Which change occurs in cast iron pipes under graphitization?

When cast iron pipes undergo graphitization, they experience a significant transformation that affects their overall structure. This process involves the conversion of the iron within the alloy to graphite, which alters the physical properties of the material.

As carbon in the cast iron segregates into graphite, the material begins to lose some of its original hardness. The transformation leads to the formation of graphite flakes, which increases the porosity of the cast iron. This porosity means that the material becomes more sponge-like, resulting in reduced structural integrity and making the pipe softer. Over time, this decreased strength can lead to further deterioration and failure of the pipe.

The changes in physical properties from graphitization can also lead to issues like increased susceptibility to cracking, but the primary characteristic of becoming more porous and soft is the hallmark of this process. Thus, the transformation is notably linked with increased porosity and a reduction in mechanical strength, making the option indicating that cast iron pipes become more porous and soft the most accurate choice regarding the changes that occur under graphitization.