Halliday & Matthiessen (1999: 260-1):
Class shift becomes metaphorical when the "shifted" term creates a semantic junction with the original. A good way of illustrating this is to bring together two instances of the same lexical item, used once as (non-metaphorical) transcategorisation and once as grammatical metaphor. Let us return to a previous example:More congruently, this would be many (pieces of glass) fail after the cracks have slowly extended, or often the cracks slowly extend and then the glass fails. The congruent form is a sequence of two figures linked by a relator; in the metaphoric form, each figure becomes a participant and the relator becomes a (relational) process to which two participants subscribe. Here, then, failure is an instance of metaphorical class shift; there is a semantic junction between two features:(1) 'process' (class meaning of verb fail),(2) 'thing/ participant' (class meaning of noun failure).Note that there is an asymmetry between the two — a time line, such that the feature 'thing' is as it were a reconstrual of the original feature 'process': we could gloss it as "a process reconstrued as a participant".
Contrast this now with failure in a technical expression such as heart failure; in origin this was no doubt a grammatical metaphor for the heart fails, but the metaphorical quality has since been lost, or at least significantly weakened (the metaphor is "dead"), and heart failure is now the only congruent form.
Likewise contrast he regretted his failure to act, agnate to that he had failed to act (or that he had not acted ), where failure is a grammatical metaphor, with he always felt that he was a failure, where failure is now the congruent form and this is not a metaphorical agnate of he always felt that he had failed.