Welding cracks are divided into hot cracks, reheat cracks, cold cracks, layered tears and so on. The following is only a specific elaboration of the causes, characteristics and prevention and control methods of various cracks.
1. Thermal cracks
It is produced at high temperatures during welding, so it is called thermal cracking, which is characterized by cracking along the proto-austenite grain boundary. Depending on the material of the welded metal (low alloy high strength steel, stainless steel, cast iron, aluminum alloy and some special metals, etc.), the form, temperature range and main cause of thermal cracks are also different. At present, thermal cracks are divided into three categories: crystalline cracks, liquefied cracks and polygonal cracks.
(1) Crystalline cracks are mainly produced in carbon steel with more impurities, low-alloy steel welds (containing S, P, C, Si cheat high) and single-phase austenitic steels, nickel-based alloys and some aluminum alloy welds. This crack is in the process of welding crystallization, near the solid phase line, due to the shrinkage of the solid metal, the residual liquid metal is insufficient, can not be filled in time, and cracking along the crystal occurs under stress.
The prevention and control measures are: in terms of metallurgical factors, appropriately adjust the composition of the welded metal, shorten the range of the brittle temperature zone to control the content of harmful impurities such as sulfur, phosphorus and carbon in the welding; refine the primary grain of the welded metal, that is, appropriately add Mo, V, Ti, Nb and other elements; in terms of process, it can be prevented and controlled by preheating before welding, controlling line energy, and reducing the binding degree of the joint.
(2) Liquefaction crack in the near-suture region is a micro-crack that cracks along the austenite grain boundary, its size is very small, and occurs in the HAZ near-fracture region or between layers. Its cause is generally due to the metal in the near seam area or the interlayer metal of the weld during welding, and the low-melt eutectic components on the austenite grain boundaries in these regions are re-melted at high temperatures, and the liquefaction crack is formed along the austenite intercrystalline crack under the action of tensile stress.
The prevention and control measures of this kind of crack are basically consistent with the crystalline crack. Especially in metallurgy, it is very effective to reduce the content of low-fused eutectic components such as sulfur, phosphorus, silicon and boron as much as possible; in terms of process, the line energy can be reduced and the concaveness of the molten pool fusion line can be reduced.
(3) Multilateralization cracks are caused by the low plasticity at high temperatures in the process of forming multilateralization. This crack is not common, and its prevention measures can be added to the weld to improve the multilateralization of the radicalization energy of elements such as Mo, W, Ti and so on.
2. Reheat the crack
It usually occurs in some steel grades and superalloys containing precipitated reinforcing elements (including low-alloy high-strength steels, pearlite heat-resistant steels, precipitation-reinforced superalloys, and some austenitic stainless steels), and they do not find cracks after welding, but cracks are produced during heat treatment. Reheat cracks occur in the overheated coarse crystal site of the weld heat affected region, which is oriented towards the expansion of the austenite coarse grain boundary along the fusion line.
Prevention and control of reheat cracks From the aspect of material selection, fine grain steel can be selected. In terms of process, a smaller line energy is selected, a higher preheating temperature is selected with subsequent thermal measures, and a low matching welding material is selected to avoid stress concentration.
3. Cold cracks
It mainly occurs in the welding heat affected zone of high, 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 welds. In general, the hardening tendency of steel grades, the hydrogen content and distribution of welded joints, and the constrained stress state of the joints are the three main factors that produce cold cracks when welding high-strength steels. The martensitic structure formed after welding is under the action of hydrogen and combined with tensile stress, a cold crack is formed. Its formation is generally through or along the crystal. Cold cracks are generally divided into weld toe cracks, weld 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 process measures. Austenitic welding consumables should be selected as much as possible; welding consumables should be selected with low hydrogen electrodes, weld seams should be matched with low strength, and austenitic welding consumables should also be used for materials with high cold cracking tendencies; reasonable control line energy, preheating and post-heat treatment are the process measures to prevent and control cold cracking.
In welding production, due to the different steel grades and welding materials used, the type of structure, the degree of steel, and the specific conditions of construction, various forms of cold cracks may occur. However, the main thing often encountered in production is delayed cracking.
Delayed cracks come in three forms:
(1) Weld toe crack - This crack originates from the junction of the base metal and the weld, and there is a clear stress concentration. The direction of the crack is often parallel to the weld bead, and generally begins to expand from the surface of the weld toe to the depth of the base metal.
(2) Crack under the weld bead - This crack often occurs in the welding heat affected zone with a large tendency to hardening and a high hydrogen content. In general, the crack direction is parallel to the fusion line.
(3) Root crack - This crack is a relatively common form of delayed crack, which mainly occurs in the case of high hydrogen content and insufficient preheating temperature. This crack is similar to a weld toe crack and originates where the stress concentration at the root of the weld is greatest. Root cracks may occur in the coarse crystal segment of the heat-affected zone or in weld metal.
The tendency of steel to harden, the hydrogen content and distribution of welded joints, and the binding stress state of the joints are the three main factors that produce cold cracks when welding high-strength steels. These three factors are interrelated and mutually reinforcing under certain conditions.
The hardening tendency of steel grades is mainly determined by the chemical composition, plate thickness, welding process and cooling conditions. When welding, the greater the hardening tendency of the steel grade, the more likely it is to produce cracks. Why does steel crack after hardening? It can be summarized into the following two aspects.
a: The formation of brittle and hard martensitic tissue - martensitic is a supersaturated solid solution of carbon in ɑ iron, and carbon atoms exist in the lattice as gap atoms, so that the iron atoms deviate from the equilibrium position, and the lattice undergoes greater distortion, resulting in the tissue being in a hardened state. Especially under welding conditions, the heating temperature in the near-seam region is very high, so that the austenite grains grow seriously, and when it is cooled rapidly, the coarse austenite will transform into a coarse martensitic. From the strength theory of metals, it can be known that martensitic is a brittle and hard tissue that consumes lower energy when fracture occurs, so cracks are easy to form and expand when the welded joint has martensitic presence.
b: 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 vacancies and dislocations. With the increase of the thermal response variable in the welding heat affected zone, under the condition of stress and thermal imbalance, the vacancies and dislocations will move and aggregate, and when their concentration reaches a certain critical value, a crack source will be formed. Under the continued action of stress, it will continue to expand and form a macroscopic crack.
Hydrogen is one of the important factors causing cold cracks in high-strength steel welding, and has the characteristics of delay, so in many literature, hydrogen-induced delayed cracks are called "hydrogen-induced cracks". Experimental studies have proved that the higher the hydrogen content of the high-strength steel welded joint, the greater the sensitivity of the crack, and when the hydrogen content in the local area reaches a certain critical value, cracks begin to appear, which is called the critical hydrogen content [H]cr that produces cracks.
The [H]cr value of cold cracking is different for various steels, which is related to the chemical composition of the steel, its rigidity, the preheating temperature, and the cooling conditions.
1: When welding, the moisture in the welding material, the rust at the slope of the weldment, oil pollution, and environmental humidity are all the reasons for the hydrogen-richness 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 skin and the moisture in the air cannot be ignored, becoming the main source of hydrogenation.
2: The solubility and diffusion ability of hydrogen in different metal tissues is different, and the solubility of hydrogen in austenite is much greater than that in ferrite. Therefore, when the transition from austenite to ferrite during welding, the solubility of hydrogen occurs a sudden decrease. At the same time, hydrogen diffuses at the opposite rate, suddenly increasing when it transitions from austenite to ferrite.
When welding at high temperatures, 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 will try to escape, but because the cooling is very fast, the hydrogen will not be able to escape and remain in the weld metal to form diffuse hydrogen.
4. Layered tearing
It is an internal low temperature cracking. Limited to the heat affected zone of base metal or weld welds of thick plates, it mostly occurs in "L", "T", "+" type joints. It is defined as a stepped cold crack in the base metal that occurs on the base metal because the plasticity of the rolled thick steel plate is not sufficient to withstand the welding shrinkage strain in that direction. Generally because the thick steel plate in the rolling process, some non-metallic inclusions in the steel into parallel to the rolling direction of the strip inclusions, these inclusions caused the steel plate in the mechanical properties of the wizarding. To prevent lamellar tearing, refined steel can be selected in the selection of materials, that is, steel plates with high z-direction performance can be selected, and the design form of the joint can also be improved to avoid unilateral welds or to open grooves on the side that is subject to z-direction stress.
Layered tearing is different from cold cracking, and its production is independent of the strength level of the steel grade, mainly related to the amount of inclusion and distribution in the steel. Generally rolled thick steel plates, such as low-carbon steel, low-alloy high-strength steel, and even aluminum alloy plates will also appear layered tears. According to the location of the laminar tear, it can be roughly divided into three categories:
The first type is a layered tear induced by the weld toe or weld root in the welding heat affected zone.
The second type is the cracking along the weld heat affected zone, which is the most common layered tear in engineering.
The third type of base material far from the heat affected zone is cracked along the edge, which generally appears in the thick plate structure with more MnS.
The morphology of the layered tear is closely related to the type, shape, distribution, and location of the inclusion. When sheet-like MnS inclusions are dominant in the rolling direction, the layered tears have a clear stepped shape, and when silicate inclusions are dominant, they are straight, and when Al inclusions are dominant, they are irregularly stepped.
When the thick plate structure is welded, especially the T-type and angle joint, under the condition of rigid constraint, the weld shrinkage will produce a large tensile stress and strain in the direction of the thickness of the base metal, when the strain exceeds the plastic deformation capacity of the base metal, the separation between the inclusion and the metal matrix will occur and micro-cracks will occur, and the crack tip will expand along the plane where the inclusion is located under the continued action of the stress, forming the so-called "platform".
There are many factors that affect layered tearing, mainly in the following aspects:
1: The type, quantity and distribution of non-metallic inclusions are the essential causes of layered tearing, which is the root cause of the anisotropy and mechanical properties of steel.
2: Z-direction binding stress Thick-wall 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 layered tearing.
3: The effect of hydrogen It is generally believed that near the thermal impact zone, it is induced by cold cracking into a layered tear, and hydrogen is an important influencing factor.
Because the impact of lamellar tears is very large and the harm is also very serious, it is necessary to make judgments on the sensitivity of lamellar tears of steel before construction.
Commonly used evaluation methods are Z-direction tensile cross-section shrinkage and latch Z-direction critical stress method. In order to prevent layered tearing, the cross-sectional shrinkage rate should not be less than 15%, generally hope = 15 ~ 20% is appropriate, when 25%, it is considered that the resistance to layered tears is excellent.
Measures to prevent lamellar tears should be taken primarily from the following aspects:
First, refined steel widely adopts the method of early desulfurization of molten iron, and vacuum degassing, which can smelt ultra-low sulfur steel with only 0.003 to 0.005% sulfur, and its cross-sectional shrinkage rate (Z direction) can reach 23 to 25%.
Second, the control of the form of sulfide inclusion is to turn MnS into sulfides of other elements, making it difficult to elongate during hot rolling, thereby reducing anisotropy. The most widely used added elements are calcium and rare earth elements. The steel after the above treatment can produce a layered tear-resistant steel plate with a Z-direction cross-section shrinkage rate of 50 to 70%.
Third, from the perspective of preventing layered tearing, the design and construction process is mainly to avoid Z-direction stress and stress concentration, and the specific measures are referred to as follows:
(1) Unilateral welds should be avoided as much as possible, and the use of double-sided welds can alleviate the stress state of the root area of the weld to prevent stress concentration.
(2) The symmetrical fillet weld with a small amount of welding is used instead of the full weld penetration weld with a large welding volume, so as not to generate excessive stress.
(3) The groove should be opened on the side subjected to Z-direction stress.
(4) For the T-joint, a layer of low-strength welding material can be pre-walled on the horizontal plate to prevent cracks in the welding root, and at the same time, the welding strain can be eased.
(5) In order to prevent layered tearing caused by cold cracking, some measures to prevent cold cracking should be used as much as possible, such as reducing the amount of hydrogen, appropriately increasing preheating, and controlling the temperature between layers.