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What are local brittle zones (LBZs)?


Local brittle zones, or LBZs, are discrete microstructural regions in a weld heat affected zone (HAZ) that exhibit significantly lower resistance to fracture initiation than surrounding material.

The term was originally coined in the mid-1980s to describe the behaviour of HAZs in certain structural grade microalloyed carbon-manganese steels during crack tip opening displacement (CTOD) fracture mechanics testing when unexpectedly low values of toughness were recorded. However, in susceptible material, LBZ behaviour can be found by any fracture mechanics (typically CTOD values very much less than 0.1mm) test (e.g. KIc, JIc) used to determine initiation fracture toughness and not just the CTOD test. Charpy impact testing, in contrast, does not normally reveal LBZ behaviour. A reason for this is that the energy recorded by the Charpy test is an integration of initiation and propagation energies for a fracture path that may include LBZs as well as tough material. A fracture mechanics test (eg. CTOD tests), on the other hand, characterises the conditions for the initiation of fracture from the lowest toughness region present at the crack tip. In addition, the standard Charpy specimen is 10mm thick and this is usually much less than the weld thickness and the thickness of the fracture toughness specimen. So the Charpy specimen could simply miss any LBZ present in the weld.

LBZ behaviour is usually associated with initiation by cleavage, but intergranular fracture has been observed in some instances. Investigations have shown that LBZs in multipass welds of ferritic steels can be either the grain coarsened HAZ (GCHAZ) close to the weld fusion boundary or the intercritically reheated grain coarsened HAZ (ICGCHAZ). Although low toughness regions could be present at the boundary between the intercritical and subcritical HAZ (IC/SCHAZ) - as a result of locally intensified strain ageing at a pre-existing crack tip in parent plate, HAZ or weld metal, or within the weld metal itself - the term is not usually used to describe these cases. LBZ behaviour depends primarily on steel chemistry, welding procedure, welding heat input, post-weld heat treatment condition and fracture toughness test temperature.

The size of a LBZ is a function of welding heat input and the coarsening behaviour of the steel. However, typically it can be less than 0.5 to 1.0mm in height and can extend parallel to the weld over a distance of tens of millimetres. The length will depend on welding process, weld straightness and frequency of stop/starts.

Sometimes 'LBZ' is used to describe a microstructural region such as the GCHAZ, ICGCHAZ or HAZ. This is an erroneous use of the term since the region is not a LBZ until fracture tests have shown it to have low toughness.

The significance of LBZs is dealt with separately.

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