Reasons and preventive measures for fracture defects of cast high-speed steel rolls heat-treated by high-frequency heating equipment
Cast high-speed steel rolls are easy to form, low in processing cost, and have the characteristics of high hardness, high thermal hardness, and good wear resistance, so they have been widely used in the past 20 years. However, the structure and performance of cast high-speed steel rolls are closely related to heat treatment. Most of the cast high-speed steel adopts high-frequency heating equipment for heat treatment. However, due to the influence of customer factors and worker operation factors during the operation process, cast high-speed steel rolls frequently occur during production. Crack fracture accidents have aroused great concern. Today, let’s look at the causes of fracture defects in cast high-speed steel and their preventive measures.
The cast high-speed steel roll is annealed by high-frequency heating equipment. After annealing, it breaks into two halves, and the fracture surface is neat, showing typical brittle fracture characteristics. No obvious casting defects such as inclusions, porosity, and porosity were found in the inspection, so the fracture of the workpiece was not caused by casting factors. According to the analysis, due to the high content of alloy elements in the roll and the fast cooling rate of the workpiece centrifugal casting, more martensite is formed in the workpiece structure, resulting in volume expansion, and rapid heating during annealing causes the surface layer of the workpiece to shrink, forming tensile stress. Cracks appear when the stress is greater than the tensile strength of the workpiece material, and expand under stress until the workpiece breaks.
Measures to reduce roll annealing crack cracking are as follows:
(1) When using high-frequency heating equipment for replacement, we need to reduce the heating rate of annealing heating to reduce the tensile stress on the surface of the workpiece.
(2) Add three heat preservation treatments during the temperature rise to relax the internal stress. And the annealing process is improved. After adopting the new annealing process, the annealing cracks of the high-speed steel roll are completely eliminated. The hardness of the workpiece after annealing is low (30-35HRC), and the processing performance is good.
When quenching and heating, the cast high-speed steel reticular carbides are not fully dissolved, which makes the performance of the workpiece uneven, resulting in a decrease in strength, toughness, and plasticity; the content of carbon and alloying elements in the carbide accumulation area is high, and its melting point is low, which is prone to overburning of the workpiece. Austenite has high stability and a low Ms point; while the area with few carbides has a high Ms point, which makes the martensite transformation produce inhomogeneity and isochronism, forms a large phase transformation structure stress, and increases the tendency of quenching cracks. big.
On the other hand, the martensitic transformation of high-speed steel rolls during cooling causes volume expansion, resulting in second-type distortion, second-type stress, and thermal stress, which is an important cause of quenching crack cracking. It is difficult to achieve workpiece quenching and cooling technology with a single quenching medium. Require.
For this reason, research and development of high-speed steel roll spray cooling technology. This technology changes the cooling capacity of the mixed cooling medium by adjusting the flow and pressure parameters of water and air.
The thermal stress generated during the cooling of the roll is relatively large, which causes the residual compressive stress to form on the surface of the workpiece. Increasing the water flow and pressure in the nozzle, reducing the air pressure, and increasing the cooling rate will help improve the residual compressive stress of the roll and prevent quenching cracks.
In the cooling below the Ms point, the flow rate and water pressure of the nozzle water flow are reduced, the air pressure is increased, the cooling rate is reduced, and the phase transformation stress is reduced, which helps to reduce and prevent the occurrence of quenching cracks in the workpiece.
Another measure to prevent quenching cracks and fractures of cast high-speed steel rolls is to reduce and eliminate carbide segregation in the workpiece structure, and through the metamorphic heat treatment process, the carbides are finely and uniformly distributed. Cracking has a good effect.
Rapid heating and tempering of high-speed steel rolls are also prone to cracks.
Because the workpiece undergoes martensitic transformation and volume expansion during quenching, rapid heating causes the surface layer of the workpiece to temper and shrink, resulting in tensile stress, which leads to the redistribution of quenching internal stress, which easily causes deformation or cracking defects in the workpiece. Before tempering, the workpiece is preheated at 350-400°C to reduce the internal stress generated during tempering, which can prevent and eliminate tempering crack defects of high-speed steel rolls.
The application of cast high-speed steel rolls is more and more extensive, and its heat treatment quality has a very important influence on cast high-speed steel rolls.
