2.2 Cause analysis of screw micro fracture
Scanning electron microscope morphology analysis was performed on the fatigue crack propagation region and transient fracture region of the screw fracture.
2.2.1 Analysis of fracture morphology in fatigue crack growth zone
The entire section of the fatigue crack growth zone is the result of fatigue crack growth. It is usually clear that fatigue stripes are directionally continuous or intermittently distributed. Stepped fatigue stripes extend parallel to the crack leading edge, and fatigue stripes are perpendicular to the crack propagation direction. These are the microscopic traces of crack growth during failure.
A fatigue fringe corresponds to a stress cycle. The fatigue fringes in the fatigue crack propagation zone of the screw fracture are basically brittle fringes, and the interval between the fringes is equivalent to the length of a stress cycle. A small crack that propagates from the fracture surface to the inside is called a secondary crack. The secondary crack is a hole defect developed from a primary crack.
2.2.2 Analysis of microscopic appearance of fractures in transient fault zone
The transient fracture zone is a brittle cleavage fracture. Cleavage steps, river patterns, cleavage tongues, and secondary cracks can be seen on the fracture surface, and no dimples can be found.
Repair of extruder screw:
The twisted screw should be considered according to the actual inner diameter of the barrel, and the outer diameter deviation of the new screw should be manufactured according to the normal clearance with the barrel. The surface of the thread with the reduced diameter of the abrasion screw is treated, and the wear-resistant alloy is thermally sprayed, and then ground to size. This method is generally processed and repaired by a professional spraying factory, and the cost is relatively low. Wear-resistant alloys are deposited on the threaded portion of the worn screw. According to the degree of wear of the screw, the thickness is 1 to 2 mm, and then the screw is ground to the size. This wear-resistant alloy is composed of C, Cr, Vi, Co, W, and B materials, which increases the ability of the screw to resist wear and corrosion. Professional surfacing plants have a high cost for this kind of processing, except for special requirements of screws, they are rarely used. The repair screw can also be plated with hard chromium. Chrome is also a wear-resistant and corrosion-resistant metal, but the hard chromium layer is more likely to fall off. .