When the square tube is quenched, the following points should be paid attention to:

(1) The slender pipe should be hung vertically in the salt bath furnace or pit furnace for heating to reduce the deformation caused by its own weight.

(2) When pipe fittings with different cross-sectional sizes are heated in the same furnace, the small pieces should be placed at the outer end of the furnace, and the large and small pieces should be timed separately and the small pieces should come out of the furnace first.

(3) The amount of furnaces to be charged each time should be compatible with the power of the furnace. When the furnace is loaded with a large amount, it is easy to press and heat, and the heating time needs to be extended.

(4) For workpieces quenched with water or brine, the quenching temperature shall be taken as the lower limit, and for workpieces quenched with oil or molten salt, the quenching temperature shall be taken as the upper limit.

(5) When performing dual-medium quenching, the time of staying in the first quenching medium is controlled by the above three methods. The time for moving from the first quenching medium to the second quenching medium should be as short as possible, preferably 0.5-2s.

(6) For pipes whose surface does not allow oxidation and decarburization, they should be heated in a calibrated salt bath furnace or a protective atmosphere furnace. If the conditions are not met, it can be heated in an air resistance furnace, but protective measures should be taken.

(7) After the square tube is vertically immersed in the quenching medium, it does not swing, but only moves up and down, and stops the agitation of the quenching medium.

(8) When the cooling capacity of the parts with high hardness requirements of the pipe fittings is insufficient, the parts can be cooled by spraying liquid while the whole is immersed in the quenching medium to increase the cooling speed.

(9) It must be placed in the effective heating area, and the amount of furnace loading, furnace loading method, and stacking form should ensure that the heating temperature is uniform and consistent, and will not cause distortion and other defects.

(10) When heating in a salt furnace, do not get too close to the electrodes to prevent local overheating, and the distance should be more than 30mm. The distance from the furnace wall and the depth below the liquid surface should be above 30mm.

(11) When quenching in stages, the quenching temperature can be increased appropriately to increase the stability of austenite and prevent it from decomposing into pearlite.

(12) Structural steel and carbon steel can be directly heated in a furnace with a quenching temperature of 20-30°C higher than the quenching temperature. High-carbon and high-alloy steel should be preheated at about 600°C and then raised to the quenching temperature.

(13) For pipes that require a deep hardened layer, the quenching temperature can be appropriately increased, and for pipes that require a shallow hardened layer, a lower quenching temperature can be selected.

(14) There must be no oil, soap liquid, or other dirt in the cooling water of the rectangular tube. Generally, the water temperature does not exceed 40°C, and the oil temperature does not exceed 80°C.