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Materials include base material and welding material.  Under the same welding conditions, the main factors determining the weldability of base metal are its physical properties and chemical composition.  

 

In terms of physical properties, the melting point, thermal conductivity, linear expansion coefficient, density, heat capacity and other factors of metals all affect the thermal cycle, melting, crystallization, phase transformation and other processes, thus affecting weldability. As to stainless steel and other materials with low thermal conductivity, while welding, in addition that their temperature gradient, residual stress, and deformation are generally high, the long residence time at high temperature and the long crystalline grains in the heat affected zone are also harmful for the joint performance.  The larger the expansion coefficient of austenitic stainless steel is, the more serious the deformation and stress of joint are.  

 

In terms of chemical composition, the most influential element is carbon, which means that the content of carbon in a metal determines its weldability. Most of the other alloying elements in steel are also not conducive to welding, but generally to a much lesser degree than carbon.  With the increase of carbon content in steel, the tendency of hardening increases， the plasticity decreases and it is easier to produce welding crack. Generally, the sensitivity of metal material to produce crack and the change of mechanical properties of welded joint area are regarded as the main indexes to evaluate the weldability of metal materials. So the higher the carbon content, the worse the weldability. Low carbon steel and low alloy steel with carbon content less than 0.25%, except for excellent plasticity and impact toughness, and good plasticity and impact toughness of welded joints usually have good weldability since it is unnecessary to carry on preheating and post-welding heat treatment during welding, and the welding process is easy to control.  

 

In addition, the smelting and rolling state, heat treatment state and structural state of steel also affect weldability to varying degrees. The weldability of steel can be improved by refining  crystalline grains and controlling rolling process.  

 

Welding materials directly participate in a series of chemical metallurgical reactions in the welding process, which determines the composition, structure, properties and defect formation of the weld metal. If the welding material is selected improperly and does not match with the base metal, it not only fails to obtain joints meeting the requirements, but also introduce cracks, other defects and changes in organizational properties. Therefore, the correct selection of welding materials is an important factor to ensure the quality of welding joints.  

 

Process factors include welding methods, welding process parameters, welding sequences, preheat, post-heat and post-weld heat treatment, etc, of which the welding method has a great influence on weldability, mainly in heat source characteristics and protection conditions.  

Different welding methods have great differences in power, energy density, maximum heating temperature and other aspects of heat sources. Metal welding under different heat sources will show different welding properties. For instance, electroslag welding power is very high, but the energy density is very low, the highest heating temperature is not high, heating is slow while welding, and residence time to high temperature is long, resulting in the crystalline s in the heat affected zone is massive , impact toughness significantly reduces, and it must be improved by normalizing treatment. On the contrary, as to electron beam welding, laser welding and other methods, the power is not large, but the energy density is very high, heating is quick, the residence time to high temperature is short, the affected zone is narrow, and there is no danger of crystalline grain growth.  

 

In order to adjusting welding process parameters, the welding thermal cycle can be adjusted and controlled by preheating, post-heating, multi-layer welding and controlling temperature between layers and other technological measures, so that the weldability of metal can be changed. If the measures such as preheating before welding or heat treatment after welding are taken, it is completely possible to obtain the welded joints without cracks and meet the performance requirements.  

It mainly refers to the impacts of the design forms of welding structure and welding joints, such as structural shape, size, thickness, joints’ groove form, weld layout and section shape, etc on weldability, mainly in heat transfer and force state. The direction and velocity of heat transfer are different with different plate thickness, different joint form or groove shape, which affect the crystallization direction and grain growth of molten pool. The structural switch, plate thickness and weld layout, etc., determine the stiffness and constraint of the joints, and affect the stress state of the joints, where bad crystal morphology, severe stress concentration and excessive welding stress are the basic conditions for forming welding cracks. In the design, reducing the stiffness of the joints, reducing the cross weld and reducing the factors causing stress concentration are important measures to improve the weldability.  

It refers to the working temperature, load conditions and working medium during the service period of the welding structure, which require the welding structure to have the corresponding performance. For the welding structure, it should have brittle fracture resistance while working at low temperature, should have creep resistance while working at high temperature, should have  good fatigue resistance while working under alternating load. For the welded vessels, if operating in acid, alkali or salt media environment, it shall have high corrosion resistance, etc. In short, the harsher the use conditions, the higher the quality requirements for welding joints, the more difficult it is to ensure the weldability of materials.