Cast iron welding is a great way to repair or refurbish old pieces of equipment. It creates a strong bond between the iron and the metal, making it a reliable way to maintain old pieces of equipment. This blog post, Frequently Asked Questions will teach you how to use a Cast iron welding technique: The secret to creating flawless welds
1. What is cast iron welding technique?
Cast iron welding technique is used mainly in repairing machinery and equipment, repairing casting molds, defective products after casting …, rarely used to connect separate parts in manufacturing such as aluminum welding. , iron or stainless steel …
2. Cast iron welding characteristics
– Cast iron welds are easily transformed, so the machinability of the weld is not good
– The formation of welds is difficult due to the high melting point of the weld, making the liquid metal easy to flow out of the welding puddle.
– Cast iron welds are prone to slag pitting because cast iron contains some elements that make slag difficult to flow such as: P, Si. On the other hand, the low melting point of cast iron and the rapid transition from the liquid state to the solid state make it difficult for gas to escape, so it is easy to create porosity in the weld.
– When welding, due to uneven heating and cooling , the metal in the weld area and near the weld is prone to cracking
3. Cast iron welding methods
3.1 Cast iron hot welding
Welding cast iron by hot welding method allows to obtain welds with good mechanical properties, good machinability, high tightness, and uniform welds in composition. But the process of hot welding cast iron is relatively complicated and expensive, so hot iron welding is only applicable to welding important details and complex structures.
Hot welding is welding with preheating. Heating before welding will reduce the temperature difference between the solder and the weld pool.
Welding preheating has two methods:
– Heating the entire solder is called hot iron welding
– Local heating of each area or only heating the welding area is semi-hot welding of cast iron
Heating before welding:
Preheating before welding must ensure proper heating temperature and heating rate. If the heating temperature is high and the heating speed is fast, the solder is easily deformed and may generate new cracks when fired. If the heating temperature is low, the weld is easily whitened after welding.
Welding can be done with a molten electrode or a non-molten electrode
– Welding with molten electrode: welding current I = (30 % 40)d
– Welding with a non-melting electrode: the welding current to be taken is larger than that of welding with a molten electrode
When welding cast iron, regardless of the electrode welding, during the welding process, the solder must not be flipped. In case you have to flip the solder, you must let the solder cool down to a temperature of about 4000C and turn it gently
Heat treatment after welding:
After welding, the solder must be cooled slowly. In case it is necessary to heat the solder to a temperature of about 6500C, then cool it slowly with the furnace so that the cast iron does not turn white and avoid cracking.
3.2 Cold welding of cast iron
Welding without preheating before welding is called cast iron cold welding. When cold welding , the weld is easy to turn white, but because cold welding of cast iron is simple and can be welded in any place, this method is quite widely applied.
Cold welding of cast iron is classified according to the welding rod used: welding with a carbon steel core; composite soldering iron; copper-nickel soldering iron
Welding with carbon steel core welding rod
Welding with carbon steel core welding rod, the weld is often hardened and cast iron is prone to whiteness, especially when welding the first layer of a multi-layer weld.
Cold welding of cast iron with carbon steel core welding rod is best done by welding with one-way current, reverse
Cold welding of cast iron with copper – steel welding rods
Welding with copper welding rods – the weld steel does not crack, has good weld connection and good machinability of the weld.
Types of copper-steel welding rods commonly used are:
– Copper soldering iron core, the coating contains a lot of iron powder
– The core of the welding rod is steel and thin copper foil is wrapped on the steel core and then coated with medicine
– Bundle the copper core and steel core together and then wrap the medicine
Cold soldering with copper-nickel electrodes
Welding with this type of welding rod has low mechanical properties, but the weld has good machinability and good crack resistance.
4. Some common difficulties when welding cast iron
– The property of cast iron is hard and brittle. Among metals, the ductility of cast iron is relatively low. In testing to look at the tensile map of cast iron, there is almost no obvious yield zone.
– Cast iron is very sensitive to heat, the cooling speed is also fast, so it is easy to change the organization during the welding process as well as the cooling process after welding, leading to the weld or cracking.
– Cast iron products are diverse in chemical composition, so it is difficult to determine the welding and heating mode.
– Difficult to perform when welding cast iron in the prone position because when in liquid form, cast iron has a high melting point.
5. Cast iron welding materials
Normally, cast iron welding materials need very high ductility, so the nickel content in it is very high, up to 90%.
Should choose a welding rod and reasonable welding mode, the type of welding rod commonly used is copper welding rod, stainless steel welding rod. The best welding is to use copper nickel alloy soldering iron and use a carbonizing flame to offset the amount of carbon in the burnt cast iron.
For the cracked cast iron surface, we can still conduct welding with a special welding rod.
Type of welding rod: Eutectic 240 (Welding rod for connecting and filling cast iron)
Welding source: AC/DC+
Ingredients: Ni Fe Mn C
Tensile Strength: 55 000 psi (380 MPa)
6. Some techniques and experiences when welding cast iron
Because cast iron is hard and brittle, the residual stress in cast iron is quite large, cracks will continue to develop during use and especially during welding. Therefore, to prevent the crack from continuing to tear, people often drill to block the two ends of the crack before welding. The next step is to use the welding sequence from the beginning of the crack to the point where they meet. Finally, proceed to weld the drill holes.
With large details, we can switch to the brake (ie create a V-groove like steel welding) then process the threaded holes which can be threaded holes 6, 8, 10 … is optional and try as much as possible. it is better to arrange in sequence. Here we take the bolts in and cut them off (remember to leave 5 to 10mm extra so we’ll weld on it later).
Thanks to the extra protruding bolts and having eaten into the details, the welding process will be much more convenient and the structure will be better (because at this time we will weld on both cast iron and steel and steel will be easy to weld).
When welding cast iron , it must be carried out in an environment isolated from the wind, otherwise the heating and welding process will cause further splitting and cracking.
For details with high rigidity, it is possible to conduct local heating (slashed part) instead of total heating. The purpose of preheating is to enable the weld metal and adjacent metal to have a uniform heating and cooling rate, avoiding the occurrence of thermal stress cracking.