The test methodology was designed to optimize the hot coiling process parameters step by step. To isolate the effects of 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 conducted with two temperature parameters: preheating temperature and quenching temperature. Following this, thermal deformation simulations were introduced to refine the parameter space, and the impact toughness along with the fracture microstructure were evaluated. The samples used for the temperature parameter tests had dimensions of 715mm × 715mm × 55mm. They were heated in a box furnace at preheating temperatures of 940°C and 920°C. After 20 seconds of air cooling, they were quenched in an oil tank and then tempered at 340°C, 380°C, and 420°C. After grinding, non-standard unnotched impact specimens of 7mm × 7mm × 55mm were prepared, and the optimal temperature range was determined based on impact toughness testing. Using the same sample size, thermal deformation simulation tests were performed at 900°C, 910°C, and 920°C, following the thermal deformation specifications under the optimized temperature parameters. Then, they were tempered at 380°C, 390°C, and 400°C. After grinding, another set of impact specimens was made, and the final optimized parameter range was selected based on impact toughness measurements. The microstructure of the fracture surfaces was analyzed using a S570 scanning electron microscope.
The test material used was SAE9259 spring steel, which is hot-rolled round bar. The study focused on how hot processing parameters affect impact properties. It was observed that temperature parameters influence impact toughness independently of thermal deformation. From the data in Figure 2, three clear trends were identified: (1) At the same tempering temperature, the impact toughness of the 920°C preheated sample was higher than that of the 940°C preheated one; (2) With the same preheating temperature, impact toughness increased as the tempering temperature rose; (3) A parabolic relationship was found between impact toughness and tempering temperature. Curve fitting showed that the impact toughness of 920°C and 940°C preheated samples followed equations (1) and (2), respectively, where y represents impact toughness (J/cm²) and x is the tempering temperature (°C). The equations are:
y = -0.10628x² + 50.1575x - 980.714 (for 920°C preheating, 340°C ≤ x ≤ 420°C)
y = 0.10444x² + 36.14x - 714.112 (for 940°C preheating, 340°C ≤ x ≤ 420°C)
When considering the effect of thermal deformation near the 920°C preheating temperature, thermal deformation simulations were carried out according to the specified thermal deformation conditions. Impact toughness was tested after tempering at different temperatures, and it was found that the preheating temperature had little effect on impact toughness after thermal deformation. However, impact toughness decreased as the tempering temperature increased, maintaining a parabolic trend:
y = -0.108x² + 59.19x - 1085.0 (for 380°C ≤ x ≤ 400°C)
The interaction between temperature and thermal deformation parameters revealed that tempering temperature was the key factor affecting impact toughness. Whether or not thermal deformation occurred directly influenced the change in impact toughness.
Comparing the impact toughness at different tempering temperatures under a preheating temperature of 920°C, both with and without thermal deformation, it was observed that after thermal deformation, the impact toughness stabilized 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, when the tempering temperature was within 385.5°C, the impact toughness fluctuated within a range of 354 ± 10 J/cm², with an error margin of less than 3%. The actual impact toughness of the 920°C preheated sample tempered at 380°C was measured as 360 J/cm². SEM analysis of the fracture surface showed a mixture of dimples and small facets, indicating enhanced microstructural resistance to impact.
The test results indicate that both temperature and thermal deformation parameters have a significant impact on the impact toughness of SAE9259 spring steel. Tempering temperature is a critical control parameter that maintains a parabolic relationship with impact toughness. In the absence of thermal deformation, reducing the preheating temperature increases impact toughness due to finer austenite grain size. This leads to a larger grain boundary area, which enhances plastic deformation resistance and absorbs more energy during slip, thus improving impact toughness. However, after thermal deformation, the added dislocation stress field, solute atoms, and precipitates create a much higher resistance to dislocation motion, surpassing the strengthening effect of grain refinement. As a result, the impact toughness becomes less sensitive to preheating temperature changes.
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