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ASME STP-PT-085-2017 — Effect of Hot Wire Filler Metal Addition on GTAW Heat Input for Corrosion Resistant Overlays
📐 ASME STP-PT-085-2017 is a technical report sponsored by the ASME Pressure Technology Codes & Standards committee, prepared under ASME ST-LLC, and carried out by ARC Specialties Technical Services, issued in June 2017. The report studies the effect of hot wire filler metal addition on GTAW heat input for corrosion resistant overlays (CRO) and the resulting effects on heat-affected-zone (HAZ) hardness and toughness and on overlay chemical composition. The trials cover two base materials — P22 (2.25Cr-1Mo) and 4130 — and include both CRO overlay welds and groove welds.
💡 Hot wire GTAW resistively preheats the filler wire, increasing deposition rate without raising arc power. But wire preheating changes the weld’s heat balance, directly affecting heat input, iron dilution (%Fe), and HAZ hardness — precisely the process-property relationships this report quantifies systematically.
1. Project Background and Objectives 📌
Corrosion resistant overlays (CRO) deposit a corrosion-resistant alloy (nickel-base or austenitic stainless steel) onto carbon- or low-alloy-steel substrates, widely used on hydroprocessing reactors, heat-exchanger tubesheets, valve and fitting internals where corrosion resistance is needed without making the whole component from the expensive alloy. GTAW is the traditional process for low-dilution, high-quality overlays, and hot wire GTAW — which dramatically raises deposition rate while preserving quality — is becoming mainstream.
The project objective is to quantify how the heat-input change caused by hot wire addition affects HAZ hardness and toughness (governing substrate cold-cracking and service performance) and overlay chemistry (governing corrosion resistance), providing data support for ASME code and API/ASME weld-procedure qualification.
2. Weld Trial Matrix and Test Program 🔧
| Trial Category | Base Material | Position / Method | Test Items |
|---|---|---|---|
| CRO overlay welds | P22, 4130 | Horizontal (2G) and flat (1G) | Microhardness, chemistry, deposit area |
| HAZ hardness mapping | P22 (as-welded / PWHT) | Grid indentation along HAZ | HAZ hardness distribution and peak |
| Chemical analysis | P22, 4130 | Per API and ASME qualification locations | Overlay %Fe and alloy content |
| Groove weld trials | Corresponding bases | Standard groove joint | Joint performance, hardness, chemistry |
Test methods include microhardness testing with indentations located along the HAZ; chemical analysis at the locations required for API and ASME qualification; deposit-area measurement by the Jackson method (image analysis of the weld-bead cross-section); and macro-section metallography of the P22 and 4130 overlay structures.
3. Key Results and Process-Property Relationships 📊
The core output of the report is the quantitative relationship among HAZ hardness, heat-input rate, and %Fe dilution:
- Heat input and %Fe dilution: hot wire addition changes the bead melting and pool-stirring characteristics, so the iron dilution (%Fe) in the overlay changes accordingly; excessive dilution weakens the corrosion-resistant alloy chemistry, while too little can affect bonding and bead shape.
- HAZ hardness: HAZ hardness for both P22 and 4130 shows clear trends with heat-input rate; in the as-welded condition the HAZ peak hardness is high, dropping markedly after post-weld heat treatment (PWHT) — the report provides the comparison curves.
- Hardness-toughness trade-off: excessive HAZ hardness signals cold-cracking and embrittlement risk (critical for hardenable steels such as 4130), so heat input and PWHT must be jointly controlled.
- Groove weld trials: verify that the same process logic applies to load-carrying joints, confirming that the hot wire process does not adversely affect the hardness or toughness of groove welds.
⚠️ Heat input and dilution are a “double-edged sword”: lower heat input can reduce HAZ influence and hardness but may cut deposition rate or affect interpass remelting; higher heat input increases dilution and HAZ peak-hardness risk. The optimum window must be locked in by a trial matrix like this report’s, in combination with PWHT control.
4. Engineering Practice Essentials 🛠️
- 🔍 Procedure qualification: hot wire GTAW CRO must be covered by weld-procedure qualification (WPQ/PQR); sample and test chemistry and hardness per API or ASME requirements to verify dilution and HAZ hardness compliance.
- 📊 HAZ hardness control: for hardenable steels such as 4130, prioritize controlling the peak HAZ hardness (e.g., below ~350 HV with PWHT), adding preheat and interpass-temperature control where needed.
- 🔧 Post-weld heat treatment: PWHT P22 overlay components per code requirements — it both lowers HAZ hardness and restores toughness and stabilizes the tempered overlay microstructure; the report’s comparison data serve as a PWHT-effect reference.
- 📋 Quality verification: use image analysis (Jackson method) to verify deposit area and thickness, ensuring overlay thickness and chemistry meet the corrosion design requirements, and record the heat-input-rate and %Fe target windows in the procedure documents.
🚨 Loss of dilution control is the number-one cause of CRO failure: an overlay %Fe that is too high means the corrosion-resistant alloy has been diluted by the substrate, sharply reducing local corrosion resistance and risking pitting or stress-corrosion cracking in service. Chemical analysis must be sampled in the upper overlay (the effective corrosion-resistant zone) at the required frequency.
5. Frequently Asked Questions ❓
❓ Q: How does hot wire GTAW differ from conventional cold wire GTAW?
A: Hot wire GTAW resistively preheats the filler wire to near-melting before feeding it into the pool, markedly increasing deposition rate at the same arc power (often more than double cold wire) while keeping low dilution and good bead shape; the cost is that wire preheating changes the heat-input composition, so the process window must be re-qualified.
A: Hot wire GTAW resistively preheats the filler wire to near-melting before feeding it into the pool, markedly increasing deposition rate at the same arc power (often more than double cold wire) while keeping low dilution and good bead shape; the cost is that wire preheating changes the heat-input composition, so the process window must be re-qualified.
❓ Q: Why does %Fe dilution in the overlay matter?
A: %Fe is the direct measure of base-metal dilution. The overlay bottom dilutes most, diluting the corrosion-resistant alloy; if %Fe in the effective corrosion-resistant zone (upper overlay per code sampling) is out of spec, corrosion resistance fails and the overlay is ineffective.
A: %Fe is the direct measure of base-metal dilution. The overlay bottom dilutes most, diluting the corrosion-resistant alloy; if %Fe in the effective corrosion-resistant zone (upper overlay per code sampling) is out of spec, corrosion resistance fails and the overlay is ineffective.
❓ Q: What risks are associated with HAZ hardness?
A: Excessive HAZ hardness indicates a high fraction of hard (mainly martensitic) microstructure — high cold-cracking sensitivity and poor toughness; especially dangerous for higher-carbon steels such as 4130. Codes usually set a HAZ peak-hardness limit, met through PWHT or process control.
A: Excessive HAZ hardness indicates a high fraction of hard (mainly martensitic) microstructure — high cold-cracking sensitivity and poor toughness; especially dangerous for higher-carbon steels such as 4130. Codes usually set a HAZ peak-hardness limit, met through PWHT or process control.
🔍 Q: What does this report mean for ASME codes?
A: It supplies systematic trial data for code provisions on CRO/hot wire GTAW qualification — hardness acceptance criteria, chemical-analysis sampling, and heat-input control — supporting the development and update of welding requirements and acceptance criteria.
A: It supplies systematic trial data for code provisions on CRO/hot wire GTAW qualification — hardness acceptance criteria, chemical-analysis sampling, and heat-input control — supporting the development and update of welding requirements and acceptance criteria.
