The first option is to shut off the water supply and replace the pipe.

If the water supply can be shut off, simply replacing the damaged pipe is a more complete solution. However, a careful calculation is required: the potential loss of service, the cost of purchasing the new pipe, the labor costs for installation, and the freight costs of transporting the pipe to the site all need to be considered.

The second option is to use a special repair pipe clamp

commonly known as a half-joint. The biggest advantage of this method is that it can be repaired under water and pressure without shutting off the water supply, and it is also simple to install. The cost depends on the leak location—whether it is a straight pipe, a tee, an elbow, or a flange. Different locations require a corresponding half-joint, and the price varies. Add in the installation and freight costs, and you have the total cost.

Let's focus on why welding is not recommended.

The risks of welding ductile iron pipes are primarily related to the material properties and welding process.

From the material itself, ductile iron contains a high content of carbon and silicon. During welding, the pipe base metal melts into the weld, resulting in a high carbon equivalent in the weld metal—a negative development. Furthermore, the area near the fusion line experiences rapid heating and cooling during welding, which makes the structure in this area hard and brittle.

Furthermore, the excellent strength and toughness of ductile iron are due to its spherical graphite. However, during welding, these graphite globules near the fusion line in the heat-affected zone (HAZ) may partially dissolve or deform, negating these inherent advantages. Even more problematic, the matrix near the fusion line also develops the brittle structure described above. Furthermore, the high carbon equivalent, combined with impurities such as sulfur and phosphorus, makes the weld particularly susceptible to hot cracking near the end of solidification. The martensite in the weld and HAZ is highly susceptible to hydrogen embrittlement. Hydrogen introduced during welding accumulates in tiny defects during cooling, and combined with residual tensile stress, cracking may occur over time, often referred to as cold cracking. Furthermore, the significant compositional disparity between the base metal and filler metal makes a hard and brittle transition layer easily form in the fusion zone, compromising overall performance. Let's talk about thermal stress and deformation. Cast iron doesn't conduct heat well, but its coefficient of linear expansion is quite high. The concentrated localized heating during welding, followed by rapid cooling, generates significant contraction stresses in the weld and heat-affected zone. However, cast iron itself has very low plasticity, and it can't release these stresses through deformation. Consequently, large residual tensile stresses act on the already brittle and hardened heat-affected zone, making the weld or heat-affected zone susceptible to cracking. Sometimes cracks occur during welding, while others only appear after cooling or after a period of aging. Furthermore, the localized heating and contraction can cause the pipe to bend and twist, resulting in uncontrollable deformation.

Thus, the risks of welding ductile iron pipes stem from the combination of these three factors: structural embrittlement caused by the high carbon equivalent, large residual welding stresses, and cast iron's inherent lack of plasticity, which prevents stress release. The resulting welded joints, especially the heat-affected zone, are often hard and brittle, prone to cracking, and their performance is significantly inferior to that of the original pipe.

Avoid welding unless standard connections are absolutely impossible or the repair is minor, with rigorous workmanship and testing procedures. Even then, seek experienced welders and strictly adhere to specific welding specifications for ductile iron, including preheating, selecting the right welding consumables, controlling the process, and performing post-heating or heat treatment. If critical pressure piping is involved, welding repair plans must undergo rigorous evaluation and approval.