The first option

is to cut the pipe and add a tee, provided the water supply can be shut off. However, the costs of shutting off the water supply, installing the pipe, replacing the pipe, drilling the hole, and transporting the pipe to the site must be considered.

The second option

is to use a dedicated repair pipe clamp (repair clamp). To add a pipe under pressure and without shutting off the water supply, a Baoshuo flanged hole tee is required. Made of Q235 carbon steel, it offers improved ductility and pressure resistance. (Stainless steel flanged hole tees are also available.) DN900 pipes can be customized with any hole size, for example, an 800 hole can be added to a DN900 pipe. Installation is simple. Costs to consider include the price of the flanged half tee, the cost of installing the repair clamp, drilling the hole, and transporting the repair clamp to the site.

The final option, welding, is also possible, but welding ductile iron pipe does carry significant risks and is generally not recommended as a primary connection or repair method. The reasons are explained in detail below.

Thermal Stress and Deformation Issues:

High Residual Stress: Cast iron has relatively poor thermal conductivity and a relatively high coefficient of linear expansion. The localized, concentrated heating and subsequent rapid cooling during welding can generate significant shrinkage stresses in the weld and heat-affected zone (HAZ).

Low Plasticity: Cast iron inherently has extremely low plasticity, making it difficult to release these stresses through plastic deformation.

Risk of Cracking: The aforementioned large residual tensile stresses acting on the already hardened and brittle HAZ can easily cause cracking in the weld or HAZ. This cracking can occur during welding, post-weld cooling, or during placement.

Severe Deformation: Localized heating and shrinkage can also cause uncontrolled bending or twisting of the pipe.

Degradation of Matrix Properties:

Degraded HAZ Performance: As previously mentioned, the HAZ develops a brittle structure with significantly lower strength, toughness, and ductility than the original ductile iron matrix. This makes the weld joint a weak link in the entire piping system.

Spheroidization Degradation: In the high-temperature area immediately adjacent to the fusion line, magnesium may burn or diffuse, causing the graphite morphology in this area to deteriorate (from spherical to flocculent or even flaky), further reducing the mechanical properties.

Process Control Difficulties:

Strict Preheating Requirements: To slow the cooling rate, reduce the formation of martensite and white cast iron, and reduce thermal stress, a higher preheating temperature (300-400°C or even higher) is typically required. Preheating large-area pipes is difficult and costly.

Strict Interpass Temperature Control: A high interpass temperature (close to the preheating temperature) must be maintained during welding.

Low Current, Short Weld Beads, and Hammering: Low current is used to reduce heat input, short weld bead lengths are used to disperse heat, and the weld is hammered immediately after welding to relieve some stress. These operations require skilled and experienced welders.

Post-weld Slow Cooling or Heat Treatment: Slow cooling or stress relief heat treatment is often required after welding to further reduce residual stress and improve the microstructure of the heat-affected zone. This can be difficult to implement on-site. Welding Material Selection: Specially designed ductile iron welding rods (such as nickel-iron-based rods) are required, which are relatively expensive. Even then, weld performance is unlikely to be completely equivalent to that of the parent material.

Unpredictability and Low Reliability:

High Defect Susceptibility: Even with strict adherence to process specifications, the risk of defects such as cracks, pores, and slag inclusions in welded joints remains significantly higher than with other weldable materials due to the inherent properties of the material.

Difficult Quality Inspection: Ensuring the internal quality of welded joints is difficult and costly.

Long-Term Performance Concerns: The long-term service performance of welded joints is uncertain, especially when subjected to pressure, shock, or vibration loads, where brittle areas can become sources of failure.

Summary and Recommendations:

The high risk of welding ductile iron pipes stems primarily from the combined effects of metallurgical embrittlement (martensite and white cast iron in the heat-affected zone) caused by its high carbon equivalent, high weld residual stresses, and the inherent low ductility of cast iron, which prevents effective stress relief. The result is often a welded joint (especially the heat-affected zone) that is brittle, prone to cracking, and exhibits performance far inferior to that of the parent material.

Welding should only be considered a last resort under specific circumstances (e.g., when standard connections are impractical, minor repairs are necessary, and strict workmanship and testing procedures are required). Welding must be performed by highly experienced welders, strictly adhering to specific welding procedure specifications for ductile iron (preheating, consumables, procedures, and post-heating/heat treatment). For critical pressure piping, welding repair plans must be rigorously evaluated and approved.