Interpass Temperature Control in Boom Weld Repairs

Industry Applications

Hydraulic excavator booms, mining shovel dipper arms, crane jib extensions, offshore support vessel cranes

Key Standards

ASME Section IX, EN ISO 15614-1, AWS D1.1/D1.5, ASTM A709/A1010, ISO 14713-2

Typical Scale

Repair welds range from 1.2 m to 8.5 m long; 20–60 passes per repair; interpass tolerance ±5°C

⚠️ Why It Matters

1

Excessive interpass temperature

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2

Prolonged time-at-temperature in heat-affected zone (HAZ)

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3

Over-tempering of martensitic/bainitic microstructures

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4

Reduced Charpy impact toughness (especially at −40°C)

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5

Premature brittle fracture in dynamic-load booms

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6

Catastrophic structural failure under cyclic loading

📘 Definition

Interpass temperature is the maximum allowable temperature of the weldment surface immediately prior to depositing the next weld pass, measured within 1 inch (25 mm) of the weld toe or fusion line. It is a controlled thermal parameter specified in welding procedure specifications (WPS) and enforced during execution to prevent excessive heat accumulation that could degrade microstructure, toughness, or residual stress distribution in high-strength low-alloy (HSLA) steels.

🎨 Concept Diagram

AI-generated illustration for visual understanding

💡 Engineering Insight

Interpass temperature isn’t just a 'maximum allowed' number—it’s the upper bound of a *thermal window* bounded below by preheat and above by the steel’s tempering threshold. In boom repairs, exceeding 230°C on S690QL doesn’t merely risk hardness loss—it triggers irreversible dissolution of Nb/V-rich precipitates that pin dislocations and stabilize yield strength. That’s why qualified welders measure *at the fusion line*, not the weld crown—and why a single out-of-spec reading invalidates the entire pass sequence.

📖 Detailed Explanation

Interpass temperature control begins with understanding how heat accumulates in thick-section HSLA steels during multi-pass welding. Unlike mild steel, high-strength grades like S690QL or ASTM A514 have narrow thermal windows between austenitizing and over-tempering temperatures. If the base metal remains too hot when the next pass is deposited, the cumulative heat input pushes the prior HAZ back into the austenite phase field—then rapid cooling forms coarse, brittle microstructures instead of fine tempered bainite.

Deeper analysis reveals that interpass interacts dynamically with preheat, travel speed, and joint geometry. For example, a deep U-groove in a 40-mm boom web creates thermal confinement—heat escapes slower than in a shallow V-groove, raising effective interpass even with identical surface readings. Modern practice uses embedded thermocouples (Type K, 0.5 mm diameter) placed 1 mm beneath the surface at critical HAZ locations to capture subsurface peaks missed by surface IR guns.

At the advanced level, interpass control must be integrated with real-time metallurgical modeling. Tools like Thermo-Calc or SYSWELD can simulate t₈/₅ and peak HAZ temperature fields across complex geometries, allowing engineers to pre-qualify interpass limits *before* field execution. This is now mandated for critical Class B/C repairs under ISO 14713-2 and OEM structural integrity programs (e.g., CAT Structural Welding Standard SW-1211).

🔄 Engineering Workflow

Step 1

Step 1: Verify WPS/PQR validity for specific steel grade, thickness, and repair geometry

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Step 2

Step 2: Calibrate and validate temperature measurement tools (contact pyrometer, calibrated IR gun, thermocouple wires)

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Step 3

Step 3: Establish preheat and interpass limits per ASME IX & EN ISO 15614-1, cross-referenced with OEM structural integrity requirements

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Step 4

Step 4: Perform real-time thermal monitoring at defined locations during each pass — record timestamp, location, and value

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Step 5

Step 5: Enforce mandatory cooldown pause if interpass exceeds limit; document duration and method (natural air, forced convection, or water mist *only if approved*)

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Step 6

Step 6: Conduct post-weld visual inspection, NDE (MT/PT), and verify PWHT compliance (if required) before load testing

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Step 7

Step 7: Archive all thermal logs, welder IDs, and NDE reports in traceable digital weld map per ISO 3834-2

📋 Decision Guide

Rock/Field Condition	Recommended Design Action
Steel Grade: S690QL, CE = 0.48, Repair Thickness > 25 mm	Max interpass = 220°C; use contact pyrometer + IR verification; mandatory 2-min dwell at ≤220°C before next pass
Cracked boom near hinge pin, ambient < 5°C, wind > 15 km/h	Raise preheat to 120°C; reduce max interpass to 180°C; shield weld zone with portable windbreak + insulation blankets
Multi-pass fillet weld on 30-mm-thick loader arm web-to-flange joint	Monitor interpass at three points per pass (toe, center, opposite toe); reject if any reading >230°C; recool with forced-air cooling only if below 200°C

📊 Key Properties & Parameters

Yield Strength (YS)

700–1100 MPa (e.g., ASTM A514/T1, S690QL)

The minimum stress at which high-strength steel begins to deform plastically, typically measured at 0.2% offset.

⚡ Engineering Impact:

Higher YS increases susceptibility to HAZ softening and necessitates tighter interpass control to preserve strength margins.

Carbon Equivalent (CE)

0.38–0.52 for boom-grade HSLA steels (e.g., S690QL, ASTM A709 Grade 100)

A weighted sum of alloying elements used to estimate hardenability and cold cracking susceptibility (e.g., CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15).

⚡ Engineering Impact:

Higher CE raises risk of martensite formation and hydrogen-induced cracking if interpass exceeds 250°C, requiring strict monitoring and preheat coordination.

Heat Input (HI)

0.5–1.8 kJ/mm for boom repairs using GMAW-S or FCAW-G processes

Energy delivered per unit length of weld, calculated as HI = (Voltage × Current × 60) / Travel Speed.

⚡ Engineering Impact:

High heat input combined with elevated interpass temperature accelerates grain coarsening and reduces notch toughness in the coarse-grained HAZ.

Cooling Rate (t₈/₅)

2–20 seconds for boom repairs (target: 5–12 s to avoid brittle phases)

Time required for the weld metal or HAZ to cool from 800°C to 500°C — a critical indicator of microstructural transformation kinetics.

⚡ Engineering Impact:

Interpass temperature directly governs t₈/₅; exceeding 250°C shortens t₈/₅ below safe thresholds, promoting untempered martensite and reduced ductility.

📐 Key Formulas

Heat Input (HI)

HI = (V × I × 60) / S

Calculates energy per unit length delivered to the weld (kJ/mm).

Variables:

Symbol	Name	Unit	Description
V	Voltage	volts (V)	Arc voltage across the weld
I	Current	amperes (A)	Welding current
S	Travel Speed	mm/min	Speed at which the welding torch moves along the joint

Typical Ranges:

GMAW-S on 25-mm S690QL

0.7–1.2 kJ/mm

FCAW-G root pass on cracked boom

0.9–1.5 kJ/mm

⚠️ ≤1.3 kJ/mm unless validated by PQR for specific CE and thickness

🔧 Open Calculator

Carbon Equivalent (CE)

CE = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

Estimates hardenability and cold cracking risk in HSLA steels.

Variables:

Symbol	Name	Unit	Description
C	Carbon content	wt%	Weight percent of carbon in the steel
Mn	Manganese content	wt%	Weight percent of manganese in the steel
Cr	Chromium content	wt%	Weight percent of chromium in the steel
Mo	Molybdenum content	wt%	Weight percent of molybdenum in the steel
V	Vanadium content	wt%	Weight percent of vanadium in the steel
Ni	Nickel content	wt%	Weight percent of nickel in the steel
Cu	Copper content	wt%	Weight percent of copper in the steel