Weld-preheating

for Crack-Safe Performance.

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Why Heating Before Welding?

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Weld-preheating is a hot topic...

It should always be considered, whatever will be finally decided.

It may not be needed in the specific application.

Searching the term "preheating" in this website only, brings more than 60 results.

That means that it is rather a popular argument, cropping up quite frequently.

It is therefore proper to dedicate this separate page to the subject, to summarize its important points.

Preheat is used:

to remove moisture and
to ease the liberation of hydrogen,
to avoid embrittlement and cracking,
to reduce thermal gradient and distortion
to reduce cooling rate after welding,
to limit hardness of the heat affected zone and improve ductility,
to reduce residual shrinkage stresses in highly restrained joints,
to improve properties and
to avoid brittle fracture.

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Hydrogen induced cracking in welded medium carbon steel structures, is responsible for a large proportion of the failures reported, with very large economic cost.

The conditions resulting in hydrogen cracking are explained in our page on Hydrogen Embrittlement.

Here such terms are briefly reminded as those presenting

sufficient hydrogen and
sufficient stress in a
susceptible microstructure,
at temperature below 150 °C.

The above essential synopsis is reported from the following reference, which includes a complete treatment of the subject.

Welding Steels Without Hydrogen Cracking
by N. Bailey and others
Woodhead Publishing Ltd. in association with the Welding Institute.

It is available also from various sources by performing a search of the above Title.

Weld-preheating requirements, if prescribed by applicable Code or by engineering considerations, must be spelled out in Welding Procedure Specifications (WPS).

Preheating is not needed in general, when welding mild steel, characterized by low carbon content and low Hardenability.

For less than 25 mm (1 in) preheat is generally not required except for drying if needed.

Low alloy steels develop higher mechanical properties upon heat treatment.

For such materials a suitable range of Weld-preheating shall be specified, to avoid rapid cooling that could dangerously affect joint microstructure and properties.

The rate of cooling in the range from 800 °C to 500 °C has the highest influence in hardenable steels.
An Article on this subject, titled t8/5, was published (2) in Issue 51 of
Practical Welding Letter for November 2007.
Click on PWL#051 to see it.

Why Weld-preheating?

For quenched and tempered high strength low alloy steels, it is imperative to establish the minimum but not to exceed the maximum temperature given by handbooks or by manufacturers.

In fact a too high temperature Weld-preheating may reduce too much the cooling rate.

That would result in the transformation of austenite in ferrite (with islands of high carbon martensite) or coarse bainite, where both structures lack the strength and toughness required.

Codes and Handbooks can give indications as to the recommended minimum preheat temperature to be applied for every thickness range.

Except that special conditions may require a thoughtful increase to improve results.

A useful method involves the calculation of a factor called the Carbon Equivalent (CE) from the composition of a given steel.

It represents an attempt to reduce the influence of the amount of each of the elements present, expressed as the weight percent results from chemical analysis, to a single number.

An accepted empirical expression of CE is that proposed by the International Institute of Welding (IIW) as follows:

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

In the various methods using CE for determining the suggested Weld-preheating temperature, appropriate ranges of CE are established, with additional factors, to be taken into account, influencing the final selection.

A summarized overview of the kind of information available from calculation of Carbon Equivalent for any given steel, and determination of Weld-preheating requirements is presented hereafter:

CE < 0.2 - not susceptible to cracking, no preheating
CE < 0.4 - cooling rates controllable by heat input, use preheating only for thick plates
CE < 0.6 - use hardness control method to select preheat temperature reducing Heat Affected Zone Hardness to less than 350 HV

Furnace Weld-preheating is the best option if available, for iron castings and small assemblies.

For larger structures other means must be provided. They are listed hereafter in order of decreasing power efficiency:

Induction Heating by using a water cooled inverter power source providing high frequency electric current. All surfaces of the item to be preheated are covered by wrapping it with an insulating ceramic fiber blanket. Then induction heating water cooled cables are wrapped around the part over the blanket. The induction heating source uses thermocouples attached to the part to monitor the temperature and control its heat output.
On this subject, an article introducing ASM Handbook Volume 4C on Induction Heating and Heat Treatment was published (2) in Issue 136 of Practical Welding Letter for December 2014.
Click on PWL#136 to see it.
An Article on Induction Heating Assisted Underwater Wet Welding was published (11) in Issue 138 of Practical Welding Letter for February 2015. Click on PWL#138 to see it.
Resistance Heating uses resistance pads made up of a resistant element woven through ceramic tiles. The pads have enough flexibility to allow for being contoured around or inside components with varying profiles. The resistance heating source controller uses one thermocouple per zone attached to the part, to monitor the temperature and control the output heat to that zone.
Gas Burners are assembled in banks or groups, and the flames impinge directly on the part, which may move or rotate to provide uniform heating. Thermocouples attached to the part must be used as before but generally the flames are manually turned on and off.

In all cases the purpose is to preheat the parts at the required temperature as quickly as possible, and then maintain that temperature for as long as necessary.

The importance of knowing exactly the attained temperature before initiating welding cannot be emphasized too much.

Taking reading from thermocouples or pyrometers, or even from temperature sensitive markers, should be possibly performed from the side opposite to that of heat application.

The temperature distribution should be as uniform as possible for a vast zone around the joint place.

Selecting suitable Weld-preheating temperature will provide sound weldments without cracks and will contribute to successful economic performance of any welding operation.

For a rich reference list of Resources on Welding Preheating,
Click on Bulletin 92.

Watch the following Video on

Tempilstik Preheat and Interpass Demo

https://www.youtube.com/watch?v=3BRZS-DN_H4

* * *

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