Why is the lower thickness limited for Impact Tested Welding Procedures? 3

[Salmet]

Hi! Time and again I stuck to the question that: Why is the lower thickness limited for Impact Tested Welding Procedures?. Where as Metallurgically, increasing the thickness usually increases the chances of retained austenite/martensite, thereby decreasing fracture toughness/impact strength.

Thanks in advance.

Salmet

 

[stanweld]

Assuming that you are refering to the 5/8" minimum thickness per ASME IX for qualified coupon T > 5/8", it was instituted in the Code in either the 1973 Addenda or the 1974 Edition. It was instituted by the Nuclear wing (ASME III) of the Code Committee membership on ASME IX. One hopes that it was based on the heat transfer characteristics during welding. Above approximately 1/2" to 5/8", heat transfer is three dimensional (fast cooling). Below those thicknesses, heat transfer is one or two dimensional (slow cooling). The slow cooling rate adversely affects impact properties in the HAZ and increases the width of the HAZ.

 

[JakeR82]

I would initially think that the Surface area per volume ratio is greater on the smaller thicknesses which should result in faster cooling.

 

[stanweld]

JakeR82,
Primary cooling during welding is through conduction (metals have high heat conductivity) and not through convection. The greater the heat sink (mass/volume) the faster the cooling rate.

 

[Salmet]

Dear Stanweld! Thanks for your kind response and time. I would be thankful if you could please explain the increase in HAZ in low thickness, metallurgically with slow cooling rates.

 

[stanweld]

The HAZ is that zone of the base material that experiences temperatures above the lower critical temperature. If the cooling rate is slow, the width of base metal that experieces those temperatures increases.

 

[Salmet]

Dear Stanweld! Hats off and thanks a lot for your professional explanation.