Ten key points of square tube heat treatment:
1. The square pipe and fixture should be removed before heat treatment of oil, residual salt, paint and other foreign objects;
2. The fixture used for the first time in the vacuum furnace should be degassed and purified in advance at a vacuum degree not lower than that required by the workpiece;
3. The workpiece that is easy to deform in the heat treatment process should be heated on the special fixture. The preheating methods are: the primary preheating is 800℃, the secondary preheating is 500-550℃ and 850℃, and the primary preheating temperature rise speed should be limited;
4. For the workpiece with complex shape or sharp changes in section and large effective thickness, it should be preheated;
5. with grooves through the hole of the workpiece, castings and welding parts and processed stainless steel workpiece, generally should not be heated in the salt bath furnace;
6. According to the effective thickness of the workpiece and the condition thickness (actual thickness multiplied by the workpiece shape factor), the square tube heating should have enough holding time;
7. Martensitic stainless steel and heat-resistant steel square tube quenching cold to room temperature can be cleaned, cryogenic treatment or tempering, welding and subsequent heat treatment interval should not exceed 4 hours;
8. According to the requirements and surface conditions of the square pipe using alkali washing, water-soluble cleaning agent, chlorine solvent sandblasting, shot blasting and other methods to clean;
9. After correction should be lower than the original tempering temperature for stress annealing, complex shape or size requirements of the workpiece, after correction in the tempering with a fixture combined with tempering to correct;
10. When the mechanical properties of the square tube are not qualified, repeated heat treatment can be carried out, but the number of repeated quenching or solid solution is generally not more than two times. Supplemental tempering of square tubes does not count 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.
