A 45# steel tempering flange crack analysis and technical countermeasures

We produce a flange part material for 45# steel; its main processing technology is forging + tempering + rough turning + fine turning + galvanized + storage. The forging blank of the part is shown in Figure 1, and the forging has a large thickness difference between inside and outside.

Our production of a batch of about 500 pieces of the flange forgings for tempering, the tempering process is: quenching insulation temperature 825 ℃, holding time of 85 minutes, quenching medium for 10% NaCl brine, cooled in brine for 15 seconds, tempering in 4 hours, tempering temperature 520 ℃ – 530 ℃, holding time of 2.5 hours. The batch of forgings, after tempering, found a large number of pieces of cracks.
Figure 2 for the forging thin-walled end face up and down the crack, can be seen along the thin-walled end face of the flange cracks were circumferential distribution, in the thin-walled end face has been cracked, cracks through the entire thin-walled end face. According to statistics, the batch of forgings after tempering cracks 274 pieces, a scrap rate of more than 50%, seriously affecting the subsequent production and processing.

Figure.1 Flange forging blank map

Figure.2 Flange forging blank after tempering cracks

1. Experimental analysis

To ensure the subsequent production for the batch of flange forgings after tempering cracks occurring in the cause of analysis.

1.1 Material analysis

45# steel raw materials need to comply with the S78002-2016 standard required by our factory, and its standard chemical composition is shown in Table 1.
The chemical composition of the forging tested in this batch is shown in Table 2.
Comparing the chemical composition of 45# steel in Table 2 with the standard in Table 1, it can be seen that the chemical composition of the batch of flange forgings meets the requirements.

1.2 Hardness test

After tempering, the hardness of flange forgings must be 25HRC – 30HRC; take a forging that has been cracked and test the hardness at the thin-walled end face; the result is 26.5HRC, which meets the process requirements.

1.3 Metallographic analysis

The forging shown in Figure 2 will be sawn along the perpendicular to the cracking direction for metallographic analysis; the results are shown in Figure 3.
As can be seen from Figure 3, away from the crack matrix organization: tempered sohnite, grade 1, in line with the national standard GB 13320-2007 tempered steel tempering organization requirements. In the vicinity of the crack, no decarburization organization is observed, indicating that the crack is not caused before quenching but should appear when quenching; combined with the crack’s macroscopic morphology, the crack is quenching crack.

Figure.3 Crack surface organization
Table.1 Chemical composition of 45# steel required by S78002 (wt%)

Grade	C	Si	Mn	P	S	Cr	Ni
45# Steel	0.42-0.50	0.17-0.37	0.50-0.80	≤ 0.025	0.015-0.030	≤ 0.20	≤ 0.30
	Mo	Cu	V	Al	Ti	Nb	B
	≤ 0.03	≤ 0.20	≤ 0.02	0.015-0.045	≤ 0.01	≤ 0.030	≤ 0.0008

Table.2 Chemical composition of 45# steel material for tempered forgings

The unit of O, N, H is ppm, the unit of other elements is wt %.
C	Si	Mn	P	S	Cu	Ni	Cr
0.49	0.22	0.67	0.011	0.018	0.075	0.028	0.09
Ti	V	Al	B	Mo	Nb	W	Sn
<0.001	<0.005	0.016	<0.0005	0.017	<0.005	<0.005	<0.002
As	Sb	Pb	Bi	O	N	H
0.01	0	0.0013	0	7.25	49.7	0.001

2. Discussion

Through the above analysis, the batch of forgings cracked after tempering is mainly due to quenching. Tempering of forgings is a heat treatment heating and cooling process due to thermal expansion and contraction and tissue phase change. When the difference between the specific volume of the old and new tissues, volume changes are bound to occur. Special attention should be paid to the parts in the cooling process of the surface. The heart of the existence of temperature differences, coupled with the organization of the phase change of the non-simultaneity and different phase variables, the volume of the steel parts of the surface and the heart of the volume changes cannot be synchronized, thus generating internal stress.
According to the differences in the causes and mechanisms of internal stresses, they can be classified into two types, i.e., thermal and tissue stresses. Thermal stress refers to the rapid cooling of steel parts in the heat expansion state; in the cooling state, different parts of the volume shrinkage are different at the same time, thus generating thermal stresses. Organizational stresses refer to the cooling process of the parts by the austenite transformation into martensite organization; martensite specific volume is greater than the austenite specific volume, the same parts of different parts due to the cooling capacity of the different parts of the different tissue transformation dynamics, which leads to the transformation of austenitic organization into martensitic Organizational speed is not the same, different parts of the organization transformation has not simultaneous. Internal stress is the compound effect of thermal stress and organizational stress.
For the flange forgings, different parts of the thickness have a large difference; the thickness of the forging hole attachment is much greater than that of the outer thin wall. As can be seen from the figure, the thickness of the thin wall at the thin wall is only 10mm, quenching the thin wall at the thin wall can be quenched completely. Quenching, thin-walled due to faster cooling, the first occurrence of cooling and martensitic transformation, and near the inner hole due to the thickness of the larger, slower heat dissipation and cooling, followed by martensitic transformation, due to the two size difference is large, thus generating a large amount of internal stress when the internal stress exceeds the fracture strength of the forging, the forging cracking occurs. A kind of steel in the same medium quenching; in the case of quenching, there is a dangerous cross-section size. 45 # steel cracking size range of 5mm-11mm. The forging is thin-walled at the thickness of 10mm, in the range of cracking size, in the quenching of quenching cracking in the quenching of tempering easily.

3. Improvement measures

Through the above analysis, for the flange forgings, you can take the method of increasing the thickness of the thin wall, reduce the thickness difference between inside and outside the forging, and promote the internal and external organization of the quenching of the transition rate tends to be the same, to reduce the rate of quenching cracking caused by the scrap rate. However, the forging blank is based on the product parts to add margin and design; the structure of the parts determines its overall structure. Increasing the part thin-walled allowance, on the one hand, increases the cost of raw materials, resulting in higher production costs of forgings; on the other hand, it will cause a significant increase in the subsequent machining workload, reducing production efficiency. Therefore, it is not desirable to seek to improve the quenching process only by increasing the allowance at the thin wall of the part. We need to change from the perspective of the tempering process, thereby improving the qualification rate of tempering.
Due to the poor hardenability of 45# steel, to ensure that qualified tempering hardness and organization can be obtained, brine is usually used as the quenching medium. Due to the special structure of the forging, using brine as a single quenching medium can meet the hardness and organization requirements but cannot eliminate the occurrence of quenching cracks.
To reduce quenching cracking, from the perspective of cooling characteristics, I hope that the quenching medium in the early stage of cooling, the cooling rate is slower to avoid in austenite and supercooled austenite state of the workpiece due to the cooling rate is fast, the contraction of the sharp and the bending distortion. In the supercooled austenite, the most unstable interval (pearlitic transformation curve “nose,” 600 ℃ – 400 ℃), I hope that the fast cooling to avoid the occurrence of pearlitic transformation. And hope to enter the martensitic transformation zone (Ms point below); the slower the cooling rate, the better to relieve the volume expansion of the martensitic transformation of the stresses generated to prevent cracking and reduce distortion. Figure 4 shows the cooling curve of the ideal quenching medium. However, due to a variety of steel overcooling austenite stability and the actual size and shape of the workpiece differences, the requirements of the quenching medium at the same time suitable for a variety of steel is unrealistic, so the ideal quenching medium does not exist.

Figure.4 Cooling curve of the ideal quenching medium
However, it is possible to take close to the ideal cooling medium of dual-media quenching for quenching. Dual-media quenching is when the heated workpiece is first quenched into the cooling capacity of the stronger medium when the workpiece temperature drops to the C curve “nose temperature” below the temperature and then quenched into the weaker cooling capacity of the medium to continue to cool, to obtain the martensitic organization.
Therefore, for the forging, the development of a new tempering process for the quenching and holding temperature of 820 ℃, holding time of 60 minutes, quenching medium for 10% NaCl brine and quenching oil, brine temperature of 20 ℃ – 40 ℃, quenching oil temperature of 30 ℃ – 60 ℃, in the brine cooled for 3-5 seconds and then quickly put into quenching oil-cooled, tempered in 4 hours, the tempering temperature of 520 ℃ -530 ℃, holding time of 2.5 hours. The new tempering process compared with the original process, the main change in the quenching medium, from a single brine, changed to a water-oil double medium. This can be done in the high-temperature zone with the rapid cooling of brine to inhibit the decomposition of supercooled austenite, in less than 400 ℃, immediately transferred to the oil for slow cooling to reduce the quenching stresses to prevent quenching cracks.
Through dual-media quenching, the first batch of 200 pieces of the test, followed by rough turning, only 9 pieces were found to have cracks, and the qualification rate of tempering was greater than 95%. The appearance of the 9 pieces may be related to the proficiency of the workers’ operation (Note: the forgings are double-media quenched at the outsourcing manufacturer). When dual-media quenching, the residence time of the workpiece in the first medium is a critical parameter. If the first medium stays too long, it becomes a single liquid quenching and cannot reduce deformation and prevent cracking. If placed into the second medium too early, the temperature of the workpiece is still high, the cooling rate of the medium is slow, in the cooling process occurs in the non-martensitic type of tissue transformation. As the test forgings are in the outsourcing manufacturers tempering, cooling time in brine is only 3-5 seconds, so workers, due to human factors in the operation, cannot be completely accurate to ensure that each piece of forging cooling time stays in brine for a little longer, it will cause the forging cracking. Subsequent need to further strict requirements for operating procedures.

4. Conclusion

Through the Division of a flange forging tempering cracks in the analysis of the causes, this paper put forward the corresponding improvement measures. Specific conclusions are as follows:

• (1) The forging thickness size difference is large, easy to quench cracking in the tempering, cracks for quenching cracks;
• (2) Using water-oil dual-media quenching to reduce quenching internal stress can reduce quenching cracking and improve the qualification rate of this forging tempering.

Author: Dang Xiaolai