Brazing Hard Carbide – A Primer

October 30, 2017

Cutting tools like drills, dies, and machine tool bits as well as masonry, rock and mining drills are frequently made with carbide.  At fast cutting speeds, carbide increases the life of the cutting edge.

While many mechanical fastening methods have been tried to secure carbide to tools, brazing with a silver alloy is the most successful and widely used method.  Using silver brazing shims, carbide can be attached to the shank on as many or as few sides as necessary, requires no additional space, and can be replaced or salvaged by melting off and re-brazing.

Care should be taken to ensure that the brazement is continuous in the critical areas that make up the boundaries of the brazement.  If not, stress concentrations at any defect may cause failure to start and to progress through the entire joint.  Voids located internally also reduce the strength of the joint; as the tool is reground, internal voids may become exposed and the defect will become located in the more critical section of the joint.  Heat developing during the cutting operation also lowers the strength of the bond, making the soundness of the brazement more vital.

Torch and induction brazing are the most adaptable brazing processes to use for joining carbide tips:  torch brazing for occasional work or where a variety of sizes are being brazed, and induction brazing in a production environment where a large number of inserts are being brazed.  While furnace brazing can be used, problems can occur related to positioning and holding the carbide during the heating cycle.  Proper fixturing can be achieved through wiring and can be economical where large production requirements must be met.

Silver shims should be placed between the carbide and the shank.  As tungsten carbide is difficult to wet with most brazing alloys, nickel helps to overcome this difficulty and is, therefore, a recommended presence in brazing alloys used.  The BrazeIt #A50N alloy is used more extensively than any other alloy, whether it be used in a solid or Plymetal form, although BrazeIt #A49NM or #A40N2 are also frequently used.  For applications where carbide inserts and heat treatment of steel is done in one operation, we recommend using BrazeIt #A40N2 with black flux, as the higher melting point and increased nickel content in #A40N2 controls “run out” of brazing alloy during longer and higher brazing temperatures.

Tungsten carbide has a thermal expansion coefficient of approximately one half that of steel; steel expands at a rate of 3-1 relative to carbide.  This means that during cooling, the contraction of the carbide will be one half the contraction of the steel shank.  The brazement, which is proportional to the size of the carbide, is then placed under internal shear stress.  During service, this internal stress will add to the load of the service stress.  On large inserts, this can lead to joint failure, and on thin carbide blanks, the carbide itself may be cracked and broken by these stresses, as microscopic cracks form due to the inability of the carbide to flex with the other contracting metal.  In some cases, the carbide will actually break off from the base to which it is brazed.  Therefore, when carbide exceeds 3/8” or ¾” in its maximum dimension, brazing with Plymetal (“sandwich brazing”) is recommended.

For questions related to brazing carbide tools, contact us for technical assistance related to alloy and process selection!

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