Main causes of thermal cracking and preventive measures

Cracking is one of the most dangerous welding defects that reduce the performance of a welded structure, and any form of cracking in the weld is prohibited. Welding cracks are gaps created by the joint action of welding stresses and other embrittlement-causing factors that break the atomic bond of the material and form a new interface. According to the conditions of welding cracks, it can be divided into hot cracks, cold cracks, reheat cracks, laminar tears and stress corrosion cracks, the following focus on the most common form of cracking - welding hot cracks.

A. What is thermal cracking

Thermal cracking is a crack that occurs during the solidification process at high temperatures and molten pools, and is the most common type of cracking in the welding process, from mild steel, low-alloy high-strength steel, to austenitic stainless steel, aluminium alloys and nickel-based alloys have the potential to produce welded thermal cracking. Thermal cracks are most commonly found in the centre of the weld and are crystalline cracks, the formation of which is mainly related to low melting point eutectics and tensile stresses.
Second, the main factors affecting the thermal cracking

1, the chemical composition of the weld metal

Weld metal in C, S, P, Cu, Zn and other low melting point elements and their compounds are more, will promote the formation of hot cracking. During the solidification process of the weld, these low melting point substances are easy to gather in the centre of the weld and deviate, when the edge of the weld crystallizes and solidifies, the impurities between the grains in the centre of the weld are still in the liquid film state, and cracks are produced under the action of the stress generated by the shrinkage of the weld.

2, weld cross-sectional shape

When the depth of the weld is larger than the width, it will make the solidification particles grow perpendicular to the centre of the weld, which is prone to thermal cracking, especially when high melt depth submerged arc welding and flux cored wire gas welding 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) be between 1 and 1.4 to improve cracking resistance.
In addition, concave welds are more prone to cracking than convex welds, while high voltage and excessive welding speed are the main causes of concave welds and should be avoided.
3.Welding stress

The rigidity of the welded parts is large, and the assembly and welding produce large welding stress, which will prompt the formation of thermal cracks.

Third, the main measures to prevent thermal cracking

1, metallurgical control aspects

(1) control the content of harmful impurities in the weld

Strictly limit the content of C, P, S and other harmful impurities in the base material and welding materials.

(2) improve the crystalline organization of the weld

Carbon steel and low-alloy steel mainly by adding certain alloying elements to the weld, such as Mo, V, Ti, etc., to change the crystalline organization of the form, refining the grain to improve the crack resistance. Stainless steel is made by adding Cr, Mo and other ferrite forming elements to form the right amount of ferrite in the weld to reduce the distribution of P, S and other harmful elements on the grain boundaries, while refining the grain, thus effectively preventing cracking.

(3) Limit the dilution rate

For some easy to transfer some harmful impurities to the weld base material, welding must minimize the dilution rate, such as opening a large bevel, reduce the depth of fusion, stacked weld isolation layer, especially in carbon steel, high carbon steel and dissimilar metals when welding.

2, stress control aspects

(1) choose a reasonable form of joint

(2) determine a reasonable welding sequence

The overall principle is to try to make most of the weld seam in the smaller stiffness conditions welding, to avoid welding the structure to produce a large constraint stress.

(3) Determine reasonable welding parameters

Welding process parameters directly affect the cross-sectional shape of the weld, appropriate reduction in current can reduce the thickness of the weld and 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 preheating can reduce the cooling rate and reduce stress, but also help to reduce thermal cracking.
Source: Chemical Equipment Man
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