In addition to the issues mentioned in hose and steel pipe assemblies, tube fittings are also prone to two specific types of failures: cracking and leakage due to poor dimensional fit.

1. Fitting Cracking
   Cracking is a common problem in tube fittings. Causes include inclusions, sand entrapment, and other material defects, as well as insufficient strength or improper microstructure in the raw material. Cast fittings with severe porosity or sand holes can significantly reduce performance. Improper material selection is another key factor.
   For example, thin-walled components made from cold-drawn stock often have elongated grain structures along the axis, resulting in weak radial strength. This can lead to cracking during assembly torque. A case involved Z14 weld-end straight connectors made from cold-drawn hexagonal steel. When welded directly onto a hydraulic cylinder, axial cracks were observed at the threaded section during pressure testing. Although the defect rate was less than 1%, rework was time-consuming.
   Material testing (including flaw detection) did not reveal internal cracks. However, when over 1,200 fittings in stock were tested using a custom-designed conical press tool (which did not damage sealing surfaces), 8 cracked under pressure. The root cause was identified as insufficient radial strength at the thread joint due to poor grain orientation. Switching to forged blanks improved the microstructure and completely eliminated cracking issues in subsequent production.
2. Leakage Due to Poor Dimensional Fit
   Dimensional mismatches can also lead to oil or fluid leakage at the connection points. Even when a fitting meets its own dimensional tolerances, neglecting compatibility with steel pipe assemblies, hose assemblies, or mating parts can still result in leaks. Therefore, precise design, tight manufacturing control, and attention to assembly fit are essential to minimize and prevent leakage.