Preheating

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I was wondering if there is a general rule for pre heating mild steel.  What are some of the factors, thickness, outside temperature?
Reply:you can do a quick preheat to remove surface moisture or counteract possible distortion, but generally i don't bother with preheating mild steel unless its thick section (ie 40mm plus). there is not enough carbon in it to cause any significant hardening from uneven cooling/heating.
Reply:At work we pre-heat carbon steel that is greater than 3/4" to 250 deg.  I think it only needs 200 but we go to 250 just be sure.  We preheat anything when it's below 50 deg.  I generally like to put a little heat in most stuff before I weld just to get the moisture out unless it's thin gauge material.SA-250DLincoln Pro-MIG 140 w/spoolgunVictor Journeyman O/A outfitWP-26, WP-17, WP-9 TIG torches run off my SA-250Metabo grinders and all the other stuff...
Reply:The AWS code book for structual states min temp is 50 deg on average with preheat starting at 150 deg on material 3/4" and goes up on thicker material.
Reply:The basic idea is to prevent cracking. The code breaks everything down by filler type (Other than Lo-Hy or Lo-Hy) and base material composition and thickness. There are minimum temperatures of 32*, 50* 150*, 225*, 300*, all different requirements. It just depends on the variables.City of L.A. Structural; Manual & Semi-Automatic;"Surely there is a mine for silver, and a place where gold is refined. Iron is taken from the earth, and copper is smelted from ore."Job 28:1,2Lincoln, Miller, Victor & ISV BibleDanny
Reply:General rules come from several issues, of which two are cracking and distortion (which are kind of related, as they can sometimes come from the same causes)Two major causes of cracking are brittle material and hydrogen cracking. At low temperature, the material loses ductility and will crack (like chaulk or concrete) rather than yield, and the strain from contracting weld metal can cause cracking as the weld cools, often in the heat affected zone adjacent to the weld. The temperature range at which many low carbon steels go brittle starts a bit below 50 deg F, and ranges down to well below zero F, depending on the exact alloy and metal condition (trace elements like sulfur and phosphorous, grain structure, any heat treatments, service history, etc) Preheating to above this range in the weld area -- 70 deg is a common minimum-- for several inches from the weld avoids this cause. Hydrogen cracking occurs when hydrogen gets trapped in the weld metal. Hydrogen is more soluble at higher temperature than at lower temp, so as the weld metal cools, hydrogen comes out of solution. At higher temperature, is can diffuse through the metal structure, but it is less able to move at lower temperature. To oversimplify, the hydrogen tends to collect at grain boundaries at lower temperature and cracking occurs (you can think of it as being like a gas collecting and the pressure causing the grains to separate, but this is a kindergarten  level explanation) . Preheat (and postheat) slows the cooling of the weld metal, giving more time for the hydrogen to find the surface and escape, rather than be trapped where it can cause cracking. The cracking may not occur for several hours or days, as the mobility of the hydrogen is very low at room temperature, and it takes time for it to collect, and more time for the material to fail. Not usually an issue with thin sections in low carbon steel (less than 1/2" or so), but as temperature drops (to the brittle range), carbon level is higher (less ductile steels at any temperature), and thickness goes up (more hydrogen gets trapped), this risk goes up, and either a low hydrogen process is needed, or sufficient pre and post heat are needed to let the hydrogen diffuse out of the material.Distortion is also controlled by preheat. Preheating reduces the difference in shrinkage of the weld metal and the base metal, and also can keep the weld metal in a more ductile state through a greater part of the shrinkage  (trapping less strain), reducing distortion in the weldment. It will not eliminate trapped strain and distortion, but in many cases can reduce it to acceptable levels, and in well restrained assemblys  and fixturings can practically eliminate distortion in the final product.