How to Weld Cast Iron?

March 2, 2021

It is a morbid nightmare for machine owners of a cast iron to break or crack a crucial cast component.

It usually can mean a considerably high expense for replacing parts and hiring cast iron welding specialists. In some cases, you may opt for either option.

How to Weld Cast Iron
man holding a welding torch

However, if you know how to weld cast iron or you are up for adding this skill to your kitty, welding it on your own can solve the issue very quickly. 

Here, we will help you answer various questions: What is a cast iron? How to weld cast iron?

Iron cast welding step-by-step guide so that you have a clear answer to all the questions you have in mind.

What Is Cast Iron?

What Is Cast Iron?

A cast-iron is an alloy of silicon, carbon, and iron, where the carbon amount generally is over 1.7% and below 4.5%.

The weldability of the cast iron is way low and depends on material kind, thickness, complexity, need for machinability, and casting complexity.

Malleable and ductile irons come with good weldability, and grey cast iron & white cast iron is just weldable for the small attachments. 

The most widely utilized cast iron type is grey iron (or gray iron).

The grey iron has various compositions and is generally such that it is particularly perlite with several flakes of graphite dispersed throughout. The name came into existence as grey color fracture forms because of the presence of graphite.

There are also alloy cast irons that contain small amounts of chromium, nickel, molybdenum, copper, or other elements added to provide specific properties.

Next, alloy iron is an austenitic cast iron that is modified owing to the inclusions of nickel and various other components to lower the transformation temperature.

This is so that the structure stays austenitic at normal or room temperatures. Cast iron of austenitic type has the same degree of resistance towards corrosion. 

In white cast iron, mostly all the carbons are in combined form. This caters to the cast iron with an excellent hardness that is used for abrasion resistance.

Cast iron, which is malleable, is made by endowing white cast iron with necessary special heat treatment. This treatment changes the carbon structure in iron.

This structure transforms to ferritic or perlitic, which enhances its flexibility. 

Ductile cast iron and nodular iron are prepared by adding aluminum or magnesium, which will either tie up carbon in combined form or endow free carbon a nodular or spherical shape instead of just the normal flake structure in the gray cast iron.

This formulation caters to a higher degree of malleability or ductility of casting. 

The primary factor that contributes to the toughness of the welding cast iron is the lack of flexibility.

In case the cast irons are ladened over their yielding point, they smash off rather than get deformed to a considerable level.

Thus, part configuration and weld filler metal should be selected to lower welding stresses. Flux core, MMA, TIG, MIG & gas welding procedures are generally utilized with nickel stationed welding consumables.

This is done to produce good quality welds. However, steel electrodes and cast iron can even produce acceptable welds, in particular alloys. 

Equipment Needed for Welding

Major components of equipment needed for welding include: 

  • Power source
  • Fume extraction
  • Welder protection
  • Electrode

Iron Cast Welding Step-By-Step

Iron Cast Welding Step-By-Step

Inhouse welding of the cast iron portions can save money and time. However, there are some challenges.

Failure in welding can mostly lead to cracking or various other damages. 

In case any crucial parts are involved, then it might be advisable to opt for the workmanship of a welding facility with highly experienced welders to make sure a successful result is generated. 

When welding is performed in house, it becomes imperative to research necessary steps/measures needed to produce welded portions efficiently.

4 key steps that should be taken before starting are: 

  • Recognize the alloy
  • Nicely clean the casting
  • Choose a preheat temperature.
  • Choose an apt welding technique

Recognize the Alloy

Recognize the Alloy

This is one of the crucial things to do before starting with the iron cast welding process. Cast iron is considered in the group of iron-carbon alloys.

Cast irons get a characteristic hardness due to their increased carbon content between 2% and 4%. Note that this hardness is at the expense of letting go of ductility. As compared to wrought iron or steel, it is not very malleable.

The cooling and heating cycles while welding leads to contraction and expansion in metal, as well as induces tensile stress.

Cast iron hardly deforms or stretches when stressed or heated – instead, they break or crack through, making them exceedingly tough to weld.

Such characteristics can be ameliorated by including various alloys. 

Cast Alloys That Are Simple to Weld

Cast Alloys That Are Simple to Weld

Grey Cast Iron

Grey cast iron (gray cast iron) is one of the most widely used cast iron varieties.

The carbon precipitates out into the graphite flakes at the manufacturing process, either into a ferrite or pearlite crystalline microstructure.

This is more weldable and ductile as compared to white cast iron.

But it still poses a great challenge to the prospective welders because the graphite flakes strewn in gray cast iron can enter the weld pool and cause weld metal embrittlement.

White Cast Iron

White cast iron stores the carbon in iron carbide with zero precipitating out as graphite. Cementite crystalline microstructure is brittle and hard. The white cast generally is considered un-weldable. 

Nodular, Malleable, or Ductile Iron

Such cast irons are less brittle because of the microstructural differences owing to their manufacture.

All the 3 have a spheroidal carbon microstructure produced by their unique manufacturing procedure. 

One of the prudent ways to know whether you have a gray or white iron is to analyze its original specifications.

A spectrochemical examination can even help you to understand. When such precise manners seem impossible, you can check in the shop. 

Clean the Casting

Clean the Casting

Like in most welding, it is said cleaner the surface of the welding cast, the better will be the welding results. Ensure to clean the surface to get rid of paint, oil, grease, etc.

Ensure to remove all kinds of foreign substances in the weld’s area for best results.

To eliminate the entrapped gases from the base metal weld zone, ensure to carefully apply the heat slowly to the area where you need to weld. 

You can test the cast iron surface’s readiness by carefully examining the cast for any deposit of weld pass o the metal; if impurities are present, then the area will be porous.

You can then grind off the porous area or even repeat the process until the porosity is gone.

Choose A Pre Heat Temperature

Choose A Pre Heat Temperature

Welding cast iron comes with two schools of thought, hot welding is one, and cold welding is the other. Masses agree that preheating is a good choice.

However, others go with minimum cold or heat weld.

You require adopting one as, unlike various other metals, cast iron in nature is brittle and comes with the minimum potential to deform when it witnesses contraction, bending, and expansion. 

Every metal comes with the property to get deformed when subjected to heat owing to thermal expansion.

Preheating lowers this impact of undergoing cracks near the welding zone, which keeps the temperature almost like the welding zone. It even makes the entire component alter uniformly.

Also, it is much better to avoid excessive heat in the component because the cast iron characteristic is to change once they are exposed to a temperature above 1400 degrees Fahrenheit. 

On the other hand, welding cast iron in a cold manner with zero preheating might be useful as it lowers the temperature.

This is a procedure, which does not produce any kind of visible cracks. Looks might be good; however, remember that the internal strength will stay poor in cold welding. 

Choose an Apt Welding Technique

Choose an Apt Welding Technique

After you determine grey cast iron requires welding, you need to select the procedure, which will go into welding.

One of the prudent ways to weld the cast iron is to opt for an appropriate welding process to choose an accurate filter material.

Welding process choice should be based on personal preference. 

If you have a machined surface, which requires welding, preference should be given to TIG weld because the spatter from stick & MIG welding cast iron might excessively damage the clean surface.

Note that TIG is spattered and an excellent choice to make. 

On curtailing the extreme heat and on swift cooling of electric welding procedure, the option available here is oxy acetylene. It helps maintain the temperature of the component. 

Stick welding is an alternative to welding cast iron, where electrodes with flux get utilized.

Machining is essential once the procedure is over. Just remember, the filler metal selection forms the base of cast iron welding. 

Extra Tips

Brazing is another procedure, which helps join cast iron.But this isn’t a welding procedure.

You can opt for it in case of any problem when welding. Rusty and dirty surfaces might not join properly, so make sure to clean the surface well. 

Conclusion

Cast iron is a very challenging element to weld.

Even a minor incorrect move can hamper your broken part to a great extent, or worse, make your welding appear externally strong while it is weak internally.

Thus, knowing what you are doing is very important.

Step by step, following the guidelines, can make sure your welds have zero cracks and that the strength internally is adequate to withstand the component’s purpose. 

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