Tempering and PWHT Parameters

Tempering – this has been described in the previous section. However, this process is critical in weld integrity.

• Applicable only to steels and used to soften hard microstructures formed during earlier heat treatments.
• Purpose:
  • Improve ductility and toughness.
  • Refine and control precipitate formation to achieve the desired mechanical properties.
  • Especially important for creep-resistant chromium-molybdenum steels.

Process Details

• The steel is heated below the lower critical temperature.
• The exact temperature depends on the alloy composition:
  • C-Mn steel: around 650°C.
  • 2¼Cr–Mo steel: around 760°C.
• Quenched steels are always tempered.
• Normalised steels are usually tempered before use, though sometimes low-carbon C–Mn steels are welded in the normalised condition, with PWHT providing the tempering effect.
• Annealed steels are not tempered.

Typical Tempering Temperature Ranges

Steel Type	Temperature Range (°C)
Tool Steels	150 and above
Constructional Steels	550–760

Post Weld Heat Treatment (PWHT)

As already described in part 1 of this article, PWHT is a specific heat treatment applied after welding. It includes both stress relief and tempering processes. Not to be confused with other post-weld treatments such as:

• Ageing of aluminium alloys.
• Solution treatment of austenitic stainless steels.
• Hydrogen release heat treatments.

Purpose of PWHT

• Mandatory under many welding codes and specifications (e.g., ISO 15614-1, ASME IX, EN 13445, BS PD 5500).
• Reduces residual stress, thereby:
  • Improving toughness.
  • Lowering the risk of brittle fracture.
  • Minimising the likelihood of stress corrosion cracking.
• Has limited effect on fatigue life, except when compressive stresses are dominant.

PWHT in Welding Procedure Qualification

• PWHT is an essential variable in procedure qualification standards such as:
  • ISO 15614 Part 1
  • ASME Section IX
• Any change in PWHT conditions (temperature, time, or omission) requires requalification of the welding procedure.

Typical PWHT Temperature Ranges (Pressure Vessel Standards)

Steel Grade	BS EN 13445 (°C)	ASME VIII (°C)	BS PD 5500 (°C)
C Steel	550–600	593	580–620
C–½Mo	550–620	593	630–670
1Cr–½Mo	630–680	593	630–700
2¼Cr–Mo	670–720	677	630–750
5Cr–Mo	700–750	677	710–750
3½Ni	530–580	593	580–620

Note:

• BS EN and PD 5500 give a range; ASME specifies a single minimum temperature.
• PWHT soak time is typically 1 hour per 25mm thickness.

Important: Never exceed the original tempering temperature unless testing confirms acceptable mechanical properties.

PWHT Requirements

• Thickness-based requirement:
  • BS EN 13445 / BS PD 5500: Mandatory for joints over 35 mm thick.
  • ASME VIII: Mandatory for joints over 19 mm thick.
• Stress corrosion environments: PWHT is mandatory regardless of thickness.
• Soak time:
  • General rule — 1 hour per 25 mm of thickness.
  • Always refer to the specific code for precise requirements.

Important Considerations

• The PWHT temperature must not exceed the original tempering temperature of the steel.
  • Exceeding it can reduce strength below the specified minimum.
  • Higher PWHT temperatures are acceptable only if mechanical testing confirms adequate properties.
• Heating and cooling rates (above 350–400°C) are limited by standards to prevent:
  • Distortion due to uneven temperature distribution.
  • Stress cracking in highly restrained components.

Application of PWHT

Factors Influencing the Method

The method used for PWHT depends on several factors:

• Available equipment and its heating capability.
• Size and configuration of the component.
• Required soaking temperature and uniformity of heating.
• Accessibility, especially for large or complex structures.

Furnace PWHT – The Preferred Method

• The most effective method is to perform PWHT in a furnace.
• Types of furnaces:
  • Permanent furnaces: Fixed installations for regular use.
  • Temporary furnaces: Erected around large or complex components (ideal for on-site PWHT).
• Common furnace designs:
  • Bogie furnace: Uses a wheeled bed to move the component in and out.
  • Top-hat furnace: Has a fixed base and a removable cover.
• Example capacity:
  • A furnace for a 150-tonne pressure vessel may be about 20 m long, with a 5×5 m door, and consume ~900 m³ of gas per hour.

Temperature Monitoring and Control

• Temperature monitoring is critical for achieving uniform heating.
• Modern furnaces use zone control, with thermocouples measuring and adjusting temperatures automatically.
• Best practice:
  • Attach thermocouples directly to the component surface at various locations.
  • These are used to control heating, soaking, and cooling rates.
  • Do not rely solely on furnace air temperature readings.
• The number and placement of thermocouples depend on the component’s size, shape, and thickness.

Component Support During PWHT

• As temperature increases, yield strength decreases, and the component may distort under its own weight.
• To prevent distortion:
  • Support the component adequately using shaped trestles or stands.
  • Support spacing depends on shape, diameter, and thickness.
  • Internal supports (for vessels) should be made of similar material to ensure matching thermal expansion.

Sectional PWHT

• When the entire vessel cannot fit inside the furnace, it may be heat treated in overlapping sections.
• Overlap requirements:
  • BS EN 13445: Overlap = 5√(R·e), where R = inside diameter and e = thickness.
  • ASME VIII: Minimum overlap = 1.5 metres.
• Considerations:
  • Areas within the overlap may experience two PWHT cycles, this must be accounted for in procedure qualification testing.
  • The transition zone between hot and cold areas must be insulated with thermal blankets to control temperature gradients.

Combined and Local PWHT

• Large vessels may be assembled and PWHT’d in sections.
• The final closure weld can be treated using a local PWHT.

Local PWHT

When full furnace heating isn’t possible (e.g., large vessels, pipelines, on-site repairs), local PWHT is used.

When Local PWHT is Used

• Applied when it is not possible to place the entire component in a furnace, for example:
  • Large or complex fabrications.
  • Circumferential welds in pipe systems.
  • On-site installation work.
• Requires careful planning to ensure:
  • Controlled heating and cooling rates.
  • Even temperature distribution.
• Uneven or rapid heating can create harmful thermal gradients, leading to new residual stresses or distortion during cooling.

Common Methods of Local PWHT

• High-velocity gas burners.
• Infrared burners.
• Induction heating.
• High or low-voltage electrical resistance heaters. Low-voltage electrical resistance heaters (ceramic pads)

Notes:

• Electrical methods are preferred on-site because they are safer and easier to control.
• Low-voltage resistance and induction heating are the most common techniques.
• High-voltage heating is rarely used on-site due to safety risks and positioning requirements for radiant heaters.

Special Applications

• High-velocity gas burners are suitable for large surface areas, such as inside pressure vessels (which can act as their own furnace).
• Internal insulating barriers can help localize the heat source in vessel circumferential seams.
• Microprocessor-controlled burners with motorized valves allow precise management of heating cycles.

Electrical and Induction Heating Setup

• Low-voltage resistance heating:
  • Uses flexible ceramic heating elements (often called “corsets”).
  • Multiple elements are positioned to cover the area being heat-treated.
• Induction heating:
  • Uses insulated induction cables wrapped around the weld or custom-shaped fittings.
• Temperature monitoring:
  • Thermocouples are attached (commonly by capacitor discharge welding) to control heating accurately.
  • The heated area is insulated with thermal blankets to reduce heat loss and control gradients.

Temperature Control Zones

Typical Setup Terms: Commonly used terms (per ASME guidance):

• Soak Band:
  • Area heated to the full PWHT temperature range.
  • Includes the weld, both HAZs, and some parent metal.
• Heated Band:
  • Area covered by heaters; edge temperature ≥ 50% of soak temperature.
• Gradient Control Band:
  • Region insulated to ensure a smooth temperature drop from PWHT temperature to ambient.
• Control Zone:
  • Group of heaters managed by one thermocouple circuit, enabling independent zone heating.

Temperature Gradients

• Axial gradient: Along the pipe or vessel length.
• Through-thickness gradient: Caused by internal heat losses; greater for large diameters.
• Inner surface insulation helps but may not be possible on small pipes.
• Codes require minimum soak and heated band widths to maintain acceptable gradients.

Code Requirements for Band Width

Additional Notes:

• No specific ASME requirement for heated band width.
• Rule of thumb (flat plate): Heated band ≈ twice the weld length.
• Insulated band width: BS EN 13445 recommends 10√(R·t).
• Always refer to the applicable code for precise dimensions.

Acceptable Temperature Gradients

• If the temperature at the edge of the heated band is ≥ 50% of soak temperature, the gradient is generally acceptable.
• BS EN 13445 limits:
  • Below 450°C: Max 150°C over 4500 mm (≈ 1°C per 3 mm).
  • Above 450°C: Max 100°C over 4500 mm (≈ 1°C per 4.5 mm).

Thermocouple Placement and Monitoring

• Number and position depend on pipe or vessel diameter:
  • <100 mm: 1 control + 1 recording thermocouple.
  • 100–200 mm: At 12 o’clock and 6 o’clock.
  • 250 mm: At 12, 3, 6, and 9 o’clock.
• Placement guidelines:
  • On weld centerline, edges of soak, and edges of heated band.
  • Ideally, also on opposite surfaces to monitor through-thickness temperature (if accessible).
  • Always double up on thermocouples to prevent data loss from failure.

Thermocouple Installation Best Practices

• Hot junction must be in good thermal contact with the component surface.
• Attachment methods:
  • Capacitor discharge (CD) welding – most accurate and reliable.
  • Mechanical fixing – must be insulated with heat-resistant putty.
• If insulation on thermocouple wire is stripped, bare wires must be re-insulated.
• Document all thermocouple positions in a formal heat treatment procedure and attach to project records.

Heat Treatment of Welded Joints Part 2 FAQs

What is tempering in welding and why is it important?

Tempering is a heat treatment for steels that softens hard microstructures formed in earlier steps. It improves ductility and toughness and controls precipitate formation. It is especially important for creep resistant chromium molybdenum steels used at high temperature. How is tempering performed?

The steel is heated below the lower critical temperature. Typical targets are about 650°C for carbon manganese steel and about 760°C for 2¼ chromium molybdenum steel. Quenched steels are always tempered. Normalised steels are usually tempered before service, though some low carbon carbon manganese steels may rely on subsequent post weld heat treatment to provide the tempering effect. Annealed steels are not tempered.

What are the usual tempering temperature ranges?

Steel type	Temperature range °C
Tool steels	150 and above
Constructional steels	550 to 760

What is post weld heat treatment (PWHT)?

PWHT is a specific post weld heat treatment that provides stress relief and tempering to the welded joint. It must not be confused with ageing of aluminium alloys, solution treatment of austenitic stainless steels, or hydrogen release heat treatments.

What is the purpose of PWHT?

• Meet code and specification requirements such as ISO 15614-1, ASME IX, EN 13445, and BS PD 5500.
• Reduce residual stress to improve toughness.
• Lower the risk of brittle fracture and stress corrosion cracking.
• Note that effects on fatigue life are limited unless compressive stresses dominate.

Why is PWHT an essential variable in welding procedure qualification?

PWHT temperature, soak time, and any change or omission influence mechanical properties. Therefore any change requires requalification per ASME Section IX and ISO 15614 Part 1.

What are typical PWHT temperature ranges in major pressure vessel standards?

Steel grade	BS EN 13445 (°C)	ASME VIII (°C)	BS PD 5500 (°C)
Carbon steel	550–600	593	580–620
C–½Mo	550–620	593	630–670
1Cr–½Mo	630–680	593	630–700
2¼Cr–Mo	670–720	677	630–750
5Cr–Mo	700–750	677	710–750
3½Ni	530–580	593	580–620

BS EN and PD 5500 provide ranges while ASME gives a single minimum temperature. A common soak time rule is 1 hour per 25 mm of thickness.

Why must PWHT temperature not exceed the original tempering temperature?

Exceeding the original tempering temperature can reduce strength below the specified minimum. Higher PWHT temperatures are acceptable only when confirmed by mechanical testing.

What thickness triggers PWHT requirements in common codes?

• BS EN 13445 and BS PD 5500 require PWHT above 35 mm.
• ASME VIII requires PWHT above 19 mm.
• For stress corrosion service, PWHT is required regardless of thickness.

What heating and cooling controls are required during PWHT?

Codes limit heating and cooling rates above about 350–400°C to prevent distortion and cracking. Uniform controlled heating avoids new residual stresses.

Which factors influence the choice of PWHT method?

Consider available equipment and its capacity, the size and configuration of the component, the soak temperature and uniformity required, and site accessibility.

Why is furnace PWHT preferred?

Furnace PWHT provides the most uniform heating and soaking. Common designs include bogie furnaces with a wheeled bed, top hat furnaces with a removable cover, and large vessel furnaces for very heavy components.

How is temperature monitored and controlled during furnace PWHT?

Use thermocouples attached directly to the component surface at several positions. Modern furnaces use zone control to adjust heating automatically based on these readings. Do not rely only on furnace air temperature.

How should components be supported during PWHT?

Yield strength drops with temperature so components must be supported on shaped trestles or stands with spacing matched to diameter and thickness. Internal supports in vessels should be of similar material so thermal expansion is compatible.

What is sectional PWHT?

When a whole vessel cannot fit in a furnace the heat treatment can be performed in overlapping sections. BS EN requires an overlap of 5√(R·e). ASME requires a minimum overlap of 1.5 m. Overlap zones may see two PWHT cycles and this must be covered in procedure qualification testing. Transition areas should be insulated with thermal blankets to control gradients.

What is local PWHT and when is it used?

Local PWHT treats only the region near the weld. It is used for large fabrications, pipe circumferential welds, and site work where full furnace processing is not possible. Careful control of heating, soaking, and cooling is required to avoid harmful thermal gradients.

What methods are common for local PWHT?

• High velocity gas burners
• Infrared burners
• Induction heating
• Low voltage electrical resistance heaters using ceramic pads

Electrical methods are generally preferred on site due to safety and control. Low voltage resistance and induction are the most common choices.

How does induction heating work for local PWHT?

Insulated induction cables are wrapped around the weld or shaped to the geometry. Alternating current induces heat in the metal. Thermocouples control the temperature and insulation blankets reduce heat loss and manage gradients.

What are the standard temperature control zones during local PWHT?

• Soak band is at full PWHT temperature and includes the weld, the heat affected zones, and some parent metal.
• Heated band is covered by heaters and has an edge temperature at least 50% of the soak temperature.
• Gradient control band is insulated to provide a smooth drop from PWHT temperature to ambient.
• Control zone is a group of heaters managed by one thermocouple circuit.

What temperature gradients are acceptable during local PWHT?

If the edge of the heated band is at least 50% of the soak temperature the gradient is usually acceptable. BS EN limits are a maximum of 150°C change over 4500 mm below 450°C and a maximum of 100°C over the same length above 450°C.

What are the band width requirements for local PWHT?

Standard	Requirement
ASME VIII	Soak band equals 2× weld thickness or 50.8 mm each side whichever is less
ASME B31.3	Soak band equals weld width + 25.4 mm each side
BS EN 13445	Heated band equals 5√(R·t)

Rule of thumb for flat plate is a heated band about twice the weld length. An insulated band width of 10√(R·t) is recommended by BS EN.

How should thermocouples be placed and monitored?

Placement depends on diameter. For less than 100 mm use 1 control and 1 recording thermocouple. For 100–200 mm place at 12 and 6 o’clock. For about 250 mm use 4 positions at 12, 3, 6, and 9 o’clock. Place thermocouples on the weld centreline, the edges of the soak band, and the edges of the heated band and duplicate them to avoid data loss if one fails.

What are best practices for thermocouple installation?

Ensure the hot junction has firm thermal contact with the surface. Capacitor discharge welding gives the most reliable attachment. If mechanically fixed insulate with heat resistant putty. Re-insulate any stripped wires. Always document all thermocouple positions and include them in the project records.

Read part 3: Heat Treatment of Welded Joints  – part 3: Precipitation or Age Hardening