The purpose of preheat:- Reduce the risk of hydrogen cracking Reduce the hardness of the weld heat affected zone Reduce shrinkage stresses during cooling and improve the distribution of residual stresses. If preheat is locally applied it must extend to at least 75mm from the weld location and be preferably measured on the opposite face to the one being welded. Got To The Pre-Heat Calculator Background To Preheating When hydrogen diffusing from a solidified weld meets a hard microstructure under a tensile stress a crack is likely! Hydrogen cracking normally occurs in the heat affected zone where hard microstructure is to be found,  occasionally it can occur in weld metal.   Hydrogen This is a very searching gas that can be liberated by oil,  grease, rust etc. and water under the right conditions. The greatest risk comes from hydrogen generated within the  arc from damp or contaminated welding consumables, mainly fluxes or electrode  coatings. Contamination on the parent metal can also be a risk unless  the heat from the welding arc can drive it away.  Moisture from condensation on the parent  metal will normally be driven off by the heat from the arc before it can get into the weld pool.  Hydrogen in the atmosphere is unlikely to penetrate the arc envelope unless welding is carried out in very damp and humid conditions. A hydrogen crack can take anything from a few hours to 24 hours to occur. After 24 hours cracking is still possible but less likely, although there have been some reported cases of  cracking at 72 hours. It is therefore good practice to allow at least 48 hours before carrying out any NDE. Hydrogen will eventually disperse from the parent metal,  within a few days at room temperature or a few hours if held at around 200°C. Hydrogen cracking is only possible at room temperature, this is why it is also referred to as cold cracking   Parent Metal A hydrogen crack requires a hard microstructure which is  created by a hardenable material subject to fast cooling  from 800°C to 500°C.  Cooling can be slowed down by:- applying preheat,  maintaining a high  interpass temperature, increasing welding power and reducing travel speed. The heat sink caused by the parent metal thickness and the  number of available paths the heat can take to escape, also influence cooling  rate.  (However once the heat sink  reaches a certain size further increases have a negligible effect on cooling  rate.).  This is why when determining preheat the term combined thickness is used, for a butt weld it is twice the thickness of the parent material and for a T fillet weld three times the thickness. The hardening of a carbon manganese steel/low alloy  steel is influenced primarily by carbon content and to a lesser extent other  constituents such as manganese, chrome, silicone etc.  The Carbon Equivalent is a formula used to express the harden-ability of a particular alloy steel in terms of an equivalent plain carbon steel.  Several such formula  exist, the one favoured for low alloy steel is the IIW formula: CEIIW = C + Mn/6 + (Cr + Mo + V)/5 + (Ni +  Cu)/15 Current steel specification do not restrict or limit the Carbon Equivalent and as most steel specs permit a wide range of composition it is possible that one batch of steel may require pre-heat and another may not. Very low sulphur ( < 0.015%) will increase hardening and special precautions are required when determining the minimum preheat level.  Additions of niobium also require special consideration. For welds subject to high restraint more preheat is advisable (suggest, Increase CE by 0.3 or go down one hydrogen scale). References. EN1011 Part 2   (English version available from British Standards) This standard is highly recommended as it gives details on this preheat method and also includes methods covering fine grain and creep resisting steels. It also includes practical guidance on the avoidance of other cracking mechanisms.  Much of the data contained in this standard comes from TWI research tempered by practical experience from industry.  (It replaces BS5135) Welding Steels Without Hydrogen Cracking. http://www.woodhead-publishing.com/  This book  is based on the original research work carried out by TWI.  It covers the avoidance of hydrogen cracking and preheat in great detail.  The preheat graphs tend to require a higher preheat than the equivalent ones in EN1011. The Welding of Structural Steels Without Preheat    The Welding Journal  April 2000 A very informative article covering recent TWI research into welding low hardenability steels without preheat.  The article won the Lincoln arc welding foundation gold award.  Preheat calculator          Lincoln arc welding foundation A simple to use and inexpensive calculator.  It is based on practical experience and tends to be very conservative when compared with the TWI method.   Got To The Pre-Heat Calculator More information on Preheat from the Lincoln Arc Foundation