Minimum Preheat Temps for PQR VS. Preheat Temps for WPS

By skerfan Date 06-23-2009 18:56

A 1" thick PQR was qualified using a minimum preheat temperature of 150 Degrees F. CVN requirements were also mandated (-40 Degrees F.) When the WPS is spun out from the PQR does the minimum preheat temperature still have to be 150 Degrees F. for all thicknesses qualified (5/8" to Unlimited)? Reference Table 4.5 Item 36 and Note d, Paragraph 5.6, and Table 4.6 Item 7. It seems to read that this is the case, it just doesn't make common sense to me that thinner Listed base metals in Table 3.2 would have to be preheated to the higher temperatures of the PQR. We would like to list a table on our WPS by thickness and in line with the Preheat Temps shown in Table 3.2.

Thanks for the help in advance,
skerfan

By 803056

Date 06-24-2009 13:31

Look at Table 4.5 in AWS D1.1-2008, item 7. It states that an increase in preheat requires requalification. Based on that, I would say you could use table 3.2 as a guide for preheating materials thinner than that used for qualification if the preheat temperature is equal to or less than that used for qualifying the WPS and items 35 and 36 from Table 4.5 are not violated.

Best regards - Al

By HgTX

Date 06-25-2009 19:19

I must say I don't understand Table 4.6 item 7. Why are they more concerned about an increase in preheat rather than a decrease in preheat? And given that preheat is thickness-governed, how can you tie the production preheat to the test preheat if the thicknesses are different?

Hg

By 803056

Date 06-25-2009 20:10

The cooling rate will influence the grain size of the weld and heat affected zones. Grain size is coarser with slow cooling, i.e., high preheat and high heat input. Small grains are promoted with lower preheat, low heat input, and faster cooling.

Coarse grains have better high temperature creep properties than small grains, but small grains have better low temperature toughness than coarse grains.

Best regards - Al

By HgTX

Date 06-25-2009 21:39

But lower preheat can mean higher residual stresses, and cooling too fast can harden the steel. In general the less optimal the welding conditions, the more preheat seems to be required by the codes, and more attention is giving to setting a minimum than setting a maximum, so it seems odd in this one location to worry about an increase to preheat.

Hg

By 803056

Date 06-26-2009 03:30

Welding at best is a balancing act of the good versus the bad.

Due consideration must be given to a number of factors and their aggregate influences on the final outcome. With welding, as with many things in life, there are more than one way of achieving acceptable results. The task of the engineer is to optimize the good and minimize the bad influences.

The saving grace in working with the steels included in D1.1 or D1.5 is their weldability is good, i.e., for the most part the carbon content and the carbon equivalency is relatively low, so the preheat temperature is relatively low compared to a higher carbon steel oe a higher carbon alloy steel. Consider the preheat requirement for 4130, 4140, etc. compared to the PH for a steel listed in D1.1 for equivalent thickness ranges.

The low carbon content and the low carbon equivalency of a steel listed in D1.1 or D1.5. Their hardenability is relatively low in comparison with other alloys having high carbon equivalencies. So, if the hardenability is relatively low, we may consider the grain size as being a more critical factor than the Ce if toughness is a concern.

That isn't to say the Ce isn't a concern, it is, and that is just one of the complications considered when the qualification of the welding procedure includes a toughness (at low temperature) requirement. As a result, the applicable code adds restrictions for the thickness range qualified, the electrode classification, heat input (weave vs. stringers), etc. are considered as essential variables.

When impact toughness is an issue, the complexity of developing a WPS becomes much more involved when compared to a WPS for "normal" welding applications.

Best regards - Al