Cast iron is a type of iron-carbon alloy containing up to 4% carbon. It is a durable, wear-resistant, and valuable metal, but it is known to be prone to cracking.
A cracked engine block presents challenges. However, if you follow the correct procedures, there is a way to fix the cracked engine block by welding – eliminating the need to sell or scrap the car.
What Causes a Cracked Engine Block?
The most common cause of a cracked engine block is an overheated engine that results in thermal stress. Cracking can also occur if the cooling system is overfilled with water and not enough antifreeze in extreme temperatures.
Another cause of cracked engine blocks is high-speed collisions. With an extremely high force impact on the engine compartment, the engine can be damaged, causing the break.
The good news is that you can weld cast iron when a block has undergone structural damage. This can be a demanding task, but it is not impossible if you use the guided welding technique that will aid in avoiding weld cracks.
Use advised heating and cooling techniques and the correct welding rods, and allow the metal to cool down slowly.
Pre-welding Steps for Cast Iron
Firstly, research the protective measures necessary for safe welding and practise the appropriate techniques before you begin the challenging task. This will give you more confidence and a successful result.
There are 4 pre-weld steps to take before you start:
- Identify the alloy
- Clean the cast iron
- Use the appropriate preheat temperature
- Choose the right welding technique for the project
1. Identifying the Alloy
Understanding the different cast iron alloys, knowing which ones are more susceptible to cracking, and utilising that knowledge will make the process a lot simpler and more effective.
Grey Cast Iron
This is the most common form of cast iron. Graphite flakes are precipitated from the carbon, which can pose a challenge as the flakes can enter into the heat-affected weld metal. Therefore, great care has to be taken when working with this alloy.
White Cast Iron
This cast iron is free of graphite. It has a brittle microstructure and is generally considered unweldable.
Ductile, Nodular, Malleable Iron
Not as brittle as white cast iron, the microstructural differences in this alloy are due to the manufacturing processes.
2. Cleaning the Cast Iron
Whatever the type of alloy, the cast iron should be properly prepared prior to welding to remove any surface paint, oil, grease or grime.
In the area of the weld, slowly preheat the block to disperse the entrapped gas and moisture from the base metal.
3. Using the Appropriate Preheat Temperature
Controlling the heating/cooling process will help to minimise the stresses that can cause cracks.
Heating causes metal to expand, and cooling too rapidly will cause stress to build up in a localised zone. If the metal is warmed and cooled at the same rate, the stress is minimised.
4. Choosing the Right Welding Technique
Relating the welding technique to the cast iron alloy is vital in choosing the right approach. The most commonly practised techniques are stick, oxy-acetylene, and braze welding.
Stick Welding
Arc or stick welding is probably the best-known process for welding cast iron. The proper welding rods have a special graphite-rich flux and can zone into the weld metal and heat-affected zone.
Braze Welding
A common method of welding, this process has a minimal impact on the metal. A unique welding rod is used that adheres to the surface of the cast iron.
Final Thoughts
If you want to attempt to weld a cast iron engine block in-house, you must use the correct techniques and take the utmost care. The pre- and post-weld heating techniques must be followed as well as the correct cooling approach.
