As discussed in our previous article, and many times in the last 50 or so, preheat is necessary in steel welding to control the cooling rate. More specifically, preheat is used to slow the cooling rate down in order to prevent excessive hardness in the heat affected zone (HAZ) which can lead to cracking.
Before getting into the details, please understand that this article is not promoting the removal of preheat when it has been specified. It is simply to show how some fabricators determine if it can be eliminated.
So if preheating is necessary to avoid cracking and other problems why would we want to eliminate it? The answer is cost. Preheating can be very expensive and certain fabricators choose not to use preheat even though they should. However, sometimes not using preheat is OK so long as the cooling rate of the weld and heat affected zone is low enough to prevent excessive hardening.
Preheat is determined based on the carbon content of the base metal to be welded. The higher the carbon, the higher the need for preheat. Other elements also affect hardenability and most formulas to calculate the need for preheat will take those into account. These calculations provide the carbon equivalent.
AWS D1.1 provides mandatory preheat and interpass temperatures which must be applied when using prequalified welding procedures. The code allows the fabricator to change the preheat and interpass temperature, or to eliminate it altogether, so long as this change is qualified by testing. However, the necessary testing goes beyond the simple procedure qualification record (PQR) tests. Testing for hardness can yield different results depending on the thickness and overall mass of the test plate even when the exact same heat input from welding and preheat temperature, or lack thereof, is used.
In the previous article we listed the factors that affect the cooling rate. When developing a welding procedure we typically only have control over the preheat temperature and the heat input from welding as a means to control the cooling rate. So if we want to eliminate the need for preheat we will need to compensate by having a high enough welding heat input.
Heat input is affected by amperage, voltage and travel speed. It is determined by the formula shown below.
Heat Input = (60 x Amps x Volts) / (1000 x Travel Speed) = KJ/in
As we increase amperage and voltage our heat input increases. As we increase our travel speed our heat input decreases. Many times, in order to increase the heat input we have to increase weld size. This is not a problem in multiple pass welds, but can be a problem in single pass welds. If ¼” fillet weld is necessary, the heat input cannot be increased much unless the weld size is increased. This is because as we increase amperage (which increases our deposition rate) we would need to travel faster to achieve a ¼” fillet weld size. If we slow down to significantly increase the heat input we would end up with a bigger weld. The bigger weld may end up being more costly than applying prehat.
Because of this, decreasing the cooling rate typically requires larger (and fewer) weld passes.
When we talk about cooling rate, the range in which we are focused is 1470F to 930F [800C to 500C], because this is the temperature range where phase transformations occur. The cooling rate is therefore expressed in degrees/second and calculated over this range.
To avoid excessive hardness we want the highest possible heat input to have a slow enough cooling rate. Unfortunately we don’t have a single cooling rate that we should aim for that covers all carbon steels. The critical cooling rate depends on the maximum hardness allowed for the heat affected zone for that specific weldment. Hardness is affected by carbon content; therefore, an acceptable cooling rate for ASTM A36 will not be the same as that for ASTM A514.
In conclusion, the steps necessary to determine if preheat can be eliminated are:
- Determine the maximum allowable hardness for the heat affected zone. A major factor on this particular value is whether or not a low hydrogen process is being used (i.e. using E6010 versus E7018). Another factor is the level of restraining to which the weldment will be subjected.
- Determine the cooling rate above which the hardness would exceed the maximum allowable value. This is the rate that you never want to exceed. Meaning that if the maximum allowable cooling rate is 10 degrees C per second you cannot cool any quicker. So 10.2 degrees per second would not be acceptable, but any less than 10 would be.
- Determine the minimum heat input from welding necessary to achieve the desired cooling rate (or slower). Many times this will not be possible. More often than not the only process that can consistently have high enough heat input to compensate for the lack of preheat is submerged arc welding.
- Develop a welding procedure that attains the heat input determined in step 3. It is critical to introduce a safety factor as there are several other factors such as ambient temperature, that affect that cooling rate. These factors may not have as much of an impact on cooling rate as preheat, but they do have an effect.
FINAL REMARKS: If preheat is necessary our recommendation is that you use preheat. If you seek to eliminate preheat be sure to set up the necessary tests which should be approved by the engineer in charge. NEVER assume that increasing the heat input is all that’s needed to prevent excessive hardening and cracking.