CWI Welding Metallurgy Study Guide: Preheat and HAZ

What this domain covers

Part A's heat control and metallurgy questions stay with carbon and low-alloy steels. You do not need to be a metallurgist. You need to understand why codes ask for preheat, interpass control, low-hydrogen practice and heat treatment. Then you can explain and enforce those requirements. Many questions come back to one idea: how fast the weld area cools, and what that does to the steel.

The basic phases of steel

Iron changes crystal structure with temperature, and carbon content changes how it behaves.

• Ferrite is body-centered cubic iron. It is soft and ductile and holds very little carbon.
• Austenite is face-centered cubic iron that exists at high temperature. It dissolves much more carbon than ferrite.
• Cementite is iron carbide (Fe3C), hard and brittle.
• Pearlite is alternating layers of ferrite and cementite. It forms when austenite cools slowly. A steel of about 0.8% carbon can be fully pearlitic.
• Martensite forms when austenite in a hardenable steel cools very fast. Carbon is trapped in a distorted structure that is very hard and has low toughness.

Martensite is the structure to worry about in welding. It is the one most susceptible to hydrogen cracking.

Cooling rate and the heat-affected zone

The heat-affected zone (HAZ) is base metal that did not melt but whose microstructure or properties changed. Within it, conditions vary with distance from the weld. Right next to the fusion line, the steel was heated far above its upper critical temperature and the austenite grains grew large. On cooling, this coarse-grained region is usually the hardest and least tough part of the HAZ.

Cooling rate depends on several things:

• Thickness: thick plate is a bigger heat sink and cools the weld faster. That is why codes raise preheat as thickness increases.
• Heat input: higher heat input slows cooling, which lowers hardness, but it also coarsens grains, which can lower toughness. The heat input formula is worked through in the welding calculations guide.
• Preheat: a warmer base metal reduces the temperature difference and slows cooling.

Hydrogen cracking

Hydrogen-induced cracking, also called cold or delayed cracking, needs four conditions at once:

1. Diffusible hydrogen in the weld area.
2. A susceptible, hard microstructure such as martensite.
3. Tensile stress, residual or applied.
4. A temperature near ambient, roughly below 200 degrees C (400 degrees F).

Remove any one and cracking does not happen. Because the cracks can appear hours after welding, some codes delay final inspection on certain high-strength steels. Typical forms are underbead cracks in the HAZ and toe cracks. How to spot them is in the weld examination guide.

Control measures map directly to the four conditions:

• Low-hydrogen electrodes, kept dry in holding ovens, and clean, dry joints reduce hydrogen.
• Preheat and controlled interpass temperature slow cooling, giving softer structures and time for hydrogen to escape.
• Good joint design and welding sequence reduce restraint.

Why preheat works

Preheat slows the cooling rate. It does not change the steel's composition. Slower cooling means less martensite, lower HAZ hardness, more time for hydrogen to diffuse out and lower shrinkage stress. All else equal, a joint in 2 in. plate needs more preheat than one in 1/4 in. plate.

Interpass temperature

Interpass temperature is the temperature of the weld area between passes in a multipass weld. A minimum interpass temperature, usually equal to the preheat, keeps the cooling rate under control on later passes as well as the first. A maximum interpass temperature protects steels and properties that suffer from too much heat. Inspectors check both with contact thermometers or temperature-indicating crayons, measured on the base metal near the joint.

Carbon equivalent

Carbon equivalent (CE) rolls carbon and weighted amounts of other alloying elements into one number that estimates hardenability. A common form is:

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

A higher CE means the HAZ forms martensite more easily, so more preheat and better hydrogen control are needed.

Alloying elements to know

• Manganese strengthens steel and ties up sulfur as manganese sulfide.
• Silicon is mainly a deoxidizer.
• Sulfur forms low-melting films at grain boundaries and is linked to hot (solidification) cracking. A good manganese-to-sulfur ratio helps.
• Phosphorus tends to embrittle steel.
• Nickel improves low-temperature toughness.
• Chromium and molybdenum add hardenability and high-temperature strength.

Heat treatments

• Stress relief heats the weldment below the lower critical temperature and holds it there, then cools it slowly. For carbon steel the temperature is commonly about 1100 to 1200 degrees F (595 to 650 degrees C). Residual stresses relax because yield strength drops at temperature, and hard structures are tempered.
• Normalizing heats steel above the upper critical temperature and cools it in still air, refining the grain.
• Annealing cools slowly in the furnace, giving a softer structure than normalizing.
• Quenching cools rapidly in water or oil to harden the steel, usually followed by tempering.

Quenched and tempered steels get their properties from controlled heat treatment. Too much heat input or a high interpass temperature can over-temper the HAZ and lower strength and toughness. That is why these steels can have a maximum interpass temperature or heat input limit as well as a minimum.

Mechanical properties

• Tensile strength: maximum stress in a tension test.
• Yield strength: stress at which permanent deformation begins.
• Ductility: ability to deform before fracture, reported as percent elongation and reduction of area.
• Toughness: ability to absorb energy and resist fracture, especially with a notch. It is usually measured with the Charpy V-notch test (see the destructive testing guide).
• Hardness: resistance to indentation.

Residual stress

As the weld and HAZ cool, they try to shrink, but the cooler surrounding metal holds them back. The result is tensile residual stress in and near the weld, often near the yield strength. Stress relief and, in limited cases, peening reduce it.

Mark J. Curry's tip: when a question asks why a code requirement exists, trace it back to cooling rate, hydrogen or residual stress. One of the three is almost always the answer.

Work through the free metallurgy practice questions. To see which Part A topic to study first, take the free CWI diagnostic test.

Sources

The Heat Control and Metallurgy questions cite these documents, by document and, where we can confirm it, chapter or section. We never reproduce them.

1. AWS D1.1 (4 questions)

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