Effect of second-step austempering temperature on mechanical damping of austempered ductile iron

Austempered ductile iron(ADI) has good strength and toughness, which is widely used in aerospace, engineering machinery and automobile manufacturing. In the above application fields, the requirements of safety and comfort are gradually increasing, so ADI is required not only to have excellent mechanical properties, but also to have high damping properties. The performance of ADI is closely related to its heat treatment method. Therefore, it provides a theoretical basis for the application of ADI, and reveals its mechanism to some extent. The influence of two-step ADI’s mechanical properties and damping properties due to two-step austempering quenching temperature is the main content of this paper. The retained austenite with obvious coarsening of bundle ferrite and increased size is the experimental result obtained after the increase of austempering temperature in the second step. After that, when the austempering temperature in the second step increases, the retained austenite content will directly decrease. The tensile strength of ADI will increase first and then decrease with the increase of austempering temperature in the second step. And the value reaches the maximum of 1350MPa at 360°C. The yield strength of ADI, which reaches the maximum value (1252MPa) at 400 °C, will follow the increase of austempering temperature in the second step, and the elongation and impact energy of ADI will gradually decrease. The elongation and impact energy of ADI will gradually decrease with the increase of austempering temperature in the second step. The minimum values of 4.8% and 42J are obtained when the austempering temperature in the second step reaches 400 °C. The impact fracture morphology shows that the fracture mechanism will change from ductile fracture to brittle fracture with the increase of austempering temperature in the second step. The reason why the internal friction value (Q−1) of ADI with different austempering temperatures in the second step increases gradually is the increase of strain amplitude. There are obvious S-K peaks and Ge peaks, that is, the damping characteristics change with frequency.


Introduction
The demand for structural materials is getting higher and higher, especially for light weight, toughness and noise reduction, because of the development of modern industry.A steel that can meet the above requirements at the same time is austempered ductile iron (ADI) [1][2][3][4][5][6][7].Therefore, among the transmission components of new energy vehicles, ADI [8][9][10] has a broader application prospect.As a transmission component, the vibration and noise of the transmission system can affect the damping performance of ADI.There are few reports on the damping proper-ties of ADI [11][12][13][14][15].The influence of matrix structure on mechanical properties and damping properties of ADI is the influence after the change of austempering quenching temperature in the second step, which is the main content of this paper.The relationship among austempering quenching temperature, microstructure, mechanical properties and damping properties in the second step has been further established.

Experimental material
Pig iron, carbon steel, ferrosilicon alloy, ferromanganese alloy, vermicular agent and electrolytic copper were all raw materials, as shown in Table 1.

Experimental instruments
The Carl Zeiss-AxioVert.A1 metallographic microscope (OM), FEI-Sirion scanning electron microscope (SEM) and JEM-2100 transmission electron microscope (TEM) were used to observe the microstructure of the samples.The tensile test was carried out by WDW-100 microcomputer controlled electronic universal testing machine, and the sample size was shown in Figure 1.

Microstructure analysis
The metallographic photos of two-step ADI with different isothermal quenching temperature in the second step are shown in Figure 2. It is in the matrix structure that two kinds of ferrite with different morphologies can be distinguished, and this result can be seen from the figure.The acicular ferrite has little change, but the bundle ferrite is obviously coarsened, and the increasing increase is the size of retained austenite, it will be deepened directly and obviously, and the area of white area will be obviously reduced when the temperature of austempering in the second step reaches 360 C. With the increase of austempering temperature in the second step, fine precipitates will appear in the light-colored massive austenite region.
The SEM photo is what is shown in Figure 3, which is the two-step ADI with different isothermal quenching temperature in the second step.When the temperature is lower than 360℃, two kinds of ferrite (acicular ferrite and bundle ferrite) can be distinguished in the matrix at the austempering temperature in the second step, which is what can be seen from the figure.At the same time, the greater the difference in morphology between the two ferrites is due to the higher the isothermal quenching temperature in the second step.When the isothermal quenching temperature in the second step reaches 400°C, the presence of massive austenite in the matrix has not been observed, the morphology of ferrite is also very blurred, and the boundaries between ferrites with different orientations are also indistinguishable.The XRD analysis data in Figure 4 and the calculated retained austenite content in Table 3 are all for more accurate analysis of the influence of austempering temperature in the second step on ADI structure.As isothermal quenching temperature in the second step increases, the content of Ar in ADI matrix will decrease significantly.As can be seen from Table 3 above, the amount of Ar is 30.87% when the isothermal quenching temperature in the second step is 320℃.TEM photos in the second step can be obtained from Figure 5.The thickness of flaky ferrite will gradually increase with the rise of isothermal quenching temperature in the second step, which is the result that can be seen from micrographs.
However, when the isothermal quenching temperature in the second step reaches 400°C, Fe3C patterns appeared in the diffraction pattern, which indicates that the film-like austenite has decomposed at this time and ferrite and carbides are formed.

Fracture analysis
The fracture morphology of ADI impact samples with different austempering temperatures in the second step can be observed in Figure 6.When the isothermal quenching temperature in the second step is 320°C, it can be seen from the figure that there are a lot of dimples along the tear undulations, and the matrix around the ductile iron at the impact fracture is torn, indicating that the material has ductile fracture.After the rise of isothermal quenching temperature in the second step,the tear-like area around the graphite ball is obviously reduced, and the degree of tearing is also reduced, and there are cleavage steps in the fracture.As the second iaustempering temperature rises to 400℃, the deformed graphite ball pits can no longer be observed, and the fracture is full of cleavage surfaces and cleavage steps.At this time, the fracture mode changes to brittle cleavage fracture.

Mechanical property analysis
The mechanical properties of two-step ADI with different isothermal quenching temperature in the second step are the main contents shown in Figure 7.The tensile strength, yield strength, elongation and impact work of ADI are 1318 MPa, 1177 MPa, 11.8% and 125J respectively at the isothermal quenching temperature in the second step of 320℃.The tensile strength of ADI will increase first and then decrease with the increase of austempering temperature in the second step.The strength of Dolores will reach the maximum value (1350MPa) at isothermal quenching temperature in the second step of 360℃ in the second step.The yield strength of ADI will gradually increase with the rise of isothermal quenching temperature in the second step, and the maximum value (1252MPa) will be reached when the isothermal quenching temperature in the second step is 400℃.The elongation and impact work of ADI will gradually decrease with the rise of isothermal quenching temperature in the second step.When the isothermal quenching temperature in the second step reaches 400 °C, the impact energy will decrease to 4.8% and directly to 42J.

Damping property analysis
The influence of strain on ADI damping performance at different austempering temperatures in the second step can be seen from Figure 8.The internal friction values of all samples increase with the increase of strain amplitude, and Q -1 increases, as can be seen from the figure.The most significant improvement of ADI's Q -1 is when it is less than 0.035% and the second austempering temperature is 320°C.This is because the retained austenite has good plasticity and toughness.The ADI with the highest retained austenite content is the austempering temperature of 320°C in the second step.The most significant improvement of ADI's Q -1 is when the strain is greater than 0.035% and the second austempering temperature is 400°C, which is obviously superior to other processes.Although it is the amount of retained austenite that decreases with the increase of temperature, the formation of a large amount of Fe3C increases the area of the Fe3C-ferrite interface, which consumes more energy during the vibration process.
Figure 8.Effect of second-step austempering temperatures on Q -1 -strain curves ADI Figure 9 is the effect of two-step austempering temperature on the storage modulus-strain curve of ADI.It can be seen from the figure that as the strain amplitude increases, the storage modulus gradually decreases.With the increase of the two-step austempering temperature, the storage modulus of ADI decreased first and then increased.When the two-step austempering temperature is increased from 320 °C to 360 °C, the matrix structure is obviously coarsened, the ferrite / austenite interface area and the grain boundary area are reduced, and the restriction of the interface on the short-range movement of the movable dislocation is reduced.ADI can produce microplastic deformation at a lower stress level, and the storage modulus is reduced.When the two-step austempering temperature rises to 400 °C, the retained austenite decomposes.The effect of frequency on ADI damping performance at different austempering temperatures in the second step can be observed in Figure 10.With the increase of frequency, the internal friction value of ADI samples fluctuates first and then decreases.The above results can be obtained from the above figure.For ADI samples with different austempering temperatures in the second step, the maximum values of internal friction all appear around 45Hz.Under the action of alternating stress, the interstitial solid solution atoms produce stress-induced reorientation.The damping-frequency curve of ADI has several weak characteristic peaks in the range of 0~30Hz.It can also be found from Fig. 9 that the damping peak near 32 Hz is S-K peak.Based on G-L dislocation pinning theory, under the action of alternating stress, dislocation lines get rid of pinning consumes energy and generates a damping peak in the low frequency range.The damping peaks of the damping-frequency curve near 45Hz and 53Hz are Ge peaks.With the increase of austempering temperature, the subsequent changes can be seen from Figure 5, retained austenite gradually decom-poses and a large amount of Fe3C is generated, which greatly increases the interface area and forms two damping peaks at 45Hz and 53Hz.Overall, when the frequency is in the range of 25~53Hz, ADI shows good damping characteristics, which indicates that ADI can show excellent damping performance in the medium frequency vibration working environment.

Conclusions
(1) The acicular ferrite changes greatly with the increase of austempering temperature in the second step, but the bundle ferrite is obviously coarsened and the retained austenite size is increased.However, when the austempering temperature changes in the second step, the retained austenite content will increase and then decrease.
(2) The tensile strength of ADI will change.With the increase of austempering temperature in the second step, it will first increase and then decrease, while the elongation and impact work will gradually decrease and the yield strength will gradually increase.
(3) Q -1 of ADI with different austempering temperature in the second step increases with the increase of strain amplitude.The greater the internal friction value, the higher the retained austenite content.
The internal friction value of ADI will first increase and then decrease because of the increase of frequency.
There are three obvious damping peaks at 32Hz (S-K peak), 45Hz and 53Hz (Ge peak).

Figure 1 .
Figure 1.Schematic illustration of tensile samples (unit: mm) According to GB/T 229-2007 standard, the impact experiment was conducted on NI300 pendulum impact tester with the sample size of 55mm×10 mm×10 mm.The damping properties were tested on the DMA Q800 with the sample size of 45 mm × 10 mm × 1 mm.

Figure 2 .
Figure 2. Metallograph of ADI with different second-step austempering temperature: (a) M1; (b) M2; (c) M3.The acicular ferrite has little change, but the bundle ferrite is obviously coarsened, and the increasing increase is the size of retained austenite, it will be deepened directly and obviously, and the area of white area will be obviously reduced when the temperature of austempering in the second step reaches 360 C. With the increase of austempering temperature in the second step, fine precipitates will appear in the light-colored massive austenite region.

Figure 9 .
Figure 9.Effect of second-step austempering temperatures on storage modulus-strain curves of ADI.

Figure 10 .
Figure 10.Effect of second-step austempering temperatures on Q -1 -f curves of ADI

Table 2 .
Isothermal quenching process parameters

Table 3 .
Ar content of ADI at different isothermal quenching temperatures