Vacuum Brazing and Heat Treatment Process of Stainless Steel Surgical Instruments

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Surgical instruments used for holding suture needles and knotting are inlaid with cemented carbide at their working ends to obtain better wear resistance, anti-skid performance and easy maintenance and replacement. In recent years, our factory has adopted the vacuum brazing heat treatment composite process on such products, that is, the original vacuum brazing and vacuum heat treatment: the two processes are combined into one, and certain results have been achieved.

The welding and quenching equipment of the workpiece is a domestic vacuum high pressure gas quenching furnace. The effective thickness of the workpiece is about 5 mm, and the quenching cooling gas is high-pressure 99.999% high-purity nitrogen. The final hardness of the workpiece matrix is ​​44 ~ 45 HRC

Although brazing and heat treatment are processes of different nature, for martensitic stainless steels, the two processes have a lot in common. Whether the brazing and quenching composite process is feasible depends on whether the characteristics of both can be taken into account. This is the case under normal conditions, but also under vacuum conditions.

Based on the vacuum quenching process of 20Cr13 steel, we have formulated a composite process of brazing heat treatment of 20Cr13 steel and cemented carbide. Vacuum brazing is an advanced process of joining parts without flux in a vacuum atmosphere. Compared with other welding methods, it has a series of advantages such as no oxidation, carbonization, decarburization and pollution, high cleanliness and strength of welded joints.” Defects include weld porosity and cemented carbide cracking. The main measure to prevent weld porosity is to precisely control the heating temperature and holding time. The heating temperature is too high or the holding time is too long, which will cause the solder to burn and evaporate; while the heating temperature is low , then the fluidity of the solder is poor, forming a virtual welding; if the holding time is too short, there is no time to form sufficient metallurgical bonding between the solder and the base metal, which will cause weld pores and seriously reduce the weld strength. Of course, to obtain the ideal welding effect , Appropriate pre-welding treatment should be carried out: remove oil stains and sundries on the surface, and ensure that the brazing surface is flat, clean and dry. The brazing surface of 20Cr13 steel uses metallographic sandpaper to remove the surface oxide layer, and the brazing surface of K40 is ground with diamond abrasive paste. Bright; the size of the brazing filler metal should be slightly larger than the brazing surface, and the solder joint should be fixed with the brazing filler metal with a special fixture to prevent the workpiece from shifting during brazing. Since brazing relies on capillary action to fill the gap with filler metal, it must be well controlled The gap size of the welding surface. The joint gap has a great influence on the compactness and strength of the brazing seam. The joint gap of nickel-based solder is smaller than that of other solders, generally: 0.02 ~ 0.10 mm.

The heat transfer during gas quenching in vacuum heat treatment is mainly the convection and radiation heat transfer of the circulating gas, and the most significant factors affecting the cooling rate are the pressure and flow rate of the cooling gas. At present, the gas quenching pressure of vacuum air quenching furnaces at home and abroad has risen from the positive pressure in the 1970s and 1980s to a high pressure of 0.6~2 MPa or more), which is enough to meet the quenching and cooling requirements of large tools and molds. The production practice of our factory shows that the small martensitic steel workpiece can be hardened by using nitrogen as the medium and the gas quenching pressure greater than 0.135 MPa under the condition of airflow circulation. Therefore, for surgical instrument products with a small effective thickness, the gas quenching pressure is controlled at about 0.15 MPa, which can not only eliminate cracking, but also meet the requirements of brazing and heat treatment processes.

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