AR400 is a quench and tempered steel and may be difficult to weld due its high strength and hardenability. The base steel around the weld rapidly heats and cools during welding, resulting in a heat affected zone (HAZ) with high hardness. Any hydrogen in the weld metal may diffuse into HAZ and may cause hydrogen embrittlement, resulting in delayed underbead or toe cracks outside of the weld. To minimize heat affected zone cracking use a low hydrogen consumable with an -H4 or -H2 designation. Preheat to slow the cooling rate. Note that excessive preheat may anneal the base material. Slow cool. More time at elevated temperatures allows the dissolved hydrogen to escape. Peen the weld beads to minimize residual weld stresses. Use the lowest strength filler metal meeting design requirements. If making fillet welds, the weld can be oversized to give the specified strength. Minimize weld restraint.

Tim,
I'm not familiar with that. Do you have a reference? Take a look at Table J2.4 Minimum Size of Fillet Welds in the ASD Manual. The minimum size fillets in this table is dependent on the thicker of the two parts joined. It indicates a 5/16" fillet for material thickness of over 3/4". That's where I get the 5/16" fillet on the 1" thick plate. Also, see Table 5.8 in D1.1. In addition to Table 5.8, Note 2 indicates that the weld size need not exceed the thickness of the thinner part joined. That same statement is made in the ASD Manual. I think what they mean here, for example, is not to put a 5/16" fillet weld along the edge of a 1/4" thick plate. On the other hand, should a 1" thick base plate be welded to a column that has a web thickness of 1/4", the minimum size fillet would be 5/16".