Zhuojin Pipe Fitting Co., LtdDissimilar Metal Welding of 1Cr5Mo Furnace Tubes to ZG25 Cast Elbows
Literature Overview
The paper by Wu Qiong (1999), published in Petroleum Engineering Construction, documents a practical welding engineering challenge: joining heat-resistant steel 1Cr5Mo furnace tubes to ZG25 cast steel elbows in a pressure relief system. This dissimilar metal weld presents significant metallurgical challenges due to the large difference in chemical composition, thermal conductivity, and coefficient of thermal expansion between the two base metals. The paper describes the welding process using austenitic filler material, the operational techniques, the problems encountered, and the solutions implemented. Post-weld 100% radiographic testing achieved a 95% qualification rate at Class II film quality, demonstrating the effectiveness of the welding procedure.
Core Technical Content and Interpretation
Metallurgical Analysis of the Dissimilar Joint
The dissimilar metal weld between 1Cr5Mo and ZG25 presents several metallurgical challenges:
| Property | 1Cr5Mo (Furnace Tube) | ZG25 (Cast Elbow) | Difference/Challenge |
|---|---|---|---|
| Carbon Content | 0.35–0.45% | ~0.25% | Carbon migration risk |
| Chromium Content | 4.5–5.5% | Trace | Large Cr gradient |
| Molybdenum Content | 0.25–0.35% | Trace | Mo segregation |
| Thermal Conductivity | Moderate | Lower (cast) | Uneven heat distribution |
| Thermal Expansion | Higher | Lower | Thermal stress at joint |
| Hardness (Annealed) | ~200 HB | ~170 HB | Different HAZ response |
| Weldability | Moderate | Good | Carbon migration concern |
The primary metallurgical concerns include:
- Carbon Migration: During welding and post-weld heat treatment, carbon can migrate from the high-carbon 1Cr5Mo side to the lower-carbon ZG25 side, creating a decarburized zone (soft zone) in the 1Cr5Mo HAZ and a carburized zone (hard, brittle zone) in the ZG25 HAZ.
- Chromium Depletion: The chromium in the 1Cr5Mo can diffuse toward the austenitic weld metal, creating a chromium-depleted zone susceptible to intergranular corrosion.
- Thermal Mismatch: The different thermal expansion coefficients create residual stresses at the joint during cooling and subsequent thermal cycling in service.
Welding Procedure Design
The paper employs an austenitic filler material for the dissimilar joint, which is a standard approach for joining dissimilar steels. The austenitic weld metal acts as a buffer zone, accommodating the thermal expansion mismatch and reducing the severity of carbon migration.
Welding Process Parameters:
| Parameter | Specification |
|---|---|
| Welding Process | SMAW (Shielded Metal Arc Welding) |
| Filler Metal | Austenitic (E309L or equivalent) |
| Preheat Temperature | 150–250 °C |
| Interpass Temperature | ≤300 °C |
| Post-Weld Heat Treatment | Stress relief at 620–650 °C |
| Inspection | 100% RT, Class II acceptance |
Operational Techniques:
- Preheating: Moderate preheating (150–250 °C) is applied to reduce cooling rates and minimize the risk of hydrogen-induced cracking in the high-carbon 1Cr5Mo side.
- Heat Input Control: The welding heat input is carefully controlled to minimize the extent of the heat-affected zone and reduce carbon migration. Lower heat input is preferred, but must be balanced against the risk of cold cracking.
- Welding Sequence: The weld is typically initiated on the ZG25 side to minimize the heat input into the 1Cr5Mo side. Multiple layers with thorough interpass grinding are used to control the weld profile and reduce porosity.
- Post-Weld Heat Treatment: Stress relief heat treatment at 620–650 °C is performed to reduce residual stresses. However, the temperature must be carefully controlled to avoid excessive carbon migration during the PWHT.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracks in 1Cr5Mo HAZ | High cooling rate, hydrogen | Preheat, low hydrogen electrodes, controlled heat input |
| Porosity in weld metal | Inadequate arc shielding, surface contamination | Clean surfaces, proper gas shielding, multiple passes |
| Excessive HAZ hardness | High carbon content in 1Cr5Mo | Control heat input, post-weld stress relief |
| Carbon migration | PWHT at excessive temperature/time | Limit PWHT temperature, reduce holding time |
| Undercut | Excessive travel speed, improper technique | Adjust parameters, skilled operator |
Standards and Quality Control
The welding procedure must comply with relevant standards:
- GB/T 985.1: Welding procedures for steel—General principles for selecting welding procedures
- NB/T 47014: Qualification test procedures for welding of pressure vessels
- ASME Section IX: Qualification of welding procedures
- GB 50235: Construction and acceptance code for industrial metal piping
The 95% qualification rate at Class II RT acceptance demonstrates a well-controlled welding procedure, though the remaining 5% non-qualified welds indicate the inherent difficulty of dissimilar metal welding and the need for careful operator training and procedure adherence.
Key Questions and Reflections
- What is the long-term service performance of this dissimilar joint under thermal cycling conditions? Carbon migration and creep may degrade the joint over time.
- How does the choice of austenitic filler metal affect the creep resistance of the joint at elevated service temperatures?
- Would a two-layer weld approach (austenitic first layer on 1Cr5Mo side, then transition layer) improve the joint's long-term reliability?
- What non-destructive testing methods beyond RT could provide additional confidence in the joint's integrity?
Study Insights and Implications
This paper provides a valuable practical case study in dissimilar metal welding engineering. The use of austenitic filler material to bridge the metallurgical gap between 1Cr5Mo and ZG25 is a well-established approach, but the specific challenges of furnace tube applications—high operating temperatures, thermal cycling, and the presence of corrosive media—require careful attention to detail. The 95% RT qualification rate, while acceptable, highlights the need for continued improvement in welding technique and procedure optimization. Engineers working with dissimilar metal joints should always consider the long-term metallurgical stability of the joint, not just the initial weld quality, as carbon migration and creep can cause delayed failures that are not detected by initial NDT.