Preheat and Interpass Temperature
Understand why steel cracks after welding, and how preheat, interpass limits and low-hydrogen practice prevent it.
Table of contents
A weld can look perfect and still crack hours later. Steel that cools too fast, with hydrogen trapped inside it, is the usual reason. Preheat and interpass temperature are how you control that cooling. They appear on welding procedures for pipe, structural steel and heavy equipment, and they are checked by inspectors.
What You Will Be Able to Do
- Explain what causes hydrogen cracking.
- Say what preheat does and when it is needed.
- Measure preheat and interpass temperature correctly.
- Explain why interpass temperature has both a minimum and a maximum.
- Handle low-hydrogen electrodes so they stay dry.
How Hydrogen Cracking Happens
Hydrogen cracking is also called cold cracking or delayed cracking, because it happens after the weld has cooled, sometimes hours or days later. Three things have to be present at the same time:
| Ingredient | Where it comes from |
|---|---|
| Hydrogen in the weld | Moisture in electrodes, flux or gas, oil, rust, or wet metal |
| Hard, brittle structure | Fast cooling of steels with more carbon or alloy |
| Tensile stress | Shrinkage of the weld, thick or stiff joints |
Heat from the arc splits water and oil into hydrogen, and the molten weld absorbs it. If the steel cools fast, it can form a hard, brittle structure called martensite in the heat-affected zone (HAZ), the metal next to the weld. Hydrogen trapped in that hard metal, with shrinkage pulling on it, starts a crack.
Remove any one of the three ingredients and the crack cannot form. Your job is to control hydrogen and cooling rate.
What Preheat Does
Preheat means heating the base metal to a set minimum temperature before you start welding and holding it there. It does three useful things:
- It slows the cooling rate, so less hard martensite forms.
- It gives hydrogen more time to escape from the metal.
- It drives moisture off the joint and reduces shrinkage stress.
Thick steel needs more preheat than thin steel because it acts like a heat sink. Cold, heavy metal pulls heat out of the weld very quickly. Steel with more carbon and alloy needs more preheat because it hardens more easily.
| Condition | Effect on preheat |
|---|---|
| Thicker metal | More preheat |
| Higher carbon or alloy content | More preheat |
| Very stiff, restrained joint | More preheat |
| Non-low-hydrogen filler | More preheat |
| Cold or damp base metal | Bring it up to temperature and dry it |
The required temperature comes from your procedure and the governing code, based on the steel grade, thickness and process. Do not guess. Some steels, such as quenched and tempered grades, can also be harmed by too much heat, which is another reason to stick to the procedure.
Even when no preheat is required, do not weld on cold, wet steel. Warm it to dry it first. Many codes require this on steel below a set temperature.
What Interpass Temperature Means
Interpass temperature is the temperature of the weld area just before you start the next pass. On a multi-pass weld, it often has two limits:
- Minimum interpass. This is usually the same as the preheat. If the joint cools below it, you lose the benefits of preheat, and hydrogen cracking becomes possible again. Reheat before you continue.
- Maximum interpass. Too much heat causes large grains in the HAZ and weld, which lowers notch toughness, meaning the metal resists sudden cracks less well. On stainless steel, too much heat can also hurt corrosion resistance. Let the joint cool before the next pass.
A maximum interpass on ordinary carbon steel is often around 500 to 550 F, but stainless steels and tough, high-strength steels often have lower limits. The procedure is the rule.
How to Measure Temperature
| Tool | How it works | Watch out for |
|---|---|---|
| Temperature-indicating crayon | Melts at its rated temperature | Use one crayon for the minimum and one for the maximum |
| Contact pyrometer | Probe touches the metal | Takes a moment to settle |
| Infrared thermometer | Reads from a distance | Shiny surfaces can read wrong |
A temperature-indicating crayon, often called by the brand name Tempilstik, is rated to melt at a set temperature. To check a minimum, mark the metal: if the mark melts, the metal is above that temperature. To check a maximum, use a crayon rated for the limit: if it melts, the metal is too hot.
Where you measure matters. Do not measure right on the weld or the flame path, because the surface there reads hotter than the metal around it. Codes say where to measure. A common rule is to measure a set distance away from the joint, such as 3 inches or the thickness of the metal, whichever is greater. Heat the full thickness, not only the surface, by giving it time to soak.
Preheat can be applied with a torch, resistance heating blankets or an induction system. Resistance and induction give more even heat on thick parts, but a rosebud torch is common for smaller work.
Warning: Hot-work rules apply to preheat as well as welding. Keep clear of fuel lines and combustibles, and do not point a torch at a cylinder or at coated or painted metal without checking what the coating is.
Keeping Electrodes and Flux Dry
Preheat cannot save a weld made with wet electrodes. Low-hydrogen electrodes, such as E7018, have a coating that absorbs moisture from the air. That moisture puts hydrogen in the weld.
- Keep opened low-hydrogen electrodes in a heated rod oven, typically 250 to 300 F. Many codes require at least 250 F.
- Only take out what you will use soon.
- Limit air exposure. A common limit for E70-class electrodes is 4 hours, and codes set shorter limits for higher-strength electrodes. If rods sit out too long, they must be rebaked or thrown away.
- Rebake only as the maker or code says, and only as many times as allowed. Rebaking is typically done at a much higher temperature than the holding oven, often 500 to 800 F for one to two hours, depending on the electrode and code.
- Never use electrodes that got wet, or that have damaged coatings.
Keep flux-cored wire and submerged arc flux clean and dry, too, and store bare filler rods in a dry location. Moisture on filler causes porosity even when cracking is not the worry.
Other Heat Terms
- Heat input is the amount of energy put into the weld. It goes up with amps and volts, and down with travel speed. Some procedures set a limit.
- Post-heat means holding the joint warm after welding to let hydrogen escape.
- Post-weld heat treatment (PWHT) means heating the finished weldment to a set temperature to relieve stress. It requires a qualified process and records.
Worked Example: Following a Preheat Requirement
Your procedure for a 1-1/2 inch thick plate says: minimum preheat and interpass 150 F, maximum interpass 500 F.
- Check the plate is dry and clean. Light the heater and heat the joint evenly, including the back of the plate if you can reach it.
- Mark the plate 3 inches from the joint with a 150 F crayon. When it melts, you have reached the minimum.
- Weld your first pass. Your arc adds heat, so the joint can get hotter.
- Before the next pass, check the weld area with a 150 F crayon and a 500 F crayon. The 150 F mark should melt, and the 500 F mark should not.
- If the 500 F crayon melts, wait for the joint to cool. If the 150 F mark does not melt, reheat.
- If you have to stop, ask how to cool the joint. Do not leave it to chill in a draft.
Write down your temperatures if the job requires a record.
Common Mistakes
- Heating only one spot. The joint needs even heat through the full thickness.
- Reading the temperature where the flame just hit. The surface reads hot while the metal below is cold.
- Letting the joint cool at lunch. Reheat to the minimum before you start again.
- Racing passes to finish. You can go over the maximum interpass.
- Leaving 7018 rods out in the open. Moisture adds hydrogen.
- Assuming that more preheat is always better. Too much heat can damage some steels.
Check Your Understanding
A thick plate is at 40 F on a cold morning and the procedure says no preheat is needed for its thickness. Should you just start welding?
Not without checking. Many codes require warming steel below a set temperature, and any moisture has to be driven off. Follow the code or ask your lead.
You are running a multi-pass weld and the procedure's maximum interpass is 450 F. Your crayon shows the joint is above it. What do you do, and why?
Stop and let it cool below 450 F before the next pass. Excess heat makes grains grow in the HAZ and lowers notch toughness.
A coworker says he dried his E7018 by leaving it on the truck dashboard in the sun. Are those rods fine?
No. That does not meet the storage or rebake rules, and the coating may still hold moisture. Rebake them as the maker or code requires, or scrap them.
Why does a thicker plate need more preheat than a thin one of the same steel?
Thick plate pulls heat away faster, so the weld cools quickly. Fast cooling forms hard metal and traps hydrogen, which raises the risk of cracking.
Key Takeaways
- Hydrogen cracking needs hydrogen, hard metal and stress at the same time.
- Preheat slows the cooling rate so that hydrogen can escape and less hard metal forms.
- Interpass temperature has a minimum to prevent cracks and a maximum to keep the metal tough.
- Measure away from the weld, and check every pass.
- Keep low-hydrogen electrodes in a rod oven and limit their time in the open.
- Your procedure and the governing code set the actual numbers.