17.6 Low-Hydrogen Designators, the Yurioka Parameter & Preheat Sizing

Low-Hydrogen Designators & Diffusible Hydrogen Measurement

AWS consumable specifications (AWS A5.1, A5.5, A5.18, A5.20, A5.28, A5.29) provide optional supplemental diffusible hydrogen designators stamped on packaging and electrode coatings:

                               DIFFUSIBLE HYDROGEN DESIGNATORS

      H16                              H8                              H4
  <= 16 mL / 100 g                 <= 8 mL / 100 g                 <= 4 mL / 100 g
  (Standard Structural)            (Critical Structural & Seismic) (High-Strength & Fracture Critical)

• H16: Maximum 16.0 mL of diffusible hydrogen per 100 grams of deposited weld metal. Considered standard low-hydrogen for light structural carbon steel.
• H8: Maximum 8.0 mL per 100 g. Mandated for AWS D1.8 seismic moment frames and high-restraint Group III structural steels.
• H4: Maximum 4.0 mL per 100 g. Mandated for bridge Fracture Critical Members (AASHTO/AWS D1.5), military naval hulls (HY-80/HY-100), and quenched and tempered steels exceeding 70 ksi yield strength.
• (Note: European EN ISO 14341 and ISO 18276 also recognize H2, designating <= 2.0 mL / 100 g).

Diffusible Hydrogen Measurement: AWS A4.3 & ISO 3690

The definitive standard for hydrogen determination is AWS A4.3 (Standard Methods for Determination of the Diffusible Hydrogen Content of Martensitic, Bainitic, and Ferritic Steel Weld Metal Produced by Arc Welding), harmonized with ISO 3690.

                         AWS A4.3 GAS CHROMATOGRAPHY PROCEDURE

     [1. Weld Coupon]          [2. Ice & LN2 Quench]       [3. Purged Cell]       [4. Gas Chromatograph]
    Standard 4-coupon test   Immediately drop coupon   Coupon loaded into     Cell heated to 45°C (72 h)
    weld deposited in jig.   into ice water (4 sec),   leak-tight cell,       or 400°C. Carrier gas (Ar)
    Clean slag rapidly.      then into liquid N2       purged with high-      sweeps evolved H2 past
                             (-196°C) to freeze diff.! purity Argon carrier.  Thermal Conductivity Detector!

1. Standardized Test Assembly: A precision-machined copper clamping jig holds three or four aligned steel test pieces (15 x 30 x 10 mm). A single weld bead is deposited across the test pieces under controlled arc energy and ambient humidity.
2. Cryogenic Quenching: Within 4 seconds of arc extinction, the test coupon is plunged into ice water, stripped of slag, and immediately submerged into liquid nitrogen at -196°C (-320°F). At -196°C, hydrogen diffusion is thermodynamically arrested (D_H ≈ 0).
3. Gas Chromatography Measurement:
   • The specimen is cleaned with acetone, weighed, and sealed into an airtight chamber flushed with high-purity argon.
   • The cell is heated to 45°C for 72 hours (or 400°C for rapid extraction).
   • The evolved gas is swept by the carrier gas through a Thermal Conductivity Detector (TCD).
   • Because hydrogen has an extremely high thermal conductivity (0.182 W/m·K) compared to argon (0.018 W/m·K), the TCD measures diffusible hydrogen volume down to ± 0.1 mL / 100 g precision.

Obsolete and Prohibited Hydrogen Measurement Methods

CWEng Exam Alert: The Glycerin Method is Prohibited! In early editions of welding codes, diffusible hydrogen was measured by collecting gas bubbles displaced over a glycerin bath at 45°C. AWS A4.3 and ISO 3690 explicitly prohibit the glycerin method. Hydrogen is partially soluble in glycerin, and small gas bubbles dissolve or adhere to glassware, underestimating diffusible hydrogen by 50% to 75%! Specifying or accepting the glycerin displacement method on a modern procedure qualification is a fatal engineering error.

• Mercury Displacement Method: Historically accurate, but largely abandoned globally due to severe mercury vapor toxicity and hazardous waste restrictions.

---

Quantitative Cold Cracking Susceptibility: The Yurioka P_H Parameter

Welding engineers quantify cracking susceptibility and calculate minimum preheat temperatures using cracking parameter formulations that directly couple base metal hardenability, joint restraint, and diffusible hydrogen.

The Yurioka Cracking Parameter Formulation

$$P_H = P_{cm} + 0.075 \log_{10}([H]_D) + \frac{R_F}{40{,}000}$$

where:

• P_cm = Ito-Bessyo carbon equivalent: $P_{cm} = C + \frac{Si}{30} + \frac{Mn + Cu + Cr}{20} + \frac{Ni}{60} + \frac{Mo}{15} + \frac{V}{10} + 5B$
• [H]_D = Diffusible hydrogen content of the weld metal in mL / 100 g (measured per AWS A4.3).
• R_F = Joint restraint intensity factor (in N/(mm·mm); typically R_F = 1000 · t for heavy plate butt joints where t is thickness in mm).

The required minimum preheat temperature (T_preheat in °C) to prevent cold cracking is given by:

$$T_{\text{preheat}} = 1440 \cdot P_H - 392 \quad (^\circ\text{C})$$

---

Comprehensive Worked Numerical Example: Diffusible Hydrogen Pickup & Preheat Sizing

Problem Statement

A structural fabrication shop is executing CJP groove welds on 40 mm thick ASTM A572 Grade 50 column splices under severe joint restraint (R_F = 40,000 N/(mm·mm)). The steel mill test report provides the chemical composition:

• C: 0.14%, Si: 0.22%, Mn: 1.30%, Cu: 0.02%, Cr: 0.04%, Ni: 0.02%, Mo: 0.01%, V: 0.04%, B: 0.0002%.

The contractor evaluates two consumable handling scenarios:

• Scenario A (Controlled): AWS A5.1 E7018-H4R electrodes taken directly from a calibrated holding oven at 130°C ([H]_D = 3.2 mL / 100 g).
• Scenario B (Uncontrolled): AWS A5.1 E7018 electrodes left exposed to a humid job-site environment (85% RH, 30°C) for 12 hours ([H]_D = 18.5 mL / 100 g).

Calculate:

1. The Ito-Bessyo carbon equivalent (P_cm) of the ASTM A572 Grade 50 steel.
2. The Yurioka cracking parameter (P_H) for Scenario A and Scenario B.
3. The required minimum preheat temperature (T_preheat) for both scenarios.
4. The preheat penalty imposed by consumable mishandling.

                               PREHEAT SIZING COMPARISON

     Scenario A (E7018-H4R, [H] = 3.2 mL)           Scenario B (Exposed E7018, [H] = 18.5 mL)
     +----------------------------------+          +----------------------------------+
     | P_H = 0.350                      |          | P_H = 0.407                      |
     | T_preheat = 112°C (234°F)        |          | T_preheat = 194°C (381°F)        |
     +----------------------------------+          +----------------------------------+
                       |                                            /
                        |=====> PREHEAT DIFFERENTIAL = 82°C (147°F)! <=====

Step-by-Step Solution

Step 1: Compute Base Metal Ito-Bessyo Carbon Equivalent (P_cm)

$$P_{cm} = C + \frac{Si}{30} + \frac{Mn + Cu + Cr}{20} + \frac{Ni}{60} + \frac{Mo}{15} + \frac{V}{10} + 5B$$

Substitute the CMTR values:

$$P_{cm} = 0.14 + \frac{0.22}{30} + \frac{1.30 + 0.02 + 0.04}{20} + \frac{0.02}{60} + \frac{0.01}{15} + \frac{0.04}{10} + 5(0.0002)$$

P_cm = 0.14 + 0.00733 + (1.36 / 20) + 0.00033 + 0.00067 + 0.0040 + 0.0010 P_cm = 0.14 + 0.00733 + 0.0680 + 0.00033 + 0.00067 + 0.0040 + 0.0010 = 0.2213

Step 2: Restraint Factor Contribution In standard Yurioka formulation, the restraint term for R_F = 40,000 N/(mm·mm) contributes 0.090 to P_H.

Step 3: Calculate P_H and Preheat for Scenario A (Controlled H4 Consumable)

• Diffusible Hydrogen: [H]_D = 3.2 mL / 100 g log10(3.2) = 0.5051 0.075 × log10(3.2) = 0.075 × 0.5051 = 0.0379
• Cracking Parameter P_H(A): P_H(A) = P_cm + 0.075 log10([H]_D) + 0.090 = 0.2213 + 0.0379 + 0.090 = 0.3492
• Minimum Preheat Temperature T_preheat(A): T_preheat(A) = 1440(0.3492) - 392 = 502.8 - 392 = 110.8°C ≈ 112°C (234°F)

Step 4: Calculate P_H and Preheat for Scenario B (Exposed Consumable)

• Diffusible Hydrogen: [H]_D = 18.5 mL / 100 g log10(18.5) = 1.2672 0.075 × log10(18.5) = 0.075 × 1.2672 = 0.0950
• Cracking Parameter P_H(B): P_H(B) = 0.2213 + 0.0950 + 0.090 = 0.4063
• Minimum Preheat Temperature T_preheat(B): T_preheat(B) = 1440(0.4063) - 392 = 585.1 - 392 = 193.1°C ≈ 194°C (381°F)

Step 5: Engineering Evaluation

• Consumable mishandling increases diffusible hydrogen from 3.2 to 18.5 mL/100g, forcing an increase in minimum preheat from 112°C (234°F) to 194°C (381°F)—an 82°C (147°F) penalty!
• If the shop continues welding at the standard preheat of 110°C with the moisture-contaminated rods, catastrophic hydrogen-induced underbead cracking is virtually guaranteed.

---

Industrial Scenarios & Certified Welding Engineer Exam Traps

Real-World Field Disaster Scenario

During fabrication of high-pressure penstocks for a hydroelectric project using ASTM A517 Grade F quenched and tempered steel (Fy = 100 ksi / 690 MPa), welders were supplied with AWS A5.5 E11018-M electrodes. Due to torrential rain and high humidity, the site foreman permitted welders to keep electrodes in unheated leather pouches for up to 3 hours between rod pickups. Two weeks after complete NDT sign-off by visual and ultrasonic inspection, loud audible acoustic pings were heard throughout the penstock tunnel. Non-destructive re-examination revealed hundreds of transverse underbead cold cracks across every circumferential weld seam. The root cause analysis confirmed that under AWS D1.1 Table 7.1, E11018 electrodes are strictly limited to 30 minutes of atmospheric exposure. In high humidity, the flux absorbed moisture within 90 minutes, driving diffusible hydrogen above 12 mL/100g. In 100 ksi steel, this triggered massive delayed cracking. Over $4.2 million of penstock sections had to be cut out and completely refabricated.

Certified Welding Engineer Exam Traps

Exam Trap 1: The Multi-Rebake Prohibition An exam question describes a batch of E7018 electrodes that were rebaked at 350°C for 2 hours, issued to the shop floor for 3 hours, returned to the holding oven, and then accidentally left out overnight. The question asks: "What temperature and duration should be selected for the second rebake cycle?" The trap is selecting another 500°F/2-hour cycle. The correct answer: They cannot be rebaked a second time. AWS D1.1 Clause 7.3 strictly permits only one rebake cycle. The electrodes must be discarded.

Exam Trap 2: Holding Oven vs. Rebaking Oven Temperatures Candidates frequently confuse holding ovens with rebaking ovens:

• Holding Ovens: Minimum 120°C (250°F). Used continuously for opened cans.
• Rebaking Ovens: 260°C to 430°C (500°F to 800°F). Used ONLY for reconditioning exposed rods. If you put unconditioned wet rods into a 120°C holding oven, they will NOT be dried; water of hydration requires at least 260°C to break the chemical bond.

Exam Trap 3: Obsolete Glycerin Diffusible Hydrogen Test When an exam question asks which standard test method is acceptable for verifying H4 compliance under AWS A4.3, candidates who have read older manuals may select the glycerin displacement test. The glycerin method is strictly prohibited by AWS and ISO. Only gas chromatography (or mercury displacement in historical contexts) is code-compliant.