To better understand the mechanisms and rate of temper embrittlement in cast HY-80 high-strength low-alloy steel, samples were subjected to embrittlement and recovery treatments after a typical industrial heat treatment. The embrittlement rate was studied between 315 and 565 degrees C at varied logarithmic time intervals up to 10000min. The embrittlement recovery rate was studied between 593 and 704 degrees C at logarithmic time intervals up to 1000min. Metallographic and computational work for these samples supports a conclusion that the observed embrittlement and subsequent recovery cannot be solely explained by the segregation of phosphorus and other embrittling elements to grain boundaries as has been previously suspected in this alloy system. Rather it is shown that alloy carbides such as M23C6 play a key role in embrittlement, whereas carbide coarsening and/or nucleation at the prior austenite grain boundaries and in the bulk in the embrittling temperature range. The evolution of these carbides serves both to create initiation sites for cleave and depletes the matrix of Mo, which is a P scavenger. Recovery from embrittlement is thus proposed to be related to both the removal of P from the boundary and the dissolution of carbides from the boundary. Earlier work showing the suspected maximum embrittlement rate occurs at 482 degrees C with the maximum recovery rate at 648 degrees C has been rigorously confirmed for a high carbon equivalent cast chemical composition.