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Vacuum brazing of surgical forceps – heat treatment process

The needle holder is a surgical instrument used for holding suture needles and knotting. The working end needs to be inlaid with cemented carbide by inlay technology to obtain better wear resistance, anti-skid performance and easy maintenance and replacement. In recent years, the vacuum brazing heat treatment composite process is used on such products, which is a process of combining the original vacuum brazing and vacuum heat treatment processes, which has been used more and more widely.

Vacuum brazing of stainless steel and cemented carbide has been widely used in recent years. Vacuum brazing is an advanced process for 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, and higher cleanliness and strength of welded joints. The vacuum heat treatment can significantly improve the toughness and strength while maintaining the high hardness and wear resistance of cemented carbide. For martensitic stainless steel, the brazing-heat treatment composite process can achieve the purpose of quenching at the same time as brazing. It has obvious advantages in quality, cost, efficiency, etc. It is a process method worthy of popularization and application.

The original processing procedure of the product: machining → quenching → high-frequency welding of the working end → surface treatment → inspection. The disadvantage of this process is that the workpiece is quenched as a whole and then partially welded. The welding heat will change the local structure of the workpiece, which not only affects the mechanical properties and corrosion resistance of the product, but also affects the performance of the product; It is manual operation, and the quality control is difficult; thirdly, welding and heat treatment are operated separately, which has a long construction period and high energy consumption. The improved processing procedure is: machining→vacuum brazing/quenching→surface treatment→inspection. The main improvement lies in the use of a composite process of brazing and quenching, that is, welding and heat treatment are completed at the same time, which not only reduces energy consumption, improves work efficiency, and improves operating environment, but also overcomes the quality defects of the original process.

Material size:

The base material of the workpiece is 20Cr13 steel; the cemented carbide is K40, which is equivalent to YG11, and the size is 15mm×5mm×0.8mm; the solder is nickel-based brazing material.

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.

Vacuum furnace technical parameters:

Maximum heating temperature 1320℃

Furnace temperature uniformity ±5℃

Maximum heating power 160kw

Ultimate vacuum degree 4×10-1Pa

Heating chamber 600×600×900mm

Evacuation time 30min

Maximum cooling pressure 0.6MPa

Process flow:

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. The basic parameters of vacuum quenching process include vacuum degree, heating speed, heating temperature, holding time and cooling method, etc.; while the parameters of vacuum brazing process usually also include vacuum degree, heating speed, brazing temperature, brazing time and cooling method, etc. . When formulating the process, the setting of the final heating temperature should not only consider the normal quenching temperature of stainless steel, but also ensure that the comprehensive performance of the cemented carbide is not affected. The quenching temperature of 20Crl3 steel is usually about 1000 °C, so nickel-based brazing filler metal with a melting temperature similar to that is selected (it is advisable that the working temperature exceeds the melting point of the filler metal by 25-60°C) to improve the fluidity and wetting of the filler metal. When heated under vacuum conditions, the degree of vacuum in the furnace is 4-4×10-2 Pa; when heated to above 800℃, an appropriate amount of nitrogen is charged into the furnace to prevent the Cr element of 20Cr13 steel. volatilization; the heating speed is based on the premise of ensuring that the evolved gas in the furnace is fully extracted and the workpiece is heated evenly, the method is to slowly heat up, and carry out two-stage preheating at 550°C and 800°C; it is determined according to the effective thickness of the workpiece and the amount of the furnace at one time. Brazing/quenching heating time; high pressure gas quenching done in furnace. The specific body is as follows:

Vacuum brazing – heat treatment points:

The defects in the actual operation of the surgical instruments treated by the vacuum brazing-heat treatment composite process include weld pores and hard alloy cracking. The main measure to prevent weld porosity is to precisely control the heating temperature and holding time. If the heating temperature is too high or the holding time is too long, the solder will be burnt and evaporated; if the heating temperature is too low, the fluidity of the solder will be poor, resulting in false welding; if the holding time is too short, sufficient metallurgy will not be formed between the solder and the base metal. Combined, it will cause weld pores and seriously reduce the weld strength.

The cracking phenomenon of cemented carbide is mainly related to the heating and cooling rate, especially in the cooling process, the stress generated by welding cannot be ignored. Due to the different expansion and contraction coefficients of the two materials, they cannot shrink synchronously after welding. During the heating and cooling process, a high residual stress will be formed in the weld area, which will reduce the shear strength of the weld. Welding and quenching are different processes, and the process parameters of the two are similar, but there are also differences. For example, from the welding point of view, in order to avoid cracking caused by the excessive temperature difference between the surface layer and the core of the cemented carbide, the heating and cooling speed should be controlled during brazing, and rapid cooling and rapid heating should not be performed. When formulating the brazing-heat treatment composite process, the characteristics of the two processes should be taken into account. The cooling rate of the workpiece after vacuum brazing should be close to but not lower than the critical cooling rate of martensitic stainless steel. The speed should reach about 10°C/s.

In order to obtain the ideal welding effect, proper 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 is used metallographic sandpaper to remove the surface oxide layer, and the brazing surface of K40 is polished with diamond grinding paste. When the workpiece is offset. Since brazing relies on capillary action to fill the gap with solder, it is necessary to control the gap size of the welding surface. The joint gap has a great influence on the compactness and strength of the brazing joint. The joint gap of nickel-based solder is smaller than other solders, generally 0.02 to 0.10mm.

Process technology must be matched with excellent equipment to play a role, and the brazing furnace selected in vacuum brazing is very important. The RHVB vacuum high temperature brazing furnace produced by SIMUWU is an excellent product for handling such processes. With its excellent temperature control accuracy and temperature uniformity, the workpiece can be brazed evenly and evenly, so as to achieve high-level brazing standards, reduce scrap rates, increase production efficiency and reduce costs.