Identification and Evaluation Indexes for Weldability of Metallic Materials

  • By:Iris Liang
  • Date:2021/12/27

During the welding machine process, the product undergoes welding thermal process, metallurgical reaction, welding stress and deformation, which brings about changes in chemical composition, metallographic structure, size and shape, so that the performance of the weld is often different from the base metal, sometimes even can not meet the requirements of use. For many active metals or refractory metals, special welding methods, such as electron beam welding or laser welding, should be used to obtain high-quality welds. The less equipment required and the less difficult the material to make a good weld, the better the weldability of the material; on the contrary, the need for complex and expensive welding methods, special welding materials and process measures, it means that this material the weldability is poor.

 

When we use the automatic welding tracking system to produce products, we must first evaluate the weldability of the materials used to determine whether the selected structural materials, welding materials, and welding methods are appropriate. There are many methods for assessing the weldability of materials, and each method can only explain a certain aspect of the weldability. Therefore, it is necessary to conduct tests to fully determine the weldability. The test method can be divided into simulation type and experimental type. The former simulates welding heating and cooling characteristics; the latter is tested according to actual welding conditions. The test content is mainly to detect the chemical composition, metallographic structure, mechanical properties, presence or absence of welding defects of the base metal and weld metal, and determine the low-temperature performance, high-temperature performance, corrosion resistance and crack resistance of the welded joint.

 

1. Indirect assessment method of process weldability

Since the influence of carbon is the most obvious, the influence of other elements can be converted into the influence of carbon, so the carbon equivalent is used to evaluate the excellent weldability.

 

Carbon equivalent calculation formula of carbon steel and low-alloy structural steel:

When CE<0.4%, the plasticity of the steel is good, the hardening tendency is not obvious, and the weldability is good. Under general welding technical conditions, the welded joints will not crack, but for thick and large parts or welding at low temperatures, preheating should be considered;

When the CE is 0.4 to 0.6%, the plasticity of the steel decreases, the hardening tendency gradually increases, and the weldability is poor. The workpiece must be preheated properly before welding, and slowly cooled after welding to prevent cracks;

When CE> 0.6%, the plasticity of the steel becomes worse. The hardening tendency and cold cracking tendency are large, and the weldability is worse. The workpiece must be preheated to a higher temperature, technical measures to reduce welding stress and prevent cracking must be taken, and proper heat treatment must be carried out after welding.

 

The larger the carbon equivalent value obtained by the calculation result, the greater the hardening tendency of the welded steel, and the heat-affected zone is prone to cold cracks. Therefore, when the CE >0.5%, the steel is easy to harden, and the welding must be preheated to prevent cracks , As the plate thickness and CE increase, the preheating temperature should also increase accordingly.

 

2. Direct evaluation method of process weldability

In the welding crack test method, the cracks generated in the welded joint can be divided into hot cracks, cold cracks, reheat cracks, stress corrosion, laminar tears, etc.

(1) T-joint welding crack test method. This method is mainly used to assess the hot crack susceptibility of carbon steel and low alloy steel fillet welds. It can also be used to determine the influence of welding rods and welding parameters on hot crack susceptibility.

(2) Pressure plate butt welding crack test method. This method is mainly used to assess the hot crack sensitivity of carbon steel, low alloy steel, austenitic stainless steel electrodes and welds. It is by installing the test piece in the FISCO test device, adjusting the size of the groove gap has a great influence on the generation of cracks. With the increase of the gap, the greater the crack sensitivity.

(3) Rigid butt joint crack test method. This method is mainly used to measure hot cracks and cold cracks in the weld zone. It can also measure cold cracks in the heat-affected zone. On the bottom plate, the test weld is applied according to the actual construction welding parameters during the test. It is mainly used for electrode arc welding. After the test piece is welded, it is placed at room temperature for 24 hours. For cracks, cracks and non-cracks are generally evaluated, and two test pieces are welded under each condition.

 

For more questions about welding or heat treatment of metal materials in the laser welding pipe production line, especially questions about stainless steel welded pipes, please contact Hangao Tech (SEKO Machinery) for consultation. We have more than 20 years of experience in the field of industrial stainless steel welded pipe manufacturing equipment welding tube production line. After several developments and integrations, our production line is currently the only equipment in China that can complete all the processes online, including: forming welding, inner weld leveling, online bright solid solution, polishing, etc. If you have any questions about welded pipe manufacturing equipment, please feel free to contact us. We will provide you with a comprehensive and reliable solution!

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