Liquid Metal Embrittlement in the Arc Brazing of Stainless Steel
When a solid metal, under stress, comes into intimate contact with a liquid metal of lower melting point a considerable increase in brittleness may result. It is generally accepted that this phenomenon is only observed in particular metal pairs, and for a given metal pair the increase in brittleness occurs suddenly at a characteristic level of stress; although at least one report offers evidence that this specificity is more a feature of the testing methods used than the materials themselves.
Test procedure to determine susceptibility to LME
A test for susceptibility to LME was devised whereby a flat specimen was held in a Creep Testing machine and loaded in tension using masses via a simple beam pivoted on a hardened knife edge. The beam was balanced initially to just eliminate the weight of the specimen clamping mechanism, the specimen itself and play in the force transfer mechanism. The arrangement is shown in Figure 1.
Initially, the liquid metal was applied as a single spot of liquid braze alloy, using a Gas Metal Arc Welding (GMAW) set with the torch securely mounted to one of the machine uprights and adjustable so the axis of the torch could be set perpendicular to the centre of specimen face and the distance from the torch to the specimen adjusted. A distance of 12mm was used in the tests. 100 per cent Argon was used as the shielding gas and an arc duration time of approximately 1.0s.
The tensile force was increased incrementally and the process repeated, covering a load range from zero up to a point where the specimen failed under test as the liquid metal was applied. The treated specimens were then tensile tested to destruction and the percentage elongation and UTS determined.
The tests were repeated using 316 Austenitic Stainless steel treated with two different brazing consumables as follows
|Braze Material||BS Number||Melting Point
/ Degrees Celsius
|SIFMIG 328||2901 C28||1030||550||8 per cent Al bal. Cu|
|SIFMIG 968||2901 C9||980 - 1020||350||1 per cent Mn 3 per cent Si bal. Cu|
Results to date
After testing, the results were gathered and in summary show
- the embrittled specimens showed a relatively small reduction in UTS of approximately 20 per cent
- the embrittled specimens showed a dramatic reduction in percentage elongation when the pre stress was above a characteristic value dependent upon the braze consumable used
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