1. Thermal cracks
It is produced at high temperatures during welding, so it is called hot crack. It is characterized by cracking along the original austenite grain boundary. Depending on the materials of the metal being welded (low alloy high-strength steel, stainless steel, cast iron, aluminum alloy and some special metals, etc.), the shape, temperature range and main reasons of hot cracks are also different. At present, thermal cracks are divided into three categories: crystal cracks, liquefaction cracks and polygonal cracks.
1) Crystallization cracks mainly occur in welds of carbon steel and low alloy steel that contain more impurities (including S, P, C, and Si), single-phase austenitic steel, nickel-based alloys, and some aluminum alloy welds. Hit the mark. This kind of crack occurs during the welding crystallization process, near the solidus line. Due to the shrinkage of the solidified metal, the residual liquid metal is insufficient and cannot be filled in time. Intergranular cracking occurs under the action of stress.
Preventive and control measures are: in terms of metallurgical factors, appropriately adjust the composition of the weld metal, shorten the range of the brittle temperature zone, control the content of harmful impurities such as sulfur, phosphorus, and carbon in the weld; refine the primary grains of the weld metal, that is, appropriately add Mo , V, Ti, Nb and other elements; in terms of technology, it can be prevented by preheating before welding, controlling line energy, reducing joint restraint, etc.
2) The liquefaction crack in the near-seam zone is a microcrack that cracks along the austenite grain boundary. Its size is very small and occurs in the HAZ near-seam zone or between layers. Its formation is generally due to the fact that the metal in the near-seam area or the metal between the weld seams during welding causes the low-melting eutectic composition on the austenite grain boundaries in these areas to be remelted at high temperatures. Under the action of tensile stress, the low-melting eutectic composition Austenite intergranular cracks form liquefaction cracks.
The prevention and control measures for this kind of crack are basically the same as those for crystal cracks. Especially in metallurgy, it is very effective to reduce the content of low-melting eutectic elements such as sulfur, phosphorus, silicon, and boron as much as possible; in terms of technology, it can reduce the line energy and reduce the concavity of the fusion line in the molten pool.
3) Polygonal cracks are caused by the low plasticity at high temperatures during the formation of polygonal cracks. This kind of crack is not common, and its prevention and control measures can include adding elements such as Mo, W, Ti, etc. to the weld to increase the polylateral excitation energy.
2. Reheat cracks
It usually occurs in certain steel types and high-temperature alloys containing precipitation-strengthening elements (including low-alloy high-strength steels, pearlitic heat-resistant steels, precipitation-strengthened high-temperature alloys, and some austenitic stainless steels). No cracks were found after welding. Instead, cracks occurred during the heat treatment process. Reheat cracks occur in the overheated coarse-grained parts of the welding heat-affected zone, and their direction is to expand along the austenite coarse-grained grain boundaries of the fusion line.
In terms of material selection to prevent reheat cracks, fine-grained steel can be used. In terms of technology, use smaller linear energy, use higher preheating temperature and follow-up heating measures, and use low-matching welding materials to avoid stress concentration.
3. Cold cracks
It mainly occurs in the welding heat affected zone of high and medium carbon steel, low and medium alloy steel, but sometimes cold cracks also occur in welds in some metals, such as some ultra-high strength steels, titanium and titanium alloys. In general, the hardening tendency of the steel type, the hydrogen content and distribution of the welded joint, and the restraint stress state of the joint are the three main factors that cause cold cracks during welding of high-strength steel. Under the action of hydrogen element and tensile stress, the martensite structure formed after welding forms cold cracks. Its formation is generally transgranular or intergranular. Cold cracks are generally divided into weld toe cracks, weld bead cracks, and root cracks.
Preventing and controlling cold cracks can start from three aspects: the chemical composition of the workpiece, the selection of welding materials and process measures. Materials with lower carbon equivalents should be used as much as possible; low-hydrogen electrodes should be used as welding materials, and low-strength matching should be used for welds. Austenitic welding materials can also be used for materials with high cold cracking tendency; linear energy, preheating and post-heating should be reasonably controlled. Heat treatment is a process measure to prevent cold cracking.
In welding production, due to the different steel types and welding materials used, the type and stiffness of the structure, and the specific construction conditions, various forms of cold cracks may occur. However, delayed cracking is mainly encountered in production.
Delayed cracking comes in three forms:
1) Weld toe crack - This type of crack originates from the junction of the base metal and the weld, and has obvious stress concentration areas. The direction of the crack is often parallel to the weld bead, and generally starts from the surface of the weld toe and extends to the depth of the base metal.
2) Cracks under the weld bead - This type of crack often occurs in the welding heat-affected zone where the hardening tendency is greater and the hydrogen content is higher. Generally, the crack direction is parallel to the fusion line.
3) Root crack - This type of crack is a common form of delayed crack, which mainly occurs when the hydrogen content is high and the preheating temperature is insufficient. This type of crack is similar to a weld toe crack and originates from the root of the weld where the stress concentration is greatest. Root cracks may occur in the coarse-grained segment of the heat-affected zone or in the weld metal.
The hardening tendency of the steel type, the hydrogen content and distribution of the welded joint, and the restraint stress state of the joint are the three main factors that cause cold cracks during welding of high-strength steel. These three factors are interrelated and mutually reinforcing under certain conditions.
The hardening tendency of steel types is mainly determined by chemical composition, plate thickness, welding process and cooling conditions. When welding, the greater the hardening tendency of the steel type, the easier it is to produce cracks. Why does steel crack after it is hardened? It can be summarized into the following two aspects.
a: Form a brittle and hard martensite structure - martensite is a supersaturated solid solution of carbon in ɑ iron. Carbon atoms exist as interstitial atoms in the crystal lattice, causing the iron atoms to deviate from the equilibrium position and causing greater changes in the crystal lattice. distortion, causing the tissue to be in a hardened state. Especially under welding conditions, the heating temperature in the near-seam area is very high, causing the austenite grains to grow seriously. When cooled rapidly, the coarse austenite will transform into coarse martensite. It can be known from the strength theory of metals that martensite is a brittle and hard structure, which consumes less energy when fracture occurs. Therefore, when martensite exists in the welded joint, cracks are easy to form and expand.
b: Hardening will form more lattice defects - metals will form a large number of lattice defects under conditions of thermal imbalance. These lattice defects are mainly vacancies and dislocations. As the thermal strain in the welding heat-affected zone increases, vacancies and dislocations will move and gather under conditions of stress and thermal imbalance. When their concentration reaches a certain critical value, crack sources will form. Under the continued action of stress, macroscopic cracks will continue to expand and form.
Hydrogen is one of the important factors causing welding cold cracks in high-strength steel, and it has delayed characteristics. Therefore, delayed cracks caused by hydrogen are called "hydrogen-induced cracking" in many literatures. Experimental studies have proven that the higher the hydrogen content of high-strength steel welded joints, the greater the sensitivity to cracks. When the hydrogen content in a local area reaches a certain critical value, cracks will begin to appear. This value is called the critical value for crack generation. Hydrogen content [H]cr.
The [H]cr value of cold cracking in various steels is different, and it is related to the chemical composition, steel strength, preheating temperature, and cooling conditions of the steel.
1: During welding, moisture in the welding material, rust, oil stains at the groove of the weldment, and environmental humidity are all causes of hydrogen-rich welds. Under normal circumstances, the amount of hydrogen in the base metal and welding wire is very small, but the moisture in the electrode coating and the moisture in the air cannot be ignored, becoming the main source of hydrogenation.
2: The dissolution and diffusion capabilities of hydrogen in different metal structures are different. The solubility of hydrogen in austenite is much greater than that in ferrite. Therefore, during the transition from austenite to ferrite during welding, the solubility of hydrogen suddenly decreases. At the same time, the diffusion rate of hydrogen is just the opposite, suddenly increasing when transforming from austenite to ferrite.
Under the action of high temperature during welding, a large amount of hydrogen will be dissolved in the molten pool. During the subsequent cooling and solidification process, due to the sharp decrease in solubility, the hydrogen will try to escape, but due to the rapid cooling, the hydrogen will not have time to escape. Remains in the weld metal to form diffuse hydrogen.
4. Lamellar tear
It is an internal low-temperature cracking. It is limited to the base metal or weld heat affected zone of thick plates, and mostly occurs in "L", "T", and "+" type joints. It is defined as a step-like cold crack that occurs in the base material because the plasticity of the rolled thick steel plate in the thickness direction is not enough to withstand the welding shrinkage strain in this direction. It is generally due to the fact that during the rolling process of thick steel plates, some non-metallic inclusions in the steel are rolled into strip-shaped inclusions parallel to the rolling direction. These inclusions cause anisotropic conductivity in the mechanical properties of the steel plate. To prevent lamellar tearing, you can use refined steel in material selection, that is, use steel plates with high z-direction performance. You can also improve the joint design to avoid single-sided welds or make grooves on the side that bears z-direction stress.
Lamellar tearing is different from cold cracking. Its occurrence has nothing to do with the strength level of the steel type, but is mainly related to the inclusion amount and distribution shape in the steel. Generally, lamellar tears can occur in rolled thick steel plates, such as low carbon steel, low alloy high-strength steel, and even aluminum alloy plates. According to the location where lamellar tears occur, they can be roughly divided into three categories:
The first type is lamellar tearing induced by cold cracks in the weld toe or weld root in the welding heat affected zone.
The second type is inclusion cracking along the welding heat affected zone, which is the most common lamellar tear in engineering.
The third type of inclusion cracking in the base metal away from the heat affected zone generally occurs in thick plate structures with more MnS flake inclusions.
The form of lamellar tearing is closely related to the type, shape, distribution, and location of inclusions. When the inclusions are dominated by flaky MnS along the rolling direction, the lamellar tear has a clear step shape, when it is dominated by silicate inclusions, it is linear, and when it is dominated by Al inclusions, it is irregular. Stepped.
When welding thick plate structures, especially T-shaped and corner joints, under rigid constraints, the shrinkage of the weld will produce large tensile stress and strain in the thickness direction of the base metal. When the strain exceeds the plasticity of the base metal, When the deformation ability is reached, the inclusions and the metal matrix will separate and micro-cracks will occur. Under the continued action of stress, the crack tips will expand along the plane where the inclusions are located, forming a so-called "platform".
There are many factors that affect lamellar tears, mainly including the following aspects:
1: The type, quantity and distribution form of non-metallic inclusions are the essential cause of lamellar tearing. It is the fundamental reason for the anisotropy and mechanical properties of steel.
2: Z-direction restraint stress. Thick-walled welded structures bear different Z-direction restraint stress, post-weld residual stress and load during the welding process. They are the mechanical conditions that cause lamellar tearing.
3: Influence of hydrogen It is generally believed that hydrogen is an important influencing factor when cold cracking induces lamellar tearing near the heat-affected zone.
Since lamellar tearing has a great impact and the hazards are very serious, it is necessary to judge the sensitivity of steel to lamellar tearing before construction.
Commonly used evaluation methods include Z-direction tensile area shrinkage and pin Z-direction critical stress method. In order to prevent lamellar tearing, the shrinkage of area should not be less than 15%. Generally, it is desired to be 15~20%. When 25%, the lamellar tear resistance is considered excellent.
To prevent lamellar tearing, measures should be taken mainly from the following aspects:
First, for refining steel, the method of early desulfurization of molten iron and vacuum degassing can be widely used to 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%.
Second, controlling the morphology of sulfide inclusions is to turn MnS into sulfides of other elements, making it difficult to elongate during hot rolling, thereby reducing anisotropy. Currently widely used additive elements are calcium and rare earth elements. Steel treated as above can produce lamellar tear-resistant steel plates with a Z-direction area shrinkage of 50 to 70%.
Third, from the perspective of preventing lamellar tearing, the design and construction process are mainly to avoid Z-direction stress and stress concentration. The specific measures are as follows:
1) Unilateral Welds Should Be Avoided As Much As Possible. Using Bilateral Welds Instead Can Alleviate The Stress State In The Root Zone Of The Weld And Prevent Stress Concentration.
2) Use Symmetrical Fillet Welds With A Small Amount Of Welding Instead Of Full-Penetration Welds With A Large Amount Of Welding To Avoid Excessive Stress.
3) A Bevel Should Be Made On The Side That Bears The Z-Direction Stress.
4) For T-Shaped Joints, A Layer Of Low-Strength Welding Material Can Be Pre-Welded On The Horizontal Plate To Prevent Welding Root Cracks And Also 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, Controlling The Interlayer Temperature, Etc.
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