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I. Reasons for excessive hardness values during the welding of carbon steel and low-alloy steel pipes. There are many reasons for excessive weld hardness values during the welding of carbon steel and low-alloy steel pipes, as listed below. 1. Improper temperature control: It is necessary to control the welding temperature during welding; either too high or too low temperatures can result in hardness levels that are above the acceptable range. If the welding temperature is too high, it will cause the microstructure in the weld to become coarser and increase its hardness ； If it is too low, it will cause cold cracking in the weld. For carbon steel and low-alloy steel, the typical welding temperature ranges from 600°C to 700°C. 2. Material mismatch: Material mismatch is also one of the reasons for excessive weld hardness values. When selecting welding materials, the compositional differences between the steel pipe and the welding material should be taken into account; otherwise, it will result in excessively high weld hardness. For low-alloy steels, appropriate alloy welding consumables containing elements such as molybdenum, chromium, and titanium can be used to improve the toughness of the weld and prevent excessive hardness values. 3. Excessively fast welding speed: A too high welding speed can also result in weld hardness levels that are above the acceptable range. Under rapid cooling, the tissue becomes harder, resulting in an increased hardness value of the weld. The welding speed should be controlled to be neither too fast nor too slow, in order to avoid excessively high hardness values. II. Solutions to Excessive Hardness Values in the Welding of Carbon Steel and Low-Alloy Steel Pipes To avoid excessive hardness values during the welding of carbon steel and low-alloy steel pipes, the following solutions can be adopted. 1. Optimize welding parameters: During the welding process, it is necessary to control the welding speed as well as the values of current and voltage, in order to regulate the welding temperature and thus prevent the hardness level from exceeding the acceptable limits. For low-alloy steel pipes, it is also possible to preheat the weld before welding in order to improve its toughness. 2. Control of welding temperature and preheating: During welding, it is necessary to control the welding temperature properly and carry out appropriate preheating, so as to achieve a finer and more uniform weld structure with lower hardness values. 3. Select appropriate welding materials. It is very important to choose the right welding materials; this means that the welding materials should match well with the steel pipe material, and their composition should be properly designed to ensure the toughness of the welds. 4. Control the cooling rate: To prevent excessive hardness values resulting from rapid cooling during welding, it is necessary to control the cooling rate after welding. Slow cooling or uniform cooling can be employed here to ensure a finer weld structure and lower hardness values. 5. Perform subsequent heat treatment: If the hardness level remains above the specified limit after welding, subsequent heat treatment can be considered. This method has high requirements, and it must be improved based on actual conditions. III. Summary: The reasons for excessive hardness values during the welding of carbon steel and low-alloy steel pipes, as well as the corresponding solutions, mainly lie in aspects such as control of welding parameters, preheating at welding temperature, matching of welding materials, control of cooling rate, and subsequent heat treatment. Welders should keep the above reasons and solutions in mind to ensure the quality and performance of the welds.