The Role of Preheat in Welding: Part 1

In carbon steels, low alloy steels, thick sections, restrained joints, and higher strength steels, preheat can be the difference between a sound weld and a delayed crack.

A weld does not fail only because of poor technique. Sometimes, it fails because the steel was too cold before the arc was even struck.

Preheat is one of the most important controls in welding. It affects how the weld cools, how the heat affected zone (HAZ) transforms, how hydrogen behaves, and how residual stresses develop after welding.

This article explains the role of preheat in welding, why it is important, how it helps prevent hydrogen assisted cold cracking, and why it must be treated as a controlled welding variable.

1. What is Welding Preheat?

Preheating is the controlled heating of the parent metal before welding begins. The aim is not just to make the metal warm. The real purpose is to control how the welded joint heats, cools, transforms, and responds to stress.

In many steels, especially thick sections, restrained joints, higher strength steels, and alloyed steels, welding without the correct preheat can create the perfect conditions for cracking.

The weld may look acceptable on the surface, but the damage can already be forming in the heat affected zone, commonly called the HAZ.

Low carbon steels may often be welded without preheat, provided the joint thickness, restraint, consumables, hydrogen level, heat input, and service requirements are not severe.

However, as carbon equivalent, material thickness, joint restraint, or hydrogen risk increases, preheat becomes one of the most important controls in the welding procedure.

2. Purpose of Preheating

The main purpose of preheating is to reduce the risk of cracking and improve the metallurgical condition of the welded joint.

Preheat helps by:

Slowing the cooling rate of the weld and HAZ
Reducing the formation of hard and brittle microstructures
Supporting the diffusion of hydrogen away from the weld area
Reducing the severity of thermal stresses
Improving control of the welding thermal cycle

Preheat is therefore not just a heating step. It is a metallurgical control.

2.1 Reducing the Risk of Hydrogen Assisted Cold Cracking

Hydrogen assisted cold cracking (HACC) is one of the most dangerous cracking mechanisms in welded steel structures. It is dangerous because it may not appear immediately after welding. In some cases, cracking can occur several hours after the weld has been completed.

Hydrogen assisted cold cracking is sometimes loosely referred to as hydrogen induced cracking. However, in welding, the more accurate term is HACC, because cracking normally requires the combined presence of hydrogen, tensile stress, and a crack sensitive microstructure.

Hydrogen can enter the weld from several sources, including:

Moisture in electrode coatings or fluxes
Cellulose type consumables
Oil, grease, paint, rust, or water on the parent metal surface
Humid welding conditions
Poorly stored welding consumables

When the weld cools, hydrogen can become trapped in the weld metal or heat affected zone. If this happens in the presence of high residual stress and a hard, brittle microstructure, cracking can occur. Figure 1 shows a typical hydrogen assisted crack in the heat affected zone.

Hydrogen assisted crack in the HAZ — Figure 1. Typical hydrogen assisted crack in the HAZ

Preheating reduces this risk in three important ways.

First, it slows down the cooling rate of the welded joint. This gives hydrogen more time to diffuse away from the weld area before the steel reaches a temperature range where cracking is more likely.
Second, it reduces the formation of hard and brittle microstructures in the heat affected zone. This is particularly important in steels with higher carbon equivalent or higher hardenability.
Third, it reduces the temperature difference between the weld and the surrounding parent metal. This helps lower thermal stress and reduces the driving force for cracking.

Preheat can also help remove surface moisture before welding, but it should not be treated as a substitute for proper material cleaning, dry consumables, and good welding practice.

The heat affected zone is where many weld failures begin. Preheat helps control what happens there before cracking gets the chance to start.

2.2 Controlling Hardness in the Heat Affected Zone

The HAZ is often where the real danger lies. The weld metal may look sound, but the surrounding parent metal may have experienced severe thermal cycling during welding.

When steel is heated above its critical transformation temperature, part of the heat affected zone can transform to austenite. The exact transformation temperature depends on the steel composition.

During cooling, this austenite may transform into ferrite, pearlite, bainite, or martensite, depending on the chemical composition and cooling rate. This is where preheat becomes critical.

If the joint cools too quickly, hard and brittle microstructures can form in the heat affected zone. In steels with sufficient hardenability, rapid cooling can promote martensite formation.

Martensite is very hard, but it is also brittle and highly sensitive to cracking, especially when hydrogen and restraint are also present.

Preheating reduces this risk by slowing the cooling rate. A slower cooling rate gives the steel more time to transform into softer and more ductile microstructures. This helps reduce heat affected zone hardness and improves resistance to hydrogen assisted cold cracking.

2.2.1 Cooling Rate and Rosenthal’s Heat Flow Relationship

The mathematics confirms what welders see in practice: a cold joint cools fast, hardens fast, and cracks more easily.

A simplified cooling rate relationship derived from Rosenthal’s heat flow theory may be expressed as:

Where:

T₀ = initial or preheat temperature

T = current temperature at the point being considered

Q = net heat input rate or net arc power, in J/s or watts (W)

V = welding travel speed

k = thermal conductivity

In this form of the equation, Q is the net heat input rate or net arc power. This is important because the welding travel speed, V, is already included in the equation.

If heat input is expressed as net heat input per unit length, the equation may be written in simplified form as:

Where:

H = net heat input per unit length

The negative sign indicates cooling. In practical welding terms, the important point is simple: as the preheat temperature T₀ increases, the temperature difference T − T₀ becomes smaller.

Since this term is squared, even a moderate increase in preheat can significantly reduce the cooling rate.

2.2.2 Sample Cooling Rate Calculation

This sample calculation uses the simplified relationship where H is the net heat input per unit length.

Assume the following values:

T₀ = 250°C; T = 800°C; H = 1.0 kJ/mm = 1000 J/mm

k = 0.035 J per mm per second per °C

Step 1: Calculate the temperature difference: T − T₀ = 800 − 250 = 550°C

Step 2: Square the temperature difference: (T − T₀)² = 550² = 302,500

Step 3: Substitute the values into the cooling rate equation – Equation 2.

Cooling rate = – 2π × 0.035 × 302,500 ÷ 1000 = – 66.5 °C/s

Therefore, the estimated cooling rate at 800°C, with a preheat temperature of 250°C, is approximately 66.5°C/s. Note that the negative value only shows that the material is cooling. In practice, the cooling rate is usually reported as a positive value.

For comparison, if the same weld was made without preheat and the initial temperature was 20°C:

T − T₀ = 800 − 20 = 780°C

(T − T₀)² = 780² = 608,400

Cooling rate = – 2π × 0.035 × 608,400 ÷ 1000 = – 133.8°C/s

Therefore, without preheat, the estimated cooling rate is approximately 133.8°C/s.

This shows that increasing the initial temperature from 20°C to 250°C reduces the estimated cooling rate from 133.8°C/s to 66.5°C/s.

The practical meaning is clear: preheat reduces the temperature gradient, slows cooling, lowers HAZ hardness, and reduces the risk of hydrogen assisted cold cracking.

Figure 2 shows a schematic of the heat affected zone and the type of microstructural changes that may occur in the parent metal because of welding temperature gradients.

Figure 2. Schematic of the of the HAZ of a 0.17% C steel indicated on Fe-Fe₃C phase diagram showing temperature gradients and microstructural changes.

2.3 Reducing Residual Stresses

Cracking does not need hydrogen alone. It needs stress, hardness, and time. Preheat helps weaken that dangerous combination.

Welding creates intense local heating. The weld area expands while the surrounding colder parent metal restrains it. As the joint cools, the hot region contracts, but this contraction is again restricted by the surrounding material.

The result is residual stress locked into the welded joint.

These stresses are not harmless. When residual stress acts together with hydrogen and a hard heat affected zone, the risk of hydrogen assisted cold cracking increases significantly.

This is why cracking often occurs not only because hydrogen is present, but because hydrogen, hardness, and stress are present at the same time.

Preheating helps reduce this risk by lowering the temperature difference between the weld zone and the surrounding parent metal. A smaller temperature difference means less severe expansion and contraction during welding.

This helps reduce the severity of thermal strain and the stress acting on the joint during cooling.

Preheat does not eliminate residual stress, but it reduces the harsh thermal conditions that promote cracking. In restrained joints, thick sections, and hardenable steels, this can be the difference between a sound weld and a delayed crack.

3. Preheat and Interpass Temperature

Carbon equivalent tells you how easily the steel can harden. Preheat is one of the controls that stops that hardness from becoming a crack.

3.1 What is Preheat Temperature?

Preheat temperature is the minimum temperature to which the parent metal is heated before welding starts.

This temperature should not only be achieved before the first arc is struck. It should also be maintained in the welding area until welding is completed or until the welding procedure allows the joint to cool.

3.2 What is Interpass Temperature?

Interpass temperature is the temperature of the weld area immediately before the next weld run is deposited in a multipass weld.

In simple terms, preheat controls the starting temperature before welding begins, while interpass temperature controls the temperature between weld passes.

Both are important. If the temperature is too low, the weld may cool too quickly and become more susceptible to cracking. If the interpass temperature is too high, the weld and heat affected zone may suffer excessive heat accumulation, loss of mechanical properties, excessive grain growth, distortion, or reduced toughness.

3.3 Carbon Equivalent and Preheat Determination

Carbon equivalent is one of the most widely used tools for assessing the weldability of carbon and low alloy steels. It gives an indication of how strongly the alloying elements may increase hardenability and cracking risk.

A commonly used formula is the International Institute of Welding carbon equivalent equation:

Where all elements are expressed in mass %.

As carbon equivalent increases, the steel generally becomes more hardenable. This means the heat affected zone is more likely to form hard microstructures during rapid cooling.

In such steels, preheat is often required to slow the cooling rate and reduce the risk of hydrogen assisted cold cracking.

A general guide is:

CE below 0.40 %: preheat is often not required for thin, low restraint joints welded with low hydrogen practice.
CE between 0.40 and 0.60 %: preheat is commonly required, especially for thicker sections, restrained joints, or higher hydrogen risk.
CE above 0.60 %: significant preheat is normally required, and welding procedure control becomes critical.

However, carbon equivalent should not be used in isolation. Preheat selection must also consider:

Material thickness
Joint restraint
Diffusible hydrogen level
Welding heat input
Welding process
Consumable type
Ambient temperature
Applicable fabrication standard
Project specification
Service requirements

A low carbon equivalent does not automatically mean no preheat is needed. A thick, highly restrained joint can still crack if welded cold.

Likewise, a higher carbon equivalent steel may be welded successfully when the correct preheat, low hydrogen consumables, heat input, and interpass controls are applied.

Preheat is therefore not a guess. It is a controlled welding variable that should be specified in the Welding Procedure Specification and verified before welding begins.

Conclusion to Part 1

Preheat is one of the most important controls in welding carbon and low alloy steels. It reduces cooling rate, lowers heat affected zone hardness, supports hydrogen diffusion, and reduces the severity of thermal stress.

However, preheat should never be treated as guesswork. It must be selected based on material type, thickness, carbon equivalent, restraint, hydrogen level, heat input, service requirements, and the applicable welding standard.

In Part 2, the discussion will continue with preheat measurement, interpass temperature control, maximum interpass temperature, relevant welding standards, and practical application on fabrication sites.

Frequently Asked Questions

What is preheat in welding?

Preheat in welding is the controlled heating of the parent metal before welding starts. It is used to slow the cooling rate, reduce heat affected zone hardness, support hydrogen diffusion, and reduce the risk of cracking.

Why is preheat important in welding?

Preheat is important because it reduces the risk of hydrogen assisted cold cracking. It also helps control the cooling rate, reduces hard microstructures in the heat affected zone, and lowers the severity of thermal stress during welding.

Does every weld require preheat?

No. Not every weld requires preheat. Many low carbon steels can be welded without preheat when the material is thin, the joint is not highly restrained, low hydrogen practice is used, and service requirements are not severe. However, thick sections, high restraint joints, higher strength steels, and steels with higher carbon equivalent may require preheat.

What causes hydrogen assisted cold cracking?

Hydrogen assisted cold cracking occurs when diffusible hydrogen, tensile stress, and a hard crack sensitive microstructure are present together. Hydrogen can come from moisture, fluxes, electrode coatings, surface contamination, or humid welding conditions.

How does preheat reduce hydrogen cracking?

Preheat slows the cooling rate of the weld and heat affected zone. This gives hydrogen more time to diffuse away from the weld area before the steel cools to a temperature where cracking is more likely. It also helps reduce HAZ hardness and thermal stress.

What is the heat affected zone in welding?

The heat affected zone is the area of the parent metal that is not melted during welding but has its microstructure and properties changed by the heat of welding. It is often one of the most critical areas in welded steel structures.

How does preheat affect HAZ hardness?

Preheat slows the cooling rate of the heat affected zone. Slower cooling reduces the likelihood of hard and brittle microstructures such as martensite forming in steels with sufficient hardenability. This helps reduce cracking risk.

What is the difference between preheat temperature and interpass temperature?

Preheat temperature is the minimum temperature required before welding starts. Interpass temperature is the temperature of the weld area immediately before the next weld pass is deposited in a multipass weld.

What is carbon equivalent in welding?

Carbon equivalent is a calculated value used to estimate the hardenability and weldability of carbon and low alloy steels. Higher carbon equivalent generally means higher cracking risk and a greater need for preheat and welding procedure control.

Can carbon equivalent alone determine preheat temperature?

No. Carbon equivalent is useful, but it should not be used alone. Preheat selection should also consider material thickness, joint restraint, hydrogen level, heat input, welding process, consumable type, ambient temperature, service conditions, and the applicable standard.

What happens if preheat is not used when required?

If preheat is not used when required, the weld may cool too quickly. This can produce hard microstructures in the heat affected zone, trap hydrogen, increase residual stress, and lead to delayed cracking.

Is preheat a substitute for clean material and dry consumables?

No. Preheat can help remove some surface moisture, but it is not a substitute for proper cleaning, dry consumables, correct storage, and good welding practice.