procedures to help you perform cast iron repairs that are predictable and reliable.

Brazing
The casting must be preheated to at least 900F before you begin increasing the temperature in the area you want to braze.  The bronze rod will melt at around 1725F.  A good bonding temperature is 1800F.  The casting should be positioned so that you are brazing on an incline of at least 30 degrees.  Avoid "in position" or flat brazing as this causes pin holes, cold laps, and burnt edges.  Brazing uphill allows you to fill the vee from bottom to top in one pass.  Small, thin steps allow you to move quickly and keep the area ahead of the puddle tinned.  Be sure to post heat to normalize the casting at 900F.  Slow cool the part over 24 hours.  

Many people believe bronze to be an inferior metal to use on cast iron.  Actually, bronze matches cast iron closely in hardness and metal to metal wear resistance.  Gray cast iron has a maximum tensile strength of 40,000 PSI.  And ductile iron (or nodular) can reach 75,000 PSI tensile strength.  Common bare bronze is 70,000 PSi tensile strength.  

Fusion welding
Fusion welding is a skill that can take years to master.  It requires actual melting and puddling of base metal as the filler material is added.  Fusion welding is used primarily on dense castings that can be machined after the welding is done.  (Cylinder heads are excellent subjects for fusion welding but engine blocks are not.)  A 1300F to 1500F preheat is required.  For a heating source, use natural gas or diesel fuel.  Do not use propane.  Use oxy-acetylene gas and bare cast iron rod for the fusion welding process.  Heat the part and maintain it at bright red during the entire welding procedure.  Cast iron melts at approximately 2300F.  A post heat of 1300F to 1500F for 15 minutes is needed.  Slow cool the part over 24 hours.

Powder welding
Preheat the casting to 900F, apply a light coat of powder to protect the surface from ferrite oxide deposits and then continue the preheat to at least 1300F.  During the build-up process, the weld affected area can reach 1800F in the area the capillary bond process takes place.  Without high temperature preheat, powder welding should be restricted to corners, ears and ends.

Corners, ears, and ends
It is impossible to create stress on corners, ears and ends.  A good illustration of this is if you welded two pieces of welding rod together.  The ends have nothing to expand against, therefore there is no confinement.  After joining the two pieces together, the heated area can contract and shrink without creating stress.  If you must use a nickel rod on cast iron, use it only on corners, ears and ends.  Preheat the area with a torch to a dull red, arc weld it and immediately post heat with the same torch.  It is best to use two people, one to preheat and post heat and the other to weld.  Timing is critical.  You will also need to use less amps when the iron is preheated.  Your best bet is to learn how to braze and forget the arc welder.  (This also applies to tig and mig welding.)

Electric arc welding
This is the poorest of all choices for welding cast iron.  It would be nice if it would work, but it causes so many problems it should be avoided if at all possible.  The heat potential is great and the process causes the heat to be too localized.  Thin sections heat faster and cool quicker than thick sections.  If a section of the casting is heated too quickly, the surrounding area does not have ample time to absorb enough heat to allow the casting to have a uniform temperature.  This causes restricted expansion and contraction.  It occurs when the weld affected area is contained by cooler iron.  This will always result in some stress.  Often it is enough stress to cause additional cracking.  There is no such thing as cold arc welding; cast iron melts at 2300F.  No professional industrial cast iron welder would ever arc weld on a casting heated to less than 1000F.

Nickel rod
In our shop, we can tell no difference between one manufacturer's rod and another.  There are repairs where we must use nickel rod, such as on compressors in the oil field.  In these cases, H2S gas is present and bronze is not acceptable.  We will preheat  to 1200F before welding, followed by a long post heat to a uniform 1200F before a 24 hour cool down.  The critical part of the cool down is from 1800F to 1200F.  

Recap
Let's add together the things you have learned about expansion and contraction along with temperature control and review the ways to prevent stress from building up in your welded cast iron parts:

bulletUse a high temperature preheat when welding in the center of a casting.
bulletLimit low temperature welding and brazing to corners, ears and ends.
bulletUse a proper preheat and maintain the part at the appropriate temperature during the entire welding process.
bulletCool the casting very slowly in order to allow the weld affected area to stretch with the contraction of the weld.

Metal stitching avoids all of these problems.  And it can be used on approximately 90% of all repairs of cracked castings.

 

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