Ten precautions for heat treatment of galvanized pipe are as follows.
1. Galvanized pipes and fixtures should be cleaned of foreign objects such as oil, residual salt, paint and so on before heat treatment.
2. The fixture used for the galvanized pipe in the vacuum furnace for the first time should be degassed and purified in advance at a vacuum not lower than the required vacuum degree of the workpiece.
3. Workpieces that are easily deformed during heat treatment of galvanized pipes should be heated on special fixtures. The preheating methods are as follows. The primary preheating is 800℃, the secondary preheating is 500-550℃ and 850℃, and the temperature rise rate of the primary preheating should be limited.
4. Galvanized pipes should be preheated for workpieces with complex shapes or sharp changes in cross-section and large effective thickness.
5. Workpieces, castings and welded parts with grooves but not through holes in galvanized pipes, as well as processed stainless steel work pieces, are generally not suitable for heating in a salt bath furnace.
6. The galvanizing pipe should be based on the effective thickness of the workpiece and the thickness of the condition (the actual thickness is multiplied by the shape factor of the workpiece), and there should be sufficient heat preservation time when the galvanized pipe is heated.
7. After galvanized martensitic stainless steel and heat-resistant steel galvanized pipes are quenched and cooled to room temperature, they can be cleaned, cryogenically treated or tempered. The time interval between welding and subsequent heat treatment should not exceed 4 hours.
8. According to the requirements and surface conditions of the galvanized tube, use alkaline washing, water-soluble cleaning agent, chlorine solvent sandblasting, shot blasting and other methods to clean.
9. After the galvanized pipe is rectified, stress relief annealing should be carried out at a temperature lower than the original tempering temperature. The workpiece with complex shape or strict size requirements should be rectified with a shaping fixture combined with tempering during tempering after correction.
10. When the mechanical properties of the galvanized pipe are unqualified, repeated heat treatment can be carried out. But the number of repeated quenching or solid solution is generally not more than twice. Supplementary tempering of galvanized pipes is not counted as repeated treatment. Martensitic stainless steel and heat-resistant steel workpieces in the quenched state or after low temperature tempering should be preheated, annealed or tempered at high temperature before repeated quenching.