The torque wrench is mainly used to fasten bolts, and its application torque is generally 20%~90% of the wrenching torque, which is continuously adjustable.
When using, set the target torque first, and pull the handle.
When the torque reaches the target value, the wrench will have a slight vibration and a clear “chatter” sound.
After a torque wrench was used for 3 months, the connecting rod between the head and the handle broke.
In order to find out the cause of the fracture, researchers conducted a series of physical and chemical inspection and analysis, and improved its heat treatment process to prevent the recurrence of such problems.
1. Physical and chemical inspection
1.1 Macro observation
The torque wrench is 1180mm long with a torque of 1200N · m.
The fracture occurred at the connection between the head and the handle, as shown in Fig. 1a).
Since it is close to the head of the wrench, the stress here is large.
Fig. 1b) is a connecting rod installed inside the wrench, which is mainly used to connect the head and handle, with a diameter of 16mm and a length of 350mm.
Fig. 1c) shows the macro morphology of the fracture surface of the torque wrench, which is divided into three areas:
Zone I is the crack source zone, located at the edge of the fracture, accounting for 1%~2% of the fracture area;
Zone II is the fatigue expansion zone, which is bright white crescent-shaped, with obvious fatigue striations, accounting for 8%~10% of the fracture area;
Zone III is a transient fracture zone with gray color and obvious tear edges, accounting for about 90% of the fracture area.
It can be seen that the connecting rod bears a large force when it breaks, which belongs to high-stress low cycle fatigue fracture.
Fig. 1 Wrench Fracture Location, Connecting Rod and Macro Morphology of Fracture
1.2 Chemical composition analysis
The connecting rod is made of 40Cr alloy steel. A cylindrical sample with a size of ϕ16mm×12mm is taken near the fracture surface.
After being ground flat with a grinder and polished with a grinder, the chemical composition of the connecting rod is analyzed with a direct reading spectrometer.
It is found that its chemical composition meets the technical requirements of 40Cr alloy steel in Alloy Structural Steels (GB/T 3077-2015).
1.3 Mechanical property test
The hardness of the connecting rod after quenching and tempering heat treatment is 22~26HRC.
A section of the sample with the size of ϕ10mm×5mm is cut off from the connecting rod.
The tensile property is measured by universal material testing machine, and the hardness is measured by Rockwell hardness tester. The results are shown in Table 1.
The tensile strength, yield strength, and elongation of the connecting rod do not meet the technical requirements.
Table 1 Mechanical Property Test Results of Connecting Rod
| Performance index | Tensile strength/MPa | Yield strength/MPa | Elongation after fracture/% | Hardness/HRC |
| Standard value | ≥960 | ≥780 | ≥11 | 22~26 |
| Measured values | 870 | 724 | 7.5 | 24 |
1.4 Microstructure observation
Cut the sample near the fracture of the connecting rod, corrode it with nitric acid ethanol, and observe it with a microscope.
It can be seen from Fig. 2 that the dark gray structure is tempered sorbite transformed from martensite after high-temperature tempering, and reticular ferrite and acicular ferrite are distributed in parallel at the grain boundary of parent austenite.
The widmanstatten structure is distributed in the grain in an inverted triangle shape.
The ferrite in the widmanstatten structure is precipitated along the habitual plane of parent austenite, and the crystal plane index of the habitual plane is {11 1} γ.
Under the quenching cooling condition, when the temperature drops to the Ac3 line, in order to maintain the stability of the structure, the surplus ferrite will be “discharged” from the solid solution to the surrounding, thus forming a network ferrite, which is a typical high-temperature transformation feature.
The slower the cooling rate is, the easier it is to form network ferrite and Widmanstatten structure.
The connecting rod undergoes high-temperature transformation during the cooling process, indicating that its heat treatment process is unreasonable.
Fig. 2 Microstructure of Connecting Rod Fracture
1.5 Fracture analysis
Fig. 3a) shows the microscopic morphology of the initial area of the fracture.
The initial area is located at the edge of the fracture. There are obvious concentric circular bainite lines near the crack source.
This is a typical feature of fatigue growth, indicating that the fracture form belongs to fatigue cracking. Energy spectrum analysis is conducted on the box of Fig. 3a), as shown in Fig. 3d).
The diffraction peaks of Fe, Cr, Mn and O are relatively obvious, indicating that the crack source is not caused by inclusions.
Fig. 3b) shows the microscopic morphology of the expansion zone, and the fatigue striation is narrow, indicating that the stress is small during expansion.
Fig. 3c) shows the micromorphology of the transient fracture zone, with many oval dimples, indicating that the connecting rod was finally fractured by tension.
Fig. 3 Micromorphology at Different Positions of Fracture Surface of Connecting Rod and EDS Spectrum of Box Area in Fig. 3a
2 Analysis and discussion
Through the analysis of the fracture surface of the connecting rod, it can be seen that the fracture surface belongs to fatigue fracture surface, with no inclusions at the fracture surface and no scratch damage on the surface, but its tensile strength, yield strength and elongation do not meet the technical requirements.
It can be seen from the microstructure observation that the microstructure at the fracture is reticulated ferrite and widmanstatten, which indicates that the temperature is high during the quenching and tempering process, and the austenite in the connecting rod has strong stability, leading to the formation of widmanstatten.
Secondly, the retention time of the parts from the heat treatment furnace to the quenching medium is long, which leads to the precipitation of ferrite at the grain boundary and the formation of a network, which reduces the strength and interface energy of the grain boundary, thus increasing the brittleness of the material.
Under the action of external force, the crack starts and extends to the grain boundary, where the ferrite hardness is low, and the grain boundary becomes the crack propagation channel.
Therefore, it is necessary to improve the heat treatment process of the connecting rod.
3. Improvement of the heat treatment process
The measures to improve the heat treatment process of connecting rods are:
(1) The quenching temperature is reduced from 880 ℃ to 830 ℃.
The lower quenching temperature can increase the inhomogeneity of the composition of the austenite micro zone, reduce the thermal stability of the austenite, reduce the probability of the high-temperature transformation of the austenite to decompose into acicular ferrite, and promote the early transformation of the austenite in the micro-zone;
(2) Shortening holding time can avoid austenite grain growth and surface decarburization at high temperature;
(3) In the original heat treatment process, after the connecting rod is heated in a compact row in the trolley furnace, it needs to be quenched after being loaded into the basket.
The transfer time is about 180s.
After the improvement, the connecting rod is dispersed and heated in the mesh belt furnace, which can quickly enter the quenching medium.
The transfer time is about 8s. Shortening the transfer time can inhibit the precipitation of the mesh ferrite, promote the austenitic structure to quickly enter the low-temperature transformation zone, and thus produce the low-temperature martensite transformation;
(4) The connecting rod is thin and long, the stress after quenching is relatively uniform, and it is not easy to crack.
The original quenching medium is ordinary quenching oil, the cooling rate of the oil is low within the range of 550~650 ℃, and the average cooling rate is only 60~100 ℃/s.
The temperature range is at the “nose” of the continuous transformation C curve, which requires rapid cooling.
After the 12% (mass fraction) PAG (polyalkylene glycol) solution is used, the cooling rate is accelerated.
The medium temperature transformation in this temperature range can be reduced, so that a more ideal low-temperature martensite structure and a larger depth of hardened layer can be obtained.
Table 2 Comparison of heat treatment process parameters before and after improvement
| Process Parameters | quenching temperature/℃ | Holding time/min | Transfer time/s | Tempering temperature/℃ | Quenching medium |
| Original process | 880 | 60 | 180 | 560 | conventional quenching oil |
| Improved process | 830 | 50 | 10 | 560 | 12% PAG solution |
The improved process was used to heat treat the connecting rod, and its mechanical properties were tested.
The tensile strength was 1054MPa, the yield strength was 880MPa, the elongation was 12%, and the hardness was 23HRC, all of which met the technical requirements.
The microstructure after heat treatment is shown in Fig. 4.
There is no reticular ferrite, Widmanstatten structure or massive ferrite, and the structure is uniform and stable.
After improved process heat treatment, the connecting rod has been in service for 18 months without fracture.
Fig. 4 Microstructure of connecting rod after improved process heat treatment
4. Conclusion
(1) The hardness of the connecting rod is qualified, and the tensile strength, yield strength and elongation do not meet the technical requirements;
The microstructure is tempered sorbite+reticulated ferrite+widmanstatten.
The fracture belongs to fatigue fracture. The crack originates from the outer surface of the connecting rod, and there is no inclusion at the crack source.
(2) The reason for the fracture of the connecting rod is that the heat treatment process of the connecting rod is unqualified, resulting in its low mechanical properties.
The heat treatment process is improved by reducing the quenching temperature, shortening the holding time and transfer time, and increasing the quenching cooling rate.
(3) After the improved process heat treatment, the mechanical properties and microstructure of the connecting rod meet the technical requirements, and the connecting rod has not been broken after 18 months of service.
