Every welding engineer knows the importance of preheat. It has an important effect on the properties of the weld. There exists a lot of literature stating the importance of preheat, ill-effects of not adopting preheat, methods of preheat, how to calculate preheat and so on. This article explores the various aspects of preheat with regard to ASME Section IX.
In Section IX, preheat is listed as an essential variable for most of the welding processes under QW-250. Addressed at QW-406.1, this variable says that a decrease by more than 100°F (55°C) in the preheat temperature qualified, requires requalification. Seemingly simple enough, it is a variable that is commonly misunderstood by welding engineers who write WPSs.
Suppose that a PQR lists the preheat temperature as 300°F. Does this mean that the preheat on the production WPS can be specified as 200°F, in line with what QW-406.1 apparently permits?
This is the mistake that most welding engineer commit. In fact, Section IX does not intend this variable implemented in this manner.
How Should the Preheat Be Reckoned Then?
Say, a procedure qualification is being carried out. The provisional WPS being used for making the test coupon specifies a preheat of 300°F. The welder lights up the flame (or switches the induction heating arrangement on, or whatever) and brings the temperature up to 300°F. Welding is commenced.
For the first pass, the preheat is recorded as 300°F. However, as the welding goes on – the temperature on the base metal can no longer be expected to be 300°F. In fact, it rises up to 350°F or 375°F, etc. for the subsequent passes. This is due to the heat of the molten metal being deposited on it.
Over the course of welding, the observed preheat temperature on the coupon ranges between 300°F to 400°F. Section IX assumes that this variation in temperature would not be more than 100°F.
When a WPS is written with the support of this PQR, a minimum preheat of 300°F must be specified. Some code users wrongly think that a minimum preheat of 200°F can be specified on the WPS, citing that QW-406.1 permits a decrease of up to 100°F.
The essence of this discussion is that the minimum preheat specified on the WPS should not be lower than the minimum observed during test coupon welding. The 100°F leeway given at QW-406.1 is merely to account for the inevitable rise in temperature during actual welding of the test coupon.
Section IX does not offer this clarity in full. Perhaps the committee ought to take cognizance, and take corrective action.
Suggestion to ASME Section IX Committee
QW-406.1 says:
“A decrease of more than 100°F (55°C) in the preheat temperature qualified. The minimum temperature for welding shall be specified in the WPS.“
Whereas, QW-406.3 says:
“An increase of more than 100°F (55°C) in the maximum interpass temperature recorded on the PQR.”
These two variables are wrongly applied by code users in the following manner:
Suppose that a minimum pre-heat of 300°F has been observed in test coupon welding. Using QW-406.1, code users think that it is alright to specify 200°F in the WPS. Likewise, for inter-pass temperature.
We know that the actual pre-heat temperature observed during test coupon welding easily varies by ̴100°F from the temperature specified on the provisional WPS. For example, if the specified preheat on provisional WPS is 300°F minimum, the observed preheat during test coupon welding would in all likelihood be 375°F-400°F, especially after the first few passes have been deposited.
Likewise, if the specified inter-pass on provisional WPS is 400°F maximum, the observed inter-pass during test coupon welding would in all likelihood be 300°F-325°F, since the welder is only aiming to stay below 400°F.
So What Should Be Done Then?
QW-406.1 can be reworded as: “The range of pre-heat temperature observed across all passes of test coupon welding shall be recorded. If this range is not more than 100°F, the preheat specified on production WPS shall not be less than the minimum preheat temperature recorded on PQR. If the range exceeds 100°F, the pre-heat specified on production WPS shall not be more than 100°F below the highest preheat temperature recorded on the PQR.”
Likewise, QW-406.3 can be reworded as: “The range of inter-pass temperature observed across all passes of test coupon welding shall be recorded. If this range is not more than 100°F, the inter-pass specified on production WPS shall not be more than the maximum inter-pass temperature recorded on PQR. If the range exceeds 100°F, the inter-pass specified on production WPS shall not be more than 100°F above the lowest inter-pass temperature recorded on the PQR.”
If this suggestion is implemented, this will eclipse interpretation IX-83-165 (qs 2).
This was all. Thank you for reading. Please do offer your take on the subject, in the comments section below.