1. Preface
Effect of alloying elements on tempering transformation
In actual production, we can generally encounter some phenomena, such as:
- To achieve the same hardness (such as 480-610HV5) after quenching, why do carbonitrided parts require a higher tempering temperature compared to carburized parts?
- Although 45 steel requires a hardness of 28-32HRC, why does 42CrMo need a higher tempering temperature?
- Why does the hardness of high-speed steel such as SKH-9 and W6Mo5Cr4V2 increase instead of decreasing after conventional high-temperature tempering? This phenomenon is attributed to the influence of alloying elements on the tempering transformation of metal parts, which will be discussed in this article.
This article provides an in-depth analysis of the topic and we hope you enjoy reading it.
2. Effect of alloying elements on martensite decomposition
The decomposition process of martensite in alloy steel is basically similar to that of carbon steel, but the decomposition rate differs significantly.
Experiments have demonstrated that the impact of alloying elements is particularly significant during the last stages of martensite decomposition.
The reasons and laws of alloying elements that affect the decomposition of martensite can be roughly summarized as follows.
1. During the decomposition phase of martensite, the supersaturated carbon in martensite undergoes desolvation, causing precipitation and aggregation of carbide particles, resulting in a decrease in the carbon content in the α matrix phase.
The role of alloying elements is mainly to influence the decomposition process of martensite, the aggregation and growth rate of carbide particles, and the diffusion of carbon. This, in turn, affects the rate of decline of carbon concentration in the α phase.
The extent of this effect varies depending on the strength of the bonding force between the alloying elements and the carbon.
2. Non-carbide-forming elements (such as Ni and Mn) have a carbon binding strength similar to that of Fe and therefore have no significant effect on the decomposition of martensite.
Strong carbide-forming elements (such as Cr, Mo, W, V, Ti, etc.) have a strong binding force with carbon, which increases the activation energy of carbon diffusion into martensite, making its diffusion difficult and delaying the decomposition rate of martensite.
Non-carbide-forming elements such as Si and CO can dissolve in ε-FexC to stabilize it and decrease the rate of carbide aggregation, thereby delaying martensite decomposition.
The complete desolvation temperature of supersaturated carbon in martensite during tempering of carbon steel is about 300 ℃. The addition of alloying elements can change the complete desolvation temperature by 100-150℃ to a higher temperature.
In other words, alloy steel can maintain a certain concentration of saturated carbon and fine carbides in the α phase even when tempered at a higher temperature, thus maintaining high hardness and strength.
Alloying elements that prevent the reduction of carbon content in the α phase and the growth of carbide particles, and maintain the high hardness and strength of steel parts, are known as alloying elements that improve tempering resistance or “strength”. to the backfire” of steel.
3. Effect of alloying elements on the transformation of retained austenite
The transformation of retained austenite to alloy steel is similar to that of carbon steel, but alloying elements can affect the temperature and rate of decomposition of retained austenite, which can change the type and nature of the transformation.
When tempering below the MS point, the residual austenite transforms into martensite.
If the MS point is high (>100 ℃), the martensite decomposition process occurs, forming tempered martensite.
When tempering above the MS point, the retained austenite can undergo three transformations:
① Isothermal transformation to bainite in the bainite formation zone;
② Isothermal transformation into 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 called “secondary quenching”.
Note: Is point ① related to secondary quenching theory applied to the high-speed steel multiple tempering process?
4. Effect of alloying elements on the transformation of carbide
Non-carbide forming elements such as Cu, Ni, Co, Al, Si, etc., and carbon do not form any single type of carbide. However, they improve the transformation of ε-FexC to θ-Fe3C, as well as the conversion of cementite to other types of special carbides.
During tempering of alloy steel, there is a redistribution of alloy elements between the cementite and the α phase with increasing tempering time or temperature. The carbide-forming elements continue to diffuse into the cementite, while the non-carbide-forming elements gradually enrich in the α phase. This results in more stable carbides replacing the original unstable carbides, causing changes in the composition and structure of the carbides.
The possible sequence of carbide transformation during tempering of alloy steel is: ε-carbide (<150 ℃) → cementite (150-400 ℃) → cementite (alloy, 400-550 ℃) → metastable special carbide → cemented carbide special stable (>500 ℃). The possibility of forming special carbides in steel depends on the properties and content of the alloying elements, the carbon or nitrogen content, and the tempering temperature and time.
Typically, during the tempering process of steel alloys, cementite is transformed into special stable carbides through metastable carbides.
For example, after quenching high-Cr, high-carbon steel, the carbide transformation process during tempering is:
（Fe,Cr） 3 C→((Fe,Cr) 3 C）+（Cr,Fe） 7 C 3 →（Cr,Fe） 7 C 3 +(Cr,Fe) 23 C 6 →(Cr,Fe) 23 C 6
Special carbides are also formed by these two mechanisms.
There are two types of carbide transformation processes. The first is the in situ transformation, where the carbide-forming elements are initially enriched in cementite. When its concentration exceeds the solubility limit of the cementite alloy, the cementite network reorganizes into a single carbide network. An example of this type is the transformation of (Fe, Cr) 3C into (Cr, Fe) 7C3 in low chromium steel. Increasing the tempering temperature accelerates the carbide transformation process.
The second type is just nucleation and growth, where special carbides are precipitated directly from the α phase, accompanied by the dissolution of the cementite alloy. Steels containing carbide-forming elements such as V, Ti, Nb, Ta and steels with high Cr content belong to this type.
For example, steel with 0.3% C and 2.1% V quenched at 1250 ℃ precipitates cementite alloy when tempered below 500 ℃ despite the low V content. As solid solution V strongly inhibits continuous decomposition of the α phase, only about 40% of the carbon precipitates in the form of cementite, and the remaining 60% is still retained in the α phase.
When the tempering temperature exceeds 500 ℃, VC is directly precipitated from the α phase. With further increases in tempering temperature, a significant amount of VC precipitates and the cementite dissolves. At 700℃, all cementites dissolve and all carbides are converted to VC.
5. Secondary hardening during tempering
In the third stage of tempering, the carbon steel will continue to soften with the growth of cementite particles, as shown in Figure 1.
Fig. 1 Hardness change of low and medium carbon steel quenched at 100-700 ℃ for 1h
However, if the steel contains strong carbide-forming elements such as Mo, V, W, Ta, Nb and Ti, the softening tendency will be weakened, resulting in greater softening resistance.
When martensite contains sufficient carbide-forming elements, fine special carbides precipitate during tempering above 500℃, causing hardening of steel that has been thickened due to the increase in tempering temperature and the hardening of θ carbides. This phenomenon is known as secondary hardening.
In some cases, the hardness of the secondary hardening peak may be higher than that of quenching.
Fig. 2 Effect of tempering temperature on martensite hardness of low-carbon molybdenum steel
Figure 2 shows the effect of molybdenum content on the secondary hardening effect of low-carbon molybdenum steel (0.1%c).
The intensity of the secondary hardening effect increases with the Mo content.
Similar effects are observed with other strong carbide forming elements such as Ti, V, W, Nb, etc.
A less distinct secondary hardening peak is observed when the Cr content is very high (more than 12%).
Carbon steel does not undergo secondary hardening.
Electron microscope observations confirmed that secondary hardening is caused by the precipitation of dispersed and fine special carbides such as Mo2C, W2C, VC, TiC, NbC, etc.
These special carbides precipitate in the dislocation zone, often in the form of very thin, small needles or sheets, and maintain a coherent relationship with the α phase.
As the tempering temperature increases, the number and size of carbides gradually increase, and the distortion of the α-phase lattice intensifies until the hardness reaches its 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, leading to a rapid decrease in hardness.
The secondary hardening effect of steel can be improved in the following ways:
- To increase the nucleation site of special carbides and improve their dispersion, the dislocation density in steel can be increased. This can be achieved through the low temperature deformation quenching method as shown in the figure.
- Some alloying elements may be added to steel to retard the diffusion of special carbide-forming elements, inhibit the growth of fine carbides, and retard the occurrence of excessive aging of such carbides.
- For example, the addition of CO, Al, Si, Nb, Ta and other elements can help maintain distortion consistent with the α phase and achieve a fine dispersion of special carbides, thereby increasing the tempering stability of the steel.
Alloy steel with secondary hardening effect can be selected to make the part work well in hot state. As long as the temperature used is lower than the tempering temperature (the temperature that produces peak secondary hardening), steel parts can maintain high hardness and strength.
6. Effect of alloying elements on the recovery and recrystallization of the α phase
When alloy steel is tempered at high temperatures, it can form special carbides with fine particles that maintain a coherent relationship with the α phase. This allows the steel to maintain a high carbon supersaturation in the α phase and significantly delay its recovery and recrystallization. As a result, the α phase remains in a highly distorted state, maintaining its high hardness and strength, leading to high tempering stability.
Alloying elements commonly used in alloy steel, such as Mo, W, Ti, V, Cr, Si, etc., can make it difficult to eliminate various distortions during tempering. They generally delay the recovery and recrystallization of the α phase (increase the recrystallization temperature), as well as the process of aggregation and growth of carbides, which helps to improve the tempering stability of the steel.
The retardant effect of alloying elements is enhanced with an increase in their content in the steel.
When several alloying elements are added to steel simultaneously, the interaction between them is intensified.
Alloy steel has high tempering stability and maintains its high hardness and strength even at higher temperatures. This makes it suitable for tool steels such as chip cutters and hot work dies, which require red hardness and heat resistance.
7. Conclusion
This article discusses five factors that can influence the tempering transformation of alloying elements. I believe that after reading it, you will have gained valuable insights and inspiration.
