🎓 Lesson 7
D4
Heat-Affected Zone (HAZ) Transformation Kinetics
The Heat-Affected Zone (HAZ) is the area of metal next to a weld that gets hot enough to change its structure—but not hot enough to melt—causing changes in strength, hardness, and toughness.
🎯 Learning Objectives
- ✓ Explain how peak temperature and cooling rate govern phase transformations in the HAZ of low-alloy steels used in farm equipment
- ✓ Calculate equivalent weld heat input (Q_eq) and estimate critical cooling rate (t₈/₅) using Rosenthal’s solution and empirical correlations
- ✓ Analyze microstructural banding and hardness profiles across the HAZ to diagnose inadequate preheat or interpass temperature control
- ✓ Design preheat and interpass temperature specifications compliant with AWS D1.1 and ISO 15614-1 for S355JO and A572 Grade 50 repair welds
📖 Why This Matters
Farm equipment—like grain augers, loader buckets, and PTO shaft housings—undergoes repeated impact, vibration, and corrosion. When cracked, they’re often repaired via arc welding. But improper heat management creates brittle zones in the HAZ, leading to premature failure under service loads. Understanding HAZ transformation kinetics isn’t academic—it’s what separates a 10-year repair from one that fails after three harvests.
📘 Core Principles
HAZ transformation kinetics hinge on two interdependent factors: thermal history (peak temperature Tₚₑₐₖ and cooling rate dT/dt) and base metal composition. As heat flows from the weld pool, successive HAZ subzones form: (1) the coarse-grained HAZ (CGHAZ), heated above Ac₃ (~910°C for plain carbon steel), where rapid cooling forms brittle martensite; (2) the fine-grained HAZ (FGHAZ), between Ac₁ and Ac₃, where partial austenitization yields mixed ferrite-pearlite; and (3) the intercritical HAZ (ICHAZ), where cycling across Ac₁–Ac₃ causes heterogeneous grain growth and softening. Kinetics follow JMAK (Johnson-Mehl-Avrami-Kolmogorov) models for diffusional transformations and Koistinen-Marburger for martensite, both highly sensitive to cooling rate—especially the t₈/₅ time (cooling from 800°C to 500°C), which correlates strongly with hardness and toughness.
📐 Critical Cooling Time (t₈/₅) Estimation
t₈/₅ is the most widely used practical indicator of HAZ microstructure. It estimates cooling rate through empirically calibrated heat flow models. For manual metal arc (MMA) and flux-cored arc (FCAW) repairs common in field shops, t₈/₅ is approximated using heat input and plate thickness. Accurate prediction enables preheat selection to avoid martensite formation in susceptible steels.
💡 Worked Example
Problem: A repair weld on a 25 mm thick S355JO bucket side plate uses FCAW with voltage = 28 V, current = 240 A, and travel speed = 12 cm/min. Preheat is 50°C. Estimate t₈/₅ and assess risk of untempered martensite.
1.
Step 1: Calculate heat input Q = (V × I × 60) / (S × 1000) kJ/mm → Q = (28 × 240 × 60) / (120 × 1000) = 3.36 kJ/mm
2.
Step 2: Apply t₈/₅ formula: t₈/₅ (sec) ≈ (5 × 10⁴ × Q × t) / (Tₚ − T₀)², where t = thickness (mm) = 25, Tₚ = assumed peak temp ≈ 1100°C, T₀ = preheat = 50°C → denominator = (1050)² = 1,102,500
3.
Step 3: Numerator = 5e4 × 3.36 × 25 = 4,200,000 → t₈/₅ ≈ 4,200,000 / 1,102,500 ≈ 3.8 sec
4.
Step 4: Compare to S355JO critical t₈/₅ threshold: < 5 sec risks >350 HV martensite; > 10 sec favors tough ferrite-pearlite. 3.8 sec indicates high martensite risk.
Answer:
The estimated t₈/₅ is 3.8 seconds—below the 5-second safety threshold—indicating significant risk of hard, crack-sensitive martensite. Increase preheat to ≥100°C or reduce heat input via faster travel speed.
🏗️ Real-World Application
In 2022, a Tier 1 agricultural OEM reported premature cracking in welded lift arm assemblies on Class 8 telehandlers. Metallurgical analysis revealed a 1.2-mm-wide CGHAZ with 420 HV hardness and transgranular cleavage—classic untempered martensite. Root cause was traced to field repairs performed at ambient temperature (−2°C) with no preheat and excessive heat input (Q = 4.9 kJ/mm). Revised procedure mandated preheat ≥120°C (per AWS D1.1 Table 3.2 for 25 mm S355), interpass ≤250°C, and maximum Q = 2.8 kJ/mm—reducing field failures by 94% over 18 months.
🔧 Interactive Calculator
🔧 Open Weld Repair Procedure Qualification for Structural Farm Equipment Calculator📋 Case Connection
📋 Tractor Frame Crack Repair at Tier-1 OEM Service Center
Crack located near rear axle mount under cyclic torsional load; customer warranty claim pending
📋 Loader Arm Fracture Repair in Sub-Zero Conditions
No shop access; extreme cold causing hydrogen cracking risk and brittle behavior