6.3 Preheating & PWHT Rules and Controls

6.3 Preheating & PWHT Rules and Controls

Thermal controls during and after welding are critical to ensuring the structural integrity, cracking resistance, and mechanical performance of metallic piping systems. Preheating and Postweld Heat Treatment (PWHT) are metallurgical tools used to manage weld-induced stresses and heat-affected zone (HAZ) microstructures. As an API 570 piping inspector, you must verify that all preheat and PWHT operations comply with the construction code (typically ASME B31.3) and that in-service repairs utilize the correct codes and exemptions.

Metallurgical Purpose of Preheating

Preheating involves heating the base metal in the vicinity of the joint to a specified minimum temperature before and during welding. Preheating serves several crucial functions:

• Reduces the Cooling Rate: Slowing the cooling rate of the weld metal and the adjacent base metal prevents the formation of hard, brittle microstructures like martensite in the heat-affected zone.
• Facilitates Hydrogen Diffusion: By keeping the weldment warm, hydrogen gas has more time to diffuse out of the steel, significantly reducing the risk of hydrogen-induced cracking (HIC), also known as underbead or cold cracking.
• Reduces Residual Stresses: Preheating reduces the temperature gradient between the weld pool and the surrounding base metal, minimizing thermal contraction stresses.

ASME B31.3 Table 330.1.1 dictates the minimum preheat temperatures based on the material's P-number and wall thickness:

• P-1 (Carbon Steel): Requires a minimum preheat of 50°F (10°C) if the nominal wall thickness is ≤ 1 in (≤ 25 mm) and the specified minimum tensile strength of the base metal is ≤ 71 ksi. If the thickness exceeds 1 in or if the tensile strength is > 71 ksi, the minimum preheat is 175°F (79°C).
• P-3 (Low-Alloy Steel, Cr-Mo ≤ 0.75%): Requires a minimum preheat of 175°F (79°C).
• P-4 (1.25% Cr-0.5% Mo): Requires a minimum preheat of 250°F (121°C).
• P-5A (2.25% Cr-1% Mo): Requires a minimum preheat of 300°F (149°C).

The preheat zone must extend at least 1 in (25 mm) or the thickness of the weld (whichever is greater) beyond each edge of the weld joint.

Metallurgical Purpose of Postweld Heat Treatment (PWHT)

Postweld Heat Treatment involves uniformly heating the completed weldment to a high temperature below the material's transformation range, holding it at that temperature for a specified period, and cooling it at a controlled rate. The primary goals of PWHT are:

• Stress Relief: Relaxing the high residual tensile stresses locked in the weld joint during solidification.
• Tempering the HAZ: Reducing the hardness of the heat-affected zone and improving ductility and toughness.
• Environmental Cracking Resistance: Lowering weld hardness is crucial for piping systems in corrosive chemical services prone to environmental cracking mechanisms, such as amine stress corrosion cracking (SCC), caustic cracking, and wet H2S (sour) service. These services often require PWHT regardless of material thickness.

ASME B31.3 Table 331.1.1 governs the temperature ranges and holding times for PWHT:

Worked Example: PWHT Holding-Time Calculation

An inspector must verify the postweld heat treatment parameters for a heavy-wall carbon steel (P-1, Gr. B) piping line during a piping modification. The engineering design requires PWHT. The production details are:

• Material: ASTM A106 Gr. B (P-1)
• Joint nominal wall thickness: 1.75 in. (44.5 mm)
• PWHT Table Reference: ASME B31.3 Table 331.1.1

PWHT Parameter Verification:

1. PWHT Temperature: From Table 331.1.1, the PWHT holding temperature range for P-1 materials is 1100°F to 1200°F (593°C to 649°C).
2. Holding Time Calculation: For P-1 carbon steel, Table 331.1.1 specifies the minimum holding time is 1 hour per inch of thickness, with a minimum holding time of 1 hour for any thickness. PWHT Hold Time = 1.75 inches * 1 hour/inch = 1.75 hours Convert 1.75 hours to minutes: 1.75 hours * 60 minutes/hour = 105 minutes
3. Exemption Check: The nominal wall thickness is 1.75 in. This is well above the 3/4 in. maximum limit for thickness exemptions in P-1 materials under B31.3. Thus, PWHT cannot be exempted, even if a preheat of 175°F was maintained.

Conclusion: The weldment must be heated to between 1100°F and 1200°F and held at that temperature for a minimum of 1 hour and 45 minutes (105 minutes). The heating and cooling rates must also comply with ASME B31.3 requirements.

PWHT Exemptions for Carbon Steel (ASME B31.3)

Under ASME B31.3, P-1 carbon steel piping can be exempt from PWHT under specific conditions:

1. The nominal wall thickness is ≤ 3/4 in (19 mm).
2. A minimum preheat of 175°F (79°C) is applied and maintained during welding (or if the thickness is ≤ 5/8 in, a preheat of ≥ 50°F is maintained).
3. The piping is not in a service where environmental cracking is a concern (e.g., sour water, amine, caustic), which would override the thickness exemptions.

API 570 In-Service Repairs: Alternatives to PWHT

In-service piping repairs often make conventional PWHT impractical due to process safety concerns, the risk of damage to internal linings, or localized thermal stresses. API 570 Section 8 provides two alternative methods to PWHT, provided the owner-user's piping engineer approves and the repair complies with the following rules:

Alternative 1: Preheat Method (No Impact Testing Required)

This method can be used if the construction code does not require notch toughness (impact) testing of the weldment. It is restricted to P-1 and P-3 (excluding Mn-Mo steels) base metals:

• The welding process must be restricted to SMAW, GTAW, or GMAW.
• The weld area must be preheated and maintained at a minimum of 300°F (150°C) during welding.
• The preheat zone must extend at least 4 inches (100 mm) or 4 times the material thickness (whichever is greater) from the edges of the weld joint.
• The interpass temperature must be monitored and must not exceed the maximum allowed by the WPS.

Alternative 2: Controlled-Deposition (Temper Bead) Method (Impact Testing Required)

If impact testing is required by the construction code, the preheat method alone is insufficient because high preheat can degrade notch toughness. Instead, a controlled-deposition (temper bead) technique per ASME Section IX QW-290 must be used:

• The welding process is restricted to SMAW or GTAW.
• A specific welding technique is used where the heat input of each weld pass is controlled so that the subsequent pass heat-treats (tempers) the heat-affected zone of the previous pass.
• The preheat and interpass temperatures must be carefully monitored. The test coupon qualification determines the maximum interpass temperature.
• After welding is complete, a post-weld soak at 450°F to 550°F (232°C to 288°C) for a minimum of 2 hours must be performed to facilitate hydrogen degassing and prevent cold cracking.

NDE and Heat Treatment Sequencing

A critical field control for inspectors is the timing of NDE relative to thermal treatments. NDE (such as visual, magnetic particle, or liquid penetrant testing) must be performed before PWHT to detect any flaws that could be repaired before heat treatment. Crucially, final NDE (specifically volumetric methods like RT or UT, and surface cracking checks on crack-sensitive alloys) must be performed after PWHT is complete to ensure that no reheat cracking or hydrogen cracking occurred during the heating cycle.