I just wanted to add this article from "the Practical Welding Letter" regarding Delta t 8/5:
Article: t8/5
You may have met the expression t8/5 and, unless you know what it is, you may have wondered on its meaning. It is simply a shorthand way of saying how long it takes, for a given weld bead including nearby material, to cool down from 800 0C to 500 0C after welding.
The reason why it is so important to know how much it is and if and which means should be taken to either increase or decrease it, depends on the great influence that this parameter has on certain mechanical properties of the finished weld, especially with high strength steels.
It is also a practical way of concentrating the influence of different factors in a single parameter for appreciating the relationship between welding conditions and mechanical properties.
In particular the Hardness of the Heat Affected Zone (HAZ) bordering on the weld will be higher the shorter is this specific cooling time. This condition might promote the formation of cold cracks.
If the cooling time is extended, possibly by increasing the Preheating temperature, the HAZ hardness will decrease, but also the impact properties of the weld may be impaired. If those are specification requirements, one has to make sure that they will not fall below the minimum acceptable value for the case in question.
The factors that influence the selection of suitable welding parameters for given situations are:
The chemical composition of the base metal
The maximum thickness
The hydrogen content of the welding consumables
The heat input of the process used
The Preheating/Interpass temperature
The chemical composition is evaluated by calculating the Carbon Equivalent, a composite number that takes into account weighted proportions of the alloying elements present.
A few common formulas are known, usually selected according to their ability to provide more accurate simulations in typical cases.
The welding processes suitable to weld large thicknesses in a limited number of passes are also those who provide the highest heat input. Therefore they should be scrutinized to determine if their influence on the weld outcome is not going to impair the mechanical properties required.
Empirical relations and formulas were proposed by various sources and their predictions were confirmed by actual testing. In general the applicability is limited to a range of validity based on the cases where basic data are quite similar to those that were used to develop the expressions in the first place.
The calculation of acceptable welding parameters can be based on the methods presented in the European Standard EN 1011-2.
See the following additional information:
1.) Software support for plate welders: ProWeld Version 5.0:
http://www.thyssenkrupp.com/en/presse/art_detail.html&eid=tk_pnid1240ProWeld can be obtained on a CD-ROM without charge from the
Heavy Plate profit center of ThyssenKrupp Stahl AG.
2.) See also, downloadable from the University of Cambridge:
Program MAP_Neural_NNWork
http://www.msm.cam.ac.uk/map/neural/programs/nnwork-e.htmland:
3.)Program MAP_Steel Weldsoft:
http://www.msm.cam.ac.uk/map/steel/programs/weldsoft-e.html4.) To readers wishing to exercise their patience and solve specific problems by applying these principles to practical cases we would suggest to look for Weldability Calculations - Nippon Steel:
http://homepage3.nifty.com/yurioka/exp.htm Well, that's enough from me for now since I'm tired and need to get some rest... I hope that this is helpful.
Respectfully,
Henry