Embrittlement

When selecting the right alloy for the respective application it is very important to pay attention to the possible tendency of the material towards embrittlement. Any embrittlement in the anchoring system that occurs during the application can easily lead to failure of the refractory lining.

Chi phase

The Chi phase is a metal phase in the Fe36Cr12Mo10 structure that is frequently precipitated prior to the sigma phase and converted to it. The Chi phase occurs at temperatures from 400 to 550° C (475° C – embrittlement). The Chi phase occurs in steel with a chrome content of above 15 %. The ductility and corrosion resistance of steel is reduced by the Chi phase.

Sigma phase

A brittle, intermetallic phase, the sigma phase, occurs in ferritic and austenitic steels at a temperature range of between around 600 to 900° C. Any sigma phase already formed can be dissolved again using a solution annealing process. The sigma phase can have various compositions, for example chrome-molybdenum-nickel-iron or chrome-iron. The formation of the sigma phase is accompanied in the periphery of precipitation by a reduction of chrome and molybdenum in the matrix. The consequence of this is that, in addition to a reduction in ductility, there is drastic worsening in corrosion resistance. Alloy components such as molybdenum, titanium and silicon promote the formation of sigma phases, while nitrogen and carbon reduce formation.

Coarse grain embrittlement

Coarse grain forms in ferritic, heat-resistant steel at temperatures above 950°C. This leads to a loss of ductility, particularly at low temperatures, i.e. when components return to a range of lower temperatures.

Hydrogen embrittlement

Atomic hydrogen can become incorporated in a metal matrix if so-called traps are available in the lattice, but also in lattice imperfections, for example forming martensite or delta-ferritic/sigma phase. The hydrogen can form hydrides with the alloy components or recombine into molecular hydrogen, building up high pressure as a result. For use in refractory construction, where austenitic materials are essentially used, the risk of hydrogen embrittlement is less significant because austenitic lattices are not at risk of hydrogen incorporation. Incorporated hydrogen effuses again at higher temperatures above 350° C.
However it is important to remember that the weld on an anchoring system can be at risk from the incorporation of hydrogen. For this reason the weld area must be dry and electrodes and welding ferrules in the case of stud welding may also exhibit no moisture.

Carburisation

Carbon atoms can be incorporated in the metal matrix as a result of an attack usually by gaseous carbon compounds. The carburisation that is created is accompanied by a loss of ductility.  Carbide formations, often with chrome, may occur at the grain boundaries. As a result chrome is bonded, and is then no longer available to prevent corrosion attacks.

Nitration

Nitrogen contained in the furnace atmosphere, for example ammoniac, may be interstitially incorporated in the metal matrix in atomic form or with formation of nitrides. The result is that ductility of the component can be reduced to the extent that the anchoring system breaks.