Recently, a group of springs of a company broke during the test, and the manufacturing process of the springs is: cooling heat treatment – grinding – curved forming – pre shot peening – gas nitoring – shot peening – hot setting.
The cracking reason of the broken spring was analyzed.
Testing method and equipment
Test content: macro morphology analysis;
- M250 stereomicroscope;
- DM6000 metallographic microscope;
- ZEISS EVO18 scanning electron microscope;
- ZEISS EVO18 energy spectrometer.
Macroscopic morphology analysis
According to the marked three samples after the spring, they are recorded as No. 4 sample, No. 3 sample and No. 3 sample.
See Fig. 1. All three samples are fatigue fracture.
According to the flatness of the fracture surface and the area of the instantaneous fracture zone, it can be determined that No. 4 sample is preferentially fractured, No. 3 sample is subsequently fractured, and No. 3 sample is finally fractured.
The light yellow color of the spring is caused by nitriding treatment. Nitriding treatment refers to a chemical heat treatment process that allows nitrogen atoms to penetrate into the surface layer of the workpiece in a certain medium at a certain temperature.
The parts after nitriding treatment have excellent wear resistance, fatigue resistance, corrosion resistance and high temperature resistance.
The fracture spring is rubbed at the fatigue source and appears bright white.
The nitride layer has been worn, thus reducing the fatigue strength of the surface and causing fatigue cracking.
Fig. 1 macro photo of fracture of broken spring
Metallographic microstructure analysis
See Fig. 2 for the metallographic micrograph of the cross section of the broken spring.
From Fig. 2, it can be seen that there is a very thin white nitrided layer on the spring surface, and the nitrided layer is corrugated and uneven in thickness.
Fig. 2 metallographic micrograph of fracture spring cross section
See Fig. 3 for the metallographic micrograph of the longitudinal section of the broken spring.
From Fig. 3, it can be seen that there is a white nitrided layer on the surface of the spring and there is an obvious strip structure.
Fig. 3 metallographic micrograph of longitudinal section of fracture spring
See Fig. 4 for the metallographic micrograph of the cross-section of the spring through fatigue test.
From Fig. 4, it can be seen that there are white pulse like structures on the surface of the spring through fatigue test.
Fig. 4 metallographic micrograph of spring cross section through fatigue test
The vein structure is a kind of network nitride, which is produced by the high temperature or nitrogen potential or the long nitriding time.
Since the nitriding layer of spring parts is very thin, the influence of too long time can be eliminated.
The possible reason is that the nitriding temperature is too high or the nitrogen potential is too high.
The vein structure will reduce the fatigue strength of the nitriding layer.
The surface microhardness test results of the spring show that the surface microhardness of the spring is about 560hv and the core microhardness is about 510HV.
SEM analysis of fracture surface
The SEM micrograph of the spring fracture is shown in Fig. 5.
From Fig. 5, it can be seen that the spring fracture is a fatigue fracture, and the fracture originates from the outside of the spring.
However, the fatigue expansion area is very small, accounting for about 20% of the spring fracture.
After the fatigue expansion area, it is a fast expansion area.
This area is mainly unstable herringbone pattern, and there are fast expanding fatigue bands locally.
There are obvious scratch marks at the spring fracture source, and pits are found locally.
This is the main reason for the fatigue fracture of the spring.
There are spherical particles in the fracture source area.
Fig. 5 SEM micrograph of fracture spring
Chemical composition analysis of fractographic particles by EDS
In order to analyze the source of fracture particles, EDS energy spectrum micro area composition analysis was carried out for fracture particles.
The analysis results show that the main components of particles include Mo and Cr, which may be lubricants in lubricating oil.
Table 1 EDS energy spectrum analysis results of particulate matter (%)
|element||% by weight|
Fig. 6 EDS energy spectrum analysis results of particulate matter
Table 2 2EDS energy spectrum analysis results of particulate matter (%)
|element||% by weight|
Fig. 7 2EDS energy spectrum analysis results of particulate matter
The fracture mode of spring is fatigue fracture;
The fatigue source of the broken spring is worn, showing bright white, and the nitride layer has been worn, thus reducing the fatigue strength of the surface and causing fatigue cracking;
The pulse like structure on the surface will reduce the fatigue strength of the material.