Research on the thermal and cold fatigue resistance of high-speed steel rolls

Roll consumption accounts for a large proportion of rolling costs. In order to ensure continuous rolling, a large number of roll spare parts must be prepared and replaced regularly. The rolls after being removed from the machine are ground to the required roll shape before being used on the machine. During the rolling process, the temperature of the hot roll will exceed 500°C, and the roll surface is subject to periodic hot and cold load impacts. As the rolling time is prolonged, material fatigue on the roll surface is inevitable.

There are two main types of surface cracks on hot rolls: thermal cracks and mechanical cracks. The depth of thermal cracks on the surface of the hot roll of the finishing rolling stand gradually decreases from the front end to the rear end of the stand and is basically unobservable at the rear end of the stand. The thermal cracks develop perpendicular to the roll surface and develop inward. Generally speaking, hot cracks will cause Peeling, spotting and changing the friction state of the roller surface. Mechanical cracks can rapidly expand in the opposite direction of rolling, often causing major rolling accidents. Therefore, once mechanical cracks occur, they must be completely removed.

Research shows that roll wear caused by thermal fatigue is much more serious than traditional wear (such as friction wear and mechanical wear). In many cases, the replacement of rolls is due to the deterioration of roll surface quality or roll surface peeling during the rolling process. Even the rolls break, so the thermal fatigue problem of hot rolling rolls has received widespread attention, and high-speed steel rolls represent the development direction of hot rolling roll materials. Therefore, it is of great practical significance to carry out research on the thermal fatigue resistance of hot-rolled high-speed steel rolls.

 

1. Test method

 

The induction heating method is usually used to study the thermal and cold fatigue resistance of rolls. This method is suitable for studying the roll’s ability to resist macro-crack expansion, especially in comparative tests. It is also suitable for evaluating the accident resistance of hot rolls, such as the thermal shock resistance of hot roll materials in the case of steel jamming and tail flicking.

There are certain limitations in using a medium-frequency thermal fatigue testing machine to study the thermal fatigue resistance of hot roll materials. The main reasons are low power frequency and too deep thermal penetration.

The thermal simulation testing machine uses a high-frequency power supply and the current only acts on the surface of the sample. The power supply and heating speed are accurately controlled. It is of more reference significance to use it to simulate and study the thermal and cold fatigue resistance performance of the roll under actual rolling conditions.

In this study, a medium frequency induction testing machine with a frequency of 2.8kHZ was used to comparatively study the cold and thermal fatigue resistance of five types of high-speed steel samples (referred to as samples 1-5), and to study the heating temperature, power frequency and material microstructure. Effect of structure on crack depth and shape. The initiation mechanism of thermal fatigue cracks under actual rolling conditions was simulated and studied on a thermal simulation testing machine. The scratch resistance of the material was tested on a submicron scratch meter.

 

2. Test results

 

2.1 Metallographic Structure Characterization

 

cold fatigue resistance of high-speed steel rolls

 

It can be seen from the metallographic structure of high-speed steel that the carbides in samples 1, 3, and 4 are relatively uniform and diffuse, while the carbides in samples 2 and 5 have an obvious network distribution.

 

2.2 Hot and cold fatigue test

 

The performance morphology of high-speed steel after 10 thermal cycles at 500°C on a thermal fatigue testing machine with a frequency of 8 kHz is shown in the figure below.

 

fatigue resistance, high-speed steel rolls

 

It can be seen from the figure that after 10 times cold and hot cycle load impact, cracks are generated on the surface of the five types of high-speed steel. However, due to differences in the material microstructure, the degree of crack generation is different. Among them, the cracks in tests 2 and 5 are larger than those in samples 1 and 3. ,4 the cracks are serious and the cracks have a reticular trend.

Taking sample 4 as an example, the following test results will be explained. The surface morphology of sample 4 after 5 thermal cycles at 500°C on a hot and cold fatigue testing machine with a frequency of 2 kHz shows that after only 5 hot and cold cycle load impacts, the surface thermal cracks on the surface of sample 4 were reticular and there were obvious cracks.

It can be seen from the cross-sectional morphology of sample 4 after five thermal cycles at 500°C on a thermal fatigue testing machine with a frequency of 2kHz.

1) The thermal cracks formed on the surface of the sample develop very rapidly in the radial direction, and the crack depth reaches about 6mm.

2) It can be seen from the partially enlarged SEM photo of the crack that the hot crack starts at the interface between the surface matrix and the carbide, and develops inward along the boundary between the matrix and the carbide.

3) During the propagation process, cracks not only develop inward along the boundary between the matrix and carbides but also develop transgranularly at local carbide breaks.

It can be seen from the cross-sectional morphology of sample 4 after 50 thermal cycles at 600°C on a thermal simulation testing machine with a frequency of 100kHz.

Cracks mainly exist at the interface between the surface matrix and carbides. There are some cracks on locally larger carbides, but they are not serious. No cracks in the matrix have been observed.

It can be seen from the cross-sectional morphology of sample 4 after 50 thermal cycles at 700°C on a thermal simulation testing machine with a frequency of 100kHz.

Cracks mainly exist on larger carbides. There are no obvious cracks at the interface between the matrix and carbides, and no cracks within the matrix are observed.

 

2.3 Scratch test

 

It can be seen from the scratch morphology of sample 4 without thermal fatigue treatment and after thermal cycling at 600°C 50 times and 700°C 50 times on a thermal simulation testing machine with a frequency of 100kHz.

The scratch morphology of the sample without thermal fatigue treatment and the sample after 50 thermal cycles at 600°C are basically similar. During the scratching process, the carbide is pressed into the matrix by the indenter from beginning to end, and there is no material peeling off during the entire scratching process. In the initial scratching process of the sample after 50 thermal cycles at 700°C, the carbide was pressed into the matrix, but in the later stage, local material peeling off was obvious at the edge of the indenter.

 

3. Analysis of test results

 

The phase transformation point of the high-speed steel used in this study is around 750°C, and the maximum test temperature is 700°C, so the test is below the phase transformation temperature and there is no phase transformation stress. Cyclic alternating thermal stress is the main stress that causes material surface fatigue. Under the impact of periodic thermal loads, the material surface repeatedly bears tensile and compressive stress. When the tensile stress exceeds the strength limit of the material, thermal cracks will occur. As time goes by, thermal cracks will expand along a certain path, causing material fatigue and failure.

On the medium-frequency thermal fatigue testing machine, high-speed steel rolls have very high crack sensitivity. Under the impact of cyclic high-temperature thermal loads, hot cracks quickly occur on the surface of the sample, and the hot cracks rapidly expand inward along the radial direction of the sample. Such cracks Once formed, it is very detrimental to the use of high-speed steel rolls. If it forms on the roll surface and is not removed in time, it will cause local peeling of the roll surface and even breakage of the roll under the combined action of other stresses. Through simulation observations on a medium-frequency thermal fatigue testing machine, it was found that during abnormal rolling processes (such as steel jamming and tail flicking, etc.), the thermal shock caused by high-temperature strips to the high-speed steel roll surface is very serious, which shows that the high-speed steel roll resistance Thermal shock performance with large penetration depth is poor.

In addition, comparative tests of five types of high-speed steel show that differences in material microstructure are the main reason for large differences in thermal shock resistance. The carbides in samples 2 and 5 exist in a network shape, while the carbides in samples 1, 3, and 4 are relatively dispersed and evenly distributed, and the cracks are relatively light, that is, the thermal cracks in samples 2 and 5 are significantly smaller than those in samples 2 and 5. 1,3,4 have serious thermal cracks.

Under the impact of alternating cold and hot loads, the sudden change in the thermal expansion coefficient at the interface between the matrix and the carbide causes the carbide to be repeatedly stretched and compressed. When the tensile stress exceeds the wrapping force of the matrix on the carbide, cracks will preferentially develop between the matrix and the carbide. The carbide junction is formed and gradually expands inward along the direction of heat transfer. When it reaches a certain position where no carbide exists, the crack growth will be temporarily blocked. However, as the number of hot and cold fatigue increases, the stress concentration at the end of the crack continues to accumulate. When the stress exceeds the strength of the matrix, the crack will expand within the matrix, showing transgranular development. Therefore, once a thermal fatigue crack is formed, it will develop rapidly along the thermal penetration direction. In addition to mainly expanding along the interface between the matrix and carbide, it will also develop locally through the grain.

Compared with traditional high-chromium cast iron rolls and infinitely chilled cast iron rolls, the working layer of high-speed steel rolls allows a larger residual compressive stress of 200~300MPa, which is beneficial to prevent the expansion of cracks on the roll surface but requires the roll body to have a relatively high residual compressive stress. Large cross-section and high strength and toughness to withstand the residual tensile stress in the core.

After specimen 4 was thermally cycled at 600°C 50 times on a thermal simulation testing machine, the cracks mainly existed at the interface between the shallow surface matrix and the carbides, and there were some cracks on the local larger carbides, but they were not serious. It can be seen that coarse carbides are detrimental to improving the hot cracking resistance of high-speed steel rolls. The most obvious difference is that no cracks are observed in the matrix. This is mainly due to the difference in test conditions. The thermal simulation testing machine uses a 100kHz high-frequency power supply, while the thermal and cold fatigue testing machine uses a medium-frequency power supply (2, 8kHz), heating below the Curie point temperature. Theoretically, the current penetration depth of the high-frequency power supply is only about 3/10 of that of the medium-frequency induction power supply. The sample is quickly heated and cooled only on the surface layer, which inhibits the cracks from extending to the inside. Therefore, controlling the heat penetration depth is one of the key links to prevent crack expansion on the high-speed steel roll surface.

After sample 4 was thermally cycled at 700°C 50 times on a thermal simulation testing machine, the cracks mainly existed on the carbides. There were no obvious cracks at the interface between the matrix and the carbides, and a large number of fine and dispersed carbides precipitated in the matrix. This is mainly because the temperature is too high and reaches the carbide precipitation temperature, which leads to the precipitation of carbides in the matrix, thereby softening the matrix, increasing elastic-plasticity, and making the matrix more capable of deforming. Therefore, no obvious cracks are observed at the interface between the matrix and the carbides. However, as the temperature increases, the deformation of the matrix increases, and the tensile and compressive impact of the matrix on the carbides becomes stronger, resulting in the appearance of large blocks of carbides with lower strength. Cracks indicate that large pieces of carbide are not conducive to improving the thermal and cold fatigue resistance of high-speed steel rolls.

Sample 4 has not been subjected to thermal fatigue treatment and thermal cycle at 600°C for 50 times. The carbides are always pressed into the matrix and no peeling of the material occurs. After sample 4 was thermally cycled at 700°C 50 times, the carbides were pressed into the matrix in the initial stage, but in the later stages of scratching, local peeling occurred, indicating fatigue of the material. During the rolling process, local fatigue spalling on the roll surface will not only deteriorate the surface quality of the roll but also intensify the friction between the roll surface and the rolled material, thereby reducing the surface quality of the rolled material. In addition, it can be seen from the characteristics of the scratching process that the matrix plays a supporting role in the carbide during the scratching process. Therefore, the hardness of the matrix will directly affect the indentation depth of the indenter during the test.

The matrix Vickers hardness of original sample 4 measured on a submicron scratch meter was 751, the maximum indentation depth was 16.10 μm, the maximum friction force was 5.29N, and the friction coefficient was 0.21 (friction force and friction coefficient are linearly related ). After 50 thermal cycles at 600°C, the matrix Vickers hardness of the sample reached 716, the maximum intrusion depth was 18.42 μm, the maximum friction force was 6.45N, and the friction coefficient was 0.25. It can be seen that at 600°C, high-speed steel can still maintain good thermal and cold fatigue impact resistance, and the matrix Vickers hardness decreases by less than 5%. After 50 thermal cycles at 700°C, the matrix Vickers hardness of the sample was only 411, the maximum intrusion depth was 22.19 μm, the maximum friction force was 12.96N, and the friction coefficient was 0.52, indicating that the material had undergone significant fatigue. The Vickers hardness of the matrix decreases, indicating that the matrix is softened, which is caused by the precipitation of a large amount of carbides in the matrix. The softened matrix leads to a substantial increase in the indentation depth, and the increase in friction force and friction coefficient leads to an increase in load during the rolling process. Under the same rolling conditions, compared with traditional high-chromium cast iron rolls, the use of high-speed steel rolls usually results in an increase in rolling force, which is mainly due to differences in the material microstructure. The increase in rolling force caused by roll surface fatigue will further increase the burden on the rolling mill, which is very detrimental to the equipment.

High-speed steel rolls have good high-temperature properties due to their high content of alloying elements such as molybdenum, tungsten, vanadium and chromium. From the above analysis, it can be seen that even at 600°C, the Vickers hardness of the matrix decreases by less than 5% compared with the original state, and no obvious fatigue spalling is observed on the surface of the sample. However, excessively high rolling temperatures (such as 700°C) will quickly cause fatigue in high-speed steel. Therefore, temperature control is another key factor to prevent high-speed steel roll surface fatigue. Optimizing the cooling environment of high-speed steel rolls and reducing the roll surface temperature is of great significance to extending the service life of the rolls and improving the quality of rolled materials.

High-speed steel has a strong thermal sensitivity. Tests conducted on a thermal fatigue testing machine with a frequency of 2.8kHz show that high-speed steel rolls are very sensitive to thermal shock. Large thermal penetration will quickly cause macro cracks in high-speed steel. , and rapidly expands along the direction of heat flow transfer, this crack is not allowed to exist on the roll surface, because once the crack occurs, it will accelerate expansion under the action of external rolling force and residual stress, resulting in roll surface peeling or even roll breakage accidents. From the tests conducted on a hot and cold fatigue testing machine with a frequency of 100 kHz, it can be seen that the cracks only exist in the carbide and the interface with the matrix, and do not spread in the matrix. Such cracks are inevitable for rolls serving under the conditions of chilling and heating.

It can be seen from the surface morphology of the actual roller of sample 1

In the actual rolling process, uniform and dispersed small pieces of carbide have better resistance to cold and thermal fatigue, while there are obvious cracks between larger carbides and the matrix, accompanied by local carbide spalling, which is consistent with the The results of the 600°C simulation test conducted on the thermal simulation testing machine are consistent. The only difference is that there is rolling shear force during the actual rolling process and the carbide that gradually loses the matrix wrapping force due to cold and thermal fatigue peels off.

In summary, the heat penetration depth and temperature of the roll surface play a crucial role in the generation and expansion speed of thermal cracks. Therefore, in the actual rolling process, it is very important to cool the roll surface evenly as soon as possible. Compared with traditional hot rolls, the thermal expansion coefficient of high-speed steel rolls is relatively large, and cooling control is more important. During the rolling process, the temperature in the middle of the roll will be higher than that on both sides. Therefore, the cooling water should be concentrated in the middle to maintain a good roll shape. In addition, in order to achieve the best cooling effect, it is also very important to optimize the flow rate, flow rate and spray angle of the cooling water nozzle according to the rolling conditions.

 

4. Conclusion

 

Cyclic alternating thermal stress is the main cause of thermal fatigue of hot roll surface materials. The specific reasons are as follows

1) The thermal expansion coefficients of the matrix and carbide are different. The alternating thermal stress causes stress concentration at the interface between the matrix and carbide. As the rolling time increases, fatigue cracks first occur at the interface between the matrix and carbide.

2) High-speed steel rolls are very sensitive to thermal shock. Larger heat penetration will quickly cause macro cracks in the high-speed steel, which will rapidly expand along the direction of heat flow transfer. The cracks will mainly expand along the interface between the matrix and the carbide and will develop transgranularly locally.

3) Network carbides can cause cracks to expand quickly, while uniformly distributed carbides can make cracks develop slowly. Larger carbides are not conducive to improving the thermal and cold fatigue resistance of high-speed steel rolls.

4) Under high-frequency conditions, high-speed steel rolls have good thermal and cold fatigue resistance. At 600°C, the Vickers hardness of the matrix decreases by less than 5% compared with the original state. No obvious fatigue spalling is observed on the surface of the sample. But if the temperature is too high, high-speed steel will quickly fatigue.

5) Compared with traditional hot rolling rolls, the thermal expansion coefficient of high-speed steel rolls is relatively large, and cooling control is more important. The cooling water should be more concentrated in the middle of the roll body to ensure that a good roll shape is maintained during the rolling process. In addition, in order to achieve the best cooling effect, it is also very important to optimize the flow rate, flow rate and injection angle of the cooling water nozzle according to the rolling conditions.

 

Source of article: Wang Yuhai, Zhou Dahang, Sun Liang, Bai Kunyan, Zheng Honglin, Zhou Li, “Research on the thermal and cold fatigue resistance of high-speed steel rolls”

One Response

  1. Good morning.
    Really interesting. Thanks for sharing.
    Is possible to get the full article?
    Thanks in advance.

Leave a Reply

Your email address will not be published. Required fields are marked *

Share:

Facebook
Twitter
LinkedIn

Most Popular

Product Categories

Contact Us

We provide world-class quality mill rolls  designed for your specific applications.