Recently, batch quenching cracks occurred in a part of the unit during heat treatment, and the scrap rate reached 20%.
The material of the parts is 27SiMn, and the heat treatment process adopted is 480 ℃ (30min) preheating + (900 ± 10) ℃ × 15min salt bath heating and circulating water cooling + 500 ℃ quenching .
In this post, a physical and chemical analysis of the causes of tempering cracks in parts is carried out, solutions are presented and good results are obtained in subsequent production.
1. Physical and chemical analysis
(1) Macroscopic observation
As shown in Figure 1, by observing the scrapped parts after heat treatment, it can be seen that all tempering cracks on the parts appear inside or close to the internal holes of the parts.
In the macroscopic state, the cracks advance along the internal holes of the parts in the form of small zigzag serrations, with a length of several millimeters to ten millimeters and a depth of 2 to 5 mm along the axial direction.
Generally, each part has several cracks.
Fig. 1 macro photo of hardening of cracked parts
(2) Chemical composition test
The cracked parts are sampled for chemical composition testing.
The test results are shown in Table 1.
The chemical composition complies with the chemical composition and mechanical properties of GB/T 3077-1999 of structural alloy steel.
Table 1 Chemical composition of 27SiMn steel parts (mass fraction) (%)
| Chemical element | W | Yes | Mn | P | s |
| GB/T 3077 | 0.24~0.32 | 1.10 ~ 1.40 | 1.10 ~ 1.40 | ≤0.35 | ≤0.35 |
| Test results | 0.32 | 1.24 | 1.25 | 0.019 | 0.005 |
(3) Metallographic analysis
Metallographic observation must be carried out by sampling the cracked part of the part.
As shown in Figure 2, under a metallographic microscope, it can be clearly seen that the crack snakes along the grain boundary;
Decarburization and oxidation are not found on both sides of the crack, which should be caused by quenching;
Cracks crack along the grain boundary, which must be an intergranular fracture;
There are a large number of white reticulated structures at the rear end and on both sides of the fissure.
The microhardness test shows that the white lattice structures are ferrite lattice structures.
The interior of the grain is a tempered sorbite structure and a small amount of pinnate upper bainite structure.
Metallographic observation is carried out on the core of the hardened parts.
As shown in Fig. 3, the matrix structure of the parts is a quenched sorbite structure + white mesh structure and a small amount of upper bainite structure.
The microhardness of the white lattice structure was tested, and the microhardness value was about 300hv, which can be considered as ferrite lattice structure.
2. Discussion
In the quenched state of steel materials, there are generally two forms of ferrite structure:
The first form is quenched at subtemperature or normal temperature, but the retention time is very short.
The structure of ferrite is retained in quenching and subsequent cooling because it does not completely transform into austenite or has no transformation time, and its shape is blocky or half-crescent at the grain boundary or within the grain;
The second way is normal temperature quenching, but due to the slow cooling rate at high temperature during cooling, the ferrite structure will preferentially precipitate in the form of a grain boundary network.
The different shapes of ferrite structure in the matrix structure have a great impact on the properties of materials.
The first way is that the ferrite structure is not fully austenitized and is distributed at the grain or grain boundary within the block-shaped or semi-crescent matrix structure.
Generally speaking, it has a good effect on relieving quenching stress, reducing quenching cracks and improving brittleness at low temperatures.
The reason is that the structural stress and transformation stress generated during martensitic transformation will be greatly relieved due to the good shape of the ferrite structure.
In addition, the ferrite structure is distributed at the grain boundary in a block or crescent shape, and more trace elements such as P can be dissolved, which has an obvious effect on eliminating the reversible quenching brittleness and improving the cold brittleness of the steel.
In the second form, due to the slow cooling speed at high temperature during the quenching process, the ferrite structure takes the lead in the network form and precipitates along the grain boundary.
This structure has a great impact on the mechanical properties of the steel, which will greatly reduce the impact resistance of the material and increase the probability of crack extinction.
The reason is that the ferrite structure is distributed along the grain boundary in the form of a network.
Due to the low strength of the ferrite structure, more trace elements can be dissolved, which greatly weakens the grain boundary strength and reduces the impact strength.
Since the structural stress and phase transformation stress of the material are very large during quenching, the crack will nucleate preferentially at the grain boundary where the ferrite network is located and crack along the grain boundary, resulting in accidents. Of Quality.
In this post, all the cracked parts of the tempered parts appear in the inner hole.
It can be clearly seen from the metallographic structure of the quenched parts that there are a large number of ferrite network structures, and the cracks snake along the grain boundary, which indicates that there are problems in the quenching and cooling process of the parts.
From the above physical and chemical tests, it can be seen that the alloying elements of 27SiMn Steel, especially the carbon content, are higher than the upper limit, therefore the hardenability is better;
In general, the critical diameter of 27SiMn steel can reach 38mm in 20℃ static water, and the effective thickness of parts is 13mm.
Therefore, under normal conditions, parts can be fully quenched in water.
The normal metallographic structure after heat treatment should be a quenched sorbite structure.
However, from the observation of the metallographic structure in Figure 2 and Figure 3, it is abnormal that there are a large number of lattice ferrite structures and a small amount of upper bainite structure in the structure.
The means of tempering the parts is the circulation of tap water.
Water cooling performance is very sensitive to temperature.
When the water temperature exceeds 40 ℃, the cooling performance will be greatly reduced, and its cooling capacity in the high temperature region (500-700 ℃) will be drastically reduced, while the low temperature cooling capacity in the low temperature region temperature. the temperature martensitic transformation region (200-350 ℃) will be less reduced.
This can be seen in Figure 4.
This will easily lead to the following consequences:
The slow cooling of the parts in the high temperature zone leads to the transformation of the austenite structure into a ferrite structure and distributed on the grain boundary in a network, weakening the grain boundary;
However, in the low temperature region, the cooling rate is very fast, the austenite turns into martensite, and the structural stress is very large.
Fig. 4 Schematic diagram of the water cooling rate curve at different temperatures (still water)
Through field investigation and visit, the author found that the temperature control system of the salt bath furnace was normal and the thermocouple detection accuracy was qualified.
However, it was found that there were three problems in the heat treatment process of the parts:
First, the joint door of the quenching water tank is damaged, and the circulating water of the quenching water tank cannot be replaced smoothly;
Secondly, the cooling water flow (high water temperature) from the main electrode of the salt bath furnace flows directly into the quenching water tank, resulting in high water temperature of the quenching water tank;
Third, when the parts are actually quenched, the quenching interval of each furnace is short, the volume of the water tank is small, and the refresh rate of tap water in the water tank is not timely.
The above three factors cause the water temperature of the parts to be very high during quenching. According to the investigation, it is inferred that the water temperature can be 50 ~ 60 ℃.
From this, it can be judged that the water cooling performance in the high temperature area of the parts is greatly reduced due to the high water temperature.
After the parts are quenched, the ferrite structure is distributed on the grain boundary in a network form, resulting in a sharp decrease in the grain boundary resistance.
However, the low-temperature cooling performance does not change much, resulting in very fast cooling speed, very large tensile stress, and cracks along the grain boundary during the transformation of martensite in the low-temperature area.
In addition, the shape of the parts and the quenching method also play a certain role in the cracking of materials.
The shape of the pieces is complex. It is heated in a salt bath oven. The quenching accessory passes through the inner hole.
The pieces are fixed in a vertical line, as shown in Fig.
There are a large number of parts in each accessory and the accessory is long.
When the crane is used for quenching, due to the speed limit of the crane, the water flow of the device is slow when the parts are quenched, resulting in different cooling speeds between the parts and different parts of a single part;
At the same time, as the quenching device passes through the inner hole of the workpiece, the water flow in the inner hole is not smooth (the diameter of the inner hole of the workpiece is 16mm) and the cooling speed of the inner hole is slow;
Furthermore, as shown in Fig. 5, when the parts enter the water, position 1 is cooled first, while position 2 is filled with air in its recess, so that the cooling of the inner hole in this position and its position 3 it's slower.
Fig. 5 Fixing part and quenching method
The slow cooling of the inner hole will facilitate the precipitation of the ferrite liquid structure, weaken the grain boundary, and make the residual thermal stress value generated by cooling in the high-temperature area relatively small, while the structural stress generated by cooling in the high-temperature area Low-temperature martensitic transformation is large.
The superposition of the two will cause a large state of tensile stress on the surface of the inner hole.
When the tensile stress value exceeds its tensile strength, cracks will be caused in the inner hole.
3. Conclusion and improvement measures
(1) The shape and quenching method of the parts lead to the slow cooling speed of the inner hole and the large tensile stress in the inner hole.
(2) During the quenching process of parts, due to damage to the quenching water tank gate and other reasons, the water flow is not smooth, the water temperature increases, and the water cooling performance is greatly reduced.
The ferrite structure is distributed on the grain boundary in a network form, resulting in a sharp decrease in grain boundary resistance.
Under the residual tensile stress, the crack breaks along the grain boundary.
To verify the above judgment, the author repaired the water tank door, controlled the cooling water temperature within 30℃ in the next trial production batch of parts (the same batch of materials), and increased the stirring force.
As a result, the tempering crack rate of the parts was greatly reduced, less than 1‰, which proved the accuracy of the above judgment.
