E7018 Electrode — Preheat Requirements, Correct Storage at 120–150°C & Rod Oven Verification with Tempilstik®
E7018 is the most widely used SMAW electrode for structural and pipeline welding in Vietnam. Its low-hydrogen designation (≤ 8 ml/100g deposited weld metal) makes it the preferred choice for critical structural applications, pressure piping, and alloy steel welding. But low-hydrogen does not mean zero-hydrogen — and this is where many welders and even some engineers make a costly assumption: E7018 still requires preheat in specific cases, and critically, requires correct rod oven storage to actually deliver its low-hydrogen performance.
Why E7018 Still Requires Preheat — Technical Explanation
The most common misconception: "low-hydrogen electrode means no preheat needed." This is technically incorrect. Per AWS D1.1 and ASME, preheat requirements are governed by steel type and wall thickness — not by electrode classification.
E7018 reduces the hydrogen content introduced by the electrode (≤ 8 ml/100g versus ≥ 20 ml/100g for E6010), which allows preheat to be reduced in some borderline cases. It does not eliminate the preheat requirement entirely.
Hydrogen-induced cold cracking (HIC) occurs when three conditions coincide: hydrogen in the weld metal, tensile stress in the HAZ, and a susceptible microstructure (hard martensite in steel with high CE). Preheat addresses the first factor: by slowing cooling it gives hydrogen time to diffuse out before being trapped in the hardened martensite microstructure. With thick steel or high CE, the remaining hydrogen from E7018 is still sufficient to cause cracking without preheat.
Preheat Requirements for E7018 per AWS D1.1
AWS D1.1 Table 3.2 classifies steel by category and specifies preheat based on CE, thickness, and electrode hydrogen classification (H4 / H8 / H16). E7018 standard grade is classified as H8 (≤ 8 ml/100g).
| AWS Category | CE | Thickness | Preheat with H16 (E6010) | Preheat with H8 (E7018) | Tempilstik® |
|---|---|---|---|---|---|
| Category I SS400, A36 |
≤ 0.40 | t ≤ 38 mm | 66°C (150°F) | Not required (or 10°C) | — |
| 38 < t ≤ 65 mm | 66°C (150°F) | 66°C (150°F) | #28006 | ||
| Category II SM490, A572 |
0.41–0.45 | t ≤ 20 mm | 66°C (150°F) | Not required (or 10°C) | — |
| 20 < t ≤ 38 mm | 66°C (150°F) | 66°C (150°F) | #28006 | ||
| Category III | 0.46–0.60 | t ≤ 20 mm | 110°C (230°F) | 66°C (150°F) | #28006 / #28312 |
| 20 < t ≤ 38 mm | 150°C (302°F) | 110°C (230°F) | #28312 |
For full preheat tables by material and thickness see: Preheat Temperature Reference Table.
Rod Oven Storage: Why 120–150°C is Critical
The E7018 electrode flux coating contains calcium carbonate (CaCO₃) and calcium fluoride — the alkaline flux system that creates the slag, generates CO₂ arc shielding, and absorbs hydrogen. Calcium carbonate absorbs moisture aggressively from ambient air. When moisture in the flux coating decomposes at arc temperature, it releases hydrogen directly into the weld pool — instantly destroying the low-hydrogen performance of the electrode.
Standard storage procedure to maintain H8 performance (≤ 8 ml/100g)
| Stage | Action | Temperature | Time Limit |
|---|---|---|---|
| After opening the hermetic can | Transfer immediately to a holding oven | 120–150°C (248–302°F) | Store continuously until use |
| Atmospheric exposure | Per AWS D1.1 D.2.2: maximum permitted exposure before rebaking | Ambient | Maximum 4 hours |
| Rebaking (if exposed or moisture suspected) | Rebake per manufacturer's specification | 250–350°C (482–662°F) | 1–2 hours (per manufacturer) |
| Rebaking cycle limit | Flux coating cracks and degrades with repeated thermal cycling | — | Maximum 2–3 rebake cycles total |
Verifying Rod Oven Temperature with Tempilstik®
Electrode holding ovens and rebaking ovens must be verified periodically — the set temperature on the controller may diverge from the actual temperature inside the oven, particularly in older ovens or when thermostats have drifted. Tempilstik® resolves this by confirming the actual temperature at the electrode tray location.
| Verification Purpose | Target Temperature | Tempilstik® Part No. | How to Interpret |
|---|---|---|---|
| Holding oven — lower bound | 110°C / 230°F | #28312 | Crayon melts → oven is ≥ 110°C ✓ — adequate holding temperature |
| Holding oven — upper bound | 150°C / 302°F | #28318 | Crayon does NOT melt → oven is still below 150°C ✓ — within safe range |
| Rebaking oven — minimum temperature | 177°C / 350°F | #28025 | Crayon melts → oven has reached sufficient temperature to initiate rebaking cycle |
- Place #28312 (110°C) in the electrode tray — it must melt to confirm the oven is adequately hot.
- Place #28318 (150°C) alongside — it must not melt to confirm the oven is not overheating.
- If both crayons have melted, the oven is running above 150°C — prolonged exposure at this temperature may damage the flux coating. Investigate the thermostat.
Measurement procedure: place the Tempilstik® crayons in the electrode basket or tray, close the oven door, wait 15–20 minutes for the temperature to stabilize, then open and check whether each crayon has melted. Record the result in the oven temperature inspection log.
For continuous monitoring without opening the oven, Tempilabel® Series 21 adhesive temperature indicators can be affixed inside the oven — they record the maximum temperature reached irreversibly, providing a permanent record without requiring manual inspection.
Three Key Points to Remember — E7018 and Preheat
Always look up preheat by steel category and wall thickness. E7018 (H8) may allow reduced preheat versus E6010 (H16) in borderline cases — it does not universally eliminate preheat.
An E7018 electrode left in ambient air for more than 4 hours may perform as H16 or worse. The 120–150°C holding oven is not optional — it is the mechanism that maintains H8 performance.
Use Tempilstik® #28312 and #28318 to confirm actual temperature at the electrode location. Controller setpoint and actual internal temperature can diverge significantly in ageing ovens.
Frequently Asked Questions
Need Tempilstik® #28312 or #28318 for rod oven verification?
Fast Group Engineering — authorized Tempil® distributor in Vietnam. Available individually or as a box of 10. C/O + C/Q + VAT invoice. Stock in HCMC and Vung Tau.
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