By 803056
Date 10-12-2007 23:04
Edited 10-12-2007 23:06
There are some very knowledgeable people that don't agree with me on this subject, but I view it as the preheat temperature is the minimum temperature that must be attained for welding to be initiated or continued, whereas the maximum interpass temperature is the maximum temperature at which welding can continue. That is, the weldment must be allowed to cool below the maximum interpass temperature before welding can resume.
The maximum interpass temperature may not be a variable that has to be controlled to attain or maintain the specified mechanical properties. For instance; when welding hot rolled carbon steel, the maximum interpass temperature is not usually controlled. There is little chance that the welding operation is going to exceed the temperature at which the steel is rolled and the time at temperature for the welding operation is never going to come close to the time at temperature of the manufacturing processes used to make the steel. If notch toughness is a concern, the steel has to be manufactured with notch toughness in mind and the welding process has to be controlled to ensure the notch toughness of the weld (selecting the correct filler metal) and base metal are not compromised. Interpass temperature and heat input are controlled to ensure cooling rates that will promote a fine (small) grain structure in the weld and HAZ to minimize the loss of notch toughness.
That isn't the case for other steels such as quench and tempered steels that include a quench operation after rolling and then a tempering operation to reduce the hardness of the martensite. The tempering operation gives the steel good toughness and ductility without excessive loss of strength. In that case, the interpass temperature and heat input are controlled to prevent an unacceptable reduction of the mechanical properties in the HAZ. High heat input and excessive interpass temperatures will allow the HAZ to cool too slowly to form martensite as the austenized metal in the HAZ cools below the transformation temperature. The high interpass temperature will also create a wider HAZ that is softer and has less strength than is required. Once again, the cooling rates have to be such that fine grain structures are attained to maintain the notch toughness of the base metal.
In the case of austenitic stainless steel, interpass temperature is controlled as a means to minimize the time at temperatures where sensitization can occur. Likewise interpass temperature is controlled when welding heat treatable aluminum alloys that can be overaged if held at "high" temperatures for too long a time.
Best regards - Al