welding-metallurgy

name	welding-metallurgy
description	Welding metallurgy — HAZ zones, grain growth, solidification, weld microstructure, martensite formation, carbon equivalent, PWHT, hydrogen cracking, preheat, interpass temperature.
metadata	{"priority":7,"promptSignals":{"phrases":["welding metallurgy","HAZ","heat affected zone","preheat","PWHT","carbon equivalent","martensite","hydrogen cracking","solidification","weld microstructure"],"minScore":3}}

Welding Metallurgy — Complete Skill

Heat Input and Thermal Cycle

Heat Input

H = (η × V × I) / v [J/mm] V = arc voltage [V], I = current [A], v = travel speed [mm/s] η = process efficiency: SMAW 0.80, GMAW 0.85, GTAW 0.60, SAW 1.00, FCAW 0.85

Rosenthal solution (point source, moving, steady state): T(r,t) - T₀ = [Q/(2πλ)] × exp(-v(r-x)/(2α)) × K₀(vr/(2α)) r = distance from weld centerline, α = thermal diffusivity, K₀ = modified Bessel function

Simplified (peak temperature for HAZ): 1/(T_pk - T₀) = (4.13 × ρ × c × t²/H) + 1/(T_m - T₀) t = plate thickness [mm], ρ × c = volumetric heat capacity [J/mm³K]

Cooling rate at 540°C (8/5 cooling time T₈₅): t₈₅ = (6700-5σ_h)/((540-T₀)²) × (1+(1/(540-T₀) - 1/(800-T₀))) × H (3D cooling, thick plate) σ_h = 0 for single pass; higher for preheated plates

HAZ Zones

Microstructural Zones (Low Alloy Steel)

Fusion zone (weld metal): melted and resolidified; dendritic microstructure; composition = base + filler Coarse-grained HAZ (CGHAZ): T > 1100°C; grain growth (ASTM 1-3 vs. base 8-10); lower toughness Fine-grained HAZ (FGHAZ): 900-1100°C; refined grains; good toughness Intercritical HAZ: 723-900°C (Ac₁-Ac₃); partial transformation; mixed martensite/ferrite Subcritical HAZ: < 723°C; tempering or sensitization only

CGHAZ problem: grain coarsening → large prior austenite grains → brittle martensite on cooling → low CVN impact toughness Mitigation: higher Mn/Ni content, lower heat input, TiN pinning (limits grain growth via TiN precipitates)

HAZ Width

CGHAZ width ≈ 1-3 mm (single pass, typical structural steel) Wider with higher heat input; narrower with laser or electron beam welding Measure by hardness traverse or metallographic etching (nital, picral)

Solidification and Weld Microstructure

Solidification Mode

Competitive epitaxial growth from fusion boundary: grains grow in direction of maximum thermal gradient (⊥ to fusion line → toward centerline) Primary solidification: δ-ferrite (most C-Mn steels) or austenite (austenitic SS)

Columnar to equiaxed transition (CET): High thermal gradient + slow solidification rate → columnar dendrites Low thermal gradient (center of wide welds) → equiaxed grains CET beneficial for low hot cracking susceptibility

Segregation

Dendritic solidification: solute rejected to interdendritic regions P, S, C, Si segregate to last-to-solidify (centerline, dendrite boundaries) Sulfur → FeS at grain boundaries → hot cracking (solidification cracking) Control: S < 0.01%, Mn/S > 20 (MnS instead of FeS); low heat input

Microstructure of Low-Alloy Steel Weld Metal

Proeutectoid ferrite (grain boundary ferrite, GBF): at prior austenite grain boundaries Widmanstätten ferrite: plates from boundary into interior Acicular ferrite (AF): interlocking, high-angle plates; highest toughness Martensite/bainite: if cooling too rapid or high carbon/alloy content

Acicular ferrite nucleation: on non-metallic inclusions (TiO, MnO, SiO₂) — key for filler wire selection (Ti deoxidation → AF) Optimal: maximum acicular ferrite fraction → controlled inclusion composition + density + cooling rate

Martensite Formation and Hardness

Martensite Start Temperature

M_s ≈ 539 - 423C - 30.4Mn - 17.7Ni - 12.1Cr - 7.5Mo [°C] (Andrews equation) M_f ≈ M_s - 215°C (martensite finish; below: untransformed austenite = retained)

Hardness of martensite: H_m ≈ 884C^0.302 [HV] (Vickers) — increases strongly with carbon

Maximum allowable HAZ hardness: General: HV 350 (NACE MR0175 for sour service) Structural: HV 400 (most codes) Above → risk of hydrogen-induced cracking

Carbon Equivalent (Hardenability)

IIW: CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15 CE < 0.40: no preheat needed (restraint-dependent) CE 0.40-0.60: preheat 100-200°C CE > 0.60: high preheat required

Pcm (Ito-Bessyo, for higher strength steels): Pcm = C + Si/30 + Mn/20 + Cu/20 + Ni/60 + Cr/20 + Mo/15 + V/10 + 5B Pcm ≤ 0.25: low cold cracking risk

Hydrogen-Induced Cracking (HIC / Cold Cracking)

Conditions for HIC

All three must be present:

1. Susceptible microstructure (martensite, HV > 350)
2. Hydrogen (from electrode moisture, surface contamination, base metal)
3. Residual/applied tensile stress

Hydrogen source in welding: cellulosic electrodes (E6010: ~40 mL/100g), basic low-H (E7018: <4 mL/100g) Classification: H4 (<4 mL/100g), H8 (<8 mL/100g), H16 (<16 mL/100g)

Prevention of HIC

Preheat temperature: T_preheat from CE + plate thickness + hydrogen level + restraint Rule: T_preheat ≈ 350 × √(Pcm - 0.08) + log(0.8H_D + 7.5δ) (Terada-Ikeda) Practical: AWS D1.1 Table 4.2 (SMAW) or calculate from Pcm

Key actions:

• Use low-hydrogen electrodes (E7018) — bake before use (300°C × 1hr)
• Maintain preheat and interpass temperature
• Slow cooling (insulated blanket) to allow H to diffuse out
• PWHT if required (stress relief + H removal)

PWHT (Post-Weld Heat Treatment)

Objectives

1. Reduce residual stresses (to 20-30% of yield)
2. Temper HAZ martensite (restore toughness)
3. Allow hydrogen to diffuse out (if not done before)
4. Dimensional stability (stress relieving)

Temperature and Hold Time

Carbon steel (ASME BPVC): 595-650°C × 1 hr/25mm thickness (min 1 hr) Low alloy (2.25Cr-1Mo, P22): 700-760°C × 2 hr/25mm Austenitic SS: solution anneal 1050-1100°C (sensitization reversal) — not standard PWHT

Heating rate: ≤ 55°C/hr for thick sections (> 25mm) Cooling rate: ≤ 55°C/hr above 315°C (prevent thermal shock)

Larson-Miller Parameter (Tempering Effectiveness)

LMP = T × (C + log₁₀(t)) × 10⁻³ T = temperature [R or K], t = time [hr], C = constant (≈ 20 for steels) Equal LMP → equal metallurgical effect: higher T × shorter t equivalent to lower T × longer t

Sensitization of Austenitic Stainless Steel

Sensitization: Cr₂₃C₆ precipitates at grain boundaries (450-850°C range) → Cr-depleted zones at boundaries → susceptible to intergranular corrosion (IGC) and SCC

Prevention: Low-C grades (304L, 316L: C < 0.03%): less sensitization tendency Stabilized grades (347 with Nb, 321 with Ti): TiC or NbC preferred over Cr₂₃C₆ Solution anneal (1050°C, rapid quench): dissolves Cr carbides, restores Cr uniformity

Weld decay: HAZ at sensitization temperature range → IGC in HAZ, not weld metal Double sensitization: if 321 is used but service temp passes through 450-850°C twice

Output

Provide: calculated CE (IIW) and Pcm, preheat temperature T_min [°C], maximum HAZ hardness prediction [HV], HAZ zone widths [mm], solidification mode (δ/γ), PWHT temperature and time [°C × hr], hydrogen level recommendation (H4/H8/H16), sensitization risk assessment for stainless.