Thick 4140 to thick steel

[76GMC1500]

Got some plans with a PE stamp on them. This is to make an H-bit for a boat. That involves two posts with a cross bar between them. The posts are to be 12" XXS steel pipe with a 7 inch by 1.5 wall 4140 crossbar welded between them which is also to have a 4 inch A36 roundbar inserted inside. There is no mention of pre-heat or post-heat and this seems like a weld that should need such a thing? I would think something with a little less carbon like 4130 would be a much better choice. Pre-heat and post-heat is a bit beyond the scope of most shipyard welders on smaller vessels such as these. Also the print calls for 4140/HG and makes no reference to what that means. The best I can think of is hot formed DOM tubing? We will most likely be using Lincoln Outershield 71M .045 for the filler. I'm just looking for some comments so I can make a more educated phone call to the design firm.

[OPUS FERRO]

76GMC1500 - Don't call them. - Opus

[el bob]

Here's something about the H designation in the 4140/HG material callout. Comes down to hardenability.

http://cr4.globalspec.com/thread/544...nd-4140H-steel

Not so sure about the G designation.

[Kelvin]

H bitts take one he|| of a load when towing with a hawser...it sounds like a fairly critical weld.

[76GMC1500]

The design load of this bitt is 375kips. They do take a hell of a beating. The shipyard that built the boats only welded the bits onto the deck of the boat (only 10 inch SCH 120) posts, not even concrete filled, and the supporting box is only 3/8ths steel and is way too short. Needless to say the bitts are collapsing because the owners are using plasma line which is a much smaller diameter than poly and doesn't distribute the load. They're also ripping the bitts off the deck of the boat. We have also ripped a bitt off the deck of one of our boats as well but we don't do much barge handling. The whole concept behind the plan is to build a box around the bits, to tie the forward bit into the tonnage frame below and make sure that in all situations the line is led off the forward bit. It's looking like the fabricator used by our competition used a solid 6 inch roundbar instead of the 4140 tube with a roundbar inside.