How do large forgings be surface hardened?

2019-03-15 17:20:36 15

First, the hardness check

       Large forgings are usually quenched by a portable Shore hardness tester or with a digital display hardness tester, as these two hardness testers can be measured on any surface of the part without moving parts, since both hardness measurements are The measuring head bounces, so the measuring surface is required to have a high surface finish, and the measuring surface can be polished with sandpaper or whetstone. If the measuring surface is flattened, the hardness tester is better in the vertical direction and reduces the error. In particular, with a Shore hardness tester, it is necessary to keep the vertical bar bounce in the vertical direction, otherwise a large error is caused. In any case, it is not allowed to measure the surface hardness using a hammer-type element-based hardness tester or a Brinell hardness tester.


      When measuring the hardness, it is necessary to uniformly distribute 3-5 measuring points in the longitudinal direction and the circumferential direction of the quenching zone, and the uniformity of the quenching hardness, especially the surface hardness of the roll, and the uniformity of the hardness of the quenching surface are required.


      For the surface quenching of large gears, the hardness is measured on the toothed surface with a circular arc. If it is fixed by one shot, the hardness can be measured at the top of the tooth. In order to accurately measure the hardness of the gear tooth surface, it can be measured with a clamp type gear hardness tester (Rockwell).


       If the narrow surface quenching surface cannot be measured by the durometer, the boring tool can also be used to test the hardness. It is best to use a standard boring tool with several calibrated hardnesses.

Second, the distribution of the hardened layer and the depth of the hardened layer

       Generally, the surface quenching depth requirement of large forgings is determined according to the proper selection of induction heating frequency and process parameters. The depth of the hardened layer exceeding the range is achieved by heat conduction. The depth is not very accurate and is not checked, but for the surface of some large forgings. Quenching, especially large forgings with contact fatigue strength requirements, because the surface contact fatigue life with large radius of curvature is closely related to the fatigue strength and shear stress ratio of the transition zone of the surface hardened layer. Therefore, the depth of the hardfacing layer directly affects the contact fatigue life of such large forgings. For example: large wheel treads, large heavy-duty gears, etc., prefer to have a lower surface hardness, but the depth of the hardened layer is deeper. For cold rolls, there is also a depth requirement for the hardened layer, and the surface quenching of the cold roll is usually deeper than the depth of conventional quenching.


       In addition, the distribution shape of the surface hardened layer has a great relationship with the residual stress distribution, and it is required to quench the surface of a large forging having high fatigue strength, and it is desired to obtain an ideal hardened layer distribution shape.


       Large-scale inspection of the depth and distribution shape of the hardened layer of small parts can be carried out by means of damaged parts, that is, a piece of cut polished section is extracted from a large number of parts of a process specification and observed with nitric acid alcohol, and large forgings can only be produced. The test piece of similar shape is cut by the same process specification after surface quenching. For example, the large shaft type adopts a length of bushing, and the large large modulus gear adopts special manufacturing of two identical modulus teeth for the test piece inspection.


      The method for checking the depth of the hardened layer can be in accordance with the ministerial standard: GB5617-85 "Determination of the effective hardened layer depth after induction hardening or flame quenching of steel". The color (black) layer thickness is usually measured after etching with nitric acid alcohol.


       Third, the appearance check

       Use magnetic particle inspection, fluorescence detection, ultrasonic flaw detection and other methods to observe whether the quenched surface is cracked. Visually inspect the surface for signs of burns, paying particular attention to the sharp corners, the areas of the holes and the edges of the end faces, as they are prone to overheating and burns or even melt.


       Fourth, metallographic examination

       Metal forging is rarely performed on the surface quenching of large forgings. If the user has requirements (for example, cold rolls are used to inspect the superheated structure), the surface of the part is observed with a mobile metallographic microscope. The metallographic examination of the surface quenching is generally only to understand Markov. The thickness of the body needle, because the coarse needle-shaped martensite is detrimental to fatigue performance.