Generally, the tension spring factory must carry out this type of research. The mechanical tension springs generally use cold-drawn carbon spring steel wire coils. The advantage is that the cost is low, the hooks are easy to form, and the hooks should not be generated during use. Fatigue fracture, the disadvantage is poor anti-relaxation ability. With the development of science and technology, many mechanical devices require higher and higher resistance to relaxation of the tension springs, forcing spring plants to use oil quenched and tempered steel wires with better resistance to relaxation. However, the oil quenched and tempered spring steel wire has poor torsion properties and is easily broken at the bending of the hook. The author studies the fracture mechanism of the hook bending at the time of making the tension spring by the oil quenching and tempering spring steel wire.
Spring plant
First, test materials and methods
The test uses φ5.5 mm 60Si2MnA hot rolled wire rods after pickling, phosphating, drawing, oil quenching and tempering heat treatment made of φ3.3 mm oil quenched and tempered spring steel wire products. The tensile strength of 1 750 ~ 1 780 MPa, the area of contraction rate of 45% to 51%, reverse 13 to 15 times. The metallographic structure is tempered troostite with no decarburization of the surface.
Second, the test results
1. Spring coiling system
A total of 90,186 tension springs were tested. Among them, 90,021 normal tension springs ere found to meet the standard requirements. Only 98 springs were found to be broken and 67 were cracked. It was found that cracks and cracks in the tension springs were generated at the bends of the hooks.
2. Fracture analysis
Fracture macroscopic appearance
The normal shape of tension spring pull hook bending place, there are obvious winding scratches, dark gray color, and the scratches around the bend of the hook hook are obviously distorted, which indicates that there is a local obvious torsional deformation in the wire. The appearance of the fracture surface at the bending section of the tension spring and the surface of the inner surface of the fracture at the bending point of the tension spring shows that there is a bright band formed by scratching around the inner side of the spring fracture, and there are many transverse cracks on the white bright band.