The process flow of heat treatment of wire rolling plate by medium frequency induction heating machine

The thread rolling plate is a special tool used to process external threads such as taps and bolts. During the working process, it has to withstand alternating pressure, shearing, bending and other stresses as well as impact and wear. Most of its tooth shapes are cold rolled with needle rollers. take shape. After heat treatment, mechanical processing is no longer performed. The process of thread processing during operation is a rotating and rolling process. By rolling with the workpiece, plastic deformation occurs and the thread is rolled out. Therefore, the teeth must withstand severe stress. Due to the effects of impact load and extrusion stress, it is required that the hardness within 3-5mm below the tooth root after heat treatment is 58-61HRC, the quenched martensite is <3 levels, and the tooth surface is not decarburized. According to its service conditions, the wire rolling plate needs to have high hardness, wear resistance, sufficient strength and certain impact toughness, and strong fatigue resistance. For this reason, we often use medium frequency induction heating machines for heat treatment. Today, let’s take a look at the analysis and implementation key points of the heat treatment process of the wire rolling plate.

(1) Mechanical processing process of thread rolling board. The usual production route of the thread rolling plate is: cutting bar material - repeated forging and forming - spheroidizing annealing - machining - stress relief annealing - cold rolling threads - heat treatment (preheating, heating, tempering or cold treatment, etc.) - grinding.

①Forging and forming. This process is to repeatedly forge and roughen the bar to break large pieces of carbide, evenly distribute the carbide, eliminate composition differences, and at the same time change the internal streamline distribution of the steel to enhance the strength of its matrix.

②Spheroidizing annealing. It also prepares the tissue for final heat treatment. The forging blank has high hardness and uneven composition. In order to reduce the hardness and facilitate cutting processing, medium frequency heating equipment is used for annealing.

The investment index after annealing is as follows:

a.9CrSi steel, hardness 197-541HB; Cr12MoV steel, hardness 217-269HB.

b. The depth of the decarburization layer is not allowed to exceed 1/2 of the processing volume.

c. The grade of pearlite is 2-4, the carbide is in granular form, and the size and distribution are relatively uniform.

(2) Analysis and implementation key points of the heat treatment process of the thread rolling board

1. In order to prevent the quenching deformation of the thread rolling plate, the following measures should be taken in actual production for the thread rolling plate made of 9CrSi material.

a. Small thread rolling plates with specifications ≤ M6 should be placed in a special quenching fixture to cool the plate teeth outward, otherwise the deformation will increase.

b. Wire rolling plates with specifications >M6 should be cooled with 10# mechanical oil below 80℃, and M8-M18 should be quenched back to back.

The reason for the large deformation of the thread rolling plate after quenching is caused by the large internal stress. Its manifestations are: concave thread surface and convex smooth surface, which need to be straightened. Therefore, the deformation of the thread rolling plate should be controlled during heat treatment, and necessary measures must be taken on the quenching medium and quenching fixture. Because when the thread rolling plate is cooled, the cooling speeds of its teeth and the back are different. The cooling rate can effectively control its deformation.

2. It should be noted that Cr12MoV steel wire rolling plate is a high-alloy steel with a large number of alloy elements and high hardenability. Like high-speed steel, there is a special austenite at around 500°C. A stable region that does not undergo tissue transformation even if it remains for a long time.

3. The thread-rubbing plate adopts a process of higher temperature preheating + low-temperature short-time heating and quenching to make the quenched martensite fine, which can reduce the carbon content in the martensite, thereby reducing the twin substructure and improving The toughness of the thread rubbing board can effectively reduce the probability of tooth chipping. However, it should be noted that in order to improve the plasticity and toughness of the material and give full play to the high strength potential of the material, the grinding performance of the material should also be reasonably utilized. Therefore, correct analysis and treatment should be carried out, and necessary verification can be carried out through experiments. Choose better process parameters to determine the relationship between steel structure, tempering temperature and strength and toughness.

4. It should be noted that the early tooth chipping failure of the thread rolling board (generally refers to the processing number of less than 3,000 pieces) is caused by brittle fracture and low-cycle fatigue damage caused by bending stress and extrusion stress. Metallographic structure analysis shows: a. The quality of spheroidizing annealing is poor. In addition to the irregular shape of carbides, there are also fine flake pearlite, which can easily cause stress concentration, crack initiation and coarsening of austenite grains, which reduces the toughness. Decrease; b. The quenching heating temperature is too high or the holding time is long, resulting in coarse martensite, uneven undissolved carbides, and intermittent distribution on the original austenite grain boundaries, which increases the microstructure of the martensite. Cracks, the carbon content of martensite increases, the twin substructure increases, and the brittleness of the thread rolling plate increases; c. Insufficient tempering results in high hardness of the thread rolling plate. The above are the main reasons for early chipping, so it should be Focus on control in these three aspects.

This article briefly introduces the medium frequency heat treatment process of the thread rolling board. When performing heat treatment according to the above process, everyone must be careful and careful to avoid defects that affect its service life.