The basics of Preheat

Preheating involves the application of heat on a fabrication or component before commencing the welding process. In this blog, we will discuss the basics around welding preheat.

Why do we need to preheat?

Preheating is used for a number of reasons, including:

• Avoiding the risk of cracking both during welding and after
• Increasing weldability
• Improving the material’s mechanical properties such as notch toughness.

Preheat provides;

• A slower cooling rate through the critical temperature range in steels (900-700C)
• Preventing microstructure hardening and lowering of ductility of both the weld and HAZ
• A slower cooling rate through the temperature range in steels 200C
• Allowing more time for hydrogen to diffuse from both the weld and HAZ
• Minimised risk of hydrogen assisted cold cracking
• A reduction of shrinkage stresses in particularly highly restrained joints

 

When do we need preheat?

This depends on many factors that may require a thorough understanding of the material and the conditions involved, such as:

• Code requirements
• Base metal chemistry, particularly the carbon or carbon equivalent values (CEQ)
• Diffusible hydrogen levels of the weld metal (levels change according to process, flux type, storage conditions etc.)
• Heat input of the welding process
• Material thickness including combined thickness
• Ambient conditions ie. temperature and humidity
• Previous cracking problems encountered
• Levels of internal and external restraint

Preheating is specified as a minimum value and should be achieved and maintained during the period of welding including tack welding. Many low carbon steels particularly those less than 25mm in combined joint thickness will often require no preheat.

 

Code requirements

In some instances, there isn’t an applicable code or standard specifying the requirements for preheat in a particular situation or application. When this situation arises, a welding engineer/specialist might be required. They will be able to determine if and how much preheat is required considering all of the factors listed above.

 

Levels of internal and external restraint

When metallic materials are heated, typically they expand and then contract during cooling. In controlled conditions, such as a furnace or oven, the expansion and contraction can be carefully controlled.

During welding it is common for uneven expansion and contraction to occur. This is generally due to the localised nature of the heat being applied from the welding arc. It can cause several problems such as:

• Hot cracking during the cooling stage, particularly when high levels of internal or external restraint are present
• Cold cracking (hydrogen cracking) following a period of time after cooling
• Distortion

Restraint may be a result of:

• Increased thickness in the material
• Joint and fabrication configuration
• External clamping type forces
• Higher-strength steels which are becoming more popular.

Measuring this level of stress can prove to be very difficult, therefore stress is often avoided. Through their experience, the fabricator or welder will be able to ensure:

• Good fit-up
• Correct joint gaps
• Avoiding forcing parts together before tack welding etc.

 

Base metal Chemistry

As stated already, not all steels require preheat before welding. The chemical composition of the steel is a key factor in determining the need for preheating. The main element of interest regarding hardenability is carbon followed by manganese and to a lesser extent, alloy additions such as:

• Chrome
• Molybdenum
• Vanadium
• Nickel
• Copper

The percentage of these elements present in steel can be found on the material’s test/mill certificate. Often, if applicable, the manufacturer will include the carbon equivalent value (CEV) or the formula required to calculate the value as a percentage.

Knowing the carbon value or CEQ is crucial in determining the need for preheating. It is primarily the carbon that transforms during rapid cooling following welding or other high-temperature applications. The result is an extremely hard and brittle microstructure called Martensite.

The high hardness and brittle nature of martensite is not too dissimilar to glass. Imagine pressurising a glass vessel with hydrogen. The vessel could contain a certain pressure but when it reaches a maximum pressure it will break suddenly and violently. this is similar to what occurs in a HAZ that has cooled too quickly due to lack of preheating.

For martensite to form there needs to be a certain amount of carbon or CEV present in the material. As indicated in the table below, a CEV below 0.45% will often require no or very little preheat depending on other contributing factors.

Typical structural steels such as AS/NZS 3678 grade 350 (min 350 MPa yield strength) may have a carbon value of approximately 0.156%. A CEQ of approximately 0.37% for example, demonstrates very good weldability.

Diffusible hydrogen levels of the weld metal

Hydrogen levels within the weld zone may build pressures great enough to exceed the material’s UTS, especially if high hardness levels are present. For this to occur, there has to be a certain level of hydrogen present. Typically, levels of diffusible hydrogen greater than 15ml per 100g of deposited weld metal (ml/100g) are considered high. These levels will likely result in hydrogen cracking if other factors are present.

Hydrogen may be introduced into the weld zone through various means such as:

• Moisture (H2O) content in fluxes and gases,
• Moisture content in the surrounding atmosphere (H2O)
• Materials in contact with the base materials and electrodes containing hydrogen such as hydro-carbons i.e. oil and grease.
• Rust – this is hydrated iron oxide (iron + oxygen + water (H2O))

Careful control of consumables may need to be employed if hydrogen absorption is to be avoided.

 

Material thickness including combined thickness

The thickness in the joint area of the material will increase during welding. When this happens, the mass of the material and therefore its quenching effect on the weld will also increase. As per the above table, increased thickness or combined thickness warrants preheat in order to counteract the higher quenching effects.

As an example, consider welding 20mm thick CMn steel and the combined thickness affect that may be considered – see table below:

Note! Many different standards, codes and specifications will be very specific about how preheat temperatures should be determined based on:

• The material being welded
• The specific joint and weld configurations.

This is why the relevant documentation for a particular fabrication or project must be followed.

 

Ambient temperature conditions

Ambient temperature is often overlooked by less experienced welding personnel.

Ambient temperatures vary from day to day, season to season and from country to country. The same applies to relative humidity levels when considering hydrogen and its effects on pickup influxes and gases. Always consider the location you are welding in, i.e. shop or site will also have an effect.
Many standards will specify that welding should not be performed on steels below a certain temperature. Further to this, steels should reach a minimum “preheat” temperature before welding commences. An example from AS 4458-1997 is shown:

“It is recommended that no welding be performed when the temperature of the base metal is lower than -20oC. At temperatures between 0oC and -20oC the surface of all areas within at least 75mm of the point where a weld is to be started shall be heated to a temperature at least warm to the hand (estimated to be above 20oC) before welding is started”.

What methods do you use to heat metal?

 

 

Pre-heat is best done in a controlled manner such as a furnace. This is because the temperature can be controlled to a greater degree. Unfortunately, a furnace isn’t applicable in many welding situations.

Fortunately, there are other processes that can be used instead. They include the use of heating rings, torches and thermal blankets.

In the out of furnace condition, welders use a variety of thermometers to monitor temperatures that include:

• Magnetic dial thermometers
• Thermocouples
• Digital thermometers
• Thermal crayons

 

 

Where should preheat temperature measurements be taken?

Different codes, standards and specifications will be quite specific about preheat, including:

• Measurement locations
• The number of areas or points to be checked before welding
• Which side of the material the points are measured.

Examples of this are:

• AS 4458-1997
• ISO 13916:2017
• AS/NZS 1554.1 – 1554.6
• AWS D1.1

 

When it is not possible to measure on the reverse side of a thick section of material, a soak period following heating is typically recommended. This allows time for temperature equalization.

A soak period essentially prevents the material from “sucking” the heat out of the area. This results in a preheat temperature lower than specified at the start of welding.

Typically, the requirements are based on the material, combined thickness, diameter and access limitations. Preheat temperature is always specified as a minimum and must be maintained during the welding operation, this includes tack welding.

Reach us at Technoweld for training, inspection, consultancy and supervision of welding procedures. We can also research and document welding procedures for your specific welding processes as well as run the procedures.