Welding of Low Carbon Quenched and Tempered Steel

The tensile strength of low carbon quenched and tempered steel is generally 600~1300MPa, which belongs to heat treatment strengthened steel. This type of steel not only has high strength, but also has good plasticity and toughness.

Types, Composition and Properties of Low Carbon Quenched and Tempered Steel

Generally speaking, the effect of alloying elements on the plasticity and toughness of steel is opposite to its strengthening effect, that is, the greater the strengthening effect, the more obvious the decrease in plasticity and toughness. Under normalizing conditions, the toughness will drop sharply when the strength is further increased by adding Hajin elements. In order to further increase the strength of the steel, quenching and tempering treatment is required.

In order to ensure good comprehensive performance and weldability, low-carbon quenched and tempered steel requires that the mass fraction of carbon in the steel is not more than 0.22%. In addition, adding some alloying elements is to improve the hardenability of steel and the tempering stability of martensite. Due to the low carbon content of this type of steel, low-carbon martensite will be obtained after quenching, and "self-tempering" will occur, with low brittleness and good weldability.

Low carbon quenched and tempered steel has high strength and good plasticity, toughness and wear resistance, especially low crack sensitivity. According to different conditions of use, low carbon quenched and tempered steel can be divided into the following categories:

● High-strength structural steel (600~800MPa) is mainly used for engineering welded structures, and the welds and welded areas are mostly subjected to tensile loads.

● High-strength wear-resistant steel (≥ 1000MPa) is mainly used for high-strength wear-resistant engineering structures that require impact wear.

● High-strength and high-toughness steel (≥700MP) This type of steel requires high strength and high toughness at the same time, and is mainly used for high-strength and high-toughness welded structures.

Weldability Analysis of Low Carbon Quenched and Tempered Steel

Low-carbon quenched and tempered steel is mainly used as high-strength welded structural steel, so the carbon content is limited to a low level, and the weldability requirements are considered in the design of the alloy composition. The mass fraction of low-carbon quenched and tempered molybdenum carbon does not exceed 0.18%, and the welding performance is far better than that of medium-carbon quenched and tempered steel. Low-carbon martensite is formed in the welding heat-affected zone of this type of steel, and the Si-site transformation temperature Ms is relatively high. The formed martensite has the characteristics of "self-tempering", which makes the tendency of welding cold cracks to be lower than that of medium-carbon quenched and tempered steel. Steel is small.

1. Weld Strength and Toughness Matching

Ensuring the strength performance of the joint area is the first problem to be considered in the analysis of low carbon quenched and tempered key weldability. Yield strength is the main basis for determining allowable stress in engineering design, while tensile strength is an important indicator of strength reserve. The ratio of yield strength to tensile strength is called the yield ratio, which is an important parameter for selecting materials, and has different requirements for welded structures for different purposes. A low yield ratio is beneficial to processing and forming, and a high yield ratio enables the strength potential of steel to be brought into full play.

Weld strength matching coefficient is one of the parameters to characterize the mechanical heterogeneity of joints, which can be divided into super strong matching, equal strong matching and low strength matching.

Weld strength matching coefficient is one of the parameters to characterize the mechanical heterogeneity of joints, which can be divided into super strong matching, equal strong matching and low strength matching.

For the selection of weld metal strength, traditionally, most of them advocate that the weld strength is equal to or greater than that of the base metal, that is, the so-called equal-strength matching or super-strength matching, and it is considered safer to have a higher weld strength. However, the higher the strength of the weld metal, the lower the toughness is often, even lower than the toughness level of the base metal. Even for low-strength steel, when welding methods with large heat input (such as submerged arc welding, electroslag welding, etc.) are used, the toughness of the weld metal is often lower than that of the base metal, and the strength and toughness of the weld metal and the base metal must be maintained. Matching is sometimes difficult. With the rapid development of high-strength steel and ultra-high-strength steel, the matching of weld strength and toughness with the base metal is becoming more and more prominent.

2. Cold Crack

The alloying principle of low-carbon quenched and tempered steel is to add a variety of alloying elements to improve hardenability on the basis of low carbon, so as to ensure the obtaining of low-carbon "self-tempering" martensite with high strength and good toughness and part of the lower carbon. Mixed structure of bainite. Due to the high hardenability of this type of steel, there is a tendency to produce cold cracks and decrease toughness in the coarse-grained zone of the welding heat-affected zone. However, the hardened structure in the heat-affected zone is low-carbon martensite with a high MS point, which has certain toughness and low crack sensitivity. For low-alloy steels with a carbon content of less than 0.12%, the maximum hardness in the heat-affected zone can be 400HV.

3. Thermal Crack and Eliminate Stress Crack

Low-carbon quenched and tempered steel has lower C content, higher Mn content, and stricter control of SP, so the tendency of hot cracking is smaller. However, there is a certain sensitivity to hot cracks for high-Ni and low-Mn steel types, which mainly occur in the overheated zone of the heat-affected zone (called liquefaction cracks). The key to avoid thermal cracks or liquefaction cracks is to control the content of C and S to ensure a high ratio of Mn and S, especially when the Ni content is high, the requirements are more stringent.

4. Performance Changes in the Heat Affected Zone

The heat-affected zone of low-carbon quenched and tempered steel is the part where the structure and properties are not uniform, and the prominent feature is the simultaneous existence of embrittlement (ie, the decrease in toughness) and softening. Even though the low-carbon quenched and tempered steel base metal itself has high toughness, microcracks are easy to generate and develop in the brittle parts of the heat-affected zone during the structural operation, and there is a possibility of brittle fracture in the joint area. Affected by the welding heat cycle, the heat effect of low carbon quenched and tempered steel may cause the loss of strengthening effect (called softening or loss of strength). The greater the strengthening degree of the base metal before welding, the greater the softening degree of the heat affected zone after welding.