Zhuojin Pipe Fitting Co., LtdFailure Analysis of TIG Welded Butt Joints Between Martensitic and Austenitic Stainless Steel
Literature Overview
This paper by Ling Wei, Ge Liaohai, Ren Zhen'an, and Sun Daqian, published in "Transactions of the China Welding Institute" (2007, Vol. 28, No. 5, pp. 89-92), presents a comprehensive failure analysis of TIG-welded butt joints between 1Cr17Ni2 martensitic stainless steel and 1Cr18Ni9Ti austenitic stainless steel used in marine equipment. The joints experienced leakage failure in service, and the investigation employed optical microscopy, scanning electron microscopy, and microhardness measurements to identify the root cause.
Core Technical Concept
The fundamental challenge in welding dissimilar stainless steels lies in the vastly different thermal responses of martensitic and austenitic grades. Martensitic stainless steel (1Cr17Ni2) undergoes phase transformation during welding—heating above the Ac1 temperature produces austenite, and rapid cooling upon weld solidification transforms this austenite back to martensite, creating a hard, brittle microstructure. Austenitic stainless steel (1Cr18Ni9Ti), conversely, remains austenitic throughout the welding thermal cycle due to its high nickel content, which stabilizes the austenite phase.
Microstructural Zones and Hardness Distribution
| Zone | Material Side | Microstructure | Approximate Hardness (HV) | Failure Relevance |
|---|---|---|---|---|
| Weld metal | Interface | Mixed martensite/austenite | 250-350 | Moderate brittleness |
| HAZ (high-temperature) | 1Cr17Ni2 side | Fine martensite (rapid cooling) | 450-550 | High brittleness, crack initiation |
| HAZ (low-temperature) | 1Cr17Ni2 side | Coarsened ferrite | 150-200 | Low hardness, weak zone |
| HAZ | 1Cr18Ni9Ti side | Grain growth only | 180-220 | Relatively ductile |
| Base metal | 1Cr17Ni2 | Tempered martensite | 300-380 | Acceptable toughness |
| Base metal | 1Cr18Ni9Ti | Solution-treated austenite | 150-180 | High ductility |
Failure Mechanism Analysis
The critical finding of this investigation is that the failure zone is located in the high-temperature region of the martensitic steel's heat-affected zone, immediately adjacent to the weld metal. In this zone:
- The base metal was heated above the Ac1 temperature, transforming the tempered martensite to austenite.
- Rapid cooling during welding (due to the high thermal conductivity of the austenitic side drawing heat away) transformed this austenite to untempered martensite.
- The untempered martensite exhibited extremely low fracture toughness, evidenced by the typical cleavage and quasi-cleavage fracture morphology observed on the fracture surface.
The fracture surface analysis revealed a classic brittle fracture pattern—flat cleavage facets with river markings—confirming that the failure was controlled by the brittle martensitic microstructure rather than by weld metal defects or base metal imperfections.
Contributing Factors to Failure
| Factor | Description | Severity |
|---|---|---|
| Untempered martensite in HAZ | Formed by rapid cooling after phase transformation | Primary cause |
| Dissimilar thermal conductivity | Austenitic side conducts heat away, accelerating cooling rate on martensitic side | Aggravating |
| Thermal stress concentration | Differential thermal expansion between martensitic and austenitic sides | Aggravating |
| Residual tensile stress | From welding shrinkage constrained by dissimilar CTE | Aggravating |
| Potential hydrogen embrittlement | Absorbed hydrogen from arc plasma in martensitic microstructure | Contributing |
Remediation Measures
Based on the failure analysis, the following corrective measures are recommended:
- Preheating: Apply preheat of 200-300°C to the martensitic steel side to slow the cooling rate and allow partial tempering of the weld-formed martensite.
- Post-weld heat treatment: Perform stress-relief annealing at 500-600°C for 1-2 hours to temper the HAZ martensite and reduce residual stresses.
- Welding consumable selection: Use a nickel-rich filler metal (such as ER309L or a custom Ni-based composition) to dilute carbon content in the weld metal and reduce the amount of martensite formed in the weld.
- Interlayer welding: Consider inserting an intermediate layer of austenitic material between the two dissimilar steels to buffer the thermal gradient.
- Welding sequence optimization: For multi-pass welds, use a back-step welding sequence to minimize peak temperatures and thermal cycles on the martensitic side.
Engineering Practice Integration
In marine and offshore equipment, dissimilar stainless steel joints are common where different corrosion resistance requirements exist in adjacent zones. The failure described in this paper is a classic example of how welding-induced microstructural changes can compromise the integrity of otherwise sound components. The key lesson is that the heat-affected zone of martensitic stainless steel is the critical weakness in any dissimilar joint, and its properties must be actively managed through process control rather than assumed to be acceptable.
Study Insights and Reflections
This failure analysis exemplifies the importance of metallurgical understanding in welding engineering. The joint failed not because of poor weld technique or inadequate fusion, but because of an inherent metallurgical incompatibility that was not adequately addressed by the welding procedure. The martensitic steel's susceptibility to cold cracking and embrittlement during welding is well-documented, but in dissimilar joints, the presence of the austenitic steel actually worsens the situation by accelerating the cooling rate through thermal conduction. Any engineering design involving dissimilar stainless steel welding must incorporate a comprehensive metallurgical assessment of the HAZ, not merely the weld metal. The recommended post-weld heat treatment is non-negotiable for this joint configuration.