Effect of alloying elements on tempering transformation
In actual production, we can generally find some phenomena, such as:
① For carburized parts and carbonitriding parts, in order to achieve the same hardness (such as 480-610HV5) after quenching, why do carbonitriding parts need higher tempering temperature?
② Compared with 42CrMo, 45 steel requires 28-32HRC hardness.
Why is the tempering temperature of 42CrMo higher?
③ Why does the hardness of high-speed steel (such as SKH-9, W6Mo5Cr4V2) increase instead of decrease after conventional high-temperature tempering?
The above phenomenon is the “influence of alloy elements on the tempering transformation of metal parts” that will be introduced in this article.
Please see the following for details.
The article is a little longer, and please enjoy it.
2. Effect of alloy elements on martensite decomposition
The decomposition process of martensite in alloy steel is basically similar to that of carbon steel, but its decomposition rate is obviously different.
Experiments show that the influence of alloy elements is very significant in the stage of martensite decomposition, especially in the late stage of martensite decomposition.
The reasons and laws of alloy elements affecting martensite decomposition can be roughly summarized as follows.
1. During the decomposition stage of martensite, the desolvation of supersaturated carbon in martensite and the precipitation and aggregation of carbide particles will occur, and the carbon content in matrix phase α will decrease.
The role of alloy elements is mainly to affect the decomposition process of martensite and the aggregation and growth rate of carbide particles by affecting the diffusion of carbon, so as to affect the decline rate of carbon concentration in phase α.
The magnitude of this effect varies with the size of the binding force between alloy elements and carbon.
2. The binding force between non carbide forming element (Ni) and weak carbide forming element (Mn) and C is similar to that of Fe, so it has no obvious effect on martensite decomposition.
The strong carbide forming elements (Cr, Mo, W, V, Ti, etc.) have a strong binding force with C, which increases the diffusion activation energy of C in martensite, hinders the diffusion of C in martensite, and slows down the decomposition rate of martensite.
The non carbide forming elements Si and CO can be dissolved into ε-FexC to stabilize ε-FexC and slow down the aggregation rate of carbides, thus delaying the decomposition of martensite.
The complete desolvation temperature of supersaturated carbon in martensite during tempering of carbon steel is about 300 ℃, and the complete desolvation temperature can be shifted by 100-150 ℃ to high temperature by adding alloy elements.
In other words, alloy steel can still maintain a certain saturated carbon concentration and fine carbides in phase α when tempered at a higher temperature, so as to maintain high hardness and strength.
Alloy elements, which prevent the reduction of carbon content in phase α and the growth of carbide particles and maintain the high hardness and high strength of steel parts, are called alloy elements that improve the tempering resistance or “backfire resistance” of steel.
3. Effect of alloy elements on Transformation of retained austenite
The transformation of retained austenite in alloy steel is basically similar to that of carbon steel, except that alloy elements can change the decomposition temperature and speed of retained austenite, which may affect the type and nature of retained austenite transformation.
When tempering below MS point, residual austenite will be transformed into martensite.
If Ms point is high (> 100 ℃), then the decomposition process of martensite will also occur, forming tempered martensite.
When tempering above Ms point, retained austenite may undergo three transformations:
① Isothermal transformation to bainite in the bainite formation zone;
② Isothermal transformation to pearlite in the pearlite formation zone;
③ It does not decompose during tempering heating and holding, but turns into martensite in the subsequent cooling process, which is the so-called “secondary quenching” phenomenon.
Note: thinking ① is the secondary quenching theory applied to the multiple tempering process of high speed steel?
4. Effect of alloy elements on carbide transformation
Non carbide forming elements (Cu, Ni, Co, Al, Si, etc.) and carbon do not form special types of carbides.
They only improve the transformation from ε-FexC to θ-Fe3C, but also the transformation from cementite to other types of special carbides.
When alloy steel is tempered, the redistribution of alloy elements between cementite and phase α will occur with the increase of tempering temperature or tempering time.
Carbide forming elements continue to diffuse into cementite, while non carbide forming elements gradually enrich into phase α, so that more stable carbides gradually replace the original unstable carbides, so that the composition and structure of carbides change.
The possible sequence of carbide transformation during tempering of alloy steel is:
ε- Carbide (< 150 ℃) → cementite (150-400 ℃) → cementite (alloying, 400-550 ℃) → special carbide (metastable) → special carbide (stable > 500 ℃)
Whether special carbides can be formed in steel depends on the properties and content of the alloy elements, the content of carbon or nitrogen, and the tempering temperature and time.
In the tempering process of alloy steel, cementite is usually transformed into stable special carbides through metastable carbides.
For example, after quenching of high Cr high carbon steel, the carbide transformation process during tempering is:
Special carbides are also formed by these two mechanisms.
One is in-situ transformation, that is, carbide forming elements are first enriched in cementite. When their concentration exceeds the solubility limit of alloy cementite, the lattice of cementite reorganizes into a special carbide lattice.
The transformation from (Fe, Cr) 3C to (Cr, Fe) 7C3 in low chromium steel belongs to this type.
Increasing the tempering temperature will accelerate the carbide transformation process.
The other is nucleation and growth alone, that is, special carbides are precipitated directly from phase α, accompanied by the dissolution of alloy cementite.
Steels containing carbide forming elements V, Ti, Nb, Ta, etc. and high Cr steels belong to this type.
For example, 0.3% C, 2.1% V steel quenched at 1250 ℃ precipitates alloy cementite when tempered below 500 ℃, in which V content is very low.
Because solid solution V strongly prevents the continuous decomposition of phase α, only about 40% of carbon precipitates in the form of cementite, and the remaining 60% is still retained in phase α.
When the tempering temperature is higher than 500 ℃, VC is directly precipitated from phase α.
With the further increase of tempering temperature, a large amount of VC precipitates and cementite dissolves.
When the tempering temperature reaches 700 ℃, all cementites are dissolved and all carbides are converted to VC.
5. Secondary hardening during tempering
In the third stage of tempering, carbon steel will continue to soften with the growth of cementite particles, as shown in Fig. 1.
Fig. 1 hardness change of low and medium carbon steel tempered at 100-700 ℃ for 1h
However, when the steel contains strong carbide forming elements such as Mo, V, W, Ta, Nb and Ti, the softening tendency will be weakened, that is, the softening resistance will be increased.
When martensite contains enough carbide forming elements, fine special carbides will be precipitated when tempering above 500 ℃, resulting in re hardening of steel coarsened by the coarsening of θ-carbides due to the increase of tempering temperature. This phenomenon is called secondary hardening.
Sometimes the hardness of secondary hardening peak may be higher than that of quenching.
Fig. 2 Effect of tempering temperature on martensite hardness of low carbon molybdenum steel
Fig. 2 shows the effect of molybdenum content on the secondary hardening effect of low carbon (0.1%c) molybdenum steel.
It can be seen that the secondary hardening effect intensifies with the increase of Mo content.
Other strong carbide forming elements (such as Ti, V, W, Nb, etc.) also have similar effects.
When the Cr content is very high (such as more than 12%), there is a less obvious secondary hardening peak.
Secondary hardening does not occur in carbon steel.
Electron microscope observation confirmed that the secondary hardening was caused by the precipitation of dispersed and fine special carbides (such as Mo2C, W2C, VC, TiC, NbC, etc.).
Special carbides with secondary hardening precipitate in the dislocation zone, often in the form of very fine needle or sheet, with small size, and maintain a coherent relationship with phase α.
With the increase of tempering temperature, the number of carbides increases, the size of carbides gradually increases, and the lattice distortion with phase α gradually intensifies until the hardness reaches the peak.
As the carbide grows, the dispersion decreases, the coherent relationship is destroyed, the coherent distortion disappears, and the dislocation density decreases as the temperature continues to rise, thus the hardness decreases rapidly.
The secondary hardening effect of steel can be improved by the following ways:
1. Increase the dislocation density in steel to increase the nucleation site of special carbides, so as to further increase the dispersion of carbides.
As shown in the figure, low temperature deformation quenching method is adopted.
2. Some alloy elements are added to the steel to slow down the diffusion of special carbide forming elements, inhibit the growth of fine carbides and delay the occurrence of over aging of such carbides.
For example, the addition of CO, Al, Si, Nb, Ta and other elements to steel can make special carbides fine dispersion and maintain coherent distortion with phase α, thereby increasing the tempering stability of steel.
Using the secondary hardening effect, the alloy steel with secondary hardening can be selected to make the workpiece working in the hot state.
As long as the use temperature is lower than the tempering temperature (the temperature that produces the secondary hardening peak), the steel parts can maintain high hardness and strength.
6. Effect of alloy elements on the recovery and recrystallization of phase α
When alloy steel is tempered at high temperature, if it can form special carbides with fine particles and maintain a coherent relationship with phase α, it can maintain a high carbon supersaturation of phase α and significantly delay the recovery and recrystallization of phase α. Therefore, α gets along in a large distortion state and still maintains a high hardness and strength, that is, it has high tempering stability.
In alloy steel, commonly used alloy elements (such as Mo, W, Ti, V, Cr, Si, etc.) can hinder the elimination of various distortions during tempering, and generally delay the recovery and recrystallization of phase α(increase the recrystallization temperature) and the aggregation and growth process of carbides, so as to improve the tempering stability of steel.
With the increase of the content of alloying elements, this delaying effect is enhanced.
When several alloying elements are added to the steel at the same time, the interaction is intensified.
Alloy steel has high tempering stability and maintains high hardness and strength at higher temperatures, making the steel have red hardness and thermal strength, which is very important for tool steels such as chip cutters and hot working dies.
This article shares with you five factors that affect the tempering transformation of alloy elements. I believe you have more inspiration after reading it.