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1. Hot crack

It is produced at high temperature during welding, so it is called hot crack, which is characterized by cracking along the prior austenite grain boundaries. Depending on the material of the metal to be welded (low-alloy high-strength steel, stainless steel, cast iron, aluminum alloy and some special metals, etc.), the shape, temperature range and main reasons for hot cracking are also different. At present, thermal cracks are divided into three categories: crystalline cracks, liquefaction cracks and multilateral cracks.

1) Crystallization cracks mainly occur in the welds of carbon steel and low-alloy steel with more impurities (including high S, P, C, Si) and single-phase austenitic steel, nickel-based alloys and some aluminum alloy welds. seam. This kind of crack is in the process of weld crystallization, near the solidus line, due to the shrinkage of the solidified metal, the residual liquid metal is insufficient and cannot be filled in time, and intergranular cracking occurs under the action of stress.

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, 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, add Mo appropriately , V, Ti, Nb and other elements; in terms of technology, it can be prevented by preheating before welding, controlling line energy, and reducing joint restraint.

2) Liquefaction cracks in the near-fracture zone are micro-cracks along austenite grain boundaries, which are small in size and occur in the HAZ near-fracture zone or between layers. It is generally caused by the remelting of the low-melting eutectic composition on the austenite grain boundaries in these areas at high temperatures in the metal near the seam area or the metal between the weld layers during welding, and under the action of tensile stress Austenite intergranular cracks to form liquefaction cracks.

The prevention measures for this kind of cracks are basically the same as those for crystalline cracks. Especially in terms of 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 of the molten pool.

3) Multilateral cracks are caused by the low plasticity at high temperature in the process of forming polygons. This kind of crack is not common, and its prevention measures can add elements such as Mo, W, Ti, etc.

2. Reheat cracks

It usually occurs in certain steel grades and superalloys containing precipitation strengthening elements (including low alloy high strength steels, pearlitic heat-resistant steels, precipitation strengthened superalloys, and some austenitic stainless steels), and no cracks are found after welding. Instead, cracks were generated during the heat treatment. Reheat cracks occur in the overheated coarse-grained parts of the weld heat-affected zone, and their direction is to expand along the austenite coarse-grain boundaries of the fusion line.

From the aspect of material selection to prevent reheat cracks, fine-grained steel can be selected. In terms of technology, choose a smaller line energy, choose a higher preheating temperature and cooperate with subsequent thermal measures, and choose a low-matching welding material to avoid stress concentration.

3. Cold crack

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 restraint stress state of the joint are the three main factors that cause cold cracks during welding of high-strength steels. The martensitic structure formed after welding is under the action of hydrogen element and combined with tensile stress, cold cracks are formed. Its formation 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 be started from three aspects: the chemical composition of the workpiece, the selection of welding materials and technological measures. Materials with lower carbon equivalent should be selected as far as possible; low-hydrogen electrodes should be used as welding consumables, and low-strength welding should be used for welding seams. Austenitic welding materials can also be used for materials with high cold cracking tendency; Heat treatment is a technological measure to prevent cold cracking.

In the welding production, due to the different steel types and welding materials used, the type of structure, the degree of rigidity, and the specific conditions of construction, various forms of cold cracks may appear. However, it is mainly delayed cracking that is often encountered in production.

There are three forms of delayed cracking:

1) Weld toe cracks - This kind of crack originates at the junction of the base metal and the weld, and has obvious stress concentrations. The direction of the crack is often parallel to the weld bead, generally starting from the surface of the weld toe and extending deep into the base metal.

2) Cracks under the weld bead - This type of crack often occurs in the heat affected zone of the weld with a greater tendency to harden and a higher hydrogen content. In general, the crack direction is parallel to the fusion line.

3) Root crack - This kind 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 kind of crack is similar to the weld toe crack and originates from the location where the stress concentration is the largest at the root of the weld. Root cracks may appear in the coarse grained sections of the heat affected zone and may also appear in the weld metal.

The hardening tendency of the steel grade, 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 steels. These three factors are interrelated and mutually reinforcing under certain conditions.

The hardening tendency of steel grades mainly depends on chemical composition, plate thickness, welding process and cooling conditions. During welding, the greater the hardening tendency of the steel, the easier it is to produce cracks. Why does steel cause cracking after hardening? It can be summarized into the following two aspects.

a: The formation of a brittle and hard martensite structure - martensite is a supersaturated solid solution of carbon in ɑ iron, carbon atoms exist in the lattice as interstitial atoms, making the iron atoms deviate from the equilibrium position, and the lattice is larger distortion, resulting in a hardened state of the tissue. Especially under welding conditions, the heating temperature in the near seam area is very high, which causes the austenite grains to grow seriously. 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 low energy when fracture occurs. Therefore, when there is martensite in the welded joint, cracks are easy to form and expand.

b: Hardening creates more lattice defects - Metals can form a lot of lattice defects under thermally unbalanced conditions. These lattice defects are mainly vacancies and dislocations. With the increase of thermal strain in the welding heat-affected zone, under the condition of stress and thermal imbalance, both vacancies and dislocations will move and aggregate, and when their concentration reaches a certain critical value, crack sources will be formed. Under the continued action of stress, it will continue to expand and form macroscopic cracks.

Hydrogen is one of the important factors that cause cold cracks in high-strength steel welding, and has the characteristics of delay. Therefore, delayed cracks caused by hydrogen are called "hydrogen-induced cracks" in many literatures. Experimental studies have shown that the higher the hydrogen content of high-strength steel welded joints, the greater the sensitivity of cracks. When the hydrogen content in the local area reaches a certain critical value, cracks begin to appear. This value is called the critical value of crack generation. Hydrogen content [H]cr.

The [H]cr value of cold cracking of various steels is different, and it is related to the chemical composition, rigidity, preheating temperature, and cooling conditions of the steel.

1: During welding, the moisture in the welding material, the rust at the groove of the weldment, the oil stain, and the environmental humidity are all causes of hydrogen enrichment in the weld. Under normal circumstances, the amount of hydrogen in the base metal and the welding wire is very small, but the moisture in the coating of the welding rod and the moisture in the air cannot be ignored and become the main source of hydrogen increase.

2: The ability of hydrogen to dissolve and diffuse in different metal structures is different, and the solubility of hydrogen in austenite is much larger than that in ferrite. Therefore, the solubility of hydrogen suddenly decreases during the transformation from austenite to ferrite during welding. At the same time, the diffusion rate of hydrogen is just the opposite, suddenly increasing during the transformation 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, hydrogen will escape as much as possible, but due to the rapid cooling, the hydrogen cannot escape in time. Retains in the weld metal to form diffusible hydrogen.

4. Laminar 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 in the base metal that occurs in the base metal due to insufficient plasticity in the thickness direction of the rolled thick steel plate to withstand the welding shrinkage strain in this direction. Generally, during the rolling process of the thick steel plate, some non-metallic inclusions in the steel are rolled into band-shaped inclusions parallel to the rolling direction, and these inclusions cause the anisotropy of the mechanical properties of the steel plate. To prevent laminar tearing, refined steel can be selected in the selection of materials, that is, the steel plate with high z-direction performance can be selected, and the joint design can be improved to avoid unilateral welds, or to open grooves on the side bearing z-direction stress.

Laminar tearing is different from cold cracking, and its occurrence has nothing to do with the strength level of the steel, but is mainly related to the amount and distribution of inclusions in the steel. Generally rolled thick steel plates, such as low-carbon steel, low-alloy high-strength steel, and even aluminum alloy plates, also have laminar tearing. According to the location of the laminar tear, it can be roughly divided into three categories:

The first type is the laminar tear induced by cold cracking in the weld toe or root of the weld heat-affected zone.

The second type is cracking along the inclusion in the welding heat-affected zone, which is the most common laminar tear in engineering.

The third type of cracking along the inclusions in the base metal far from the heat-affected zone generally occurs in the thick plate structure with more MnS flaky inclusions.

The morphology of laminar tear is closely related to the type, shape, distribution and location of inclusions. When the lamellar MnS inclusions are dominant 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 the thick plate structure is welded, especially the T-shaped and fillet joints, under the condition of rigid restraint, when the weld shrinks, a large tensile stress and strain will be generated in the thickness direction of the base metal. When the strain exceeds the plasticity of the base metal. When the deformation capacity is increased, the separation between the inclusion and the metal matrix will cause micro-cracks. Under the continuous action of stress, the crack tip will expand along the plane where the inclusion is located, forming a so-called "platform".

There are many factors that affect laminar tearing, including the following:

1: The type, quantity and distribution of non-metallic inclusions are the essential causes of laminar tearing, which are the root cause of the difference in anisotropy and mechanical properties of steel.

2: Z-direction restraint stress Thick-walled welded structures are subjected to different Z-direction restraint stress, residual stress and load after welding during the welding process, which are the mechanical conditions that cause laminar tearing.

3: Influence of hydrogen It is generally believed that in the vicinity of the heat-affected zone, hydrogen is an important influencing factor for laminar tearing induced by cold cracking.

Due to the great influence and serious harm of laminar tearing, it is necessary to judge the sensitivity of laminar tearing of steel before construction.

Commonly used evaluation methods are Z-direction tensile reduction of section and bolt Z-direction critical stress method. In order to prevent lamellar tearing, the area shrinkage rate should not be less than 15%, and it is generally expected to be 15~20%. When it is 25%, it is considered that the lamellar tearing resistance is excellent.

To prevent laminar tearing, measures should be taken mainly from the following aspects:

First, the method of pre-desulphurization of molten iron is widely used in refining steel, and vacuum degassing can be used to smelt ultra-low sulfur steel with only 0.003~0.005% sulfur, and its section shrinkage rate (Z direction) can reach 23~25%.

Second, the morphology of sulfide inclusions is controlled by changing 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. After the above-mentioned treatment of the steel, the anti-lamellar tearing steel plate with the Z-direction area shrinkage rate of 50-70% can be produced.

Third, from the perspective of preventing laminar tearing, the design and construction process are mainly to avoid Z-direction stress and stress concentration. The specific measures are as follows:

1) One-sided welds should be avoided as much as possible, and double-sided welds should be used to alleviate the stress state in the root zone of the welds, in order to prevent stress concentration.

2) Symmetrical fillet welds with a small amount of welding are used instead of full-penetration welds with a large amount of welding to avoid excessive stress.

3) Bevels should be made on the side bearing the Z-direction stress.

4) For T-joints, a layer of low-strength welding material can be pre-surfacing on the horizontal plate to prevent root cracks and ease welding strain.

5) In order to prevent laminar tearing caused by cold cracking, some measures to prevent cold cracking should be adopted as far as possible, such as reducing the amount of hydrogen, appropriately increasing the preheating, and controlling the temperature between layers.