Brazing Procedures Specifications

Posted on Monday, December 5th, 2011 at 5:53 pm.

Example: Brazing Tungsten Carbide to Steel

Note: this is a general overview. It is, in no way, meant to be suitable for any specific application. It is to serve as a guide and a base for brazing applications in general

1.  Safety

A.  List of dangers – flux fumes, perhaps cadmium in the braze alloy, Zinc fumes

B.  Methods of exposure – inhalation, smoking or eating without washing hands

C.  Avoiding exposure – work under the hood. Use smoke sticks to verify hood effectiveness. . Wash your hands.

D.  Other safety considerations.  Do not get burned. Even parts that have cooled considerably can still be hot enough to burn you.

2.  Surface condition

A.  Is plating necessary

B.  Roughness

C.  Free carbon

D.  Oxides

E.  Oils and greases

F.  Surface cleaning methods

3.  Test methodology for surface conditions

A.  Eyedropper method of water break test – use an ordinary eyedropper and gently put four drops of water onto the surface to be tested.   If the water forms a high bubble the surface needs to be tested. If the water forms a flat puddle the surface should braze well.

4.  Ambient conditions

A.  Cold shop – A cold shop, common on Monday mornings, can affect heat transference and cycle times.

B.  Cold fixturing – Cold fixturing can suck the heat out of the braze joint.  Typically the fixturing is on the under side or the back side. Quite often there is a good joint on the visible side and a cold joint on the fixturing sign.

5.  Alloy selection – alloy selection may be the most under-appreciated part of brazing.

A.  Suitable for materials being joined

B.  Desire temperature range

C.  Desired strength

D.  Desired cost

E.  Safety e.g. Cadmium

6.  Flux selection

A.  Suitable for braze alloy temperature range

B.  Robustness – some fluxes hold up longer than others.

C.  External fluxing – fluxing outside the braze joint only

D.  Internal fluxing – fluxing incised the braze joint. A more purified flux may be needed

7.  Braze joint thickness

A.  Tensile strength versus impact strength – a thinner joint typically has higher tensile strength while a thicker joint gives more strength against impact particularly where there is a great difference in thermal expansion between the two materials being joined.

B  Amount of alloy

F   Fixturing pressure – too much pressure can squeeze the liquid alloy out of the joint

8.  Internal temperature measurement

A.  “Rock in shoe” method – when brazing large parts it can be difficult to tell when the proper temperature is been achieved in the thermal center. One method to determine this is to put a bit of braze alloy wire in the center of the joint. When the wire bit in the Center melts the parts will shift and you know the temperature.

9.  Heating

A.  Heating method – torch, induction, oven, atmosphere, friction

B.  Maximum temperature – how hot can you get the braze alloy?

C.  Heat of transformation – (50º F above flow point) (Much more than heat of transformation is involved here.)

D.  Cycle time

E.  Maintenance of equipment e.g. Fluoride based flux fumes etching optical sensors

10. Movement of parts

A.  Keeping parts where you want them

B.  Moving parts at temperature – sometimes desirable to release trapped flux fumes

C.  Amount of movement

D.  Movement pressure

11.  Post braze analysis

A.  Gaps and voids

B.  Uniform flow

C.  Braze joint thickness

D.  Difficulty of cleanup – brazing fluxes are typically easy to clean up with warm water and a little scrubbing. If this is not the case, quite often it is indicative of the fact that not enough flux was used.

12.  Cleanup

A.  Method – typically warm water and general agitation

B.  Materials

C.  Tools – e.g. brush, tumbler, strainer

D.  Safety – wear eye protection if brushing, wash hands afterwards

Carbide Processors, Inc.

3847 S. Union Ave

Tacoma, WA 98409

800 346-8274

president@carbideprocessors.com