Preheating a part before welding is common practice. Sometimes we apply heat after we are done welding. We sometimes apply heat between passes. A lot of us do this simply because it is written in the WPS or PQR, but not really understand why it is necessary.
Preheating is used for the following reasons:
- Reduces shrinkage stresses in the weld and heat affected zone (HAZ). This is especially important in joints that are highly restrained. When a weld bead cools it shrinks and wants to pull the base material towards it. If the joint is restrained it creates a lot of stress because the weld is trying to pull the base material. By preheating the HAZ and the weld cool at similar quenching rates thus reducing the stresses created by shrinkage.
- Preheating slows down the cooling rate in the 1600⁰F to 1330⁰F which is the critical temperature range. This prevents excessive hardening and reduced ductility of both the weld and the HAZ. When a part is quenched rapidly it forms martensite. Martensite is hard and very brittle and this will negatively affect the mechanical properties of the weld. Reduced elongation and lower charpy v-notch values.
- Preheating slows down the cooling rate down through the 400⁰F range. This allows for more hydrogen to diffuse out of the weld and HAZ and avoid cracking. Hydrogen can produce cold cracking while it is present in the weld and HAZ. See post on Hot Cracking vs Cold Craking for a more detailed explanation.
It is important to keep in mind that preheating is not as simple as applying heat to the area to be welded until it is at a specific temperature, say 350⁰F. The bigger the base material the more of a heat sink it is. So we can heat around the weld area but it will cool very quickly because the rest of the part is still at room temperature or colder if we are outside. Preheat is essential when welding on low ambient temperature or when bringing in material from outside on cold winter days.
An easy way to explain this is to look at what happens when you use a bucket of water to quench welding coupons. Say you are doing 8-inch long fillet welds on two pieces of 3/8” steel. You weld it, pick it up with pliers and dip it in your bucket of water. You pull it out and you can hold it with your bare hand. If you take the water temperature you will see that it increased. As you do this many times the water gets hotter and the part you quench is not as cool as the first one when it comes out of the water. In the same way as you preheat a part all the way through the weld will cool slower than if you just welded without preheat.
The amount of preheat necessary varies depending on the type of material and thickness. Fortunately the American Welding Society and the American Institute of Steel Construction have established preheat and interpass temperature requirements for commonly welded steels. Interpass temperature refers to the temperature near or around the weld between passes. The D1.1 Structural Welding Code specifies minimum preheat and interpass temperatures based on the base material, the thickness and the process to be used. This is on table 3.2 of such code. A similar table can be found on the Procedure Handbook for Arc Welding (Table 3.3) – Minimum Preheat and Interpass Temperature.
Not sure if preheat is necessary? Here are some things that you should look for, and if present, consult with your welding engineer, production manager, or whoever is in charge of welding operations.
- The base material is over 1” thick
- The base material has recently been brought in from outside during cold winter months
- You are welding outside and the temperature is below freezing.
- You suspect the base material is very high in carbon