Which materials have a tendency to develop delayed cracks?

Thread Content

Standard specifications often contain provisions stating that materials prone to delayed cracking (such as 12Cr2Mo1R) should undergo non-destructive testing at least 24 hours after welding is completed (GB/T150.4 P335). So the question arises: what is delayed cracking? Which materials have a tendency to develop delayed cracks? What are the non-destructive testing methods performed 24 hours after welding is completed? Definition of delayed crack: Delayed cracks belong to cold cracks. Cold cracks refer to cracks that occur after welding when the material cools below the martensite transformation temperature, M3 point; they generally appear after a period of time following welding (several hours, days, or even longer), which is why they are also known as delayed cracks. Formation of texture: The delay in crack formation is related to the hardening tendency of the steel, the hydrogen content in the weld joint and its distribution, as well as the stress state of the weld joint. Delayed cracks propagate either along grain boundaries or through the grains, which is related to the microstructure of the welded joint, the stress state, and the hydrogen content. Generally, delayed cracks are crack types that propagate through the grain boundaries (i.e., cracks that go from one grain boundary to another). They arise either because the low-plasticity structure in the heat-affected zone of the weld cannot withstand the stresses resulting from volume changes and structural transformations during cooling, or because hydrogen atoms combine to form molecules that enter the tiny pores in the metal; these molecules generate high internal stresses together with the welding-induced restraint stresses, leading to crack formation in the weld (this is known as hydrogen-induced cracking). Cause: The hardened microstructure is the determining factor for cold cracking. When the welding cooling rate is high, bainite and a large amount of martensite structure appear in the heat-affected zone. Especially when coarse twin martensite is formed, its notch sensitivity increases, leading to severe embrittlement and the formation of cold cracks under welding stresses. Furthermore, the enrichment of diffused hydrogen in the hardened and embrittled zone causes microcracks. The triaxial stress region formed at the crack tip further induces hydrogen diffusion and enrichment, causing the microcracks to propagate into macrocracks; this is what is known as delayed cracking. Classification of delayed cracks: Delayed cracks are divided into weld toe cracks, cracks under the weld bead, and root cracks. (1) Weld toe cracks occur at the junction between the base metal and the weld, showing a distinct tendency for stress concentration; the direction of these cracks is generally parallel to the weld bead. (2) Cracks beneath the weld bead occur in the welding heat-affected zone, which has a high tendency to harden and a high hydrogen content; the direction of these cracks is generally parallel to the weld seam. (3) Root cracks may occur in the coarse-grained area of the heat-affected zone, or they may appear in the weld metal, depending on the mechanical properties of the base material and the weld, as well as the shape of the weld root. The content of diffused hydrogen in the deposited metal is directly related to the delay in crack formation, and the hydrogen content in the welded joint plays a key role in this regard; temperature is an important factor affecting the diffusion and escape of hydrogen from the welded joint. During the welding process, some of the hydrogen absorbed by the liquid metal can escape during the crystallization of the molten pool. However, due to the fast crystallization rate of the molten pool, a considerable amount of hydrogen does not have time to escape and remains in the solid weld metal. This has a significant impact on the properties of the weld, generating substantial internal stresses that cause the weld to crack. In fact, the hydrogen that actually causes delayed cracking is known as diffusible hydrogen, so named because it can diffuse freely within solid metal. Diffused hydrogen is an important factor causing defects such as hydrogen embrittlement, hydrogen cracking, and hydrogen-induced cracks, namely delayed cracks. Sources of hydrogen in welds: The hydrogen that enters the molten pool during welding primarily comes from moisture in the welding materials, hydrogen-containing substances, water vapor in the air surrounding the arc, as well as impurities such as water, rust, and oil on the surfaces of the welding wire and the base metal groove. Measures to prevent delayed cracking: 1. From a process perspective, create a low-hydrogen welding environment by using low-hydrogen welding materials and processes; for example, CO2 gas shielded welding can produce low-hydrogen welds. Try to choose low-strength welding materials in order to reduce the stress in the welded joint; when the strength of the welded joint is lower than that of the base material, tensile stress can be released through plastic deformation. Adopt a reasonable welding sequence: weld the joints with greater contraction first, and those with smaller contraction later, allowing the welds to contract freely in order to reduce welding restraint stress. (1) Establishing reasonable welding specifications and heat input, as well as controlling the cooling time, can improve the microstructure and properties of the weld and the heat-affected zone. (2) Preheating before welding, controlling the interlayer temperature, and reducing the cooling rate facilitate hydrogen diffusion, which can effectively prevent the formation of delayed cracks. (3) Low-temperature dehydrogenation treatment accelerates hydrogen diffusion. Delayed cracks generally occur a few hours or days after welding. If heat treatment (dehydrogenation treatment) is applied to the weld in a timely manner before cracks form, by heating the weld at 300°C to accelerate the diffusion and escape of hydrogen from the remaining solid weld metal, it is possible to reduce welding residual stresses to some extent, improve the microstructure of the welded joint, and effectively prevent delayed cracks. (4) Multiple layers and passes of welding are used for thick steel plates. The preceding layer preheats the subsequent layer, while the subsequent layer performs heat treatment on the preceding layer, reducing residual stresses and improving the microstructure of the welded joint. (5) Strike the weld surface to generate compressive stress, improve the weld stress field, and reduce the likelihood of delayed crack formation. (6) For materials prone to delayed cracking, inspection should be carried out 24 hours after welding to prevent the failure to detect such cracks. 2. In the field of metallurgy: (1) Control the source of hydrogen. Try to choose low-hydrogen or ultra-low-hydrogen welding materials. These materials should be dried before use, and impurities such as water, rust, and oil from the welding wire and the weld groove must be removed. (2) Use basic low-hydrogen electrodes; adding fluorite to the flux can reduce the hydrogen content in the weld. (3) Reasonably select the alloy composition of the weld metal, and appropriately add certain alloying elements to improve the toughness of the weld metal as well as its crack resistance. (4) For steels that are difficult to weld and prone to cracking, austenitic stainless steel electrodes should be used; since austenite has a high solubility for hydrogen, cracks will not occur in the welds if there is no hydrogen present. 3. Reduce welding stress: (1) Design reasonable welding joints; prefer butt joints over lap joints. The shape of the welds should be properly designed, and they should be placed as far away as possible from areas where the cross-section changes. The weld height should not be too large, and the transition at the weld joint should be smooth in order to reduce weld restraint and avoid excessive stress concentration. (2) Avoid dense welds and adopt a reasonable welding sequence to reduce weld stiffness. Which materials have a tendency to develop delayed cracks? Materials prone to delayed cracking mainly refer to various low-alloy high-strength steels. As the steel strength grade increases and the amount of alloying elements rises, its tendency to harden upon quenching gradually increases, as does the likelihood of delayed cracking. It is generally believed that materials with high sensitivity to cold cracking are mainly low-alloy high-strength steels with a yield strength of 450 Mpa or higher, or a tensile strength of 540 Mpa or higher; grades include 15MnVNR, 18MnMoNbR, 13MnNiMoNbR, 07MnCrMoVR, 07MnNiCrMoVDR, etc ； As well as some low-alloy heat-resistant steels, with grades such as 1.0Cr0.5Mo (15CrMo), 1.25Cr0.5Mo (14Cr1Mo), 1Cr-0.5Mo-V, 2.25Cr-1Mo (12Cr2Mo1), etc ； As well as some martensitic stainless steels, such as 1Cr13, 2Cr13, 4Cr13, 2Cr12WMoV, 2Cr12MoV, 2Cr12Ni3MoV, etc. How to conduct non-destructive testing? Materials prone to delayed cracking (such as 12Cr2Mo1R) should undergo non-destructive testing at least 24 hours after welding is completed; this non-destructive testing primarily refers to surface inspection. This article is from the Design Institute Website www.shejiyuan.com (There are 10 design projects for fixed-tube-sheet heat exchangers currently under quotation on the website; designers are welcome to participate)