Introduction To Several Kinds Of Welding Cracks

Welding cracks on its nature to points, can be divided into hot cracks, reheat cracks, cold cracks, laminated tearing and so on. The following are only on the causes of various cracks, characteristics and prevention methods for specific elaboration.
1. Thermal cracks
Is produced at high temperatures during welding, so called thermal cracking, which is characterized by cracking along the original austenite grain boundaries. According to the weld metal material (low-alloy high-strength steel, stainless steel, cast iron, aluminum alloys and some special metals, etc.), the form of thermal cracking, temperature range and the main reason is also different. At present, the thermal cracks are divided into three major categories such as crystallization cracks, liquefaction cracks and multilateral cracks.
(1) crystallization cracks are mainly produced in carbon steel containing more impurities, low alloy steel weld (containing S, P, C, Si is high) and single-phase austenitic steel, nickel-based alloys and some aluminum alloy weld. This crack is in the welding process of crystallization, in the vicinity of the solid phase line, due to the solidification of the metal contraction, the residual liquid metal is insufficient, can not be added in a timely manner, under the action of the stress occurs along the crystal cracking.
Preventive measures are: in metallurgical factors, appropriate adjustment of the weld metal composition, shorten the range of brittle temperature zone to control the weld in sulfur, phosphorus, carbon and other harmful impurities; refine the weld metal grain, that is, the appropriate addition of elements such as Mo, V, Ti, Nb, etc.; in terms of technology, can be preheated before welding, control the line of energy, reduce the joints constraints and other aspects to prevent and control.
(2) Near-seam zone liquefaction crack is a kind of microcrack that cracks along the austenite grain boundary, which is very small in size and occurs in the near-seam zone of the HAZ or the interlayer. Its cause is generally due to welding near seam area metal or weld interlayer metal, at high temperatures so that these regions of the austenite grain boundaries on the low-melting eutectic constituents are re-melted, under the action of tensile stress along the austenite intergranular cracking and the formation of liquefaction cracks.
This kind of crack prevention and control measures and crystallization cracks are basically the same. Especially in metallurgy, as far as possible to reduce the sulfur, phosphorus, silicon, boron and other low-melting eutectic constituent elements of the content is very effective; in the process, you can reduce the line energy, reduce the concavity of the melting line of the melt pool.
(3) Polygonization cracks are caused by very low plasticity at high temperatures during the formation of polygonization. This crack is not common, and its prevention and control measures can be added to the weld to improve the polygonization excitation energy of elements such as Mo, W, Ti, etc..
2. Reheating cracks
Usually occurs in some containing precipitation strengthening elements of steel and high temperature alloys (including low-alloy high-strength steel, pearlitic heat-resistant steel, precipitation strengthened high-temperature alloys, as well as some austenitic stainless steel), they did not find cracks after welding, but in the heat treatment process cracks. Reheat cracks arise in the weld heat affected zone of the superheated coarse crystal parts, the direction of which is along the fusion line of the austenite coarse crystal grain boundary extension.
Prevention and control of reheat cracking from the selection of materials, you can choose fine grain steel. In terms of process, choose a smaller line energy, choose a higher preheating temperature and with the later heat measures, choose a low matching welding material to avoid stress concentration.
3. Cold crack
Mainly occurs in high, medium carbon steel, low, medium alloy steel welding heat affected zone, but some metals, such as some ultra-high strength steel, titanium and titanium alloys, etc. Sometimes cold cracking also occurs in the weld. In general, the hardening tendency of the steel grade, the hydrogen content and distribution of welded joints, as well as the joints are subjected to the state of the confining stress is the three main factors of high-strength steel welding to produce cold cracks. The martensitic organization formed after welding under the action of elemental hydrogen, together with the tensile stress, cold cracks are formed. His formation is generally through the crystal or along the crystal. Cold cracks are generally categorized as toe cracks, under-weld cracks, and root cracks.
Prevention and control of cold cracks can be from the chemical composition of the workpiece, the choice of welding materials and process measures in three aspects. Should try to choose materials with lower carbon equivalent; welding consumables should be selected with low hydrogen electrodes, welds should be matched with low strength, for high cold cracking tendency of the material can also be selected austenitic welding consumables; reasonable control of the line energy, preheating and post-heat treatment is to prevent and control the cold cracking of the process measures.
In welding production due to the use of steel, welding materials, different types of structures, steel, as well as construction of different specific conditions, there may be a variety of forms of cold cracks. However, the main thing that is often encountered in production is delayed cracking.

There are three forms of delayed cracking:
(1) Weld toe cracks - This type of crack originates at the junction of the base metal and the weld, and there is an obvious stress concentration area. The direction of the crack is often parallel to the weld channel, generally starting from the surface of the weld toe to the depth of the parent material.
(2) Cracks under the weld channel - this crack often occurs in the hardening tendency, higher hydrogen content of the weld heat-affected zone. Generally the crack direction is parallel to the fusion line.
(3) root crack - this crack is a more common form of delayed cracking, mainly occurs in the case of higher hydrogen content and insufficient preheating temperature. This type of crack is similar to weld toe cracks and originates in the part of the weld where the stress concentration is greatest at the root of the weld. Root cracks may occur in the coarse grain section of the heat affected zone or in the weld metal.
The hardening tendency of the steel grade, the hydrogen content of the welded joint and its distribution, as well as the state of the joint subjected to the confining stress are the three main factors that produce cold cracks when welding high-strength steel. These three factors are interrelated and mutually reinforcing under certain conditions.
The hardening tendency of the steel grade 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 to produce cracks. Why does steel hardening cause cracking? It can be summarized in the following two aspects.
a: the formation of brittle hard martensite organization - martensite is carbon in ɑ iron supersaturated solid solution, carbon atoms with interstitial atoms exist in the lattice, so that the iron atoms deviate from the equilibrium position, the lattice undergoes a large aberration, resulting in the organization in a hardened state. Especially in welding conditions, near the seam area of the heating temperature is very high, so that the austenite grain growth occurs seriously, when the rapid cooling, coarse austenite will be transformed into coarse martensite. From the theory of strength of metals can be known, martensite is a brittle and hard organization, the occurrence of fracture will consume less energy, therefore, welded joints with the presence of martensite, cracks are easy to form and expand.
b: Hardening will form more lattice defects - A large number of lattice defects are formed when the metal is subjected to thermally unbalanced conditions. These lattice defects are mainly vacancies and dislocations. With the increase of thermal stress in the welded heat affected zone, under the conditions of stress and thermal imbalance, both vacancies and dislocations will move and gather, and when their concentration reaches a certain critical value, a crack source will be formed. Under the continued action of stress, expansion will occur continuously and form macroscopic cracks.
Hydrogen is one of the important factors causing cold cracking of high strength steel welding, and has the characteristic of delay, therefore, in many literatures the delayed cracking caused by hydrogen is called "hydrogen cracking". Experimental studies have proved that the higher the hydrogen content of high-strength steel welded joints, the greater the susceptibility to cracking, when the local hydrogen content reaches a certain critical value, cracks will begin to appear, and this value is called the critical hydrogen content of cracks [H]cr.
Various steel cold cracking [H]cr value is different, it is related to the chemical composition of steel, steel, preheating temperature, and cooling conditions.
1: When welding, moisture in the welding material, rust and oil at the bevel of the weldment, and ambient humidity are all causes of hydrogen enrichment in the weld. Generally the amount of hydrogen in the base material and wire is very small, while the moisture in the flux skin of the electrode and the moisture in the air can not be ignored, and become the main source of hydrogen enrichment.
2: Hydrogen in different metal organizations in the solubility and diffusion capacity is different, hydrogen in the austenite solubility is much larger than the ferrite solubility. Therefore, when welding from austenite to ferrite transition, the solubility of hydrogen occurs a sudden drop. At the same time, the diffusion rate of hydrogen is the opposite, from austenite to ferrite transition suddenly increased.
Welding at high temperatures, there will be a large amount of hydrogen dissolved in the molten pool, in the subsequent cooling and solidification process, due to the sharp decrease in solubility, hydrogen is trying to escape, but because of the cooling is very fast, so that the hydrogen is too late to escape and retained in the weld metal in the formation of diffusion hydrogen.

4. Laminar tearing
Is an internal low-temperature cracking. Limited to the base metal of the thick plate or weld heat-affected zone, mostly occurring in the "L", "T", "+" type joints. Defined as rolled thick steel plate along the thickness of the direction of plasticity is not enough to withstand the direction of the welding contraction strain and occurred in the base metal of a step-like cold crack. Generally due to thick steel plate in the rolling process, some non-metallic inclusions within the steel rolled parallel to the rolling direction of the band inclusions, these inclusions caused by the steel plate in the mechanical properties of the conductivity of each. Prevention and control of laminar tearing in the selection of materials can be selected from refined steel, that is, the selection of z to the high performance of the steel plate, you can also improve the joint design form, to avoid unilateral weld, or to bear z to the side of the stress out of the bevel.
Laminar tearing and cold cracking is different, it produces and steel strength level has nothing to do, mainly with the amount of inclusions in the steel and the distribution of morphology. Generally rolled thick steel plate, such as low carbon steel, low alloy high-strength steel, and even aluminum alloy plate will appear in the laminar tear. According to the location of laminar tearing can be roughly divided into three categories:
The first category is the formation of laminar tearing induced by cold cracks in the weld toe or weld root in the heat affected zone of welding.
The second category is the welding heat affected zone along the inclusions cracking, is the most common engineering laminar tearing.
The third category is away from the heat-affected zone in the base material along the inclusions cracking, generally more in the thick plate structure with more MnS flake inclusions.
Laminar tearing morphology and inclusions of the type, shape, distribution, as well as the location of a close relationship. When the rolling direction along the flaky MnS inclusions are dominant, laminar tearing has a clear step, when the silicate inclusions are dominant in a straight line, such as Al inclusions are dominant in an irregular step.

Thick plate structure welding, especially T-type and angle joints, in the rigid constrained conditions, the weld contraction will be in the direction of the thickness of the base material to produce a lot of tensile stress and strain, when the strain exceeds the plastic deformation capacity of the base metal, the inclusions and the metal matrix will be separated from the metal matrix and microcracking occurs, in the stress continues to play the role of crack tip along the plane of the expansion of the inclusions are located, the formation of the so-called "platform".
There are many factors affecting laminar tearing, mainly in the following aspects:
1: non-metallic inclusions of the type, quantity and distribution of morphology is the essential cause of laminar tearing, it is caused by the anisotropy of steel, mechanical properties of the fundamental differences.
2: Z-direction confinement stress thick-walled welded structures in the welding process to withstand different Z-direction confinement stress, post-weld residual stress and load, they are caused by the mechanical conditions of laminar tearing.
3: The effect of hydrogen is generally believed to be in the vicinity of the heat-affected zone, induced by cold cracking to become laminar tearing, hydrogen is an important influencing factor.
Because the impact of laminar tearing is very large, the harm is also very serious, so it is necessary to make a judgment on the susceptibility of steel to laminar tearing before construction.
Commonly used evaluation methods are Z-direction tensile section shrinkage and pin Z-direction critical stress method. In order to prevent laminar tearing, section shrinkage should not be less than 15%, generally hope that = 15 ~ 20% is appropriate, when 25%, that the anti-laminar tearing excellent.
To prevent laminar tearing, measures should be taken mainly from the following aspects:
First, refining steel widely used iron desulfurization methods, and vacuum degassing, can be smelted out of the sulfur content of only 0.003 ~ 0.005% of ultra-low-sulfur steel, its section shrinkage (Z direction) can reach 23 ~ 25%.
Second, control the form of sulfide inclusions is to turn MnS into other elements of sulfide, so that it is difficult to elongate in hot rolling, thus reducing anisotropy. At present, the widely used added elements are calcium and rare earth elements. With the above treatment, the steel can be manufactured with a Z-direction section shrinkage of 50 to 70% to resist laminated tearing steel plate.
Thirdly, from the point of view of preventing laminar tearing, the design and construction process is mainly to avoid Z-direction stress and stress concentration, and the specific measures are referred to in the following example:
(1) should try to avoid unilateral weld, instead of bilateral weld can ease the stress state of the root zone of the weld, in order to prevent stress concentration.
(2) The use of symmetric fillet welds with less welding instead of welded large amount of full weld through the weld, so as not to produce excessive stress.
(3) The bevel should be made on the side subjected to Z-direction stress.
(4) For T-type joints, a layer of low-strength welding material can be pre-stacked on the cross plate to prevent weld root cracks, and also moderate the welding strain.
(5) In order to prevent laminar 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, increasing the preheat and controlling the interlayer temperature.