Explained: What and How’s of Preheating and Post Weld Heat Treatment (PWHT) of Welds

A well-defined preheating and post weld heat treatment procedure improves the mechanical properties, thereby the weldability of the welding portion.

Heating and cooling procedures are critical requirement, particularly in the case of a high strength low alloy steel, Cr-Mo Steels and most high thickness materials. 

In this article, I will highlight the importance of pre-heat and PWHT of welds, and how to check the requirement for it in code.

Weld Joint Preheating and post weld heat treatment

Weld Joint Pre-heat Treatment

What is Pre-heating in Welds?

Pre-heating is the process of heating the weld and nearby base metal area to the required minimum temperature prior to welding. And, may have to maintain that temperature until the job is complete, as required by procedure to avoid cracking.

Heat-treating is mostly by using a heating device with a high temperature, such as heating coils. The preheating temperature depends on the proposed base material and the welding method.

What is the purpose of Pre-heating?

Pre-heating will slow down the cooling rate of the weld, the Heat Affected Zone (HAZ), and the adjacent base metals. This will produce a more ductile metallurgical structure, which will increase the resistance to cracking.

A preheat can help prevent martensite formation in the metal grain structure, which is what causes brittleness.

When the rate of cooling is low, the hydrogen diffuses out slowly, thus avoiding cracks. Pre-heating thus removes diffusible hydrogen in base metal.

Pre-heat reduces the chance of cracking during and after welding. Also, the shrinking stresses are reduced in the weld and adjacent base metal.

Moreover, reduces expansion and contraction rate, and improves mechanical properties by burning unwanted impurities in the base metal before welding.

When to apply preheat for welds?

We often find pre-heat requirements stated in the Welding procedure (WPS) or a referring a PQR (Procedure Qualification Record). 

These are basically by their code, however, they mainly depend on factors like base metal chemistry, section thickness, previous cracking history, hydrogen content in the filler metal and ambient temperature.

Preheat is mostly not for low carbon steels less than 1 in, (25 mm) thick. The demand for preheat increases as the chemistry, diffusible hydrogen level, restraint and/or section thickness of the base metal increases. 

How to find the pre heat temperature for piping welds?

In general, the pre-heat minimum temperatures are available in most of the Codes and standards. There may be some variations as per recommendations to that in codes in the relevant WPS.

Some of the codes with pre heat details are specified here under:

• ASME Section VIII Div.1 Non Mandatory Appendix R
• AWS D1.1 , Structural Steel Welding Code Table 3.3
• ASME B31.1, Power Piping code Table 331.4.1
• ASME B31.3, Process Piping code Table  330.1.1

For instance, to find the pre heat temperature required for piping welds:

Considering the process ASME B31.3 Table 330.1.1 (requirements for piping typically found in petroleum refineries)

For Base Metal as Alloy Steel with P No.3 and Thickness T≤ 13mm & Specified Minimum Tensile Strength (SMTS) < 450 Mpa – the required minimum temperature is 10⁰C.

Similarly, if the same metal has thickness T>13mm and SMTS < 450 Mpa – the required minimum temperature is 95⁰C.

And, if it has thickness T>13mm and SMTS > 450 Mpa – the required minimum temperature is 95⁰C.

The temperature requirements as per the nominal thickness of materials (higher thickness in case of combination materials). The requirement applies to all types of welding (tacks, repair or on threaded welds).

What is the pre-heat procedure for welds?

As a normal practice, is to uniformly heat three times the distance either side of weld joint (2″ min.). Distance from welds may differ, as in case of API RP 582 it is 3″ (76mm), refer to relevant construction codes.

Refer API RP 582 for maximum preheat interpass temperature ranges

Refer AWS D10.10 for comparison of soak band widths (through thickness volume of metal) for pre-heat and interpass pre-heating.

In case of different chemistry, the metal with the highest preheat requirement governs. Ensure welds are pre-heated to 10° C if the ambient temperature is below 10° C.

There are basically four methods for preheating of welds. Flame, Resistance, Convection Ovens and Induction types.

Flame Pre-heating: A normal or Oxy Acetylene torch (fuel gas mix with oxygen) with its flame directed on the parts for heating. The process is not under practice, as it mostly provides less efficient in heating the part uniformly, and comparatively slower.

Resistance Pre-heating: Resistance pre-heating is a procedure for pre heating of welds in most oil and gas industries. Resistance pre heat method delivers uniformity in heating all through the area of concern. The procedure incurs high costs, heat waves and setup time, and hence, mostly used in the PWHT of steels.

Convection Pre-heating: Convection pre-heat ovens mostly found in stand-alone workshops where large equipment’s or a certain items have to batch treated. This is a uniform heating procedure, however, not applicable in the in-service or site construction due to high costs and substantial electrical requirements.

Induction Pre-heating: This pre-heat technique generates heat in the part by generating eddy current due to the magnetic field. The procedure gives high uniformity in heating, and safe. Mostly used for localized heating, and also for varied shape and sizes. The alternating magnetic field created by the coil current stimulates the eddy currents inside the part and creates internal heat. As a result, the work-piece becomes the source of heat rather than the heating element.

How to check the pre-heat temperature for welds?

Most WPS define the method applicable for checking the pre-heat and interpass temperature. Normally, an infrared temperature gun at 75mm from the weld centerline is recommended. Other measurement methods include a digital contact thermometer and Tempilstick.

For interpass pre-heat checks, it is recommended to measure immediately before the next pass and as nearer to the weld as possible.

What is Post weld heat treatment in Welding?

Post weld heat treatment (PWHT) or Stress Relieve (SR) is basically a residual stress relieving procedure. Due to localized heat at weld joints significant temperature changes occur that leads to stresses in joint. PWHT helps to relieve these stresses that may lead to failure or undesirable output. 

However, compared to PWHT, in stress relieving we can achieve treatment below the minimum transformation temperature of the steel where no micro-structural change of the material occurs. For instance, heat treatments like normalizing, annealing, quench and tempering, etc.

Post heating is a heat treatment done during and/or after welding. However, the temperature limit is lower, at 149 °C compared to PWHT at 548 °C and higher.

Post weld treatment is a procedure wherein, a welded component is subject to heating to a specific temperature and then held at that temperature for a period of time. Further, cooled gradually to ambient to produce a desired physical or chemical effect.

What is the purpose of post weld heat treatment in welds?

The purpose of post-weld heat treatment is to reduce or eliminate the distortion and residual stresses, controlling material hardness, and enhancement of mechanical strength, thus reducing the risk of cracking.

PWHT releases residual stresses (due to differences in thermal expansion coefficients) by allowing the metal to creep slightly at the elevated temperature, thereby reducing the likelihood of brittle failure.

Post weld treatments improves ductility of material, and also helps prevent Hydrogen Induced Cracking (HIC) by eliminating the diffusible hydrogen in steels.

When to apply PWHT for welds?

In general, the clients scope, procedure and specifications, the material used, and susceptibility to failure determines the requirements for PWHT. 

The chemistry of the metal, thickness of the parts to join, joint design, welding processes and service or process conditions are some of the factors that also influence the application of PWHT.

The heat treatments are done after the completion of all weld or repair operations. No welding to be performed after PWHT.

How to find the post weld heat treatment for piping welds?

The fabrication Standards, design details and construction drawings to which the piping , vessel or other structural parts are constructed contains the requirements for post weld heat treatment.

The relevant construction codes may refer the requirements stated to AWS D1.1 for structural parts, ASME Section VIII for pressure vessels or ASME B31.3 for Piping, as applicable. 

Considering the process ASME B31.3 Clause 331.1.1, Table 331.1.1 and Table 331.1.2  (requirements for piping typically found in petroleum refineries)

An alloy steel piping, P No.3 Group 1 and temperature rage 595 – 650⁰C is 1 hr/25mm (15 min. minimum) for upto 50mm. Above 50mm thickness, the minimum holding time is 2 hr plus 15min for each additional 25mm thickness. The temperature range may have to increase as per procedure requirements, however, may not exceed the lower critical temperature of the control thickness (lesser thickness, including the combination materials). 

The post-weld heat treatment may be exempt, if the proper pre-heat temperature is applied during the welding, and the criteria represented in Table 331.1.3 of ASME B31.3.

Before applying the requirements of B 31.3 to the above, the general note section of the tables and the reference paragraphs have to be checked.

What is the heat treatment procedure for welds?

Post weld heat treatment is a controlled process where the welding joints are subject to heat treatment and maintained at a temperature below its lower critical transformation temperature, followed by slow and controlled cooling. The procedure is time consuming and expensive to execute.

Below listed are some of the types of post weld heat treatments:

Local and 360⁰ PWHT – An electrical resistance or induction heating method, wherein, circumferential weld seams can be post weld heat treated by heating a band around the weld.

Gas Firing – Vessels with internals (non-dismantled) are subject to heat-treatment by internal firing through nozzles or manways. Specific care is a consideration as to not damaging the internals.

Furnace Heating- These may be temporary or fixed type pre-heat methods. Temporary furnaces are specifically constructed and customized for the job, rather than transport the equipment to a fixed heating furnace. Fixed furnaces are good for high temperatures, also high cost, but can do normalizing, annealing, and stress relieving.

The most applied method is the electrical resistance heating method for industrial construction, and for large equipment’s or batch materials, the furnace heating and finally, if agreed by client, local flame heating or Gas firing.

In any heat treatment procedure, it is necessary to have thermocouples, as per diameter equally placed as nearer to weld possible, in order to ensure proper PWHT of components.

The thermocouples has to be tack welded (approved filler max 2.5mm dia). Additionally, sagging due to localized heat treatment should be taken care of by providing adequate supports.

The minimum number of thermocouples is 1 for up to 3′′ diameter, 2 for up to 6′′ diameter, and 3 for up to 10′′ and above. If necessary, the minimum number can be increased.

Heating resistance elements are normally over the attached thermocouples throughout of heating band before insulation. Insulating materials shall be mineral wool/glass wool which can overcome the temperature employed safeguards the thermocouples to avoid incorrect temperature readings due to direct radiation.

In general, the minimum insulation thickness shall be 50 mm. To hold the insulation material in position, need a wire mesh to wrap around and/or tied with other suitable means. The entire PWHT arrangement should be not less than 40 times the pipe thickness.

The soak band width (through thickness volume of metal) for some of the codes as per AWS D10.10

The heating and cooling rate should be as per the WPS and relevant standards but in no case, more than 200°c/hr and the difference between the temperatures recorded with thermocouples shall be within the range specified.

How to check the PWHT temperature for welds?

Temperature-indicating crayons and paints, thermocouples, resistance temperature detectors (RTDs), infrared instruments, bi-metallic switches, expansion bulbs, or other temperature-sensitive devices may be selected to measure the temperature, depending upon the application and required accuracy. This applies for both preheating and post weld heat treatment methods.

Although temperature can be measured using these various methods, temperature indicating crayons/paints and thermocouples are most common for local heating. 

Conclusion

Hope this article sufficiently covers your area of interest and an overall understanding of Preheating and post weld heat treatment practices in piping welds.

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