Formation and control of heat treatment cracks in high-speed steel rolls

Abstract: This article studies the causes of cracks during the heat treatment of centrifugally cast high-speed steel rolls and proposes specific solutions. By applying the improved process to heat treat high-speed steel rolls, the roll crack rate has been reduced by more than 90%, significantly improving the roll production qualification rate.

Cast high-speed steel rolls have the characteristics of high hardness, red hardness and good wear resistance. Since their advent, they have been widely used in steel rolling, especially hot rolling production. The structure and performance of high-speed steel rolls are directly related to heat treatment. Due to the large differences in composition and process conditions between high-speed steel for rolls and traditional high-speed tool steel, the heat treatment process of high-speed steel rolls cannot copy the heat treatment process of traditional high-speed tool steel. Based on a detailed study of the effects of quenching temperature, cooling method, tempering temperature and tempering times on the structure and performance of high-speed steel rolls, the optimal heat treatment process for high-speed steel rolls was obtained.

There are still crack problems in the actual production process of high-speed steel rolls, which significantly reduce the quality of high-speed steel rolls and affect the production and use of high-speed steel rolls. Based on the analysis of factors affecting the cracks of high-speed steel rolls, a new heat treatment process for high-speed steel rolls was developed and It has been applied in actual production and achieved good results.

1. Formation and prevention of annealing cracks in high-speed steel rolls

The cast high-speed steel roll has high hardness and is difficult to process directly, so it needs to be softened and annealed. High-speed steel rolls have poor thermal conductivity, large internal stress in castings, and are prone to cracking after ordinary annealing treatment. Figure 1 is a picture of the cracking of a high-speed steel roll after ordinary annealing. The roll cracked directly into two halves in the annealing furnace. The crack surface is very neat, which is a typical brittle cracking. There are no obvious casting defects such as inclusions, pores, and looseness on the crack surface. , indicating that the annealing cracks of high-speed steel rolls are not caused by poor casting quality.

 

roll crack

Fig 1 Crack morphology of the annealing roll

 

This is because the content of alloy elements in the roll is high, the centrifugally cast high-speed steel roll cools quickly, and the casting structure contains more martensite. The cast high-speed steel roll has already experienced volume expansion, and rapid heating during annealing causes the surface layer of the roll to temper and shrink. , produces tensile stress, and when the tensile stress exceeds the critical fracture strength, cracks appear.

Effective measures to prevent annealing cracks in high-speed steel rolls, in addition to reducing the heating rate, add three times of heat preservation during the heating process to continuously relax the internal stress during the heating of the rolls. The high-speed steel rolls adopt the annealing process shown in Figure 2, which can eliminate the cracks in the rolls. Annealing cracks, and the hardness of the roll after annealing is low (30~35HRC), and the processing performance is good.

 

annealing process, high speed steel roll

Fig 2 Annealing process of high-speed steel roll

 

2. Formation and prevention of quenching cracks in high-speed steel rolls

The formation of quenching cracks in steel materials mainly includes cracks formed by dislocation accumulation, cracks formed by dislocation reactions, and microcracks formed by inclusion boundaries. For cast high-speed steel rolls, they contain a considerable amount of second-phase particles, such as various carbides and non-metallic inclusions. Most of them are hard and brittle phases, and their shape and distribution will affect the occurrence of roll cracks. Some scholars have proposed a cleavage fracture model based on the influence of carbide particles.

Suppose there is a thickness C on the grain boundary. For carbide, under the action of external force, a dislocation accumulation group will be formed in the matrix in front of the carbide interface, which will cause stress concentration at the front end of the accumulation head, and cracks will form in the carbide under the action of this stress. For this crack to expand into the adjacent matrix, it is also necessary to overcome the surface energy in the matrix. This is a fracture controlled by crack expansion, and its criterion is

Griffith

C in the formula. is the carbide sheet thickness, C. The larger it is, the lower σf is. After the crack is formed, if the crack size reaches the critical crack length a=2Eγ/πσ² specified in the Griffith formula, the crack will become unstable and expand rapidly.

Heat treatment cracks in high-speed steel rolls mainly include longitudinal cracks and transverse cracks. Longitudinal cracks are mainly caused by tangential tensile stress, and transverse cracks are mainly caused by axial tensile stress. The internal factors that affect the quenching cracks of high-speed steel rolls are mainly that high-speed steel rolls contain more alloy elements, which have poor thermal conductivity and a high tendency to crack. Carbide segregation also has a significant impact on cracks in high-speed steel rolls. Under quenching and heating conditions, the coarse network carbides are difficult to fully dissolve, resulting in uneven roll performance. The greater the unevenness of the carbides, the lower the strength, toughness and plasticity.

Under the same quenching heating conditions, the carbon and alloy element content in the carbide accumulation area is high, the melting point is low, overburning is prone to occur, the austenite stability is great, and the Ms point is low; while the Ms point is high in the location where the carbide is less distributed, so that This leads to the inhomogeneity and anisochrony of martensite transformation. When the enriched area of carbon and alloy elements transforms to martensite, the low concentration area has completed the martensite transformation and is in a hardened state, resulting in a larger structural stress that increases the tendency of quenching cracks. When the thermal conductivity of high-speed steel rolls cannot be significantly improved, reducing and eliminating carbide segregation, and using modification treatment measures to make the carbides in the roll structure finer and evenly distributed, will undoubtedly have a good effect on improving heat treatment cracks in high-speed steel rolls.

When the high-speed steel roll is quenched and cooled, below the Ms temperature, due to the transformation of austenite to martensite and volume expansion, second-type distortion, second-type stress and macroscopic heat treatment stress are generated, which can easily lead to quenching cracks. If the quenching cooling rate is too slow, it will affect the degree of hardening and fail to meet the requirements for the hardened layer of the roll. The ideal cooling method is shown in Figure 3. The cooling medium has the strongest cooling ability at the temperature where supercooled austenite decomposes the fastest, and the cooling ability becomes gentler when it is close to the Ms point, which ensures hardening. requirements, and reduces the quenching stress, which can prevent the high-speed steel roll from deformation and cracking during quenching.

It is difficult for a single quenching cooling medium to meet the quenching cooling requirements of high-speed steel rolls. High-speed steel roll spray cooling equipment and technology have been developed. The high-speed steel rolls are placed in the spray cooling circle, and then the pressure and flow rate of water and air in the nozzle are adjusted. , can change the cooling capacity of the mixture. Rapid cooling of high-speed steel rolls from the high-temperature austenite zone will produce large thermal stresses, form residual compressive stress on the roll surface, increase the water pressure and flow rate in the nozzle, reduce the air pressure, increase the cooling rate, and increase the residual compressive stress. Helps prevent quenching cracks. Below the Ms point, reducing the water pressure and flow rate in the nozzle, increasing the air pressure, reducing the cooling rate, and reducing the phase change stress can all help to prevent quenching cracks in high-speed steel rolls.

When quenching and heating ordinary high-speed steel rolls, to prevent the occurrence of cracks, measures such as reducing the heating rate and adding 1 to 3 times heat preservation at 550~850°C are taken, which not only increases the heat treatment energy consumption but is also not conducive to preventing heat treatment cracks in the rolls. The rapid heating of high-speed steel rolls is beneficial to preventing heat treatment cracks in the rolls. This is because rapid heating causes the surface temperature of the roll to increase and expand, while the cold core gives compressive stress to the surface layer of the roll. The surface softens due to the increase in temperature and can be plastically deformed to relax the stress. Moreover, rapid heating can also reduce oxidative decarburization, improve productivity, and save energy.

However, rapid heating and tempering of high-speed steel rolls are prone to cracks. This is because the volume of the quenched high-speed steel rolls has already experienced volume expansion. Rapid heating causes the tempering shrinkage of the surface of the rolls, resulting in tensile stress and affecting the redistribution of quenching internal stresses, which may lead to Roller deformation or cracking. Preheating high-speed steel rolls at 350~400℃ before tempering can reduce the internal stress generated during the rolls and prevent tempering cracks of steel rolls.

 

3. Conclusion

 

(1) The cast high-speed steel roll contains a lot of martensites.

During annealing, the surface layer of the roll tempers and shrinks, generating tensile stress and promoting cracks in the roll. Measures such as reducing the heating rate and adding three times heat preservation during the heating process can continuously relax the internal stress during the roll’s heating, eliminating the roll’s annealing cracks. Moreover, the hardness of the roll after annealing is low and the processing performance is good.

(2) The high-speed steel roll adopts spray quenching.

By adjusting the pressure and flow rate of water and air, the cooling capacity of the mixture can be changed. Above the Ms point, increasing the water pressure and flow rate in the nozzle, reducing the air pressure, and increasing the cooling rate will increase the residual stress of the roll, which is beneficial to preventing quenching cracks. Below the Ms point, reducing the water pressure and flow rate in the nozzle, increasing the air pressure, reducing the cooling rate, and reducing the phase change stress are also beneficial to preventing quenching cracks.

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