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The Zero-Tolerance Rule

D1.1:2025 Table 8.1 organizes 目視 受入基準 into eight discontinuity categories. Seven of those categories have quantitative 限界値 — a 最大 depth, a maximum length, a percentage of weld length. Cracks are different. Item (1) Crack Prohibition reads: “Any crack shall be unacceptable, regardless of size or location.”

The “X” in both the statically 読み込み済み and cyclically loaded columns means this criterion applies to every structural connection type covered by D1.1. There is no connection type, no loading condition, and no 溶接サイズ for which a crack is acceptable. A hairline crack visible only under magnification is held to the same 規格 as a full-penetration crack through the weld cross-section.

This absolute standard reflects the fracture mechanics reality: under cyclic loading, even a very small crack is a stress concentration that will propagate. Under static loading, a crack indicates a failure in the 溶接工程 that may signal other quality problems. The zero-tolerance rule eliminates any judgment call about whether a crack is “small enough.”

Crack Types in Structural Welds

Hot Cracks (Solidification Cracks)

Form during solidification of the 溶接金属 while it is still at elevated 温度. Low-melting-point impurities — primarily sulfur and phosphorus — segregate to grain boundaries as the metal solidifies. When the surrounding weld metal contracts on cooling, these weakened grain boundaries tear apart. Hot cracks typically run longitudinally along the weld centerline or through the crater at the weld termination. They are visible immediately after 溶接.

Cold Cracks (Hydrogen-Induced Cracking)

Form after the weld has cooled to below approximately 300°F, driven by three factors acting together: 拡散性水素 in the weld metal, a susceptible ミクロ組織 (hard 熱影響部), and residual tensile stress. Cold cracks may not appear until hours or days after welding — which is why D1.1 Table 8.1 item (5) requires delaying 外観検査 of A514, A517, and A709 HPS 100W welds for 48 hours. Using 低水素 electrodes (H8, H4) and adequate 予熱 are the primary 予防 methods.

Crater Cracks

Form at unfilled craters at weld terminations. When the arc is extinguished without filling the crater, the small weld pool solidifies rapidly under high restraint, creating a star-shaped crack pattern. D1.1 Table 8.1 item (3) requires all craters to be filled to the specified weld size. Crater cracks are one of the most preventable crack types — proper arc termination technique eliminates them.

Lamellar Tearing

A 母材 cracking mode, not a weld metal crack. Occurs in rolled plate when through-板厚 tensile stresses (from weld shrinkage in T継手 and 角継手 configurations) separate the low-ductility sulfide 介在物 planes parallel to the plate surface. Appears as a step-like crack beneath the weld. More common in older steels with high sulfur content. Modern steels with controlled sulfur content (Z-grade steels per ASTM A770) are significantly more resistant.

Toe Cracks

Initiate at the 溶接止端 — the junction between the weld face and the base metal surface. The weld toe is a geometric stress concentration and a site where hydrogen from the weld metal can diffuse into the HAZ. Toe cracks are a form of hydrogen-induced cracking and are prevented by the same methods: low-hydrogen process, adequate preheat, and avoiding excessive restraint.

Root Cracks

Initiate at the 溶接ルート in groove welds, typically in the first pass where the cross-section is smallest and restraint is highest. 融合不良 at the root combined with hydrogen and residual stress creates the conditions for root cracking. Back-gouging and re-welding from the second side eliminates any root crack before it is enclosed in the completed weld.

Prevention: The Four Controls

Low-hydrogen electrodes. Diffusible hydrogen is the primary driver of 低温割れ. Using electrodes with low hydrogen designations (H8 = max 8 mL/100g, H4 = max 4 mL/100g, H2 = max 2 mL/100g) and keeping them dry removes the hydrogen source. D1.1 Table 5.11 requires H8 or better for several base metal categories, and H4 for A913 Grade 80 (カテゴリー G).

Preheat and パス間温度. Preheat slows the 冷却速度, reducing the hardness of the heat-affected zone and giving hydrogen more time to diffuse out of the weld before the microstructure becomes susceptible. D1.1:2025 条項 5.7 and Table 5.11 establish minimum preheat temperatures as mandatory “shall” 要求事項 for 事前認定 WPSs — not recommendations.

Proper base metal cleanliness. Mill scale, moisture, oil, and paint introduce contaminants that increase hydrogen content and promote 高温割れ. D1.1 Clause 7.14 requires base metal preparation before welding.

Correct arc termination. Filling craters before extinguishing the arc prevents crater cracks. A runoff tab or backstep technique at weld terminations ensures the crater is filled. D1.1 Clause 7.30 addresses the use and removal of weld tabs.

Repair

D1.1:2025 Clause 7.25 governs repair of defective welds including cracks. The sequence: identify and mark the full extent of the crack (磁粉探傷試験 or dye 浸透探傷 試験 helps define crack ends), then per Clause 7.25.1.4 remove the crack and sound metal 2 in [50 mm] beyond each confirmed crack tip by grinding or gouging. This 2-inch extension is mandatory — crack tips are often not visible and may extend further than they appear. Inspect the excavated cavity to confirm removal, then re-weld using an approved WPS with appropriate preheat. The repaired area is re-inspected by VT against Table 8.1 item (1).

For cold cracks, the repair WPS must address the root cause — typically by increasing preheat above the minimum or switching to a lower-hydrogen 電極. Re-welding without addressing the hydrogen or restraint condition that caused the original crack invites recurrence.