Influence of heat treatment process on properties of HSS roll

1. Common components and characteristics of forged high-speed steel rolls

Before the end of the 1980s, forged high-speed steel rolls had been used to manufacture work rolls and intermediate rolls of multi-roll mills.
It is a standard type of tungsten-molybdenum high-speed steel, such as American M2, M4 and high-carbon type T15. High-speed steel rolls use high-speed steel with high hardness, especially good red hardness, wear resistance and hardenability, as the working layer of the roll, and use forged steel, cast steel, high-strength grey iron or balls that meet toughness requirements. As the core material of the roll, iron is a high-performance roll that combines the working layer and the core in a metallurgical combination.

Its main features are as follows:

In the working layer of the roll used in the past, most of the matrix is distributed with M3C or M7C3 eutectic carbides, with coarse structure and low hardness. The working layer of the high-speed steel roll is generally made of high-carbon, high-vanadium high-speed steel, and the matrix of the working layer is distributed with high-hardness M6C and MC carbides. The outer layer of the high-speed steel roll contains more elements such as tungsten, chromium, molybdenum, vanadium, etc., so it has good thermal stability and high hardness at high temperatures. It has good wear resistance when used as a hot rolling work roll.

 

heat treatment of HSS rolls

 

2. Research on heat treatment of high-speed steel rolls

The key to producing rolls, especially large rolls, is heat treatment. The key to the heat treatment of rolls is to prevent cracking under the premise of ensuring performance. Usually, the heat treatment of high-speed steel is quenching + tempering. When heated to a high temperature, the secondary carbides in the steel are fully dissolved, and the primary eutectic carbides are partially dissolved.

The carbon and alloying elements contained in these carbides dissolve into the austenite, increasing the content of carbon and alloying elements in the austenite. They are dissolved in bainite and martensite during quenching, and dispersed carbides are precipitated during tempering, making the steel exhibit a “secondary hardness” that is higher than that during quenching. Therefore, the heating during quenching of high-speed steel can increase the heating temperature as much as possible under the principle of ensuring that the grains do not grow.

Through the experiment, the Fe-1.8%C-4%Cr-6%Mo-6%V high-speed steel roll ring that has been modified by 1.0% ferrovanadium was cut into 9 samples (20x2Ox120) by wire cutting, respectively, at 1050 ° C, Quenching at three temperatures of 1100 °C and 1180 °C, after quenching, tempering at three different temperatures of 530 °C, 550 °C and 570 °C, the tempering heating and cooling speed should be slow to prevent cracking, tempering 3 to 4 times. The effect of heat treatment conditions on the amount of retained austenite and the hardness of high-speed steel was investigated.

The experiment shows:

The hardness of Fe-1.8%C-4%Cr-6%Mo-6%v high-speed steel increases with the increase of quenching temperature, and its change law is to increase first and then decrease, and the hardness reaches the peak when quenching at about 1100 °C.

The quenching temperature exceeds 1100°C, and as the quenching temperature increases, the hardness decreases instead.

Because the hardness value after quenching is not only related to the structural factors of the alloy but also determined by the saturated carbon and alloy element content in the martensite and the untransformed retained austenite.

2.1 Effect of heat treatment process on impact toughness and hardness

When the high-speed steel is treated with an appropriate heat treatment process, the impact toughness can be significantly improved by increasing the heat treatment temperature.

Raise the quenching temperature and keep warm at a higher temperature quenching temperature, which is beneficial to the diffusion of alloy elements and the dissolution of carbides, which can be dispersed and precipitated during tempering, reducing the uneven distribution of carbides and the splitting effect on the matrix;

In addition, the tempering characteristics of general high-speed steel are: the plasticity tends to decrease at the tempering temperature at which the hardness and strength peak, increasing the tempering temperature, due to the decomposition of martensite and the accumulation of alloy carbides, will make the hardness, The strength decreases but the plasticity increases.

The tempering at 530°C used in this experiment is exactly the temperature range where the hardness peak appears, so it may also be a temperature with low plasticity and toughness. The tempering at a higher temperature of 570°C improves the hardness of the high-speed steel with a small loss of hardness. toughness. Therefore, increasing the quenching temperature and tempering temperature can improve the impact of toughness.

 

2.2 Effect of heat treatment process on the red hardness

Under the premise of ensuring that the austenite grains do not grow, increasing the quenching temperature can effectively improve the hardness and red hardness after tempering, and may make the red hardness higher than other high-speed steels.

After quenching, adding a low-temperature tempering at 380°C before conventional tempering can improve the hardness and red hardness after tempering.

The high-speed steel was air-quenched at the peak hardness and tempered at 570°C. The test results showed that with the increase in tempering temperature, the hardness decreased, and the decrease in hardness became smaller with the increase of vanadium and carbon content. It is not difficult to understand that carbon, content of vanadium is high, and the number of carbides is large, so the supporting force at high temperatures is large, and the hardness at high temperatures decreases slowly.

Some data show that the hardness of high-speed steel after quenching after modification decreases continuously with the increase of quenching temperature due to the increase of retained austenite. After tempering, the hardness increases with the increase of quenching temperature. When the quenching temperature is 1080°C, the hardness can reach above 66HRC. This is because the high-speed steel used in the test has a high equilibrium carbon content and a low carbon saturation concentration. Considering the austenite grain size, the quenching temperature of the steel is 1080°C.

For example, after quenching and before conventional tempering, a low-temperature tempering at 380°C can be performed first, and the hardness and red hardness after tempering can be improved because the low-temperature tempering at 380°C can promote subsequent conventional tempering. The precipitated carbides are more dispersed, so the hardness and red hardness after tempering are improved.

 

2.3 Effect of heat treatment process on wear resistance

Tests have shown that the hardness of high-carbon and high-vanadium high-speed steel with various components is relatively high, and its wear resistance is better than that of high-chromium cast iron. In high-carbon and high-vanadium high-speed steel, alloys with dispersed MC carbides ( See Figure 3) the wear resistance is obviously better than alloys with other components, and the wear resistance is better when the vanadium content is 8%.

It is also reflected from the worn surface that the wear marks of 2M028V high-speed steel are shallow and thin, and the ploughing is not obvious, so the wear resistance is the highest. The main reason is that there are a large number of fine and dispersed high-hardness carbides in its structure, which can effectively protect the matrix, prevent the cutting of abrasive particles, reduce the cutting depth, and reduce fatigue shedding.

 

3. Conclusion

The roll is made of high-speed steel, and the heat treatment temperature for obtaining the best structure of the tungsten-free high-carbon Fe-1.8%C-4%Cr-6%Mo-6%V high-speed steel roll ring is 1100°C quenching + 550°C Temper.

As long as the quenching temperature is higher than 1050°C, the hardness of high-speed steel after quenching + reheating treatment is quite high, and a relative peak appears at around 1050°C. With the increase of quenching temperature over 1050 °C, the hardness value decreases.

Article source: “Technology Innovation Herald”

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