leftyeng;
The main conmparison between these two material specifications is going to be the notch toughness for the intended service. A 320 provides assurance for adequate notch toughness at low temperature. If you are going to use A 325, you would need to have impact testing performed to assure sufficient notch toughness.

Great - thanks!

I will suggest we have the vendor do some charpy tests on both types and present us with the results.

The issue is more complicated than running a single set of tests.  ASTM A 320 requires parts to be impact tested from every lot.  Here is an excerpt from the standard:

6.1.2.1 For heat-treated bars, one tension test and one
impact test consisting of three specimens shall be made for
each diameter of each heat represented in each tempering
charge. In the continuous type treatment, a charge shall be
defined as 6000 lb [2700 kg].

This frequency is used to guarantee impact results are not affected by lot-to-lot variations in chemical composition, melting practices, microstructure, heat treatment, etc.

In similar climates, we have been using regular ASTM A325 Type 1 bolts and they perform fine.  The client is requesting ASTM A320 L7.  The goal is to find a compromise that the client is comfortable with, and thats why I would request testing to confirm the comparison of qualities, before making a decision of which type to use. I doubt that they will choose continue doing extra tests outside the usual requirements for the chosen bolt type throughout production.

Does anyone know the temperature ratings for A325M Type 1 and Type 3 bolts? I couldn't find it for these two, but the A320 L7 temperature ratings are overkill (-150F/-100C) for our application.

ASTM A 320 only lists recommended temperature - you are free to specify - 45 °C.

If you stay with ASTM A 325, your impact results can vary widely.  This standard allows a part to have a sulfur concentration of 0.058 % by mass, which is very high for toughness-critical part.

If you can work with your suppliers (preferably a single one) to use best practices and produce parts that have impact results in mind, you may be able to use ASTM A 325.

For reference, a similar standard to ASTM A 325 is ISO 898-1, property class 8.8.  The minimum impact strength is 30 J at room temperature using a Charpy U-notch specimen.