Analysis of the Globe Valve Crack in the Main Steam Pipe of the Boiler
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3. Analysis and Discussion
3.1 Heat Treatment During Valve Casting
The heat treatment process for ZG20CrMoV requires two normalizing stages and one tempering stage. The first normalizing stage is intended to homogenize the various components and dissolve the carbides as much as possible, with a temperature range of 940–960°C. The second normalizing stage aims to refine the grains, with a temperature range of 920–930°C. Finally, tempering within the temperature range of 690–710°C results in a relatively stable structure and eliminates internal stress. Valve manufacturers generally use coal-fired furnaces to heat-treat the valve body. Typically, dozens of valve bodies are placed in a single furnace. Since thermocouples are installed in the walls on both sides of the furnace, the measured temperature reflects the furnace temperature rather than the actual temperature of the valve bodies, which is much lower. As a result, the desired heat treatment is not achieved. The primary issues include poor plasticity of the cast structure, non-compliant valve plasticity, abnormal metallographic structure, and extremely low impact toughness. Based on the results of metallographic and hardness analysis, an improper heat treatment process caused the abnormal metallographic structure and high hardness of valves.
3.2 Elements of Valves
The normal content of the element Si in ZG20CrMoV ranges from 0.17% to 0.37%. When the Si content exceeds the upper limit, both strength and toughness decrease simultaneously. During the process of adding elements, impurities like the element P are particularly easy to introduce. In the tempering process of heat treatment, the grain boundary segregation of impurity elements such as P can be easily accelerated, making the material more prone to fracture.
3.3 Preheating of Valves Before Welding
The formula for calculating carbon equivalent, as recommended by the International Welding Association, is shown in Formula (1):
Note: The element symbols in Formula (1) represent the mass fraction of each element.
Formula (1) primarily applies to non-quenched and tempered low-alloy high-strength steel. When the thickness is less than 20 mm and Ceg < 0.40%, the steel exhibits little tendency to harden, possesses good weldability, and does not require preheating. When Ceg = 0.40% to 0.60%, especially when Ceg > 0.5%, the steel is prone to hardening and requires preheating before welding. The method is used to indirectly assess the steel's brittle hardness tendency under air-cooled welding conditions. For ZG20CrMoV steel, based on the average percentage of each element, the carbon equivalent Ceg is calculated to be 0.692. Additionally, due to the significant wall thickness, the steel has a strong tendency to quench, meaning it is susceptible to developing a hardened structure or cold cracks in the parent material after welding and in the heat-affected zone. This cast steel exhibits poor weldability, and improper welding can cause cracks in the heat-affected zone, making preheating before welding essential. However, the actual preheating temperature during the welding process was insufficient; the heating method did not meet process requirements. Relevant units did not provide adequate pre-job training for welding personnel; welding personnel lacked a comprehensive understanding of the welding process, and necessary monitoring measures were absent, all of which can lead to crack formation.
This paper analyzes the cracks in the main steam pipe globe valve of a boiler under construction at a specific enterprise through on-site investigation, chemical element analysis, metallographic structure analysis, hardness testing, mechanical property testing, scanning electron microscope analysis, and other methods, and draws the following conclusions:
Note: The element symbols in Formula (1) represent the mass fraction of each element.
Formula (1) primarily applies to non-quenched and tempered low-alloy high-strength steel. When the thickness is less than 20 mm and Ceg < 0.40%, the steel exhibits little tendency to harden, possesses good weldability, and does not require preheating. When Ceg = 0.40% to 0.60%, especially when Ceg > 0.5%, the steel is prone to hardening and requires preheating before welding. The method is used to indirectly assess the steel's brittle hardness tendency under air-cooled welding conditions. For ZG20CrMoV steel, based on the average percentage of each element, the carbon equivalent Ceg is calculated to be 0.692. Additionally, due to the significant wall thickness, the steel has a strong tendency to quench, meaning it is susceptible to developing a hardened structure or cold cracks in the parent material after welding and in the heat-affected zone. This cast steel exhibits poor weldability, and improper welding can cause cracks in the heat-affected zone, making preheating before welding essential. However, the actual preheating temperature during the welding process was insufficient; the heating method did not meet process requirements. Relevant units did not provide adequate pre-job training for welding personnel; welding personnel lacked a comprehensive understanding of the welding process, and necessary monitoring measures were absent, all of which can lead to crack formation.
This paper analyzes the cracks in the main steam pipe globe valve of a boiler under construction at a specific enterprise through on-site investigation, chemical element analysis, metallographic structure analysis, hardness testing, mechanical property testing, scanning electron microscope analysis, and other methods, and draws the following conclusions:
- The heat treatment of the valve does not meet the standard, leading to the formation of a martensitic structure, which reduces the material's plasticity and increases its brittleness.
- The addition of chemical elements during the valve casting process does not meet the standard, resulting in grain boundary segregation, making the material more prone to fracture.
- The main steam pipe and valve are not sufficiently preheated before welding, which increases the likelihood of cold cracks.
Based on the above conclusions, the following recommendations are made:
- For valve manufacturers: (1) The steel used in casting should be smelted in an open furnace or electric furnace, and the relevant process regulations should be strictly followed during smelting. (2) The chemical composition of the finished steel should be checked for each furnace according to the melting furnace. If the content of individual elements in the casting exceeds standard requirements, a mechanical property test should be conducted. If the mechanical properties are found to be qualified and align with the casting's usage requirements, it can be approved for use by the relevant technical department. (3) The casting should be accompanied by a quality certificate (including chemical composition, mechanical properties, and microstructure) upon leaving the factory.
- For the on-site construction unit: (1) Replace the ZG20CrMoV globe valve with abnormal metallographic structure and hardness at the site, and require the manufacturer to provide the corresponding quality certification documents. If necessary, send personnel to the manufacturer to witness the casting process and heat treatment. (2) Conduct on-site construction in accordance with regulations and specifications, strengthen supervision of welder training, preheating before welding, and operating procedures, and address any issues in a timely manner.
