Welding capacity of medium and high carbon steels is low, this causes cracks at the end of the welding pool or in the heat-sensitive areas. In case this situation happens, it requires welders to preheat and local heat the areas of welds. Preheating is at a temperature of 250 - 300 degrees Celsius. If local heat occurs around the welding site, the temperature is at 650 - 700 degrees Celsius.
Flame welding and flame capacity
- Welding flame:
Use a normal flame or you can use a carbonization flame.
- Flame capacity
Flame output in welding
of medium carbon steel must be smaller than that of low carbon steel welding, formula is: W = (75 ÷ 90) S (liter / hour)
- Welding rod:
Types of welding rod: CB-15, CB-15T, CB-10T2, CB-08A, CB-08TA
- Welding flux:
The welding flux used is an embryo (Na2B4O7)
Note: After welding, it is necessary to cool down the weld with covering asbestos on the surface of the weld and heat treatment after welding according to the requirements and functions of parts
Welding of medium carbon steel
Welding technique of alloy steel
Because of the influence of alloy elements in steel, the following phenomenons usually occur during the welding time:
-Welds usually have hardenability since alloy steel has the high hardenability
- The low thermal conductivity of alloy steel is the reason that leads to overheating and cause distortion seriously
In the welding process, carbon and combustible alloy elements decrease their content in the weld, therefore, reducing the mechanical properties of the weld.
In order to minimize the size of heat affected zone as well as the melting depth, welding of the alloy steel should be performed at a rapid speed and the flame output required for each type of steel.
- Welding of Mo and Cr - Mo Steel
The Mo and Cr - Mo steel used for welding include the codes: 15M, 20M, 30M, 20XM, 30XM.
Prior to welding,the weld beads must be preheated at 250 to 300 degrees Celsius and remained at that temperature during welding procedure
Flame welding and flame capacity
The flame to weld is a neutral flame. Flame power for 1mm welding thickness is:
W = (100 ÷ 150) (liter / hour)
Welding rod grades: CB-12M, CB-12XM or low carbon steel welding rod.
Diameter of welding rod: Diameter of welding rod is chosen according to weld thickness. For thick objects, welders should select a large diameter rod. Regarding the thickness of welds as the table below, welders can pick rod diameters invoved:
Note: If weld beads is less than 5mm thick, a layer should be welded. While thickness is greater than 5mm, workers must weld in multiple layers
Heat treatment must be carried out after welding. For 15M and 20M steel, it should be at temperatures (900 ÷ 930) degrees C as usual
Welding of Cr and CR - Mo - Si steel
Prior to welding, the preheating should be done at a temperature of (200 ÷ 250) degrees C.
Welding flame and flame capacity
Flame to weld Cr and Cr - Mo - Si Steel is neutral flame. Flame power for 1mm welding thickness is: W = (70 ÷ 75) (liter / hour)
For Chrome steel: OX13, 1X13 ... CB - OX18H9 welding rods used
For Cr - Mo - Si steel: it should use CB - 08; CB-08A, CB-18XTCA welding rods
Welding of Cr and Cr - Mo - Si steel needs to be welded at a rapid speed and must limit the horizontal movement of the weld.
After welding, cool down the welding torch.