6.3 Welding Processes, Metallurgy & Weld Discontinuities (API 577)

Welding Processes, Metallurgy & Weld Discontinuities (API 577)

Welding inspection in the refining, chemical, and process industries demands a deep foundation in both mechanical execution and physical metallurgy. An API 510 Inspector must not only detect flaws during visual and nondestructive examinations, but also understand the metallurgical root causes—such as phase transformations, thermal stresses, hydrogen embrittlement, and solidification dynamics—that produce those flaws.

API Recommended Practice 577 (Welding Processes, Inspection, and Metallurgy) provides practical guidance on welding operations, filler metal classifications, metallurgical phenomena, and defect acceptance criteria.

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1. Industrial Welding Processes & Operating Characteristics (API 577 Section 5)

Five primary electric arc welding processes dominate pressure vessel fabrication, alteration, and repair:

+-----------------------------------------------------------------------------+
|                   PRIMARY PRESSURE VESSEL WELDING PROCESSES                 |
|                                                                             |
|   [SMAW] "Stick" Welding     ---> Flux-coated consumable electrode          |
|   [GTAW] "TIG" Welding       ---> Non-consumable tungsten + inert shield gas|
|   [GMAW] "MIG/MAG" Welding   ---> Continuous solid wire + shielding gas     |
|   [FCAW] Flux-Cored Arc      ---> Continuous tubular wire with flux core    |
|   [SAW]  Submerged Arc       ---> Continuous bare wire under granular flux  |
+-----------------------------------------------------------------------------+

Comparative Analysis of Welding Processes

GMAW Modes of Metal Transfer

1. Short-Circuiting Transfer (GMAW-S): Low heat input, small weld pool; electrode touches work 20–200 times/sec. Excellent for thin sheet and root passes, but carries an extreme risk of lack of side-wall fusion ("cold lap") on thick vessel materials.
2. Globular Transfer: Larger droplets than wire diameter fall under gravity; produces high spatter and rough weld appearance.
3. Spray Transfer: High current and voltage with Argon-rich shielding ($> 80%\text{ Ar}$); fine droplets sprayed across arc. High deposition, excellent fusion, limited to flat/horizontal positions.
4. Pulsed Spray (GMAW-P): Current pulses between low background level and high peak spray level. Allows all-position welding with spray-quality fusion and low heat input.

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2. AWS Filler Metal & Electrode Classification Nomenclature

American Welding Society (AWS) specifications classify electrodes and filler metals with standardized alphanumeric designations:

+-----------------------------------------------------------------------------+
|                  SMAW ELECTRODE DESIGNATION SYSTEM (AWS A5.1 / A5.5)        |
|                                                                             |
|        E     70     1     8     -     H4     R     -     1                |
|        |     |      |     |           |      |           |                  |
|   Electrode  |   Position |       Diffusible |        Improved Low-Temp     |
|              |            |        Hydrogen  |        Toughness (-50°F)     |
|        Min Tensile   Flux Coating &  (<= 4 mL/100g)                         |
|         (70 ksi)       Current/Polarity    Moisture Resistant               |
|                      (Low Hydrogen, DCEP)                                   |
+-----------------------------------------------------------------------------+

Breakdown of Electrode Classifications

• Tensile Strength Designator (First 2 or 3 digits): Indicates minimum tensile strength in ksi (e.g., 70 = $70,000\text{ psi}$, 80 = $80,000\text{ psi}$, 110 = $110,000\text{ psi}$). For stainless steel (e.g., E308, E316), the 3 digits represent the AISI alloy grade.
• Position Designator (Third or Fourth digit):
  • 1: All positions (flat, horizontal, vertical, overhead).
  • 2: Flat position and horizontal fillet welds only.
  • 4: Vertical-down progression and flat/horizontal (specialized electrodes).
• Coating & Current Type (Last digit):
  • 0: High cellulose sodium (DCEP, e.g., E6010 — deep penetration).
  • 1: High cellulose potassium (AC/DCEP, e.g., E6011).
  • 5: Low hydrogen sodium (DCEP, e.g., E7015).
  • 6: Low hydrogen potassium (AC/DCEP, e.g., E7016).
  • 8: Low hydrogen iron powder (AC/DCEP, e.g., E7018 — high deposition, crack resistant).
• GTAW/GMAW Solid Wire Designation (e.g., ER70S-6):
  • ER: Electrode or Rod (usable for GMAW wire or GTAW cut rod).
  • 70: Minimum tensile strength ($70\text{ ksi}$).
  • S: Solid bare wire.
  • 6: Chemical composition with high silicon/manganese deoxidizers for welding over mill scale and light rust.

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3. Welding Metallurgy: Heat-Affected Zone (HAZ) & Microstructure

When steel is welded, the localized thermal cycle creates dramatic metallurgical transformations across three distinct regions:

+-----------------------------------------------------------------------------+
|                        WELD JOINT METALLURGICAL ZONES                       |
|                                                                             |
|   +-------------+--------------+------------------+---------------------+   |
|   | WELD METAL  | FUSION LINE  |   HEAT-AFFECTED  | UNTREATED BASE METAL|   |
|   | (Cast Solid)| (Bond Line)  |     ZONE (HAZ)   | (Original Mill Mtl) |   |
|   +-------------+--------------+------------------+---------------------+   |
|          |             |                 |                   |              |
|          v             v                 v                   v              |
|      As-cast       Maximum         Microstructural      Unaltered base      |
|     dendritic      thermal         zones: Coarse-Grain  grain structure     |
|     structure      gradient        Fine-Grain, Inter-   and mechanicals     |
|                                    critical, Subcritical                    |
+-----------------------------------------------------------------------------+

The Sub-Zones of the Heat-Affected Zone (HAZ)

1. Coarse-Grained HAZ (CGHAZ): Adjacent to the fusion line; heated above $2000^\circ\text{F} (1100^\circ\text{C})$. Austenite grains grow excessively large. Upon rapid cooling, large grains transform into hard martensite or bainite, resulting in high hardness, low ductility, and reduced notch toughness (most susceptible to cracking).
2. Fine-Grained HAZ (FGHAZ): Heated just above the $Ac_3$ upper transformation temperature ($1650^\circ\text{F} / 900^\circ\text{C}$). Undergoes grain refinement (normalization), yielding excellent toughness and strength.
3. Intercritical HAZ (ICHAZ): Heated between $Ac_1$ and $Ac_3$ ($1350^\circ\text{F} \text{ to } 1650^\circ\text{F}$). Incomplete transformation creates mixed microstructures with localized soft or hard zones.
4. Subcritical HAZ (SCHAZ): Heated below $Ac_1$ ($< 1350^\circ\text{F}$). No phase transformation; undergoes minor tempering or stress relief.

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4. Carbon Equivalent ($CE$), Hydrogen Cracking Triad & Preheat Control

The Hydrogen-Assisted Cracking (HAC) Triad

Hydrogen-Assisted Cracking (also called cold cracking, delayed cracking, or underbead cracking) is the single most hazardous defect mechanism in carbon and low-alloy steel welding. It requires the simultaneous convergence of three critical factors:

+-----------------------------------------------------------------------------+
|                    HYDROGEN CRACKING CRITICAL TRIAD                         |
|                                                                             |
|                    [HARD, SUSCEPTIBLE MICROSTRUCTURE]                       |
|                    (High CE, Martensite in CGHAZ)                           |
|                                  /\\                                         |
|                                 /  \\                                        |
|                                /    \\                                       |
|                               /      \\                                      |
|   [DIFFUSIBLE HYDROGEN] <------------- [HIGH RESIDUAL TENSILE STRESS]       |
|   (Moisture in flux, oil,              (Restraint, joint geometry,          |
|    grease, humid atmosphere)            weld shrinkage stresses)            |
+-----------------------------------------------------------------------------+

Carbon Equivalent ($CE$) Formula (API 577 Section 10.3)

The Carbon Equivalent quantifies the hardenability of steel by converting the combined effect of various alloying elements into an equivalent percentage of carbon:

$CE = \text{\%C} + \frac{\text{\%Mn}}{6} + \frac{\text{\%Cr} + \text{\%Mo} + \text{\%V}}{5} + \frac{\text{\%Ni} + \text{\%Cu}}{15}$

Hardenability & Preheat Threshold Guidelines

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5. Post-Weld Heat Treatment (PWHT) Principles & Hardness Controls

Post-Weld Heat Treatment (PWHT), governed by ASME Section VIII Div 1 paragraph UCS-56 and API 577 Section 10.6, involves heating the completed weldment uniformly to a subcritical temperature (typically $1100^\circ\text{F} \text{ to } 1250^\circ\text{F} / 590^\circ\text{C} \text{ to } 675^\circ\text{C}$ for P-1 carbon steels) and holding for a specified duration (typically $1\text{ hr per inch}$ of thickness).

Primary Metallurgical Benefits of PWHT:

1. Residual Stress Relief: Thermal relaxation reduces peak weld residual stresses (which can reach base metal yield strength) down to $15% \text{ to } 20%$ of original levels.
2. Tempering of Hard Microstructures: Tempers hard martensite in the coarse-grained HAZ into tempered martensite, restoring ductility and impact toughness.
3. Hydrogen Bake-Out: Accelerates the diffusion of trapped hydrogen out of the weldment before cracking can initiate.
4. Dimensional Stability: Minimizes distortion during subsequent machining or service.

Hardness Limits for Environmental Cracking Prevention

In refinery wet $H_2S$ (sour) and caustic service, high HAZ hardness drastically accelerates sulfide stress cracking (SSC) and stress corrosion cracking (SCC).

• NACE MR0175 / SP0296 and API RP 582 establish a maximum hardness limit of 200 HBW (or 248 HV / 22 HRC) for carbon steel welds in sour service.

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6. Weld Discontinuities, Flaws, and NDE Inspection Selection (API 577 Section 9)

A discontinuity is any interruption in the normal physical structure of a weldment. A discontinuity becomes a defect only when its size, shape, or orientation exceeds the specific acceptance criteria of the applicable construction or in-service code.

+-----------------------------------------------------------------------------+
|                    PLANAR VS. VOLUMETRIC WELD DISCONTINUITIES               |
|                                                                             |
|   [PLANAR DISCONTINUITIES (CRITICAL)]         [VOLUMETRIC DISCONTINUITIES]  |
|   - Sharp crack-like tip geometry             - Rounded, three-dimensional  |
|   - Severe stress concentration risers        - Lower stress concentration  |
|   - Cracks (Hot, Cold, Lamellar, Crater)      - Porosity (Spherical, Cluster)|
|   - Lack of Fusion (LOF / Cold Lap)           - Slag Inclusions             |
|   - Incomplete Penetration (LOP)              - Tungsten Inclusions         |
|   - Detected best by UT / PAUT / MT / PT      - Detected best by RT         |
+-----------------------------------------------------------------------------+

Comprehensive Discontinuity & NDE Selection Matrix

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7. Delta Ferrite & Ferrite Number (FN) in Austenitic Stainless Steel Welds

When welding austenitic stainless steels (such as AISI Types 304, 304L, 316, 316L, 321, 347):

+-----------------------------------------------------------------------------+
|                     AUSTENITIC STAINLESS STEEL FERRITE BALANCE              |
|                                                                             |
|   [TOO LOW FERRITE (< 3 FN)]              [OPTIMAL BALANCE (3 - 10 FN)]     |
|   - Fully austenitic solidification       - Solidifies as primary delta     |
|   - Segregation of P and S to boundaries    ferrite                         |
|   - EXTREME HOT CRACKING / MICROFISSURES  - Impurities dissolve in ferrite  |
|                                           - EXCELLENT CRACK RESISTANCE      |
|                                                                             |
|   [TOO HIGH FERRITE (> 10 - 12 FN)]                                         |
|   - High-temperature service (1000°F - 1500°F) transforms ferrite to brittle |
|     SIGMA PHASE (sigma embrittlement)                                       |
|   - Degrades low-temperature cryogenic toughness and specific acid corrosion|
+-----------------------------------------------------------------------------+

Ferrite Control Guidelines:

1. Prevention of Hot Cracking: Austenitic stainless weld filler metals (e.g., E308L, ER316L) are intentionally alloyed with a slightly higher chromium-to-nickel ratio to ensure that $3% \text{ to } 10%$ delta ferrite (3 to 10 FN) forms in the as-deposited microstructure. Delta ferrite dissolves low-melting phosphorus and sulfur impurities, preventing them from forming liquid films along grain boundaries during cooling.
2. Measurement of Ferrite: Measured using magnetic instruments (Ferritescope) or calculated via the DeLong, Schaeffler, or WRC-1992 constitution diagrams using Chromium Equivalent ($Cr_{\text{eq}}$) and Nickel Equivalent ($Ni_{\text{eq}}$).

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8. Common Exam Traps & Pitfalls

[!WARNING] Trap 1: GMAW-S and Radiographic Examination: Questions often ask why GMAW short-circuiting mode (GMAW-S) cannot be qualified for welder testing via Radiography alone. Reason: GMAW-S is notorious for cold lap (lack of side-wall fusion). Because cold lap produces zero gap and no significant density change, RT frequently misses it. Ultrasonic testing (UT) or mechanical guided bend tests are required.

[!IMPORTANT] Trap 2: Coarse-Grained HAZ vs. Weld Metal Hardness: When hydrogen-assisted cracking occurs, it almost always initiates in the Coarse-Grained Heat-Affected Zone (CGHAZ) rather than the weld metal because the CGHAZ undergoes extreme grain growth followed by rapid quenching into hard martensite.

[!TIP] Trap 3: Austenitic Stainless Steel Preheat Myth: Carbon and low-alloy steels require preheat to prevent hardening and hydrogen cracking. In contrast, austenitic stainless steels (P-8) DO NOT undergo phase transformation to martensite and must NEVER be preheated. Applying high preheat or excessive interpass temperature ($> 350^\circ\text{F}$) to austenitic stainless steel increases distortion, promotes carbide precipitation (sensitization), and elevates hot cracking risk.