The main causes of thermal cracks and preventive measures
Cracks are one of the most dangerous welding defects that reduce the serviceability of a welded structure, and any form of cracking is prohibited in a weld. Welding crack refers to the joint action of welding stress and other embrittlement factors, so that the atomic bond of the material is destroyed, the formation of a new interface and the resulting gap. According to the conditions under which welding cracks occur, they can be categorized into hot cracks, cold cracks, reheat cracks, laminar tearing and stress corrosion cracks, and the following focuses on the most common form of cracks - welding hot cracks.
I. What is hot cracking
Thermal cracking is produced in the process of high temperature and solidification of the molten pool cracks, is the most common type of cracking in the welding process, from mild steel, low alloy high-strength steel, to austenitic stainless steel, aluminum alloys and nickel-based alloys, etc. There is a possibility of welding hot cracks. Thermal cracks are most commonly found in the center of the weld, belonging to crystalline cracks, the formation process is mainly related to low melting point eutectic and tensile stress.
II The main factors affecting thermal cracking
1, the chemical composition of the weld metal
Weld metal C, S, P, Cu, Zn and other low melting point elements and their compounds more, will promote the formation of hot cracks. During the weld solidification process, these low melting point substances are easy to gather in the weld central precipitation, when the edge of the weld crystallization solidification, the weld center intergranular impurities are still in the liquid film state, in the weld contraction of the stress generated by the action of cracks.
2, the shape of the weld cross-section
When the depth of the weld is greater than the width, it will make the solidified particles grow perpendicular to the center of the weld, which is prone to thermal cracking, especially the high melting depth of submerged arc welding and flux cored wire gas-shielded welding used for thick plate narrow gap welding is more likely to occur. It is recommended that the weld width to depth ratio (weld width / weld depth) between 1 ~ 1.4 is conducive to improving crack resistance.
In addition, concave welds are more prone to cracking than convex welds, and high voltage and excessive welding speed are the main causes of concave welds, which should be avoided as much as possible.
3, welding stress
Weldment rigidity, assembly and welding produce large welding stress, will promote the formation of thermal cracks.
III、Metallurgical control aspects
(1) Control the content of harmful impurities in the welding seam
Strictly limit the content of C, P, S and other harmful impurities in the base material and welding material.
(2) improve the weld crystalline organization
Carbon steel and low alloy steel mainly by adding certain alloying elements to the weld, such as Mo, V, Ti, etc., in order to change the morphology of the crystalline organization, grain refinement and thus improve crack resistance. Stainless steel through the addition of Cr, Mo and other ferrite-forming elements, so that the weld to form a moderate amount of ferrite, in order to reduce P, S and other harmful elements in the distribution of grain boundaries, while refining the grain, thereby effectively preventing cracks.
(3) limit the dilution rate
For some easy to transfer certain harmful impurities to the weld base material, welding must minimize the dilution rate, such as opening a large bevel, reduce the depth of fusion, surfacing isolation layer, etc., especially in the medium-carbon steel, high-carbon steel, as well as dissimilar metal welding
2、Stress control aspects
(1) Select a reasonable joint form
(2) determine a reasonable welding sequence
The general principle is to try to make most of the weld in the smaller stiffness conditions of welding, to avoid the welding structure to produce a large constraint stress.
(3) to determine the reasonable welding parameters
Welding process parameters directly affect the shape of the cross-section of the weld, properly reduce the current can reduce the thickness of the weld, improve the shape of the weld; the use of low voltage is conducive to the formation of convex weld; avoid high-speed welding can reduce the dilution rate and promote the formation of convex weld; necessary to take the preheating can reduce the cooling rate and reduce the stress, but also help to reduce thermal cracking.
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