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A Brief Discussion on the Causes of Weld Cracks in Valves and Their Prevention Methods 1. Introduction Valves are used in various industries, including petroleum, chemicals, photovoltaics, hydrogen energy, offshore engineering, wind power projects, shipbuilding, aerospace, and the automotive industry. Common welding defects in valve welding include porosity, slag inclusions, lack of fusion, incomplete penetration, and cracks; among these, cracks have the greatest impact on the structure. Welding cracks can easily lead to structural fractures, leakage of media, and even explosions. They also affect the fatigue life of structures; for instance, the formation of fatigue cracks in jacket platforms can have serious consequences. To ensure the stability and reliability of welding, welding procedure qualifications are conducted prior to welding in accordance with relevant standards. Nevertheless, it is still impossible to completely prevent the occurrence of welding cracks. The characteristics of valve weld joints include limited space for welding, uneven wall thickness, high restraint at certain locations, numerous T-joints, large wall thickness, and a variety of materials used. The main welding applications include welding of valve bodies, welding of jacketed valves, surfacing of valve sealing surfaces, wear-resistant surfacing, corrosion-resistant surfacing, etc. Valve welding methods include TIG welding, shielded metal arc welding, submerged arc welding, flame spray welding, plasma welding, laser welding, and supersonic spray welding. Welding quality control is particularly important. Based on the causes of their formation, welding cracks can be broadly classified into four categories: cold cracks, hot cracks, lamellar tearing, and reheat cracks. Although there are numerous studies and methods regarding the causes and mechanisms of crack formation, various factors such as the conditions at the actual construction and installation sites, the methods of installation, the skill level of the personnel, and the quality of the materials can all affect the welding results and lead to crack formation. Only by combining theory with practice can we better identify the causes, resolve problems at their root, and formulate relevant solutions. 2. Crack cause analysis: For valve welding, among the above four types of cracks, cold cracks and layered tearing are generally more common during welding in factories or on-site. It is particularly important to know how to analyze and identify cracks; identifying the specific cause is necessary before taking any action. However, there are many interfering factors during the identification process, and it is crucial to eliminate these factors. Tracking the welding process is an important prerequisite for analyzing the causes of cracks. Through the analysis of crack types and formation causes. When cracks occur, it is necessary to examine the situation from five aspects: people, machinery, materials, methods, and environment. The analysts need to have extensive experience; they must first understand the mechanisms behind the formation of different types of cracks, as well as the welding processes in use on site and the factors that can lead to crack formation. The analysis methods for the causes of welding cracks in different structures vary. In relatively mature processes and structures, cracks may suddenly appear during welding. First, it’s necessary to determine what type of crack it is by examining its morphology—whether it formed after welding or during the welding process. Generally, the reasons lie in human factors and materials: the personnel may not be familiar with the welding process, or they need to oversee the entire welding procedure to ensure that the correct welding techniques are being used, that preheating and post-heating are carried out as required, that the paint is properly cleaned, and that any oil residues are removed before welding. Material analysis must be comprehensive; both the base material and the welding materials need to be inspected. Regarding welding materials, it is necessary to check whether the wrong welding materials were used, whether they were baked and stored as required, and whether welders obtained and used them in accordance with regulations. Analyze whether the properties of the base material meet the requirements, and whether the wrong base material has been used, etc. For crack types that cannot be determined, it is also possible to analyze the microstructure through microscopic metallographic examination and determine the chemical composition at the crack openings via mass spectrometry; the causes can then be analyzed by combining these findings with the actual conditions. For new or complex structures, this analysis becomes more complicated. There are also factors caused by human error, such as welding out of sequence, failure to preheat as required, inadequate post-weld heat insulation, and non-compliance with technical requirements during the welding process ； The impact of machinery is generally relatively small; the cracks caused by equipment failures are minor, and welders can usually detect such failures in a timely manner ； Materials must also be comprehensively tested; for example, in the case of T-shaped thick-wall joints, it’s necessary to determine whether the material exhibits Z-direction properties and whether its strength meets the requirements. It is necessary to consider whether the welding procedures and methods are appropriate. Prior to any welding, all required preparatory processes are carried out as stipulated; under normal circumstances, this prevents any problems. However, there are exceptional cases where factors such as the structural restraint and stress levels are not taken into account during the welding process, which can adversely affect the welds. For example, when welding the valve jacket welding frame, factors such as the complex sequence of structural assembly must also be taken into account when determining whether the weld design is appropriate; issues like weld toe cracks at intersecting welds can occur, and in some cases, no weld holes are provided at the intersections of structural elements. Environmental influences, improper wind and rain protection measures, unsuitable welding conditions, failure to clean the weld area prior to welding, etc. When analyzing the degree of internal stress restraint, software can be used to simulate the formation of cracks in welded joints. The causes of welding cracks during the valve welding process also depend on the raw materials used—for instance, whether the material is coiled pipe or seamless steel pipe, forged steel or cast steel, or composite pipes. Additionally, the type of crack matters: whether it’s lamellar tearing or cold cracking. Lamellar tearing is generally attributed to metallurgical factors in the base material or to the properties of the plate in the Z-direction. It’s also important to determine whether the crack occurs at locations where multiple weld seams intersect. The issue with composite material surfacing: cracks occur upon being reheated. Welding valve bodies generally starts with the base material and welding materials, followed by an investigation of the welding process. For jacket welding, the analysis generally starts with the distribution of welds, followed by considerations related to the material, and finally by the welding methods and procedures used by the personnel. Regarding hot cracks and reheat cracks, appropriate measures can generally be taken during the manufacturing process to prevent them; they rarely occur in on-site welding. In such cases, it’s necessary to consider whether there are any issues with the base material used in practice, whether the sulfur and phosphorus content exceeds the allowable limits, and whether there are excessive impurities present. If the paint is not removed thoroughly before welding, the chemical components in the paint can affect the metallurgical properties of the weld, leading to cracking. At the same time, it is necessary to consider whether thermal cracks are caused by excessive heat input and segregation of the material’s chemical composition. 3. Preventive measures for welding cracks: When welding valves, for different types of defects, corresponding measures can be taken by combining their formation mechanisms with the actual welding process. These specific measures can be considered from the following aspects. 3.1 Regarding materials, a specific analysis is required for different structures and materials. When developing welding processes, it is necessary to take into account the thickness of the materials; steel with low sulfur and phosphorus content should be chosen, as such materials have a lower tendency to develop welding cracks. By calculating the carbon equivalent, appropriate steel grades and preheating temperatures are selected. Preheating can reduce the occurrence of cold cracks, but it is not applicable to all welds. When the restraint is excessive, using preheating to prevent cold cracks may not necessarily be effective. Since hydrogen is also a cause of cold cracks, low-hydrogen welding materials are selected; the welding materials are baked as required, and on-site insulation is maintained as specified. 3.2 In terms of design, when the welding of jacketed valves is affected by restraint, a weld with lower stress should be considered in the weld design; whenever possible, butt welding should be used instead of lap welding. The groove angle of the weld joint also needs to be properly designed, to avoid situations where the groove is either too large or too small. Weld seam arrangement, welding sequence, transition between intersecting weld seams, rationality of weld seam distribution, and avoidance of stress concentration at welding joints. When designing welded structures with complex geometries, relevant software can be used to simulate their stress conditions, conduct finite element analysis, simulate temperature fields and stress concentrations, and assess the stress on the welds. For structures at crossover welds, a weld access hole can be used in the design to facilitate the transition of the weld joint. 3.3 Various factors must be considered in terms of the process to evaluate welding quality. Depending on the various welding conditions, appropriate welding materials should be selected, taking into account the material’s overall mechanical properties as well as the balance between strength and toughness. Appropriate preheating and postheating temperatures and methods should be chosen based on the thickness and type of material. The welding process must be carried out strictly in accordance with the specified welding procedures, timely post-weld heat treatment must be applied, and control over the welding energy input is also essential. The tendency for cold cracking can also be tested by welding tests on inclined Y-groove joints; preheating and post-weld heat treatment can also help remove hydrogen, thereby preventing hydrogen-induced cracks. At the same time, it is also necessary to take into account the actual structure of the product and its degree of restraint, and appropriately increase the strength of the welding material. Choose an appropriate welding sequence; especially for complex structures where stress is often concentrated, adopt segmented welding or symmetric welding, starting with the welds that are under the greatest constraint. Currently, many welding-related codes have eliminated the requirement for preheating of ordinary carbon steel. When the thickness exceeds the specified limit, it is mandatory to use multi-layer and multi-pass welding. The principle behind this is to apply post-heat treatment to the previous weld bead in the form of a hot weld bead; this is also known as a temper bead. When the thickness exceeds the specified limit, if heat treatment or post-heating is not employed to prevent crack formation, the fracture resistance of the material and welded joints must be verified through a CTOD (Crack Tip Opening Displacement) test. 3.4 Implementation of the welding quality control system: control of the welding process, management of the use of welding materials, control of preheating temperatures during welding and post-weld heating temperatures, to ensure that all requirements are met and to prevent welding cracks caused by human factors. During the assembly inspection, it is checked whether the base material is correct, whether the groove angle meets the requirements, and whether there are any factors such as paint or water in the weld area that could cause cracks. Improving welding quality involves personnel process supervision and control, the welding environment to ensure appropriate welding conditions, as well as welding protection measures such as protection against wind and rain, rust removal, and prevention of drafts. Control of heat input during welding, storage and insulation measures for welding materials, and the execution of welding procedures, etc. For materials with a high tendency to cold cracking, has heat treatment or post-heating been carried out immediately as required? In manufacturing or processing enterprises, steel is generally a purchased material; therefore, controlling it at the source is crucial. The selection of materials is of great importance. Material inspection upon arrival is related to the content and distribution of inclusions within the material, as well as the rolling direction of the steel plates. When cutting, welds should be avoided. In the design and layout of raw materials, Z-direction stress concentration must be prevented. The inspection of forgings and castings is carried out using appropriate non-destructive testing methods such as UT/RT/PT/MT, in order to prevent cracks caused by defects in the base material.