Over the years we have heard many things about preheating steels before welding. Preheating pulls the moisture out of the material. Preheating burns the contaminants away. Preheating makes the material more ductile and reduces cracking. Preheating is only recommended not required. But how much of this is True?
The first item ‘Preheating pulls moisture out of the material,’ is false. Preheating with a flame actually causes a chemical reaction that generates water. Therefore the water on the surface of the material is actually generated by your preheating process. The water will dissipate when the material gets to a certain temperature because the material it hot enough to evaporate the water before it can generate on the surface. When using induction heating for preheat you will not witness the water generating on the surface of the material.
Occasionally you will hear the term hydrogen cracking, preheating does allow this type of cracking to decrease by allowing more time for the material to diffuse hydrogen after welding. The best way to reduce hydrogen cracking is to use a low hydrogen welding process. What is a low hydrogen welding process? GMAW and GTAW are the lowest hydrogen processes, but SMAW with low hydrogen electrodes will also suffice. SAW and FCAW can also be low hydrogen when the consumables are kept in dry environments or hermetic containers before use. Although SAW and FCAW consumables will readily absorb moisture.
The second item ‘Preheating burns the contaminants away’, is true to a point. When using a flame to preheat you can burn oils or paint off the material. Although when these materials burn they produce hydrocarbons that can also contaminate the weld pool. When using an induction heating process it may not reach temperatures high enough to vaporize paint and oil. Grinding, wire brushing, vapor degreasing, and other cleaning processes are the best way to reduce contaminants.
The third item ‘Preheating makes the material more ductile and reduces cracking’ is also true to a point. Preheating lowers the internal stresses in materials that may be too constrained, this is what reduces cracking. According to D1.1 when calculating preheating using the hydrogen control method joint constraint is one of the main factors. The higher the constraint the higher the required preheat.
The last item ‘Preheating is only recommended not required’ is false. All these items missed the main reason for preheating. The number one reason for preheating is to reduce the cooling rate. By reducing the cooling rate harder microstructures are less likely to form. When materials are very thick they act as a heat sink, which substantially accelerates the cooling of materials after welding. When cooling is accelerated the microstructures are froze quickly and not allowed to dissipate and form more ductile microstructures. Higher carbon steels are more likely to form brittle microstructures since brittle microstructures have saturated carbon content. These high carbon content materials require substantially higher preheat due to their susceptibility of forming these microstructures. They do require a very slow cooling rate to reduce likelihood of cracking. The main microstructure that causes cracking in steels is martensite. Martensite is very hard and brittle, so when over constrained it is very prone to cracking especially when cooling.
Now that you have a better understanding of when and why to preheat, go to bakersgas.com to find everything you need to make your next job, your best job!