Our next article will discuss a question we are asked quite often: Can we eliminate preheating before welding? Preheating is necessary to slow the cooling rate down after welding which in turn avoids excessive hardening of the weld and heat affected zone. Eliminating preheat can lead to hydrogen induced cracking and catastrophic failure.
A big problem we have in our industry is that many times welding procedure specifications (WPSs) are changed without proper testing and engineering approval. If a WPS calls for preheat and we don’t preheat, and the weld does not fail, we immediately start questioning the need for preheat. We then weld 10 more parts without preheating, have no failures, and incorrectly determine that preheating was never necessary. Then the 11th part fails and having eliminated preheat proves to be a very costly mistake.
Preheating can be very expensive and reducing the amount of preheat or eliminating it all together can save a fabricator a lot of money, so long as nothing fails. If we can control our cooling rates in other ways we may be able to reduce or eliminate preheat. But it is very important to perform adequate testing before changing the required preheat procedure.
Regardless of what you do, what is important is the cooling rate. If you know the cooling rate under which you need to be, you can do your testing and determine if preheating can be eliminated. Many factors affect the cooling rate. Some can be tightly controlled, others not so much. Here are some of the factors that will affect cooling rate to different degrees.
- Preheat and interpass temperature
- Material thickness
- Heat sinks – presence of chill bars or other ways to drive heat away from the welded part
- Heat input from welding – certain process, such as submerged arc welding, can have high enough heat inputs that make preheat not necessary
- Use of cooling blankets
- Ambient conditions (ambient temperature)
- Quenching (air, water, oil, furnace)
It is possible to eliminate preheat by developing a welding procedure that has high enough heat input to slow the cooling rate down enough. However, as stated above, this should be proven by testing. It is important to note that the standard test for procedure qualification from AWS D1.1 Structural Welding Code – Steel is probably not sufficient. The test plates are not restrained and they are relatively small compared to welding heavy sections in production. There is software available that very accurately predicts cooling rates. This may be necessary.
The purpose of listing all these factors is to make you aware that preheat is not the only factor that affects the cooling rate. And if you eliminate preheat and no failures occur to understand that these other factors may have marginally kept the cooling rate at an acceptable level. But unless we tightly control all of them, eliminating preheat may not be a good idea.