Welding of Common Metal Materials

I. Weldability of metal materials

Weldability refers to the difficulty of obtaining excellent welded joints for the welded metal materials under certain welding conditions. It includes two aspects:

(1)Process performance: Welded joints are prone to process defects, especially the formation of weld cracks.

(2)Application performance: Adaptability of the welded joints to the requirements of use.

Good weldability: This means that excellent welded joints can be achieved with the simplest and most common welding process conditions.

Poor weldability: This means that special and complex welding procedures must be used to obtain excellent welded joints.

The main factors that determine the weldability of metal materials are the chemical composition of the base metal.

II. Weldability of carbon steel

1. Welding of mild steel

Mild steel welded joints plasticity and impact toughness is good,welding generally without preheating, control the temperature and heat between the layers, and good weldability. The whole process is not required to take special process measures, and does not required preheating or post-welding heat treatments. If there is force complexity of the important structure,when using manual arc welding, you should use alkaline electrodes, such as J426, J427 or J507, etc. However, there are two exceptions:

(1) If the welding thickness is greater than 50 mm, the welded parts must be annealed after welding.

(2) When welding large structural steels with high rigidity at low temperature conditions, the surface of the steel should be preheated. If the ambient temperature is below 0℃, it should be preheated to 100-150℃.

Mild steel welding methods are: manual arc welding, submerged arc automatic welding, electroslag welding, gas shielded welding and resistance welding.

2. Welding of medium carbon steel

Steels with a medium carbon content have between 0.30 and 0.45 percent carbon. Increased carbon means increased hardness and tensile strength, decreased ductility, and more difficult machining.

Welding properties of medium carbon steels:

(1) Quenched and partially quenched zones are prone to hardened tissue and cold cracks.

(2) Weld metal is more prone to thermal cracking.

Preventive measures:

(1) Preheating before welding, and slow cooling after welding. Can slow down the cooling rate, to avoid hardened tissue and reduce the tendency of cold cracking; at the same time, the temperature difference is reduced to reduce the stress. For example, when welding 45# steels, usually preheated to about 150-200 ℃.

(2) Use alkaline welding electrodes (manual arc welding) with good crack resistance, that is, electrodes with low hydrogen content, such as J507.

(3) Use thin welding rods, small current, open bevel, and multi-layer welding processes. Prevent the C, S, P high base material in the weld too much to avoid hot cracks.

Medium carbon steel welding methods are manual arc welding, electro-slag welding, etc.

3. Welding of high-carbon steels

High-carbon steels are poorly weldable and generally limited to repairs.