1, breaking Hammer Uniom State Description
Hammer Uniom nut started in July 2004, in December 2006 rupture occurred, as shown in FIG. Hammer Uniom model is equivalent to the SY / T5211-2003 YR70-3, ie, the maximum working pressure of 70MPa, nominal diameter of 76.2mm, tightening torque of 2400 ~ 3000 N · m. Fracture occurs at a trapezoidal thread third buckle at the lateral fracture.

Figure 1

Figure 2
2, Hammer Uniom fracture analysis
Macroscopic morphology of fracture is shown in Figure 3, the macroscopic morphology was typical of fatigue fracture morphology. Right side of the fracture is smooth, the crack area, the left rough as a short-break zone, crack source at the root of a nearby third button on the right ear of the thread. Under the alternating stress, cracks (smooth section) continues to expand until the section is insufficient to withstand the effects of stress and sudden fracture, fracture surface clearly visible.

Figure 3 macroscopic morphology of fracture Hammer Uniom
Hammer Uniom from broken when cut metallographic sample, found the root of the thread starting end to the outer has a through crack, the cutting of the thread portion has been broken (see Figure 4 and Figure 5). Remove it from the body, he found that the fracture was smooth arc shape, but also showing fatigue fracture type appearance. Since breaking the thread length of only about 40mm, it can be inferred fault is caused by defects in its original, that thread roots have original micro-cracks, in alternating stress, the crack continues to expand until it penetrates to the outer circle.
3, breaking Hammer Uniom Material Analysis
1, the chemical composition
Material samples were taken for analysis from the Hammer Uniom, each element measured content are given in Table 1.

Figure 4 screw fracture heel

Figure 5 off the thread
Table 1 chemical composition of the material coupling

From the material point of view, it should be the United States of steel No. 4140, similar to domestic 40CrMnMo, P, S and other harmful elements without excessive phenomenon.
2, microstructure
Failure of the fracture surface in the vicinity of Uniom Hammer sampling for metallurgical analysis, were used to measure grain size and morphology. Wherein the grain size assessment by GB / T 6394 standards, microstructure analysis according to GB / T 13298 standard. Test observe and captured on the OLYMPUS microscope, the results in Table 2 and Figure 4.
Metallurgical analysis results in Table 2

After testing the hardness is HRC28. As it can be seen from Figure 6, the grains uniform, small grain size, indicating normal tissue.

Figure 6 Hammer Uniom microstructure
4, Hammer Uniom Failure Analysis
As can be seen from Figure 3 Hammer Uniom the fracture surface, Hammer Uniom fast suddenly completely broken, the right of the crack extended source region and the source region darker, indicating that the crack is growing progressively extended from the crack initiation, in the role of alternating load extruded to form a smooth surface. When the crack to a certain stage, is not sufficient to withstand the external forces, suddenly brittle fracture. Visible fracture tearing ridge, the remaining parts of relatively flat, and the surface of the fractured porcelain-like presentation, characterized by a brittle fracture. Although not screw off the nut Hammer Uniom cleavage site, but can prove Hammer Uniom nut thread roots genetic defects exist.
5. Conclusions
Hammer Uniom chemical composition, microstructure and hardness of normal; the thread root of the original defect is an important reason Hammer Uniom nut breakage.