Analysis of Causes of Cracks in Welding Tools and Improvement of Hardware Tool Technology

1 Mechanism and type of crack formation in welding knives
1 Mechanism and type of crack formation in welding knives

1) Effect of heating on the formation of cracks in cemented carbide

The thermal expansion coefficient of cemented carbide inserts and steel (cutter) differs greatly, and the thermal conductivity of the alloy is also worse than that of the cutter body. If it is heated during welding, it will generate a large internal stress, which will promote the thermal stress of the insert at the weld layer. Too large causes the blade to crack.

Therefore, the soldering temperature is controlled to be about 30 to 50 ° C greater than the solder melting point. The melting point of the selected solder should be lower than the melting point of the cutter bar by 60 ° C. When welding, the flame should be uniformly heated from the bottom to the outside and slowly preheated for welding. Therefore, the sipe is required to be in conformity with the welded surface of the blade. Local overheating can cause a large temperature difference between the blade itself or the blade and the shank (larger and thicker blades), and thermal stress will cause the blade edge to crack. Therefore, it is required to preheat the arbor first when preheating. If the blade is heated together with the arbor, it should be heated back and forth to the left and right to move the flame, so as to avoid localized overheating and cracking caused by heat concentration.

2) Effect of sipe shape on crack formation

The shape of the sipe is inconsistent with or different from the welding surface of the shank, forming a closed or semi-closed groove shape, which is likely to cause excessive welding surface and excessive welding layer. Since the shrinkage rate is inconsistent after thermal expansion, it is also easy to be welded in the blade. The stress is too large to form a crack. The area of ​​the brazed surface is minimized as required to meet the required weld strength requirements for use.

3) Effect of cooling on the formation of cracks in cemented carbide

Cooling or rapid cooling during welding or after welding and poor dehydration of the solder will cause the blade to burst and penetrate. Therefore, the solder is required to have good dehydration properties. After welding, it must not be cooled rapidly in water. It should be slowly cooled in lime, asbestos powder, sand, etc. It is best to keep it cool at about 300 ° C for more than 6 hours after cooling.

4) Influence of defects on the bottom surface of the sipe on crack formation

The contact surface between the blade and the sipe is not flat. If there is a black pit and a local inequality, the welding cannot form a planar bond, resulting in uneven solder distribution, which not only affects the weld strength but also causes stress concentration, resulting in blade breakage. Therefore, the blade should be ground to the contact surface, and the welding surface of the blade sipe should be cleaned.

In the process of the milling insert groove and the blade, the blade is required to extend the support portion of the cutter bar to be no more than 0.5 mm. If the blade extends beyond the support portion of the cutter bar or the support portion of the cutter bar is weak, the cutter is in the welding process. It is subjected to tensile force and fracture.

5) Effect of secondary heating of the blade on crack formation

After the blade is brazed, the copper brazing filler metal does not completely fill the gap, and some individual welding occurs. In some processes, the blade falls on the arbor during the tapping process, so secondary heating is required, so that the binder Co Severe burning, WC grain growth, may directly lead to blade cracks.

2 Characteristics of cracks caused by welding stress

Cracks appear on the carbide inserts, in some cases due to excessive weld stresses that exceed the strength of the carbide insert. When welding a tool, the height hc of the body should be greater than ht3 times the height of the blade. Such as hc / ht, after welding, it is easy to cause the alloy blade to break; if hc / ht http://news.chinawj.com.cn edit: (Hardware Business Network Information Center) http://news.chinawj.com.cn

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