The test method was conducted through step-by-step optimization based on the process parameters of hot coil springs. To isolate the interaction between temperature and thermal deformation during hot coiling, air cooling was used to simulate the temperature changes from preheating to oil quenching. Initially, orthogonal experiments were carried out using two temperature parameters—preheating temperature and quenching temperature. Then, thermal deformation simulation was introduced to refine the optimization space of these parameters. The impact toughness and microscopic fracture morphology were evaluated as part of the analysis.
The sample size for the temperature parameter test was 715mm × 715mm × 55mm. The samples were heated in a box furnace at two preheating temperatures: 940°C and 920°C. After being air-cooled for 20 seconds, they were quenched in an oil tank and then tempered at 340°C, 380°C, and 420°C. Following grinding, non-standard unnotched impact specimens measuring 7mm × 7mm × 55mm were prepared. Impact toughness testing was used to identify the optimal temperature range.
Using the same sample size, thermal deformation simulations were performed at 900°C, 910°C, and 920°C according to the thermal deformation specifications under the optimized temperature parameters. Tempering was then done at 380°C, 390°C, and 400°C. After grinding, similar impact specimens were created, and final optimization was achieved by testing impact toughness. Microscopic fracture analysis was conducted using an S570 scanning electron microscope.
The test material, SAE9259 spring steel, is a hot-rolled round bar. The influence of hot processing parameters on impact properties was studied. The effect of temperature parameters was found to be independent of thermal deformation. From the results in Figure 2, three key regularities were observed: (1) At the same tempering temperature, samples preheated at 920°C showed higher impact toughness than those preheated at 940°C; (2) With the same preheating temperature, impact toughness increased as tempering temperature rose; (3) A parabolic relationship was observed between impact toughness and tempering temperature.
Curve fitting revealed that impact toughness for 920°C and 940°C preheating followed equations (1) and (2), respectively. Here, y represents impact toughness (J/cm²) and x is the tempering temperature (°C). The equations are:
y = -0.10628x² + 50.1575x - 980.714
y = 0.10444x² + 36.14x - 714.112
When thermal deformation was simulated near the 920°C preheating temperature, impact toughness after tempering at different temperatures was tested. Although the preheating temperature had little effect on impact toughness after thermal deformation, the impact toughness decreased with increasing tempering temperature. The relationship remained parabolic, described by the equation:
y = -0.108x² + 59.19x - 1085.0 (for 380°C ≤ x ≤ 400°C)
The interaction between temperature and thermal deformation parameters was analyzed, with tempering temperature identified as the key factor influencing impact toughness. Thermal deformation itself directly affected the change in impact toughness.
Comparing impact toughness at different tempering temperatures under a preheating temperature of 920°C, both with and without thermal deformation, revealed a stable zone around 380°C. By solving the simultaneous equations (1) and (3):
y = -0.10628x² + 50.1575x - 980.714
y = -0.108x² + 59.19x - 1085.0
The intersection point was found at x = 384.15°C, corresponding to an impact toughness of y = 354 J/cm². Using this point as the midpoint of the boundary zone, the impact toughness fluctuated within 354 ± 10 J/cm² when the tempering temperature was between 385.5°C, with a fluctuation error of less than 3%. The actual impact toughness of 920°C preheated samples tempered at 380°C was 360 J/cm². SEM observation of the fracture revealed a mixture of dimples and small facets, indicating improved microstructural resistance to impact.
Analysis of the test results shows that both temperature and thermal deformation parameters affect the impact toughness of SAE9259 spring steel. Temperatures play a critical role in maintaining a parabolic relationship between impact toughness and tempering temperature. In the absence of thermal deformation, lower preheating temperatures increase impact toughness due to finer austenite grain sizes. This increases grain boundary area, enhancing plastic deformation resistance and energy absorption.
After thermal deformation, the added dislocation stress field, solute atoms, and precipitates create significant frictional resistance to dislocation movement, surpassing the strengthening effect of fine grains. Therefore, the impact toughness becomes less sensitive to preheating temperature.
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