Welding cracks can be divided into hot cracks, reheat cracks, cold cracks, lamellar tears, etc. The following is a specific explanation of the causes, characteristics and prevention methods of various cracks.
01
Thermal cracking
It is produced at high temperatures during welding, so it is called thermal cracking, which is characterized by cracking along the grain boundaries of the original austenite. Depending on the material of the metal being welded (low-alloy high-strength steel, stainless steel, cast iron, aluminum alloy and some special metals, etc.), the morphology, temperature range and main causes of thermal cracks are also different. At present, thermal cracks are divided into three categories: crystalline cracks, liquefaction cracks and multilateral cracks.
(1) Crystalline cracks
It is mainly produced in the welds of carbon steel and low-alloy steel with more impurities (including S, P, C, Si) and single-phase austenitic steel, nickel-based alloys and some aluminum alloys. This kind of crack is in the process of welding and crystallization, near the solidus, due to the shrinkage of the solidified metal, the residual liquid metal is insufficient, can not be filled in time, and cracks along the grain occur under the action of stress.
The prevention and control measures are: in terms of metallurgical factors, appropriately adjust the metal composition of the weld, shorten the range of the brittle temperature zone, and control the content of harmful impurities such as sulfur, phosphorus and carbon in the weld; Refine the metal grains at one time, that is, add Mo, V, Ti, Nb and other elements appropriately; In terms of process, it can be prevented by preheating before welding, controlling line energy, and reducing the constraint degree of joints.
(2) Liquefaction crack in the proximal seam area
It is a microcrack that cracks along austenite grain boundaries, which is small in size and occurs in the proximal seam zone or interlayer of HAZ. Its cause is generally due to the remelting of the low-melting eutectic components on the austenite grain boundaries in these regions at high temperatures due to the metal in the near-seam zone or the interlaminar metal of the weld during welding, and the formation of liquefaction cracks along the austenite intergranular crack under the action of tensile stress.
The prevention and control measures of this kind of crack are basically the same as that of crystalline cracks. Especially in metallurgy, it is very effective to reduce the content of low-melt eutectic elements such as sulfur, phosphorus, silicon, and boron as much as possible; In terms of process, the line energy can be reduced and the concavity of the melt pool fusion line can be reduced.
(3) Multilateralized cracks
It is caused by the low plasticity at high temperatures in the process of multilateralization. This kind of crack is not common, and its prevention and control measures can add elements such as Mo, W, Ti and so on to improve the multilateralization and intensification energy to the weld.
02
Reheat cracks
Typically occurs in certain steel grades and superalloys that contain precipitation-strengthening elements, including low-alloy high-strength steels, pearlitic heat-resistant steels, precipitation-strengthened superalloys, and certain austenitic stainless steels, where cracks are not found after welding, but rather during heat treatment. Reheat cracks arise at the superheated coarse-grained sites in the weld heat-affected zone, and their strike is austenitic coarse-grained grain boundaries along the fusion line.
In terms of material selection, fine-grained steel can be used to prevent and control reheat cracks. In terms of process, small line energy is selected, higher preheating temperature is selected and later heating measures are selected, and low matching welding materials are selected to avoid stress concentration.
03
Cold cracks
It mainly occurs in the welding heat-affected zone of high and medium carbon steel, low and medium alloy steel, but some metals, such as some ultra-high strength steels, titanium and titanium alloys, sometimes cold cracks also occur in the weld. In general, the hardening tendency of the steel grade, the hydrogen content and distribution of the welded joint, and the confined stress state of the joint are the three main factors for the occurrence of cold cracks during the welding of high-strength steel. The martensitic structure formed after welding is combined with tensile stress under the action of hydrogen element to form cold cracks. His formation is generally transcrystalline or along-crystalline. Cold cracks are generally divided into toe cracks, underpass cracks, and root cracks.
The prevention and control of cold cracks can start from three aspects: the chemical composition of the workpiece, the selection of welding materials and the process measures. Materials with lower carbon equivalent should be selected as much as possible; The welding consumables should be low-hydrogen electrodes, the welds should be matched with low strength, and austenitic welding consumables can also be selected for materials with high cold cracking tendencies; Reasonable control of line energy, preheating and post-heat treatment are the technological measures to prevent and control cold cracking.
In welding production, due to the different steel grades, welding materials, different types of structures, different stiffness, and different specific conditions of construction, various forms of cold cracks may occur. However, it is mainly delayed cracks that are often encountered in production.
There are three types of delay cracks:
(1) Weld toe crack - this crack originates from the junction of the base metal and the weld, and there is an obvious stress concentration position. The direction of the crack is often parallel to the weld bead, generally starting from the surface of the weld toe and propagating deep into the base metal.
(2) Crack under the weld bead - this crack often occurs in the welding heat-affected zone with a large hardening tendency and a high hydrogen content. In general, the crack strike is parallel to the fusion line.
(3) Root crack - this crack is a more common form of delay crack, which mainly occurs when the hydrogen content is high and the preheating temperature is insufficient. This crack is similar to a toe crack and originates in the area where the stress concentration at the root of the weld is greatest. Root cracks may occur in coarse-grained segments in the heat-affected zone, or in weld metals.
The hardening tendency of the steel grade, the hydrogen content and distribution of the welded joint, and the confined stress state of the joint are the three main factors for the occurrence of cold cracks during the welding of high-strength steel. These three factors are interrelated and mutually reinforcing under certain conditions.
The hardening tendency of steel grades is mainly determined by chemical composition, plate thickness, welding process and cooling conditions. When welding, the greater the hardening tendency of the steel grade, the more prone it is to cracks. Why does steel cause cracking when hardened? It can be summarized into the following two aspects:
(1) The formation of brittle and hard martensitic structure - martensite is a supersaturated solid solution of carbon in ɑ iron, and the carbon atoms exist in the crystal lattice as interstitial atoms, which makes the iron atoms deviate from the equilibrium position, and the crystal lattice undergoes large distortion, resulting in the structure in a hardened state. Especially under welding conditions, the heating temperature in the proximal seam zone is very high, which causes the austenite grains to grow severely, and when cooled rapidly, the coarse austenite will transform into coarse martensite. From the strength theory of the metal, it can be known that martensite is a brittle and hard structure, and it will consume low energy when it breaks, so when martensite is present in welded joints, cracks are easy to form and propagate.
(2) Hardening will form more lattice defects - the metal will form a large number of lattice defects under the condition of thermal imbalance. These lattice defects are mainly vacancy and dislocation. With the increase of the thermal strain in the welding heat-affected zone, under the condition of stress and thermal imbalance, vacancy and dislocation will move and aggregate, and when their concentration reaches a certain critical value, a crack source will be formed. As the stress continues, macroscopic cracks will continue to propagate.
Hydrogen is one of the important factors causing cold cracks in welding of high-strength steel, and it has the characteristics of delay, so the delay cracks caused by hydrogen are called "hydrogen-induced cracks" in many literatures. Experimental studies have proved that the higher the hydrogen content of high-strength steel welded joints, the greater the sensitivity to cracks, and cracks begin to appear when the hydrogen content in the local area reaches a certain critical value, which is called the critical hydrogen content [H]cr of cracks.
The [H]cr value of cold cracking is different for different steels, and it is related to the chemical composition, stiffness, preheating temperature, and cooling conditions of the steel.
(1) When welding, the moisture in the welding material, the rust and oil at the groove of the weldment, and the environmental humidity are the reasons for the hydrogen enrichment in the weld. In general, the amount of hydrogen in the base metal and welding wire is very small, but the moisture of the electrode coating and the moisture in the air cannot be ignored, which has become the main source of hydrogen increase.
(2) The dissolution and diffusion ability of hydrogen in different metal structures is different, and the solubility of hydrogen in austenite is much greater than that in ferrite. As a result, the solubility of hydrogen decreases abruptly during the transition from austenite to ferrite during welding. At the same time, the diffusion rate of hydrogen is just the opposite, with a sudden increase in the transition from austenite to ferrite.
During welding, under the action of high temperature, a large amount of hydrogen will be dissolved in the molten pool, and in the subsequent cooling and solidification process, due to the sharp decrease in solubility, hydrogen escapes as much as possible, but because of the rapid cooling, the hydrogen does not have time to escape and remains in the weld metal to form diffuse hydrogen.
04
Laminar tears
It is a kind of internal low-temperature cracking. It is limited to the heat-affected zone of the base metal or weld of the thick plate, and mostly occurs in "L", "T", and "+" joints. It is defined as a stepped cold crack that occurs in the base metal due to the rolling thick steel plate that is not plastic enough to withstand the welding shrinkage strain in that direction. Generally, it is due to the rolling process of thick steel plate, some non-metallic inclusions in the steel are rolled into ribbon inclusions parallel to the rolling direction, and these inclusions cause the conductivity of the steel plate in mechanical properties. To prevent and control lamellar tearing, refined steel can be selected in the material selection, that is, the steel plate with high Z-direction performance can be selected, and the joint design form can also be improved to avoid unilateral welds or grooves on the side that bears Z-direction stress.
Laminar tear is different from cold cracking, and its occurrence has nothing to do with the strength level of the steel grade, but mainly related to the amount of inclusions and distribution morphology in the steel. Laminar tears can occur in generally rolled thick steel plates, such as low-carbon steel, low-alloy high-strength steel, and even aluminum alloy plates. Laminar tears can be broadly divided into three categories according to the location of their occurrence:
The first type is a lamellar tear induced by cold cracks in the welding toe or root in the welding heat-affected zone.
The second type is the cracking along the weld heat affected zone, which is the most common laminar tear in engineering.
The third type is the cracking along the inclusions in the base metal far from the heat-affected zone, which generally occurs in the thick plate structure with more MnS flake inclusions.
The morphology of lamellar tears is closely related to the type, shape, distribution, and location of the inclusions. When the lamellar MnS inclusions are dominant along the rolling direction, the lamellar tears have a clear stepped shape, and when the silicate inclusions are dominant, they are linear, and when the Al inclusions are dominant, they are irregular stepped shapes.
When the strain exceeds the plastic deformation capacity of the base metal, the inclusions and the metal matrix will be separated and microcracked, and the crack tip expands along the plane where the inclusion is located under the continuous action of stress, forming the so-called "platform".
There are many factors that affect lamellar tears, mainly the following:
(1) The type, quantity and distribution of non-metallic inclusions are the essential causes of lamellar tearing, which are the fundamental causes of the anisotropy and mechanical properties of steel.
(2) Z-direction constraint stress
Thick-walled welded structures are subjected to different Z-direction binding stresses, residual stresses and loads after welding during the welding process, which are the mechanical conditions that cause laminar tearing.
(3) The effects of hydrogen
It is generally believed that hydrogen is an important influencing factor in the vicinity of the heat-affected zone, which is induced by cold cracking to lamellar tearing.
Because the impact of lamellar tearing is very large, and the hazard is also very serious, it is necessary to judge the sensitivity of the lamellar tear of steel before construction.
The commonly used evaluation methods include Z-direction tensile section shrinkage and bolt Z-direction critical stress method. In order to prevent lamellar tear, the section shrinkage should not be less than 15%, generally hope = 15~20% is appropriate, when 25%, it is considered that the resistance to lamellar tear is excellent.
To prevent lamellar tears, measures should be taken mainly from the following aspects:
(1) Refined steel
Widely using the method of early desulfurization of molten iron, and using vacuum degassing, can smelt ultra-low sulfur steel with a sulfur content of only 0.003~0.005%, and its section shrinkage (Z direction) can reach 23~25%.
(2) Control the morphology of sulfide inclusions
It is a sulfide that turns MnS into other elements, making it difficult to elongate during hot rolling, thereby reducing anisotropy. The most widely used additives are calcium and rare earth elements. After the above treatment, the steel plate with Z-section shrinkage rate of 50~70% can be manufactured.
(3) From the perspective of preventing lamellar tearing, the design and construction process are mainly to avoid Z-direction stress and stress concentration, and the specific measures are referred to as follows:
1) Single-sided welds should be avoided as much as possible, and double-sided welds should be used instead to ease the stress state in the root area of the weld to prevent stress concentration.
2) Symmetrical fillet welds with less welding amount are used to replace the full penetration welds with large welding quantities, so as to avoid excessive stress.
3) The bevel should be opened on the side that bears the Z-direction stress.
4) For T-joints, a layer of low-strength welding material can be pre-surfacing on the transverse plate to prevent cracks at the welding root and at the same time ease the welding strain.
5) In order to prevent lamellar tearing caused by cold cracking, some measures to prevent cold cracking should be adopted as much as possible, such as reducing the amount of hydrogen, appropriately increasing preheating, and controlling the interlayer temperature.