🎓 Lesson 15
D5
Interpreting Hardness Traverse Results Across the HAZ
Hardness traverse results show how the hardness of metal changes across the heat-affected zone (HAZ) near a weld, helping us see if the repair weakened or strengthened the material.
🎯 Learning Objectives
- ✓ Analyze a hardness traverse plot to identify HAZ sub-zones (CGHAZ, FGHAZ, ICHAZ, SBHAZ) and label critical transitions
- ✓ Explain the metallurgical cause of hardness peaks/valleys in the HAZ using Fe–C phase diagram principles
- ✓ Apply AWS D1.1 Section 4.2.3 limits to determine compliance of measured hardness values relative to base metal and weld metal specifications
- ✓ Calculate the width of softened or hardened regions from raw traverse data using interpolation and threshold criteria
📖 Why This Matters
When repairing cracked tractor booms or loader arms—components subjected to cyclic fatigue and impact loading—welding introduces localized heating and rapid cooling. Without proper control, the heat-affected zone (HAZ) can become too hard (brittle fracture risk) or too soft (yield failure under load). Hardness traverses are your 'thermal fingerprint': they tell you whether the repair will survive harvest season—or fail catastrophically in the field. This isn’t just lab work—it’s the difference between warranty claims and decades of service.
📘 Core Principles
The HAZ is not uniform—it comprises sub-zones defined by peak temperature and cooling rate: coarse-grained HAZ (CGHAZ, >1100°C), fine-grained HAZ (FGHAZ, 900–1100°C), intercritical HAZ (ICHAZ, 723–900°C), and subcritical HAZ (SBHAZ, <723°C). Each zone develops distinct microstructures (e.g., martensite in CGHAZ of AISI 4140; spheroidized ferrite in SBHAZ of normalized S355), directly influencing hardness. Hardness traverses quantify this gradient. A compliant traverse shows no region exceeding 350 HV (for non-PWHT low-alloy steels) and no drop below 85% of base metal hardness—unless justified by qualified procedure. ASTM E384 governs test method; AWS D1.1 defines acceptance thresholds for structural farm equipment.
📐 Hardness Compliance Ratio
This ratio evaluates whether a measured HAZ hardness value meets minimum strength retention requirements relative to base metal. It is used during procedure qualification review to flag soft zones that may compromise yield strength.
Hardness Retention Ratio
R = H_{HAZ} / H_{base}Quantifies percentage of base metal hardness retained in a given HAZ location; used to assess strength degradation risk.
Variables:
| Symbol | Name | Unit | Description |
|---|---|---|---|
| R | Retention ratio | dimensionless | Decimal ratio of HAZ hardness to base metal hardness |
| H_{HAZ} | Measured HAZ hardness | HV or HRC | Vickers or Rockwell hardness value at specified HAZ location |
| H_{base} | Base metal hardness | HV or HRC | Average hardness of unaffected parent metal, measured per ASTM E10/E18/E384 |
Typical Ranges:
AISI 4140, as-welded: 280–340 HV
S355J2 +N, PWHT: 190–220 HV
💡 Worked Example
Problem: Base metal hardness = 220 HV; measured HAZ point at 2 mm from fusion line = 178 HV. Does this meet AWS D1.1 minimum retention?
1.
Step 1: Identify base metal hardness (H_base = 220 HV) and measured HAZ hardness (H_HAZ = 178 HV).
2.
Step 2: Compute ratio: H_HAZ / H_base = 178 / 220 = 0.809.
3.
Step 3: Compare to AWS D1.1 Sec 4.2.3 requirement: ≥ 0.85 for as-welded non-PWHT joints in structural steel ≤ 25 mm thick.
Answer:
The result is 0.809 (80.9%), which falls below the safe limit of 85%. This indicates unacceptable softening and requires either PWHT or procedure revision.
🏗️ Real-World Application
In 2022, a Tier-1 agricultural OEM qualified a GMAW repair procedure for cracked rear axle housings made of ASTM A572 Gr. 50 (base metal hardness = 190–210 HV). The hardness traverse revealed a 1.8-mm-wide soft zone (162 HV) in the SBHAZ due to excessive preheat (>150°C) combined with slow cooling. Per AWS D1.1, this violated the 85% rule (162/210 = 77%). Engineers reduced preheat to 100°C and added controlled-air cooling—revising the WPS—and requalified. Post-revision traverse showed uniform 185–205 HV across HAZ—fully compliant.
🔧 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