Dissimilar Material Welding of 1Cr5Mo Furnace Tubes to ZG25 Elbows

Literature Overview

The paper by Wu Qiong, published in Petroleum Engineering Construction in 1999, addresses a practically significant challenge encountered in refinery depressurization furnace systems: the welding of 1Cr5Mo heat-resistant steel furnace tubes to ZG25 cast steel elbows. This is a classic dissimilar material welding scenario where the two parent metals differ substantially in chemical composition, thermal expansion coefficient, and high-temperature mechanical behavior. The author reports that after welding, 100% radiographic testing was performed, and the Grade II film pass rate reached 95%, which is a commendable result for such a challenging joint.

Core Technical Challenge

The fundamental difficulty in joining 1Cr5Mo to ZG25 lies in the incompatibility of the two materials. 1Cr5Mo is a low-alloy heat-resistant steel containing approximately 0.4-0.6% Cr and 0.25-0.35% Mo, designed for high-temperature service up to 600-650°C. ZG25 is a medium-carbon cast steel with approximately 0.20-0.30% C, having significantly lower alloy content and different thermal expansion characteristics. When these two materials are welded together, the resulting joint must withstand cyclic thermal loading typical of furnace duty without cracking, excessive distortion, or premature degradation.

The critical metallurgical concerns include:

• Carbon diffusion from the higher-carbon ZG25 side into the weld zone, leading to carburization of the 1Cr5Mo side and decarburization of the ZG25 side
• Differential thermal expansion causing high residual stresses at the weld interface
• Sensitivity of the 1Cr5Mo HAZ to low-temperature cracking due to its higher hardenability
• Molybdenum segregation and potential Mo-rich phases at the weld boundary

Welding Process Selection and Key Parameters

The author selected austenitic filler metal for this dissimilar joint, which is a well-established approach for joining dissimilar steels. The austenitic weld metal acts as a buffer zone, accommodating the mismatch in thermal expansion and reducing the risk of cracking. The choice of filler material is critical and must be carefully considered.

The use of austenitic filler metal introduces its own set of considerations. The coefficient of thermal expansion of austenitic stainless steel (approximately 17×10⁻⁶/°C) is significantly higher than both 1Cr5Mo (approximately 12×10⁻⁶/°C) and ZG25 (approximately 11×10⁻⁶/°C). This means that while the austenitic weld metal reduces cracking susceptibility, it introduces additional thermal mismatch stresses during cooling and subsequent service. The welder must balance the benefit of reduced cracking against the cost of increased residual stresses.

Common Defects and Countermeasures

Based on the author's analysis and supplemented by engineering experience, the following defects are most commonly encountered in this type of joint:

1. Hot cracking in the weld metal: The high sulfur and phosphorus content in ZG25 can promote solidification cracking. Countermeasures include strict control of preheat temperature, use of low-sulfur filler metal, and minimizing arc time per pass.
2. Cold cracking in the HAZ: The 1Cr5Mo side is susceptible to hydrogen-induced cracking, particularly in the coarse-grained HAZ. Countermeasures include adequate preheating (150-200°C), low hydrogen electrode selection, and controlled cooling rate.
3. Undercut and incomplete fusion: The geometry mismatch between the furnace tube and the cast elbow often creates difficult access angles. Proper joint preparation and multi-pass welding with careful root pass control are essential.
4. Porosity: Gas porosity from moisture in electrodes or contamination on the ZG25 casting surface. Thorough surface cleaning and electrode baking are mandatory.

Engineering Practice Insights

Having worked extensively with furnace tube systems, I find this paper particularly instructive for several reasons. First, the 95% RT pass rate on 100% radiography is a strong indicator of process control, but it also highlights that even 5% rejection in dissimilar material welding represents significant rework cost and schedule impact. In my experience, the key to achieving high first-pass quality in such joints lies in meticulous joint preparation and consistent preheating practice.

The depressurization furnace application is particularly demanding because these systems experience rapid thermal cycling during startup and shutdown. The weld joint must survive thousands of thermal cycles without degradation. I would recommend that any engineer approaching a similar welding challenge consider performing a thermal cycling qualification test on a coupon weldment before committing to production welding.

The choice of austenitic filler metal, while effective for crack resistance, introduces a long-term concern: the formation of a brittle sigma phase at the weld/HAZ interface during prolonged high-temperature exposure. For service temperatures above 550°C, this could become a significant issue. Alternative approaches, such as using 309L-type filler with controlled dilution, or applying a thin transition layer of austenitic material followed by a 1Cr5Mo-compatible overlay, should be evaluated for longer service life.

Study Reflections

This paper, though published in 1999, remains highly relevant to contemporary practice. The fundamental metallurgical challenges of dissimilar steel welding have not changed, even as welding equipment and consumables have evolved. The author's systematic approach—selecting appropriate filler metal, controlling thermal input, performing 100% NDT, and analyzing defects—is exactly the methodology that should be followed in modern engineering practice. The paper serves as a valuable reminder that in dissimilar material welding, the selection of filler metal is not merely a matter of matching mechanical properties but a strategic decision that must account for thermal expansion mismatch, metallurgical compatibility, and long-term service conditions.