That said, if it's a structural weld, odds are you'll want to avoid formation of untempered martensite in the HAZ. Not sure of the exact hardness range, but I'd guess if you apply pre- or post-weld heat treat and see hardness readings at or under 35 HRC in the HAZ (converted from either Vickers or Knoop) you are probably free of untempered martensite. Examining the microstructure is will confirm this. This still doesn't answer the question of whether or not the weld is sufficient for your application - that's something you'll need to determine.
200 HB is NOT a good rule of thumb. That would only be a 93 HRB threshold.
HAZ hardness should be close to the base metal hardness. This is achieved in 1045 with preheating and possible post-weld heat treatment. I agree with Kaufmj42 that you are really concerned with untempered martensite formation. However, you need to look at HAZ hardness relative to the base metal instead of an absolute value because 1045 can be heat treated to differing strengths/structures/hardness. Instead, focus on whether the HAZ hardness is either too high relative to base metal (implying possible formation of a brittle structure) or too low (indicating a lower strength region adjacent to the weld more prone to cracking by fatigue).
