"Cold welding" technique

1. Advantages:

• Fastest repair capabilities
• Virtually no deformation
• Limited dismantling

2. Applications:

• For welding of rigid parts
• For welding machined parts which will not tolerate any distortion
• For welding where pre-heating is impractical.

3. Preparation:

• Cleaning / degreasing of the workpiece
• Dye-check for presence of cracks
• Parts must be protected from cool draughts
• Cracks must be stopped (by gouging or drilling a hole)
• Cracks and impurities must be removed by gouging
• Grinding for removing hardened zones
• Round off sharp edges
• Dye-check for presence of cracks
• Cleaning / degreasing
• Slight preheating (50°C), if necessary

4. Preheating:

• Up to a wall thickness of 25 mm: preheating is generally not necessary for microcrack-free welds.
• For wall thickness above 25 mm: preheating can be necessary for microcrack-free welds (see "semi-hot welding")
• In repair welding, a slight preheating of 50°C can improve bonding on cast iron.
• This preheating is only applied for the bonding layer (first layer).
• Advantages of slight preheating: better bonding, no porosity / no cracks

5. Consumable:

• Heterogeneous: different to cast iron

6. Welding:

• Select smallest reasonable electrode diameter
• Deposit of short (25mm) stringer bead (lowest welding current)
• Peen immediately each bead deposited (red heat)
• Air cool back to approx. 50°C (hand temperature)
• Apply the next stringer bead in backstep sequence

This procedure limits heat input considerably, and so reduces the risk of cracking or distorting the workpiece.

7. Machining:

• The heat-affected zone (HAZ) is hard and brittle.

"Semi-hot welding"

1. Advantages:

• Excellent mechanical properties
• Quick and economical

2. Preparation:

• Cleaning / degreasing of the workpiece
• Dye-check for presence of cracks
• Parts must be protected from cool draughts
• Gouging
• Grinding for removing hardened zones
• Round off sharp edges
• Dye-check for presence of cracks
• Cleaning / degreasing
• Preheating

3. Preheating:

• Grey cast iron 200 to 250°C
• Spheroidal graphite cast iron 250 to max. 300°C

In repair welding, we speak of "semi-hot welding" if the preheating and the weld-induced heating remains localised only.
Local heating is acceptable only if the workpiece can expand and contract freely.

4. Consumable:

• Heterogeneous: different to cast iron

5. Welding:

• Deposition in stringer beads
• Deposition in a continually warm environment
• No pauses are made between beads
• Preheating temperature is maintained
• Slow cooling, in a refractory material such as vermiculite

7. Machining:

• The heat-affected zone (HAZ) is hard and brittle.

"Hot welding"

"Hot welding" is carried out on workpieces preheated to 550 to 700°C.
This technique can generate deformation and surface oxidation.
Finish machining can therefore become a significant factor.
The high level of heat radiation can make this process quite uncomfortable for the welder.
In repair welding, "hot welding" is applied if the workpiece is highly stressed.

1. Advantages:

• The mechanical properties and / or colour match sought are similar to those of the base metal.
• No machining problems in the weld area.
• Heat treatment can be employed after welding.

2. Preparation:

• Open grooves
• Round off sharp edges.

3. Preheating:

• The entire workpiece should be slowly preheated to 550 to 700°C maximum.
• The most common types of furnace used are electrical or gas.
• In repair welding, there is a tendency to keep the preheating temperature at a minimum.
• Cooling: Slow furnace cooling

4. Consumable:

• Homogeneous: filler metal is similar to cast iron
• Heterogeneous: different to cast iron in case of braze-welding

5. Welding:

• The interpass temperature limit is equal to the preheating temperature and must be strictly observed.
• Mould welding: Welds can be built up against copper or graphite support plates.
• Slow furnace cooling after welding.

Safety: Arc welding

see chapter safety

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