Intergranular Phase Precipitation Mechanism in Ultra-Low Phosphorus Austenitic Stainless Steel Strip Electroslag Weld Overlay

Literature Overview

Published in the journal Welding in 1999, this paper by Wang Jiachun and Chen Yong investigates the intergranular phase precipitation mechanism in ultra-low phosphorus austenitic stainless steel strip electroslag welding (ESW) overlay layers. The research originates from the Beijing General Research Institute of Nonferrous Metals and addresses a critical issue in the production of clad plate and bimetallic pressure vessels: the formation of brittle intergranular phases that compromise the corrosion resistance and mechanical integrity of the overlay layer.

Core Technical Points

Phosphorus Segregation and Its Effects

Phosphorus is a well-known detrimental element in austenitic stainless steels because it segregates to grain boundaries during solidification and subsequent cooling. Even at low concentrations, phosphorus can significantly reduce the pitting resistance equivalent number (PREN) and promote intergranular corrosion. The study demonstrates that ultra-low phosphorus (< 0.015%) base strip material, when used in ESW overlay, still exhibits intergranular phase precipitation due to the high thermal cycle of electroslag welding.

Parameter	Conventional Strip	Ultra-Low P Strip	Effect
Phosphorus content (wt%)	0.020-0.045	< 0.015	Ultra-low P reduces but does not eliminate intergranular precipitation
Sintering temperature	1000-1200°C	1000-1200°C	Higher sintering promotes grain boundary phase formation
Slag composition (CaF2/CaO)	40/60	40/60	Slag basicity affects P partitioning
Cooling rate (°C/s)	1-10	1-10	Slower cooling increases segregation

Mechanism of Intergranular Phase Precipitation

The authors identified that the primary intergranular phases are chromium-rich M23C6 carbides and, in some cases, sigma (σ) phase. The precipitation sequence follows: solidification → chromium depletion at grain boundaries → chromium carbide nucleation → possible sigma phase transformation at elevated temperatures. The ESW process, with its high heat input and slow cooling rate, creates conditions favorable for these transformations.

The study further revealed that the strip composition, particularly the ratio of chromium to carbon, plays a decisive role. When the C/Cr ratio exceeds a critical threshold, chromium carbides precipitate preferentially at grain boundaries, creating chromium-depleted zones susceptible to intergranular corrosion.

Process Optimization

Optimization Parameter	Recommended Range	Rationale
Slag temperature	1100-1200°C	Lower temperature reduces P and S partitioning to slag
Electrode strip C content	< 0.03%	Low carbon limits chromium carbide precipitation
Interpass temperature	< 150°C	Prevents sigma phase formation
Post-weld heat treatment	1050°C × 1h + water quench	Dissolves intergranular phases and restores chromium homogeneity

Standards and Quality Control

For pressure vessel applications governed by ASME VIII Division 1 or GB/T 150, the overlay layer must pass intergranular corrosion testing per ASTM A262 Practice A (65°C oxalic acid) or Practice E (80°C sulfuric acid-copper sulfate). The findings of this paper emphasize that relying solely on low phosphorus content is insufficient; the carbon content and post-weld heat treatment must also be controlled to prevent intergranular phase formation.

Key Questions and Reflections

A notable contribution of this work is the recognition that ultra-low phosphorus does not guarantee freedom from intergranular phases. Engineers sometimes assume that reducing phosphorus to below 0.015% eliminates the risk, but this study clearly shows that chromium carbide precipitation driven by carbon and the thermal history remains a significant concern. This has direct implications for the procurement of ESW strip materials: specifications should include both P and C limits, along with mandatory intergranular corrosion testing.

Another reflection concerns the post-weld heat treatment. While solution heat treatment at 1050°C can dissolve intergranular phases, it may also cause grain coarsening and reduce the bond strength at the overlay/base metal interface. A careful balance must be struck between corrosion resistance and mechanical integrity.

Study Insights and Implications

This paper is a valuable resource for engineers designing ESW overlay processes for austenitic stainless steel clad plates used in pressure vessels and heat exchangers. The understanding of intergranular phase precipitation mechanisms enables more effective process control and quality assurance. In practice, this means that the overlay process specification should include not only chemical composition requirements but also thermal cycle parameters and mandatory post-weld heat treatment and intergranular corrosion testing protocols.