Brief Description of Welding Heat Affected Zone

  • By:Iris Liang
  • Date:2021/12/01

Hangao Tech (SEKO Machinery), which has 20 years of experience in the development and production of stainless steel industrial welded pipe production line equipment, will take you to understand the different conditions of the welding heat-affected zone during the welding process and the impact on the quality of the weld.

 

The heat-affected zone (HAZ) of welding is different from the weld. Welding seams can be adjusted, redistributed and proper welding process through the chemical composition of the base metal in order to ensure the performance requirements. However, it is impossible to adjust the performance of the heat-affected zone through chemical composition. It is a problem of uneven tissue distribution that only occurs under the action of thermal cycling. For general welded structures, the four issues of embrittlement, toughening, hardening, and softening of the heat-affected zone are mainly considered, as well as comprehensive mechanical properties, fatigue properties, and corrosion resistance. This should be decided according to the specific use requirements of the welded structure.

 

1. Hardening of welding heat-affected zone

 

The hardness of the welding heat-affected zone mainly depends on the chemical composition and cooling conditions of the base material to be welded. The essence is to reflect the properties of the metallographic structure of different metals. The hardness test is more convenient. Therefore, the highest hardness Hmax of the commonly used heat-affected zone (usually in the fusion zone) is used to judge the performance of the heat-affected zone. It can be used to indirectly predict the toughness, brittleness and crack resistance of the heat-affected zone. In recent years, the Hmax of HAZ has been regarded as an important mark for assessing weldability. It needs to be pointed out that even in the same organization, there are different hardnesses. This is closely related to the carbon content of the base metal, alloy composition and cooling conditions. Therefore, it is recommended to use steel produced by a reliable and regular manufacturer for welding to ensure stable quality.

 

2. Embrittlement of welding heat affected zone

 

The embrittlement of the welding heat-affected zone often becomes the main cause of cracking and brittle failure of welded joints. According to the current production data and information, the embrittlement forms include coarse crystal embrittlement, precipitation embrittlement, thermal strain aging embrittlement, hydrogen embrittlement, structure transition embrittlement, and graphite embrittlement.

 

1) Coarse crystal embrittlement. Due to the effect of thermal cycling, grain coarsening occurs near the fusion line and overheated area of ​​the welded joint. Coarse grains will seriously affect the brittleness of the base metal structure. Generally speaking, the larger the grain size, the higher the brittle transition temperature.

2) Precipitation and embrittlement. During the aging or tempering process, carbides, nitrides, intermetallic compounds and other metastable intermediates will be precipitated in the supersaturated solid solution. These precipitated new phases increase the strength, hardness and brittleness of metals or alloys. This phenomenon is called precipitation embrittlement.

3) Tissue embrittlement. The embrittlement caused by the appearance of brittle and hard structure in welding HAZ is called structure embrittlement. For commonly used low-carbon low-alloy high-strength steels, the structure embrittlement of welded HAZ is mainly caused by M-A component, upper bainite, and coarse Widmanstatten structure. But for steels with higher carbon content (generally ≥0.2%), the structure embrittlement is mainly caused by high-carbon martensite.

4) The thermal strain aging embrittlement of HAZ. The welding structure needs to be processed in the manufacturing process, such as material, shearing, cold forming, gas cutting, welding and other thermal processing. The local strain and plastic deformation caused by these processing have a great influence on the embrittlement of the welded HAZ. The embrittlement caused by these processing steps is called thermal strain aging embrittlement. Strain aging embrittlement can be divided into static strain aging embrittlement and dynamic strain aging embrittlement. Generally speaking, "blue brittleness" belongs to the phenomenon of dynamic strain aging.

3. Toughening of welding heat affected zone

Welding HAZ is a non-uniform body in structure and performance. The fusion zone and the coarse-grained zone are particularly prone to embrittlement and belong to the weak area of ​​the entire welded joint. Therefore, it is necessary to improve the toughness of the welded HAZ. According to research, the following two methods can be used to toughen the HAZ.

1) Control the organization. Low-alloy steel should control the carbon content, so that the alloying element system is a strengthening system of low-carbon traces of multiple alloying elements. As a result, under the cooling conditions of welding, the HAZ is distributed with dispersion-strengthened particles, and low-carbon martensite, lower bainite and acicular ferrite are produced in its structure with better toughness. Secondly, the segregation of grain boundaries should be controlled as much as possible.

2) Toughening treatment. Some important structures often use post-weld heat treatment to improve the performance of the joint. However, some large and complex structures adopt local heat treatment, which is more difficult in actual operation. Therefore, the correct selection of the welding heat input, the formulation of a reasonable welding process, and the adjustment of preheating and post-heating temperatures are effective measures to improve welding toughness.

In addition, there are other ways to improve the toughness of the HAZ. For example, fine-grained steel adopts a controlled process to further refine the ferrite grains, which will also improve the toughness of the material. This depends on the element content of the base metal itself and is related to the smelting technology.

Fourth, the softening of the welding heat-affected zone

For metals or alloys strengthened by cold work hardening or heat treatment before welding, different degrees of vector strength will generally occur in the welding heat-affected zone. The most typical are the high-strength steels that have been modulated and the alloys with precipitation strengthening and dispersion strengthening, and the softening or vector strength produced in the heat-affected zone after welding. When welding quenched and tempered steel, the softening degree of HAZ is related to the heat treatment state of the base material before welding. The lower the tempering temperature of the quenching and tempering treatment before welding of the base metal, the greater the degree of strengthening, the more serious the post-welding softening will occur. A large number of practical research data shows that when different welding methods and different welding wire powers are used, the most obvious position of softening in the HAZ is the temperature between A1-A3.

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