Der welding heat affected zone is the area where the solid base metal on both sides of the weld undergoes obvious changes in structure and properties under the action of the welding heat cycle, which is called the welding heat-affected zone. A welded joint is a welding process consisting of three parts: the weld seam, the fusion zone and the heat-affected zone.

1.Definition

Under the action of a welding heat affected zone fusion welding, the area where the structure and properties change within a certain range near both sides of the weld is called the “welding heat affected zone“, or the “Near Weld Zone” (Near Weld Zone). ). The welded joint is mainly composed of two parts, the weld seam and the hot shadow zone, and there is a transition zone in between, which is called the fusion zone. Therefore, in order to ensure the quality of welded joints, it is necessary to make the structure and properties of the weld and the heat affected zone meet the requirements at the same time. With the continuous use of various high-strength steels, stainless steels, heat-resistant steels and some special materials in production, the problems existing in the welding heat-affected zone become more complicated and have become the weak area of welded joints. Therefore, researchers in many countries have paid great attention to the welding heat affected zone.

The extent of the HAZ varies depending on the heat input of the welding process, the thermal conductivity of the material, and the cooling rate. Higher heat input or slower cooling rates typically result in a larger HAZ.

2.Tissue Distribution

According to the welding heat affected zone of the steel, the steel for welding is divided into two categories, one is the steel with a small quenching tendency, such as low carbon steel and some low-alloy steel, which is called hard-quenching steel; the other is the hardening tendency. Larger steel grades, such as medium carbon steel, low and medium carbon quenched and tempered alloy steel, etc., are called easily quenched steel. Due to the different quenching tendency, the structure of the welding heat affected zone of the two types of steels is also different.

3.Performance

The microstructure distribution of the welding heat-affected zone is non-uniform, and thus the performance is also non-uniform. The welding heat-affected zone is different from the welding seam, and the welding seam can meet the performance requirements by adjusting the chemical composition and matching the appropriate welding process. The performance of the heat-affected zone cannot be adjusted in composition, and it is a non-uniformity problem that occurs under the action of welding thermal cycles. For general welded structures, the hardening, embrittlement, toughening and softening of the heat-affected zone, as well as comprehensive mechanical properties, corrosion resistance and fatigue properties are mainly considered, which are determined according to the specific use requirements of the welded structure.

Hardening

The hardness of the welding heat-affected zone mainly depends on the chemical composition and cooling conditions of the steel to be welded, and its essence is to reflect the performance of different metallographic structures. Because the hardness test is more convenient, the highest hardness HMAX of the heat-affected zone is often used to judge the performance of the heat-affected zone, which can indirectly predict the toughness, brittleness and crack resistance of the heat-affected zone. In the project, the HMAX of the heat-affected zone has been used as an important index for evaluating the weldability. It should be pointed out that even the same structure has different hardness, which is related to the carbon content of the steel and the alloy composition. For example, the hardness of high-carbon martensite can reach 600HV, while the hardness of low-carbon martensite is only 350-390HV.

Embrittlement

The embrittlement of the welded heat-affected zone is often the main cause of cracking and brittle failure of welded joints. Brittleness and toughness measure the ability of a material to resist fracture under impact loads, and are a comprehensive reflection of material strength and plasticity. The higher the brittleness of the material, the lower the toughness of the material and the poorer the ability to resist shock loads. Because the microstructure distribution on the heat-affected zone is not uniform, and even in some parts, the strength is much lower than that of the base metal, that is, serious embrittlement occurs, so that the welding heat-affected zone becomes a weak point of the whole joint. part. Therefore, the embrittlement of the welding heat-affected zone is studied, and the embrittlement phenomenon mainly involves the embrittlement mechanisms such as coarse grain embrittlement, microstructure embrittlement, and thermal strain aging embrittlement, so as to improve its toughness and improve the mechanical properties of the entire joint. 

Toughened

The welding heat-affected zone, especially the fusion zone and the coarse-grained zone, are the weak areas of the whole welded joint. Therefore, measures should be taken to improve the toughness of the welding heat-affected zone. However, the toughness of the welding heat-affected zone cannot be adjusted and improved by adding trace alloying elements like the weld. It is inherent in the material itself, so it can only be improved within a certain range by improving the toughness of the material itself and some technological measures. be improved within. According to the research, the toughening of the welding heat affected zone can adopt the following two measures.

Soften

Metals or alloys strengthened by cold work or heat treatment generally have different degrees of strength loss in the heat-affected zone of welding. Softening or loss of strength in the heat-affected zone. The softening of cold-work-strengthened metals or alloys is caused by recrystallization. The softening or loss of strength in the heat-affected zone has relatively little effect on the mechanical properties of welded joints, but it is not easy to control.

4. Effects on Material Properties

●Strength and Hardness: The HAZ can have varying levels of hardness and strength compared to the base metal. Typically, areas closer to the weld experience higher temperatures and may become harder and more brittle, especially in high-carbon steels.

●Toughness: The toughness of the HAZ can be reduced due to the formation of harder and more brittle microstructures. This is particularly critical in applications where impact resistance is important.

●Corrosion Resistance: In some materials, like certain stainless steels, the heat can cause changes that affect corrosion resistance, such as the precipitation of carbides at grain boundaries.

5. Importance and Control

Structural Integrity: 

Understanding and controlling the welding heat affected zone is crucial for ensuring the structural integrity of welded components, especially in critical applications such as in aerospace, automotive, and structural engineering.

Optimierung Welding Parameters: 

①Heat Input: Lowering the heat input by adjusting the voltage, current, and welding speed can help reduce the size of the HAZ. Lower heat input results in faster cooling rates and less time for adverse microstructural changes.

②Interpass Temperature: Controlling the interpass temperature (the temperature between each weld pass) can influence the microstructure of the HAZ, improving properties like toughness.

Preheating and Post-Weld Heat Treatment (PWHT):

①Preheating: Applying heat to the material prior to welding can reduce the cooling rate, minimizing the risk of forming undesirable microstructures such as martensite in steels. It also helps in reducing residual stresses.

②PWHT: Applying heat after welding can help in tempering hard microstructures formed in the HAZ, thereby improving toughness and reducing residual stresses.

Videodemo