In drilling, damage or fracture will occur in the shank, shank shoulder, and body of drill rod. The fatigue breakage at 200~400mm before shank shoulder is usually called “normal failure” of drill rod, and the breakage or brittle breakage in other parts is called “abnormal failure”. Fracture is the natural phenomenon of separation of two adjacent parts of the metal material locally or in the whole section when the force reaches a certain value under the action of external force. In practical engineering, the complete fracture of metal material is usually called fracture, and the local fracture of metal material is called crack.

The factors affecting manufacture and rock drilling of drill rods are complex, and the causes of breakage are also multifaceted. It often requires scientific and rigorous analysis of the chemical composition of raw materials, rolling state, manufacturing process, geometric structure, surface quality, mechanical properties, microstructure, and applicable conditions of rock drilling, as well as macroscopic and microscopic characteristics of fracture. What are the characteristics of the fatigue fracture of drill rod?

Macroscopically

1. Shank shoulder root fracture

Fracture characteristics

There are two characteristics, one is a fatigue fracture and has multiple fatigue source points around the fracture. The other is a brittle fracture, and the source point of crack occurs mostly in the corner.

Reason

①Radius of the transition arc of shank shoulder

At the root of shank shoulder, stress concentration is easy to occur near the transition arc due to the sudden change of the geometric shape section of shank shoulder.

②Forging quality

Forging quality has a great influence on service life of shank shoulder roots. When mold is used to a hot state to a certain extent, its hardness decreases, and local deformation occurs.

③ Heat treatment process

To eliminate the “metallurgical gap” caused by the forging process of shank shoulder, induction heating and normalizing should be carried out after forging. During the heating, to ensure that the large section of shank shoulder is not overheated, a low-temperature zone will appear at the root and front of shank shoulder. Such normalizing treatment becomes a high-temperature tempering state. If the hardness is reduced, the yield point and normal fracture resistance are reduced, which creates conditions for brittle normal fracture failure.

④Quality of rock drill rotating sleeve

Rock drill rotating sleeve is a vulnerable part. Especially after using it for some time, the size of hexagonal inner hole of sleeve is large after wear. On the other hand, inner chamfer of rock drill rotating sleeve is a straight chamfer, not a circular chamfer, which cuts the two sides of the transition arc into sharp dents when repeatedly impacting the root of shank shoulder. This indentation aggravates the stress concentration.

2. Middle of shank shoulder is broken

Fracture characteristics

Usually, the fracture in the middle of shank shoulder is an internal fatigue fracture. The fracture is smooth.

Reason

①Inner hole diameter sudden change

During forming of shank shoulder, free upsetting deformation occurs first, and the diameter of inner hole increases with the increase of outer diameter of shank shoulder. Then, when upsetting metal part touches the inner cavity wall of mold, the flow of metal to outside receives obstruction and changes to direction of an inner hole with less resistance, resulting in thickening of wall thickness and a reduction of inner hole diameter.

②Iner tube is poorly bonded to base steel, and inner hole forms a trumpet shape

In drilling, a local upsetting of liner tube forces a “flared” hole to form in the middle of shank shoulder. The “flared” water hole is easy to cause strong stress concentration, coupled with a decarburization layer on an inner wall of base, and will lead to fatigue strength is reduced. Under the action of tensile (compressive) stress, a fatigue crack source is generated, which propagates radially outward into the matrix steel.

Microscopically

1. Metallographic organization

Fracture characteristics

Fatigue cracks start from inner hole and expand outward, fatigue arc band is visible, and there are more visible steps.

2. Microscopic fracture

Observing fracture of drill rod under a scanning electron microscope, fatigue streaks in the fatigue zone are not visible but showed friction traces in different degrees.

Although there are many reasons leading to drill rod breaking, it is always inseparable from a selection of materials and technology in the manufacturing process of drill rods. To produce high-quality drill rods, it is necessary to control material selection, forming, and heat treatment. Through scientific and strict analysis of fatigue fracture of drill rods, technology, and material selection method are continuously adjusted, to prolong drill rod service life.