A complete set of quenching heat treatment process for the spline shaft of 40Cr steel precision machine tool

① Workpiece name: Spline shaft of precision machine tool, the material used is 40Cr steel.

②Heat treatment technical conditions. It is required that the hardness of the spline shaft after quenching and tempering is 250HBW, and the quenching hardness of the 30mm spline tooth outer circle is 48HRC.

③Processing process: blank - quenching and tempering treatment - cold pressing straightening - stress relief annealing - rough machining - stress relief annealing - fine machining - spline part high frequency induction heating surface quenching - overall workpiece low temperature tempering - inspection and Straightening, low temperature tempering, cleaning and sand blasting.

①Heat treatment process specifications

a. Quenching and tempering treatment. Heating and quenching temperature (840~860) ± 10°C, holding time 2 hours, oil cooling; high temperature tempering temperature 600°C ± 10°C, holding time 2 hours, air cooling.

b. Stress relief annealing. Heating temperature 580℃, holding time 2h, furnace cooling or air cooling after coming out of the furnace.

c. Stress relief annealing treatment. The heating temperature is sso°C ± 10°C, the holding time is 2 to 4 hours, and then slowly cooled in the furnace.

d. High frequency quenching of splines. Heating temperature 860~900℃, water spray cooling.

e. Low temperature tempering. The heating temperature is 280~300℃, the holding time is 1 hour, and then it is air-cooled out of the oven.

f. Temper at low temperature again. The heating temperature is 160°C, the holding time is 4 hours, and then it is air-cooled out of the oven.

⑤Heat treatment process analysis

a. Quenching and tempering treatment. Improve the comprehensive mechanical properties such as strength, hardness, toughness, plasticity and other cross-sections of the spline shaft overall parts.

b. Stress relief annealing. Its purpose is mainly to eliminate the residual internal stress caused by cold press straightening and reduce the deformation of the workpiece.

c. Stress relief annealing treatment. In order to minimize or eliminate the residual internal stress caused by mechanical processing, prevent the workpiece from bending deformation during subsequent high-frequency quenching, and stabilize the dimensional accuracy of the workpiece.

d. High frequency quenching of splines. Its purpose is to ensure the high hardness and high wear resistance of the spline tooth circumferential surface. Since all the teeth of the spline teeth are stressed at the same time, unlike gears that only mesh with a few teeth, in fact, as long as both sides of the spline teeth are hardened, there is no need to have too much hardened layer depth, so high frequency is used Just surface quenching.

e. Low temperature tempering. In order to eliminate the internal stress of the spline teeth and other parts, stabilize the size and shape of the workpiece, ensure the high hardness and high wear resistance of the spline teeth and reduce their brittleness.

f, low temperature tempering again. For workpieces that have been pressurized and straightened after low-temperature tempering, in order to eliminate the internal stress caused by pressurized straightening and prevent the spline shaft from deforming out of tolerance, low-temperature tempering must be performed again.

⑥Key points for the implementation of heat treatment technology

a. The blanks are generally heated to the quenching and tempering temperature in a box-type electric furnace, and batches of blanks should be stacked in parallel and neatly; after heat preservation, the workpieces should be quenched in oil one by one, and cooled to low temperatures to produce oil.

b. After the blank is quenched and tempered, it is cold-pressed and straightened, and then subjected to stress relief annealing at 580°C and heat preservation for 2 hours; if it is unqualified after straightening, stress relief annealing needs to be repeated until it is qualified; it should be noted that two After straightening, the temperature should be further reduced by 10 to 20°C to eliminate the straightening stress.

c. Stress relief annealing should be done in a well-type tempering furnace, and the workpieces should be hung vertically in the loading basket, preferably in a close row and upright. d. During stress relief annealing, the workpiece should be hung vertically with the small end on top. Heating should be done in a well-type electric furnace with air circulation or with a fan. This process is very important. It is an important measure to prevent bending deformation for all quenched and tempered parts, and later for slender parts that require partial surface quenching. e. When performing high-frequency surface quenching of spline teeth on a high-frequency quenching machine tool, for spline teeth that are tall and wide, the workpiece must be rotated slowly to ensure uniform heating along the circumference of the workpiece. If the heating is uneven, uneven hardness of the hardened layer on the tooth portion of the spline shaft will easily occur, that is, one side will be hardened while the other side will have insufficient hardness, as shown in Figure 2-60. It is the main cause of increased bending deformation.

⑦Heat treatment quality inspection

a. After quenching and tempering, the workpiece should be inspected to see whether the hardness meets the requirements (hardness is 250HBW).

b. After the hardness reaches the requirements, cold press straightening is carried out. Within the entire length of the workpiece, the full circle runout is ≤0.60mm.

c. After the workpiece has been stress-relieved and annealed, the total runout of the outer circle of the entire length of the workpiece should be ≤1mm to be qualified. For those that are >1mm, the above process still needs to be repeated. After two straightenings, the annealing temperature to eliminate the straightening stress should be further reduced by 10~ 20℃.

d. After stress-relieving annealing treatment, the full-circle runout of the entire length of the workpiece should be checked to be ≤1mm. Pressurized alignment should be avoided as much as possible to avoid unnecessary additional stress during alignment.

e. After high-frequency quenching of splines, qualified workpieces should be inspected for bending immediately, and the full-length circular runout should be ≤0.25mm. Those that exceed the limit should be immediately pressurized and straightened. Generally, the bending deformation is not serious after high-frequency quenching; if it is out of tolerance, it can be straightened immediately after high-frequency quenching, and it can be easily straightened.

f. The hardness should be checked after low-temperature tempering. The quenching hardness of the outer circle of the 30mm spline teeth is 48HRC. Each piece should be inspected for bending, and the full-length circular runout should be ≤0.25mm. For those that exceed the limit, pressure should be straightened immediately, and if necessary, hot spot straightening should be applied to the bottom of the tooth.