Preheat and post-weld heat treatment of steel structure welds: when they are needed

Where cracks that cannot be seen at once come from

The main reason for preheat is cold, or hydrogen, cracking. It appears not during welding but after it: when the weld has already cooled almost to room temperature, sometimes a few hours later, sometimes the next day.

Such a crack needs three conditions at once. The first is a hardened zone next to the weld: if the metal in the heat-affected zone cools too fast, it forms a hard brittle structure. The second is hydrogen in the weld metal, which came from moisture, rust, grease, a damp electrode or flux. The third is tensile stress from weld shrinkage and the restraint of the joint. Removing one of the three is enough.

Preheat acts on the first condition and partly on the second: the part cools more slowly, the structure comes out softer, and the hydrogen has time to leave the weld. The rest is done by dry low-hydrogen consumables and a sensible weld sequence. The standard on arc welding of ferritic steels itself considers reducing the amount of hydrogen entering the weld the most effective measure.

What decides whether preheat is needed

There is no single figure of the kind «preheat from such-and-such thickness». The need and the temperature follow from a combination of factors, which is why they are recorded in the welding procedure specification (WPS) and not in the general technical specifications.

Cold, cutting and dissimilar steels

The execution standard for steel structures mentions temperature directly: if the metal is colder than 5 °C, suitable preheat may be applied to it. That is not «you must heat to 5 °C» but a permission and a signal: in the cold the need for preheat grows, and both the shop and the site welder must act by the WPS. The preheat determination methods in the annex to the standard on arc welding of ferritic steels are, in addition, written for metal above 0 °C; at sub-zero temperatures special requirements may be needed.

Preheat also occurs outside welding. After thermal cutting the surface of the free edge may harden; the standard allows preheating the material to limit this hardening.

Preheat temperature and interpass temperature

These are two different numbers, and they must not be confused. Preheat temperature is the temperature of the metal in the weld zone immediately before welding starts. Interpass temperature is the temperature of the weld and the adjacent metal before the next pass is laid. There is also the maintenance temperature: the minimum that is held if welding is interrupted (shift change, lunch, moving the piece).

Preheat applies not only to the main welds. The standard requires it to be done to the valid WPS and applied throughout the welding process, including tack welds and welding on of temporary attachments: stops, gussets, run-off plates. A tack weld is the shortest weld with the lowest heat input, a cold part quenches it at once, and a crack may start from it.

The interpass temperature is limited both below and above. Below, so that the weld does not cool faster than it should. Above, because if the weld is not allowed to cool and passes follow one another, the heat-affected zone stays hot for a long time, the grain grows and the impact toughness falls. Normalized and thermomechanical steels and structures working in the cold are especially sensitive. That is why a WPS has both a minimum preheat and a maximum interpass temperature.

The place and method of measurement are set by the standard on measuring preheat and interpass temperature (EN ISO 13916), to which the execution standard refers. Usual means: temperature-indicating crayons, a contact thermometer, a thermocouple. An estimate by eye from the colour of the metal or the smoke is not a measurement.

What else reduces the risk besides preheat

Preheat rarely works alone. In the WPS, next to the temperatures, the consumables and the heat input are recorded: the execution standard requires the welding planning documents to state the requirements for consumables, control of heat input to avoid local hardening on short runs, and special handling of materials, for example low-hydrogen ones.

In practice it means this. Electrodes and fluxes are stored, dried and used as the manufacturer recommends; electrodes with cracks or flaking coating and wire with traces of rust are rejected. Edges are cleaned of rust, grease and moisture. Welding is carried out so that preheat is not interrupted.

If the risk of cold cracking is high, the weld is sometimes kept warm for some time after completion: either the minimum interpass temperature is maintained, or the joint is heated right after welding, before it has cooled below the minimum temperature. The aim is to let the hydrogen out. The duration depends on thickness and is set by the WPS.

Post-weld heat treatment: what it is

It is controlled heating of already welded parts with a hold and slow cooling. Most often this means tempering (annealing) to relieve stress: the joint is heated to a temperature below the transformation point of the steel, held, and cooled to a set schedule.

What it is done for. It reduces residual stress from welding. It softens the structure in the zone next to the weld. It lets out the remaining hydrogen. It stabilizes the geometry if the joint will be machined after welding: without it, stress can «lead» the part off after machining.

Normalizing is a different heat treatment: heating above the transformation temperature and cooling in air, after which the structure becomes uniform and fine-grained. As an operation after welding building structures it is rare. More often it is met as the delivery condition of the plate or section (the letter N in the steel designation) or after hot forming, when cooling cannot be controlled.

Heat treatment itself is not harmless. With an unfavourable combination of steel composition and tempering regime, carbides and nitrides precipitate in the metal, ductility falls, and cracks appear while the stress is relieved. That is why the regime is not made up on the spot but confirmed by a procedure.

When post-weld heat treatment is required

The execution standard for steel structures does not name a thickness from which tempering is mandatory. It only requires that, when heat treatment of welded components is needed, the technologies used are confirmed. So the requirement itself comes from the design, the specifications or the product standard. If heat treatment is needed and there is no product standard, the standard on arc welding of ferritic steels prescribes setting the details in the design requirements, taking into account the effect on the parent metal, the heat-affected zone and the weld metal.

The grounds on which it is specified are usually these:

• thick restrained joints where large stresses remain without tempering;
• joints working at low temperatures or under fatigue loading, when guaranteed toughness next to the weld is needed;
• an environment that together with stress gives stress corrosion cracking: then stress and hardness are reduced;
• parts that are precision machined after welding.

For typical frames, poles and masts in S235–S355 steels in ordinary thicknesses tempering is usually not specified: the problem is solved by preheat, consumables and weld sequence. It appears in special joints and where the design directly requires it.

It is an expensive operation. The piece must fit into a furnace or be heated locally with temperature measurement; after heating it may change geometry; a record and calibrated instruments are needed. The standard on heat-treatment quality lists the parameters set in the procedure: loading temperature, heating rate, holding temperature and time, cooling rate, unloading temperature. The procedure can be included in the WPS or the WPS can refer to it; a record is kept for each piece. «Heat treatment just in case» in the KM is an expensive mistake, because without a justified regime it achieves nothing.

Steel grade and thickness

What follows is a comparison by composition and delivery condition, not a table of temperatures. Figures for a particular steel and thickness are taken from the WPS.

Thickness and choice of grade

Thickness acts twice. It determines the cooling rate of the weld, and so the preheat. And it is one of the reasons why for thick plate a steel with higher impact toughness or a better class is chosen: that is a design question, covered in the article How to choose a steel grade. The limiting carbon equivalent values from DBN are also given there and are not repeated here.

Different thicknesses at one joint also need attention: preheat is determined by how much metal meets at the weld, and one structure may contain several different conditions. Weld types and edge preparation are covered separately: weld joint types and designations.

The pause before weld inspection

Cold cracks appear with a delay, so the execution standard does not assume that additional non-destructive testing of a weld is completed before a minimum hold time after welding has passed. That time depends on the weld size (for a fillet weld its throat thickness, for a full-penetration butt weld the material thickness), the heat input and the steel grade. The table below is for the case where the preheat temperature was determined by method A of the standard on arc welding of ferritic steels (carbon-manganese steels).

How to read the hold-time table

«Cooling period only» means that testing starts when the weld has cooled enough. For preheat by method B (low-alloy high-strength steels) the standard gives other values: for S275–S690 steels with a weld size up to 20 mm, cooling period only, over 20 mm 24 h; for steels above S690, 24 and 48 h respectively.

If the piece is kept preheated for some time after welding, these periods may be shortened. The interval between completion of the weld and the start of testing is entered in the test report. If later work will cut off access to the weld, it is inspected before that work starts.

For the customer the practical consequence is: in the schedule, on thick welds in high-strength steel, a pause of up to two days lies between welding and ultrasonic testing. Visual and ultrasonic inspection are ours; we do not carry out radiography. On inspection in general see quality control of steel structures.

WPS, KM and KMD: what is recorded where

The execution standard requires the welding procedure specifications, defined by the qualification of the procedure, to contain requirements for consumables, any preheat, the interpass temperature and post-weld heat treatment. The execution class (EXC) sets which part of EN ISO 3834 applies to welding planning; for EXC2–EXC4 tack welds are made to a qualified procedure.

So the temperatures live in the WPS, not on the drawings. The designer owns something else: stating the steel grade and class, thicknesses, weldability and inspection requirements, and, if post-weld heat treatment is needed, its purpose and scope.

Site welds

Everything said applies to welds made on site too, but there the conditions are worse: wind, rain, sub-zero temperature, awkward access. Site welds are marked separately from shop welds in the KMD. Preheat, weather protection and the corresponding WPS for them are the responsibility of the organization that welds on site: erection is carried out by a partner construction company under its work execution project. So the work execution project should contain a reference to the WPS for site welds, especially in winter.

Typical mistakes

The mistakes are usually not in a formula but in organization:

• the main weld was preheated and the tack welds and temporary attachments were forgotten, although the standard includes them in preheat;
• temperature is measured by eye or not where the WPS requires, and the torch heats so fast that the edge is overheated while the metal deeper down is still cold;
• the upper interpass limit is not watched, especially on normalized and thermomechanical steels;
• electrodes and flux were not dried or were stored open;
• welding in wind and on damp metal without protection, on site especially in winter;
• a weld is repaired without preheating again: repair welding works under the same conditions as the main weld, and the metal around it is already stressed;
• the pause before inspection was not kept and a crack was «missed» that would have shown the next day;
• the KM specifies heat treatment without a purpose and a regime, so it can be neither confirmed nor accepted;
• it was not taken into account that thermomechanical steel does not tolerate heating beyond what the steel producer permits.

What this means for an order with us

We do the welding ourselves, with visual and ultrasonic inspection in-house; we do not carry out radiographic inspection. So that we allow for the right conditions, state in the enquiry or in the design: steel grade and class, thicknesses, execution class (EXC), inspection requirements and, as a separate item, whether post-weld heat treatment is needed, and if so, why and for which joints. Heat treatment affects lead time and cost, so it must be mentioned in the request.

How drawings are designed and what the KMD must contain: KM and KMD design and steel fabrication. If a crack has appeared on already welded structures, its repair is covered in the article on typical damage to steel structures.

Standards and validity

The requirements for welding execution, including preheat, interpass temperature, post-weld heat treatment and the minimum hold before inspection, are given by DSTU EN 1090-2:2019 «Execution of steel structures and aluminium structures. Part 2. Technical requirements for steel structures» (in force from 01.07.2020, replaced DSTU B EN 1090-2:2014). The article uses clauses 7.1, 7.2.2, 7.5.3, 7.5.5, 7.5.6, 7.5.7, 7.5.15, Table 23 and clause 12.4.2.1.

For preheat and interpass temperatures the execution standard refers to DSTU EN 1011-2:2022 «Welding. Recommendations for welding of metallic materials. Part 2. Arc welding of ferritic steels» (identical to EN 1011-2:2001, in force from 31.12.2023). For heat treatment quality it is DSTU EN ISO 17663:2022 «Welding. Quality requirements for heat treatment in connection with welding and allied processes» (based on EN ISO 17663:2009, in force from 31.12.2023). The international ISO 17663 was already updated in 2023, so check which edition applies to your project.

The Ukrainian DSTU EN ISO 13916:2015 on temperature measurement was cancelled from 01.01.2024 with no replacement named, and DSTU EN 1090-2 still refers to it. Which edition to apply needs to be checked in the project.

Preheat and heat treatment temperatures are not given in the article on purpose. They are taken from a qualified WPS or from the current text of the standards, for your steel grade, thickness and joint. The limiting carbon content and the carbon equivalent of steels under DBN V.2.6-198:2014 are given in the article on choosing a steel grade.