ZHENGZHOU KETCHAN ELECTRONIC designs an induction quenching machine for customers, which is used to produce a batch of slender pieces with a cross-section of 50mm×25mm, with a total length of 1000mm, and 5 15mm through holes evenly distributed on the surface of 50mm×1000mm, as shown in FIG. 1.The material is 55 steel. Three adjacent faces are required to be quenched (A, B, and C faces are quenched, while D faces are not quenched). The quenching hardness is 55 ~ 58HRC.
1. Traditional surface induction quenching process
Since the three quenched surfaces are flat, while the other surfaces do not need to be quenched, and this material belongs to the induction hardening material, continuous induction hardening is adopted. The traditional surface induction quenching process is as follows: A surface quenching → B surface quenching → C surface quenching → straightening → tempering → hardness and deformation detection.
U-shaped inductor (see FIG. 2) is used in induction quenching to continuously heat and quench each adjacent surface by utilizing an external magnetic field. The inductor is made of a hollow copper pipe with a cross-section of 15mm×15mm. The u-shaped tube of the inductor is equipped with a row of the horseshoe-shaped magnetic conducting body to enhance the magnetic induction intensity of the quenched surface. There is a row of 45° water jet holes on the corners of the inductor u-shaped tube, which continuously cools the heating surface. The workpiece is suspended vertically, the inductive machine tool moves upward from the bottom, the sensor U-shape surface is parallel to the workpiece’s quenching surface, and the constant spacing is kept as the inductive machine tool moves upward at a constant speed (see Figure 3).
Figure 2 U-shaped Square induction coil（inductor）diagram
FIG. 3 Induction heating mode
After quenching the workpiece by using the traditional surface induction quenching process, the hardness of the quenching surface is measured to be 58 ~ 60HRC. All the workpiece is bent out of shape after quenching, the unquenchable surface is convex on surfaced, the quenchable surface is concave on surface b, and some adjacent quenchable surface edges and corners have cracks. For the deformed workpiece pressure straightening, the quenching surface B in straightening is easy to crack due to tensile stress, and the scrap rate is as high as 60% due to bending straightening fracturing or angular cracking. After quenching, the workpiece is tempered at 140℃, and the hardness of 56 ~ 58HRC is detected.
2. Improved surface induction quenching process
The quenching process of this batch of slender parts faces two difficulties:
(1) As the elongated parts are prone to bending deformation after quenching, the quenching surface is prone to crack when straightening after deformation, and the through-hole on the quenching surface is larger, so the shape and structure determine the great difficulty of quenching.
(2) The technology requires three-side quenching, and the edge edges are prone to quenching cracks due to the secondary quenching caused by the quenching of adjacent faces.
In order to improve the surface quality and improve the pass rate, the surface induction quenching process route is changed to press the appropriate amount on Surface D (pre-deformation 0.2-0.3mm) → simultaneously quench the surface of Surface A, B and C → straighten → temper → detect hardness and deformation.
Square type inductor (see FIG. 4) is used in induction quenching to heat three adjacent surfaces continuously at the same time by using an internal magnetic field. The inductor is made of a hollow copper tube with a cross-section of 15mm×15mm, and a row of 45° water-jet holes are measured inside the bottom edge of the copper tube to continuously cool the heating surface. The workpiece is suspended vertically, and the inductor moves upward with the machine tool. In the inductor, the square surface of the inductor is perpendicular to the workpiece, the copper tube of the inductor is parallel to the quenched surface, and the constant spacing between the inductor and the workpiece is subject to the machine tool moving uniformly from the bottom to the top (see Figure 5).
FIG. 4 Square induction coil（inductor）
FIG. 5 Induction heating mode
When the workpiece is heated and quenched in the square inductor, due to the small gap between surface A, Surface B, and surface C and the inductor, the three surfaces can be rapidly heated and quenched at the same time, while the gap between surface D and the inductor is large and the electromagnetic induction has little influence, so it cannot be rapidly heated to high temperature and quenched. Due to the pre-deformation of 0.2-0.3mm under the pressure of Surface D before quenching, the deformation of surface B during quenching is opposite to the pre-deformation and cancels with each other, and the bending deformation of the workpiece after quenching is small. When the pre-pressure reduction of Surface D is greater than or equal to the quenching deformation, the unquenched surface D is concave, and the quenched surface B is convex. When the deformation workpiece is straightened, the high point of surface B is pressed, then the quenched surface B is in the state of compressive stress and it is not suitable to produce cracks. The scrap rate is less than 5% due to bending straightening fracturing or angular cracking.
After quenching the workpiece using the improved surface induction quenching process, the hardness of the quenching side is measured to be 58 ~ 60HRC, and the hardness is measured to be 56 ~ 58HRC after tempering at 140℃. The workpiece deformation is small, with no crack on the surface after straightening, and the one-time quenching of three surfaces at the same time avoids the occurrence of edge crack. The qualified rate of finished products can reach more than 95%, greatly reducing the scrap rate.
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