The "k" area, on a wide flange beam, for example, is the dimension from the outside of the flange to the web where the radius ends. The "t" area picks up where the radius ends. For example, a W14 x 22 has a nominal depth of 13 3/4". The two "k" dimensions are 7/8", and the "t" dimension is 12".
Welding should always be avoided at or near the "k" area of the shape. The "k" area may have reduced ductility due to cold straightening at the mill. Also, the "k" area is the slowest to cool after the shape is hot rolled, so it will contain more of a course microstructure. This combination can sometimes lead to brittle crack propagation within the "k" area.