Responding to paulandrews, if you cannot impact test coupons taken from the existing pipe, the following is recommended to help guestimate the low temp transition temp.
1. Obtain the chemistry for C, Mn, P, S, Si, Al, V , Nb.
2. In-situ microscopically examine for grain size.
If the grain size is fine (ASTM 7 or finer) and the Mn:C ratio is > 6 and the material is fully deoxidized (Si > .12), the X46 pipe should have toughness properties at 0C and below. Lower transition temperatures can be expected if P and S are low (<0.015%).