Yes, using thermal imagers for measuring temperatures during welding requires the adaption of the soectral sensitivtiy of the camera to the measured metal. For the best choice the emissivity of the metal shouldbe as high as possible - and stable - in the spectral range of the thermal imagers. Usually, the mid-infrared range (5 -10 µm) is well suited - for steel 2 - 3 µm are better -, but depents on material and surrounding temperature. You may have a look at the following reviews.
Chapter A Review and Preliminary Experiment on Application of Infrar...
Chapter Chapter 1 Industrial Applications of Radiation Thermometry
Article Infrared Thermography for Condition Monitoring- A Review
Fluke camera is almost the best but it's a little costly however ensure that the camera measuring capacity could cover you welding temperature application