Brazing materials and technology for shield cutters

---

The shield cutter is usually brazed with cemented carbide on the steel cutter body to cut rock and soil as the working edge.

Principle of solder selection

Solder is an important factor for the welding quality of cemented carbide tools.

Solder can be divided into copper based solder and silver based solder. Silver based solder (such as solder containing 5% to 65% silver) has low melting point, good high temperature plasticity and good fluidity, but its price is high and its welding strength is poor. The chemical composition and characteristics of common copper based solders. The fluidity and wettability of solder can be increased by adding trace CO, Ni and Sn, and the strength of weld can be improved by adding Fe and Mn. The melting point of solder not only affects the welding productivity, but also affects the welding stress. When the welding temperature exceeds 950 ℃, the phase transformation of cemented carbide will occur, which reduces the performance of the alloy. The solder with good high temperature plasticity can play the role of buffer and balance in the process of welding cooling, reduce the thermal stress caused by the difference of linear expansion coefficient and thermal conductivity between cemented carbide and steel, and can be heat treated after welding, which provides the conditions for post welding heat treatment for some tools with certain hardness requirements. The uneven thickness of the weld is also the cause of welding cracks in cemented carbide. When welding, the solder with good fluidity and wettability can flow across the whole welding surface to obtain uniform and thin weld and improve the welding strength.

(1) When the working temperature is higher than 400 ℃ and the mechanical load is large, red copper and high strength copper solder are selected

(2) When the working temperature is lower than 400 ℃ and the mechanical load is not large, brass and low silver solder (solder containing less than 30% silver) should be selected. Silver solder can be replaced by high strength solder.

(3) When the blade is longer and thinner, or the shape is complex, low melting point solder, such as low temperature silver solder or silver based solder with higher silver content, should be used.

Selection of flux (flux)

The amount of flux should ensure that the brazing filler metal, cemented carbide and steel matrix will not be oxidized in the brazing process, because the shield cutters are relatively large and the brazing area is large. This requires flux to have the ability to remove oxide for a long time.

Special flux for shield cutter, hard alloy, high strength copper solder for shield cutter industry, special flux.

2. Control of heating rate and cooling rate

During rapid heating, the compressive stress in the outer layer and tensile stress in the center of cemented carbide may appear visible cracks or invisible cracks in the inner when the heating speed exceeds the allowable value. During rapid cooling, tensile stress appears on the outside, which causes cracks. If it is heated too fast, the temperature of the tool bar will be higher than that of the alloy blade, then the molten solder will wet the tool bar but not the blade, the welding strength will be reduced, and the uneven heating will occur. If the heating is too slow, the opposite phenomenon will appear, and the phenomenon of welding surface oxidation will be caused. Different brands and specifications of the blade welding heating speed is different, the lower the cobalt content and specifications of the larger, more complex, heating speed should be slower. The heating speed of high frequency welding can be adjusted by adjusting the heating power or the distance between the tool and the induction ring.

After welding, cool down

The cooling rate after welding also has a very obvious effect on the generation of welding cracks. Especially when the temperature is below 300 ℃, the rapid cooling will force the sharp plastic deformation of the cutter bar and cause the blade to crack. Generally speaking, the cooling rate of the blade is about 8 times lower than the allowable heating rate. The welding tool should be placed in a 200 ℃ ~ 300 ℃ furnace, kept for 6 ~ 8h, and cooled to room temperature with the furnace. Or put it in dry mica powder, asbestos powder, hydrated lime or similar insulation material for cooling. When welding in autumn and winter, the working environment temperature should be kept above 15 ℃ to avoid the direct blowing of cold air, which can promote the cooling speed to produce internal stress.

3. The better welding temperature should be 40 ℃ to 80 ℃ higher than the melting point of solder. Too high will lead to oxidation, too low will lead to false welding. When white smoke and blue smoke are found during welding, it means that the temperature is too high, which causes zinc volatilization, and produces bubbles in the welding layer to form pores, which affects the welding quality.

4. The flux should be dehydrated, otherwise it will bring difficulties in welding operation and affect the welding quality. It should be stored in a sealed container to prevent moisture.

5. The thickness of the welding part of the steel body and the cutter bar should be 3-5 times greater than the thickness of the blade. When the steel body and cutter bar need to be quenched, it is better to be heated at the same time with welding (i.e. once), and the quenching agent temperature should be controlled at 180 ℃ ~ 220 ℃. When welding a large blade, a small groove should be milled on the plane of the groove to reduce the internal stress, or a few small holes should be made on the plane of the groove to increase the thickness of the welding layer.

Welding procedure

1. Clean the steel body and blade thoroughly

(1) Cemented carbide shall be sandblasted

(2) The steel body, cutter body, blade and solder shall be degreased

(3) Grind off the oxide layer on the welding surface and make it smooth.

2. For large steel substrate, borax should be boiled after preheating to cover the surface with flux.

(1) Correct selection of flux.

(2) The rust, grease, etc. can be cleaned with flux to prevent the oxidation of knife bar and blade.

3. Soaking

(1) When brazing, first heat the bottom of the tool bar

(2) Soaking to prevent hot cracking

(3) Do not overheat

(4) After reaching the brazing temperature, the temperature should be kept for 10-30 seconds according to the size of the alloy sheet to make the temperature on the welding surface uniform.

4. Clean the adhesive of alloy sheet and welding part, and carefully check whether the blade and groove are wetted by solder.

5. Slow cooling

6. Sandblasting after welding

The shape, toughness and coefficient of linear expansion of cemented carbide tool body material are quite different from that of steel matrix. When the shield cutter has welding cracks and excessive residual stress, the alloy is easy to peel off.

Because of the large brazing area of the tool, the welding seam will produce large residual stress, deformation and cracks due to the mismatch of material parameters in the brazing cooling process.

Eliminating the residual stress in the brazing process is an important factor to ensure the alloy breaking and falling off during the cutting process of shield cutter.

vacuum braze furnace