The Tapered Drilling Rod's body, shank shoulder, and shoulder will all sustain damage or fracture while drilling. It is customary to refer to the fatigue breaking of the Tapered Drilling Rod at 200–400mm before the shank shoulder as "normal failure" and the breakage or brittle fracture in other areas as "abnormal failure." When the force under the influence of an external force reaches a specific value, a natural occurrence known as fracture occurs, which causes two adjacent metal pieces to separate locally or throughout the entire section. In actual engineering, the localized fracture of metal material is referred to as a crack and the total rupture of metal material is typically referred to as a fracture.
Tapered Drilling Rod production and rock drilling are influenced by various complicated elements, and breaking has many different root causes. The chemical makeup of the raw materials, rolling state, manufacturing procedure, geometric structure, surface quality, mechanical properties, microstructure, and applicable rock drilling conditions, as well as macroscopic and microscopic fracture characteristics, frequently require thorough, scientific analysis. What are the features of the Tapered Drilling Rod fatigue fracture?
Macroscopically: 1. shoulder root fracture in the shin
characteristics of fracture
Two characteristics stand out: the first is a fatigue fracture with numerous fatigue source points surrounding it. The source point of the other fracture, which is brittle, is primarily in a corner.
Reason
1. Shank shoulder's transition arc radius
Due to the abrupt change in the geometric shape of the shank shoulder section at the root, stress concentration is simply happens near the transition arc.
2. Forging quality
The service life of shank shoulder roots is significantly influenced by the forging quality. Mold loses hardness and experiences localized deformation when heated to a certain degree.
3. Heat treatment process
Induction heating and normalizing should be done after forging to close the "metallurgical gap" left by the forging of the shank shoulder. An area with a low temperature will develop at the root and front of the shank shoulder during heating to prevent the vast portion from becoming overheated. An environment of high temperature is created during such a normalization process. The yield point and normal fracture resistance both decrease with decreasing hardness, which produces the circumstances for brittle normal fracture failure.
4. Quality of rock drill rotating sleeve
The revolving sleeve of a rock drill is a weak point. The inner hexagonal hole of the sleeve has a significant after-wearing size, especially after prolonged use. However, the spinning sleeve of a rock drill has an inner chamfer that is straight, not round, and when it repeatedly strikes the root of the shank shoulder, it repeatedly slices the two sides of the transition arc into sharp dents. The stress concentration is exacerbated by this indentation.
2. Middle of the shank shoulder is broken
Fracture characteristics
Usually, the fracture in the middle of the shank shoulder is an internal fatigue fracture. The fracture is smooth.
Reason
1. Inner hole diameter sudden change
During forming of the shank shoulder, free upsetting deformation occurs first, and the Tapered Drilling Rod diameter of the inner hole increases with the increase of the outer diameter of the shank shoulder. Then, when the upsetting metal part touches the inner cavity wall of the mold, the flow of metal to the outside receives obstruction and changes to the direction of an inner hole with less resistance, resulting in a thickening of wall thickness and a reduction of the inner hole diameter.
2. The inner tube's weak attachment to the base steel causes the inner hole to take the shape of a trumpet.
A "flared" hole forms in the center of the Tapered Drilling Rod shank shoulder during drilling due to a localized upsetting of the liner tube. The "flared" water hole makes it simple to concentrate severe stresses, and this, along with a decarburization layer on the inner wall of the base, will result in lower fatigue strength. A fatigue crack source is created by the action of tensile (compressive) stress, and it spreads radially outward into the matrix steel.
