01 The main factors affecting the mechanical properties of bainite
02 Strength and hardness of bainite
03 Toughness of bainite
一、The main factors affecting the mechanical properties of bainite
01 Influence of ferrite in bainite
The strength of bainite and the grain size of ferrite in bainite conform to the Hall-Petch formula, that is, the smaller the ferrite grains (or subgrains) in bainite, the higher the strength of bainite, and Toughness is sometimes improved.
The grain size of ferrite in bainite mainly depends on the austenite grain size (which affects the length of the ferrite strips) and the formation temperature (which affects the thickness of the ferrite strips), but the latter is predominant. The lower the bainite formation temperature, the smaller the overall size of the bainitic ferrite grains, and the higher the strength and hardness of the bainite.
Bainitic ferrite tends to have a slightly higher carbon content than equilibrium ferrite, but generally less than 0.25%. The supersaturation of bainitic ferrite is mainly affected by the formation temperature. The lower the formation temperature, the greater the supersaturation of carbon, and its strength and hardness increase, but the toughness and plasticity decrease less.
The substructure of bainitic ferrite is mainly entanglement dislocations. As the phase transition temperature decreases, the dislocation density increases, and the strength and toughness increase. As the substructure size of bainitic ferrite decreases, strength and toughness also increase.
02 Influence of cementite in bainite
According to the mechanism of dispersion strengthening, the smaller the size of carbide particles and the greater the number, the greater the contribution to the strength.
With the same size of cementite, the more cementite in bainite, the higher the hardness and strength, and the lower the toughness and plasticity.
The amount of cementite mainly depends on the carbon content in the steel. Cementite in bainite can be flake, granular, interrupted rod or layered. Generally speaking, the toughness of bainite is higher when the cementite is granular, the strength is higher when the cementite is in the form of fine flakes, and the brittleness is higher when it is discontinuous rod or layered.
When the composition of the steel is constant, with the decrease of the transformation temperature, the size of the cementite decreases and the number increases, and the shape of the cementite also changes from intermittent rod-like or layered to fine flakes, and the hardness and strength increase, but the toughness is increased. and less reduction in plasticity. With the extension of the isothermal time or the tempering at a higher temperature, the cementite will be transformed into granular.
Generally, when the cementite is uniformly dispersed in the same direction, the strength is higher and the toughness is better. If the cementite orientation is not uniformly distributed, the strength is lower and the brittleness is higher. In the upper bainite, the cementite is easily oriented and distributed unevenly, and the particles are relatively coarse, while in the lower bainite, the cementite distribution is relatively uniform, and the particles are smaller, so the strength and toughness of the upper bainite are higher than those of the lower bainite. Bainite is much lower.
03 Influence of other factors
Due to the different austenitizing temperatures, the chemical composition and grain size of austenite will change, which will also affect the properties of bainite.
In addition, due to the incompleteness of bainitic transformation, non-bainite structures such as retained austenite, pearlite and martensite (tempered martensite) appear in bainitic ferrite conditions, which will also affect properties of bainite.
二、Strength and hardness of bainite
According to the above analysis, it can be concluded that the strength and hardness of bainite increase as the transformation temperature decreases. The yield strength of bainite can be expressed by the following empirical formula:
In the formula, d (mm) is the grain size of ferrite in bainite; n is the number of carbide particles per square millimeter of section.
Equation 1 is only applicable to the distribution state of finely dispersed carbides, and carbide dispersion becomes an effective strengthening factor only when the carbide spacing is smaller than the thickness dimension of strip ferrite in bainite.
Therefore, the strength of bainite on low carbon is actually completely controlled by the grain size of bainitic ferrite. Only in the lower bainite or high-carbon upper bainite, the dispersion strengthening of carbides has a more obvious contribution.
In addition, since the lower bainite structure of medium and high carbon steel, especially high carbon steel, has high strength and toughness, it is expected to have high wear resistance. Tests show that lower bainite in steel is one of the most wear-resistant microstructures.
三、Toughness of bainite
Toughness is an important performance index of high-strength materials. In low carbon steel, the impact resistance of the upper bainite is lower than that of the lower bainite, and the brittle transition temperature suddenly decreases when the bainite structure transitions from the upper bainite to the lower bainite. The reasons may be:
In the upper bainite, there are coarse carbide particles or intermittent strip carbides, and there may also be high-carbon martensite (formed from untransformed austenite during cooling), so it is easy to form cracks larger than the critical size, And once the crack propagates, it cannot be stopped by the small-angle grain boundaries between the ferrites in the bainite, but only by the large-angle bainite “bundle” boundary or the original austenite grain boundary. Therefore, cracks spread rapidly in the upper bainite structure.
When many medium carbon alloy steels are isothermally treated to obtain the upper bainite structure, their impact toughness decreases sharply, which is called “bainite brittleness”.
The reason for this is due to the uneven distribution of carbides between the ferrite bars in the upper bainite. In addition, the macroscopic hardness of the steel increases in the transformation temperature range where bainitic brittleness occurs, indicating that this brittleness is also not completely transformed with the supercooled austenite in this temperature range, and partially transformed into martensitic in the subsequent cooling process body related.
In the lower bainite structure, small carbide particles are not easy to form cracks, and even if cracks are formed, it is difficult to reach the critical size, and even if cleavage cracks are formed, their propagation will be blocked by a large number of dispersed carbide particles and dislocations.
Therefore, cracks are not easily propagated even after they are formed, and new cracks are often suppressed and new cracks must be formed so that the brittle transition temperature is lowered. Therefore, although the lower bainite structure has higher strength, its impact toughness is much higher than that of the lower bainite structure.
For steels with temper brittleness, compared with the bainite obtained by isothermal quenching and the martensite obtained by tempering after quenching, if tempered within the tempering brittleness temperature range, when the temperature or strength is the same, the bainite structure The impact toughness is higher than that of tempered martensite; ①When the isothermal quenching temperature is low and the lower bainite structure is obtained, it can maintain a high impact toughness, which is better than the quenching and tempering treatment; ②When the isothermal quenching temperature is high, the When the bainite structure is added, not only the strength but also the impact toughness is significantly reduced, even lower than the quenching and tempering treatment. Therefore, only when the lower bainite and retained austenite structure is obtained during austempering, the steel can have higher impact toughness and lower brittle transition temperature.
If the carbon content or alloying element content of the steel is high, the Ms point is low, and when twinned martensite is obtained after quenching, the lower bainite structure obtained by isothermal quenching is often higher than that obtained by low-temperature tempering after quenching. impact toughness.
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