Preheat, heat input and cracking for the CWI

The heat-affected zone

When you weld, the weld metal melts. The base metal beside it doesn't melt, but it gets hot enough for its internal structure to change. That strip of changed base metal is the heat-affected zone, or HAZ. It's often the weakest link in a welded joint, because you didn't choose its properties. The welding heat did.

What happens in the HAZ depends on how hot each spot got and how fast it cooled:

• Steel right next to the fusion line gets hottest, then cools fastest because the cold plate around it pulls the heat away.
• Fast cooling can lock the steel into a hard, brittle structure called martensite.
• Slower cooling lets softer, tougher structures form instead.

For study, keep one sentence: the HAZ is base metal that was changed by heat but never melted, and fast cooling makes it harder and more brittle.

Hardenability and carbon equivalent

Hardenability is how easily a steel turns hard when it cools quickly. More carbon raises it. So do other alloying elements, such as manganese, chromium, molybdenum and nickel. A steel with high hardenability makes a hard HAZ even at modest cooling rates, which makes it more crack-prone.

Carbon equivalent (CE) boils the chemistry down to one number so you can compare steels. The common IIW form weights carbon fully and the other elements less:

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

A higher CE means higher hardenability and a greater need for care. Another form, Pcm, is used for low-carbon steels. You don't need to memorize what number is "high". You need to know that a higher CE points toward preheat and tighter control, and that the weld procedure and governing code set the actual requirements. Work a few by hand, then check yourself with the carbon equivalent calculator, which shows the IIW and Pcm steps. Part A is closed book, so the formula has to be in your head.

Hydrogen-induced cracking in three conditions

Hydrogen-induced cracking is also called cold cracking or delayed cracking. It can show up hours or days after the weld cools, and it's the crack that metallurgy questions love. It needs three things at once:

1. Hydrogen dissolved in the weld metal and HAZ.
2. A susceptible microstructure, meaning hard steel in the HAZ.
3. Tensile stress, from restraint, thick sections or residual stress.

Take away any one and the crack doesn't form. That's why every countermeasure maps to a condition:

Hydrogen cracks tend to start at the toe or root and sit in the HAZ, they're often hard to see on the surface, and they're planar, so ultrasonic and magnetic particle testing are the usual finders. The discontinuities explainer shows how cracks fit among the other flaws.

How low-hydrogen practice works

"Low-hydrogen" means the electrode covering, flux or wire is made to put as little hydrogen as possible into the weld, and that you keep it that way. The catch is that low-hydrogen coverings absorb moisture from air, and moisture is hydrogen. So the practice is about handling:

• Store electrodes in the way the manufacturer and your procedure specify.
• Take out only what you'll use, and put the rest back in a holding oven.
• Don't use electrodes that sat open past the allowed exposure time.
• Clean the joint of rust, paint, oil and moisture before you weld.

A damp low-hydrogen electrode is a classic exam stem, and the answer is always the same: it adds hydrogen, which raises the cracking risk. For how the arc processes differ in hydrogen risk, see welding processes for the CWI exam.

What preheat and interpass temperature do

Preheat is heating the base metal before welding. Interpass temperature is how hot the weld area is when you start the next pass, and the usual job is to keep it from going too high. Preheat does three useful things:

• It slows the cooling rate, so the HAZ stays softer and tougher.
• It gives hydrogen more time to diffuse out before the joint is cold.
• It drives off surface moisture and takes the chill off thick or cold steel.

Interpass control is the other side of the same coin. A minimum keeps the benefit of preheat through the whole job. A maximum protects toughness, because too much heat across many passes can leave the weld area with poorer properties.

We give no preheat or interpass values here. They depend on the steel, the thickness, the process, the hydrogen level and the restraint, and the welding procedure specification and the governing code set them. As an inspector you read the values from the WPS and check that the welder met them. Your job is to know why they exist. The WPS, PQR and WPQ explainer covers where those numbers sit in the paperwork.

Heat input in concept

Heat input is the energy put into each length of weld. Higher heat input means a hotter, wider weld that cools more slowly. Lower heat input means a smaller, faster-cooling weld. The arc voltage, current and travel speed set it, and the formula is short:

heat input = (volts × amps × 60) ÷ (travel speed × 1000), in kJ/in when speed is in inches per minute

That sets up a trade-off worth understanding rather than memorizing:

• Too low raises the cooling rate, and the HAZ gets harder, so cracking risk goes up. It also invites lack of fusion.
• Too high slows cooling and can cost toughness and strength in the weld and HAZ.

So the procedure usually holds heat input inside a range, and preheat is a second lever when the steel is crack-prone. Try the numbers in the heat input calculator, and the CWI math calculator puts heat input and carbon equivalent next to the other Part A formulas.

How Part A asks about it

Metallurgy questions on Part A tend to arrive in a few shapes, and none needs a code book:

• Cause to effect: a steel is thick, restrained and high in carbon, and the stem asks what cracking risk you've added.
• Countermeasure: the stem names a crack and you pick what prevents it, such as dry low-hydrogen consumables or preheat.
• Formula: a short heat input or carbon equivalent calculation, worked by hand.
• Flaw match: a crack in the HAZ after cooling, and you pick the type or the NDE method.

A good habit is to answer from the three conditions. Ask which of hydrogen, hard microstructure or stress the stem is changing, and the right option usually shows itself.

Test yourself

Write the chain on one card: heat, HAZ, hardness, hydrogen, stress, crack, then the fix for each link. Then see whether it holds up under questions. The free 10-question Part A check shows your weak areas by topic, so you know whether metallurgy needs more work or you should be spending the time elsewhere. Take the free 10-question check, then use the Part A guide and the study guide to build the plan around what it shows.

FAQ

What is the heat-affected zone?

It's the strip of base metal beside the weld that never melted but was changed by the heat. It often has different hardness and toughness than the rest of the plate, which is why cracks and failures start there.

What causes hydrogen cracking?

Three conditions together: hydrogen in the weld area, a hard microstructure in the HAZ and tensile stress. Remove any one and the crack doesn't form.

Why does preheat prevent cracking?

It slows the cooling rate, which keeps the HAZ from getting as hard, and it gives hydrogen time to escape. It also removes surface moisture.

What preheat temperature should I use?

The one in the welding procedure specification, which follows the governing code for your steel and thickness. This article doesn't give values because they come from the code and the WPS.

Does a higher carbon equivalent mean more risk?

Yes. A higher CE means higher hardenability, so the HAZ is more likely to turn hard and crack, and preheat and tighter control usually become more important.