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Effect of different post-weld insulation on vacuum brazing strength

PDC/45 steel brazing parts are widely used in cutting tools, oil drilling, coalfield geology, coal mine underground drilling construction and many other fields. During the vacuum brazing process of PDC tools, due to the uneven heating and cooling, as well as the constraints or external constraints caused by the welding base material itself, welding stress will always occur in the welds of PDC/45 steel tools after the welding work is completed. The existence of welding stress in components will reduce the actual load-bearing capacity of the welded joint area and produce plastic deformation. In severe cases, it will also cause damage to the component. Vacuum heat treatment after welding can effectively improve the yield strength and relax the welding stress. Especially for weldments made of two base metal materials with large differences in thermal expansion coefficients, appropriate post-weld insulation can well release the thermal stress of the weld, thereby ensuring the quality of the weld. It is generally believed that PDC tools should be cooled slowly after welding to reduce or eliminate the welding stress caused by the inconsistent thermal expansion coefficient of the tool base material. They cannot be placed in water or oil for rapid cooling. There are two commonly used methods: one is overall high-temperature tempering, and the other is local high-temperature tempering, which is mainly determined according to the process requirements of the workpiece.

The determination of different post-weld holding times is mainly based on the characteristics of the filler metal and base metal used. When the interaction between the filler metal and the base metal has unfavorable tendencies such as strong dissolution, generation of brittle phases, and intergranular infiltration, the vacuum brazing holding time should be shortened as much as possible; on the contrary, if the interaction between the two can eliminate the When there is a brittle phase or low melting point structure, the heat preservation time of vacuum brazing should be appropriately extended. In general, the holding time and method should be determined by taking into consideration the size of the weldment and the gap between the welds. Large and thick welding parts require a longer holding time than small and thin welding parts to ensure that the welding parts are heated. The filler metal and base metal can be used uniformly and fully. In the heat preservation method, the natural cooling heat preservation time is short, and the furnace cooling heat preservation time is long. The cooling speed of the heat preservation sand is between the two.

This experiment mainly studies the effects of natural cooling, insulation sand cooling and furnace cooling on the shear strength of the weld.

1 Test materials and methods

This experiment uses vacuum furnace heating to vacuum braze PDC/45 steel. The size of the composite sheet is 13.44mmx4.5 mm, and the size of the 45 steel sample is $15 mmx20mm. The welding surface is sandblasted and cleaned with detergent before welding. After welding, natural cooling, insulation sand cooling (the equipment is a sand insulation tank) and furnace cooling (the equipment is a vacuum brazing furnace with a power of 40kW) are performed. The initial temperature of the sand insulation tank for cooling the insulation sand after welding and the insulation furnace for cooling with the furnace is set to 500°C.

A universal material testing machine was used to conduct a shear strength test on the vacuum brazed specimens; a secondary electron scanning electron microscope was used to analyze the structural components of the brazing seam.

2 Test results and analysis

2.1 Shear strength of weldment

According to the shearing method shown in Figure 1, shear tests were conducted on weldments with different post-weld treatments. The shear results of natural cooling, thermal insulation sand insulation, and furnace cooling are obviously different, and the shear results show an increasing effect. The main reason for this phenomenon is that during natural cooling after welding, the temperature gradient of the weldment changes too quickly, the weld temperature cools rapidly, the elements in the tissue do not effectively diffuse, the particles of the tissue components become larger, and the stress cannot be effectively released. Therefore, The strength of the weld is reduced. The temperature gradient of the weld seam changes slowly as the furnace cools, and the diffusion time of the elemental components in the weld structure is sufficient. The structural components can be evenly refined under long-term heat preservation, and the thermal stress of the weld seam can be well released, so the weld seam is well formed. , the shear strength is significantly improved. Judging from the shear test results, among the three cooling methods, the shear strength of the weld is the highest after furnace cooling.

2.2 Structural composition of weldments

From the above shear test results, it can be seen that the shear strength value of naturally cooled welds is the worst. Therefore, the two structural components of the welds, furnace insulation and insulation sand insulation at 500°C, were analyzed using a scanning electron microscope. As can be seen from Figure 2 It can be seen that there are some evenly distributed gray tissues and white tissues in the weld. It can be seen from Figure 2(a) that the active ingredients of the solder in the weld are immersed in the PDC alloy and inside the 45 steel matrix, and are tightly combined with almost no pores. In the welds insulated with thermal insulation sand (Figure 2(b)), it can be seen that there are obvious large pores, the immersion of the solder into the base metal area is not obvious, and the uniformity of the weld structure composition is significantly lower than that in Figure 2 (a).

When maintaining heat after welding, the different elements distributed in the weld are beneficial to the change of the weld structure. Cu element has good thermal conductivity and can form α solid solution with various metal elements within a suitable temperature range to improve the structural properties. The presence of Zn element can greatly reduce the melting point of Cu-Ag alloy and improve the fluidity and wettability of the alloy. Research shows that Co and Ni can significantly improve the high-temperature strength of the solder, have good wettability to diamond, and can improve the plasticity of the weld. In addition, the effective presence of N element can better improve the connection strength of the weld. Combining experiments and theoretical analysis, it is shown that reasonable post-weld holding temperature and appropriate post-weld holding time can effectively promote the mutual diffusion of effective metal element components in the weld, thereby improving the comprehensive mechanical properties of the weld. The higher temperature during furnace cooling of the PDC composite drill bit after welding can effectively reduce the degree of segregation of elements in the solder, improve the diffusion of elemental components in the weld, and uniformize the component distribution. When the post-weld holding temperature is low, the segregation of elements in the weld will not only cause local stress concentration in the weld structure, but also reduce the bonding strength, toughness and plasticity of the weld. Therefore, the post-weld holding temperature is subject to the performance requirements of the welding material. The higher the temperature, the more conducive to improving the welding strength of the diamond composite sheet.

3 Experimental conclusions on the effects of different post-weld insulation on vacuum brazing strength

(1) Furnace insulation and cooling after welding can effectively ensure the uniformity of the structure within the weld, enable the components in the solder to effectively diffuse into the base metal structure, improve the uniform distribution of metal elements within the weld, and increase the resistance of the weld. High shear strength.

(2) When the initial temperature is 500°C, cooling and insulating the sample with the furnace can effectively reduce the degree of segregation of elements in the weld structure, which is beneficial to improving the welding strength of the weld. At the same time, furnace cooling after welding can effectively ensure the release of stress in the weld after vacuum brazing of dissimilar base materials and improve the welding strength.