Over the coming months, we will be highlighting and explaining some of the most interesting and obscure failure mechanisms which we have witnessed at R-Tech Materials. Today we kick off our #epicfail series of articles with a look at Liquid Metal Embrittlement…
Liquid metal embrittlement (LME) can lead to catastrophic failure of various products across a wide range of industries. The phenomenon occurs when molten metals come into contact with susceptible materials. Crack growth rates can be incredibly rapid and failure can occur within seconds of exposure. The fast crack propagation associated with LME can have serious consequences; a plant explosion at Flixborough UK plant in 1975, which killed 28 people, was attributable to LME of a stainless steel pipe in contact with molten zinc. In fact, products which contain mercury are prohibited by all airlines as they pose a serious risk to the structural integrity of the aircraft which is produced predominantly from aluminium alloys.
Liquid Metal Embrittlement (LME) can occur during fabrication or in service. Galvanised steel products have been found to exhibit LME upon welding. In addition, brazed stainless steel products are also commonly associated with this phenomenon. In service, a susceptible material in contact with a low melting point metal at low temperatures may crack when the temperature rises above the melting temperature of the low melting alloy. In refineries, mercury is found in some crude oils and can condense in the atmospheric tower overhead system thereby embrittling Monel 400, titanium and aluminium components.
The mechanism occurs initially with the molten metal weakening the surface layer, disrupting the stress equilibrium. Areas of high stress between the grains of the base material are relieved by the liquid filler metal, which quickly causes visible cracks along the grain (see Figure 1) and/or failure in the base material. There are three conditions necessary for LME to occur:
a) A susceptible alloy
b) The presence of a tensile stress (applied or residual)
c) The presence of a liquid filler metal
Figure 1 – Copper LME of a steel product
Numerous materials can be affected by this phenomenon such as austenitic stainless steel, carbon steel, alloy steel, nickel alloys, aluminium alloys and copper alloys. Liquid Metal Embrittlement (LME) tends to occur in specific combinations of metals in contact with low melting point metals such as mercury, zinc, copper, cadmium, tin and lead. Typical combinations are given in the table below.
|
Susceptible Alloy |
Molten Metal |
|
Carbon steel |
Zinc, lithium, copper |
|
Austenitic stainless steel |
Zinc, lithium, copper |
|
Copper alloys |
Mercury, zinc |
|
Monel 400 |
Mercury |
|
Aluminium alloys |
Mercury, zinc, tin |
|
High strength steels |
Cadmium, lead |
|
Titanium |
Mercury, cadmium |
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