Analysis of four common steel pipe welding cracks

Welding cracks

The most common serious defect in welded parts. Under the combined effect of welding stress and other brittle factors, the bonding force of metal atoms in local areas of the welded joint is destroyed, resulting in a new interface gap. It has the characteristics of sharp notches and large aspect ratios. According to the formation conditions, it can be divided into four categories: hot cracks, cold cracks, reheating cracks, and layered tears.

First, cold cracks

Cold cracks are generated during or after welding at a lower temperature, approximately near the martensite transformation temperature (i.e., Ms point) of steel, or below 300-200°C (or T<0.5Tm, Tm is the melting point temperature expressed in absolute temperature), so it is called cold cracks. Cold cracks can be divided into delayed cracks, quenching cracks, and low-plasticity brittle cracks.

(I) Generation conditions: The welded joint forms a hardened structure. Due to the large hardening tendency of steel, a large amount of brittle, hard, and large martensite is generated during the cooling process, forming a large internal stress. Hardening tendency of joints: The influence of carbon is the key. The more carbon and chromium content, the thicker the plate, the larger the cross-sectional area, and the smaller the heat input, the more severe the hardening.

Steel and welds contain more diffusible hydrogen. Hydrogen atoms accumulate (concentrate) at defects (holes, dislocations) to form hydrogen molecules. The volume of hydrogen molecules is larger than that of hydrogen atoms and cannot continue to diffuse. They continue to accumulate, generating huge hydrogen molecule pressure, even reaching tens of thousands of atmospheres, causing the welded joint to crack. In many cases, hydrogen is the most active factor in inducing cold cracks.

Cracks occur when the welding tensile stress and restraint stress are large (or stress concentration) and exceed the strength limit of the joint.

(II) Causes: It can be divided into two aspects: material selection and the welding process.

1. Material selection

(1) Improper matching of parent material and welding material, resulting in a huge strength difference;

(2) The material contains too many elements such as carbon, chromium, molybdenum, vanadium, and boron, which increases the hardening sensitivity of steel.

2. Welding process

(1) The welding rod is not fully dried, and there is moisture (free water and crystal water) in the coating; there is oil, rust, water, paint, etc. on the groove of the welding material and the base material; the ambient humidity is too high (>90%); the groove is polluted by rain and snow. The above moisture and organic matter decompose under the action of the welding arc to produce H, causing supersaturated hydrogen to dissolve in the weld.

(2) The ambient temperature is too low; the welding speed is too fast; the welding line energy is too low. It will cause the joint area to cool too quickly, causing great internal stress.

(3) Improper welding structure, resulting in great restraint stress.

(4) Cracks have already occurred at the spot welding point, but they were not removed during welding; stress concentration points such as undercuts cause weld toe cracks; stress concentration points such as incomplete penetration cause weld root cracks; stress concentration points such as slag inclusions cause weld cracks.

(III) Prevention methods: It can start from two aspects: material selection and the welding process.

1. Correct material selection

Use basic low-hydrogen welding rods and fluxes to reduce the content of diffused hydrogen in the weld metal; select and match the parent material and welding material; if technical conditions permit, choose materials with good toughness (such as welding materials with a lower strength grade), or implement "soft" cover to reduce surface residual stress; if necessary, conduct chemical analysis, mechanical properties, weldability, and crack sensitivity tests on the parent material and welding material before manufacturing.

2. Welding process

(1) Strictly perform welding operations by the correct process specifications obtained from the test. Mainly include: strictly drying the welding rod according to the specifications; selecting appropriate welding specifications and line energy, reasonable current, voltage, welding speed, interlayer temperature, and correct welding sequence; inspecting and processing spot welding; cleaning double-sided welding, etc.; carefully cleaning the groove and welding wire to remove oil, rust, and moisture.

(3) Select a reasonable welding structure to avoid excessive restraint stress; correct groove form and welding sequence; and reduce the peak value of welding residual stress.

(4) Preheating before welding, slow cooling after welding, controlling the interlayer temperature, and post-weld heat treatment are effective methods to prevent cold cracks in high-strength steel with poor weldability and unavoidable high-constraint structures. Preheating and slow cooling can slow down the cooling rate (extend the dwell time of △t 800-500℃), improve the microstructure of the joint, reduce the hardening tendency, and reduce the microstructure stress; post-weld heat treatment can eliminate welding residual stress and reduce the content of diffused hydrogen in the weld. In most cases, stress relief heat treatment should be performed immediately after welding.

(5) Hammering immediately after welding to disperse residual stress and avoid high-stress areas is one of the effective methods to prevent cold cracks during local repair welding.

(6) At the root of the weld and the weld surface where stress is relatively concentrated (the heat-affected zone is subject to lower constraint stress), using electrodes with lower strength levels often achieves good results under high constraint.

(7) Inert gas-shielded welding can maximize the control of the hydrogen content of the weld and reduce the sensitivity of cold cracks, so TIG and MIG welding should be vigorously promoted.

Second, layered tearing

Layered tearing is a special form of cold cracking. This is mainly due to the presence of layered (along the rolling direction) inclusions (especially sulfides) in the steel plate. Under the tensile stress perpendicular to the rolling direction (plate thickness direction) generated during welding, "step"-type layered cracks parallel to the rolled surface of the parent material are generated in the heat-affected zone or slightly farther away in the steel plate. It occurs in the corner weld joints of T-shaped and K-shaped thick plates.

Improving the quality of steel plates, reducing layered inclusions in steel, and taking measures from the aspects of structural design and welding technology to reduce the welding tensile stress in the plate thickness direction can prevent layered tearing. Before welding thick plates, ultrasonic and groove penetration testing of the plates is performed to check the layered inclusions. If layered inclusions exist, try to avoid them or repair or grind them in advance.

Third, thermal cracks

Thermal cracks are generated at high temperatures, from the solidification temperature range to temperatures above A3, so they are called thermal cracks, also known as high-temperature cracks.

If there are more low-melting eutectic impurity elements (P, S, C, etc.) and more lattice defects in the material, grain boundary segregation is likely to occur during the crystallization of the welding molten pool. The segregated substances are mostly low-melting eutectics (such as FeS-Fe, Fe3P-Fe, NiS-Ni, Ni3P-Ni) and impurities. They exist in the liquid interlayer during the crystallization process, forming a liquid film with very low deformation resistance. The corresponding liquid phase exists for a longer time, and finally crystallizes and solidifies. The strength after solidification is also extremely low. When the welding tensile stress is large enough, the liquid interlayer will be pulled apart, or it will be broken shortly after solidification to form cracks.

In addition, if there are low-melting eutectics and impurities on the grain boundaries of the parent material, these low-melting eutectics will melt into liquid interlayers in the heat-affected zone when the heating temperature exceeds its melting point. When the welding tensile stress is large enough, they will also be pulled apart to form liquid cracks in the heat-affected zone.

Hot cracks are all cracked along the austenite grain boundaries and are serrated, so they are also called intergranular cracks. They often appear in the middle of the weld, especially in the arc pit. Most of them are at the junction of the columnar crystals of the weld, that is, the final position of the weld solidification, which is also the position most likely to cause low-melting eutectic segregation; a few appear in the heat-affected zone. Longitudinal cracks in the weld generally occur in the center of the weld, parallel to the length of the weld; transverse cracks generally occur along the columnar grain boundaries and are connected to the grain boundaries of the parent material, perpendicular to the length of the weld. When the crack penetrates the surface and communicates with the air, the fracture surface is oxidized (such as blue-gray, etc.), and some macro cracks on the surface of the weld are filled with slag.

(I) Causes of occurrence

1. Material selection: excessive sulfur content in the material produces "hot brittleness"; excessive copper content produces "copper brittleness"; excessive phosphorus content produces "cold brittleness".

2. Welding process: nickel-based stainless steel, improper welding sequence or too high interlayer temperature, excessive heat input, too slow cooling speed; improper groove form (narrow and deep weld with weld shape coefficient ψ=b/h≤1), single-layer single-pass welding is prone to weld center segregation cracks; poor arc pit protection, due to segregation, arc pit hot cracks are prone to occur; multiple repairs will cause lattice defects to aggregate and form polygonal hot cracks.

(II) Prevention methods

Since the occurrence of hot cracks is related to stress factors, prevention methods should also start from both material selection and the welding process.

1. Material selection

(1) Limit the content of elements and harmful impurities that are prone to segregation in steel and welding materials, especially the content of S, P, and C, because they not only form low-melting eutectics but also promote segregation. C≤0.10%The sensitivity of hot cracking can be greatly reduced. If necessary, chemical analysis and low-power inspection (such as sulfur printing, etc.) of the material should be carried out.

(2) Adjust the chemical composition of the weld metal, improve the structure, refine the grains, improve the plasticity, change the form and distribution of harmful impurities, and reduce segregation, such as using a dual-phase structure of austenite plus less than 6% ferrite.

(3) Increase the basicity of the welding rod and flux to reduce the content of impurities in the weld and improve the degree of segregation.

2. Welding process

(1) Choose a reasonable groove form, weld forming coefficient ψ=b/h＞1, avoid narrow and deep "pear-shaped" welds, prevent columnar crystals from meeting in the center of the weld, produce central segregation and form a brittle fracture; use multi-layer and multi-pass welding to disrupt segregation aggregation. It is worth noting that excessive welding current will also form a "pear-shaped" weld.

(2) Control welding specifications:

a. Use smaller (appropriate) welding line energy. For austenitic (nickel-based) stainless steel, use smaller welding line energy as much as possible (no preheating, no swing or less swing, fast welding, small current), and strictly control the interlayer temperature, to shorten the residence time of the weld metal in the high-temperature zone; b. Pay attention to the protection when closing the arc, close the arc slowly and fill the arc pit to prevent arc pit segregation and thermal cracks;

c. Avoid multiple repairs as much as possible to prevent lattice defects from agglomerating and generating polygonal thermal cracks;

d. Take measures to minimize joint stress, avoid stress concentration, and reduce the stiffness near the weld. Properly arrange the welding sequence, and try to make most welds welded at a smaller stiffness to allow them to shrink.

Fourth, reheat cracks

Reheat cracks refer to the welded joints of some low-alloy high-strength steels and heat-resistant steels containing alloy elements such as vanadium, chromium, molybdenum, and boron. During the heating process (such as stress relief annealing, multi-layer multi-pass welding, and high-temperature work, etc.), they occur in the coarse-grained area of the heat-affected zone, and cracks along the original austenite grain boundaries. They are also called stress relief annealing cracks (SR cracks).

Reheat cracks originate from the coarse-grained area of the heat-affected zone of the weld, and have the characteristics of grain boundary fracture, and most cracks occur in the area where stress is concentrated.

Preventive measures

1. When selecting materials, attention should be paid to carbide-forming elements that can cause precipitation, especially the V content. When high-V steel must be used, special attention should be paid during welding and heat treatment.

2. Avoiding reheat-sensitive areas during heat treatment can reduce the possibility of reheat cracks. If necessary, heat treatment process tests should be performed before heat treatment.

3. Minimize residual stress and stress concentration, reduce excess height, eliminate undercut, incomplete penetration, and other defects, and grind the excess height and weld toe to smooth if necessary; increase the preheating temperature, slow cooling after welding, and reduce residual stress.

4. Appropriate line energy to prevent overheating of the heat-affected zone and coarse grains.

5. On the premise of meeting the design requirements, select a welding rod with a lower strength grade to release part of the stress eliminated by the heat treatment process, so that the stress is relaxed in the weld, which is good for reducing reheat cracks.