The main reasons for the lack of hardness of seamless steel pipes (smls) are the following five points:
1. The heat treatment temperature is too high
This is mainly caused by the poor temperature setting of the processing technology or the error of the temperature control system. The temperature of the processing technology should be adjusted, and the temperature control system software should be repaired and calibrated.
2. Shorter cooling time for heat treatment
Because the refrigeration time is short, the temperature of the seamless square tube after the specific heat treatment is high, but the heat treatment itself is not carried out, and the strength after quenching is less than the standard value. When the heat treatment is completed, the temperature is generally lower than the Ms of the raw material, otherwise the strength will be lower than the standard value after quenching. When the heat treatment is completed, let the seamless square tube be air-cooled again for a while, and observe the temperature. If the temperature is high, it should be filled with refrigeration to reduce the temperature to a suitable level. Naturally, those with working experience can identify the temperature from the time when the water disappears when the water is discharged.
3. Overquenching
It is caused by too high quenching temperature setting, abnormal difference of temperature control system or mixed burning when temperature control is too high. The processing temperature should be adjusted, and the software of temperature control system should be repaired and calibrated, not higher than the set temperature.
4. Carburizing
Caused by residual carburized layer of raw material or heat treatment. To prevent such problems of
seamless square and rectangular tubes, the countermeasures used are controllable atmosphere heating, salt bath heating, annealing furnaces and box furnaces are selected for loading maintenance or application of anti-oxidative architectural coatings, and the seamless rectangular The remainder of the tube machining is increased by 2-3 mm.
5. The cooling time is too long
Poor selection of refrigeration materials, temperature of heat-treating materials increases and refrigeration performance decreases, mixing is not good or the temperature of the trough is too high. Therefore, it is necessary to release and enter the tank quickly, and to grasp the refrigeration characteristics of the heat-treated material. The water temperature is 60-80 °C, and the temperature is below 30 °C. When the heat treatment amount is large and the temperature of the refrigeration material is increased, the refrigeration heat treatment material should be added or other refrigeration tanks should be used for refrigeration. In addition, pay attention to improving the mixing of the seamless pipe coolant.
