Interfacial crevice corrosion is a particular type of failure where silver brazed joints in stainless steel are exposed to water or humidity in service. In these conditions joint failure may result along the stainless steel - brazing filler metal interface.
To produce a joint failure by interfacial corrosion three criteria need to be satisfied:
All types of stainless steel are susceptible to attack by interfacial corrosion. The nickel free, or low nickel ferritic and martensitic type stainless steels (e.g. types 403, 410, 416, 420, 430, 431) are most susceptible to interfacial crevice corrosion.
Austenitic grades of stainless steel (e.g. types 302, 303, 304, 316, 321) are more resistant. Failures with these grades are rare provided the correct brazing alloys are chosen.
There are several other types of filler metals that can be used to avoid the potential problem of interfacial corrosion:
Please contact us for more details about these products or on any other matter relating to this problem.
Here are three steps that can be taken to avoid cracking in tungsten carbide segments:
1. Joint design to help avoid cracking
Increasing the joint clearance will provide a thicker layer of ductile brazing filler metal capable of accommodating the stresses from differential expansion.
On large carbides the use of thin backing materials can lead to cracking because they are not able to withstand the high stress resulting from contraction on cooling. Thicker backing materials or bodies to hold the carbide can reduce the incidence of failure. With long lengths of carbides bending or cracking can be a problem. Consideration should be given to the use of multiple pieces of carbide to overcome this.
2. Filler metal selection to help avoid cracking
Conventional free-flowing brazing filler metal alloys such as Silver-floTM 55 capable of filling joint gaps of 0.05mm are satisfactory for brazing carbide with a length of up to 9mm.
Larger sized carbide pieces can be stressed to such an extent that cracking occurs during brazing or in a subsequent grinding operation. For these applications the joint must be artificially thickened. Using either a less free-flowing brazing filler metals such as Argo-brazeTM 49H or a tri-foil brazing material can do this.
3. Cooling and finishing
Slow uniform cooling of the carbide is always recommended to avoid stressing and possible cracking. Quenching in water is not recommended. It is advisable to avoid thermal stresses during grinding and finishing of the carbide component.
The cemented carbide will be more easily wetted by the molten brazing filler metal if the surface is ground shortly before brazing then degreased and kept clean before applying flux.
The degree of wetting of brazing filler metal onto a cemented tungsten carbide piece will depend on its composition. Cemented tungsten carbides with small additions of titanium or tantalum carbide are more difficult to wet than standard carbides.
Wetting can be improved by the use of brazing filler metals containing nickel or manganese (e.g. Argo-brazeTM 49H) and special boron modified fluxes such as TenacityTM No.6 Flux Powder or Paste. Plating or coating the carbide with an easy to wet metal such as copper or nickel can also help overcome this problem.
In general when brazing cemented tungsten carbide a heating pattern should be employed which brings both components to brazing temperature at the same time. Care should be taken to avoid overheating the cemented tungsten carbide component as this will increase stresses. Once the brazing filler metal is molten it is advisable to move the carbide slightly to improve wetting and displace trapped gas or flux.
Tin containing filler metals are 'hot-short' and may be prone to cracking if quenched from high temperatures (in excess of 300˚C). This is particularly the case with filler metals with medium or high brazing temperatures. They should not be quenched when used to braze components with widely differing coefficients of thermal expansion.
Filler metals containing silicon may be used to braze steel assemblies but are not recommended where steel components are subject to high impact or fatigue stress in service as the silicon forms a brittle iron intermetallic.
When brazing assemblies which come into contact with sea water and other aqueous solutions with a high ion concentration, it is important that both the parent metal and the brazing filler metal are resistant to dezincification.
Johnson Matthey undertook research work during the 1940's and discovered that the filler metals Silver-floTM 60, 56, 55, 452, 44 and 43 all offer excellent resistance to dezincification. Consequently these filler metal alloys have found uses in brazing of marine pipes and fittings in offshore installations and in shipbuilding.
Of these filler metals Silver-floTM 55 is the most widely used because it has a low brazing temperature and excellent flow characteristics.
To build up an assembly in two or more brazing operations, it may be necessary to use brazing filler metals with successively lower melting points. This technique will avoid disturbing the previously brazed joints. There are a number of filler metals which have short melting ranges and are ideal for this work:
Silver-floTM 24, 20 and 16 are also suited to step brazing.
For some applications a colour match between the brazing filler metals and the parent metals is clearly desirable. The following Silver-floTM alloys should be considered.
Nickel and nickel-based alloys are susceptible to cracking during brazing with silver brazing filler metals. This cracking is often known as inter-granular penetration or stress cracking. High nickel content copper alloys, such as 70:30 cupro-nickel, are also prone to this type of failure. Removing the source of stress will eliminate the problem.
Silver-floTM 60 is recommended where freedom from stress cannot be guaranteed. The relatively low zinc content and high brazing temperature make it less likely to initiate stress cracking.
Today the potential danger from cadmium oxide fume is widely recognised. The localised extraction of fumes during brazing is now virtually obligatory. The filler metal should never be heated directly with a flame. Indirect heat from the components being joined should be used to melt the filler metal. If cadmium containing filler metals are felt likely to cause a health hazard then use a cadmium free filler metal from our Silver-floTM range.
Full details on exposure levels (OES) are given in the specific Material Safety Data Sheets (MSDS). These should always be consulted before use of a cadmium bearing brazing filler metal.
The use of cadmium containing filler metals is not permitted in the manufacture of food and drink handling equipment, medical instruments, automotive components, electrical and electronic goods.
More information can be found in the UK Department of Health and Safety Information Sheet 'Cadmium in Silver Soldering or Brazing' (PDF, 145KB, opens in a new tab).