1. Hot cracks
Hot cracks are generated at high temperatures in the welding process of valves, which have characteristics of cracking along the original austenite grain boundary. Different welding metals such as low-alloy high-impact steel, stainless steel, cast iron aluminum alloys and special metals can make forms, temperatures and the main causes of hot cracks different. Hot cracks are divided into crystal cracks, liquefied cracks and multilateral cracks at present.
Crystal cracks
They are mainly generated in the welding seam of carbon steel containing more impurities, low-alloy steel containing much S, P, C, and Si, single-phase austenitic steel, nickel-based alloys and certain aluminum alloys. This kind of crack is in the crystallization process of the welding seam, near the solidus line. Intergranular cracking occurs due to the shrinkage of the solidified metal and insufficient residual liquid metal not being filled in time under the action of stress. These cracks can be eliminated by appropriately adjusting the metal composition of the welding seam, shortening the range of the brittle temperature zone, and controlling the content of harmful impurities such as sulfur, phosphorus, and carbon in the welding seam; refine the primary grain of the weld metal, that is, appropriately add Mo, V, Ti, Nb and other elements; crystal cracks can also be prevented by preheating before welding, controlling heat input, and reducing joint restraint.
Liquefied cracks near the welding seam zone
A liquefied crack is a kind of micro-crack along the austenite grain boundary. Its size is very small and it occurs near the welding seam zone or between layers of the HAZ. It is generally caused by the low-melting eutectic composition on the austenite grain boundary in areas near the welding seam or between layers in the welding process being remelted at high temperatures, and the liquefied crack happens along austenite crystals after cracking. The measure to prevent this kind of crack is the same as crystal cracks. It is very effective to reduce the content of low-melting eutectic elements such as sulfur, phosphorus, silicon and boron as much as possible; reduce the heat input and the concavity of the fusion line of the molten pool.
Multilateral cracks
They are caused by low plasticity at high temperatures in the process of forming multilateralization. This kind of crack is not common to be seen, which can be prevented by adding elements such as Mo, W and Ti to increase the activation energy of multilateralization into the welding seam.
2. Reheat cracks
They usually occur in certain steel and high-temperature alloys containing precipitation strengthening elements, including low-alloy high-impact steel, pearlitic heat-resistant steel, precipitation strengthened high-temperature alloys, and certain austenitic stainless steel. Cracks are not found for these steels after welding, but they are found in the heat treatment process. The reheat cracks are generated in the overheated coarse-grained parts of the welding heat-affected zone, which expand along the austenite coarse grain boundaries of the fusion line. Fine-grained steel can be used to prevent reheat cracks. Select a smaller heat input, a higher preheating temperature and cooperate with subsequent thermal measures, and choose low-matching welding materials to avoid stress concentration.
3. Cold cracks
They mainly occur in the welding heat-affected zone of high and medium carbon steel, low and medium alloy steel, but some metals, such as certain ultra-high strength steel, titanium and titanium alloys, sometimes cold cracks also occur in the welding seam. Under normal circumstances, the hardening tendency of steel, the hydrogen content and distribution of welded joints, and the restrained stress state of joints are the three main factors that cause cold cracks in the welding of high-impact steel. The martensite structure formed after welding forms cold cracks under the action of hydrogen element and combined with tensile stress. The forming of the cold crack is generally transgranular or intergranular. Cold cracks are generally divided into weld toe cracks, under-bead 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 technological measures. Materials with lower carbon equivalent should be used as much as possible; welding rods containing low-hydrogen should be selected, and the welding seams should be matched with low strength. For materials with great cold cracking tendency, austenitic welding materials can be used; reasonable control of heat input, preheating and post heat treatment is a technological measure to prevent cold cracks. In the welding process, cold cracks with various forms may appear due to different steel grades, welding materials, structure types, steel degrees, and specific construction conditions.
The hardening tendency of steel, the hydrogen content and distribution of welded joints, and the restrained stress state of joints are the three main factors that cause cold cracks in the welding of high-impact steel. These three factors are interrelated and mutually promoted under certain conditions. The hardening tendency of steel grades is mainly determined by chemical composition, plate thickness, welding processes and cooling conditions. When welding, the greater the hardening tendency of the steel grade is, the more likely it is to produce cracks. Why does cracking happen for the steel after hardening? Brittle and hard martensite structure and more lattice defects formed are the two reasons. Martensite is a supersaturated solid solution of carbon in ɑ iron. The carbon atoms exist in the crystal lattice as interstitial atoms, which causes the iron atoms to deviate from the equilibrium position, and the crystal lattice to have a distortion, making the tissue to be in a hardened state. The heating temperature in the near-joint zone is very high especially under welding conditions, causing serious growth of austenite grains. When rapidly cooled, the coarse austenite will transform into coarse martensite. From the strength theory of metals, it can be known that martensite is a brittle and hard structure, which consumes lower energy when fracture occurs. Therefore, when there is martensite in the welded joint, cracks are easy to form and expand. Metals will form a large number of lattices under thermally unbalanced conditions. These lattices are mainly vacancies and dislocations.
With the increase of the thermal strain in the welding heat-affected zone, vacancies and dislocations will move and aggregate under the conditions of stress and thermal imbalance. When their concentration reaches a certain critical value, crack sources will form. Under the continuous action of stress, the crack source will continue to expand and form macroscopic cracks. Hydrogen is one of the important factors that cause cold cracks in the welding of high-impact steel and has delayed characteristics. Therefore, delayed cracks caused by hydrogen are called "hydrogen-induced cracks" in a lot of literature. Experimental studies have proved that the higher the hydrogen content of high-impact steel welded joints is, the greater the sensitivity of cracks becomes. When the hydrogen content of a partial area reaches a certain critical value, cracks begin to appear. This value is called the critical hydrogen content value for cracking.
The [H]cr value for cold cracking of various steels is different, and it is related to the chemical composition, degree, preheating temperature, and cooling conditions of the steel. When welding is performed, the moisture in the welding material, rust and oil stain at the groove of the weldment, and the environmental humidity are all the reasons for the hydrogen enrichment in the welding seam. Under normal circumstances, the amount of hydrogen in the base metal and the welding wire is very small, but the moisture in the electrode coating and the air cannot be ignored, becoming the main source of increasing hydrogen. The dissolution and diffusion capabilities of hydrogen in different metal structures are different, and the solubility of hydrogen in austenite is much greater than that in ferrite. Therefore, when welding is conducted, the solubility of hydrogen suddenly drops, transforming from austenite to ferrite. Meanwhile, the diffusion rate of hydrogen is just the opposite, and it suddenly increases when it transforms from austenite to ferrite. Under the action of high temperature in the welding process, a large amount of hydrogen will be dissolved in the molten pool. In the subsequent cooling and solidification process, a lot of hydrogen tries to escape as much as possible due to the sharp decrease in solubility. Hydrogen doesn't have time to escape and remains in the weld metal to form diffusible hydrogen because of the rapid cooling.
Hot cracks are generated at high temperatures in the welding process of valves, which have characteristics of cracking along the original austenite grain boundary. Different welding metals such as low-alloy high-impact steel, stainless steel, cast iron aluminum alloys and special metals can make forms, temperatures and the main causes of hot cracks different. Hot cracks are divided into crystal cracks, liquefied cracks and multilateral cracks at present.
Crystal cracks
They are mainly generated in the welding seam of carbon steel containing more impurities, low-alloy steel containing much S, P, C, and Si, single-phase austenitic steel, nickel-based alloys and certain aluminum alloys. This kind of crack is in the crystallization process of the welding seam, near the solidus line. Intergranular cracking occurs due to the shrinkage of the solidified metal and insufficient residual liquid metal not being filled in time under the action of stress. These cracks can be eliminated by appropriately adjusting the metal composition of the welding seam, shortening the range of the brittle temperature zone, and controlling the content of harmful impurities such as sulfur, phosphorus, and carbon in the welding seam; refine the primary grain of the weld metal, that is, appropriately add Mo, V, Ti, Nb and other elements; crystal cracks can also be prevented by preheating before welding, controlling heat input, and reducing joint restraint.
Liquefied cracks near the welding seam zone
A liquefied crack is a kind of micro-crack along the austenite grain boundary. Its size is very small and it occurs near the welding seam zone or between layers of the HAZ. It is generally caused by the low-melting eutectic composition on the austenite grain boundary in areas near the welding seam or between layers in the welding process being remelted at high temperatures, and the liquefied crack happens along austenite crystals after cracking. The measure to prevent this kind of crack is the same as crystal cracks. It is very effective to reduce the content of low-melting eutectic elements such as sulfur, phosphorus, silicon and boron as much as possible; reduce the heat input and the concavity of the fusion line of the molten pool.
Multilateral cracks
They are caused by low plasticity at high temperatures in the process of forming multilateralization. This kind of crack is not common to be seen, which can be prevented by adding elements such as Mo, W and Ti to increase the activation energy of multilateralization into the welding seam.
2. Reheat cracks
They usually occur in certain steel and high-temperature alloys containing precipitation strengthening elements, including low-alloy high-impact steel, pearlitic heat-resistant steel, precipitation strengthened high-temperature alloys, and certain austenitic stainless steel. Cracks are not found for these steels after welding, but they are found in the heat treatment process. The reheat cracks are generated in the overheated coarse-grained parts of the welding heat-affected zone, which expand along the austenite coarse grain boundaries of the fusion line. Fine-grained steel can be used to prevent reheat cracks. Select a smaller heat input, a higher preheating temperature and cooperate with subsequent thermal measures, and choose low-matching welding materials to avoid stress concentration.
3. Cold cracks
They mainly occur in the welding heat-affected zone of high and medium carbon steel, low and medium alloy steel, but some metals, such as certain ultra-high strength steel, titanium and titanium alloys, sometimes cold cracks also occur in the welding seam. Under normal circumstances, the hardening tendency of steel, the hydrogen content and distribution of welded joints, and the restrained stress state of joints are the three main factors that cause cold cracks in the welding of high-impact steel. The martensite structure formed after welding forms cold cracks under the action of hydrogen element and combined with tensile stress. The forming of the cold crack is generally transgranular or intergranular. Cold cracks are generally divided into weld toe cracks, under-bead 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 technological measures. Materials with lower carbon equivalent should be used as much as possible; welding rods containing low-hydrogen should be selected, and the welding seams should be matched with low strength. For materials with great cold cracking tendency, austenitic welding materials can be used; reasonable control of heat input, preheating and post heat treatment is a technological measure to prevent cold cracks. In the welding process, cold cracks with various forms may appear due to different steel grades, welding materials, structure types, steel degrees, and specific construction conditions.
The hardening tendency of steel, the hydrogen content and distribution of welded joints, and the restrained stress state of joints are the three main factors that cause cold cracks in the welding of high-impact steel. These three factors are interrelated and mutually promoted under certain conditions. The hardening tendency of steel grades is mainly determined by chemical composition, plate thickness, welding processes and cooling conditions. When welding, the greater the hardening tendency of the steel grade is, the more likely it is to produce cracks. Why does cracking happen for the steel after hardening? Brittle and hard martensite structure and more lattice defects formed are the two reasons. Martensite is a supersaturated solid solution of carbon in ɑ iron. The carbon atoms exist in the crystal lattice as interstitial atoms, which causes the iron atoms to deviate from the equilibrium position, and the crystal lattice to have a distortion, making the tissue to be in a hardened state. The heating temperature in the near-joint zone is very high especially under welding conditions, causing serious growth of austenite grains. When rapidly cooled, the coarse austenite will transform into coarse martensite. From the strength theory of metals, it can be known that martensite is a brittle and hard structure, which consumes lower energy when fracture occurs. Therefore, when there is martensite in the welded joint, cracks are easy to form and expand. Metals will form a large number of lattices under thermally unbalanced conditions. These lattices are mainly vacancies and dislocations.
With the increase of the thermal strain in the welding heat-affected zone, vacancies and dislocations will move and aggregate under the conditions of stress and thermal imbalance. When their concentration reaches a certain critical value, crack sources will form. Under the continuous action of stress, the crack source will continue to expand and form macroscopic cracks. Hydrogen is one of the important factors that cause cold cracks in the welding of high-impact steel and has delayed characteristics. Therefore, delayed cracks caused by hydrogen are called "hydrogen-induced cracks" in a lot of literature. Experimental studies have proved that the higher the hydrogen content of high-impact steel welded joints is, the greater the sensitivity of cracks becomes. When the hydrogen content of a partial area reaches a certain critical value, cracks begin to appear. This value is called the critical hydrogen content value for cracking.
The [H]cr value for cold cracking of various steels is different, and it is related to the chemical composition, degree, preheating temperature, and cooling conditions of the steel. When welding is performed, the moisture in the welding material, rust and oil stain at the groove of the weldment, and the environmental humidity are all the reasons for the hydrogen enrichment in the welding seam. Under normal circumstances, the amount of hydrogen in the base metal and the welding wire is very small, but the moisture in the electrode coating and the air cannot be ignored, becoming the main source of increasing hydrogen. The dissolution and diffusion capabilities of hydrogen in different metal structures are different, and the solubility of hydrogen in austenite is much greater than that in ferrite. Therefore, when welding is conducted, the solubility of hydrogen suddenly drops, transforming from austenite to ferrite. Meanwhile, the diffusion rate of hydrogen is just the opposite, and it suddenly increases when it transforms from austenite to ferrite. Under the action of high temperature in the welding process, a large amount of hydrogen will be dissolved in the molten pool. In the subsequent cooling and solidification process, a lot of hydrogen tries to escape as much as possible due to the sharp decrease in solubility. Hydrogen doesn't have time to escape and remains in the weld metal to form diffusible hydrogen because of the rapid cooling.
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