Preheat is the temperature the base metal is heated to before welding can commence. The maximum interpass temperature of the base metal adjacent to the point of weld initiation that welding can continue. If the maximum interpass temperature is higher than that listed by the WPS, welding must be delayed to allow the heat to dissipate.
The maximum interpass temperature is usually a concern when toughness is a concern. Allowing the interpass temperature to exceed some value will reduce the cooling rate and potentially encourage grain coarsening in the HAZ. Generally, coarse grains exhibit lower toughness when compared to the toughness of a HAZ with fine grain.
The minimum preheat and interpass temperature, as well as the maximum interpass temperature is established when the WPS is qualified by testing. If, in production, the contractor decides it is necessary to increase the interpass temperature above the interpass temperature established by testing by a certain amount, the WPS must be requalified.
ASME Section IX provides more detailed information with regards the permitted ranges for preheat and interpass temperature and when a new WPS has to be qualified. Generally, the WPS has to be requalified if a lower preheat is required or if a higher interpass temperature is needed to meet production requirements. ASME Section IX, Article II lists the Essential, Supplementary Essential, and Nonessential Variable for most of the recognized welding processes used for the fabrication of pressure vessels or pressure piping.
One can think of preheat as the minimum temperature required before welding commences and the maximum interpass temperature is the maximum temperature where welding is allowed to continue. Once welding starts, the lower interpass temperature is usually required to be at least equal to the minimum preheat temperature. Welding can continue as long as the maximum interpass temperature isn't exceeded.
