Weld Heat Input Calculation and Control for Duplex Stainless and CrMo Pipe Fittings

The Heat Input Formula

Heat input (Q) is calculated as: Q = (V × I × 60) / (S × 1000) kJ/mm, where V is arc voltage (volts), I is welding current (amps), S is travel speed (mm/min), and the factor 60 converts minutes to seconds. An additional arc efficiency factor (η) is applied for processes other than GTAW: η = 1.0 for GTAW, 0.8 for SMAW, 0.9 for FCAW, 0.8–0.99 for SAW depending on polarity. The qualified heat input range is established during WPS/PQR qualification and is an essential variable — exceeding the maximum or falling below the minimum qualified heat input requires re-qualification of the welding procedure.

Why Heat Input Limits Matter for Duplex Stainless

Duplex stainless steel requires a precise balance of ferrite and austenite (35–55% ferrite for 2205, 35–65% for 2507) in the weld metal and HAZ. This balance is controlled by cooling rate through the 1000–800°C range — which is directly determined by heat input:

• Too high heat input (>2.5 kJ/mm for 2205): slow cooling promotes ferrite growth and sigma phase formation at 700–950°C. The weld HAZ has excess ferrite (above 65 FN) and reduced toughness. Sigma phase precipitates can cause Charpy energy to drop below 20 J at −40°C.
• Too low heat input (<0.5 kJ/mm): rapid cooling suppresses the ferrite-to-austenite transformation, leaving excess ferrite (above 70 FN) with insufficient austenite reformation. The weld has low toughness and reduced corrosion resistance because the nitrogen-stabilised austenite that carries much of the PREN has not formed properly.

The correct range for duplex 2205 is 0.5–2.5 kJ/mm; for super duplex 2507, the range is tighter at 0.2–1.5 kJ/mm because 2507's higher alloy content makes sigma formation faster.

Why Heat Input Limits Matter for P91

P91 (9Cr-1Mo-V) is the most heat-input-sensitive of the CrMo alloy steels. The target microstructure is fully tempered martensite with fine, uniformly distributed carbide precipitates (M₂₃C₆, MX carbonitrides). Heat input outside the qualified range produces:

• Too high heat input (>3.0 kJ/mm): wider HAZ, more time at high temperature causes grain growth in the coarse-grained HAZ, reducing toughness. Slow cooling can allow delta ferrite formation, which reduces creep strength and resists PWHT tempering.
• Too low heat input (<1.5 kJ/mm): rapid cooling produces a harder, more brittle HAZ that is more susceptible to cold cracking (despite preheat) and Type IV cracking initiation in service.

The EPRI P91 welding guidelines recommend 1.5–3.0 kJ/mm as the qualified heat input range, tighter than ASME IX alone requires.

Interpass Temperature: The Partner Control

Heat input and interpass temperature are complementary controls. Interpass temperature is the temperature of the weld joint before each subsequent pass is started. For duplex 2205: maximum 150°C; for 2507: maximum 100°C. Exceeding these limits allows heat from successive passes to accumulate, effectively increasing the equivalent heat input and causing the same sigma phase and excess ferrite problems as a single high-heat-input pass. For P91: maximum 300°C interpass, but the joint must not be allowed to cool below the preheat minimum (typically 200°C) between passes — a narrow control band that requires continuous temperature monitoring with contact thermometers or thermocouple pyrometers.

Production Monitoring and Records

The WPS must specify the qualified heat input range (kJ/mm) and interpass temperature limit. Production welding records must document the actual voltage, current, and travel speed for each pass, enabling calculation of actual heat input and comparison against the qualified range. For P91 and duplex welds in critical service, third-party inspection at a Hold point during welding — not just after completion — is the only reliable way to verify that interpass temperature and heat input are being maintained in production.